BACKGROUND OF THE INVENTION 
     This invention relates to a method and apparatus for determining the position of a guided vehicle relative to a buried wire carrying an alternating current signal. 
     Prior art wire guidance devices are primarily hardware devices using analog circuits and devices which is subject to drift and noise. Further, prior methods and circuits do not make the calculations of the distance and angle determinations sufficiently quickly enough and to a high enough accuracy to provide precise control of the vehicle, particularly at higher speeds of vehicle travel. 
     SUMMARY OF THE INVENTION 
     This invention relates to a method and apparatus for determining the position of a guided vehicle relative to a buried wire carrying an alternating current signal by sensing the distances of individual sensors and an array of sensor coils using this distance information to calculate both the angle and distance of the vehicle&#39;s centerline from the wire. 
     In the preferred embodiment of this invention, a pair of sensor bars are carried by the vehicle, both centered on the longitudinal axis of the vehicle, one called the steered wheel (front) sensor bar and the other called the load wheel (rear) sensor bar. Each bar carries four coils installed in a straight line perpendicular to the longitudinal axis, two coils on either side. 
     The vehicle&#39;s position, as measured by the distance of each coil from the buried wire, is determined from the amplitude of the signal received. Before making a distance or position measurement, there are two preliminary procedures that are completed, and the information generated is stored for later use. The first is to correlate coil voltage measurements and distance. The second is to normalize or calibrate the coil output to accommodate for different signal levels, variation in sensor coil sensitivity, the height of the sensor bars from the floor, different electronic circuits, etc. and thereby provide a voltage output that is truly representative of the distance of the coil from the wire. 
     The first preliminary procedure, or the correlation of nominal coil voltage to distance, is done once in the laboratory by measuring and recording the coil outputs of different coils at various distances and then recording the results in a look-up table. In the preferred embodiment of this invention, the look-up table has recorded therein voltages for each of 64 distances ranging from zero to maximum sensor range--about 7 inches. In use, the normalized voltage from a sensor coil will be compared to the voltages in the look-up table, and interpolation procedures will be used to provide a digital value of distance to a high degree of precision, in the order of 1/64 inch. 
     The second preliminary procedure is the calibration of the sensor coils. This procedure is performed whenever there is a change in circumstances warranting it, such as the initial installation of a vehicle in an existing warehouse, changes in any of the vehicle electronics components that are used in the wire guidance system, changes in the buried wire power source, or other similar changes that might affect the output of the coils. 
     The calibration procedure is simply to move the vehicle in a path that causes each coil to pass directly over the wire at least once, and to record the peak voltage observed. The maximum voltage recorded is then divided into a number that represents the maximum expected output of the coil, and the resulting number, or gain factor of that particular coil, is stored in electronic memory for later use. 
     Once the voltage-to-distance data is stored in the look-up table, and the gain of each coil is determined and stored, the vehicle position portion of the guidance procedure is ready to be implemented. 
     The guidance electronics will sense the distance of the vehicle from the buried wire by reading the voltage output of each sensor coil individually. The sensor coil outputs are first processed to eliminate unwanted, spurious noise signals using a dynamic clipper described in copending U.S. patent application Ser. No. 446,502, now U.S. Pat. No. 5,008,604. 
     Thereafter, the actual coil output voltage is multiplied by the gain factor to obtain an adjusted or normalized coil voltage measurement. The normalized coil voltage measurement is then compared to the values in the look up table to obtain an actual distance measurement for each coil. 
     Once each coil distance is determined, or at least those coils within range of the wire, the distance of the center line or reference point on the sensor bar is determined. This is done for both sensor bars. Finally, the distance of a predetermined point on the vehicle, called the virtual point, is calculated. The angle of the vehicle&#39;s center line relative to the wire is also calculated by comparing consecutive distance measurements and the position of the steered wheels. 
     It is therefore an object of this invention to provide an improved wire guidance system for vehicles, such as lift trucks and the like. 
     It is a specific object of this invention to provide a method for determining the distance of a guided vehicle from a wire carrying an alternating current signal, said method comprising the steps of providing the vehicle with a set of sensor coils for detecting the signal carried by the wire where the space between the sensor coils is divided into regions, normalizing the output from each coil, and converting the normalized output from each coil into a distance value by reference to a look up table. 
     It is another object of this invention to determine which coil has the highest normalized output, to determine which region the wire is in by determining which adjacent coil has a normalized output greater than a predetermined minimum value, and combining the distance measurements from the selected coils defining the region to derive a sensor bar-to-wire distance measurement. 
