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AMIR: Erin Mohan, DOL
April 17, 2014 12:45pm - 2:00pm
April 17, 2014 12:45pm - 2:00pm
Room 272
After completing a federal appellate clerkship, Erin Mohan (JD '11) joined the Department of Labor through the Office of the Solicitor Honors Program. At DOL, Erin provides legal advice and litigates cases involving a range of worker protection issues, including minimum wage, family leave, immigrant guestworker programs, employee benefit plans, and worker health and safety.
Related Media
- Anna Wang
- annawang@
Admission Restrictions
This event is open to the Stanford community.


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The current BMW E90 generation is soon coming to an end, including the E90 M3 sedan. Based on the official BMW production schedule (see below), all E90 3-series and E90 M3 sedan production will cease in October 2011. M3 coupe and convertible fans have some more time, however. The E92 M3 and E93 M3 will continue to be produced until at least June 2012 (along with the 3-series coupe and convertible). Whether they will be produced beyond June 2012 as 2013 model years is not … CONTINUED
We recently posted official BMW studio photos of the M3 Coupe Competition Package in Frozen Black Matte, but for those who haven't yet seen the Frozen Black Matte M3 in real life photos, these are the best ones yet.

subset: commoncrawl


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High Energy Costs have Communities Reconsidering their Lighting Technology
The engineer for the City of Fairbanks, Alaska recently conducted an energy audit and found that the cities' 2646 street lights cost $535,000 to operate each year. The number was staggering, considering that the communities total electric bill was $880,000. Now the city engineer and other city officials are studying the situation, and trying to determine what can be done to reduce the energy costs.
Should we just turn out the lights?
One proposed option is to simply eliminate many of the street lights - an option which the local law enforcement officials do not favor, considering the established link between lighting and crime prevention. For a community like Fairbanks, where daylight is scarce in winter time, this could mean a very dark city. Community officials and other interested parties are also considering the possibility of transition to LED lighting, or other energy efficient lighting technology, but fear that the price tag may be too steep. Communities like Fairbanks should not dismiss LED lighting technology as too expensive. LED lighting options are quickly becoming more readily available, and have become more cost-effective. The City of Ann Arbor, Michigan recently just retrofitted all of its street lights with LED units. That retrofit cost about $600,000--just a little more than Fairbanks paid to operate its street lights in one year. The City of Ann Arbor expects to save over $100,000 each year in energy costs. That would be about a 5 year payoff if the City of Fairbanks did the same thing.
Is LED lighting really more expensive?
This illustrates a good point of LED lighting technology. The general perception is that LEDs are expensive. This is true. An LED light is more expensive, than a comparable incandescent or similar lamp, when one considers the upfront cost of the fixture. However, in most cases, LEDs are less expensive than incandescent and other older lighting technologies, when one considers the overall cost savings - which includes energy use, replacement lamps, and maintenance costs. In many situations, the return on investment is 3-5 years and the LED lamps can last for 10 years or more.
Should Christmas lights be eliminated to reduce energy costs?
Fairbanks is so concerned about its energy consumption that it is considering foregoing installing Christmas lights next year. There has been a lot of discussion about this, as it is perceived by most as an "unnecessary" energy consuming luxury. In some respects, it is hard to argue against that last point; however, its seems to me that this is simply taking a swipe at the low hanging fruit. We could eliminate all of the holiday lighting displays across the country, and we would conserve a substantial amount of energy, but it would be a drop in the bucket when considering the big picture. This is not to say that we should ignore small items like Christmas lights, when looking for ways to conserve energy--the point is that eliminating Christmas lights in Fairbanks isn't going to trim much from the utility bill, and it will trim a lot from the happiness and holiday spirit of the cities' residents.
At the end of the day, we need to look at the big picture when considering energy conservation, and we also need to take an approach that is realistic and will be accepted by the majority. Fortunately, we have viable and cost-effective options in lighting, that make these extreme measures unnecessary.


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9 Shouts - 7,432 Scrobbles
M26.7 is a jonglo-punk-rock'n'roll group from Cambrai (6 musicians of which a clown disguised juggler). After many dates which saw them going up on scene with Banlieue Rouge, René Binamé, Infraktion, Bomb Factory, Toxic Waste, Les Vieilles Salopes, Trouble Juice,... one 6 titles out in September 2005.
M26.7 on Last.fm.
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Luke Hancock averaged 10.9 points and 4.3 assists last season at George Mason.
June 7, 2011
Rick ~nbsp>Pitino Press Conference | Rick ~nbsp>Pitino on Podcast
LOUISVILLE, Ky. - Kevin Ware, a 6-4, 180-pound guard from Rockdale County High in Conyers, Ga., and Luke Hancock, a 6-5, 190-pound guard/forward who has played the last two seasons at George Mason, each have signed a financial aid agreement to attend the University of Louisville.
Ware averaged 13.2 points, 4.1 assists, 3.9 rebounds and 2.2 steals as a senior last season in helping Rockdale County post a 26-6 record and finish as the Georgia 4-A state runner-up. A first-team all-state selection, he is rated by Rivals.com as the No. 70 overall prospect in the class of 2011, No. 94 in the ESPNU Top 100 and 99th nationally by Scout.com.
"Kevin is someone who can play three positions," said U of L coach Rick Pitino. "He gives us great length and height at the point spot. He'll be outstanding with our full-court press and great with our offensive transition game. We're very excited to have him as a part of our program."
As a junior at Rockdale County, Ware averaged 14.9 points and 4.0 assists per game while earning 4-A second-team all-state honors as the Bulldogs won the regional championship and reached the second round of the 4-A state playoffs with a 28-2 record.
Ware totaled eight points and four rebounds for the White team in the 2011 Derby Festival Basketball Classic All-Star game, played in Louisville's KFC Yum! Center in April. He scored a season-high 28 points as a senior against Meridian (Miss.) High School.
Ware originally signed a national letter-of-intent with Tennessee before being granted a release, and had narrowed his choices to Louisville and Georgia before joining the Cardinals.
Ware is the fifth member of the Cardinals' 2011 stellar recruiting class, which is ranked among the nation's top 10. He joins fellow U of L signees Chane Behanan, a 6-7, 250-pound forward from Bowling Green (Ky.) High School; Wayne Blackshear, a 6-5, 205-pound guard from Morgan Park High School in Chicago; Angel Nunez, a 6-7, 190-pound forward from Bronx, N.Y. who played last season at Notre Dame Prep in Fitchburg, Mass.; and Zach Price, a 6-10, 230-pound center from Jeffersontown High School in Louisville, Ky.
Luke Hancock
"Luke is exactly the type of player that we need for our basketball team," said U of L Coach Rick Pitino. "He's a young man who understands how to play the game. He's a true triple-threat player -- he can catch, pass and shoot well -- and he really has a great basketball IQ. If it comes down to someone needing to make the pass, make the shot or go to the foul line, you want him doing all three in crucial situations."
Hancock, who hails from Roanoke, Va., averaged 10.9 points, 4.6 rebounds and a team-high 4.3 assists as a sophomore last season in helping George Mason post a 27-7 record and reach the second round of the NCAA Tournament. He connected on 49.4 percent from the field, 35.9 percent from three point range, and 81 percent at the free throw line. He will sit out the 2011-12 season and have two years of eligibility remaining.
He totaled 18 points, five assists and three rebounds against Villanova in the first round of the 2011 NCAA Tournament and his 3-pointer with 21 seconds left put the Patriots ahead for good in a 61-57 upset victory. He was a third-team All-Colonial Athletic Association selection as a sophomore when he scored in double figures on 19 occasions and handed out five or more assists 11 times.
Hancock earned all-rookie team honors in the CAA as a freshman when he averaged 7.7 points, 3.5 rebounds and 3.0 assists for the Patriots. He gathered 59 steals in his initial two collegiate seasons.
Hancock played high school basketball at Hidden Valley High School before playing for current U of L assistant Kevin Keatts for one season at Hargrave Military Academy when it was the national prep runner-up. He earned all-state honors while at Hidden Valley and was selected to the all-tournament team at the National Prep Tournament with Hargrave.
Hancock was an All-Timesland selection as a senior at Hidden Valley High School in 2007-08 when he averaged 13.9 points and 7.1 rebounds and helped the Titans reach the state AA semifinals for the second straight season.
Before selecting U of L, Hancock also considered transferring to Michigan, South Carolina, Virginia and Virginia Tech.


