Guidance System For Self-propelled Trackless Carriages

Abstract

A trackless carriage with two individually drive lateral traction wheels has four sensors, disposed in a diamond-shaped array, which coact capacitively or inductively with a continuous guide strip on the floor to steer the vehicle along a predetermined path by controlling the transmission of driving torque to these wheels. When the two sensors on the longitudinal diagonal of the array register with the strip, both traction wheels are driven; when either or both of these sensors are out of line and one of the two other sensors, on the transverse diagonal, detects the proximity of the strip, the opposite traction wheel is driven to realign the vehicle with the guide path.

Parent Case Text

This application is a continuation-in-part of my pending application Ser. No. 633,871, filed 26 Apr. 1967, now U.S. Pat. No. 3,474,877.

Description

In that copending application I have disclosed a transportation system utilizing trackless carriages which are steered along a predetermined path with the aid of a strip, e.g., of iron, provided on the floor over which the carriage is to travel, the proximity of this strip being sensed (e.g., electromagnetically) by a pair of detectors which form part of a control circuit for a steering motor driving a pair of dirigible wheels. A pair of traction wheels are separately powered by e.g., motor.

The broad object of my present invention is to provide an improved guidance system of this general type dispensing with the need for separate steering and traction motors.

As further disclosed in my copending application, there is advantageously provided aboard each carriage a reader in the form of an array of magnetic switches designed to detect the presence of a corresponding set of code markers, such as permanent magnets, imbedded in the floor alongside the guidance strip in order to switch the carriage onto a branching guide path or to arrest it at an assigned station. Reference in this connection is also made to my copending application Ser. No. 626,197, filed 27 Mar. 1967, and to my prior U.S. Pat. No. 3,340,821, wherein similar code markers and reading switches are used for the control of a trackborne carriage, the code magnets being mounted in that case on a vertical panel alongside the track instead of being sunk into the floor.

A more specific object of my present invention is to adapt such a reader for use with my improved guidance system whereby the same basic control circuit can be used to hold the carriage on its course and to let it veer onto a branch path at selected junction points.

The wheel base of a carriage embodying my present improvement again includes two traction wheels, i.e., a right-hand wheel and a left-hand wheel as viewed in the direction of locomotion, along with one or more idler wheels or casters which advantageously are swivelably mounted so as to follow any course change dictated by the transmission of different torques to the two independently powered traction wheels. Two separate drive means, one for each traction wheel, may be constituted by individual motors or by independently controllable clutches coupling the shafts of these wheels to a common drive motor. These drive means are jointly and alternately actuatable by a control circuit which includes central sensing means as well as right and left sensing means designed to detect the proximity of the guide strip. Upon alignment of the central sensing means with the guide strip, the control circuit operates both drive means to power the two traction wheels whereby the vehicle is maintained on a substantially straight course. If the vehicle strays off the guide path, one or the other of the two lateral sensing means flanking the central sensing means will respond to the proximity of the strip and transmit a steering signal to the control circuit. A response of the right sensing means, indicating a deviation to the left, applies torque to only the left-hand traction wheel if the deviation is sufficient to discontinue the normal steering signal normally emitted by the central sensing means. Conversely, if the left sensing means responds, the steering signal applies power only to the right-hand traction wheel under like circumstances. This unilateral transmission of torque to only one traction wheel results in a yawing motion toward the opposite side which realigns the central sensing means with the guide strip.

If, at a junction, another strip branches off the strip previously followed, one of the lateral sensing means will detect the presence of the branching strip and will emit a steering signal which, however, will be ineffectual as long as the drives for the two traction wheels are concurrently energized by steering signal from the central sensing means. Thus, the carriage will continue along its straight course unless a reader aboard the carriage has been preset to detect a combination of dode markings just ahead of the branching point so as to modify the operation of the control circuit, as by making the drive means nonresponsive to the central sensing means whenever the branch strip is picked up by the corresponding lateral sensing means. This modified state of the control circuit may be allowed to persist until, at a further junction point, the carriage is intended to travel straight on in lieu of veering off on a branch path; thus, the reader may be preset to respond to another code combination at this further junction so as to restore the original mode of operation of the control circuit.