     It is a further object of this invention to provide a method comprising the steps of employing four sensor coils arranged in a straight line on a sensor bar which is perpendicular to the axis of the vehicle, dividing the sensor bar into five regions, determining which region the wire is in by determining which adjacent coil has a normalized output greater than a predetermined minimum value, combining the distance values from the selected coils defining the region to derive a sensor bar-to-wire distance measurement, and combining sensor bar distance measurements to derive a vehicle-to-wire distance measurement. 
     It is a further object of this invention to provide a method and apparatus for determining the angle of a guided vehicle relative to a wire carrying an alternating current signal wherein the sensor bar distance, the angle of the steered wheels of the vehicle, and the distance the steered wheels travel from one distance measurement to a second distance measurement is used in calculating the current sensor bar distance and angle of the vehicle. 
     It is a further object of this invention to provide a method for calibrating the output of wire sensor coils in a guided vehicle comprising the steps of placing a vehicle containing sensor coils over a wire carrying a sinusoidal signal of a predetermined frequency and amplitude, moving the vehicle over wire in a path that will cause each coil to pass directly over the wire at least once, determining the maximum strength of the voltage induced into each coil as an indication that the coil has passed directly over the wire, dividing the maximum output voltage of each coil into a predetermined number to derive a gain factor for that coil, storing the gain factor for each coil, and thereafter using the coil output voltage multiplied by its gain factor to provide a normalized coil output voltage representing the distance of that coil from the wire. 
     It is another object of this invention to provide an apparatus for determining the distance of a sensor coil carried by a wire guided vehicle from a buried wire carrying an alternating current guidance signal comprising at least one sensor coil mounted on a sensor bar, means for recording a gain factor for each sensor coil, means for normalizing the output of each sensor coil including means for multiplying the gain factor by the coil output, look up table means for recording previously determined data correlating normalized voltage outputs of each sensor coil with the actual distance of the coil from the wire at a plurality of selected distances, and means for providing an output representing the distance of each coil from the wire including means for comparing the normalized output of each coil with the data recorded in said look up table means and interpolating between recorded data positions. 
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is perspective view of a turret stock picker which is representative of the type of vehicle that includes automatic guidance; 
     FIG. 2 is a schematic plan view showing some of the major components comprising the vehicle control system; 
     FIG. 3A is a plan view showing a vehicle approaching a buried wire in the forward direction; 
     FIG. 3B is a plan view showing a vehicle approaching a buried wire in the reverse direction; 
     FIG. 4 is an elevational view showing the relationship between a sensor bar that includes four sensor coils relative to a guidance wire buried in a concrete floor; 
     FIG. 5 is an elevational view showing the radiation pattern emanating from a sinusoidal signal carried by a buried wire; 
     FIG. 6 is a plan view illustrating a buried guidance wire placed between storage racks in a typical warehouse; 
     FIG. 7 is a simplified electrical block diagram showing the major components comprising of a guidance system; 
     FIG. 8 is a electrical block diagram showing the basic components needed to convert the signal sensed by a single sensor coil to a distance measurement; 
     FIG. 9 is a curve showing the relationship between normalized sensor coil voltage and the distance of the coil from the buried wire; and 
     FIG. 10 is a plan view of the sensor bar showing the regions formed by the four coils in the bar. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and particularly to FIG. 1 which illustrates a forklift truck of the type including a wire guidance system, the truck may include a power unit 10, a platform assembly 20, and a load handling assembly 30. 
     The power unit 10 includes a power source, such as a battery unit 12, a pair of load wheels 14 positioned under the platform assembly, a pair of steered wheels 15 (FIG. 2) positioned under the rear end of the power unit 10 with each wheel being driven by a traction motor 16, a mast 17 on which the platform assembly 20 rides, and a power unit electronic control unit 18 (FIG. 2). 
     The platform assembly 20 includes a seat 22 from which the operator can control a steering tiller 23, traction motor control 24, brake pedals 25 and forklift controls 26. The platform assembly 20 includes an electronics package 28 which is interconnected with the power unit electronics package 18 by means of appropriate electrical cables. 
     The load handling assembly 30 includes a pair of lift forks 32 which may be raised and lowered, and also rotated relative to the platform assembly by the controls 26. 
     As illustrated in FIG. 2, the power unit 10 supports two sensor bars 40 and 42. Sensor bar 40 is located between the steerable wheels while the sensor bar 42 is placed between the load wheels 14. Both sensor bars are designed to detect a wire 50 embedded in the floor of the warehouse. 
     As shown in FIG. 6, the embedded wire 50 is placed down the center of the narrow aisles between the storage racks 51 in a warehouse. A line driver 52 causes current to pass through the wire at a frequency in the range of from 4-12 kHz. As shown in FIG. 5, the wire 50 is embedded in a saw cut 53 made in the floor 54, and it is held in place by epoxy 55 which fills the remainder of the cut after the wire has been placed in the bottom thereof. The wire will radiate a signal, shown by the dashed lines 56 in FIG. 5, which signal may be detected by sensor coils carried by either or both of the sensor bars 40 or 42. 