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Coping in a Violent World (02)
by Ken Good
Aikido Journal #101 (1994)
Random acts of violence seem to be on the increase, at least in the United States. Although many of us hope to improve our self-defense skills as part of our aiki training, just how realistic is this hope? Aikido Journal has asked four law enforcement professionals to answer a series of questions about how each of us can cope when confronted by violence. Using as a starting point the December 1993 Long Island Commuter train incident, in which a gunman gone berserk killed and wounded dozens of his fellow passengers, we asked our experts the following:
- What advice would you give to a passenger seated in a train car in which someone has begun shooting?
- Should the untrained individual attempt to disarm the gunman?
- What steps might you personally take to deal with the gunman?
- Are there any distracting maneuvers that could divert the gunman's attention in another direction?
- Are there techniques taught in the dojo that might be of use in such a situation?
- What steps should be taken by bystanders afterwards, while waiting for the police and emergency personnel to arrive?
- Is there any way to minimize the panic among passengers?
- What common sense steps can we take to protect ourselves in crowded publics places such as a train, subway, or bus?
- Do you favor banning or placing restrictions on the sale of handguns in an effort to reduce the number of gun-related incidents?
- What should people traveling abroad keep in mind when visiting large cities with frequent violence?
Addressing this particular situation is challenging to say the least. In writing this article, I would like to acknowledge the help of many friends, especially David Maynard and James Williams. Mr. Maynard has been a significant force in shaping my perspective through his insightful tactical concepts and close personal friendship. Mr. Williams, whose background includes thirty years of study in various martial disciplines such as Shorin-ryu Okinawa-te, tae kwon do, Tang Soo Do, judo, Amis de Mano, boxing, kick-boxing, Hung Gar Kung Fu, Temple tai chi of Wayson Liao, Grade Jiu Jitsu, aikijujutsu (14 years), as well as serving as an officer with the US Army infantry, has been my teacher.
Dave Maynard and I have spent the last fifteen years working with firearms and their tactical employment in a wide variety of situations, including military applications with Naval Special Warfare at SEAL Team One. In my present capacity as course director of Security Forces Training for US Naval Fleet security, I have had the unique opportunity to work with many military units, federal agencies, and police SWAT teams conducting high-risk entry type training. Our department conducts gun fight simulations utilizing semi-automatic paint projectile rifles and pistols. We have trained thousands of students, some of whom have been in actual gun fights following their training. Discussions with these individuals reveal the effectiveness of this approach to training.
I have been forced to draw my weapon many times on human beings in actual self-defense situations. In the training environment, I can only estimate, but I believe that I've been in over thirty thousand mock gun battles. During these training sessions, typically involving twenty participants, nearly ten thousand rounds of ammunition are fired a day in a scenario environment The sheer number of engagements has literally pounded gun fighting principles into my body and mind. The logical extension has been to develop Combative Concepts, Inc., a consulting company that facilitates training for those who carry weapons professionally, or are interested in their application in self-defense situations. Our concept of gun fighting and the associated tactical principles has steadily evolved since we decided to scrap the "play book" originally given to us by our military training. It has been interesting to note the many parallels between aiki and our proven gun fighting principles. Constant involvement in simulated gun fights forced us to re-evaluate our original "hard" methods. A precise, frame by frame motion analysis significantly changed our approach to training and our tactical employment of firearms. No one can sustain prolonged punishment from firearms or edged weapons. It is best to read your opponent, blend with him and allow him to assert himself or force him to make the mistake. Mistakes such as over-extension resulting from improper target analysis, erratic movement, speed control problems, and over-aggression are all evident in the many students we have encountered.
Pistols are highly effective weapons as demonstrated by the tragedy in New York. Could things have been different? Quite possibly yes. The primary focus of this discussion will be on the unarmed individual facing an armed attacker. This is nothing new to anyone studying a fighting system. In this case, the armed attacker is a gunman. How would you react?
Before we determine what must be done in relation to the gunman, let us spend some time determining how the gunman may think and what the gunman must overcome to carry out his attack successfully.
In this case, the total weapon system constitutes a disturbed male in possession of a handgun. The psychotic gunman starts with many advantages. He selects his weapons and he decides the time and place he intends to initiate his attack. He may spend considerable time planning, to give himself the maximum tactical advantage. He selects his victims, giving himself the power to decide who lives or dies. He is not interested in the consequences of his actions, only in the destruction of those he has targeted. He has no regret, and is not concerned that your loved ones will mourn your death. If you believe you will be exempted, spared because of your caring personality, then you become exactly the victim he was looking for in the first place. You will fall like the others when the gun begins to speak.
The attacker must deliver a lethal blow to a vital area, which is not as easy at it appears. Please keep in mind, I am not advocating running into bullets!!! But, remember-bullets kill by rupturing major organs, disrupting the blood transport system or by shocking the central nervous system. Statistically, gunshot wounds are not always fatal, although many believe they are. True accounts of the amazing resiliency of gunshot victims are too numerous to cite. Also, it is interesting to note that in conflicts involving two combatants with handguns, often at ranges closer than ten feet, 85% of the shots fired miss the intended target entirely!
A well publicized gun fight took place in early February in San Diego, California between a store owner and robbery suspects. This fight illustrates my point. A tremendous volume of fire was exchanged without one shot hitting its intended target. Why does this consistently happen? Lack of range time for those in possession of firearms? Lack of intent? I do not believe these are the answers. The duress created by the engagement itself makes simple tasks exceedingly difficult. Factors such as lighting conditions, movement, and the adrenaline rush caused by extreme stress all significantly change the event Simply stated, someone was fighting off the attack. No static range situation can simulate all the conditions of an actual dynamic engagement.
Recently in San Diego, California, a felon aimed a weapon at two police officers, but never fired. The officers fired several shots, killing a bystander. Further investigation revealed that one officer's bicycle (the mode of transportation in this case) was struck by the officer's own gunfire, at angles nowhere near the desired target line. What happened? The officers were placed under duress, and this created unwanted and tragic physical results.
Again in San Diego, a SWAT police officer from a neighboring city, who we have had the privilege of training, was in a shooting involving three suspects all armed with handguns. As he relayed the events, foremost in his mind was purposeful movement and target analysis. Initially in an open alley area, the officer faced the first suspect who was less than ten feet away. He moved as the first suspect turned to fire a handgun. The officer displaced himself with proper lateral movement, never allowing the attacker to keep the weapon on his center line, the lower body articulating to keep the firing platform steady, head and eyes moving. The first suspect did not move and was shot five times by the officer. The second suspect appeared on a stairway above. Again movement and proper maintenance of body control allowed the delivery of an incapacitating blow to the second suspect. Suspect #3 produced a weapon through a hinged window and could not bring it to bear as the officer again moved from the potential target line. This suspect was apprehended. Both the first and second suspects survived the encounter although they had been struck with projectiles. Suspect #1 was struck in the "center of mass area." This is significant, as we are discussing the difficulty in actually delivering lethal blows to non-compliant adversaries. Incidentally, the officer had a partner who never fired a round and confided he felt that he was never "in the fight." He was caught one step behind in the decision cycle.
The keys to the first officer's success were purposeful movement and body control facilitated by control of the mental state, achieved through proper training. Purposeful movement is that action undertaken based on analysis of the attacker to enable the application of the appropriate technique.
What can bystanders do?
It is fundamental to maintain the proper state of mental awareness at all times. One must constantly evaluate the physical surroundings and the people who occupy those spaces. No defense system or individual technique, regardless of its sophistication or potential effectiveness, can be useful if it is never initiated because of mental laziness or ignorance. A military commander can be forgiven for being defeated but never for being surprised. This mind-set of always asking yourself, "What is wrong with this circumstance?" sets you on the path to recognize threats and threatening situations before they become completely unmanageable. As a functioning member of society surrounded by technical sophistication and many lifestyle comforts, we can easily forget the lurking, potential savagery of other human beings.
The defender must constantly analyze the ever-changing situation. Timing is a critical element This means maintaining maximum aware ness of the attacker at all times. Only then can you determine the answers to these critical questions: Where is the attacker focusing his attention? In what direction is the weapon pointed at this moment in time? In what direction is he moving? How is his body positioned? Is he specifically aware of me? Does he appear to be calm or under extreme duress? Is he reloading? Is anyone working with him? Realistically, processing all this information in a compressed time frame requires proper training.
Those who master techniques for defeating a swordsman do not focus on the sword itself, but on defeating the one who uses the sword. To defeat the weapon system, we don't focus on the weapon, but the gunman himself.
During any engagement a phenomenon known as "target fixation" occurs. Essentially, during periods of extreme duress, the mind has a tendency to sharply focus on a small percentage of the total situation. If you have ever been in conflict or been involved in a traumatic episode such as a car accident, you may have experienced this phenomenon. The debilitating effects of this phenomenon can only be minimized by training in stressful environments while attempting to perform associated tasks. Target fixation is potentially deadly to anyone operating in a multiple threat environment Undoubtedly, the gunman in New York experienced this phenomenon and had to overcome its effects. Target fixation can be exploited. Herein lies a key to possible success. Unmolested and faced with little or no pressure, the attacker can operate with a great deal of effectiveness. Furthermore, if all targets are compliant and non-moving, target acquisition and bullet delivery is relatively easy. Elusive, thinking, and moving targets are an all together different problem.
Keep in mind that the gunman can place the muzzle in only one point in space at any given time, thus creating a defined trajectory line Projectiles travel along this line for very brief periods of time and are not everywhere at all times. Granted, the weapon's placement in space can be altered fairly quickly, but you as the defender must note this line and move accordingly. During this repositioning, time has passed. As time passes, opportunities rise and fall for both combatants. The attacker is now presented with a myriad of decisions previously unfaced. His simple assault has now become complex. The gunman is forced to focus further. The negative effects of target fixation are now multiplied against the gunman. Windows of opportunity may open, but it must be stressed that no guaranteed solution exists. All one can hope for in a situation such as this is a reduction in the probability of becoming a victim. The alternative is panicked motion or paralysis, further tipping the balance in favor of the gunman.
To defeat the gunman, we must understand that he is subject to the constraints of human reflex and affected by the psychological pressures and phenomenon associated with stressful situations. The attacker must keep the weapon within its effective range. If his victim gets too close, the gun might be turned against him. If he allows his target to move too far away, he faces difficulty in target acquisition and shot placement.
If you are close (within ten feet), displace and close the gap. Close whether or not the weapon is discharging. You may be shot diminishing your physical capabilities. Once committed, you must close undeterred. Remember, the vast majority of people shot with handgun rounds survive the encounter. A significant factor contributing to the survival of these people was their strong will to live. When you are within range, disrupt the attacker's balance while simultaneously turning the weapon toward the gunman The weapon is the attacker's primary source of strength and psychological terror. He may have selected this weapon specifically for its intimidating qualities. Maintain control of all weapons and turn them over to the authorities.
If you are at an extended distance, the simplest solution may be exiting the space. Leaving the area to contact authorities can assist in the apprehension of fleeing suspects and facilitate the arrival of emergency medical response teams. You may have loved ones or other constraining reasons compelling you to stay in the conflict. At this point use timing and well conceived movement to close the gap.
To train for these encounters, obtain an operating replica weapon that has a functioning trigger and hammer. Through experimentation, determine the distance at which you can successfully disarm an attacker. For even more realistic training, utilize a paint projectile weapon or Simu-nition ammunition along with the associated safety gear. As the defender you must be pro-active. Learn to use body displacement when moving. You will soon discover the shooter faces distinct disadvantages at close ranges.


