WO1985002267A1

WO1985002267A1 - Infra-red position sensor system

Info

Publication number: WO1985002267A1
Application number: PCT/AU1984/000238
Authority: WO
Inventors: Alec Martin Stevens; Stephen Richard Williams
Original assignee: Alec Martin Stevens
Priority date: 1983-11-21
Filing date: 1984-11-21
Publication date: 1985-05-23
Also published as: IT8449195A0; EP0162086A1; IL73582A0; IT1184262B

Abstract

A sensor system applicable to agricultural booms (16) to control extension so as to enable work heads (46, 60) to be extended between trees off a vehicle travelling parallel to a row of trees with an infra-red detector (75) operative to detect reflection of a pulsed infra-red signal off a nearby tree trunk to signal need to withdraw boom. The sensor system indicates approach of the boom to within a selectable distance of the tree trunk by detecting the level of the reflected pulse.

Description

INFRA-RED POSITION SENSOR SYSTEM

TECHNICAL FIELD OF THE INVENTION THIS INVENTION relates to sensor systems. In one application it is applied to a boom which may be powered for extension and retraction in application on tractors and other like work vehicles to enable operations such as spraying, mowing, etc.

BACKGROUND ART A wide variety of equipment has been developed by which sprays, mowers, etc., may be operated off transport vehicles, such as tractors and other like • vehicles, in order to cover extensive areas. Sprays, etc. , are commonly mounted off booms in order that tractors travelling along access routes may apply sprays to areas off the access route. Similarly mowers, slashers, etc., working off roadways, utilise booms in order to reach roadside verges and cut down unwanted vegetation. These operations require deployment of boom type structures and fixed booms are of little use. Booms are known which may be swung out of the way of obstacles. Hydraulically extendible booms similar to those used in crane construction have been used. However, no ready and simple solution has yet been found in the quest for an economical and easily maintained boom, for application off tractors and other like vehicles, to enable operations such as spraying, mowing, etc., particularly in orchards between rows of trees where the position of the boom in relation to the tree must be measured if automatic operation is to be achieved. OUTLINE OF THE INVENTION It is an object of the present invention to provide a sensor system which is able to provide posit¬ ional detection of a variety of elements in a variety of applications. It is another object of the present invention to provide a sensor system suitable for an extendible boom which lends itself to control of motorised work heads during operation off mobile carriers therefor. It is a further object of the present invention to provide a sensor system for an extendible boom which comprises a simple construction, and an efficient and reliable mode of operation. An additional object of the present invention is a sensor system for a boom which is effective to automatically control a boom moving along a row of trees. Other objects and advantages will hereinafter become apparent.

The present invention provides a sensor system for enabling an indication of relative position comprising an infra-red detector and infra-red transmitter, both oriented generally in the same direction, said infra-red transmitter being operated with an output pattern having a predetermined distinctive characteristic, and said infra-red detector being coupled to a detection circuit examining the output of said detector to indicate the reception of a reflected signal having the distinctive characteristic indicating a reflection off a nearby object.

The present invention also provides a sensor system for an extendible boom for the operation of work heads supported thereon off supporting vehicles therefor comprising a boom assembly having an elongated work head carrying arm mounted on a supporting means therefor which enables movement of said elongated arm in a direction extending along the length thereof, a source of motive power for application to said elongated work head carrying arm to move said arm back and forth, at least one sensing element mounted to the end of said boom to detect its approach towards an obstacle such as a tree and signal generating means triggered by said sensing element operable to provide a signal to said source of motive power to cause its operation to retract said boom.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more

OMP readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a preferred embodiment thereof and wherein:- Fig. 1 shows schematically the features of an extendible boom which may be controlled by a sensor system according to the present invention;

Fig. -2 is a section through the boom of Fig. 1; Fig. 3 shows an extendible boom as shown in Figs. 1 and 2 mounted to a tractor;

Fig. 4 shows the disposition of sensor elements as/might be used in the sensor system of the present invention;

Fig. 5 shows an alternate style of sensor system employing audio sensor elements;

Fig. 6 is a front elevation of a series of interconnected infra-red sensor modules in accordance with the present invention;

Fig. 7 is a sectional view of two extrusions that may be employed in the embodiment of Fig. 6;

Fig. 8 is a plan view of two end caps to be used with the extrusions of Fig. 7;

Fig. 9 is a block diagram indicating components that may be employed to operate the sensor modules of Fig. 6; and

Fig. 10 is a circuit diagram capable of performing the operation described with respect to Fig. 9.

