This application claims the benefit of Provisional Application No. 60/250,621, filed Dec. 1, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to methods and apparatus for monitoring the operation and use of a machine. More particularly, the present invention relates to methods and apparatus for monitoring the operation and use of a machine for the purpose of diagnosing the mechanical condition of the machine based thereon. 
     It is known to monitor machine operation parameters, such as, for example, machine vibration, temperature, sound, force or pressure, motor current or lubricant oil temperature, viscosity or composition, and to evaluate such monitored parameters for the purpose of providing an indication of the condition of the machine. For example, a thermocouple may be located within the lubricant “bath” so as to alert the machine operator when the temperature of the oil has exceed some predetermined threshold value. In another example relating to machine vibration, techniques employed to monitor and to analyze machine vibration for the purpose of providing an indication of the condition of the machine range from the very simple, such as touching or listening to a machine, to the very complicated, such as detecting time-domain signals, converting those signals to frequency-domain signals, and computer-processing the resulting frequency domain signals. Techniques at the lower range of the monitoring and analysis spectrum are overly-simplified and do not provide reliable results whereas techniques at the upper range of the spectrum are overly-complicated and are typically so expensive so as to prohibit their widespread use by most machine operators. 
     Accordingly, there is a need for new and improved monitoring and analysis techniques for providing an indication of the condition of a machine, such as an indication of the general “health” of the machine by providing a running history or profile of the machine&#39;s operation or usage. Preferably, such monitoring and analysis techniques would provide reliable results, would be easy to use, would be relatively inexpensive and would be adaptable to improve the operating practices of the machine, such as, for example, as one component of the machine operator&#39;s predictive/preventative maintenance procedures. There also is a need for new and improved analysis techniques for providing an indication of the useful “life” remaining in a machine, based upon actual past machine operation and usage. 
     SUMMARY OF THE INVENTION 
     These and other needs are met by the present invention wherein analog signals generated in response to one or more operating parameters of a machine, such as, for example, vibration, temperature, sound, force, pressure, motor current or lubricant oil temperature, viscosity or composition, are converted into root-mean-square (“rms”) values which are periodically sampled, sorted into predefined rms value bands and stored as an evolving history log of the actual operation of the machine. Data stored in a machine operation history log may be analyzed to evaluate past machine operation and usage, to provide an indication of the general “health” of the machine, or to provide an indication of the useful “life” remaining in the machine. In addition, data stored in the log may be analyzed as a component of a machine operator&#39;s predictive/preventative maintenance procedure. 
     In accordance with one aspect of the present invention, a machine is monitored by coupling a transducer to the machine at a location thereon likely to detect changes in the level of one or more operating parameters of the machine&#39;s operation. The transducer preferably is adapted to generate analog signals representing the level of the operating parameter detected. The analog signals are converted to rms signals which are periodically sampled to generate rms sample values. The rms sample values are processed according to each sample value&#39;s respective level or magnitude and thereafter sorted into two or more signal magnitude bands, where the number of rms samples detected in each of the at least two signal magnitude bands is accumulated over time to provide an indication of the usage and condition of the machine. 
     In accordance with another aspect of the present invention, a machine is monitored by coupling a transducer to the machine at a location thereon likely to detect changes in the level of one or more operating parameters of the machine&#39;s operation. The transducer preferably is adapted to generate analog signals representing the level of the operating parameter detected and is operatively coupled to a machine controller of the machine. The analog signals are converted to rms signals, which the controller periodically samples and generates rms sample values corresponding thereto. The controller then processes the rms sample values according to each sample value&#39;s respective level or magnitude, sorts the rms sample values into sample magnitude bands, accumulates the number of rms sample values into each of at least two magnitude bands and uses the accumulated numbers of accumulated rms samples in the at least two magnitude bands to provide an indication of the usage and condition of the machine. 
