Reverse direction guidance system for lift truck

Abstract

An automatic guidance device for a self-powered cargo moving vehicle operated by a vehicle-borne sensor which follows a buried, energized wire path and which includes sensor means mounted either between the fixed and steerable axles or in front of the fixed axle of the vehicle for guiding the vehicle when it travels in a direction such that the fixed axle precedes the sensor by effectively generating a position error signal relative to the direction of travel. In one preferred embodiment the sensor includes a pair of sensing coils whose outputs are combined to generate an error signal V=(1+K)R-KF where R and F are the difference of outputs of a pair of rear and forward sensors, respectively, and K is equal to the ratio of the distance of the rear pair of coils to the virtual sense point and the distance between the rear and forward pairs of coils.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending application Ser. No. 629,491 filed Nov. 6, 1975, feed 7.Baddend. 5 volt output connected to the signal ground.

The output of the lowpass filter 184 is supplied to one input of a differential amplifier 204 whose other input is supplied with the output of the lead filter 186 comprised of a parallel RC circuit connected in feedback configuration to the amplifier 204.

The output of the differential amplifier circuit 170 172 is also supplied to the signal amplitude detector 192 which is comprised of an input resistance 206 connected to the cathode of a diode 208 whose anode is connected to the input of a differential amplifier 210. The other input of the amplifier 210 is connected through a resistance 212 to the circuit ground and through a capacitor 214 to the anode of the diode 208. Plus 12 volts bias is supplied through a resistor 216 to the anode of the diode 208. The output signal from the amplifier 210 may be designated as the threshold signal and it is supplied via a line 220 to the enable logic circuit 190. The "MAN." terminal of the single pole double throw switch 146 is connected to the line 220. The contact arm of the switch 146 is connected to the circuit ground. Thus when the switch 146 is in the "MAN." position the line 220 is grounded and no threshold signal is supplied to the enable logic circuit 192 190 just as if no threshold signal has been produced. Both of these conditions will be designated for the purposes of this discussion as a logic low.

The line 220 is connected to the input of an inverter 222 whose output is fed to one input of a NOR gate 224. The output of the NOR gate is fed to one input of a second NOR gate 226 and to the controlling input of a CMOS switch 228 and the input of an inverter 230. The other input of the NOR gate 224 is the output of the NOR gate 226. The output of the inverter 230 is connected through a resistance 232 to the base of NPN transistor 234. The emitter electrode of the transistor 234 is connected to the circuit ground. The LED 138 is connected in series between the plus 24 volt supply and the collector of the transistor 234.

The output of the inverter 230 is also connected to the controlling input of a second CMOS switch 236 whose input is supplied with the output of the power steering tachometer 46. The outputs of the CMOS switches 228 and 236 are combined and fed to one input of a differential amplifier 238.

The other input of the NOR gate 226 is supplied from the output of an exclusive OR gate 240. As will be explained in greater detail hereinafter, the output of the OR gate 240 is a signal representative of whether or not the signs of the slope and polarity of the error signal after synchronous detection are the same to "enable" the logic, i.e. to make the guidance system acquire the buried wire 34.

As was stated before, when the switch 146 is in the manual mode or when no threshold signal is present on the line 220, a logic high is placed on the corresponding input to the NOR gate 224. When this happens the NOR gates 224 and 226 act as a flip-flop in which the high input from the inverter 222 to the NOR gate 224 is an overriding reset. The result is that the output of the NOR gate 224 will be a logic low and the output of the NOR gate 226 will be a logic high regardless of the output of the exclusive OR gate 240. The logic low apearing at the output of the NOR gate 224 will cause the transistor 234 to become conductive to energize the LED 138. This same logic low will also cause the CMOS switch 228 to be open and, because of the inverter 230, the CMOS switch 236 will be closed.

With the CMOS switch 228 open and the CMOS switch 236 closed the output from the power steering tachometer 46 will be fed to the input of the differential amplifier 238. The output of the amplifier 238 may be taken as the velocity command or, in effect the steering control signal to the motor. The polarity of the signal will determine which way the motor control rotates.

