AU6223899A

AU6223899A - Control apparatus and a method for steering an electric vehicle

Info

Publication number: AU6223899A
Application number: AU62238/99A
Authority: AU
Inventors: Gavin Scott Pugh
Original assignee: Elliott Industries Ltd
Current assignee: Elliott Industries Ltd
Priority date: 1998-10-21
Filing date: 1999-10-21
Publication date: 2000-05-08
Also published as: GB2342903A; CA2347688A1; GB9823055D0; WO2000023297A1; EP1123223A1

Description

WO 00/23297 PCT/GB99/03480 - 1 CONTROL APPARATUS AND A METHOD FOR STEERING AN ELECTRIC VEHICLE The present invention relates to electric 5 vehicles, and in particular to electrically powered vehicles such as electric wheelchairs in which steering is achieved by differentially driving a left wheel and a right wheel (i.e. driving them at different angular velocities). 10 Electric vehicles in which steering is achieved by driving left and right wheels at different respective angular velocities are well-known, and an example is shown schematically in Fig. 1. The vehicle 40 comprises input means 1, connected to drive 15 apparatus 2 which is arranged to drive a left wheel 32 and a right wheel 33. The left and right wheels are typically arranged to rotate about a common axis. The input means 1 is operable by a user or driver of the vehicle to generate signals 11,12 indicative of 20 a desired forward/backwards motion of the vehicle and a desired turning motion. These two signals may be separate components of a single signal. In the case of electric wheelchairs the input means 1 is typically a single joystick, to facilitate 25 control. The joystick is typically biased to a centre position and for a given deflection away from this centre position generates two signals, an X signal and a Y signal, respectively indicative of the components of the deflection along two orthogonal axes, the X and 30 Y axes. The Y axis is aligned with the forward direction of the vehicle and the Y signal is used as the desired forward motion signal 12. The Y signal is typically proportional to the magnitude of the component of 35 joystick deflection along the Y axis, and is positive for deflections in the forward direction and negative WO 00/23297 PCT/GB99/03480 - 2 for deflections towards the reverse. The X signal is used as the desired turning motion signal 11. The X signal is typically proportional to the magnitude of the component of 5 joystick deflection along the X axis, and is positive for deflections to the right of the vehicle, and negative for deflections to the left. Thus, when X and Y are both positive, the user is indicating a desire to move forwards and turn to the 10 right. When X is zero no turning motion is required, and when Y is zero and X is non-zero the user is indicating a desire to turn the vehicle "on the spot" i.e. substantially with no forward or reverse movement. 15 The joystick design may inherently limit the maximum deflections (and hence the maximum values of the X and Y signals),but it is also known for the joystick assembly to incorporate a template having an aperture which defines and limits the range of 20 deflections available. An example of such a template and the available movement of the joystick within it is shown in Fig. 2. The nominal X and Y axes are shown, with the sprung centre position located at the origin. The joystick is shown at an arbitrary 25 position P, and the current "forward" and turning motion signals are Y, and X. respectively. The aperture 142 in the template 141 in this example is square and the joystick movement is bounded by the sides 14 of the aperture 142. In this example the X 30 signal is constrained to lie in the range -X 0 to +X 0 and the Y signal is constrained in the range -Y 0 to +Yo Returning now to Fig. 1, the drive apparatus 2 comprises a battery 24, a controller 21 and two 35 electric motors 22,23. In response to the input signals 11,12 from the input means l,the controller 21 WO 00/23297 PCT/GB99/03480 - 3 controls the supply of power from the battery 24 to each motor, 22, 23. When the X signal is zero, the controller 21 determines the supply of power to the motors according to the Y signal such that the two 5 wheels are driven substantially at the same angular velocity, assuming of course that the wheels are of the same diameter. Usually, the two motors, 22,23 and the drive trains to the wheels are nominally identical and so the controller simply controls the motors to 10 rotate at the same speed, usually controlling the motor speed with reference to a demand voltage derived from the X and Y signals. When the X signal is non-zero, the drive apparatus 2 drives the two wheels at different 15 respective angular velocities to provide turning or steering motion. In the past, the drive apparatus has been arranged to drive the left and right wheels substantially according to the following 20 relationships: =R = - A = OL + A) where Aw = KX 25 = K Y GR and cL are the angular velocities of the right and left wheels respectively, A represents the average angular velocity of the two wheels (which is 30 proportional to the Y signal), 2Ae is the difference between the angular velocities of the left and right wheels (which is proportional to X) and K x and K are constants. Thus the angular velocity of each wheel is a 35 function of both the X and Y signals, but the difference between the left and right angular WO 00/23297 PCT/GB99/03480 - 4 velocities is a function of the X signal only. Similarly, the average angular velocity of the left and right wheels is a function of Y only. For Y = zero, a non-zero value of X results in 5 the two wheels being driven at the same speed but in opposite directions. For sufficiently large values of Y, a non-zero value of X will result in the two wheels being driven at different speeds in the same direction. 10 Figure 3 shows the variation of Aw with X for the joystick and template arrangement of Fig. 2. As is a linear function of X and full deflection of the joystick to the right results in a maximum angular velocity difference of magnitude 2 K x