METHOD OF PERFORMANCE In Fig. 1, item 11 is a boom carrier bracket by which the boom assembly may be mounted to a tractor or other like vehicle. Supported on the boom carrier bracket 11, on a shaft passing therethrough, is a pinion 12 which may be a chain type sprocket. The pinion 12 engages with a rack 13 which is a roller chain when a chain sprocket pinion is employed. Also supported on the boom carrier bracket 11 are boom support means 17 which may be freely rotatable wheels on roller bearings on axles 18. The boom 16 may be a hollow length of channelling in which boom support means 17 is located to support the channel section thereupon. The boom and support means 17 is more clearly seen in Fig. 2.

Where a roller chain is employed as the rack 13, it may be supported upon the boom and connected at its ends so as to be held in place by suitable connectors 20 which may be through bolts which also serve to prevent the boom support means 17 from running out of the boom at the ends thereof. The connector 20 may provide an end stop means for the boom's travel.

Fig. 2 is a view of the boom assembly as shown in Fig. 1, from another angle, being the section indicated in Fig. 1 with like integers similarly numbered. The pinion 12 is shown mounted on the output shaft of a suitable source of motive power 14 which may be an hydraulic motor. Also employed in relation to the pinion 12 as an optional feature is a torque limiter assembly 15 as is further described below. Boom carrier bracket 11 supports at its lower extremities, boom carrier means 17 on axles 18 retained thereto, by retaining clip 19. The boom 16 encompasses the boom support means to be supported thereby. Fig. 3 shows one possible application of the boom as a rear mounted extendible arm by which a work head may be supported off to the side of a tractor 21 or other like support vehicle. Boom bracket 11 is supported off the back of the tractor and rack 13, and pinion 12, enable movement of the boom 16 from a retracted position shown dotted, behind the tractor 21, and an extended position projected out to the side of the tractor.

The above described assembly enables a motorised extension and retraction system operable off a hydraulic power take off point, or other suitable type of take off point, of a tractor or other like vehicle. Such an assembly may be used as an orchard boom spray, for roadside or parkland grass mowing, and many more diverse applications. The fitting of the attachments for spraying, grass mowing, etc., will be obvious to persons skilled in the art and these are not described here.. A complete unit may comprise a bracket, wheels, hydraulic motor, torque limiting coupling, chain sprocket and roller chain. The integers of the assembly are all readily available items and their construction into the boom of the present invention is quickly and. cheaplyachieved. The boom is a slotted steel boom which may simply be a rolled steel section. The hydraulic motor may be a conventional type and may be operated at varying speeds in either the forward or reverse direction. The torque limiter may be a friction clutch type of torque limiting coupling to protect the rack and pinion against overloading whenever the travel limit is reached, or jamming occurs.

Engagement of the chain and sprocket is assured by restraining the vertical and sideways movement of the boom which may be controlled by the limited clearance between the diameter of the wheels and the internal height of the boom, and also the limited clearance between the edges of the slot and the bracket passing therethrough. Additionally where a rolled section is used, or an extrusion having rounded internal corners, the radiused corners act on the wheels to centre them therebetween.

The selection of wheel diameter, boom, and axle length are crucial to maintaining positive engagement of the rack and pinion.

With the above described apparatus, operating for example with a mower head under a row of trees, it is possible to mow around the trees with two overlapped passes, one along each side of the rows. To achieve overlap of the cut swarth, the mower work head is moved into the space between the trees and withdrawn just before making contact with the next tree down the row, to be reinserted back between the trees once passed it. By mowing down both sides of the row in this manner, the cut achieved can be made to overlap, so as to cover all the ground between the .trees. What little grass around the tree trunk is not cut may be sprayed with a weedicide, herbicide, etc., so as to kill off that grass which the mower cannot reach for fear of doing damage to the tree trunk. This might be done automatically with spray heads turned on whenever the boom is retracted, or partially retracted, to be switched off when it is again extended. Alternately, such sprays may be .entirely operator controlled.