     Referring now to FIG. 4, four sensor coils 60 are carried by each of the sensor bars 40, 42. The axis of each coil is horizontal to the floor 54. Coil 60a is placed six inches to the left of the center line of the sensor bar, coil 60b is three inches to the left, coil 60c is three inches to the right, and coil 60d is 6 inches to the right of the center line. The centers of each coil are a nominal 4 inches above the wire 50. Each coil 60 is approximately 1 inch in length, and 1/2 inch in diameter. 
     Referring again to FIG. 2, the steering tiller 23, traction motor control 24, brakes 25 and fork lift controls 26, as well as the other controls on the platform assembly, are provided With position sensors, encoders and switches, and signals from these devices are transmitted to the electronic control package 18 located in the power unit 10. For example, the platform assembly 20 includes a steering encoder 23a, steering indicator lights 23b to show the operator which direction the wheels have been turned, a guidance switch 65, a signal strength light 65, guidance indicator lights 67, and an alarm or horn 68. 
     A serial link 70 electrically connects the platform electronic control package 28 to the power unit electronic control package 18, which also receives further input signals from a steering feedback encoder 72 which indicates the actual position of the steering wheels, the steered wheeled sensor bar 40, the load wheel sensor bar 42, and the brake switch 74. It provides output signals to control the steering servomotors 81 and 82 through a steering motor control circuit 80, a steering contactor 84, a brake relay 85, and a traction motor control circuit 86. 
     The electronic components forming the present invention, are more clearly described in the block diagram of FIG. 7. A microcomputer wire guidance system 90 includes a microprocessor 92 that is provided with inputs from two sensor amplifiers 95a and 95b, and other inputs from the steered wheel position sensor 72, the steering encoder 23a, and the guidance control switch 65. Outputs from the microcomputer 90 are provided to the traction motor control 86 for controlling the speed of the vehicle, to brake control 100 and to the steering servomotor control 80. Other outputs are provided to indicate to the operator when the vehicle is near an operating wire, that is, a wire which has the proper signal for use in a guidance system by means of the alarm horn 68 and field strength indicator light 66. 
     When the guidance selector switch 65 is in the manual position, the operator controls the steering of the vehicle directly by means of the steering tiller 23. When the guidance selector switch 65 is placed in the automatic position, the microcomputer 90 will automatically enter the search mode, and when either sensor 40 or 42 detects the guidance signal, the microcomputer 90 will automatically take the vehicle into an Acquisition Mode where the vehicle is guided into alignment with the wire. Once the vehicle is properly aligned, the microcomputer 90 will go into an Automatic Tracking Mode where the vehicle alignment is maintained automatically under computer control. 
     Each of the sensor coils is connected to its own amplifier 95, as illustrated in FIG. 8. Since the magnitude of the sinusoidal signal sensed by each sensor coil 60 is a function of the distance of that coil from the wire 50, the sensor amplifier 95 includes a dynamic clipper circuit to reduce the amount of unwanted noise and other signals not emanating from the wire itself. The output of the sensor amplifier 95 is then applied to the wire guidance computer 92. 
     The block diagram of FIG. 8 illustrates several components of the microcomputer 92 used in processing the signal with respect to a single sensor coil 60. One is an analog to digital (A/D) converter 105, which provides a digital representation of the voltage sensed by the sensor coil. A second is the gain factor memory 110 and multiplier circuit 115. The raw voltage output from the A/D 105 is normalized by multiplying it by a gain factor established for that coil during a calibration procedure. The output of the multiplier 115 is then compared by circuit 120 to the voltages in a look up table 125, which holds previously determined voltage-to-distance data. The output of the comparing circuit 120 therefore represents the distance of that particular coil from the wire 50. 
     The look up table 125 holds only a limited number of data points (only 64 in the preferred embodiment), and therefore a straightforward interpolation procedure is used to obtain distance measurements when the voltage output of a coil falls between these data points. The relationship between voltage and distance is represented in the curve of FIG. 9. 
     The microcomputer 92 will therefore sense the output of each of the sensor coils 60 individually, and by reference to a voltage level established during a calibration procedure, and the voltage-to-distance data in the look up table 125, the distance of the sensor coil from the buried wire can then be calculated. 
     The center point 130 of each sensor bar 40, 42 is used as the reference from which later vehicle distance measurements are calculated. The distance of each coil from the wire is determined, as described above, and the coil that has the highest output (the coil closest to the wire) is identified. 
     Next, the region where the wire is either located or closest to is identified by checking the output of adjacent coils. As shown in the plan view of FIG. 10, the sensor bars are divided into five separate regions: Region 1 is to the left of coil 60a (as viewed in the drawing), Region 2 is between coils 60a and 60b, Region 3 is between coils 60b and 60c, Region 4 is between coils 60c and 60d, and Region 5 is to the right of coil 60d. If the wire 50 is in the position shown by the line 50a, it is clearly in Region 5, and if it is in the position shown by line 50b, it is in Region 3. 
     Once the region is determined, the distance D of the wire relative to the sensor bar&#39;s center point 130 (referred to hereinafter as either XFRONT for bar 40 or XREAR for bar 42) is calculated from one of the following formulas. 
     