X

0 being applied 15 between the two wheels. It is convenient to represent the dependence of Aw on joystick position by plotting lines of constant A 15 on the field of joystick movement bounded by the sides 14 of the aperture in the template. Such lines 20 15 are shown in Fig. 2 in the forward half (i.e. Y > 0) only for convenience, and broken lines 15B link these "contours" 15 to corresponding positions on the aw versus X graph in Fig. 3. Similar contours can, of course, be drawn in the reverse half. The lines 15 in 25 Fig. 2 are parallel to the Y axis, indicating that Am has no dependence on Y. The aw interval between any two adjacent contours 15 is constant, and as aw is a linear function of X the contours are equally spaced along the X axis. 30 Figure 4 shows schematically the motion of a vehicle such as that shown in Figure 1 in which the left and right wheels are driven according to the above equations, i.e. Fig 4 shows the motion of a vehicle driven in accordance with the prior art. For 35 simplicity, K. and K have been set to 1. Position A represents the start position of the left and right WO 00/23297 PCT/GB99/03480 - 5 wheels 32, 33 of the vehicle at time T = 0. The wheels are parallel, are arranged to rotate about a common axis and are separated by a distance of 2L. Positions B and C represent the positions of the two wheels 5 after the same time interval t for respective different joystick positions, i.e. they represent the positions resulting from different inputs to the controller. B indicates the position of the two wheels after 10 time t for X = -1, Y = +3. Thus: OR = 3 + 1 = 4 (R = 3 - 1 = 2 w = 3 15 C indicates the position of the two wheels after time t for X = -1, Y = +6. Thus: CR = 6 + 1 = 7 OR = 6 - 1 = 5 20 w = 6 So, in the given time interval t, in the first case the right wheel 33 travels twice as far as the left wheel 32 and for the wheel separation 2L the vehicle 25 travels along a circular path RB of radius 3L. The right wheel 33 follows a circular path RRB of radius 4L and the left wheel 32 follows a circular path RLB of radius 2L. For the second case,case C, the ratio of 30 distances travelled by the right and left wheels in the given time interval t is 7:5. The centre of the vehicle travels along a circular path R e of radius 6L, with the right and left wheels following circular paths Rrc, R 1 c of radii 7L and 5L respectively. 35 Comparing these two cases, although the turning signal was the same in each (X = -1), by increasing the Y WO 00/23297 PCT/GB99/03480 - 6 signal from 3 to 6 necessarily results in the vehicle following a circular path of increased radius. In other words, for a given turning signal, the greater the forward motion signal the less tightly can the 5 vehicle turn. This effect has been an inherent feature of prior art vehicles and in many cases is undesirable. For example, if values of the constants K and K, are chosen to give appropriate steering and turning 10 rates at low speeds, then the vehicle may suffer from understeer at high speeds. Similarly, if the constants are chosen to give appropriate steering at high speed operation of the vehicle, then the low speed steering may be too sensitive. 15 Understeer at high speed can be exacerbated by the use of joystick templates whose apertures 142 have the shapes shown in Figures 5 and 6. The template aperture 142 of Figure 5 is a truncated diamond shape with the diagonals of the 20 diamond aligned with the X and Y axes. This template aperture shape is well-known in the art and acts to restrict the left/right movement of the joystick as Y increases from zero. The front edge 14f of the aperture is substantially parallel to the X axis and 25 ensures that for maximum forward deflection (Y = Ymax) a certain degree of steering is still available. If the diamond were not truncated, of course, for maximum forward deflection the joystick would not be able to move in the X direction. An advantage of this shape 30 template is that it provides well defined positions for the joystick, corresponding to particular motions, and which can be "felt" by the operator. For example, if just turning motion is required, the joystick can be pushed into the left-hand corner of the diamond and 35 can easily be held there without generating a non-zero Y signal.

