Steering control system for an unmanned vehicle
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to a steering control system for an unmanned vehicle. 
     A programmed turning system employing a trapezoid function turning program as shown in FIG. 1 is known in the art as a steering control system in which at the intersection of guide cables an un-manned vehicle is turned so as to be shifted from one guide cable to the other guide cable. 
     In this conventional system, after the guide travelling operation of the vehicle along one of the guide cables has been released, a programmed turning operation of the vehicle is carried out for the other guide cable in accordance with a vehicle turning program. When the turning operation is finished (or the steering angle becomes zero), the guide travelling operation along the other guide cable is effected immediately. In this system, it is difficult to smoothly switch the programmed turning operation over to the guide travelling operation. Therefore, the conventional system is disadvantageous in that after the programmed turning operation has been switched over to the guide travelling operation along the other guide cable, the vehicle is liable to move along a wandering course. 
     In view of the foregoing, an object of this invention is to provide a steering control system for an unmanned vehicle in which the programmed turning operation can be smoothly changed over to the guide travelling operation. 
     It is another object of the invention to provide a steering control system for an unmanned vehicle which is capable of transferring the vehicle smoothly between a main cable and any one of cables intersecting with the main cable by utilizing the above described steering control system. 
     In this invention, the conventional system in which th programmed turning operation is switched over to the guide travelling operation when completed is not employed. That is, in the steering control systems according to the invention, irrespective of the completion of the programmed turning operation (even during the programmed turning operation, or even if it passes more or less after the completion of the progammed turning operation), the programmed turning operation is switched over to the guide travelling operation when the vehicle reaches the guide cable, thereby to smoothly change the operations. 
     In the following description of this invention, the term &#34;turning angle&#34; as used herein is intended to mean the amount of change in direction of the vehicle with respect to the state of the vehicle which is about to start the turning operation. The term &#34;predetermined turning angle&#34; is intended to mean a turning angle required for the vehicle to shift to the other guide cable in such a manner that the vehicle is in parallel with the other guide cable. Furthermore, a turning program such that a programmed turning is completed with the vehicle shifted onto the other cable, being in parallel with the other cable, will be referred to as &#34;a turning program with the predetermined turning angle&#34;. In addition, a turning program such that a program turning is completed before the vehicle is shifted onto the other cable will be referred to as &#34;a turning program having a turning angle less than the predetermined turning angle&#34;. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graphical diagram showing a trapezoid function used commonly for a programmed turning of a vehicle; 
     FIG. 2 is a schematic plan view showing a state of travelling of the vehicle turning in accordance with the present invention; 
     FIGS. 3a to 3c are views for explaning operations of posture angle detection coils; 
     FIG. 4 is a graphical diagram showing trapezoid functions used for the programmed turning according to the invention; 
     FIG. 5 is a block diagram showing an embodiment of the steering control system according to the invention; 
     FIG. 6 is a block diagram showing another embodiment in which a part of the embodiment shown in FIG. 5 is modified; 
     FIG. 7 is a schematic plan view showing forward and reverse turning of the vehicle; 
     FIG. 8 is a graphical diagram showing trapezoid functions used in the forward and reverse turning of the vehicle; 
     FIGS. 9 and 10 are block diagram showing examples of constructions for producing the respective functions used in the forward and reverse turning of the vehicle; 
     FIG. 11 is a schematic plan view showing right and left turning of a fork lift truck; 
     FIG. 12 is a graphical diagram showing examples of trapezoid functions used for the right and left turning of the fork lift truck shown in FIG. 11; 
     FIGS. 13 and 14 are block diagrams showing examples of constructions for obtaining the trapezoid functions used for the right and left turning; 
     FIGS. 15 and 16 are views showing other examples of the posture angle detection coils; 
     FIG. 17 is a block diagram showing a modified embodiment of the steering control system in which the posture angle detection coil shown in FIG. 16 is used; 
     FIG. 18 is a plan view showing a state of travelling of the vehicle along a curved portion of a guide cable; 
     FIG. 19 is a diagram showing a travelling path of an unmanned vehicle between a main guide path and a plurality of lanes crossing at right angles with the main guide path; and 
     FIG. 20 is a side view of an unmanned fork lift truck mounting a load detection device. 
     FIG. 21 is a block diagram showing a construction of the load detection device. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     One embodiment of this invention using the turning program having a turning angle less than the predetermined turning angle will first be described. 
     Consider the case where, as shown in FIG. 2, an unmanned vehicle is moved from a guide cable a to a guide cable b at a place where the cables a and b intersect each other at right angles (the predetermined turning angle being 90 degrees). 
     In FIG. 2, the guide cables a and b are laid along a course along which the unmanned vehicle 1 is guided, with an AC current of a predetermined frequency being applied to the cables. 