In a preferred embodiment, the central sensing means comprises a first and a second sensor spaced apart in the direction of travel, thus along the longitudinal median of the vehicle, the right and left sensing means comprising a third and a fourth sensor spaced apart on a line transverse to that direction and defining with the first two sensors a diamond-shaped array of square of rhombic configuration. With the array disposed forwardly of the common axis of the traction wheels, the latter will perform the required course changes even if their associated drive means respond with a certain lag to the steering signal from the sensors.

The invention will be described in greater detail with reference to the appended drawing in which:

FIG. 1 is a perspective view of a trackless carriage adapted to be guided by my present system;

FIG. 2 is a somewhat diagrammatic bottom view of the carriage shown in FIG. 1;

FIG. 3 is a view similar to FIG. 2, illustrating a modified carriage;

FIG. 4 is a fragmentary cross-sectional view taken on the line IV--IV of FIG. 3;

FIG. 5 is a plan view of a branched guide path for carriages of the type illustrated in FIGS. 1-4;

FIG. 6 is a diagram of the control circuit associated with the drive means for the carriage of FIGS. 1 and 2;

FIG. 7 shows a partial modification of the lower portion of the circuit diagram of FIG. 6, adapting same for use with the vehicle illustrated in FIGS. 3 and 4;

FIG. 8 is a more detailed circuit diagram of a sensor forming part of the system of FIG. 6; and

FIG. 9 is a circuit diagram similar to FIG. 8, illustrating an alternate type of sensor.

Reference is first being made to FIG. 1 showing a carriage 1 with a chassis 2 supporting a loading platform 3 for goods to be transported. A removable cover 4, here shown to be transparent, overlies the platform 3 to form a receptacle for the goods. As more fully described hereinafter with reference to FIG. 2, carriage 1 has a pair of swivelably mounted rear wheels 5a, 5b and a pair of nonorientable front wheels 6a and 6b, only the right-hand wheel 5b, 6b of each pair being visible in FIG. 1. The vehicle is steered by the selective transmission of driving power to either or both if its two front wheels, also described in detail hereinbelow. A panel 7 aboard the vehicle carries switches for presetting a reader to respond in certain ways to code markings which the vehicle is to encounter at various points along its path. This path is marked by a continuous guide strip 17, of conductive and/or highly permeable (ferromagnetic) material, whose distinctive character-- as compared with the adjoining floor-- evokes the response of a set of sensors 21, 22, 23, 24 (FIG. 2) projecting downwardly from the underside of the carriage.

A bumper 11, projecting forwardly from the carriage 1, is formed from a fluid-filled flexible tube whose deformation upon contact with an extraneous obstacle actuates one or both of two pressure switches 12a, 12b to deactivate the wheel drive, as more fully explained in the subsequent description of FIG. 6. This arrangement is representative of any obstacle detector, including those of the echo type, adapted to be placed aboard the carriage.

In FIG. 2 I have shown a pair of separate drive motors 20a, 20b coupled with the shafts of traction wheels 6a and 6b by speed-reducing transmissions here shown as worm drives 25a, 25b. Also disposed on the underside of the vehicle 1 are a set of magnetic switches 30, 31, 33, 34, 35 designed to coact with respective magnetic markers imbedded in the floor as described hereinafter with reference to FIG. 5. Switches 32, 33 are assumed to have been rendered inoperative by the selective actuation of switches on panel 7 (FIG. 1).