WO 00/23297 PCT/GB99/03480 - 7 Another advantage of restrictive templates is associated with the fact that some joysticks have non linear responses when used in the far corners of their travel. At these extremes, the X and/or Y signals may 5 no longer be substantially proportional to the X & Y deflections of the joystick. Advantageously, a template may be used to restrict the movement of the joystick to the "linear" region, i.e. prevent its use at these extremes. 10 Contours 15 representing constant values of Aw are shown in the figure for positive Y only,and it is clear that the largest values of am are not available at the highest forward deflection. At the maximum forward deflection Ymax the maximum range of X 15 deflection is AX, which is significantly less than the range -X 0 to +X 0 available for Y = 0. Thus, the truncated diamond template further increases the understeer problem at high speeds associated with the prior art vehicles. 20 Referring now to Figure 6, another well-known template aperture shape is a circle. This shape may be chosen to give a uniform joystick feel which in some applications may be desirable. As with the truncated diamond however the circular template 25 aperture 142 restricts the left-right movement of the joystick as Y increases and in the past has exacerbated the high speed understeer problem. It is therefore desirable to provide control apparatus, a controller, an electric vehicle, and a 30 method of steering an electric vehicle which address the problems associated with the prior art. According to a first aspect of the present invention there is provided control apparatus for an electric vehicle, the apparatus comprising: 35 input means operable to generate a signal indicative of a desired forward motion of the vehicle WO 00/23297 PCT/GB99/03480 - 8 and a signal indicative of a desired turning motion of the vehicle: and drive apparatus arranged to drive a left wheel and a right wheel of the vehicle, and operable to 5 drive the left and right wheels at different respective angular velocities in response to the desired turning motion signal to provide turning motion, the drive apparatus being further arranged to 10 determine the difference between said different respective angular velocities according to the desired forward motion signal. The input means may be a joystick and the desired forward motion signal and desired turning motion signal may be components of a 15 single signal. In prior art control apparatus the difference between the angular velocities at which the left and right wheels were driven was a function of the desired turning motion signal (i.e. the X signal) only. In 20 contrast, according to the present invention, the difference is now determined in accordance with both the desired turning motion signal and the desired forward motion signal. Thus, in embodiments of the present invention the angular velocity difference is a 25 function of both the X and Y signals. The drive apparatus may comprise a controller which includes a microprocessor. Conveniently, the microprocessor may be arranged to calculate a quantity indicative of the angular velocity difference using 30 the two input signals, and, using that quantity, the drive apparatus may be arranged to set or adjust the difference. In the past, because the angular velocity difference was determined only by the steering signal, 35 the steering or turning characteristics of the vehicle for certain values of the desired forward motion WO 00/23297 PCT/GB99/03480 - 9 signal were necessarily a compromise. Advantageously, by determining the angular velocity difference in accordance with the desired forward motion signal, the present invention enables the steering or turning 5 characteristics to be optimised or at least improved over the entire range of values of the desired forward motion signal. As stated above, the drive apparatus may advantageously comprise a microprocessor, and a wide variety of dependencies of angular velocity 10 difference on desired forward motion signal may be achieved by appropriate programming. Advantageously, the desired forward motion signal may be indicative of a desired forward component of velocity of the vehicle, and in response to a constant 15 desired turning motion signal the drive apparatus may be arranged to increase the angular velocity difference in response to an increase in the desired forward component of velocity. Thus, in embodiments where the input means is a joystick, for a given X 20 deflection, as the joystick is pushed further forwards the angular velocity difference applied to the left and right wheels may be increased, thus counteracting the understeer which would result if the angular velocity difference remained constant. 25 Advantageously, therefore, the present invention may provide increased steering at high speeds compared with conventional arrangements. It will be apparent that the control apparatus may be suitably arranged so that for a given turning signal the vehicle may follow 30 substantially the same path, albeit at different speeds, for a range of different values of the desired forward motion signal. Advantageously, the input means may be a joystick incorporating an apertured template arranged to 35 increasingly restrict the left-right movement of the joystick as its forward deflection is increased. The WO00/23297 PCT/GB99/03480 - 10 drive apparatus may be arranged to increase the angular velocity difference with increasing forward joystick deflection to counteract the limiting effect of the template, and so enables templates to be chosen 5 for their "feel" and operational convenience without necessarily reducing the amount of steering available at high speeds. For some electric vehicles it may not be safe to provide full (maximum) steering and full speed 10 simultaneously. However it may still be desirable or necessary to provide increased steering gain (i.e. increased angular velocity difference for a given joystick X deflection) at high speed in order to compensate for a very strong understeer 15 characteristic, especially if a restrictive template is being used. Embodiments of the present invention may provide this increased steering gain. Alternatively the drive apparatus may be arranged to reduce the angular