     In accordance with yet another aspect of the present invention, a machine is monitored by monitoring analog signals representative of operation of the machine. The analog signals are converted to rms signals that are periodically sampled to generate rms sample values. The rms sample values are processed according to each rms sample value&#39;s magnitude or level and sorted into two or more sample magnitude bands. The numbers of rms sample values are accumulated in each of at least two sample magnitude bands and the numbers of accumulated rms sample values in the at least two sample magnitude bands are used to provide an indication of the usage and condition of the machine. 
     In accordance with still another aspect of the present invention, an apparatus for monitoring a machine having a controller comprises a transducer coupled to the machine. Circuitry is provided for converting analog signals received from the transducer to rms signals. The machine controller is then operated to perform the functions of: periodically sampling the rms signals to generate rms sample values; sorting the rms sample values according to each rms sample value&#39;s level or magnitude; accumulating the number of rms sample values in each of at least two magnitude bands; and, providing an indication of the usage and condition of the machine from the accumulated numbers of rms sample values accumulated in at least one of the at least two magnitude bands. 
     These and additional objects, features and advantages of the present invention will become apparent to those reasonably skilled in the art from the description which follows, and may be realized by means of the instrumentalities and combinations particularly pointed out therein, as well as by those instrumentalities and combinations equivalent thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be had upon reference to the following description in conjunction with the accompanying drawings in which like numerals refer to like parts, and wherein: 
         FIG. 1  is a schematic side view of an exemplary machine tool having an apparatus according to a preferred embodiment of the present invention installed thereon; 
         FIG. 2  is a flow chart showing operation of a preferred method of practicing the present invention; 
         FIG. 3  is an exemplary illustration of a display showing accumulated machine tool spindle vibration acceleration collected and displayed in accordance with a preferred method of practicing the present invention; and, 
         FIG. 4  is an exemplary illustration of a display showing real-time machine tool spindle vibration acceleration and peak-value maximum vibration acceleration monitored, collected and displayed in accordance with a preferred method of practicing the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described with reference to the various drawing figures, wherein  FIG. 1  is a schematic side view of an exemplary machine tool, such as, for example, a vertical machining center  100 . It will be apparent to those of ordinary skill in the art, upon reading the within description, that the apparatus and method according to the various embodiments of the present invention may be practiced with respect to machines of alternative configurations, structures and operations without departing from either the spirit or the scope of the present invention. For example, the apparatus and method according to the various embodiments of the present invention may be used on any combination of other machine tools, such as horizontal machining centers, turning centers, milling machines, profilers, routers, planers, boring mills, drills, broaches, gear-cutters, screw-cutters, shapers, grinders, press brakes or reamers, on any combination of composite-material forming machinery such as fiber-placement machines or composite tape-laying machines, on any combination of robotic machinery or on any combination of any other type of machinery where there is a need or desire to obtain an indication of the usage and condition thereof. 
     According to a preferred embodiment of the present invention, one or more signal transducers (which may be analog or digital, but preferably analog) are used to monitor any combination of one or more operating parameters or characteristics of a machine&#39;s operation, such as, for example, vibration, temperature, sound, force, pressure, motor current or lubricant oil temperature, viscosity or composition. Output signals from the transducers are processed to provide an inexpensive view of the running history of the usage and operation of the machine  100 . For ease of illustration, a preferred embodiment of the present invention will be described with reference to a system wherein a single transducer is used and is selected to be an accelerometer  102  mounted to a machine tool spindle  104  for the purpose of monitoring acceleration forces exerted on the spindle during machining operations. Additional transducers may be coupled to other portions of the machine to monitor other characteristics of machine operation as required for a given application, as will be apparent to those of ordinary skill in the art. In addition to the history of machine operation, real-time machine operating conditions may be monitored, viewed and analyzed to provide an operator with an indication of the real-time operating conditions of the machine and to alert the operator of any significant operating events, such as excessive vibration acceleration, lubricant oil temperature or the like. 