The motors 20a and 20b could be reversible for backward driving, in which case a second array of sensors would have to be provided near the rear edge of the carriage bottom. An arrangement of this type has been illustrated in FIG. 3 wherein, furthermore, the traction wheels 6a, 6b have been aligned with the transverse median of the rectangular carriage frame while the idler wheels 5a and 5b of FIGS. 1 and 2 have been replaced by two longitudinally spaced wheels 5c and 5d on the other median. Wheels 6a and 6b are driven, as illustrated in FIG. 4, from a single reversible motor 20 via a speed reducer 25 and a pair of electromagnetic couplings 26a, 26b as well as chain drives 126a, 126b. Couplings 26a and 26b are independently energizable, as described hereinafter with reference to FIG. 7, to complete the power train from motor 20 to traction wheels 6a and/or 6b. These couplings may include magnetic clutches as well as magnetic brakes, actuation of one clutch and of the opposite brake affording a more positive steering action. The front sensors 21-24 shown in FIG. 3 are supplemented by an array of rear sensors 21', 22', 23', 24' which can be substituted for the first-mentioned sensors, upon actuation of a reversing switch on panel 7, with concurrent reverse-energization of motor 20; a rear bumper 11' may take over the deactivation of the drive upon the encounter of an obstacle in the path of the vehicle.

In FIG. 3 the axis of traction wheels 6a, 6b is well behind the diamond-shaped array of sensors, regardless of the direction of travel. The reading switches 30-35 are the same as in FIG. 2 but may be supplemented by other switches for rearward travel; one such switch, whose specific function will be described hereinafter, has been shown at 35'.

FIG. 5 shows a portion of the guiding strip 17 along with two branch strips 17a and 17b leading to the right and to the left, respectively. At the first branching point 29a, a set of markers 130a, 131a and 133a in the form of permanent magnets, substantially level with the floor surface and the strip 17, enable an oncoming vehicle to continue on a straight path along strip 17 or to veer to the right along strip 17a, depending on the setting of its selector panel 7 (FIG. 1). At the second branching point 29b, a similar array of markers 130b, 132b and 134b enable an analogous selection between a straightforward continuation along strip 17 and a turning to the left along strip 17b. With reading switches 32 and 33 deactivated as described hereinbefore with reference to FIG. 2, the vehicle will move straight at junction 29a (arrow A) and to the left at the junction 29b (arrow B).

It will be noted that the central marker 130a or 130b coincides with the strip 17. This is entirely feasible when the strip consists of nonmagnetic conductive material designed for capacitive sensing as described hereinafter with reference to FIG. 8. In the case of a ferromagnetic strip, designed for inductive pickup as later described with reference to FIG. 9, all the markers would have to be well spaced from the strip.

Since, as also described below, the preferred system according to my present invention does not require any conductive contact between the carriage-borne sensors and reading switches, on the one hand, and the guide strip and magnetic markers, on the other hand, the latter may be covered with a thin protective layer of varnish or the like.

I shall now describe, with reference to FIG. 6, the electric circuitry used for controlling the drive motors 20a, 20b of FIG. 2 in order to propel the vehicle 1 along the path denoted by arrows A and B in FIG. 5.

The two motors 20a, 20b are energizable by a source of direct current aboard the carriage, illustrated as a battery 40. A single circuit breaker 12 in series with the ungrounded negative terminal of that source represents the two series-connected bumper switches 12a, 12b of FIG. 1. Upon closure of this circuit breaker, a bus bar 41 carries negative operating voltage to four signal generators 121, 122, 123, 124 respectively associated with sensors 21, 22, 23 and 24. The nature of these signal generators and the mode of their operation by the sensors will be described later in connection with FIGS. 8 and 9. For the present purpose, and as illustrated diagrammatically in FIG. 6, each signal generator is assumed to comprise a normally open switch which closes whenever the corresponding sensor is aligned with the strip 17.

Four relays 221, 222, 223 and 224 are energizable from bus bar 41 upon actuation of a respective signal generator 121-124. These relays control the operation of two further relays 120a, 120b to energize the two drive motors 20a, 20b, respectively. With sensing relays 221 and 222 concurrently operated, an energizing circuit is completed for both driving relays 120a and 120b via the upper armatures and front contacts of these sensing relays, upper armature and back contact of the sensing relay 223 in the case of driving relay 120a, and lower armature and back contact of sensing relay 224 in the case of driving relay 120b. The upper armature and back contact of relay 223 are normally shunted by the outer right-hand armature and back contact of a steering relay 220a; similarly, the lower armature and back contact of relay 224 are normally shunted by the outer left-hand armature and back contact of a steering relay 220b. The front contacts of the armatures of relays 120a and 120b are connected to bus par 41 via a potentiometer 42 whose slider may be adjusted for equalizing the torque exerted by the two driving motors 20a, 20b upon the respective traction wheels 6a, 6b (FIG. 2) so as to insure travel on a straight course.