velocity difference for a given 20 desired turning motion signal in response to an increase in the desired forward motion or velocity signal. Advantageously, this may help prevent the vehicle from overturning at high speeds. Furthermore, the amount of steering available at high speeds can 25 thus be reduced without the need to use an increasingly restrictive template. It will be apparent that the constraining effects of known templates may be replicated by the appropriate arrangement of the dependency of the 30 angular velocity difference on the desired forward motion signal and the desired turning motion signal, in conjunction with a simple square template. The drive apparatus may be further arranged to determine the difference independently of the desired 35 forward motion signal over a predetermined range of values of the desired forward motion signal. Thus, the WO00/23297 PCT/GB99/03480 - 11 angular velocity difference may be a function of the desired turning motion signal only in a first range, and a function of both the desired turning motion signal and the desired forward motion signal over a 5 second range. In the second range the difference may increase with increasing desired forward motion signal, and, in this way, increased turning or steering can be provided at high speeds whilst leaving the low speed steering characteristics (i.e. in the 10 first range) unchanged. The control apparatus may further comprise means for generating a signal indicative of the forward component of velocity of the vehicle (its forward speed) and the drive apparatus may be further arranged 15 to determine the angular velocity difference applied to the wheels according to the forward component of velocity signal. According to a second aspect of the present invention there is provided control apparatus for an 20 electric vehicle, the apparatus comprising: input means operable to generate a signal indicative of a desired turning motion of the vehicle: and drive apparatus arranged to drive a left wheel 25 and a right wheel of the vehicle and operable to drive the left and right wheels at different respective angular velocities in response to the turning signal to provide turning motion; and means for generating a signal indicative of a 30 forward component of velocity of the vehicle, the drive apparatus being further arranged to determine the difference between said different respective angular velocities according to the forward component of velocity signal. The input means may 35 also be operable to generate a signal indicative of a desired forward motion of the vehicle. However, this WO00/23297 PCT/GB99/03480 - 12 desired forward motion signal need not be proportional to a desired speed (demand speed). Instead, this signal may essentially have two states, on and off, representing a desire to accelerate or brake 5 respectively. Advantageously,by determining the angular velocity difference used to achieve steering according to the actual speed of the vehicle, this aspect of the present invention can provide appropriate safe 10 steering characteristics independently of the desired forward motion input signal. Again, the input means may be a joystick, or alternatively may be a steering wheel. The desired forward motion signal may be generated by a joystick, 15 or alternatively by a pedal, switch or other means. Advantageously the drive apparatus may be arranged to increase the difference for a given desired turning motion signal in response to an increase in the forward component of velocity. For 20 constant desired turning motion signal, the angular velocity difference may be linearly dependent on the desired forward motion signal or forward component of velocity signal. Again, this may provide the advantage of 25 increased steering at high speeds compared with prior art arrangements. Alternatively the angular velocity difference may be arranged to decrease in response to an increase in velocity (desired or actual). This can be a safety 30 feature, preventing overturning at high speeds. The control apparatus may further comprise means for generating a signal indicative of an acceleration of the vehicle, and the angular velocity difference may be a function of the acceleration signal. 35 According to a third aspect of the present invention there is provided a controller for an WO00/23297 PCT/GB99/03480 - 13 electric vehicle, the controller comprising: a first input for receiving a speed demand signal indicative of a desired speed of the vehicle; a second input for receiving a steering signal 5 indicative of a desired turning motion of the vehicle; a first output for outputting a first signal to control a motor driving a left wheel of the vehicle; and a second output for outputting a second signal to 10 control a motor driving a right wheel of the vehicle, the controller being arranged to generate said first and second signals and to determine a difference between said first and second signals according to both the received speed demand signal and steering 15 signal. The speed demand signal will typically be the Y output of a joystick and the steering signal will be the X output from a joystick, although alternative input means may be used. 20 The controller may conveniently comprise a microprocessor programmed to calculate a quantity indicative of the difference from the speed demand signal and the steering signal. The circuitry of the controller is then arranged to set the output signals 25 accordingly. The output signals will typically be control voltages, and so the difference calculated and set by the controller may be a voltage difference. According to a fourth aspect of the present invention there is provided a method of steering an 30 electric vehicle comprising the steps of: generating a first signal indicative of a desired turning motion of the vehicle; generating a second signal indicative of a desired forward motion of the vehicle; 35 driving a left wheel and a right wheel of the vehicle at different respective angular velocities in WO00/23297 PCT/GB99/03480 - 14 response to the first signal; and determining the difference between said different respective angular velocities according to both the first signal and the second signal. 