     Posture angle detecting coils 2a and 2b are provided in the vicinity of left and right wheels 4a and 4b in the front part 1a of the unmanned vehicle 1, respectively. Each of the detecting coils 2a and 2b is disposed at an angle or in parallel with the center line of the vehicle body, and the coils are connected in series so as to cancel the induced voltages therein as shown in part (a) of FIG. 3. The posture angle detecting coils 2a and 2b are employed to detect a vehicle posture angle (which the vehicle center line forms with the guide cable a or b) when the vehicle 1 is guided along the guide cable a or b (hereinafter referred to as &#34;a guide travelling period&#34; when applicable). When the vehicle body is on the guide cable a or b in such a manner that it is in parallel with the guide cable a or b (posture angle being zero) as shown in the part (a) of FIG. 3, voltages induced in the detecting coils 2a and 2b are equal to each other and are therefore cancelled by each other. As a result, no output voltage is provided by the series circuit 2 constituted by the detecting coils 2a and 2b. However, if the vehicle body forms an angle with the guide cable a or b as shown in the part (b) or (c) of FIG. 3, different voltages are induced in the coils 2a and 2b, and therefore the difference voltage therebetween (which is a voltage having a phase and a magnitude corresponding to the direction and magnitude of the posture angle) is provided by the series circuit 2. As each of the coils 2a and 2b is arranged to form an angle with the center line of the vehicle body as was described with reference to FIG. 2, it is possible to detect not only the posture angle but also the sideward displacement of the vehicle with respect to the guide cable a or b. When the vehicle body is displaced sidewardly of the guide cables a (or b), the induced voltage of the coil closer to the guide cable a (or b) becomes higher than that of the other coil, and therefore the difference voltage therebetween is outputted by the series circuit 2. Thus, in the case where the detecting coils 2a and 2b are arranged as shown in FIG. 2, forming an angle with the vehicle center line, a voltage corresponding to the posture angle and sideward displacement of the vehicle body is provided (the phase corresponding to the direction of posture angle and the direction of sideward displacement, while the amplitude corresponding to the extent of posture angle and the extent of sideward displacement). 
     Accordingly, if steering control is made so that the output voltage of the series circuit 2 is zero during the guide travelling then it is possible to run the vehicle 1 along the guide cable a or b. 
     In FIGS. 2 and 3, reference character 3 designates a reference phase detecting coil which is disposed on the vehicle center line in such a manner that it forms right angle with the vehicle center line. The detecting coil 3 operates to detect the phase of the AC current applied to the guide cable a or b. The phase of a voltage induced in the reference phase detecting coil 3 is maintained unchanged irrespective of the direction of posture angle and the direction of sideward displacement. 
     In this embodiment, the vehicle 1 is turned from the guide cable a to the guide cable b by the use of a turning program of a trapezoid function as indicated by the solid line in FIG. 4. This turning program indicates a steering angle corresponding to a travelling distance from the turning start point. More specifically, first the vehicle is moved to a distance point X1 by being steered gradually, and thereafter it is moved to a distance point X2 with the steering angle being maintaining unchanged. Then, the vehicle is moved from the distance point X2 to a distance point X3 with the steering angle being restored gradually to zero at the distance point X3. With the turning program indicated by the solid line in FIG. 4, the vehicle 1 is moved along a line l in FIG. 2. In other words, when the vehicle is turned in accordance with a turning program having the predetermined turning angle (as indicated by the dotted line in FIG. 4), the posture angle with respect to the guide cable b becomes zero at the time instant when the steering angle is restored to zero; however, in the case where a turning program whose turning angle is smaller (for instance, 70°-80°) than the predetermined turning angle, as indicated by the solid line in FIG. 4, the vehicle cannot be shifted onto the cable b in such a manner that it is in parallel with the cable b at the time instant when the steering angle is restored to zero (or at the distance point X3). When the vehicle reaches the distance point X3, it is moved straightly for a suitable distance (or to the distance point X4) by crossing the guide cable b obliquely with the steering angle maintained at 0°. 
     In this invention, when the vehicle 1 is shifted to the guide cable while it is straightly moved from the distance point X3 to the distance point X4, the turning program operation is switched over to the guide travelling operation using the guide cable b, so that the vehicle is smoothly moved to go along the guide cable b. The switching operation will be described later. 
     A control system for realizing the above-described vehicle turning method will be described with reference to FIG. 5. 
     Referring to FIG. 5, the detection signals of the posture angle detecting coils 2a and 2b are applied through a band-pass filter 6 to an analog switch 9. The detection signal of the reference phase detecting coil 3 is applied through a band-pass filter 7 to a comparator 8, which only when the detection signal applied thereto is positive, outputs a voltage to turn on the analog switch 9. As was described before, the phase of the output signal of the series circuit 2 comprising the posture angle detecting coils 2a and 2b is inverted in response to the posture angle and sideward displacement, and the phase of the output signal of the reference phase detecting coil 3 is maintained constant irrespective of the direction of posture angle. Therefore, the analog switch 9 outputs a detection signal having the polarity and amplitude which correspond to the directions and extents of the posture angle and sideward displacement of the vehicle. The detection signal outputted by the analog switch 9 is applied to a smoothing circuit 22 where it is smoothed, and it is applied, as a steering instruction signal S1 for instructing the direction of steering and the amount of steering in a steering mechanism 17 in the guide travelling period, to a contact 10a  of a switch 10. 
     A distance detector 11 produces pulses in proportion to the travelling distance of the vehicle 1. The pulses are counted by a counter 12 after the turning operation is started, and the count value of the counter 12 is applied to a function generator 13. The function generator 13 operates to obtain a steering instruction value corresponding to this count value in accordance with the turning program indicated by the solid line in FIG. 4, and to apply a signal corresponding to the instruction value, as a steering instruction signal S2 in the turning travelling period, to a contact 10b of the switch 10. 
     The switch 10 operates to select one of the steering instruction signal S1 for guide travelling applied to the contact 10a and the steering instruction signal S2 for program turning applied to the contact 10b and to apply it to an addition point 14. A steering angle detector 18 operates to detect an actual steering angle to provide a detection signal which is applied to the addition point 14. The addition point 14 applies the difference value between the steering instruction value and the steering instruction value through an amplifier 15 to an actuator 16 to operate the steering mechanism 17 so that the difference value becomes zero. 