Steering relays 220a and 220b are energizable upon the concurrent closure of reading switches 30, 34 and 30, 33, respectively. Closure of any of reading switches 31, 32, 35 simultaneously with switch 30 brings on a further steering relay 220c whose armatures and back contacts are connected in respective holding circuits for relays 220a and 220b. The first of these holding circuits extends from the winding of relay 220a via its outer left-hand armature and front contact, lower back contact and armature of relay 220c, and inner left-hand armature and back contact of relay 220b to bus bar 41; the second holding circuit extends to this bus bar from the winding of relay 220b by way of its outer right-hand armature and front contact, upper armature and back contact of relay 220c, and inner right-hand armature and back contact of relay 220a. Thus, each of relays 220a and 220b can lock only when neither the opposite steering relay 220b, 220a nor the "straightforward" relay 220c is operated.

The width of the guide strip 17 should be so chosen that, whenever the vehicle veers sufficiently off course to either side to disalign either or both central sensors 21, 22 with that strip, one of the lateral sensors 23, 24 finds itself in registry with the strip to generate a corrective signal. This will always be the case if the strip width is substantially equal to the length of a side of the square or rhomb defined by the array 21-24. Since, however, the vehicle is not expected to swerve sharply from its original path, it will generally suffice to make this width equal to about half the transverse diagonal 23-24 of the array. As the length of the longitudinal diagonal 21-22, relative to the width of the strip, determines the maximum yawing angle before corrective action occurs, the latter diagonal advantageously is mode somewhat longer than the transverse diagonal. Reducing the strip width, furthermore, increases the precision with which the reading switch 30-35 will line up with the corresponding markers so that the width of the latter may also be reduced.

If, with relays 220a, 220b, 220c unoperated, the vehicle deviates from its guide path sufficiently to let one of the central sensors (generally the leading sensor 21) move off the strip 17, the corresponding sensing relay (e.g., 221) will be released so that the circuit of relays 120a and 120b is broken. With these relays deenergized, the armature windings of motors 20a and 20b are open-circuited at the inner left-hand armature of relay 220a and the inner right-hand armature of relay 220b, respectively. One of the driving relays, however, is held operated by the substantially concurrent response of one of the lateral sensors, such as sensor 23 if the deviation has been to the right. With signal generator 123 operating the relay 223, an alternate energizing circuit is closed for driving relay 120b from bus bar 41 via the lower armature and front contact of relay 223. Thus, motor 20b remains under power to drive the associated traction wheel 6b while the opposite traction wheel 6a just idles; this results in a gradual course correction with the vehicle veering slightly to the left until both central sensors 21 and 22 are again aligned with the strip 17 to restore rectilinear motion.

As the vehicle, traveling on a straight line, passes the junction 29a of FIG. 5 in the direction of arrow A, sensors 21, 22 and 24 will concurrently respond to the proximity of strips 17 and 17a. The response of sensor 24 brings on the relay 224 whose upper armature and front contact close an energizing circuit for driving relay 120a. Since this relay, along with companion relay 120b, is already operated via sensing relays 221 and 222, the presence of strip 17a underneath the carriage is without effect.