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Member in Bhutan
Ministry of Agriculture and Forests (MoAF)
Mission: To ensure sustainable social and economic well-being of the Bhutanese people through adequate access to food and natural resources.
Contact Person: Dr Lyonpo Pema Gyamtsho, Ministrer
Address: Thimphu
Tel: +975 323765 / 321142
Fax: +975 324520
Email: firstname.lastname@example.com
Website: http://www.moaf.gov.bt/moaf/
Royal Society for Protection of Nature RSPN
Mission: To inspire personal responsibility and actively involve the people of Bhutan in the conservation of the Kingdom`s environment through education, applied research and information dissemination; in collaboration with concerned agencies and indigenous institutions.
Contact Person: Dr Lam Dorji, Executive Director
Address: Deki Lam, Changangkha
PO Box 325
Tel: +975 (02) 322-056
Fax: +975 (02) 323-189
Email: firstname.lastname@example.com
Website: http://www.rspnbhutan.org


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People's Critic: Film Reviews
This blog is my look at movies, pop culture and entertainment. It's a blog by the people that speaks for the people who love entertainment. I love to write and to share my opinion. Hope you enjoy what I have to say.
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Guy Pearce – Iron Man 3 Character Poster
Guy Pearce, one of Hollywood's most consistent actors, lends his talents to Iron Man 3 as Dr. Aldrich Killian. The rumor is Killian develops the nanotech technology that allows Tony Stark to use the Extremis suit. I'm not sure how big of a role Pearce will have, but adding him to the cast doesn't hurt. I'm hoping […] [Read More]
Super Bowl Movie Trailers
Super Bowl Sunday is full of football, stress, pro bets, and of course trailers. Here are the trailers that debuted this year. The making of World War Z has been almost as big of a nightmare as a real zombie apocalypse. Despite all the red flags screaming form the set, watching Bard Pitt rescue his […] [Read More]