         If in Region 1, D=-(X1+X2)/2-SC 
    
     
         If in Region 2, D=(X2-X1)/2-SC 
    
     
         If in Region 3, D=(X2-X3)/2 
    
     
         If in Region 4, D=(X3-X4)/2+SC 
    
     
         If in Region 5, D=(X3+X4)/2+SC 
    
     where D is the distance (XFRONT or XREAR) in units of 1/64 inch of the wire from the center point 130; Xl, X2, X3, and X4 are the distance values for coils 60a, 60b, 60c, and 60d, respectively; and SC is a scale constant. It is clear that when the wire is to the left of the center point, the distance value D is negative. Using the dimensions given in FIG. 4 for a sensor bar, the scale constant SC will be 4.5 inches. 
     If the wire is very close to one coil, a weight factor routine will be used to determine the distance of the coils in a region from the wire since accuracy of the coil&#39;s voltage to distance curve deteriorates, as illustrated in FIG. 9. 
     Once the distance of the wire from the center point of either or both sensor bars is finally determined, the position of the vehicle&#39;s virtual point 140 (FIG. 3A) may be calculated, and also the angle of the vehicle&#39;s axis relative to the wire (assuming the vehicle is in motion), by using one of the following procedures. The first procedure, V1CALC, is used when both sensor bars are reading the wire; the second procedure, V2CALC, is used when only one sensor bar senses the presence of the wire. In either procedure, the result is a measurement of the vehicle&#39;s angle and distance from the buried wire 50. 
     V1CALC Procedure (two sensor bar data known) 
     