If the switch 146 is switched to the auto position, as shown in FIG. 8, and a threshold signal appears on the line 220, the output of the inverter 222 will be a logic low. Assuming that the output from the exclusive OR gate 240 is also a logic low, indicating that the sign of the slope is not equal to the sign of the polarity of the synchronously detected error signal, and that the output of the NOR gate 224 continues to be a logic low, then the output of the NOR gate 226 will be a logic high. At this point, even though the switch 146 is a "AUTO," the OPV 10 will continue under the power steering mode until the signs of the slope and polarity of the modified error signal are equal. When this happens the output of the exclusive OR gate 240 will be a logic high, causing the output of the NOR gate 226 to be a logic low. With two logic lows to the input of the NOR gate 224 its output will change to a logic high and latch the flip flop.

A logic high at the output of the NOR gate 224 will cause the CMOS switch 228 to become conductive and the CMOS switch 236 to become non-conductive. The LED 138 supplied from the output of the inverter 230 will also be extinguished. Thus the input signal to the amplifier 238 will be the guidance control input derived from the sensing coils and the OPV 10 will be steered automatically.

In order to determine the polarity and slope of the error signal the output of the amplifier 204 is fed to one input of an amplifier 242 whose other input is connected to the chassis signal ground and whose output is fed to one input of the exclusive OR gate 240. The output from the amplifier 204 is also fed to one input of a differential amplifier 244 and, through a resistor 246 to the other input of the differential amplifier 244. This other input is also connected to the circuit ground through a capacitor 248. The output of the amplifier 244 is supplied to the other input of the exclusive OR gate 240. The output of the amplifier 242 is representative of the polarity of the output of the amplifier 204 and the output of the amplifier 244 is representative of the slope of the same signal. When the OPV 10 has come sufficiently close to the buried wire 34 for the threshold signal to be established at the output of the amplifier 210 then the two amplifiers 242 and 244 together with the exclusive OR gate 240 will determine whether the sign of the slope of the error signal is equal to the sign of its polarity, indicating that the OPV 10 is goind away from the wire. When this happens the output of the exclusive OR gate 240 will be a logic high.

It should be noted that the guide flip-flop made up of the NOR gates 224 and 226 is effectively a latching flip-flop. Once the flip-flop 224 has gone into the auto mode, it will only reset on a change in state of the signal applied from the output of the inverter 222, which indicates either that the switch 146 has been thrown to the manual mode or that the threshold signal has been lost. Provided the threshold signal is present and the switch 146 is in the auto position, no changes at the output of the exclusive OR gate 240 will affect the state of the flip-flop.

In order to warn the operator that the guide flip-flop has changed state, such as if the threshold signal should somehow be lost, the output from the NOR gate 224 is fed through a series RC circuit 246 to one input of a low true NAND gate 248. This same input of the NAND gate 248 is also supplied with appropriate plus 12 volt bias. The other input of the NAND gate is connected directly to the auto terminal of the switch 146 and through a resistance 250 to the LED 136. The output of the NAND gate 248 is supplied to the base electrode of an NPN transistor 252 whose emitter electrode is connected to the circuit ground and whose collector electrode is connected in series with an alarm 254 to a plus 24 volt source.

In operation, the input to the NAND gate 248 supplied by the switch 146 is a logic low. When the output of the NOR gate 224 also goes to a logic low, indicating that the guide flip-flop has somehow reset itself, then the output of the NAND gate 248 will become a logic high, triggering the alarm 254 through the transistor 252. An amplifier 256 having one lead connected through a diode 258 to the plus 12 volt source and its output connected through a resistance 260 to the base electrode of the transistor 252 will activate the alarm 254 if there is a power failure.

Referring more particularly now to FIG. 9, the velocity command output signal from the amplifier 238 is fed to one input of a comparator 262 and to the corresponding input of a second comparator 264. The output of the amplifier 262 is fed to one input of an exclusive OR gate 266, the input of an inverter 268 and through a parallel diode resistance circuit 270 to one input of an amplifier 272. The same input of the amplifier 272 is connected through a capacitor 274 to the circuit ground. The output of the inverter 268 is connected through a similar parallel diode resistance circuit 276 to one input of an amplifier 278. This same input of the amplifier is connected through a capacitor 280 to the circuit ground. The other inputs of the amplifier 272 and 278 are connected to a plus 12 volt source.

The output of the amplifier 278 is connected to the base electrode of an NPN transistor 282 whose collector electrode is connected through a resistance 284 to the collector electrode of a PNP transistor 286. The emitter electrode of the transistor 286 is connected directly to the plus 24 volt source for the motor. The base electrode of the transistor 286 is forwardly biased through appropriate resistances from the plus 24 volt source.