     The switching operation of the switch 10 will be described. The switch 10 is operated by an instruction signal from a control circuit 21. In the guide travelling period, the control circuit 21 causes the armature of the switch 10 to trip over to the contact 10a so that the steering instruction signal S1 from the posture angle detecting coils 2a and 2b is applied to the addition point 14 to guide the vehicle 1. Upon application of the turning instruction signal, the armature of the switch 10 is tripped over to the contact 10b, whereupon the counter 12 starts its counting operation, and the function generator 13 generates the steering instruction signal S2 in accordance with the turning program indicated by the solid line in FIG. 4, the signal S2 being applied to the addition point 14. The vehicle 1 carries out the program turning operation in accordance with this steering instruction signal S2. With the program turning operation carried out, the guide travelling operation is effected if the following conditions are satisfied. One of the conditions is that (1) the signal S1 from the posture angle detecting coils 2a and 2b is in the range of a predetermined low value ±Va. That is, while the vehicle is moved from the distance point X3 to the distance point X4, the posture angle is not change at all because the steering angle is maintained at 0°; but since the distance between the guide cable b and the vehicle 1 is changed, the signal S1 is changed according to that distance. More specifically, initially the coil 2b approaches the guide cable b and the voltage induced in the coil 2b is higher than the voltage induced in the coil 2a, as a result of which the signal S1 has the polarity corresponding to the coil 2b. As the vehicle is further moved, the voltage induced in the coil 2a is increased, so that the signal S1 approaches zero. At the vehicle is further moved, the voltage induced in the coil 2a becomes higher than the voltage induced in the coil 2b, and accordingly the signal S1 has the polarity corresponding to the coil 2a. Thus, if the value of the signal S1 is close to zero, it can be determined that the vehicle 1 is substantially on the cable b. 
     In this embodiment, the signal S1 is applied to a window comparator 20. When the signal S1 is within the range of the predetermined small value ±Vs in the window comparator 20, it is determined that the vehicle 1 is substantially on the cable b, and the window comparator applies a signal to the control circuit 21 as one condition to trip the armature of the switch 10 over to the contact 10a. A switch 26 is provided on the input side of the window comparator 20. This switch 26 is to operate the window comparator 20 only when the vehicle 1 is straightly moved to cross the guide cable b, and it is operated (on and off) by a signal from a comparator 27. The comparator 27 receives the count value of the counter 12, and (2) turns on the switch 26 to supply the signal S1 to the window comparator 20 when the count value of the counter 12 corresponds to farther than the distance point X3. The signal from the reference phase detecting coil 3 is also employed as one condition to trip the armature of the switch 10 to the contact 10a. As the vehicle 1 is moved away from the guide cable b, the value of this signal is decreased; and as it approaches the guide cable, the value of the signal is increased. Therefore, (3) if the value is smaller that a preset value, it is determined that the vehicle 1 is out of the cable a or b, and tripping the armature of the switch 10 over to the contact 10a is not carried out. More specifically, the signal from the reference phase detecting coil 3 is applied through the band-pass filter 7, a rectifier 23 and a smoothing circuit 24 to a comparator 25. When the value of the signal applied to the comparator 25 is smaller than the preset value, it is determined that the vehicle is out of the cable b. As a result, a courseout signal is applied from the comparator 25 to the control circuit 21 to inhibit the switching of the switch 10. 
     Thus, under the three conditions that the vehicle 1 is moved farther than the distance point X3, the signal S3 from the posture angle detecting coils 2a and 2b is within the range of ±Vs and no course-out signal is outputted, the control circuit 21 determines that the vehicle is substantially on the guide cable b and causes the armature of the switch 10 to trip over to the contact 10a thereby to carry out the guide travelling operation by the guide cable b. 
     One example of the case where a turning program having a predetermined turning angle is employed will be described with reference to FIG. 6, in which some components similar to those in FIG. 5 are omitted for simplification, and those components which have been previously described with reference to FIG. 5 have therefore been similarly numbered. 
     The turning program employed in the example shown in FIG. 6 is as indicated by the dotted line in FIG. 4. More specifically, the turning program is such that when the steering angle is restored to 0°, the vehicle is on the guide cable b in such a manner that it is in parallel with the cable b. In this example, when the vehicle is brought to be substantially on the guide cable b during the program turning operation, the program turning operation is replaced by the guide travelling operation. Even if the turning program having the predetermined turning angle is employed, it is not always possible to bring the vehicle to be on the guide cable b and in parallel therewith when the travelling is completed because of some reasons (the vehicle is turned at a considerably low speed, the posture angle with the guide cable a is not zero, when the turning is started). Under such condition, if the program turning operation is switched to the guide travelling operation, the vehicle may be moved in zigzag state. Therefore, when it is detected that the vehicle is brought to be substantially on the guide cable b even before the program turning operation is completed. The program turning operation is switched over to the guide turning operation according to the invention. 