         XMEAS=AFS(XFRONT-XREAR)-XFRONT 
    
     
         TMEAS=(XREAR-XFRONT)/SENDIS 
    
     Where XMEAS is the measured distance from the virtual point 140 to the wire; AFS is the scaled value of the distance between the load wheel and front sensor bar divided by the distance between the sensor bars; TMEAS is the measured value of the angle of the vehicle relative to the wire; and SENDIS is a scale value used in converting units from distance to radians. 
     LIB5 Formulas 
     
         TDEAD=TACT(OLD)+[NEWD * (SIN(TWHEEL))]/WB 
    
     
         XDEAD=XACT(OLD)+[NEWD * (COS(TWHEEL)) * SIN(TDEAD))] 
    
     Where TDEAD is the dead reckoning value for the truck angle; TACT is the filtered value of the truck angle, in radians; NEWD is the new value of the distance moved during the present pass; TWHEEL is the value of the wheel angle; WB is the value for the wheel base, in inches; XDEAD is the distance from the wire during dead reckoning; XACT is the filtered value of the truck virtual reference center position with reference to the wire. 
     LIB4 Formulas 
     
         __________________________________________________________________________if  TMEAS + TOFFS = TDEAD  TACT = TDEADif  TMEAS + TOFFS &gt; TDEAD  TACT = TDEAD + [K6 * (TMEAS + TOFFS - TDEAD) + K2]if  TMEAS + TOFFS &lt; TDEAD  TACT = TDEAD + [K6 * (TMEAS + TOFFS - TDEAD) - K2]if  XMEAS + XOFFS = XDEAD  XACT = XDEADif  XMEAS + XOFFS &gt; XDEAD =  XDEAD + [K1 * (XMEAS + XOFFS - XDEAD) + K4]if  XMEAS + XOFFS &lt; XDEAD =  XDEAD + [K1 * (XMEAS + XOFFS - XDEAD) - K4]__________________________________________________________________________ 
    
     where TOFFS is the truck angle offsets due to sensors; K1, K2, K4 and K6 are constants. 
     The LIB4 AND LIB5 equations are used to calculate XACT &amp; TACT only if &#34;X&#34; and &#34;theta&#34; were previously known, thus a filtered result. If &#34;X&#34; &amp; &#34;theta&#34; not previously known then XACT=XMEAS &amp; TACT=TMEAS. 
     V2CALC Procedure (one sensor data known 
     Front Sensor Only 
     
         CA=XFRONT-XFO+CB 
    
     
         CB=DIST * SIN(TWHEEL) * AFLT 
    
     
         CD=DIST * COS(TWHEEL) 
    
     Rear Sensor Only 
     
         CA=XREAR-XRO+CB 
    
     
         CB=DIST * SIN(TWHEEL) * ARLT 
    
     
         CD=DIST * COS(TWHEEL) 
    
     Where CA, CB and CD are intermediate calculations; XFO is the distance between the front sensor bar 40 and the wire during the previous pass; ARLT is the scaled value of the distance between the load wheel and the rear sensor divided by the wheel base; and DIST is the value of the distance moved since the last microcomputer update. 
     
         __________________________________________________________________________TMEAS = [CA * COS(TMEAS)] + CB + [CD * SIN(TMEAS)]TACT =  [TACT + 1/WB * DIST * SIN (THWEEL)] * [(N - 1)/N] -   TMEAS/NXMEAS(F) =   -AF * SIN(TMEAS) - XFRONT * COS(TMEAS)XMEAS(R) =   -AR * SIN(TMEAS) - XREAR * COS(TMEAS)XACT =  [XACT + DIST * COS(TWHEEL) * SIN(TACT)] *   [(N - 1)/N] - TMEAS/(N)__________________________________________________________________________ 
    
     where N is a sample count used in the averaging of a single sensor and AF and AR are constants: AF is the distance from the load wheel to the steered wheel sensor bar and AR is the distance from the load wheel to the load wheel sensor bar, in inches. 
     XTRANS 
     Lateral deviation of front/rear sensors from guidewire 
     