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In a stepping motor having phase windings A, B, etc. the current through the phase windings is switched by a drive circuit including switching transistors T1A, T1B etc. in series with each individual phase winding. The transistors are controlled by a phase sequence logic circuit 14. A current flow characteristic is indicative of rotor position and is measured and used to operate the phase sequence logic circuit. The drive circuit may incorporate a chopper-type control operated through further transistors T2A, T2B etc. also in series with each phase winding. The current flow characteristic that is measured may be current rise time, current decay time or chopping frequency. The current characteristic being monitored may be the characteristic in an energized winding or else in an unenergized winding at low current level. The drive circuit may alternatively provide a constant voltage power supply to the phase windings in sequence with current being repeatedly switched at a low level to an unenergized winding to monitor rotor position.
1. A drive circuit for a stepping motor having at least one stator phase winding comprising:
phase switching means associated with said at least one phase winding for switchably connecting said at least one phase winding to supply terminals;
phase control means for operating the phase switching means;
means for repeatedly chopping the electrical connection from the supply terminals to said at least one phase winding when current flow in such phase winding has risen to a predetermined value;
a by-pass circuit associated with each of said at least one phase winding for allowing current flow through the winding to decay when the chopping means operates;
means for restoring the electrical connection from the supply terminals to said at least one phase winding after a delay time;
means for measuring a characteristic of current flow in the motor to derive an indication of rotor position, said characteristic being selected from the group of characteristics comprising current rise time, current decay time and chopping frequency; and
means for utilizing the value of the characteristic measured in said measuring means to operate the phase control means.
2. A drive circuit for a stepping motor having a plurality of phase windings comprising:
phase switching means associated with each one of said plurality of phase windings for switchably connecting each phase winding to supply terminals;
phase control means for operating the phase switching means;
means for repeatedly switching current to an unenergized phase winding of said plurality of phase windings;
means for switching off said current when it has reached a predetermined low value;
means for measuring the time for such current to rise to said value, said measured time being indicative of rotor position; and
means for utilizing the value of said measured time to operate the phase control means.
3. A drive circut as claimed in claim 1 in which the delay time is the time required for the current to decay to a predetermined value.
4. The drive circuit as claimed in claim 1 in which the characteristic of current that is measured is a characteristic of current flow through an energized phase winding.
5. The drive circuit as claimed in claim 1 in which the characteristic of current flow that is measured is a characteristic of current flow at a low level through an unenergized phase winding.
6. The drive circuit as claimed in claim 2 in which switching means are provided for switching a constant voltage supply to each phase winding in sequence.
This invention relates to stepping motors and drive circuits therefor. It is concerned with stepping motors operating with closed-loop or rotor-position control.
Closed-loop stepping motor control has the attractive properties of guaranteed step integrity and optimal performance for varying load conditions, but this form of control has failed to attain widespread popularity because existing methods of rotor position detection based on optical, inductive or capacitive effects or the Hall effect are expensive or unreliable. The same problem has also occurred in the development of switched reluctance motors where, in traction applications for example, an optical position encoder may be operating in a particularly harsh environment.
In an attempt to eliminate the optical encoder a so-called "waveform detection" of rotor position has been suggested. With this technique the modulating effect of the motional emf on the current waveform is measured and, since the instantaneous motional emf is dependent on rotor position, it is possible to deduce the rotor position. Waveform detection has the advantage of being low cost but is unreliable in most situations because, for example, it is unable to operate at low speeds where the motional emf component approaches zero. Furthermore it appears that waveform detection can only be applied to the series resistance/constant voltage drives, which are commonly used for small motors of less than 1 kW output.
Most large stepping and switched motors are driven by chopper-type circuits. Such circuits incorporate means for repeatedly chopping the electrical connection from the supply terminals to an excited phase winding when current flow has risen to a predetermined value, which will normally be higher than the rated current of the phase winding. The current flowing in the now disconnected phase winding decays through a by-pass circuit and the connection to the supply terminals is restored after a delay time, which may be the time for the current to decay to a predetermined lower value, normally lower than the rated current of the winding. Such a control may be termed a "bang-bang" current control and it functions to maintain the average current in an excited phase winding near its rated level. Because the average current is normally constant techniques of waveform detection for deducing the position of the rotor are inapplicable.
It is an object of the invention to provide a closed-loop control for a stepping motor without the need for additional electrical or mechanical connections to the motor.
According to the invention a drive circuit for a stepping motor having at least one stator phase winding comprises phase switching means associated with the or each phase winding for connecting its associated phase winding to supply terminals, and phase control means for operating the phase switching means, wherein means are provided for measuring a characteristic of current flow in the motor to derive an indication of rotor position, the value of the characteristic measured in said measuring means being used to operate the phase control means.
As applied to a motor with a chopper type drive circuit means are included for repeatedly chopping the electrical connection from the supply terminals to a phase winding when current flow in a phase winding has risen to a predetermined value, a by-pass circuit associated with each phase winding for allowing current flow through the winding to decay therethrough when the chopping means operates, and means for restoring the electrical connection from the supply terminals to a phase winding after a delay time. The delay time may be the time required for the current to decay to a predetermined value.
In carrying out the invention the characteristic of current flow that is measured may be a characteristic of current flow through an energized phase winding. Alternatively, a phase winding other than the energized phase winding may be supplied with a low level of current and a characteristic of that current flow may be measured. Alternatively again, additional windings may be provided wound on the same poles as the phase windings and a characteristic of current flow through the additional windings is measured.
In the case of a chopper type drive circuit the characteristic of the chopped current flow, whether through an energized or an unenergized winding, that is measured may be current rise time, current decay time or the chopping frequency.
In the case of a stepping motor switched by a constant voltage supply the characteristic of current flow that is measured may be the magnitude of the current switched to an unenergized winding after a predetermined time.
The invention is based on utilization of the fact that the inductance of a phase winding is dependent on rotor position and varies substantially sinusoidally from a maximum to a minimum as the rotor advances over a pole pitch. This dependence is modified at high current levels by a magnetic saturation and at high speeds by the effect of motional emf. Nevertheless, despite these modifying effects the variation of inductance causes corresponding variation in certain characteristics of current flow an appropriate one of which may be selected to derive an indication of rotor position and thus be used to provide closed-loop control.
In order that the invention may be more fully understood reference will now be made to the corresponding drawings in which:
FIG. 1 is a circuit diagram of one phase of a known chopper drive circuit,
FIG. 2 shows calculated variations with rotor position for: (a) phase inductance, (b) phase torque, (c) rise time, (d) decay time, and (e) chopping frequency,
FIG. 3 shows (a) measured phase inductance variation, and (b) measured and calculated chopping frequency variation, with rotor position,
FIG. 4 gives experimental results for chopping frequency/rotor position for various values of phase current,
FIG. 5 shows at (a) torque, at (b) the effects of motional emf and at (c) rise time for low and high speeds,
FIG. 6a is a diagram of a circuit for detecting when a rotor has reached a particular position,
FIG. 6b shows waveforms explanatory of the operation of the circuit of FIG. 6a,
FIG. 7 is a diagram illustrating an embodiment of the invention, and
FIG. 8 is a diagram illustrating another embodiment of the invention.
A circuit diagram for one phase winding A of a stepping motor is shown in FIG. 1. A supply source 3 to the stepping motor provides a unipolar or unidirectional voltage V between terminals 1 and 2 and winding A is connected in series with two transistors T1 and T2 to these terminals. Transistor T1 is controlled to switch the phase excitation to winding A and transistor T2 is controlled to chop the excitation current. A small resistor Rc is provided in series with the collector T1 and the voltage Vc dropped across this resistor acts as a monitor of the instantaneous phase current. A diode D1 is connected from terminal 2 in parallel with transistor T1 and winding A and a diode D2 is connected to terminal 1 in parallel with transistor T2 and winding A. Voltage V is of high value which if applied continuously to winding A would result in a current flow of many times the rated value. When winding A is first energized both transistors T1 and T2 conduct and the current through winding A builds up at a rate dictated by the effective inductance of the winding. When the phase current reaches a level which slightly exceeds the rated current transistor T2 is switched off to disconnect winding A from the supply and the inductive energy in winding A causes current to circulate around the closed path comprising winding A, transistor T1, resistor Rc and diode D1. This path has a high inductance and a low resistance and the current decays slowly. The value of the current is monitored by measurement of voltage Vc and when it has decayed to a predetermined value transistor T2 is turned on to restore the supply voltage V to the phase winding causing the current to rise again.
This cycle of operations repeats with chopping being effected by repeated turning on and off of transistor T2 until the end of the phase excitation interval is reached. When this occurs transistor T1 is switched off and the energy in the winding decays around a path which includes winding A, diodes D1 and D2 and the path between terminals 1 and 2 through the supply source 3. As this path includes the supply voltage V the current is forced rapidly to zero and a large proportion of the inductive energy stored in winding A is returned to the supply.
From the above description it is apparent that the rise and fall of phase current as T2 turns on and off occurs at a rate which is dictated by the phase inductance. In a variable-reluctance motor this phase inductance varies with rotor position and therefore the chopping characteristics of the drive circuit vary during the phase excitation interval. This effect is quantified below assuming initially that magnetic saturation does not occur at any rotor position and that the motor is operating at a speed where the motional emf is small in comparison to the supply voltage. A qualitative treatment of the modification required to deal with saturation and motional emf is presented later.
Throughout the analysis the effects of secondary coupled circuits in the stator and rotor iron have been neglected, because the motors under consideration are laminated and, in any case, the chopping characteristics are dominated by the highly-inductive multi-turn phase winding circuit, which has a much longer electrical time constant than any, effectively single-turn, circuit distributed over a lamination.
It is assumed in the analysis that the mutual inductance between phases is negligible, as is usually the case in motors where each phase winding is concentrated on separate stator teeth. In cases where there is significant mutual inductance between phases, the chopping characteristics may be modified by coupling between phases, but even in these situations the chopping characteristics remain position dependent and can be used as a basis for position detection.
In the absence of magnetic saturation, as occurs for operation below rated current, the self inductance of one phase of a variable-reluctance stepping motor can be approximated by a sinusoidal function of rotor position:
La =Lo +L1 sin pθ (1)
where La is the self inductance of winding A, Lo is the average inductance thereof, L1 is the fundamental component of inductance variation with rotor position, p is the number of rotor teeth and θ is the rotor position. The torque Ta produced by this phase, when carrying a current I equal to the average current in the excited phase is:
Ta =1/2I2 dLa /dθ=1/2pI2 L1 cos pθ(2)
Now consider a time interval trise in which the phase current rises from (I-δI/2) to (I+δI/2), as a result of the high supply voltage applied to the phase winding. Neglecting at present the motional emf, the phase voltage equation is:
V=Ri+La di/dt (3)
where i is the instantaneous value of current, resistance R includes the resistance of phase winding A and the monitoring resistance Rc. If the phase current excursion δi is small enough to ensure approximately linear current changes, equation (3) can be re-written:
V=RI+La δI/trise (4)
substituting from equation (1) for La and re-arranging: ##EQU1##
Similarly for a time interval tdecay in which the phase current decays from (I+δI/2) to (-δI/2):