The output from the amplifier 272 is connected to the base electrode of an NPN transistor 288 whose collector electrode is connected through a resistance 290 to the collector electrode of a PNP transistor 292. The emitter electrode of the transistor 292 is connected directly to the plus 24 volt motor source and its base electrode is forwardly biased by appropriate resistance from the plus 24 volt source. The base electrodes of the transistors 286 and 292 are also connected to the collector electrode of an NPN PNP transistor 294 whose emitter electrode is connected to the circuit ground.

To control the direction of the current supply to the motor 44 the collector electrode of the transistor 286 is connected to the base electrode of a PNP transistor 296 whose emitter electrode is connected to the plus 24 volt motor source. The collector electrode of the transistor 296 is connected to a junction point 298 and to the collector electrode of an NPN transistor 300. The base and emitter electrodes of the transistor 300 are connected to the collector of the transistor 288 and to a junction point 302, respectively. The emitter electrode of the transistor 282 is connected to the base electrode of an NPN transistor 304 whose emitter electrode is connected to the point 302 and whose collector electrode is connected to a junction point 306.

The collector electrode of the transistor 292 is connected to the base electrode of a PNP transistor 308 whose emitter electrode is connected to the plus 24 volt battery source for the motor. The collector electrode of the transistor 308 is connected to the junction point 306. The point 302 is connected in series with a very low resistance wire 310 to the minus terminal of the motor battery. The motor 44 is connected at one side to the junction point 298 and through the resistor 198 to the junction point 306.

The output of the exclusive OR gate 266 is fed to one input of a NOR gate 314. The output of the NOR gate 314 is supplied to a combination of inverters and operational amplifiers designated generally as 316 which convert the NOR gate 314 into a 200 microsecond, one shot multi-vibrator. The output of the NOR gate 314, which is effectively the output of the multi-vibrator, is fed through an inverter 318 to the base electrode of the NPN PNP transistor 294.

If any of the inputs to the NOR gate 314 is a logic high its output is a logic low and the transistor 294 will be conductive to forwardly bias the transistors 286 and 292. When the transistors 286 and 292 are forwardly biased, i.e. conductive, they short together the base and emitter electrodes of the transistors 296 and 308, respectively, making them non-conductive so that the motor will now run. As long as all the inputs to the NOR gate 314 are logic lows, its output will be a logic high and the transistors 286 and 292 will be non-conductive.

Assuming that the output of the amplifier 62 is a logic high, the output of the amplifier 272 will cause the transistor 288 to become conductive thereby making the PNP transistor 308 and NPN transistor 300 conductive by connecting their base electrodes together through the resistor 290, which can have a value of 600 ohms, for example. It can be seen that this causes a current path to flow from the 24 volt battery source through the transistor 308, the resistor 198, the motor 44, the transistor 300 and the resistor 310 to the minus terminal of the battery. Thus the motor will run in a preordained direction determined by the path of current flow. Similarly, when the output of the amplifier 262 is the equivalent of a logic low, these same transistors will be turned off and, through the inverter 268 and the amplifier 278, the transistors 282, 304 and 296 will become conductive to supply current to the motor 44, though in the opposite direction to cause the motor to rotate in the opposite direction. Thus, the polarity of the output of the amplifier 262 is determinative of the direction in which the motor will run. As will be described in greater detail hereinafter, the polarity of the output signal from the amplifier 262 depends on the polarity of the velocity command signal from the amplifier 238 as well as the output of the electronic tachometer 200.