Manual operation of the boom requires concentr¬ ation which may be avoided by providing an automated operation. This is achieved by providing suitable sensors at the end of the boom to operate a control system which automatically moves the boom whenever the sensor, or sensors detect the presence of an obstacle.

In Fig. 4, tractor 40 carries, possibly at a front end thereof, a boom system upon a support 41 which mounts two boom carriage means 42 and 43 which include the motive power units by which the booms 44 and 45 are moved reciprocately between withdrawn and extended positions. On booms 44 and 45 is a work head 46 which may be a mower of spray unit to be moved between trees say in an orchard.

Work head 46 must be moved so as not to collide with obstacles, and in order to detect their presence, a cats whisker type sensor element 66 may extend across the leading edge of the work piece 46. Sensor element

48 is a feeler element mounted resiliently on a pivot

49 at which a microswitch may operate to detect movement of the element 48 on striking an obstacle. Microswitch operation can provide a suitable indication to a control system of the presence of an obstacle so as to cause retraction of the. boom upon activation of the boom motive means. It is possible to operate an automatic system with a single sensor with an operator maintaining a

OMPI set vehicle speed, and a suitable time delay being introduced by the control system, prior to the boom's being re-extended, so as to send the boom to its extended position once sufficient time has elapsed to pass the tree.

To account for operator error in speed control which might have the work head returning to its extended position too soon and banging into a tree trunk, another sensing element 52, pivoted at 53 and set to operate a suitable microswitch, provides a means to prevent damage to the tree as a result of such error in speed control. This extra sensor enables the operator to vary his speed without concern as to the position of the boom. In order to improve the speed of retraction in the event of its being necessary, a third sensing element 50 mounted at 51, behind element 48, may operate to cut off power to one motor of the pair of booms 44 or 45, to boost the pressure applied to one motor and double its speed, and the speed of retraction, where an hydraulic system is in operation. Similar results might be achieved with other drive arrangements with sensor 50 signalling the need to speed up retraction and boost motor speed. Fig. 5 shows an alternate arrangement with the sensor elements of Fig. 4 replaced by audio signal generators producing two wide angle, forward and sideward looking beams, 63 and 64 produced by generators 61 and 62. Suitably placed audio sensors will detect returned signals, bounced off obstacles, to signal an obstacle's presence. Beam 63 replaces element 48, and beam 64 replaces 52. Operation of the control system is the same to enable a fully automatic operation. As a back up, cat whisker sensors, such as 66 mounted at 65 might be employed. The range of beams may be set as is known so as to provide a close cut up to an obstacle such as a tree with control over beam intensity or detector

OMPI sensitivity. The particular beam generators and detectors for audio signal control to be used in the invention are any one of the known arrangements of such devices with beam geometry and detector sensitivity configured to respond to a particular obstacle such as a particular species of tree.

An advantage of an audio system is the ease with which 'it may follow the contours of odd shaped trunks such as occurs in some orchard species. The following is an infra-red system for operating the boom.

Fig. 6 is a front view of a series of abutted sensor modules 81, 70 and 83 joined together at 71 and 72. The modules are preferably built using extrusions to be described below to have the same cross-sections to enable their simple interconnection. The module extrusion may be provided with a forwardly extending flange 78 to shade and protect the sensors, and grooves 79 and 80 are provided to enable screwed application of end caps as is described below.

Module 70 comprises two infra-red transmitters 73 and 74 spaced apart on either side of a photo diode receiver 75. The front face is provided with a means of operating a BCD switch whose shaft may be advantageously passed through a hole in the extrusion to provide external operation. This switch is used to reset sense distances of individual modules. The transmitters 73 and 74 provide a pulsed output, typically of the order of 2 kilohertz. Modules butt together with adjacent transmitters forming pairs. To balance the ends of a series of modules an end module 81 may be used with a single transmitter 82 provided to enable sufficient illumination for receiver 75. The end module may be powered from the adjacent module as will become clear below. The sensor modules may be daisy chained with a connection bus passing

^{" *} through all the modules. Each module may be provided with connecting sockets at an end to mate with corresponding plugs at the other adjoining end of an abutted module. The connection bus may be terminated in a multi-pole socket at one of the end modules to enable plugging in of external control lines. A weatherproof type socket might be employed for this purpose.