O=RI-La δI/tdecay (6)
and substituting from equation (1) for La : ##EQU2##
Finally, the instantaneous chopping frequency fchop can be found in terms of the rotor position:
fchop =1/(trise +tdecay) (8)
and substituting from equations (5) and (7):
fchop =(V-RI)(RI)/VδI(Lo +L1 sin pθ) (9)
Equations (5), (7) and (9) show that current rise time, current decay time and chopping frequency all depend on rotor position, so any of these three parameters may be chosen for position detection. Typical variations of the parameters, along with the variations of phase induction and torque, over a complete rotor tooth pitch are shown in FIG. 2.
Of particular interest in FIG. 2 are the relationships between the torque produced by the phase shown at (b), the rise time shown at (c), the decay time shown at (d) and the chopping frequency shown at (e) at each rotor position. Suppose, for example, that the motor is accelerating, so each phase must produce maximum torque, and that the excitation scheme is chosen so that each phase is excited for half the total excitation cycle (the `half-stepping` mode for three-phase motors). From FIG. 2(b) it can be seen that the phase being considered must be excited for rotor positions in the range -π/2p<θ<π/2p. The corresponding variations of the chopping parameters over this range of rotor position in FIG. 2 in curves (c) to (e) show that the rise and decay times increase over the range and that the chopping frequency decreases over the range. So to detect the appropriate rotor position for the phase to be switched off it is sufficient, in this case, to find the maximum rise/decay time or minimum chopping frequency.
A similar argument can be applied to deceleration conditions in which the phase must produce negative torque, and for which the minimum rise/decay time or maximum chopping frequency must be detected.
The variations of phase inductance and chopping frequency with rotor position for a 3-phase, single-stack, 15� step-angle motor (p=8) were investigated experimentally using phase currents which were small (typically 5%) compared to the rated value. The results of FIG. 3(a) were obtained by measuring the inductance at 1 kHz of a phase winding on an ac bridge at various rotor positions. Although there is some small departure from the sinusoidal characteristic assumed in FIG. 2(a), it is possible to extract the constant and fundamental components of inductance variation, giving:
Lo =12.4 mH
L1 =4.2 mH
This phase was then used in a chopper drive circuit having the following parameters:
i V=50 V
The variation of chopping frequency with rotor position was measured over the complete range of position with the motor stationary, giving the results shown by the full line in FIG. 3(b). Also shown by a chain dotted line in FIG. 3(b) is the theoretical variation of chopping frequency obtained by substituting the inductance and drive parameters into equation (9). There is good agreement between theory and experiment at all chopping rates, even though the departure of the inductance variation from the assumed sinusoid has an evident effect.
In a well-designed variable-reluctance motor there is considerable saturation of the stator and rotor teeth in aligned position and therefore the effect of saturation on the chopping characteristics must be considered. At rotor positions where the iron teeth on the stator and rotor are fully aligned (e.g θ=π/2p) the rate of change of flux linkage with phase current decreases rapidly as the rated current is approached, due to magnetic saturation; the incremental inductance of the phase is decreasing. Similar effects are observed over a wide range of rotor positions, because local saturation can occur at the tooth `corners` even when the teeth are substantially out of alignment. However, as the rotor teeth approach positions at which they are completely mis-aligned with respect to the stator teeth (e.g. θ=3π/2p), the flux linkage/current characteristic becomes linear; saturation effects are eliminated, and the incremental inductance is constant (and equal to the average inductance) over the full current range.
These observations have implications for chopping characteristics. If the chopper drive is operating around the rated current level then the effective phase inductance for the small current excursions is the incremental inductance at rated current. At mis-aligned rotor positions, therefore, the chopping characteristics are independent of current level, because the incremental inductance is invariant. However at aligned rotor positions the incremental inductance decreases with increasing current and, referring back to equations (5), (7), (9), it can be seen that the current rise and decay times undergo a corresponding decrease and that the chopping frequency increases above the value predicted by the theoretical treatment above. Indeed, because the incremental inductance for aligned rotor positions is less than for the mis-aligned positions, the chopping frequency/rotor position characteristic can be reversed with the maximum chopping frequency occurring when the stator/rotor teeth are fully aligned (θ=π/2p).
Experimental verification of these observations was obtained by operating the system described above at various current levels up to the motor's rated current. The results are illustrated in FIG. 4 at (a), (b), (c) and (d) all at a supply voltage V=50 volts for various average current levels I as follows:
______________________________________ I amps δamps______________________________________(a) 1.40 0.30(b) 0.72 0.20(c) 0.33 0.07(d) 0.10 0.16______________________________________
The results show a local maximum in the chopping rate at pθ=3π/2, which corresponds to the maximum predicted by the earlier theory (note: the absolute value of the frequency maximum has altered, by comparison to FIG. 3, because of the change in current level). However there is also a much larger maximum in this characteristic at the aligned teeth position (pθ=π/2) because of the low incremental inductance.
Detection of rotor position is now subject to the additional complication that even within the range of rotor positions corresponding to positive torque production a given chopping frequency may indicate two rotor positions e.g. a frequency of 3.25 kHz for a current of 0.72 A (FIG. 4(b)) is obtained at θ/p=-20� and θ/p=+60�. However any ambiguity in detected position can be eliminated quite simply by considering whether the chopping frequency is increasing or decreasing with rotor position.
The theoretical treatment of chopping characteristics presented above requires modification for operation at high speeds, where the motional emf, arising from flux linkages varying with rotor position, must be taken into account. Expressed algebraically the motional emf e is:
e=idLa /dt=I(dLa /dθ)�θ (10)
and substituting from equation (1):
e=pIL1 cos pθ�θ (11)
This motional emf appears as an additional term on the right of the phase voltage equation (3):
V=Ri+La di/dt+e (12)
At high speeds, therefore, the expressions for current rise time in equation (5) decay time in equation (7) and chopping frequency in equation (9) must be modified:
trise =(Lo +L1 sin pθ)δI/(V-RI-e) (13)
tdecay =(Lo +L1 sin pθ)δI/(RI+e) (14)
fchop =(V-RI-e)(RI+e)/VδI(Lo +L1 sin pθ) (15)
As the motional emf, e, in equations 13-15 is itself a function of rotor position, the chopping characteristics are now complex functions of position and therefore may be less reliable as sources of position information. However the value of equations 13-15 lies not in this complexity (which would be worsened by including saturation effects), but in evaluating the potential of each characteristic for position detection.
In a practical chopper drive the high supply voltage is many times greater than the voltage required to circulate the rated current for continuous excitation, i.e. V>>RI. Referring to equation (14) it is apparent that the decay time is affected by the motional emf at speeds where this emf is comparable to the winding rated voltage (e≃RI) and, since this latter voltage is comparatively small, the decay time is affected at very modest speeds. However the denominator of equation (13) includes the factor (V-RI-e) and since V>>RI this factor can be approximated by (V-e), so it is apparent that the rise time is only affected by the motional emf at the very high speeds where this emf is comparable in magnitude to the supply voltage. Therefore by using current rise for position detection a considerable degree of immunity from motional emf effects is obtained.
Further immunity can be gained by correct choice of excitation scheme, as illustrated in FIG. 5, which shows at (a) the variation of torque, at (b) the motional emf and at (c) the rise time with rotor position. In FIG. 5(c) the chain dotted line is rise time at low speed and the full line is the rise time at high speed. Comparing FIGS. 5(a) and 5(b), it can be seen that the motional emf and torque have the same position variation, so that at positions where the torque is zero the motional emf is also zero. Therefore if the motor is accelerating or decelerating under maximum torque conditions, when each phase is excited for half of the total excitation cycle, it is necessary to detect rotor positions at which the motional emf is zero and therefore the current rise time is independent of speed. For instance, FIG. 5(c) shows that, even though the current rise time increases as the rotor passes through the region of positive torque production (-π/2p<θ<π/2p) at increasing speeds, the current rise times at each end of the region (θ=-π/2p, π/2p) are constant.
Should it be necessary to operate with a different excitation scheme (e.g. one-phase-on for a three-phase motor) the detected rotor position, for a fixed current rise time, would occur slightly earlier in the excitation cycle as speed increased. However this small error could be compensated, if necessary, by using the techniques for variation of switching angle described in an article entitled "Closed-loop control of stepping motors: prediction and realization of optimum switching angle," I E E Proc., Vol. 129, Part B, No. 4, July 1982.
From the above it is shown that the chopping characteristic on which the motional emf has least effect--and therefore the most reliable characteristic for position detection--is the current rise time. Depending on the chosen mode of excitation (one-phase-on, half-stepping etc.) and the degree of magnetic saturation present in the motor, the rise time corresponding to the rotor position being detected may occur at an earlier point in the phase excitation interval and it is then necessary to distinguish between increasing and decreasing rise times.
A circuit suitable for the detection of a specified current rise time T is shown in FIG. 6a. The system monitors the chop control signal to determine the latest rise time, which is compared to the period of a positive-edge triggered monostable using a D-type flip-flop, producing a `low` output if the latest rise time is longer than that specified.
The circuit of FIG. 6a comprises a Schmitt trigger circuit 10 the input of which is connected to the emitter of transistor T1 so as to receive the voltage Vc, representative of the instantaneous value of current from the supply terminals to winding A. The output of the Schmitt trigger is used as the chop control to switch off transistor T2. The threshold at which the Schmitt trigger 10 operates is predetermined to correspond to a maximum value of instantaneous phase current. The output of circuit 10 is also applied in parallel to two monostable circuits 11 and 12. Circuit 11 is triggered by the positive edge of its input pulse and remains `high` for a period T. Monostable circuit 12 is triggered by negative or falling edge of the input pulse and is `high` for a very brief period.
The outputs of circuits 11 and 12 are both fed to a D-type flip-flop circuit 13. The output of circuit 11 is fed to the D terminal input of circuit 13 while the output of circuit 12 is fed to the clock input of circuit 13. Circuit 13 provides an output at its Q terminal which is an indication of a predetermined rotor position.
A complete cycle of operation is illustrated in the timing diagram of FIG. 6b, which has been made clearer by portraying operation at a much lower frequency than would be chosen in practice. At the beginning of the cycle the current rise time is short, so that output of positive-edge triggered monostable 11 is still `high` when the current rise is complete. The transition of the chop control signal `high` to `low` produces a short duration pulse from the negative-edge triggered monostable 12 and this pulse acts as a clock signal for the D-type flip-flop 13, which, as its D input is also `high`, produces a `high` output.
The circuit remains in this state until time t1, when a current rise time longer than T occurs. The chop control signal `high` to `low` transition, and therefore the flip-flop clock pulse from circuit 12, now occur when the monostable output from circuit 11, and so the flip-flop D input, are `low` so the flip-flop output changes `high` to `low`. After a further two chop cycles, however, at time t2 the current rise time falls below T and flip-flop output returns to the `high` state.
The system detects a specific rotor position determined by the period T of the positive-edge triggered monostable circuit 11. Where any ambiguity in detected position could occur it is a straightforward matter to ensure that the resulting excitation change is triggered by the appropriate transition of the D flip-flop output, since a `high` to `low` transition indicates that current rise times are increasing while a `low ` to `high` transition indicates decreasing rise times.
A circuit diagram illustrating an embodiment of the invention is shown in FIG. 7. A stepping motor has three phase windings A, B, and C of which winding A is shown in full line and winding B as a chain dotted line. The motor is designed to operate with each phase excited in sequence. The chopper drive circuit for phase winding A is as decribed with reference to FIG. 1 and like parts have similar reference numerals with the addition of a suffix `A` where a part is specific to phase A. Phase winding A is connected to supply terminals 1 and 2 to which an unidirectional supply source 3 supplies a voltage V. Two transistors T1A and T2A are connected in series with winding A. The base electrode of transistor T1A is controlled from a phase sequence logic circuit 14 which has three outputs each of which is connected to an associated drive circuit of one of the phase windings A, B or C. Each output is energised in sequence to excite its associated phase. Transistor T2A acts as the chopping control and its base electrode is connected to the junction between two resistors R1A and R2A which form a potential divider chain and which is connected across terminals 1 and 2 through a transistor T3A. The base electrode of transistor T3A is supplied from an AND gate GA the inputs to which are derived from Schmitt trigger circuit 10 and from the phase sequence logic circuit 14. By-pass circuits for the inductive energy for winding A when it is open circuited are provided by diodes D1A and D2A as described with reference to the FIG. 1 circuit.