         ABSXF=-XACT-(AF * TACT) 
    
     
         ABSXR=-XACT-(AR * TACT) 
    
     where ABSXF and ABSXR are the absolute filtered values of XFRONT and XREAR, respectively. 
     LIB2 - STEER COMMAND 
     
         CMDU,CMD [FORWARD]=G3(XACT+XOFSET)+G2(TACT+TOFSET) 
    
     
         [REVERSE]=G3(XACT+XOFSET)-G2(TACT+TOFSET) 
    
     TCMD 
     Contribution of wheel theta to servo command 
     
         OCMD=(G1* TWHEEL)+(CMDU,CMD) 
    
     where CMDU, CMD are intermediate steered wheel commands which include the position (XACT) and angle (TACT) of the vehicle, OCMD is the output command, G1 is the feedback gain for the steered wheel angular position (volts/radians), G2 is the feedback gain for the vehicle angular position (volts/radians), and G3 is the feedback gain for the vehicle&#39;s position (XACT) (volts/inches). The total steered wheel output command OCMD, which is a pulse width modulated signal applied to the steered wheel motors, is therefore a combination of signals, taking into account the vehicle&#39;s location and angle of the steered wheels. 
     These formula comprise the microcomputer based mathematical representation of the wire guidance response system. The inputs to these equations include the vehicle&#39;s position and angular displacement from the guide wire (derived from sensor coil values), steered wheel angle (derived from the steered wheel encoder 23a), and the distance traveled since the last microcomputer update (from equations V1CALC, V2CALC, LIB4, LIB5 and XTRANS). The output from these equations is an error signal which is converted to a pulse width modulated steering command OCMD (from equations LIB2 and TCMD) which positions the steered wheels to a location in order to maintain the straight line wire guide travel condition. 
     A simulated wire guide path signal is provided by a wire 132 connected to generator 135 (FIGS. 8 and 10) that produces a 7 kHz signal during calibration. The wire is permanently placed near each of the sensor coils and when the generator 135 is activated, a test signal is generated. The signal thus produced is detected by the sensor coils, with the output of each coil being processed by its respective amplifier. If the output of each is above a predetermined magnitude during the test, then a signal is generated indicating that the guidance hardware is present and working. 
     During the wire guidance calibration mode, the peak coil values are monitored, and if any coil indicates that it is saturated, a signal is generated to warn the operator to verify that the proper equipment is installed and that the wire guidance signal is functioning properly. During the wire tracking mode, if any sensor coil reads a value in excess of the value detected during the calibration, the vehicle&#39;s brakes will be applied and the wire guidance system shut down. 
     Once the wire has been detected and the system enters the Acquisition Mode, the LIB5 formulas are used to calculate the position of the vehicle based on previous location, speed and the present wheel angle. When in the overshoot mode of operation, the guidance equations continue to drive the vehicle toward the centerline of the wire. 
     In order to enter the overshoot mode, the vehicle must not have been in the tracking mode. Also, the angle of the vehicle relative to the wire must be below a predetermined maximum limit. To remain in the overshoot mode, the vehicle must be making progress toward wire acquisition by decreasing the distance to the wire during each cycle. During an overshoot, when the vehicle approaches and then passes over the wire, the position information is not as accurate as when the vehicle is aligned with the wire, and the speed of the vehicle will be limited to 1.5 mph. 
     While the method herein described, and the form of apparatus for carrying this method into effect, constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to this precise method and form of apparatus, and that changes may be made in either without departing from the scope of the invention, which is defined in the appended claim.
A wire guidance system for a materials handling vehicle, such as a turret stockpicker, detects an alternating signal carried by a buried wire. Two sets of four sensors each are carried at either end of the vehicle. These sensors define regions, and a procedure is provided to determine in which region the wire is located. The distance and angle of the vehicle is determined by reference to the distance measurements of each sensor set, as calculated by a microcomputer. A self testing circuit is provided to insure proper operation of the sensors and associated circuitry.