Note: Descriptions are shown in the official language in which they were submitted.
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CONTROL APPARATUS AND A METHOD FOR STEERING AN
ELECTRIC VEHICLE
The present invention relates to electric
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).
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
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
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
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
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
joystick deflection along the Y axis, and is positive
for deflections in the forward direction and negative
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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
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
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.
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
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
position P, and the current "forward" and turning
motion signals are YP and Xe, 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
signal is constrained to lie in the range -Xo to +Xo
and the Y signal is constrained in the range -Yo to
+Yo .
Returning now to Fig. 1, the drive apparatus 2
comprises a battery 24, a controller 21 and two
electric motors 22,23. In response to the input
signals 11,12 from the input means l,the controller 21
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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
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
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
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
relationships:
(JL=CJ+~CJ
where oca = ICxX
2 5 c.~ = KYY
c~R and caL are the angular velocities of the right and
left wheels respectively, c~ represents the average
angular velocity of the two wheels (which is
proportional to the Y signal), 2oca is the difference
between the angular velocities of the left and right
wheels (which is proportional to X) and K,~ and K~, are
constants.
Thus the angular velocity of each wheel is a
function of both the X and Y signals, but the
difference between the left and right angular
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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
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.
Figure 3 shows the variation of 4w with X for the
joystick and template arrangement of Fig. 2. oc~ 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 F~ Xo being applied
between the two wheels.
It is convenient to represent the dependence of
oc~ on joystick position by plotting lines of constant
oca 15 on the field of joystick movement bounded by the
sides 14 of the aperture in the template. Such lines
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
ow versus X graph in Fig. 3. Similar contours can, of
course, be drawn in the reverse half. The lines 15 in
Fig. 2 are parallel to the Y axis, indicating that ow
has no dependence on Y. The ow interval between any
two adjacent contours 15 is constant, and as oca is a
linear function of X the contours are equally spaced
along the X axis.
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
simplicity, Kx and K7, have been set to 1. Position A
represents the start position of the left and right
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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
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
time t for X = -1, Y = +3. Thus:
wR = 3 + 1 - 4
wR = 3 - 1 - 2
w = 3
C indicates the position of the two wheels after time
t for X = -1, Y = +6. Thus:
wR = 6 + 1 - 7
wR = 6 - 1 = 5
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
travels along a circular path RB of radius 3L. The
right wheel 33 follows a circular path RRa of radius
4L and the left wheel 32 follows a circular path R~
of radius 2L.
For the second case,case C, the ratio of
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~ of radius 6L,
with the right and left wheels following circular
paths Rr~, Rl~ of radii 7L and 5L respectively.
Comparing these two cases, although the turning signal
was the same in each (X = -1), by increasing the Y
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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
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 I~, and K,
are chosen to give appropriate steering and turning
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.
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
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
ensures that for maximum forward deflection (Y = Y",aX)
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
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
can easily be held there without generating a non-zero
Y signal.
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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
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.
Contours 15 representing constant values of oc~
are shown in the figure for positive Y only,and it is
clear that the largest values of oca are not available
at the highest forward deflection. At the maximum
forward deflection Ymax the maximum range of X
deflection is oX, which is significantly less than the
range -Xo to +Xo available for Y = 0. Thus, the
truncated diamond template further increases the
understeer problem at high speeds associated with the
prior art vehicles.
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
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
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:
input means operable to generate a signal
indicative of a desired forward motion of the vehicle
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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
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
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
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
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
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
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,
the steering or turning characteristics of the vehicle
for certain values of the desired forward motion
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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
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
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
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-
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.
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
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
increasingly restrict the left-right movement of the
joystick as its forward deflection is increased. The
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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
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
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
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
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
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
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
forward motion signal over a predetermined range of
values of the desired forward motion signal. Thus, the
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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
second range. In the second range the difference may
increase with increasing desired farward 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
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
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
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
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
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
also be operable to generate a signal indicative of a
desired forward motion of the vehicle. However, this
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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
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
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,
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
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
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
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.
According to a third aspect of the present
invention there is provided a controller for an
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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
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
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
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.
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
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
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;
driving a left wheel and a right wheel of the
vehicle at different respective angular velocities in
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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.
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;
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
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
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
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
first signal and the second signal.
The difference may be determined substantially
according to the relationship
D = kl X + k2X (Y - YT)
for Y>YT, where D is said difference, X is the value
of the first signal, Y is the value of the second
signal, K1 and Kz are constants and YT is a
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predetermined threshold value of the second signal.
For values of Y < YT the difference may be
determined according to the relationship D = K1 X.
Embodiments of the present invention will now be
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;
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)
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;
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
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
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
position of angular velocity difference in accordance
with another embodiment of the present invention;
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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
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
with another embodiment of the present invention;
Fig. 11 is a schematic diagram of the forward
halt of another known joystick template aperture and
contours representing the dependence on joystick
position of angular velocity difference in accordance
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
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
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:
3 0 wR = w - ow
wL = w + ow
where ow = kl X + kz XY
and kl & kz are constants .
Thus, for constant X,ow is proportional to Y, and as Y
increases so does the angular velocity difference set
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between the left and right wheels.
w may be proportional to Y, or may be a different
function of Y. Contours 15 indicating lines of
constant ow are plotted within the template aperture
142 to show the dependence of ow on joystick position.
The ow interval between adjacent contours is
equal, and, for a given Y value, as ow is proportional
to X, the contours are equally spaced along the X
axis.
Unlike the prior art arrangements in which the ow
contours were parallel to the Y axis, in embodiments
of the present invention the dependence of ow on Y
over at least a range of its values necessarily means
that the ow contours are at some point non-parallel to
1S the Y axis. In the above embodiment the dependence of
ow 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.
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
the figure. The controller is programmed to calculate
a quantity D from the input X and Y signals according
to the equations:
D=k1X + k2X (Y - YT) for Y z YT
and D=k1X f or Y s YT
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
according to the relationships:
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CJR - GJ - k3D
c.~~, = w + k3D
i . a . ate = k3D where k3 is a constant .
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
increasing function of Y.
Contours of constant oca are plotted on the figure
and the two regimes are clearly distinguishable.
K1 may be adjusted, and alters the separation of
the contours along the X axis.
KZ 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.
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.
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
is adjusted according to the relatianship
oca = K1X + kZ VX
In a further embodiment, ow is set according to:
ow = k1X + k2X (V-VT) for VzVT
and ow = K1X for VsVT
where VT is a predetermined threshold value of the
velocity signal V.
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In yet a further embodiment, oc~ = Kl X + Kz XY +
K4XV ,
i.e. oc~ is a function of X, Y and the vehicle's
velocity.
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
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 = YTZ) . Below the lower threshold YT1 and
above the upper threshold YTZ 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.
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
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
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.
Referring now to Fig. 11, in another embodiment
of the present invention the input means comprises a
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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
apparatus determines the difference between the
different respective angular velocities of the left
and right wheels according to the relationship;
oc~.~ = kl X - kz XY
Thus,for a given value of X, as Y increases the
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
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
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
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
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
arranged to generate a steering signal 11 and a
forward motion signal generating means 102 comprising
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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
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
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
gearboxes 222, 232.
The drive apparatus further comprises velocity
sensors 252,253 which are arranged to generate signals
VL and VR indicative of the forward components of
velocity of the left and right hand sides of the
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
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
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
according to both the current steering signal and the
actual average velocity of the vehicle, the controller
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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
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
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,
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
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
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
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
independently of other disclosed and/or illustrated
features.