Both of the other phase windings have associated with them components similar to the components described above and some of the components associated with phase winding B are shown in FIG. 7 by chain dotted lines. It should be noted, however, that all of the phase windings are returned to terminal 2 through a single resistor Rc the voltage across which provides an indication of the instantaneous current flow from supply source 3 and a current monitor connection is taken from resistor Rc to the Schmitt trigger 10 and the subsequent position detection circuits which are common to all phases and which have already been described with reference to FIGS. 6(a) and 6(b). It will also be understood that while a 3 phase stepping motor is shown in FIG. 7 the invention is equally applicable to stepping motor having any other number of phases.
In the circuit of FIG. 7, consider the situation where winding A is to be excited by a certain required current. The phase sequence logic produces a signal which is at logical level `1` on its A output line, the logical level of the other outputs being `0`. Transistors T1A is switched on by the logical `1` signal applied to its base (in some, high-power, situations it may be necessary to include one or more stages of switching amplification between the phase sequence logic and the base of transistor T1). The current flow through winding A causes a voltage drop across the small monitoring resistor Rc. If the current monitor level is below that corresponding to the required winding current then the Schmitt trigger 10 produces a logical `1` output. This output is passed to the AND gate, GA, where it is gated with the phase sequence logic output A. In the situation, therefore, where winding A is to be excited and winding A current is below the required level, the gate GA produces a logical ` 1` output, which switches on transistor T3A. With transistor T3A conducting the resistors R1A and R2A set the base potential of transistor T2A to a level where T2A is switched on. The full supply voltage V is then applied to the series combination of the winding A and resistor Rc.
The current in winding A increases until the required level is attained. The Schmitt trigger produces a logical `0` output and the output of gate GA changes to logical `0`, hence turning off transistor T3A. The base potential of transistor T2A rises to the high supply voltage, so this transistor also turns off. Current flow through the winding is maintained via a path which includes T1A, Rc and D1A. The current decays until the error between the actual and required current level is exceeded and the Schmitt trigger produces a logical `1` output, leading to the turn-on of T2A and a consequent rapid rise in winding current. The rates of winding current rise and decay during this cycle of operation are dependent on the effective winding inductance, which in turn, is dependent on rotor position in a machine with magnetic saliency.
The above description of the chopper drive of FIG. 7 so far refers to a principle which is well-known and widely-used. Additionally in the FIG. 7 circuit the Schmitt trigger output signal is used to detect rotor position and hence provide a suitable input signal for the phase sequence logic.
In the particular example illustrated in FIG. 7 the current rise time has been taken as the basis for position detection, but alternative implementations may utilize current decay time or chopping period or chopping frequency. At the beginning of the winding excitation interval the current rise time, when the Schmitt triger output is logical `1`, is relatively short. The positive-edge triggered monostable 11 is triggered at the beginning of each rise time interval and initially will still have a logical `1` output at the end of the interval, when the `1` to `0` transition at the Schmitt trigger output triggers the negative-edge triggered monostable 12. This monostable's output acts as a clock signal for the D-type flip-flop 13, which has a logical `1` at its D input when the clock signal occurs and therefore produces a logical `1` output.
Later in the winding excitation interval the current rise times increase until they attain the period T of the positive-edge triggered monostable 11. In these circumstances the D-type flip-flop clock signal occurs after the output of the positive-edge triggered monostable 11 has fallen to logical `0`, so the output D-type flip-flop 13 falls to logical `0`. This transition of the flip-flop output is arranged to occur at a rotor position where a change of winding excitation would be advantageous to motor operation.
The flip-flop output transition initiates a change in the outputs of the phase sequence logic circuit 14, so that, for example, output A changes to logical `0` and output B changes to logical `1`, causing winding A to be switched off and winding B to be switched on. With phase sequence logic output A at `0` both T1A and T2A are switched off and the current in winding A decays via a path including diodes D1A and D2A, but not including Rc.
In an alternative implementation of a chopper drive the current level in the winding is set by a single comparator and a monostable. When the current monitor reaches the comparator reference level transistor T2A is switched off for a fixed period determined by the monostable. During this fixed period the winding current decays to a level depending on the winding inductance. Detection of rotor position with this chopper drive may be implemented by monitoring the minimum current level during chopping, in addition to use of current rise time, chopping period or chopping frequency.
As an alternative to the detection of rotor position of a stepping motor by measurement of current rise time, or other current flow characteristic in an energized winding, it is possible to take measurements in an unenergized phase winding. During normal operation of the motor the current in such a winding would be zero but by setting the required current level to be a small fraction of the rated current of the motor, chopping behaviour can be measured without diminishing the torque producing capability of the motor from its energized winding. For chopping at currents of near zero value the variation of the phase incremental inductance with rotor position is a smooth near-sinusoidal curve and therefore continuous monitoring of rotor position is possible.
In this situation the phase current increases from zero to a small value and the voltage dropped across the winding resistance by this small current is negligible in comparison to the high dc supply voltage. Furthermore, the flux linkage with the winding tends to zero as the current approaches zero so that the rate of change of the flux linkage with rotor position and hence the motional emf can be neglected. When the small current in an unenergized winding, for example in phase winding A in FIG. 7, is chopped its natural decay is exponential with a long time constant since the free-wheeling path comprising transistor T1A and diode D1A has a high inductance and a low resistance. In the case of chopping at low current this slow current decay may be unacceptable if the rotor position is to be continuously monitored. However rapid current decay can be obtained by switching off both transistors T1A and T2A in an unenergised phase winding being used for monitoring and forcing the phase current back to zero by causing it to flow against the dc supply voltage via diodes D1A and D2A.
If the time of current decay is being monitored the phase current does not flow through the current sensing resistor RC because transistor T1A is now non-conducting. Consequently the end of the decay interval cannot be found by detecting zero current in the current monitor RC. To monitor the current an additional current sensing resistor may be inserted in series with diode D1A and similarly in series with the corresponding diodes in each of the other phase windings to detect the end of current decay directly.
Alternatively, it can be shown that the rise times and the decay times of current at a given rotor position in an unenergized winding are equal so that the drive transistors in the unenergised winding can be turned off for a time at least equal to the previous rise time.
Alternatively again, since the largest incremental inductance and therefore the longest decay times occur at the teeth-aligned position the transistors can be switched off for fixed times which are sufficiently long to ensure that current decay to zero is completed even in this position.
Stepping motors, and especially stepping motors of low power, may be driven by switching a constant voltage supply to even phase in sequence through phase resistors in series with each phase winding. At low speeds the current in an excited phase rises rapidly to the rated current level and subsequently the current gradient is zero so that it is not possible to detect rotor position from the current gradient in a turned-on phase. At high speeds the effects of motional emf produce a very different current waveform in which the changes of current gradient are a function of rotor position and speed.
As an alternative to monitoring the current throughout the period of excitation of a phase it is therefore possible to concentrate on measurements in a phase when it is first turned on. In this condition the phase current and flux linkage are both zero so that the resistive voltage drop and motional emf are negligible and therefore the initial rate of current rise is proportional to the applied voltage divided by the incremental inductance. This provides a well-defined unambiguous relationship between current gradient and rotor position independent of operating speed.
To implement closed loop control with a series resistance drive the next phase in the excitation sequence to the energized phase is repeatedly turned on and the initial gradient is measured. If the gradient matches the gradient obtained by turning on at the optimum rotor position for excitation change then this change is allowed to proceed. However, if the optimum position has not yet been obtained the current gradient is incorrect and the small phase current is reduced to zero in preparation for another attempt at turn on shortly.
An embodiment of the invention showing its application to a series resistance drive is illustrated in FIG. 8. In this Figure like parts have like reference numerals to FIG. 7. In FIG. 8 there is shown two phase windings A and B of a 4-phase stepping motor. There is a respective resistor RA, RB in series with each winding. Current through the phase windings is controlled by individual phase transistors T1A, T1B etc. in each phase and in addition in order to monitor the magnitude of the current through each phase winding small measuring resistors R3A, R3B are included in the emitter circuits of the respective transistors T1A, T1B. Each transistor has its base electrode supplied through two logic gates. The logic gates G1A and G2A associated with phase A of the machine are illustrated in FIG. 8. G2A is an OR gate and G1A is an AND gate. G2A is supplied with one input from the output of gate G1A and another input from the phase sequence logic circuit 14 which corresponds to the similar referenced logic circuit in FIG. 7. A phase monitor logic circuit 15 is also provided which has four outputs A, B, C and D corresponding to the four phases of the stepping motor. Output A of circuit 15 is connected to the input of AND gate G1A which is also supplied with clock pulses. An output is taken from AND gate G1A to a negative-edge-triggered monostable circuit 16A the output of which controls a switch SWA which is connected between the current monitoring resistor R3A and a hold circuit 17. Circuit 17 also has inputs from corresponding switches such as SWB connected to the other current monitoring resistors R3B etc. in the remaining phases. The output of hold circuit 17 is taken to a comparator 18 the other input of which is supplied from a preset voltage source V' through an adjustable potentiometer P. The output of comparator circuit 18 is taken both to the phase sequence logic circuit 14 and the phase monitor logic circuit 15.
In operation of the circuit shown in FIG. 8 phase sequence logic circuit 14 determines which phase or phases are excited at any one time. The phase monitor logic circuit 15 selects the next phase that is to be excited in the sequence. During a continuously excited phase a signal is provided from phase sequence logic circuit 14 to the relevant OR gate G2 and will therefore maintain the transistor T1 connected to that gate in a conducting state. No further input signal is required and the corresponding phase winding is excited for as long as an output is obtained from the phase sequence logic circuit 14 to that phase.
The phase monitor logic circuit 15 acts to monitor the phase which is about to be turned on and it provides a signal to the AND gate G1 of that phase, for example AND gate G1A if it is phase A which is being monitored. A clock pulse train is continuously provided to the AND gates of every phase and the rate of this clock signal is set to be very much higher than the stepping rate of the motor. When AND gate G1A is enabled from phase monitor logic circuit 15 the clock pulses will pass through both AND gate G1A and OR gate G2A to switch phase switching transistor T1A on and off at a high rate.
At each turn on of the phase the current through the phase winding is allowed to increase for a fixed time determined by the clock pulse length. The value of current attained at the end of this fixed time is therefore indicative of the rate of current rise and hence of rotor position. Negative-edge-triggered monostable circuit 16A is triggered by the trailing edge of each clock pulse from enabled AND gate G1A to close switch SWA connected to the phase current monitoring resistor. The value of the phase current as indicated by the voltage on the current measuring resistor R3A is thus applied to the hold circuit 17 which will hold this value and allow it to be compared in the comparator 18 with a predetermined value of voltage from potentiometer P. If this preset value is reached or exceeded the comparator provides an output signal both to the phase sequence logic circuit 14 to switch the energization to the next phase and likewise to the phase monitor logic circuit 15 to start monitoring the next phase after that.
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Conference Champion: 0 Times (Reg. Seas.), 0 Times (Tourn.)
Seasons where the school was classified as a major
school (i.e., Division I or equivalent).
Record does not reflect forfeits or vacated games.