     In this example, the magnitude of the difference between the steering instruction signal S1 from the posture angle detecting coils 2a and 2b and a steering instruction sgnal S2&#39; from a function generator 13&#39; is employed as one condition to switch the program turning operation to the guide travelling operation. As shown in FIG. 6, a subtractor 29 receives the steering instruction signal S2&#39; from the function generator 13&#39; through a switch 28 and the steering instruction signal S1 through a switch 26 operated in associated with the switch 28 to provide the difference between the two signals S2&#39; and S1. This difference is applied to the window comparator 20. If the value of this difference is within the predetermined range ±Vs, a signal is applied from the window comparator 20 to the control circuit 21. When the vehicle is sufficiently remote from the guide cable b, the value of the signal S2&#39; is great while the value of signal S1 is approximately zero. Therefore, the difference between the two signals is great, and no output signal is provided by the wind comparator 20. However, as the turning operation reaches its end, the difference between the signals S2&#39; and S1 is decreased to be within the range of ±Vs and the signal is outputted by the comparator 20. When, under the conditions that the signal is outputted by the comparator 20 and no course-out signal is outputted, the vehicle has been moved for farther than the predetermined distance point after the start of the turning operation, the control circuit 21 causes the armature of the switch 10 to trip over to the contact 10a to carry out the guide travelling operation. Thus, the program turning operation is switched over to the guide travelling operation when the vehicle almost completes its turning operation or the signal S1 is substantially equal to the signal S2&#39;. Accordingly, the steering instruction signal is not suddenly changed, that is, the program turning operation can be smoothly switched to the guide travelling operation. 
     The term &#34;predetermined distance&#34; mentioned above is intended to mean a value set by the comparator 27&#39;, which is the distance point (X2&#39; in FIG. 4) where restoring the steering mechanism 17 is started. 
     One modification of this invention will be described with reference to FIGS. 7 to 10. In the above description, the vehicle 1 is moved forward and turned from the guide cable a to the guide cable b. If the same unmanned vehicle is moved backward and turned from the guide cable b to the guide cable a, with the same function as that in the above-described case, when the turning operation is ended, the vehicle has an angle with respect to the guide cable a as indicated by the broken line in FIG. 7; that is, the position of the vehicle 1 is not coincident with the initial position. That drawback is eliminated by the modification of the invention in which different functions are employed for the forward turning and the backward turning, respectively. 
     Described below is the case where the vehicle 1 at the position A on the guide cable a is shifted to the position B on the guide cable b in accordance with a forward-going program turning and is then moved back to the position A in accordance with a backward-going program turning. 
     In the forward-going program turning operation, a steering angle θ is controlled according to a function as indicated by the dotted line I in FIG. 8. That is, the vehicle is moved from the position A by being steered at a certain rate. When the vehicle reaches the distance point 1 1 , the steering angle θ 2  at that time is maintained until the vehicle travels to the distance point 1 2 . Then, while the steering angle is decreased at a certain rate, the vehicle is moved to the distance point 1 3 , so that the vehicle on the cable b and in parallel therewith at the position B. 
     In the backward-going turning operation from the position B to the position A, the steering angle θ is controlled according to a function II as indicated by the solid line in FIG. 8. The steering angle is abruptly changed to θ 1  at the position B. Thereafter, the steering angle is controlled in accordance with a function obtained by shifting the trapezoid function (I in FIG. 8) employed in the forward-going turning operation left along the distance axis (or the horizontal axis in FIG. 8). In this case, the steering delay caused during the backward-going turning operation can be compensated and the unmanned vehicle 1 can be brought to be on the guide cable a and in parallel therewith at the position A. 
     One example of a control device employed for this modification is as shown in FIG. 9. 
     In FIG. 9, a distance detector 11 and a counter 12 are similar to those in FIG. 2. 
     A trapezoid function as indicated by the dotted line I in FIG. 8 is programmed in a function generator 13a for forward-going turning. The function generator 13a outputs a signal representative of a steering angle in response to a position signal from the counter 12. 
     A function as indicated by the solid line II in FIG. 8 is programmed in a function generator 13b for backward-going turning. The function generator 13b outputs a signal representative of a steering angle in response to a position signal from the counter 12. 
     Switches S1 and S2 are to select steering angle instruction signals in response to the forward-going and backward-going operations of the vehicle 1. In the forward-going turning operation, the switch S1 is turned on to permit a signal from the function generator 13a to be applied to a line L. In the backward-going turning operation, the switch S2 is turned on to apply a signal from the function generator 13b to the line L. 
     The steering angle is controlled according to the signal introduced to the line L. 
     The invention has been described with reference to the case where the front wheels are steering wheels. Therefore, the program function in the backward-going turning operation is obtained by advancing the program function in the forward-going turning operation (or shifting it left along the distance axis). However, in the case where the rear wheels are employed as the steering wheels as in the case of, for instance, fork lift trucks, the program function in the backward-going turning operation is obtained by delaying the program function in the forward-going turning operation (or shifting it right along the distance axis). 
     In the example shown in FIG. 9, two function generators are provided for the forward-going and backward-going operations, respectively. However, in practice, the same object can be achieved by only one function generator. For instance, as shown in FIG. 10, one function generator 13&#39; programmed with automatic turning is provided to store a trapexoid function such as indicated by the dotted line I in FIG. 8 which is employed as the function in the forward-going turning operation. In the forward-going operation, upon application of a forward-going turning signal, the function generator 13&#39; outputs a steering instruction signal according to the count value of the counter 12 at the initial distance point 1 0  (FIG. 8). For the backward-going turning operation, a function such as indicated by the solid line II in FIG. 8 is required. However, since this function is obtained by advancing the function indicated by II in FIG. 8 by a predetermined distance 1 S  (FIG. 8) (or shifting it in the negative direction along the distance axis), the function generator 13&#39; having stored this function I can be commonly used. That is, in the backward-going operation, a backward-going turning signal is applied to the function generator 13&#39; so as to read the function as indicated by the solid line II in FIG. 8. This can be achieved by reading a function value at an address corresponding to the predetermined distance 1 S  instead of reading a function value at an address corresponding to the initial distance 1 0 . 