       FIG. 1  shows an apparatus coupled to the machine  100  for use according to a preferred embodiment hereof wherein the accelerometer  102  is powered from a power source located inside an electrical cabinet  106  coupled to the machine  100  in a conventional manner. Analog signals generated by the accelerometer  102  are conditioned by a first electrical circuit  108  and passed to voltage measuring and analog-to-digital converter (“ADC”) circuitry  110  so that digital signals representative of the acceleration forces are passed to a computerized numeric controller  112  for further processing. The first electrical circuit  108  includes amplification and root-mean-square (“rms”) signal generating components. It is noted that circuitry performing the functions of the first electrical circuitry  108  can be included in the accelerometer  102  so that rms signals are generated directly by the accelerometer  102  rather than by a separate circuit such as first circuit  108 . Accelerometers of both types are commercially available from a number of manufacturers including, for example, PCB Peizotronics of Depew, N.Y. Normal control sampling of the rms signals at a relatively low sample rate is adequate to represent, over time, the operating conditions of the machine  100 . For this reason, ADC circuitry  110  may take the form of an inexpensive, commercially-available computer interface card since a high sample rate digital signal processing board (as is commonly employed for sophisticated vibration signal analysis in existing systems) is not required. For example, the present invention contemplates adequate sampling rates ranging from one sample every millisecond to one sample every 10 seconds. Alternatively, rather than providing separate ADC circuitry  110 , rms sample values may be supplied by first circuit  108  directly to the machine controller  112  and processed in accordance with the present invention. Even further, rather than providing separate first circuit  108 , rms sample values may be supplied by accelerometer  102  directly to the machine controller  112  and processed in accordance with the present invention. 
     In any event, rms sample values representative of acceleration forces exerted on the spindle  104  are received by the controller  112  and processed to quantify the general severity of operation of the machine  100 . Since rms signal values are used, short duration events, such as impulses, may not be detected, due to the low sampling rate. Similarly, peak amplitudes of some events may go undetected. However, the general trend of the characteristics and conditions of machine operation and use can be ascertained from the rms sample values acquired at some predetermined sample rate over time. 
     Currently available analysis systems known to applicants require sophisticated computer processors equipped with a great amount of computer memory to store data for multiple vibration spectra. If only limited memory is available to acquire, store and analyze sample data, only the overall levels or frequency band levels can be monitored to trend the data over time. As overall levels increase, an alarm is provided to indicate a deteriorating condition of the machine. While such monitoring is adequate to detect wear- or fatigue-induced equipment failures of machinery having relatively stable and consistent operating conditions, it generally is not adequate for machines that are operated under non-stable or varying conditions. That is, traditional fatigue-analysis procedures presuppose a substantially continuous operating condition with operating characteristics that do not vary significantly over time. On the other hand, typical machine tool usage subjects the machine  100  to widely-varying operating conditions. For example, utilization of different tooling and/or different depths of cut may place different levels of vibration acceleration on the machine  100  to create greater or lesser degrees of use or abuse thereof. To this end, the present invention is suitable to monitor machine operations and operating severity for stable and consistent operating conditions, as well as for non-stationary or varying operating conditions. An advantage of the present invention is that it does not require the use of a standard “maintenance cycle” to assess the condition of the machine, but rather may assess operating conditions based on accumulated data representing the actual use and operation of the machine  100 . 
     An exemplary method according to a preferred embodiment of the present invention may be implemented on a computer or computer processor having only a limited amount of computer memory, although the methods and apparatus described herein can be implemented on more powerful computers as well. As will be discussed is greater detail below, the computer need only be suitable for storing single numbers for each of two or more acceleration force bands and/or for incrementing two or more counters to accumulate data within selected bands of vibration acceleration. Moreover, by using the sampling rate of the controller  112  such as in one alternative embodiment hereof, time periods of exposure of the machine  100  to measure acceleration levels of the spindle  104  can also be estimated. 