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FPS staff report
December 31, 2012
Winter officially arrived Dec. 21 but made itself known last week when a sleet and snow mixture covered the county.
A bit of preventative maintenance on your home and car could save you a big hassle and even money once winter arrives.
"As winter weather starts arriving in Ohio, now is the time to prepare your home and car," said Mary Taylor, Ohio lieutenant governor and Department of Insurance director. "Left unaddressed, these small but important tasks can lead to expensive repairs, insurance claims, or even safety concerns."
Though last year was unseasonably warm, Ohio winters have historically produced some big storms. A record amount of snowfall hit the state in 2010. Several other recent winter storms left behind millions of dollars in losses.
In preparation for winter, Ohioans should review their insurance policies with an agent. Consumers should also utilize the Department's new auto and homeowner insurance guides available at www.insurance.ohio.gov. Experts at the Ohio Department of Insurance are available at 1-800-686-1526 to answer any insurance-related questions and you can follow the Department on twitter @OHInsurance and Facebook,
In addition, the free mobile smartphone insurance apps myHome Inventory and WreckCheck are available in your carriers' downloadable apps marketplace. They will help maintain inventory of your possessions and simplify the information exchange process after an auto accident. The apps will be of significant help during a potential claim filing situation.
To best prepare for winter, Taylor recommends Ohioans take the following steps:
Home Safety Tips:
*Clean out gutters and downspouts.
*Look for missing shingles and broken, overhanging tree limbs.
*Inspect the underside of the roof from the attic for signs of leaks.
*Check water pipe insulation to prevent winter freezes.
*Make sure dryer vent is unobstructed.
*Assemble emergency supplies, including non-perishable food, water, flashlights, and a battery-powered radio.
Vehicle Safety Tips:
*Have a mechanic check your vehicle to make sure all systems are running properly.
*Make sure tires have adequate tread and air pressure.
*Keep your windshield wiper fluid tank filled and maintain at least a half tank of gasoline.
*Get a windshield scraper and brush for ice and snow removal.
*Carry a snack, water, blankets, and a first aid kit.
Claim Tips for Home Damage:
*Call your insurance agent/company as soon as possible.
*Be sure your agent knows how to contact you if you cannot stay in your home.
*Try to protect your property and salvage what you can.
*Closely inspect your property for damage. Note and photograph any damage and losses. This will assist in settling claims.
*If required to seek temporary housing, check your profile for "loss of use" coverage. Many policies cover such expenses up to a stated amount.
*Be sure everything is considered in the claim. Back up claims with written estimates.
If You're in an Accident:
*Be Safe - Make sure you stay away from moving traffic after an accident. Do not put yourself in a dangerous situation.
*Assist others if necessary - Check on passengers in your car and in other vehicles and see if they need help, but only if it doesn't put you in a dangerous situation. Call emergency personnel if necessary.
*Take photos - Try to take as many pictures as possible of the damage and accident scene.
*Exchange information - Get the name, address, phone number, date of birth from the other drivers involved in the accident. Get the driver's insurance information,including policy number. Do not discuss whose fault the accident was.
*Get witness information - Get contact information of those who may have seen the accident. Your insurance company and/or police may want to speak to them later.
*File a police report - If the accident is not serious, you do not need to call the police to have them do a report at the scene. You can move your vehicles to the side of the road, exchange information and then file a report later.
Important safety information is available on the Ohio Committee for Severe Weather Awareness' website at www.weathersafety.ohio.gov.
The Free Press Standard invites you to post your thoughts on the story in the box below.
These posts do not reflect the views of the The Free Press Standard or its employees.
©2013 The Free Press Standard
Carrollton Free Press Standard, P. O. Box 37, Carrollton, OH