5 According to a fifth aspect of the present invention there is provided a method of steering an electric vehicle comprising the steps of: generating a first signal indicative of a desired turning motion of the vehicle; 10 generating a second signal indicative of speed of the vehicle; driving a left wheel and a right wheel of the vehicle at different respective angular velocities in response to the first signal; and 15 determining the difference between said different respective angular velocities according to both the first signal and the second signal. The determining step may comprise the step of increasing the difference in response to an increase 20 in the second signal, or, alternatively, the step of decreasing the difference in response to an increase in the second signal. Thus, the method can counteract the tendency to understeer at high speeds, or reduce the steering at high speeds to prevent overturning and 25 therefore improve safety. The electric vehicle may comprise a microprocessor and the step of determining the difference may include the step of calculating a quantity representative of the difference using the 30 first signal and the second signal. The difference may be determined substantially according to the relationship D = k i X + k 2 X (Y - YT) for Y>Y ,T where D is said difference, X is the value 35 of the first signal, Y is the value of the second signal, K i and K2 are constants and YT is a WO00/23297 PCT/GB99/03480 - 15 predetermined threshold value of the second signal. For values of Y < YT the difference may be determined according to the relationship D = K i X. Embodiments of the present invention will now be 5 described with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of an electric vehicle in accordance with the prior art and an embodiment of the present invention; 10 Fig. 2 is a schematic diagram of the range of joystick movement within a template suitable for use in embodiments of the present invention; Fig. 3 is a graph of angular velocity difference as a function of X (left-right joystick deflection) 15 determined by control apparatus in accordance with the prior art; Fig. 4 is a schematic diagram of the motion of an electric vehicle steered in accordance with the prior art; 20 Fig. 5 is a schematic diagram of a known joystick template and contours representing a known dependence of angular velocity difference on joystick position; Fig. 6 is a schematic diagram of another known joystick template and contours representing a known 25 dependence of angular velocity difference on joystick position within the template; Fig. 7 is a schematic diagram of the forward half of a known joystick template aperture and contours representing the dependence of angular velocity 30 difference on joystick position in accordance with an embodiment of the present invention; Fig. 8 is a schematic diagram of the forward half of another known joystick template aperture and contours representing the dependence on joystick 35 position of angular velocity difference in accordance with another embodiment of the present invention; WO00/23297 PCT/GB99/03480 - 16 Fig. 9 is a schematic diagram of the forward half of another known joystick template aperture and contours representing the dependence on joystick position of angular velocity difference in accordance 5 with another embodiment of the present invention; Fig. 10 is a schematic diagram of the forward half of another known joystick template aperture and contours representing the dependence on joystick position of angular velocity difference in accordance 10 with another embodiment of the present invention; Fig. 11 is a schematic diagram of the forward half of another known joystick template aperture and contours representing the dependence on joystick position of angular velocity difference in accordance 15 with another embodiment of the present invention; and Fig. 12 is a schematic diagram of an electric vehicle in accordance with a further embodiment of the present invention. Referring now to Fig. 7, in a first embodiment of 20 the present invention the input means is a joystick incorporating a template having a rectangular aperture 142. The joystick is moveable within the confines of this aperture in the X and Y directions. Fig. 7 shows just the forward half of the template aperture 142 for 25 convenience. In this embodiment the drive apparatus sets the difference between the angular velocities of the left and right wheels in response to the input signals according to the relationships: 30 WR= A) - A(A) (A L A + A where aw = k 1 X + k 2 XY and k 1 & k 2 are constants. 35 Thus, for constant X,aw is proportional to Y, and as Y increases so does the angular velocity difference set WO00/23297 PCT/GB99/03480 - 17 between the left and right wheels. c may be proportional to Y, or may be a different function of Y. Contours 15 indicating lines of constant aw are plotted within the template aperture 5 142 to show the dependence of Aw on joystick position. The aw interval between adjacent contours is equal, and, for a given Y value, as aw is proportional to X, the contours are equally spaced along the X axis. 10 Unlike the prior art arrangements in which the aw contours were parallel to the Y axis, in embodiments of the present invention the dependence of o on Y over at least a range of its values necessarily means that the A contours are at some point non-parallel to 15 the Y axis. In the above embodiment the dependence of aw on Y is clearly illustrated by the convergence of the contours with increasing Y. In certain embodiments, similar contours may be drawn in the "reverse half" of the template. 20 Moving on to Fig. 8, in a second embodiment of the present invention a controller is arranged to receive signals from a joystick incorporating a conventional "truncated diamond" template. The forward half of the template aperture 142 is shown in 25 the figure. The controller is programmed to calculate a quantity D from the input X and Y signals according to the equations: D=klX + k 2 X (Y - YT) for Y YT and D=k 1 X for Y s YT 30 where YT is a predetermined threshold value of the Y signal. Drive apparatus is then arranged to adjust the angular velocities of the left and right wheels 35 according to the relationships: WO00/23297 PCT/GB99/03480 - 18 CR = - k 3D