With reading switch 31 enabled and switch 33 deactivated, relay 220c was momentarily energized by the simultaneous closure of switches 30 and 31 upon the approach of junction 29a to release steering relay 220a or 220b if either of them had previously actuated. On a differently programmed carriage, having switch 33 operative and switch 31 disabled, relay 220b would have been energized via simultaneously closed switches 30 and 33 on the approach of junction 29a. Locking over its outer right-hand armature and front contact, this relay would have opened at its outer left-hand armature and back contact the shunt path across the lower armature and back contact of relay 224 whereby the latter relay, on being operated by the response of sensor 24 to strip 17a, would have broken the energizing circuit of relay 120b notwithstanding the continued operation of relays 221 and 222. The armature winding of motor 20b would then have been short-circuited by the grounding of its left-hand terminal via the armature and back contact of relay 120b and the inner right-hand armature and front contact of relay 220b. This would have resulted in the exertion of a braking force upon the right-hand traction wheel 6b, with motor 20a continuing to drive the left-hand wheel 6a; the vehicle would then have turned onto the branch path 17b, its rightward swing being interrupted only momentarily during passage of sensor 23 across the strip 17 with brief reoperation of relay 120b and motor 20b.

As the vehicle originally considered (with active switches 31, 34, 35 as shown in FIGS. 2 and 3) approaches the junction 29b along strip 17, switch 34 passes over marker 134b concurrently with the passage of switch 30 over marker 130b so that steering relay 220a is actuated. In a manner analogous to that described above with reference to relay 220b, relay 220a causes the deenergization driving relay 120a as soon as sensor 23 picks up the branch strip 17b whereby motor 20a is braked by the short-circuiting of its armature winding via the armature and back contact of relay 120a in series with the inner left-hand armature and front contact of relay 220a. With relay 120b remaining operated under the control of sensor 23, the right-hand traction wheel 6b is powered while the left-hand wheel 6a is braked. The resulting leftward swing, which turns the vehicle onto the branch path defined by strip 17b, is again momentarily interrupted by a brief reenergization of relay 120a as the sensor 24 detects the forward extension of strip 17; thereafter, if-- as is likely-- the vehicle has not yet fully aligned itself with the strip 17b so that the front sensor 21 is not above that strip, sensor 23 again takes over the exclusive control of the driving circuit so that motor 20b, alone, remains operated.

With relay 220a locked operated, the steering of the vehicle along strip 17b would proceed in essentially the same fashion as heretofore described with reference to strip 17, except for a sharper corrective action (owing to the braking of motor 20a) whenever it strays off course to the left of its guide path; a similar situation exists along strip 17a, with regard to rightward deviation, for a vehicle whose relay 220b is energized. To restore the more gradual and symmetrical mode of guidance explained above, therefore, I may provide beyond each junction a combination of markers 130', 135' and 130", 135" to actuate the relay 220c by simultaneous closure of reading switches 30 and 35 to release the relays 220b and 220a, respectively. If the speed of the carriage in passing over these junctions is low enough, the braking circuits leading over the front contacts of relays 120a and 120b could be omitted, thus eliminating the need for these additional markers on branch paths 17a, 17b. Conversely, the back contacts of relays 120a and 120b could be permanently grounded if the nondriven traction which is to be braked under all circumstances.

Whenever the carriage reaches the end of its guide path so that at least the sensors 21, 23 and 24 no longer confront an underlying strip, both motors 20a, 20b are deactivated so that the vehicle comes to a halt. Such stoppage also occurs, naturally, if the vehicle is thrown completely off its course, e.g., by an encounter with an extraneous object.

If the carriage is conditioned for rearward travel, as by operation of a reversing key on the panel 7 of FIG. 1, another set of relays similar to those shown in FIG. 6 take over the control of motors 20a, 20b. Reading switch 35' may then control the counterpart of relay 220c, in a manner analogous to that of switch 35, upon encountering a marker on its side of the guide strip.

It will be noted that, pursuant to FIG. 5, the marker arrays at junctions 29a and 29b differ from each other by the relative position and spacing of the several bar magnets serving as markers. If the number of junctions to be successively traversed by one carriage is small, the steering commands to be executed at these junctions can be generated by different groupings of selectively activable reading switches. In a more complex system a single group of sensing switches may be selectively activated, at successive junctions, by a programmer aboard the carriage which could be stepped, for example, by each actuation of reference switch 30 or by the operation of relay 220c upon passage over any branching point.

