GB2261070A

GB2261070A - Skid-steered vehicle with heading detector

Info

Publication number: GB2261070A
Application number: GB9220391A
Authority: GB
Inventors: Peter James Gibson; Peter John Cox
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1991-10-30
Filing date: 1992-09-28
Publication date: 1993-05-05
Anticipated expiration: 2012-09-28
Also published as: GB2261070B; GB9123002D0; GB9220391D0

Abstract

A skid steered vehicle includes at each side an odometer wheel (10) rotatably mounted on a shaft (11) which passes through an arm (12) to a rotary encoder (18). Signals relating to the distance travelled by each odometer wheel (10) are conveyed from each encoder (10) by a wire (20) to a common heading detector (22) which includes a first computational means for subtractively comparing the said signals and a second computational means for calculating a chassis heading related output from output generated by the first computational means. In order to function over rough terrain the vehicle preferably includes means to limit the downward displacement of the odometer wheel (10) and moans to provide the detector (22) with alternative vehicle displacement information to generate heading related output when the wheel is no longer in contact with the ground. <IMAGE>

Description

SKID STEERED VEHICLE WITH HEADING DETECTOR The present invention relates to heading detection for skid steered vehicles and is particularly applicable to tracked vehicles but is also applicable to skid steered wheeled vehicles.

When there is a requirement for operations to be performed in a hazardous environment it is often desirable to perform these operations with an autonomous vehicle. Skid steered vehicles are particularly suitable for such operations as they are able to turn in very confined spaces which means that their use inside buildings and other confined environments is possible. The traditional method used for ascertaining the direction of travel of a skid steered vehicle relies on the use of a magnetic compass. However problems frequently arise with direction determination when a vehicle relying on a magnetic compass is used in a building or other environment in which the earth's magnetic field is significantly disturbed. Autonomous operation of vehicles is difficult enough without the added problem of the operator not knowing exactly in which direction the vehicle is orientated.One alternative to a magnetic compass is a gyro compass which uses the inertia of a freely pivoted mass in order to detect direction changes. However gyro compasses are expensive and delicate items of equipment and thus add considerably to the cost and sensitivity of any item of equipment in which they are utilised.

The object of the invention is to provide a skid steered vehicle with a heading detector which will not be affected by disturbance of the earth's magnetic field and which is cheaper and less sensitive than a gyro compass.

Thus according to the invention there is provided a skid steered vehicle comprising a chassis, two odometer devices each having a ground contactable odometer wheel mounted on the chassis and laterally spaced thereon, which odometer wheels are rollingly engageable with a surface over which the vehicle passes in use, and a heading detector which is adapted to receive signals generated by the odometer devices and which includes a first computational means for subtractively comparing the said signals and a second computational means for calculating a chassis heading related output from output generated by the first computational means.

A vehicle constructed according to the invention will be able to detect heading changes regardless of disturbance to the earth's magnetic field and can be designed to be considerably more robust than a vehicle which relies on a gyro compass for detecting heading changes.

The use of odometry to determine the overall distance travelled by a vehicle is normally only applied to wheel steered vehicles in which only limited wheel slip occurs during normal operation. For this reason it is possible to use an existing ground contacting wheel of the vehicle to act as an odometer wheel. The inventors have however recognised that odometry may be employed for determining the heading of a skid steered vehicle despite the facts that the wheels or tracks used to propel such a vehicle will frequently slip with respect to the ground and also that odometers have hitherto only been used to determine the overall distance travelled by a vehicle.

In order to minimise the sideways scrub experienced by the odometer wheels when the vehicle is undergoing a turning manoeuvre the odometer wheels are preferably located within a distance of 0.15L from half way along the base length L of the vehicle. Since a skid steered vehicle can turn about an axis passing vertically through the vehicle the sideways scrub experienced by the odometer wheels will be reduced if they are located as described above. Reducing this scrub will in turn reduce the rolling resistance of the odometer wheels upon turning of the vehicle thus increasing the sensitivity of the heading detector.

The differential between the distances travelled by the two odometer wheels for a given turning manoeuvre will be increased if the lateral distance between the wheels is increased so increasing the sensitivity and accuracy of the heading detector. Thus each odometer wheel is preferably located a distance of more than 0.15W from the vehicle's centre line where W is the vehicle's wheel base or track base width.

In order that each odometer device will only generate signals relating to a forward (or backwards) resolved component of the movement of the vehicle both odometer wheels are preferably constrained to rotate about axes which are substantially perpendicular to a central longitudinal axis of the vehicle.

Each odometer device preferably includes an optical or magnetic encoder for generating signals relating to the distance travelled by its associated wheel.