As explained above in reference to FIGS. 7A and 7B, the electronic tachometer 200 is connected in parallel with the motor and across the resistance 198. As shown in FIG. 9, these connections are made by way of lines 312, 320 and 322 connected to points 298, the junction of the motor and the resistor 198, and the point 306, respectively. The lines 312, 320 and 322 are the three inputs to the electronic tachometer 200, which is comprised of a differential amplifier 324 whose inputs are supplied by the lines connected to the motor and whose cutput is connected to the inputs of the amplifiers 262 and 264 other than the inputs connected to the output of the amplifier 238. As mentioned above, the outputs of the amplifiers 262 and 264 are supplied to the inputs of an exclusive OR gate 266. The exclusive OR gate acts as a controlled inverter whose output will be low whenever the absolute magnitude of the velocity command signal exceeds the absolute magnitude of the tachometer output signal, provided that the two signals are of the same polarity. If the two signals are of opposite polarity, then the output of the OR gate 266 will be low. For any other condition the output of the OR gate 266 will be a logic high with the result that the motor 44 will be turned off. The minimum time during which the motor 44 will be turned off is approximately 200 microseconds, which is determined by the circuit values within the multi-vibrator circuit 316. The duration during which the motor 44 will be turned on is determined by the length of time required for the output signal from the electronic tachometer 200 to match the velocity command signal from the amplifier 238. In order to guard against the possibility that a pair of series connected power transistors such as transistors 296 and 300 or 308 and 304 might be simultaneously made conducting, the parallel resistor diode circuits 276 and 270 together with their associated capacitors 280 and 274 insure that when there is a change in polarity of the velocity command signal that all the power transistors will be turned off before any other set is turned on.

A differential amplifier 326 has its two inputs connected in parallel with the resistor 310 to act as a torque limiting sensor to shut off the motor in the event that, because of some physical binding in the guide wheel mechanism, the motor is forced to draw an excess of current which would damage the motor. When the voltage across the resistor 310 increases beyond a predetermined value the output of the amplifier 326 reaches what amounts to a logic high which is fed to one input at the NOR gate 314. This logic high will cause the motor to be deenergized. Similarly, the power failure signal from the output of the amplifier 256 is also supplied to one input of the NOR gate 314 to shut off the motor in the event there is a failure in power to the guidance circuit.

Referring now more particularly to FIGS. 11 and 12, a modified embodiment of the reverse guidance system according to the invention is illustrated. In this embodiment there are no rear sensing coils, but instead there is a single angle sensing coil 125 which is positioned more or less between the right and left reference coils 158 and 160, respectively. The direction sensing coil 125 may take the form of a long ferrite coil not unsimilar to the type which is sometimes used as an antenna coil in portable radios or portable radio direction finders. The coil 125 is positioned on the bottom of the vehicle to be normally directly over the buried wire 34. Assuming that the vehicle is positioned correctly over the wire 34 but is heading in a direction at an angle φ to the wire, the output of the coil 125 will be A sine φ, where A is the maximum amplitude of the coil output. It will be appreciated that for the very small angles of φ the sine of φ will approximately equal the tangent of φ.

If it is desired to steer the vehicle with respect to a virtual sense point 29' spaced to the rear of the vehicle and beyond the fixed axle wheels when the vehicle makes an angle φ with the wire 34 this virtual sense point will be displaced off of the wire by a distance d. If the virtual sense point is at a distance D_p from the center of the coil 125, by geometric construction the error d is equal to approximately D_p sine φ, or ##EQU2##

Since A sin φ is simply the output of the coil 125 and since (D_p /A) is a constant, the error signal can be rewritten as Kφ where φ is the output of the coil 125. Added to this must be the normal, positional error signals from the coils 162 and 164 which indicate how far away the vehicle is from the buried wire 34. Thus the final error signal is R-L+Kφ where R and L are the outputs from the error coils 162 and 164.

Referring now more particularly to FIG. 12, the circuit depicted in FIG. 7A is modified so that the circuit elements 107, 108, 113, 115, 116 and 120 are omitted. Furthermore the gain control signal for the amplifiers 100 and 103 is taken directly from the output of the AGC rectifier/amplifier 111 rather than from the terminal of the switch 112. The output of the direction sensing coil 125 is fed to the inputs of a transconductance amplifier 126 whose output is connected to the plus input terminal of the summing junction 109 and to the plus input terminal of the summing junction 117. Thus it can be appreciated that when the switch 112 is in the forward direction mode, the output from the amplifier 118 will be RFO-LFO and when the switch 112 is in the reverse direction mode, thus supplying a gain signal to the amplifier 126, the output of the amplifier 118 will be RFO-LFO+Kφ. As in the embodiment depicted in FIG. 7A the output of the amplifier 118 is supplied to the summing junction 119 where it is combined with the feedback signal FB. The output of the junction 119 goes to the switch 123 as in the embodiment depicted in FIG. 7A.