     With reference now to  FIG. 2 , a method of practicing the present invention according to a preferred embodiment thereof will be described in further detail. As described above, the transducer is in the form of an accelerometer  102  coupled to a machine tool spindle  104  and is adapted to output an analog signal  103  representing the level or magnitude of acceleration forces exerted on the spindle  104 . First circuit  108  continuously receives the analog signal  103  and conditions the analog signal  103 , for example, by amplifying it. Circuit  108  also periodically calculates time-averaged root-mean-square values  109  of the signal  103  and stores the rms value  109  until such time as a new rms value  109  is calculated, at which time, the new rms value  109  replaces the old rms value. 
     At some predetermined sample rate, for example, one sample per second, programmed into ADC circuit  110 , the ADC circuit  110  obtains, acquires or otherwise samples the current value of the rms value  109  of the analog signal  103  and determines the level or magnitude of the rms value  109 . The ADC circuit  110  then supplies the machine controller  112  with the sampled rms value  109 , and more preferably, with the absolute value of the sampled rms values  109 . 
     The machine controller  112  is programmed to include two or more bandwidth counter “bins”, which store counter values corresponding to preselected bands—or ranges—of sampled rms values  109  and to sort the magnitudes of the sampled rms values  109  into the two or more bandwidth counters. For example, machine controller  112  provides a first bandwidth “bin” for counting the number of rms sample values  109  falling within a first range between 0 g and 15 g, a second bandwidth “bin” for counting the number of rms sample values  109  falling within a second range between 15 g and 30 g, a third bandwidth “bin” for counting the number of rms sample values  109  falling within a third range between 30 g and 40 g and a fourth bandwidth “bin” for counting the number of rms sample values  109  greater than 40 g. As the machine controller  112  receives each rms sample value  109 , the machine controller  112  compares the rms sample value  109  to the predetermined bandwidth “bin” ranges. Depending on which bandwidth “bin” the rms sample value  109 , falls in, the machine controller  112  increments the corresponding bandwidth “bin” counter accordingly. 
     For example, if the machine controller  112  determines that the rms sample value  109  is between 0 g and 15 g, a first bandwidth counter CTR 1  is incremented by a value of one. If, instead, the machine controller  112  determines that the rms sample value  109  is between 15 g and 30 g, a second bandwidth counter CTR 2  is incremented by a value of one. Similarly, if the machine controller  112  determines that the rms sample value is between 30 g and 40 g, or is greater than 40 g, a third or fourth counter CTR 3 , CTR 4 , respectively, is incremented by a value of one. As each counter is incremented according to the rms sample values  109 , the ADC circuit  110  obtains additional rms sample values  109 . In this manner, the counter values accumulated in each of the bandwidth “bins” represent measures of the trends of the machine  100  to operate under certain operating conditions. Any of the counters may be reset to zero, for example, where a new spindle  104  is installed on the machine  100  and where retention of operational history data acquired theretofore concerning the prior spindle is not required. 
     While the preferred embodiment hereof has been described with reference to four bandwidth “bins”, any number of “bins” representing greater or lesser ranges may be utilized as the machine operator deems fit for the particular type and general operating environment of the machine. One advantage of the present invention is that substantial computer resources, such as memory, are not required in order to practice the method hereof. For example, because the computer is storing only the counter values, a single memory register for each of the bandwidth “bins” is all that is required. Vast computer memory resources (such as are used in the prior art to store vibration spectra) are not necessary. 
     Referring now to  FIG. 3 , data accumulated for each of these bands  120  may be displayed on a display  114  of the controller  112  where cumulative data is displayed in the form of a first histogram  114   a . In addition, referring also to  FIG. 4 , a second histogram  114   b  may be displayed on the display  114  to show the magnitude of the last-sampled rms level  130 , the maximum rms level  132  sampled during some predetermined period, as well as other desired data (not shown) relating to machine operation. To provide the machine operator with useful information concerning the nature of operating conditions exceeding some predetermined threshold value, a limited number of event type descriptions can be stored with the data. Similarly, since the system is embedded on the machine, i.e., incorporated into the controller  112  rather than provided in a separate computer control, alarms and alerts can be generated by the controller  112  based upon data sampled from the transducer to notify the machine operator of critical operating events, such as a spindle vibration acceleration exceeding some predetermined threshold value. 