text: Prev | Next Purchase this photo Billy Summers/The Putnam Herald Some serious looking fishermen line the banks during the Kid�s Fishing Derby on Saturday, May 31, 2014, at Valley Park. Gallery: Putnam in Pictures, Friday, June 6 Jun. 06, 2014 @ 03:53 PM The Herald-Dispatch / 2015 Did you miss something in Putnam County this week? Chances are, one of our photographers was there. ()


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The Department of Defense announced today the death of a soldier who was supporting Operation Iraqi Freedom.
Spc. Brandon M. Read, 21, of Greeneville, Tenn., died Sept. 6 in Qayyarah West, Iraq, when his convoy came under attack by enemy forces using an improvised explosive device. Read was assigned to the 125th Transportation Company from Lexington, Ky.
The incident is under investigation.
For further information related to this release, contact Army Public Affairs at (703) 692-2000.


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The academy product was a consistent performer during a disappointing last campaign and has reportedly caught the eye of Arsenal, Fulham and Marseille.
But though fellow stopper Fabricio Coloccini's Tyneside future remains in the balance, Taylor insists he remains settled on Tyneside.
"Being at Newcastle is the best thing that has ever happened to me," Taylor, who has three years to run on his current contract, told The Chronicle.
"I am still living the dream of every young lad that is in this city.
"I still cannot describe the feeling when you walk over that line in front of 52,000 fans in a stadium like ours.
"It is what I have always wanted to do. I have to keep pinching myself."


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Backup drives are usually pretty bland and uninteresting, consisting of just a drive mounted inside a plastic shell. Not this time! Maxtor has managed to tickle our Geek-spot by wedging two 500GB drives inside a sexy rubber lunchbox, and adding a dash of RAID for spice. It all amounts to one hell of a drive, and it's the new end-all, be-all backup drive as far as we're concerned.
The Turbo's small footprint belies its behemoth capacity, which totals 930GB formatted, compliments of two Maxtor DiamondMax 11 500GB drives. While having two drives instead of one isn't that exciting on its own, we were stoked to learn that you can run the drives in either RAID 0 (striped) for maximum capacity, or RAID 1 (mirrored) for redundancy. You set up the array using the included software, which we found extremely easy to use. Note that if you run RAID 1, there's no way to swap out a drive in the event of a disk failure. Maxtor will send you a new unit, to which you would copy your data, and then you'd return the defective unit to Maxtor.
The Turbo's configuration utility sports some options we love—such as the ability to password-protect access to the drive in case of theft, the option to reduce seek times for quieter operation (although you can't adjust the integrated fan's speed), and the ability to easily tell the drives to spin down after a certain period of time.
The included backup software is Dantz Retrospect, but Maxtor has added a custom front-end that makes the software simple to use. You can make images of your drives, duplicate key files, or just keep certain folders synced with your backup drive at all times.
We're impressed. Way impressed. If Maxtor added a media reader, a USB port, and silenced the fan, this drive would have been worthy of a Kick Ass award.
Month Reviewed: May 2006
Verdict: 9
URL: www.maxtor.com


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Ask lil Wayne to apologize for Stomping on American flag
Someone please post a valid contact to Rapper Lil Wayne "My country 'tis of thee/Sweet land of kill 'em all and let 'em
Someone please post a valid contact to Rapper Lil Wayne "My country 'tis of thee/Sweet land of kill 'em all and let 'em
Please call your members of Congress today and keep calling, until the Benghazi select committee starts their
Belivers should sign to let the lawmakers and the so called powers that be know we should be allowed to openly pray, and
Atome' n'est qu'un passage entre deux discipline,disciplines est ci qui reste quand on tout savent P5+1.
We all need God this world is not getting better on it's own.God is very mad because people are trying to take Him away!
An international highway that physically connects all people of the world JOIN & BUILD, DONATE & SHARE www.ivacademy.ru
The National Motto should be displayed In Every City, County Chamber, and State Capitol In America.
Remove the pictures of Prophet Muhammad (Peace be upon him). Assalaamu 'alaikum Wa rahmatullahi Wa barakatuhu,Dear
This is America
A good thing should be shared by everyone... If ObamaCare is good for average Americans, ObamaCare should be good


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Poor old rhythm section
In the news:
Aston "Family Man" Barrett, the 60-year-old guitarist who believes he is the author of the bass line which gave Marley's late-1970s hits their inimitable rhythm, failed yesterday in his latest legal bid for a multimillion-pound slice of the Marley musical empire.How badly does that suck? The contributions of rhythm sections are so woefully underestimated in copyright law, which seems to be designed to mostly benefit vocalists and treble players.
Re: Poor old rhythm section
I think he previously settled for some sum earlier, hence why he couldn't more money again.
Hence the previous settlement.
Althought the rhythmn section kills it on every marley tune, apprently this was already done and over with.
Re: Poor old rhythm section
It's the fact that it could be settled so easily.
It's the fact that no one could have known the way Bob Marley's rhythm section would transform the western musical landscape.
It's the fact that they created something special and didn't get commensurate reward.
Re: Poor old rhythm section
I doubt anyone will question his authorship of the bass lines on the recordings he played on. But the question in this case is whether he is entitled to royalties. For whatever reason, he gave that up. He took the money. In hindsight, it was the wrong move. But life goes on.
Re: Poor old rhythm section
I guess the way drummers who create parts that are intrinsic to a song's success are left off songwriting credits rankles. I think this is also true for players who develop harmonies that raise a basic song to some special.
Re: Poor old rhythm section
But while authorship and copyright may intersect, they are not the same thing. Authorship is established at the moment of creation, and cannot be taken away. But copyright can be given away, sold, assigned, inherited, etc. In popular music, authorship should be negotiated as early as possible in the process to avoid claims such as this.
Re: Poor old rhythm section
Re: Poor old rhythm section
Yeah, being a drummer sucks. First they rip you off, then they sit around and tell all those lame drummer jokes. (Sigh.)
My 2 cents re: the Barretts? Those guys were the best, and they will never be eclipsed. Anybody wanna take issue with that? Take your Sly & Robbie and insert them someplace where it's not sunny.
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