Fig. 7 shows in sectional form, the shape of two extrusions that might be used in the construction of sensor modules. The two parts 91 and 92 are press¬ ed together with ridges 93 and 94 engaging in grooves 95 and 96 to create an enclosure for a PC circuit board 102 clipped behind flanges up againsc abuttments as shown. In Fig. 7, the PC circuit board carries an indicating LED 101 which, when the parts are interconnected, locates behind a lens housing 100 at the rear of the module. On the front face of the module is a lens holder 75 for an infra-red photo diode receiver and the switch shaft 76 operating switch 103. Providing shade and protection against impact with objects is extension 78. To enable mounting of the sensor module a groove 104 is formed with inner ridged surfaces to engage with the screw thread of a bolt head. In order to be able to screw on end caps to seal the end modules, grooves 97, 98, 79 and 80 are provided to receive self tapping style screws therein. The two extrusions combine to provide a weatherproof enclosure. Fig. 8 shows two end plates to be used with the extrusions of Fig. 7. The plates 110 and 115 are provided with holes at 111 to 113 to give access to grooves 97, 98, 79 and 80 in fixing the end plates to the extrusions with screws. End plate 115 is shown with a hole 116 for receipt of a connector plug for attach¬ ment of an external line to the common bus running through the modules. Both the end plates and the extrusions are conveniently formed of aluminium.

Fig. 9 indicates the components of a circuit to operate the infra-red transmitters and receivers of a module as shown in Fig. 6. Connections 120 extend between end sockets to be joined as a common bus for the system of interconnected modules. Six lines are provided in Fig. 9 with a twelve volt source, ground, transmit clock signal, and three outputs. The combination of activated output lines will signal to a control unit which of the sensors is detecting an object.

In Fig. 9 a constant current source 125 powers a transmitter 123 with transmit indicator 124 to indicate operation. Infra-red transmitter 122 is an optional transmitter fitted to the system to balance the ends where required. A buffer/driver circuit 121 completes the transmit circuit with clock buffer driver 132 providing the clock input to control the output frequency. The clock signal is also fed to a signal comparator and decoder 139.

An object indicator 127 powered by a suitable driver 129 signals an object sensed on outputting of a signal from comparator/decoder circuit 139. A regulated voltage supply 126 provides an appropriate environment for the circuitry incorporated in the sensor modules. Photo diode 128 outputs its signal to an amplifier 130 and a discriminating filter/ amplifier 131 whose characteristics will become more clear below. Circuit 134 enables an operator to set, via suitable inputs 133, a sense distance level, and this output might be processed through linear amplifiers (inverters) 135, 136, signal sampler 137 and pulse stretcher 138. By comparison of respective signals, comparator 139 provides an output, via suitable delay 140, buffer 15 and driver 151. Fig. 10 shows a specific circuit for performing these functions. In Fig. 10, the circuit of the sensor module contains the transmitters and receiver that are utilised to sense the object. Pulsed infra-red is emitted through the lenses and any reflected signal is detected by the receiver within the module. In normal applications the sensor module/s are connected to a control unit (not shown) which provides the common supply voltages, a common clock @ 2.2KHz (TXCLK O) and the interface required for the sensor outputs (O/P 0,1,2) to activate drive functions, solenoid drivers to actuate the various hydraulic functions. The transmit clock (TXCLK 0) is detected by a CMOS Schmitt Trigger Inverter. The inverted clean clock output from I.C.4c is then applied to the inputs I.C. 4 a & b, CMOS Schmitt Trigger Inverters, to derive two clocks, one for transmit, the other used for verification of the received signal. The output of I.C.4a is used to drive the transistor switch TRl at the clock frequency. The IRLEDS 1,2 (and^' 3 if left/right Ext P.C.B. is used) are in the collector circuit of TRl, along with the transmit indicator LED 1. The LED is used to warn of transmit failure which may affect the operation of the unit by non location of an approaching object. The IRLEDS and LED are powered from the supply voltage via a constant current source to ensure constant I.R. emissions. The constant current source REG 2 and its limit resistor R35 utilise the inbuilt limiting of the regulator to provide a 200mA current. The supply voltage is regulated to 7.5 volts by REG 1/R33/R34 and filtered by C9/C10 to supply the Integrated Circuits. The reflected signal is detected by the Photo-diode PD 1 which is connected to the inputs of I.C.3a and biased by R1/R2/C1. The gain/frequency response of I.C.3a is set by R3/R4/R5 and C2. The output is passed via R6/C3 to the non- inverting input of I.C.3b which is biased to the mid-point by R7/R8. The gain is set by R9 and the frequency is a function of C3/C4. The output of