In order to accommodate unevenness in the ground over which the vehicle is passing and to reduce the chance of the full weight of the vehicle being borne by an odometer wheel each odometer wheel is preferably vertically displaceably connected to the chassis. This may be conveniently achieved by means of a downwardly dependable arm which is pivotably connected to the chassis.

Preferably each odometer wheel is downwardly biased in order that the said wheel will follow unevenness in the ground rather than skip over discontinuities which would give rise to errors in heading detection.

As there will in practice be a limit to the possible downward displacement of each odometer wheel and ground contact of an odometer wheel may be lost when an odometer wheel reaches this position the vehicle preferably includes a sensor associated with each odometer wheel, which sensor is actuable when its associated odometer wheel reaches its lower displacement limit and is connected to the heading detector so as to indicate when an odometer wheel has reached the said lower displacement limit. By providing the computational device with this information the errors resulting from an odometer wheel losing contact with the ground can be reduced.Preferably the heading detector is provided with alternative vehicle displacement information (such as from wheels or tracks used for vehicle propulsion) which is used to generate the heading related output when an odometer wheel is no longer in contact with the ground. By using information relating to the forward displacement of one side of the vehicle from such an alternative source in such circumstances the errors resulting from an odometer wheel losing contact with the ground can be largely eliminated.

The vehicle may advantageously include a heading detection system comprising complementary odometer and magnetic heading detection capabilities. By combining two such systems and switching between them at appropriate times it is possible to overcome the disadvantages of having a purely odometer based system which would be prone to cumulative errors while maintaining the advantage of being able to ascertain heading in environments in which the earth's magnetic field is disturbed.

The invention will now be described by way of example only with reference to figures 1 to 3 in which; Figure 1 shows a side elevation of a vehicle according to the invention, Figure 2 shows a partial transverse cross section on the line AA of the vehicle shown in figure 1, Figure 3 shows a diagrammatic representation of the movement of the two odometer wheels of the vehicle shown in figure 1 as it turns.

The vehicle shown in figure 1 has transverse symmetry and the components shown in figure 1 on the left hand side of the vehicle are mirrored by similar components on the right hand side of the vehicle.

The vehicle has a chassis 1 supported on forward support rollers 4 and rear support rollers 6. On each side of the vehicle a drive track 2 passes round the support rollers and a drive sprocket 3. At each side of the vehicle an odometer wheel 10 is provided which is rotatably mounted on a shaft 11. The shaft 11 is journalled for rotation in an arm 12 and passes through the arm 12 into a rotary encoder 18. The shaft 11 is connected to a rotating part of the encoder and the arm 12 is connected to a non-rotating part of the encoder. Each arm 12 is connected to the chassis 1 by a pivot bolt 14 which passes through a pair of lugs 16 which are rigidly connected to the chassis. A spring 17 biases each arm downwardly in order to urge each odometer wheel against ground over which the vehicle is passing.Signals relating to the distance travelled by each odometer wheel 10 are conveyed from the rotary encoder 18 on each side of the vehicle by a wire 20 to a common heading detector 22 which is accommodated inside the vehicle. The heading detector 22 includes a first computational means for subtractively comparing the signals generated by the rotary encoders on each side of the vehicle and a second computational means for calculating a chassis heading related output from output generated by the first computational means.

Each arm 12 has a forward extension 24 which projects forwardly of the lugs 16. Above the free end of each extension arm a limit stop and contact switch 26 is connected to the vehicle chassis 1 and an electrical connection is provided between the contact switch and the heading detector 22. The limit stop serves to limit the possible downward displacement of the odometer wheel 10.

Information relating to the displacement of the track on each side of the vehicle is conveyed from a rotary encoder associated with the drive sprocket 3 via an electrical connection 28 to the heading detector 22.

The vehicle has a track base length L the centre of which is indicated in figure 1 by the centre line 8. The distance d that each odometer wheel is located behind the centre line 8 is 10% of the total distance L.

The vehicle has a track base width W between the outer edges of the tracks 2. The vehicle's transverse centre line 9 is marked on figure 2 and half of the track base width W/2 is indicated between this centre line and the outer edge of the vehicle's left hand track 2. The transverse centre line of the left hand odometer wheel is indicated at line 13 and the distance of this centre line from the outer edge of the track 2 is x which is 17% of the total distance W.

Figure 3 which represents a schematic view of the two odometer wheels of a vehicle undergoing a turning manoeuvre shows the axis of rotation 24 of the two odometer wheels 10 which is perpendicular to the central longitudinal axis 26 of the vehicle.