As mentioned at the beginning of this application, this embodiment is somewhat less advantageous than the multicoil arrangement depicted in FIG. 7A because this embodiment is overly sensitive to variations in the straightness of the buried wire. A slight wiggle in the wire is greatly magnified and causes a spurious error signal. These effects can be reduced somewhat by using a long direction sensing coil 125 and by only using the system on a vehicle with a relatively small wheel base. This has the effect of reducing the projection distance D_p with the consequency that variation in the straightness of the buried wire are not magnified as greatly. This is apparent from the fact that the constant K is actually equal to (D_p /A). Thus any reduction in D_p will also reduce the effect of any variations in the straightness of the buried wire.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is understood that certain changes and modifications may be practiced within the spirit of the invention as limited only by the scope of the appended claims.

Claims

1. An improved, self-guided vehicle of the type which automatically follows an externally defined path in a forward direction and which has at least one ground engaging steerable wheel, sensor means mounted on the vehicle for generating a position error signal representative of the position of the vehicle with respect to the path, steering actuator means attached to the ground engaging steering wheel for steering the vehicle in response to a steering control signal to the steering actuator means, and steering circuit means supplied with the position error signal for generating a first steering control signal for the steering actuator means to cause the steering actuator means to automatically steer the vehicle along the external path, wherein the improvement comprises:

sensor means for generating the position error signal relative to a virtual sense point to the rear of the vehicle to guide the vehicle when it travels in a backward direction along the path.

2. An improved self-guided vehicle as recited in claim 1 wherein the path is defined by a buried, energized wire and wherein the sensor means comprise a pair of forward sensor coils and a pair of reverse sensor coils with the terms forward and rear taken in the sense of the direction of forward vehicle travel, each of the pairs of coils being mounted on the vehicle so as to normally straddle the path and each pair of the coils producing an output signal representative of the difference of the outputs of the coils of each pair, and means for generating the position error signal (V) with respect to the virtual sense point according to the formula:

V=(1+K)R-KF

where

R=difference of outputs of the rear pair of sensor coils;

F=difference of outputs of the forward pair of sensor coils; and

K=constant=ratio of the distance between the rear pair of coils and the virtual sense point to the distance between the rear and forward pairs of coils.

3. An improved self-guided vehicle as recited in claim 1 wherein the path is defined by a buried, energized wire and wherein the sensor means comprise right (R) and left (L) error coils mounted on the vehicle on opposite sides of the path and a direction sensing coil for sensing the angle the vehicle makes with respect to the wire, the direction sensing coil being aligned with the wire so that its output (φ) is proportional to the tangent of the angle made with the wire for small angles, and means for generating the position error signal (V) with respect to the virtual sense point according to the formula

V=L-R+Kφ

where

K is a constant determined in part by the ratio of the distance between the direction sensing coil and the virtual sense point to the maximum output of the direction sensing coil

L=output of left error coil

R=output of right error coil

φ=output of the direction sensing coil.

4. An improved self-guided vehicle as recited in claim 1 further comprising

enabling logic means supplied with the sensor signal for producing an enabling output signal when the vehicle is being manually steered toward the external path and has crossed it, or passed within a predetermined distance of it, and thereafter is moving away from the external path, and

switching means supplied with the enabling output signal to supply the first steering control signal to the steering actuator means only after the enabling output signal is generated, whereby the vehicle will become automatically guided along the external path only after it has been manually steered across or just up to the external path and is starting to move away from it.

5. An improved self-guided vehicle as recited in claim 4 wherein the external path is defined by an energized guide wire and the improvement to the vehicle further comprises:

coil means within the sensor means and straddling the wire to produce output signals proportional to the vehicle's lateral distance from both sides of the wire, the coil means including means for producing an error signal from the difference of the coil means output signals, and

means within the enabling logic means for triggering in part the enabling output signal when the arithmetic signs of the slope and polarity of the error signal are equal, thereby indicating that the vehicle is moving away from the wire.

6. An improved self-guided vehicle as recited in claim 5 wherein the coil means further include means for producing a reference signal from the sum of the coil means output signals and the enabling logic means include means for sensing when the reference signal reaches a predetermined magnitude and for thereafter generating a threshold signal which triggers in part the enabling logic means to produce the enabling output signal.

7. An improved self-guided vehicle as recited in claim 6 wherein the enabling logic means will continue to produce the enabling output signal when once triggered provided that the threshold signal continues to be generated.