I.C.3b is then applied to a voltage divider formed by RIO - R18. Eight points on the divider are then passed to I.C.2 which is an eight line analog multiplexor. The output is selected from one of the eight inputs or if required, one of two off (no output) positions by the setting of the selector switch B.CD.Switch 1. Four pulldown resistors R19 - R22 ensure correct selection. The eight positions provide eight distinct sense distances, positions

/ nine arid ten give a no output state. The sense distance is a function of the signal level from the output of the multiplexor being applied to a Schmitt Trigger following two CMOS inverters which are configured in the linear mode to amplify the signal. A capacitor C5 decouples the multiplexor output before passing to the two stage amplifier formed by the CMOS inverters I.C.la & b and their associated gain setting resistors R24 - R26. The output of

I.C.lb is decoupled by C6 and passed to I.C.4d. The signal at this point is sinusoidal and the peak to peak voltage level increases as an object is approached. The sense distance set by the switch is the switching point of the input circuitry of the CMOS Schmitt Trigger I.C.4d. When the switch point is reached the output of I.C.4d becomes a square wave at the detected frequency. The square wave is passed to the comparator formed by I.C. 5 a, c & d. and also via diode D1/R27/C7 to I.C.4e which functions as a pulse stretcher. The output of the pulse stretcher is therefore active high whenever a signal is detected. The comparator is comprised of three CMOS Exclusive or gates and utilises the detected square wave signal, the clock signal (from I.C.4b) and the object signal from the pulse stretcher to perform a comparison operation. On receipt of a correct signal, vis : the reflected transmit signal, the output of I.C.5d goes active high, this signal is passed to a delay circuit I.C.lc/D2/R28/C8/I.C.4f which removes any transient spikes from random signals. The final signal then drives the object indicator LED2 via TR2 and R29. Resistor R31 in the emitter circuit of TR2 limits the drive to LED 2. The object verified signal also drives the output drivers I.C.le & d via I.C.lf/R30. The combined outputs of I.C.le & d are connected to the output line O/P by diode D3.

The sensor may be configured as a single unit or connected in multiples as required. The P.C.B. has provision for a 10 way connector at each end. This enables the 6 way common bus to be extended through multiple modules and also carries the connections for the left/right Ext P.C.B. , with its IRLED (3) if it is required. The IRLED is connected into the transmit circuit by strapping of the plug which mates to the 10 way connector. The left/right Ext is utilised where signal strength needs to be maintained for maximum sense distance,, usually at the extremities of a string of modules. When multiple sensors are used the adjacent IRLEDS (on different modules) give a combined signal of the required strength between the Photo diode receivers. To connect the sensor modules to the control unit the 6 way common bus may be accessed via a multipole automotive socket mounted on the end of the unit, if so desired groups of modules may also be connected by this means the required module connect- ors being wired to the sockets as required, they can be fitted to either end or both if so desired, providing versatility. There are 3 output lines (0/P 0, 1 & 2) and each module can be set to give an output code on these lines. Eight signal conditions are possible of which one is the normal condition where all output lines are low (OV) . The decoding of these lines may be performed by a control unit.