     When an unmanned vehicle which is an ordinary four-wheel vehicle (two wheels being the steering wheels) is turned right or left, the same function can be employed both in right turning and in left turning (although the steering angles are different in polarity). However, if the same function is employed in both right turning and left turning a vehicle such as a reach fork lift truck which has only one steering wheel off the center line, it is impossible to shift the vehicle onto a guide cable in one of the right and left turning operations, because the turning radius in the right turning operation is different from that in the left turning operation. 
     Shown in FIGS. 11 through 14 is another modification of this invention in which a vehicle of this type is so designed as to satisfactorily turn both right and left. 
     Referring to FIG. 11, an unmanned vehicle 31 is provided with fixed wheels 33 and 34 on the right and left sides of the front part thereof and a steering wheel 32 on the left side of the rear part thereof. 
     In the case where, as shown in FIG. 11, the vehicle 31 on a line a is turned right around the point A so that it is brought to be at a position (indicated by the dotted line 1a) on a line b, the locus of the steering wheel 32 is as indicated by a line La. On the other hand, in the case where the vehicle 1 is turned left around the point B of symmetry of the point A with respect to the line a so that it is brought to be at a position (indicated by the one-dot chain line 1b) on the line b, the locus of the steering wheel 2 is as indicated by a line Lb. The lines La and Lb are different from each other in curve dimension because the steering wheel 32 is disposed off the center line of the vehicle body. The locus La in right turning is an arc described with the point A as the center and with the distance between the point A and the center O of the steering wheel 32 as the radius. The locus Lb in left turning is an arc described with the point B as the center and with the distance between the point B and the center O as the radius. Accordingly, in order to satisfactorily shift the vehicle 31 onto the line b from the line a, in both of the right and left turning operations, it is necessary not only to reverse the steering direction but also to control the steering angle in accordance with the locus described by the steering wheel 32. 
     One example of the relation between steering angle and travelling distance in the right and left turning operations is shown in FIG. 12 in which the solid line X is for the right turning operation and the one-dot chain line Y is for the left turning operation. 
     First, the right turning operation will be described. The vehicle 31 is at a distance point X 0  at which the turning operation is started. The vehicle is so controlled that it is steered right gradually while being moved until the steering angle reaches θ 1  at a distance point X 1 . After the steering angle has θ 1  at the distance point X 1 , the vehicle is moved to a distance point X 2  with the steering angle θ 1  being maintained. After the vehicle has reached the distance point X 2 , the steering direction is gradually reversed (or changed to the left). When the steering angle becomes zero at a distance point X 3  the vehicle reaches the position indicated by the dotted line 1a on the line b (FIG. 11). In this manner, the steering angle is controlled. 
     In the left turning operation, the vehicle is so controlled that when the vehicle is moved to the distance point X 1  while being gradually steered left, the steering angle is -θ 2 . After the steering angle has -θ 2  at the distance point X 1 , the vehicle is moved to the distance point X 2  with the steering angle -θ 2  being maintained. After the vehicle has reached the distance point X 2 , the steering direction is gradually reversed (or changed to the right). When the steering angle becomes zero at the distance point X 3 , the vehicle reaches the position indicated by the one-dot chain line 1b on the line b. In this way, the steering angle is controlled. 
     In FIG. 12, the values of the distance points X 0  through X 3 , and the steering angles θ 1  and -θ 2  are predetermined in accordance with the position of the steering wheel 32 in the vehicle or the like. 
     FIG. 13 shows one example of a control system for carrying out the steering control described with reference to FIG. 12. 
     Referring to FIG. 13, a distance detector 11 and a counter 12 operates in combination to detect a travelling distance as measured by the center line of the vehicle body. Accordingly, the detector 11 is mounted on the center line of the vehicle body. If it is difficult to mount the detector 11 on the center line of the vehicle body, two detectors 11 may be provided on the right and left fixed wheels 33 and 34, respectively, to obtain the wheel travelling distances so that the average value of these travelling distance is employed as the vehicle travelling distance. A function is indicated by the solid line X in FIG. 12 is programmed in a right turning function generator 13c, which outputs a signal instructing a steering angle in response to distance information from the counter 12. Similarly, a function is indicated by the one-dot chain line Y in FIG. 12 is programmed in a left turning function generator 13d, which outputs a signal instructing a steering angle in response to distance information from the counter 12. 
     Switches S1a and S2a are to select a steering angle instruction signal separately according to the turning direction. In the right turning, only the switch S1a is closed by a right turning signal to apply the steering angle instruction signal from the right turning function generator 13c to a line l 10 . On the other hand, in the left turning, only the switch S2a is closed by a left turning signal to apply the steering angle instruction signal from left turning function generator 13d to the line l 10 . A steering mechanism (not shown) is operated so that the steering angle is controlled by the signal thus applied to the line l 10 . 
     In the example shown in FIG. 13, two function generators are provided for the right and left turning operations, respectively. However, the two function generators may be replaced by one function generator to achieve the same object. 
     FIG. 14 illustrates one example of the steering control system in which only one function generator is employed for both the right and left turning operations. 