S

= + k 3 D i.e. am = k 3 D where k 3 is a constant. 5 Thus, in this embodiment there is a range of values of Y (in fact 0<Y<YT) over which the angular velocity difference determined by the controller and drive chain (train) is independent of the Y signal. Above this range the angular velocity difference is an 10 increasing function of Y. Contours of constant aw are plotted on the figure and the two regimes are clearly distinguishable. K may be adjusted, and alters the separation of the contours along the X axis. 15 K2 may be adjusted to alter the slope of the contours with respect to the Y axis in the second, upper range. YT may be adjusted to alter the position of the transition from one dependence to the other. 20 By adjusting the constants in this way, satisfactory steering performance may be achieved over the entire range of values of X and Y. Increasing aw with Y above YT counteracts the restricting effect of the template aperture 142. 25 In a further embodiment, an electric vehicle includes a velocity sensor arranged to generate a signal V indicative of the speed of the vehicle in the forward direction. In this embodiment the angular velocity difference between the left and right wheels 30 is adjusted according to the relationship aw = KiX + k 2 VX In a further embodiment, aw is set according to: aw = k 1 X + k 2 X (V-VT) for VVV T and aw = K X for V5VT 35 where VT is a predetermined threshold value of the velocity signal V.

WO00/23297 PCT/GB99/03480 - 19 In yet a further embodiment,Am = K i X + K 2 XY +

K

4 XV, i.e. A) is a function of X, Y and the vehicle's velocity. 5 It will be apparent that many other dependencies may be utilised by embodiments of the present invention to suit particular circumstances. Fig. 9 shows the forward half of a circular template aperture incorporated in the joystick of a 10 further embodiment of the present invention. In this embodiment, the drive apparatus is arranged such that the angular velocity difference is a function of both X and Y over a limited range of Y values (i.e. between Y = YT1 and Y = YT 2 ). Below the lower threshold YT and 15 above the upper threshold YT2 the angular velocity difference is independent of Y. The three different regimes can be clearly seen on Fig. 9 in which contours representing constant angular velocity differences are again plotted. 20 Fig. 10 shows yet another template aperture incorporated in an embodiment of the present invention. In this embodiment the angular velocity difference is a non-linear function of both X and Y over their entire range of values in the forward half 25 of the template aperture. For any given value of X, the angular velocity difference increases as Y increases. By appropriate arrangement of the dependence of angular velocity difference on X and Y and the shape of the template aperture the steering 30 characteristics achieved with a square apertured template and drive apparatus in accordance with the prior art can be reproduced. The template aperture shown in Fig. 10 will of course provide a different feel. 35 Referring now to Fig. 11, in another embodiment of the present invention the input means comprises a WO00/23297 PCT/GB99/03480 - 20 joystick incorporating a template having a rectangular aperture 142. The figure shows the forward half of this aperture and contours 15 of constant angular velocity difference. In this embodiment the drive 5 apparatus determines the difference between the different respective angular velocities of the left and right wheels according to the relationship; Am = kzX - k 2 XY Thus,for a given value of X, as Y increases the 10 angular velocity difference is reduced. Therefore, it is not possible to turn the vehicle as sharply at high speed as it is at low speeds. Although in certain applications this reduction in high speed steering may be undesirable, in other applications it may be a 15 beneficial safety feature, preventing the vehicle from overturning. Although a similar restriction in high speed steering has been achieved in the prior art by using appropriately shaped (i.e. tapering) template 20 apertures, the present embodiment provides the advantage that this high speed steering reduction can be achieved by appropriate programming of a drive apparatus controller without the need to change the hardware (i.e. the rectangular template can be 25 retained). In Fig. 11, the fact that the angular velocity difference decreases with increasing Y is characterised by the divergence of the contours 15. Referring now to Fig. 12, an electric vehicle 40 30 embodying the present invention comprises input means 1 inputting steering and forward motion signals 11,12 to drive apparatus arranged to drive left and right wheels, 32, 33 of the vehicle. In this embodiment, the input means 1 includes a steering wheel 101 35 arranged to generate a steering signal 11 and a forward motion signal generating means 102 comprising WO00/23297 PCT/GB99/03480 - 21 a switch S having two states. The driver of the vehicle places the switch S in the on state to indicate a desire to accelerate the vehicle, and switches to the off state to indicate a desire to 5 brake. The switch S may be in the form of a pedal. In alternative embodiments, rather than being a two state device, the pedal may be operable progressively to give a continuously variable forward motion signal. A controller 21 in the drive apparatus controls the 10 motion of the vehicle according to the steering and forward motion signals. According to the signals, the controller controls the supply of power from a battery 24 to motors 22, 23 arranged to drive the left and right wheels of the vehicle 32,33 via respective 15 gearboxes 222, 232. The drive apparatus further comprises velocity sensors 252,253 which are arranged to generate signals VL and V R indicative of the forward components of velocity of the left and right hand sides of the 20 vehicle. The signals VL and VR are received by averaging means 25 which generates and outputs a signal V indicative of the average forward component of velocity of the vehicle. In response to a non-zero steering signal 11 the controller outputs different 25 control voltages to the two motors 22,23 in order to drive the left and right wheels at different respective angular velocities. The controller determines the difference between these two control voltages according to the input steering signal 11 and 30 the average velocity signal V. The controller 21 incorporates a microprocessor and is programmed to control the motion of the vehicle in an appropriate manner. For example, by adjusting the angular velocity difference between the two wheels 35 according to both the current steering signal and the actual average velocity of the vehicle, the controller WO00/23297 PCT/GB99/03480 - 22 can ensure that a safe degree of steering is provided at all speeds, regardless of the operation of the input controls. Although the above description refers to 5 embodiments of the present invention in which the angular velocity difference is determined according to the desired forward motion signal and/or a forward component of velocity signal, it will be apparent that the present invention may also be applied to reverse 10 motion. Indeed, throughout this specification (which term includes the claims) the term "forward" may be replaced by "forward or reverse" or "reverse". Also, it will be apparent that the present invention may be applied to forward motion only, 15 reverse motion only, or both forward and reverse motion. In certain embodiments, a dependency of angular velocity difference on desired (demand) or actual reverse motion may not be required as the vehicle may 20 be arranged to travel more slowly in reverse for a given Y axis deflection of the joystick than for the same deflection in the forward direction. Thus, understeer may not be a problem. Also, when using diamond or truncated diamond 25 templates, the template typically tapers to a central point in reverse, so that the vehicle reverses in a straight line at full reverse speed. However, it may be desirable to employ the present invention for reverse motion where the 30 joystick has a circular limiter (template) and a high reverse speed (giving a tendency to understeer). Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention 35 independently of other disclosed and/or illustrated features.