OMPI The sensors may be mounted in a two piece custom aluminium extrusion, which holds the lenses and has provision for the B.C.D. switch shaft to extend through for access. The lenses may locate the IRLEDS and the photo diode and the P.C.B. can be held securely by the clips provided in the extrusion. The cover extrusion may have the bezels, for the rear facing transmit and object indicating LED's, mounted to it and clips to the main unit. Mounting for the complete unit can be provided by a full length screw flute along the bottom face which accepts M6 bolts up to 10mm in length. The end covers may be mounted by M4 self-tapping screws which locate into the full length flutes provided, two in each part of the two piece custom aluminium extrusion to enable removal of the cover should it prove necessary. The configuration of the P.C.B. *s and their mounting is designed to allow field replacement of any module if it becomes faulty.. Low power CMOS devices may be used, these also have a built in immunity to noise generated on the supply voltage lines from other devices using the same power source. The control unit used might contain overload/overvoltage protection for the sensor modules as part of its power regulation/distribution circuitry. Power to a control unit can be derived from an external supply, e.g. the vehicle ignition.

The sensors may be set up by selecting the required sense distance for each module in use, this allows the adoption of a "staggered" sense distance with a multiple sensor unit to achieve an appropriate reaction for a given application. This is done simply by setting the switch on each unit to the desired range with a small screwdriver. The settings may be altered at will for different uses of the unit. The above described circuit is able to sense small trunks of orchard trees (down to a few centimetres in diamater) and discriminate against grass tussocks. The 15. system is immune to solar reflections and changing natural heat patterns.

The following tables itemise specific values and types of various components of the circuit of Fig. 10.

TABLE 1

RESISTORS. - 1%;0.25W (11

Rl - 15K

R2 - 10K

R3 - BM2

R4 - 10K

R5 - 4K7

R6 - 100R

R7 - 100K

R8 - 100K

R9 - 3M3

R10 - 100R

R19 - 100K

R20 - 100K

R21 - 100K

R22 - 100K

R23 - 1K21

R24 - 8M2

R25 - 10K

R26 - 220K

R27 - 8M2

R28 - 8M2

R29 - 46K4 (NOM)

R30 - 46K4 (NOM)

R31 - 237R

R32 - 4K7

R33 - 1K21

R34 - 237R

R35 - 10R (NOM)

R36 — 100K

[Values marked (NOM) may change in use] TABLE 2

CAPACITORS

C Cll - lOuF 16VW PC AL. C C22 - lOuF 16VW PC AL.

C C33 - 2n2 63VW CD. C C44 -r 2n2 63VW CD.

C C55 - - lOn 3VW CD. C C66 - lOOn 63VW CD.

C C77 - 2n2 63VW CD. C C88 - 2n2 63 W CD.

C C99 - 2.2uF 16VW TAG TANT C CIIOO -- lOuF 16VW TAG TANT

TABLE 3

SEMICONDUCTORS

I.C. *S

IC 1 - CD 4069

IC 2 - CD 4051

IC 3 - LF 353

IC 4 - CD 40106

IC 5 - CD 4030

REGULATORS

REG 1 - LM 317 T

PHOTODIODE

P.D. 1 - BPW 50 TRANSISTORS TRl - BD 139 TR2 - BC 547

It will be clear from the above that the sensor system operates to detect nearness to an object and it will be clear that the same sensor system can be used to detect position in relation to an object such as a truck backing up to a loading bay. It will also be clear that the sensor system, in array form can be used to detect the degree of movement of an object moving parallel to the array so as to provide an indication of the position of an object along the length of the array. This form of the system can be used in positioning objects s'_.ch as in loading containers which must be precisely located so as to enable engagement of locking mechanisms between them and a trailer to carry them.

Other sensing elements may be employed with detectors such as light, microwaves as produced by small solid state generators, magnetic with suitable magnetic labels in place, as well as the above described mechanical and audio variations. In all situations, the sensor element is employed to provide a detection signal, such as the tripping of a microswitch, to cause the generation of a control signal which operates to switch a motive power source into movement to retract or extend a boom arrangement. The equipment may employ any of the known control technologies such as electro-mechanical, electronic, etc. While the above has been given by way of illustrative example, many modifications and variations "as would be apparent to persons skilled in the art may be made thereto without departing from the broad scope and ambit of the invention as herein set forth and claimed in the following claims.