     A trapezoid function as indicated by the solid line X (or the one-dot chain line Y) in FIG. 12 is programmed in a function generator 13, which outputs a signal according to distance information from a counter 12. This output signal is applied through a resistor r to an amplifier 35. When a switch S1b is closed by a right turning signal, the gain of the amplifier 35 is decreased, and the steering angle indicated by the signal introduced to a line l 20  is as indicated by the solid line X in FIG. 12. Similarly, when a switch S2b is closed by a left turning signal, the gain of the amplifier 35 is increased, and the steering angle indicated by the signal introduced to the line l 20  is as indicated by the one-dot chain line Y in FIG. 12. A steering mechanism (not shown) is operated so that the steering angle is controlled by the signal applied to the line l 20 . 
     The invention has been described with reference to the posture angle detector which detects a posture angle with one pair of detecting coils. 
     Now, the invention will be described with reference to one modification thereof in which two pairs of detecting coils are employed. 
     Referring to FIG. 15, course-out detecting coils 36a and 36b and posture angle detecting coils 35a and 35b are provided in the vicinities of the rear wheels (or front wheels) of an unmanned vehicle 1, respectively. Each of the course-out detecting coils 36a and 36b is disposed perpendicularly to the center line of the vehicle body so that the detection direction is perpendicular to a guide cable a. Accordingly, voltages are induced in the detecting coils 36a and 36b at all times, respectively, but they are connected as shown in FIG. 16 so that the induced voltages are cancelled by each other (opposite-phase series connection). When the guide cable a is at the center of the distance between the two coils 36a and 36b, no voltage is provided across the output terminals a 2  and b 2 , but as the guide cable leaves the center, the voltage across the output terminals is increased. The phase of the output voltage is coincident with the voltage induced in the detecting coil closer to the guide cable a. Therefore, the direction and magnitude of displacement (indicated by l in FIG. 15) of the vehicle body rear part 1b can be detected from the phase and magnitude of the vehicle developed across the output terminals a 2  and b 2 . 
     The posture angle detecting coils 35a and 35b are provided in parallel with the vehicle body so that the detection directions are in parallel with the guide cable a. the detecting coils 35a and 35b are connected so that a voltage developed across the output terminals a 1  and b 1  is the sum of voltages induced in the coils 35a and 35b (coincident-phase series connection). Accordingly, when the tangential line of the guide cable a at the vehicle body rear part 1b is in parallel with axes of the coils 35a and 35b, no voltage is induced in the coils 35a and 35b. As this parallel state is established, the voltages induced in the coils are increased. The phase of the voltage is changed according to the direction of the posture angle of the vehicle body. Therefore, the direction and magnitude of an angle θ (hereinafter referred to as &#34;a posture angle&#34; when applicable) which the tangential line of the guide cable a at the vehicle body rear part 1b forms with the center line of the vehicle body can be detected from the phase and magnitude of the voltage developed across the output terminals a 1  and b 1 . 
     As shown in FIG. 17, the output voltage of the course-out detecting coils 36a and 36b is applied through an amplifier 37 to an addition point P1, and the output voltage of the posture angle detecting coils 35a and 35b is applied to an amplifier 38 having a gain of +K 2  and to an amplifier 39 having a gain of -K 2 . The output voltages of the amplifiers 38 and 39 are equal is magnitude but opposite in phase (out of phase by 180°), and the former is employed for posture angle control in the forward-going operation, while the latter is employed for posture angle control in the backward-going operation. The signals opposite in phase are employed in the forward-going operation and the backward-going operation, respectively, because even if the vehicle body posture angles are equal, it is necessary to reverse the steering direction separately according to the forward-going and backward-going operations. 
     The output voltages of the amplifiers 38 and 39 are applied to switches SW 1  and SW 2 , respectively. The switches SW 1  and SW 2  are turned on and off by a signal from a forward-going and backward-going switching device 40. For instance, in the forward-going operation, the switch SW SW 1  is closed, as a result of which a signal for forward-going turning is applied from the amplifier 38 to the addition point P1; and in the backward-going operation, the switch SW 2  is closed on, as a result of which a signal for backward-going turning is applied from the amplifier 39 to the addition point P1. 
     In the addition point P1, the course-out detection signal from the amplifier 37 is added to the posture angle detection signal from the amplifier 38 or 39. The resulting addition output corresponds to the steering angle which is required for correcting the displacement from the course of the vehicle 1 and the posture angle thereof. That is, the phase of the addition output is representative of the composition (or the steering direction) of the direction of the posture angle and the direction of the displacement from the course; and the level of the addition output is representative of the composition (or the amount of steering) of the magnitude of the posture angle and the amount of the displacement from the course. 
     The case where the unmanned vehicle 1 shown in FIG. 15 is run along a sharp curve will be described with reference to FIG. 18. 
     In FIG. 18, a guide cable a is laid in the form of an arc with the center O extended from the position Ps to the position Pf, and the other portions thereof are extended straightly from the position Ps and Pf. An unmanned vehicle 1 is run from the position N 0  to the position N 4 . 
     First, the vehicle 1 at the position N 0  is run towards the curve starting position Ps. Even if the vehicle front part 1a reaches the position Ps, the vehicle 1 is run straightly because the aforementioned posture angle detecting coils 35a and 35b course-out detecting coils 36a and 36b are provided in the rear part 1b of the vehicle body. The dotted line m designates the locus of the center of the vehicle front part 1a. 