The manner in which the vehicle's heading variation is computed will now be described with reference to figure 3 in which y is the distance between the two odometer wheels Z is the distance covered by the left odometer wheel z is the distance covered by the right odometer wheel q is the angle through which the vehicle has turned Each encoder produces a digital output which when multiplied by a calibration constant k gives the distance travelled by each odometer wheel. During the turning manoeuvre shown in figure 3 the left and right odometer wheels produce N and n pulses respectively.The first computation means in the heading detector 22 computes the difference in distance travelled by the left and right odometer wheels (Z-z) according to the formula; Z-z = k(N-n) The second computational means within the heading detector then generates the angle q (in radians) turned by the vehicle during the manoeuvre shown in figure 3 according to the formula; q = Z-z y A positive value of q will indicate an increase in the bearing of the heading of the vehicle and a negative value of q will indicate a decrease in the bearing of the heading of the vehicle. The encoders and the heading detector are designed so that negative values of Z and z will be generated when the vehicle is travelling backwards.

Output from the heading detector relating to the vehicle's heading variation can be used for on board navigational purposes in either a manned or autonomous mode or may alternatively be transmitted externally of the vehicle for use by a person who is teleoperating the vehicle. The output may be in the form of the absolute heading of the vehicle or the heading variation.

If at any point the odometer wheel on one side of the vehicle is lowered far enough with respect to the chassis for the extension arm to come onto contact with the limit stop and contact switch 26 then a signal will be sent to the heading detector 22 which will then use information relating to track displacement (conveyed to it via wire 28) on that particular side of the vehicle in preference to information from the odometer device in order to generate the heading related output.

The system described above may also be combined with a different system for ascertaining the heading of a vehicle such as a magnetic system. By combining two such systems and switching between them at appropriate times it is possible to overcome the disadvantages of having a purely odometer based system which would be prone to cumulative errors while maintaining the advantage of being able to ascertain heading in environments in which the earth's magnetic field is disturbed.

Although the heading detector is shown located on board the vehicle it may alternatively be located remotely and be provided with signals from the odometer devices by means of a radio link, wires or some other form of telecommunication.

Claims

1. A skid steered vehicle comprising a chassis, two odometer devices each having a ground contactable odometer wheel mounted on the chassis and laterally spaced thereon, which odometer wheels are rollingly engageable with a surface over which the vehicle passes in use, and a heading detector which is adapted to receive signals generated by the odometer devices and which includes a first computational means for subtractively comparing the said signals and a second computational means for calculating a chassis heading related output from output generated by the first computational means.

2. A vehicle as claimed in claim 1 having a longitudinal wheel or track base length L wherein the odometer wheels are located within 0.15L from half way along the base length L of the vehicle.

3. A vehicle as claimed in claim 1 or claim 2 wherein each odometer wheel is located more than 0.15W from the vehicle's centre line where W is the vehicle's wheel or track base width.

4. A vehicle as claimed in any preceding claim wherein both odometer wheels are constrained to rotate about axes which are substantially perpendicular to a central longitudinal axis of the vehicle.

5. A vehicle as claimed in any preceding claim wherein each odometer device includes an optical encoder for generating signals relating to distance travelled by its associated wheel.

6. A vehicle as claimed in any preceding claim wherein each odometer device includes an magnetic encoder for generating signals relating to distance travelled by its associated wheel.

7. A vehicle as claimed in any preceding claim wherein each odometer wheel is vertically displaceably connected to the chassis.

8. A vehicle as claimed in claim 7 wherein each odometer wheel is mounted on the chassis by means of a downwardly dependable arm which is pivotably connected to the chassis in order to allow the wheel to be vertically displaceable.

9. A vehicle as claimed in claim 7 or claim 8 wherein each odometer wheel is downwardly biased.

10. A vehicle as claimed in any one of claims 7 to 9 further including a sensor associated with each odometer wheel which sensor is actuable when an odometer wheel reaches its lower displacement limit and is connected to the computational device so as to indicate when the odometer wheel has reached the lower displacement limit.

11. A vehicle as claimed in claim 10 wherein the computational device is provided with alternative vehicle displacement information which is used to generate the heading related output when an odometer wheel has reached the lower displacement limit.

12. A vehicle as claimed in any preceding claim wherein the heading detector is mounted on the vehicle.

13. A vehicle as claimed in any preceding claim wherein vehicle propulsion is effected by tracks.

14. A vehicle as claimed in any preceding claim including a heading detection system comprising complementary odometer and magnetic heading detection capabilities.

15. A vehicle substantially as hereinbefore described with reference to figures 1 to 3 of the accompanying drawings.