Claims

1. Control apparatus for an electric vehicle, the 5 apparatus comprising: input means operable to generate a signal indicative of a desired forward motion of the vehicle and a signal indicative of a desired turning motion of the vehicle: and 10 drive apparatus arranged to drive a left wheel and a right wheel of the vehicle, and operable to drive the left and right wheels at different respective angular velocities in response to the desired turning motion signal to provide turning 15 motion, the drive apparatus being further arranged to determine the difference between said different respective angular velocities according to the desired forward motion signal. 20

2. Control apparatus in accordance with claim 1, wherein the desired forward motion signal is indicative of a desired forward component of velocity of the vehicle, and in response to a constant desired 25 turning motion signal the drive apparatus is arranged to increase said difference,in response to a change in the desired forward motion signal, said change indicating an increase in the desired forward component of velocity. 30

3. Control apparatus in accordance with claim 1, wherein the desired forward motion signal is indicative of a desired forward component of velocity of the vehicle, and in response to a constant desired 35 turning motion signal the drive apparatus is arranged to decrease said difference in response to a change in WO00/23297 PCT/GB99/03480 - 24 the desired forward motion signal, said change indicating an increase in the desired forward component of velocity. 5

4. Control apparatus in accordance with any one of claims 1 to 3 wherein the drive apparatus is arranged to determine said difference independently of the desired forward motion signal over a predetermined range of values of the desired forward motion signal. 10

5. Control apparatus in accordance with any one of claims 1 to 4 further comprising means for generating a signal indicative of the forward component of velocity of the vehicle, the drive apparatus being 15 further arranged to determine said difference according to the forward component of velocity signal.

6. Control apparatus for an electric vehicle, the apparatus comprising: 20 input means operable to generate a signal indicative of a desired turning motion of the vehicle: and drive apparatus arranged to drive a left wheel and a right wheel of the vehicle and operable to drive 25 the left and right wheels at different respective angular velocities in response to the turning signal to provide turning motion; and means for generating a signal indicative of a forward component of velocity of the vehicle, 30 the drive apparatus being further arranged to determine the difference between said different respective angular velocities according to the forward component of velocity signal. 35

7. Control apparatus in accordance with claim 5 or claim 6 wherein the drive apparatus is arranged to WO00/23297 PCT/GB99/03480 - 25 increase said difference in response to an increase in the forward component of velocity.