8. An improved self-guided vehicle as recited in claim 5 wherein the coil means comprise at least a pair of sensing coils on opposite sides of the wire and a multilayered assembly for interconnecting the coils and for mounting the coils horizontally, the multilayered assembly including a printed circuit board, a sheet of insulating material, a mu metal strip and a strip of ferrous material, the printed circuit board having a metallic circuit printed thereon and the coils having leads attached to the printed circuit board and interconnected by the printed circuit, the mu metal strip providing a low reluctance return path for lines of magnetic flux generated by the energized wire and passing through the coils.

9. An improved self-guided vehicle as recited in claim 4 wherein the steering actuator means include an electric motor for positioning the ground engaging steering wheel and the steering circuit means include

an electric tachometer for sensing the voltage drop across the motor and the electrical current flowing through it for generating a tachometer feedback signal representative of the response of the motor to the steering control signal, and

means for subtracting the tachometer feedback signal from the steering control signal before it is supplied to the motor.

10. An improved, self-guided vehicle of the type which automatically follows a path defined by a buried, energized wire and which has at least one ground engaging steerable wheel, and a pair of wheels on a fixed axle, sensor means mounted on the vehicle for generating a sensor signal representative of both the lateral position of the vehicle with respect to the wire and the inclination of direction of vehicle travel with respect to the wire, steering actuator means attached to the ground engaging steering wheel for steering the vehicle in response to a steering control signal to the steering actuator means, and steering circuit means supplied with the sensor signal for generating a first steering control signal for the steering actuator means to cause the steering actuator means to automatically steer the vehicle along the wire path, wherein the improvement comprises:

the sensor means is mounted on the vehicle between the steerable wheel and the fixed axle and guides the vehicle along the wire path when the vehicle is traveling in a direction such that the fixed axle wheels precede the steerable wheel, the sensor means including a pair of first sensor coils and a pair of second sensor coils, the first sensor coils being mounted closest to the fixed axle wheels and each of the pairs of coils being mounted on the vehicle so as to normally straddle the wire path and each pair of the coils producing an output signal representative of the difference of the outputs of the coils of each pair, and means for generating the position error signal (V) with respect to a virtual sense point, located beyond the fixed axle wheels and in the direction of vehicle travel, according to the formula:

V=(1+K)A-KB

where

A=difference of outputs of first pair of sensor coils;

B=difference of outputs of the second pair of sensor coils; and

K=constant=ratio of distance between the first pair of sensor coils and the virtual sense point to the distance between the first and second pairs of coils.

11. An improved, self-guided vehicle of the type which automatically follows an externally defined path, and which has at least one ground engaging steerable wheel, a pair of wheels on a fixed axle, a path sensor mounted on the vehicle for generating a position error signal representative of both the lateral position of the vehicle with respect to the path and the inclination of direction of vehicle travel with respect to the path, a steering actuator attached to the ground engaging steerable wheel for steering the vehicle in response to a steering control signal to the steering actuator, and a steering circuit controller supplied with the position error signal for generating said steering control signal for the steering actuator to cause the steering actuator to automatically steer the vehicle along the path, wherein the improvement comprises a first and a second pair of sensors, both pairs of sensors being mounted on the vehicle between the steerable wheel and the fixed axle to guide the vehicle along the path when the vehicle is traveling in a direction such that the fixed axle wheels precede the steerable wheel, the first pair of sensors being mounted closest to the fixed axle wheels and each of the first and second pairs of sensors being mounted on the vehicle so as to normally straddle the path, each of the first and second pairs of sensors producing an output signal representative of the difference of the outputs of the sensors of each pair, and means supplied with output signals of the sensor pairs for generating a position error signal (V) with respect to a virtual sense point, located beyond the fixed axle wheels and in the direction of vehicle travel, according to the formula:

V=(1+K)A-KB

where A=difference of outputs of first pair of sensors;

B=difference of outputs of the second pair of sensors; and

K=constant=ratio of the distance between the first pair of sensors and the virtual sense point to the distance between the first and second pairs of

sensors, this ratio being greater than 1.

12. An improved, self-guided vehicle as recited in claim 11 wherein the improvement further comprises separate automatic gain controlled amplifying means for the first and second pairs of sensors whereby the first and second pairs of sensors are desensitized to variations in the vertical distance between the path and each of the sensor pairs.