^*£URE _mOM?I _- WIFO ^**

Claims

1. A sensor system for enabling an indication of relative position comprising at least one infra-red detector (75) and at least one infra-red transmitter (73,74) characterised in that both are mounted adjacent one another on a support (70) and are oriented generally in the same direction, said infra-red transmitter (73,74) being operated with an output pattern having a predetermined distinctive characteristic and said infra-red detector (75) being coupled to a detection circuit examining- the output of said detector (75) to indicate the reception of a reflected signal having the distinctive characteristic indicating a reflection off a nearby object.

2. A sensor system as claimed in Claim 1 , wherein the output of said infra-red transmitter is variable (76) so as to provide a useful reflected signal at a preset distance to indicate the degree of approach of an object towards said detector (75) .

3. A sensor system as claimed in Claim 1 , wherein a plurality of transmitters (73,74) and interposed detectors (75) are linearly arranged, each being oriented in the same general direction along parallel axes so as to generate a positional indication of an object positioned along the direction of the array.

4. A sensor system as claimed in Claim 3, wherein said array comprises a plurality of modules (70,81,83) joined end to end, each module carrying spaced apart transmitters (73,74) outputting a distinctive, characteristic signal, and a central detector (75) oriented to receive any reflection of said distinctive, characteristic signal.

5. A sensor system as claimed in Claim 4, wherein said array is connected to a common bus and each sensor module (70,81,83) is operative to output signals on different distinctive combinations of lines in said bus.

6. A sensor system as claimed in either one of Claims 4 or 5, wherein each module contains a comparator means (139) operative to detect a coincidence of output and received signals.

7. A sensor system as claimed in any one of Claims 1 to 6, wherein said support is an extendible boom (16) for the operation of work heads (60) supported thereon off a supporting vehicle therefor comprising a boom assembly having an elongated work head carrying arm (16) mounted on a supporting means (12) therefor which enables movement of said elongated arm (16) in a direction extending along the length thereof, a source of motive power (14) for application to saiύ elongated work head carrying arm to move said arm back and forth, said sensor system being mounted to the end of said boom to detect its approach towards an obstacle such as a tree and signal generating means triggered by said sensor system operable to provide a signal to said source of motive power (14) to cause its operation to retract said boom.

8. A sensor system as claimed in Claim 7, wherein a plurality of sensors (70,81,83) are located along a length of the boom (16) spaced apart one from the other.

9. A sensor system as claimed in Claim 8, wherein a linearly spaced array of infra-red detectors (75) are separated by interposed infra-red transmitters (73,74) oriented in a forward direction in the sense of travel of the boom.

10. A sensor system as claimed in Claim 9, wherein the sense distance of each infra-red detector (75) may be independently preset to control the action of the boom in a predetermined manner.

11. A sensor system as claimed in Claim 4, wherein the linearly spaced array is comprised of interconnected modules (70,81,83), each module having an infra-red detector (75) on a front face thereof flanked by two transmitters (73,74) adjacent each end of the front face

OMPI so as to form closely spaced pairs of transmitters when a series of modules are interconnected linearly.

12. A sensor system as claimed in Claim 11, wherein each module comprises circuitry to generate and transmit said predetermined signal, process the signal received at said detector and compare (139) the two signals to determine if a reflection is recorded.

13. A sensor system as claimed in Claim 12, wherein the circuitry of each module is constructed on a circuit board (102) provided with edge connectors at opposed ends to mate with corresponding edge connectors of adjacent boards of adjacent modules in the assembly of said linear array.

14. A sensor system as claimed in Claim 12, wherein the output of said infra-red photo diode (128) is fed via a discriminating filter/amplifier (131) to a signal comparator (139) where it is compared with the transmit signal to detect a reflection.

15. A sensor system for an extendible boom (16) for the operation of work heads (60) supported thereon off a supporting vehicle therefor comprising a boom assembly having an elongated work head carrying arm (16) mounted on a supporting means (12) therefor which enables movement of said elongated arm (16) in a direction extend¬ ing along the length thereof, a source of motive power (14) for application to said elongated work head carrying arm to move said arm back and forth, at least one sensing element mounted to the end of said boom to detect its approach towards an obstacle such as a tree and signal generating means triggered by said sensing element operable to provide a signal to said source of motive power (14) to cause its operation to retract said boom.

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