     When the rear part 1b of the vehicle 1 reaches the position Ps, the vehicle starts steering and runs towards the curve finishing position Pf. In this curve, the vehicle 1 is run in such a manner that the center of the vehicle rear part 1b goes along the guide cable a with the aid of the pick-up coils 36a and 36b. Furthermore, the vehicle 1 is run with the aid of the pick-up coils 35a and 35b in such a manner that the axes of the pick-up coils 35a and 35b are in parallel with the tangential line of the guide cable a at the vehicle rear part 1b, that is, the rear part 1b of the vehicle body is perpendicular to the guide cable a (cf. the positions N 2  and N 3 ). 
     Therefore, the locus m of the center of the vehicle front part 1a forms an arc whose radius is larger than the radius of the curve of the guide cable a. When the vehicle front part 1a goes ahead of the curve finishing position Pf, the vehicle rear part 1b is still over the curve, and therefore the vehicle is run being maintained steered. When the rear part 1b reaches the curve finishing position Pf, the steering is restored, and the vehicle is run straightly along the guide cable a. 
     In the case where the vehicle is moved backwardly along the same path, the locus of the vehicle is the same as that described by the vehicle moving forwardly as described above. That is, similarly as in the above-described forward-going operation, the vehicle 1 is run with the aid of pick-up coils 36a and 36b and 35a and 35b in such a manner that the center position of the rear part 1b is perpendicular to the guide cable a at all times. 
     FIG. 19 shows another embodiment of the invention in which the above described turning is applied to steering of an unmanned mobile vehicles between a main guidance path and a plurality of lanes crossing at right angles with the main guidance path. 
     According to the illustrated embodiment, by turning by 90 degrees in a reverse moving of the vehicle, the posture of the vehicle is brought in coincidence with the direction of the lane and the position of te vehicle is brought approximately in alignment with the center of the lane and thereafter an exact alignment of the vehicle with the lane is established by forwardly moving the vehicle. 
     Referring to FIG. 19, a plurality of lanes L 0  through L n  crossing at right angles with a main guidance path ML are provided. The lane L 0  is provided with a vehicle standby station 42 and a load loading and unloading table 43. Two guide cables MC 1  and MC 2  spaced suitably from each other are provided in the main guidance path ML. There are also provided leader cables C 0  through C n  in the center of each of the lanes L 0  through L n . Low frequency guide signals of frequencies f 0 , f 1  and f 2  are applied respectively to an associated one of the guide cables C 0 , MC 1  and MC 2 . Low frequency guide signals of frequencies f 3  and f 4  are applied alternately to the leader cables C 1  through C n . One side of the lanes L 1  through L n  in relation to the main guidance path is termed an &#34;A side&#34; lane and the other side thereof a &#34;B side&#34; lane. The lanes L 1  through L n  are provided for placing load thereon. 
     Assume now that the unmanned vehicle 1 travels from the standby station 42 to the loading and unloading table 43, load a cargo therefrom and unload it on the A side lane L 2  and returns to standby station 42. The vehicle 1 proceeds forwardly from the standby 42 to the table 43 with the frequency f 0  applied to the guide cable C 0  being used as the guide signal. After loading goods from the table 43 the vehicle 1 moves reversely along the cable C 0 . Upon detection of the frequency signal f 2  of the cable MC 2  for the main guidance path ML, the vehicle decelerates its speed. Then upon detecting the frequency signal f 1  of the cable MC 1 , the vehicle 1 turns by 90 degrees reversely in accordance with a preset programmed function and subsequently moves reversely in the direction of the lane L 2  (i.e. arrow C) along the main guidance path ML guided by the guide cable MC 1 . In other words, the vehicle 1 is moved reversely along the cable MC 1  which is nearer to the A side lane if it is desired to cause the vehicle 1 to proceed to the A side lane. If it is desired to cause the vehicle 1 to proceed to the B side lane, the vehicle 1 is moved along the leader cable MC 2  of the main guidance path ML which is nearer to the B side lane. The value of the function in the programmed turning of the vehicle 1 is so selected that a center line of the vehicle 1 is substantially in alignment with the leader cable after the 90 degree reverse turning of the vehicle 1. 
     During travelling of the vehicle 1 along the main guidance path ML, the vehicle 1 sequentially detects the signals f 3 , f 4 , f 3 , etc, being applied to the lanes L n , L n-1 , L n-2  etc. which the vehicle 1 passes and counts these signals. In accordance with the count of the signals, the vehicle 1 detects the signal f 3  of the lane L 3  which is located immediately before the lane L 2  into which the vehicle is programmed to be guided and thereupon decrease the vehicle speed. The vehicle 1 subsequently detects the signal f 4  of the lane L 2  and thereupon is turned by 90 degrees reversely in accordance with the preset programmed function and stops at a position immediately before entering the B side lane L 2  after running some distance. In this state, the posture of the vehicle 1 is already in substantial alignment with the direction of the lane L 2 . In other words, the center line of the vehicle 1 approximately coincides with the cable C 2 . Accordingly, the vehicle 1 in this state assumes a posture according to which the load placed in the lane L 2  can be detected radily and accurately. 
     Nextly, the vehicle 1 proceeds forwardly using the signal f 4  of the lane L 2  as the guide signal while effecting an accurate aligning of the vehicle body with the lane L 2 . The vehicle 1 therefore can proceed to the lane L 2  accurately and unload the goods at a predetermined position in the lane L 2 . 