     The data accumulated in the present invention can also be used to calculate machine “expended life” or “life remaining” measure similar to the known “accumulated fatigue damage factor (“AFDF”) and/or “L 10 ” or “B 10 ” bearing life calculations. Typical “expended life” or “life remaining” calculations are based on cyclical stress, so the number of cycles at a particular stress level is required. In the present invention, the number of stress cycles is not established exactly, since data is not sampled continuously and as such, the number of cycles is not recorded precisely. However, a number of methods may be used to estimate accumulated damage based upon data sampled according to the present invention. 
     One method presumes that the cycles are predominantly due to such factors as: the rotational speed of the spindle  104 , the number of impacts of the cutting edge of the tool on the workpiece, and/or the natural frequency of the machine  100 . Because the apparatus is embedded within the controller  112 , other measurements can be associated with the measured spindle vibration acceleration, such as spindle speed, horse power, feedrate of axes, axis motor current and the like and used alone or in combination with one another to generate a stress profile. The selection of a particular tool used and its “force profile” can also be associated with vibration measurements. 
     Machine evaluation techniques based on the baseline empirical measurements can be utilized in a “learn cycle” for use in a damage assessment algorithm. Fuzzy logic type algorithms would be appropriate for such machine evaluation techniques. For example see: 
               Accumulated   ⁢           ⁢   Fatigue   ⁢           ⁢   Damage   ⁢           ⁢     Factor   ⁡     (     “   AFDF   ”     )         =       ∑     j   =   1       σ   =   x       ⁢           ⁢     (         n   1     ⁢     σ   1   b       +   …   +       n   x     ⁢     σ   x   b         )               (     Eq   .           ⁢   1     )             
         where: n=number of stress events;    σ=magnitude of the stress event; and,    b=slope of log-log S-N curve       

     For the present invention, the use of information readily available within the controller  112  is expanded. That is, the number of stress events, n, will be defined to include stress events estimated from several measures, such as spindle speed, number of teeth on cutter, empirically-determined predominant vibration, and frequency. The number of stress events, n, then, can be used in Eq. 1 to calculate the AFDF, which may then used in a conventional manner to determined the accumulated fatigue damage suffered by the machine. 
     For example:
         n (rpm, m)=number at assumed cycles of the acceleration margin predominant vibration assumed to be tooth impacts so, if spindle speed=2000 rpm and number of cutter teeth m=6 then:
           n=2000 rpm×6 teeth=12,000 cycles/min   
               

     Thus, for every minute at 10 g&#39;s rms operating at 2000 rpm with this cutter, 12,000 stress cycles of 10 g magnitude are presumed. 
     Similarly, the stress events can be defined by operation severity calculations, based not only on the vibration acceleration level, but also the combined dosage of spindle horsepower, axis thrust, vibration, etc. 
     σ(x, hp, I axis ) 
     Such an embedded algorithm is then be used to indicate remaining life: 
     Expected life−AFDF use=remaining life. 
     While the invention has been illustrated with reference to one or more preferred embodiments hereof, and such preferred embodiments have been described in considerable detail with reference to the drawings, it is not the intention of applicants that the invention be restricted to such detail. Rather, it is the intention of the applicants that the invention be defined by all equivalents of the preferred embodiments falling within the scope hereof.
Analog signals are generated in response to one or more operating parameters of a machine such as vibration and used to provide a log of the operating history of the machine. The analog signals are converted into root mean square (rms) values which are periodically sampled. The sampled signals are sorted into predefined rms value bands. Each occurrence of a signal in an rms value band is used to increment a counter in order to keep track of the number of occurrences of signals in a particular rms value band. The number of accumulated signals in each of the rms value bands provides an indication of the usage and condition of the machine.