In FIG. 7 I have shown part of the circuit arrangement of FIG. 6, including the steering relays 220a and 220b, modified to serve for the control of a clutch-type carriage drive as illustrated in FIGS. 3 and 4. The single drive motor 20 is connected directly between ground and bus bar 41 which is again energized from the negative terminal of battery 40 via bumper switch 12. As in the preceding embodiment, the switches for reversing the motor (if desired) and connecting it to an alternate control circuit have not been illustrated.

In FIG. 7 the driving relays 120a, 120b have been each provided with a second armature to control both the clutch portions 27a, 27b and the brake portions 28a, 28b of the associated electromagnetic couplings 26a, 26b (cf. FIG. 4). When energized, relay 120a or 120b operates the clutch 27a or 27b to power the corresponding traction wheel 6a or 6b, respectively. In its unoperated state, relay 120a connects via its right-hand armature and back contact the brake 28a to the inner left-hand armature of relay 220a whose front contact is tied to bus bar 41; thus, brake 28a becomes effective only with relay 120a released and relay 220a energized. In like manner, unoperated relay 120b connects via its left-hand armature and back contact the brake 28b to the inner right-hand armature of relay 220b whose front contact is tied to bus bar 41 whereby the brake 28b is actuated only when the unoperated state of relay 120b coincides with the energized condition of relay 220b. Thus, the operation of this system is analogous to that of the system of FIG. 6 inasmuch as either traction wheel, when nondriven, is left floating on straightforward steering but is brakes when the corresponding steering relay 220a or 220b has been set for turning to one side or the other.

Again, the back contacts of relays 120a and 120b could be permanently connected to bus bar 41 if a nondriven traction wheel is to be invariably braked.

In FIG. 8 I have shown details of a signal generator 121A, representative of any of the units 121-124 in FIG. 6, and an associated sensor 21A, also typifying any one of the elements 21-24. Sensor 21A is a conductive plate overlying with close spacing a grounded conductive guide strip 17A to form therewith a capacitor connected in parallel with an inductance 43A as part of a resonant circuit in the feedback path of a high-frequency oscillator 44A. Upon disalignment of plate 21A with conductive strip 17A, the shunt capacitance is greatly reduced as the sensor confronts only the nonconductive floor beneath it; this results in a harp detuning or a complete deactivation of the oscillator 44A. When, however, the sensor plate 21A registers with strip 17A, oscillator 44A has an operating frequency in the pass band of a band-pass filter 45A whose output, after rectification at a diode network 46A, energizes the corresponding sensing relay here designated 221A.

FIG. 9 shows a broadly similar signal generator 121B whose sensor 21B is a length of horizontal wire closely spaced from a highly permeable guide strip 17B so as to introduce a supplemental inductance into a resonant circuit of a high-frequency oscillator 44B, this resonant circuit including a coil 43B and a condenser 47B in series with wire 21B. As before, the absence of the guide strip detunes the oscillator 44B which otherwise operates at a frequency passed by a band filter 45B to actuate a sensing relay 221B after rectification at 46B.

The arrangements illustrated in FIGS. 8 and 9 are representative of a variety of ways in which a sensor may capacitively or inductively detect the proximity of an electrically conductive and/or magnetically permeable guide strip by modifying the reactance of a circuit controlling the operating frequency of an oscillator. In its broader aspects, however, the invention can also be realized with other types of sensors, e.g., photoelectric detectors for a reflected light beam if the guide path is marked by a reflecting strip and if there is no substantial danger of contamination to an extent making optical pickup impractical. Similar capacitive, inductive or optical pickup means may also be used as readers, in lieu of the magnetic switches 30-35. Naturally, the electromagnetic relays shown in FIGS. 6-9 could be replaced by equivalent electronic circuitry.

In lieu of two sets of sensors for forward and reverse motion, as illustrated in FIG. 3, a single set may be used in combination with the aforementioned reversing switch to guide the carriage in either direction of travel. In such a case it will be convenient to position the sensor array centrally of the carriage, with its transverse axis in line with the axes of the wheels 6a and 6b. Naturally this would require a relocation of the reading switches 30, 35 and 35'.