8. Control apparatus in accordance with claim 5 or 5 claim 6 wherein the drive apparatus is arranged to decrease said difference in response to an increase in the forward component of velocity.

9. Control apparatus in accordance with any one of 10 claims 6 to 8 wherein the drive apparatus is arranged to determine said difference independently of the forward component of velocity signal over a predetermined range of values of the forward component of velocity signal. 15

10. Control apparatus in accordance with any preceding claim further comprising means for generating a signal indicative of an acceleration of the vehicle, the drive apparatus being further 20 arranged to determine said difference according to the acceleration signal.

11. A controller for an electric vehicle, the controller comprising: 25 a first input for receiving a speed demand signal indicative of a desired speed of the vehicle; a second input for receiving a steering signal indicative of a desired turning motion of the vehicle; a first output for outputting a first signal to 30 control a motor driving a left wheel of the vehicle; and a second output for outputting a second signal to control a motor driving a right wheel of the vehicle, the controller being arranged to generate said 35 first and second signals and to determine a difference between said first and second signals according to WO00/23297 PCT/GB99/03480 - 26 both the received speed demand signal and steering signal.

12. A controller in accordance with claim 11 further 5 arranged to increase said difference in response to an increase in the speed demand signal.

13. A controller in accordance with claim 11 further arranged to reduce said difference in response to an 10 increase in the speed demand signal.

14. A controller in accordance with any one of claims 11 to 13 further arranged to determine said difference independently of the speed demand signal over a 15 predetermined range of values of the speed demand signal.

15. An electric vehicle comprising control apparatus in accordance with any one of claims 1 to 10. 20

16. An electric vehicle comprising a controller in accordance with any one of claims 11 to 14.

17. A method of steering an electric vehicle 25 comprising the steps of: generating a first signal indicative of a desired turning motion of the vehicle; generating a second signal indicative of a desired forward motion of the vehicle; 30 driving a left wheel and a right wheel of the vehicle at different respective angular velocities in response to the first signal; and determining the difference between said different respective angular velocities according to 35 both the first signal and the second signal. WO00/23297 PCT/GB99/03480 - 27

18. A method of steering an electric vehicle comprising the steps of: generating a first signal indicative of a desired turning motion of the vehicle; 5 generating a second signal indicative of a speed of the vehicle; driving a left wheel and a right wheel of the vehicle at different respective angular velocities in response to the first signal; and 10 determining the difference between said different respective angular velocities according to both the first signal and the second signal.

19. A method in accordance with claim 17 or 18 15 wherein the determining step comprises the step of increasing said difference in response to an increase in said second signal.

20. A method in accordance with claim 17 or 18 20 wherein the determining step comprises the step of decreasing said difference in response to an increase in said second signal.

21. A method in accordance with any one of claims 17 25 to 20 further comprising the step of determining said difference independently of the second signal over a predetermined range of values of the second signal.

22. A method of steering an electric vehicle in 30 accordance with any one of claims 17 to 21 wherein said difference is determined substantially according to the relationship: D = k i X + k 2 X (Y - YT) 35 for Y>YT, where D is said difference, X is the value WO00/23297 PCT/GB99/03480 - 28 of the first signal, Y is the value of the second signal, K i and K2 are constants and YT is a predetermined threshold value of the second signal. 5

23. A method in accordance with claim 22 wherein said difference is determined according to the relationship: D =k X 10 for Y <Y .

24. A method in accordance with claim 22 or claim 23 wherein X is proportional to a forward component of a 15 deflection of a joystick and Y is proportional to a sideways component of said deflection.

25. Control apparatus in accordance with any of claims 1 to 10 wherein the input means comprises a 20 joystick assembly including an aperture in a template arranged to limit the movement of the joystick.

26. Control apparatus in accordance with claim 25 wherein the template is arranged to increasingly 25 restrict left-right movement of the joystick with increasing forward deflection of the joystick.

27. An electric vehicle comprising control apparatus in accordance with claim 25 or 26. 30

28. Control apparatus substantially as hereinbefore described with reference to figures 1 and 7 to 12 of the accompanying drawings. 35

29. A controller substantially as hereinbefore described with reference to figures 1 and 7 to 12 of WO00/23297 PCT/GB99/03480 - 29 the accompanying drawings.

30. An electric vehicle substantially as hereinbefore described with reference to figures 1 and 7 to 12 of 5 the accompanying drawings.

31. A method steering an electric vehicle substantially as hereinbefore described with reference to figures 1 and 7 to 12 of the accompanying drawings.