     After finishing unloading of the goods, the vehicle 1 moves reversely toward the main guidance path ML and, upon detection of the signal f 1  of the cable MC 1 , the vehicle 1 decreases the vehicle speed. As the vehicle 1 has detected the signal f 2  of the cable MC 2 , the vehicle 1 is turned by 90 degrees reversely in accordance with the programmed function and subsequently is moved reversely along the main guidance path ML in the direction of the lane L 0  (i.e. arrow D) being guided by the signal f 2  of the cable MC 2 . Upone detection of the signal f 0  of the cable C o , the vehicle 1 is turned by 90 degrees reversely in accordance with the programmed function and thereafter is moved reversely along the lane L 0  to the standby station 2 being guided by the signal f 0 . The loading and unloading operation by the vehicle 1 is completed in the foregoing manner. 
     In a case where it is desired to guide the vehicle 1 to the B side lane, the vehicle 1 is moved reversely to a predetermined lane along the cable MC 2  of the main guidance path ML, then is moved forwardly into the desired lane in the same manner as has previously been described and, after finishing the unloading operation, is moved reversely along the cable MC 1  of the main guidance path ML in the direction of the lane L 0 . 
     According to this embodiment, since the vehicle posture is in substantial alignment with the direction of the lane in a state in which the vehicle is displayed forwardly after turning, detection of load which is placed in front of the vehicle 1 in the lane can be effected radily and accurately and, besides, the dead space can be reduced to a maximum possible extent. Despite the fact that the present embodiment employs a two side lane type layout, the dead space can be made smaller than the one available in the conventional one side lane type layout. Accordingly, a larger space for load can be obtained in the lane whereby the most efficient utilization of the store can be achieved. 
     FIG. 20 shows an example of construction by which a unmanned vehicle such as a fork lift truck detects load placed in a lane. 
     In FIG. 20, a fork lift truck 1 is provided with a pulse generator 53. This pulse generator 53 generates one shot of pulse signal P s  every preset travel distance, e.g. 10 cm, of the vehicle. The travel distance of the vehicle can therefore be measured by counting the number of the generated pulses Ps. The period of the pulse signal Ps varies with the vehicle speed. The pulse signal Ps is applied as a down pulse signal to a travel distance pulse counter 65 of a load detection unit 60 shown in FIG. 21. 
     A time pulse generator 62 generates a pulse signal Pt every constant time t and supplies this pulse as a down pulse signal to a time counter 66. 
     On the other hand, as shown in FIG. 20, the unmanned fork lift truck 1 proceeds forwardly along a preset travel path at a certain vehicle speed, maintaining a lift position of the fork 52 nearly at a ground level, that is, a position wherein a pallet 55a carrying load 56a is not dragged on the ground and yet can come into an abutting engagement with an opposite pallet 55b placed on the ground. In this stage, the travel distance counter 65 and the time counter 66 do not perform their counting operation through they receive the pulses Ps and Pt from the travel distance pulse generator 53 and the time pulse generator 62. 
     Upon entering the fork lift truck into a predetermined lane 54, a sequence control circuit 61 outputs preset data Da, Db. The data Da is applied to a multiplicator 63. This multiplicator 63 multiplies the data Da by a suitable coefficient K in accordance with the running speed of the fork lift truck 1. More specifically, coefficient K of a relatively large value is used if the running speed is high and coefficient K of a relatively small value is used if the running speed is low. This arrangement is made for varying the value of the data Da in accordance with the running speed of the fork lift truck 1 in order to reduce a count value of the travel distance pulse counter 65 to 0 in precedence to the time counter 66. A frequency-voltage converter 64 produces a signal e s  which is proportionate to the frequency of the pulse signal Ps and supplies this signal to the multiplicator 63. The multiplicator 63 multiplies the data Da by a coefficient proportional to the signal E s  and thereupon produces data Da&#39; which is preset in the travel distance pulse counter 65. The data Db is directly preset in the time counter 66. These travel distance counter 65 and the time counter 66 are counted down each time the travel distance pulse Ps and the time pulse Pt are applied thereto. 
     While the fork lift truck 1 is running at a constant speed, the count of the travel distance pulse counter 65 becomes 0 in predecence to the time counter 66. A reset signal thereupon is produced from a NAND gate N1 and applied to the travel distance pulse counter 65 and the time counter 66 via an OR gate OR to reset these counters. Thereafter the data Da&#39; and Db are respectively preset in the associated counters again. The above described operation is repeated while the fork lift truck 1 is travelling at a constant speed. 
     As the pallet 55a of the fork lift truck 1 comes into abutting engagement with the pallet 55b placed in the lane 54, the speed of the fork lift truck 1 is dropped. This causes the pulse period of the output pulse P s  of the travel distance pulse generator 53 to be prolonged whereby the counting down operation of the travel distance pulse counter 55 is made slower. Since the time pulse generator 62 is producing the pulse signal Pt at the constant time interval t, the count of the time counter 66 becomes 0 in precedence to the travel distance pulse counter 65. A NAND gate N2 produces an output signal when the count of the time counter 66 has become 0. This output signal of the NAND gate N 2  is used as a load detection signal. 
     This load detection signal is applied to a running control unit (not shown) of the fork lift truck 1 to stop the truck 1 immediately. 
     It should be noted that the running speed of the fork lift truck 1 in the lane, the period t of the time pulse Pt and the preset data Da, Db are so selected that the fork lift truck 1 can stop within a sufficiently short time after the pallet 55a abuts against the opposite pallet 55b without displacing the pallet 55b on which the load is placed.