Claims

I claim:

1. A system for guiding a self-propelled carriage, comprising:

a substantially continuous strip of distinctive character at the level of a floor establishing a predetermined path for the guidance of a carriage traveling over said floor;

a right-hand traction wheel and a left-hand traction wheel on said carriage;

first and second drive means aboard said carriage for transmitting torque to said right-hand and left-hand traction wheels, respectively;

and a control circuit for said first and second drive means including central sensing means and flanking right and left sensing means responsive to said guide means for emitting a steering signal upon detecting the proximity of said strip, said first and second drive means being jointly responsive to a steering signal from said central sensing means to power both said traction wheels simultaneously, said first drive means being responsive to a steering signal from said left sensing means to power said right-hand traction wheel, said second drive means being responsive to a steering signal from said right sensing means to power said left-hand traction wheel;

said central sensing means comprising first and second sensors spaced apart in the direction of travel, said right and left sensing means comprising third and fourth sensors spaced apart on a line transverse to said direction of travel; said control circuit including an individual oscillator for each of said sensors, said oscillator being provided with a reactive circuit, each of said sensors forming together with said strip a reactance connected in said reactive circuit for resonating same at a predetermined operating frequency whereby the alignment of any sensor with said strip tunes the associated oscillator to said operating frequency indicating such alignment.

2. A system as defined in claim 1 wherein said sensors form a diamond-shaped array.

3. A system for guiding a self-propelled carriage, comprising:

a substantially continuous strip of distinctive character at the level of a floor establishing a predetermined path for the guidance of a carriage traveling over said floor;

a right-hand traction wheel and a left-hand traction wheel on said carriage;

first and second drive means aboard said carriage for transmitting torque to said right-hand and left-hand traction wheels, respectively;

and a control circuit for said first and second drive means including central sensing means and flanking right and left sensing means responsive to said guide means for emitting a steering signal upon detecting the proximity of said strip, said right and left sensing means comprising individual sensors spaced apart on a line transverse to said direction of travel, said first and second drive means being jointly responsive to a steering signal from said central sensing means to power both said traction wheels simultaneously, said first drive means being responsive to a steering signal from said left sensing means to power said right-hand traction wheel, said second drive means being responsive to a steering signal from said right sensing means to power said left-hand traction wheel;

said control circuit including an individual oscillator for each of said sensors, said oscillator being provided with a reactive circuit, each of said sensors forming together with said strip a reactance connected in said reactive circuit for resonating same at a predetermined operating frequency whereby the alignment of any sensor with said strip tunes the associated oscillator to said operating frequency indicating such alignment.

4. A system as defined in claim 3 wherein said central, right and left sensing means are disposed ahead of said traction wheels as viewed in the direction of travel.

5. A system as defined in claim 4 wherein said first and second drive means are reversible, further comprising alternatively operable duplicates of said central, right and left sensing means disposed symmetrically thereto on the opposite side of a common axis of said traction wheels.

6. A system as defined in claim 3 wherein said traction wheels are coaxial, said individual sensors being disposed between said traction wheels substantially in line with the common axis thereof.

7. A system as defined in claim 3 wherein the spacing of said individual sensors is substantially equal to twice the width of said strip.

8. A system as defined in claim 3 wherein said control circuit further includes filter means in the output of each oscillator tuned to said operating frequency for generating the corresponding steering signal.

9. A system as defined in claim 3 wherein said strip splits into different branches on at least one junction point, further comprising marker means at said junction point and detector means aboard said carriage settable to respond to said a market means for modifying the operation of said control circuit to direct the carriage onto a selected branch.

10. A system as defined in claim 9 wherein said marker means comprises an array of magnets on said floor, said detector means comprising magnetic switches positioned to respond to certain of said magnets upon alignment therewith.

11. A system as defined in claim 3, further comprising obstacle-detecting means aboard said carriage and switch means responsive to said obstacle-detecting means for deactivating said first and second drive means.

12. A system as defined in claim 3 wherein said carriage is provided with two swivelable nondriven wheels supplementing said traction wheels.