Note: Descriptions are shown in the official language in which they were submitted.
'~"° ~ 94/26551 2 ~ 6 2 6 8 l PCT~CA94/00232
1
ELECTRIC POWERED SMALL TRACTOR
This invention relates to the manner of providing and controlling power
transfer from a battery pack to the electric drive motors, and to the road
wheels of an electric vehicle such as a lawn and garden tractor, for example
in
the 16 HP size range.
Such vehicles are common and are powered by gasoline or engines, but are
not economical or efficient. The noise, fumes, etc of a gasoline tractor can
be
obtrusive to many users, especially since many lawn and garden tractors are
used around buildings and people. Several attempts have been made to
provide an electric tractor, with re-chargeable batteries, in that size range.
Given that a lawn and garden tractor is purchased because the lawn and
garden is quite sizeable, and therefore the tractor will be subjected to a
demanding duty cycle, it is recognised that there is sufficient space on a
small
tractor to contain the quantity of batteries needed for the duty cycle, if
used
economically. That is to say, the size available for the battery pack is
sufficient for the typical heavy duty cycle, but only if the power is utilised
efficiently.
In previous designs of small tractor, the power to the drive motors has been
controlled, as to the speed of the wheels, by means of switched resistors.
This type of control does not use the electricity economically, in that some
of
the power goes in heating the resistors; it is also mechanically jerky; and it
also has the disadvantage that the control system employs small moving parts
which can pose a service problem, given that a lawn and garden tractor can
suffer long periods of mechanical neglect.
In previous designs of small electric tractors, the mechanical drive from the
electric motor to the road wheels has been complex, involving drive axles,
chain drives and sprockets, final drive units, differential gears, etc.
The present invention is aimed at providing a power, and power control,
system for a lawn and garden tractor, in which the system uses solid state
components, with no exposed moving parts {other than the road wheels). In
WO 94/26551 'CT/CA94/00232
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2
the invention, individual motors are provided for each road wheel. Preferably,
each motor feeds power to its road wheel through a respective simple parallel-
shaft, enclosed, speed-reduction gearbox, and the road wheel is attached
directly to the output shaft of the gearbox.
The invention thus provides respective drive motors, one for each driven wheel
of the tractor. The road wheels preferably are mounted directly on the output
shafts of gearboxes connected directly to, and housed with, the drive motors.
The driven wheels are not connected by means of a drive axle, and no
differential gear or final drive unit is required.
In the invention, the left drive motor drives the left road wheel and the
right
drive motor drives the right drive wheel. The drive control system is such
that
each motor is driven to rotate at a particular speed. The motor is supplied
with the current or power it needs to maintain that speed, even if load
conditions should vary. Thus, the tractor tends to travel in a straight line
at a
constant speed, or a steered curve, even if one wheel should encounter a
small obstacle. Also, the tractor maintains wheel speed even if one wheel
should run over a low-grip surface.
Preferably, the motors are arranged to feed energy back into the batteries if
the motors are over-running the wheels (when the tractor is decelerating, for
example.) Such regenerative braking provides a useful improvement in overall
efficiency. Not only that, but an electric tractor contains a battery pack of
considerable mass, and the inertia of the tractor might make close
manoeuvring a little difficult if the tractor were able to run on during over-
run.
The resistance on over-run caused by regenerative braking substantially
improves the feel of the controls, whereby the driver does not feel that the
tractor is running away with him.
Besides, on a lawn and garden tractor, braking requirements are not very
demanding, and providing regenerative braking means that the only other
braking required, if indeed any other is required, can be met by a rudimentary
parking brake which can be incorporated into the gearbox if desired.
~"'~ 94/26551 216 2 6 ~ ~ PCT/CA94/00232
3
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
By way of further explanation of the invention, exemplary embodiments of the
invention will now be described with reference to the accompanying drawings,
in which:
Fig 1 is a diagrammatic view showing, in plan, a small tractor, of the type
generally referred to as a lawn and garden tractor, having a drive system
which embodies the invention;
Fig 2 is a corresponding view to Fig 1 of another tractor, which also embodies
the invention;
Fig 3 is a rear elevation of the Fig 2 tractor;
Fig 4 is a block diagram of a control system for the Fig 2 tractor.
The apparatuses shown in the accompanying drawings and described below
are examples which embody the invention. It should be noted that the scope
of the invention is defined by the accompanying claims, and not necessarily
by specific features of exemplary embodiments.
Fig 1 shows a tractor 10 having a conventional steering system. The non-
driven wheels 12L,12R are mounted at the ends of an axle 14, and are swivel-
able about king-pins 16. A steering wheel 18 turns the wheels 12L,12R for
steering, as will be apparent from the diagram.
The driven wheels 20L,20R are not mounted on a common axle. Each wheel
has its own axle 23L,23R, which is mounted directly in the respective
gearboxes 25L,25R. The gearboxes are bolted to respective electric motors
27L,27R, and to the chassis 29 of the tractor 10. The (two-stage) gearboxes
provide a gear ratio between the motor and the road wheel of 30:1. The
gearboxes provide a firm mounting and a good bearing spread for the wheel
axle bearings.
WO 94/26551 21 6 2 6 8 7 PCT/CA94/00232
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The tractor 30 of Figs 2 and 3 has the same arrangement of driven wheels as
in Fig 1, and the same reference numerals are used. However, in Figs 2 and
3 the non-driven wheels 32L,32R are mounted as castors. A cross-beam 34
carries swivel-able stirrups 36L,36R, in which the wheels are mounted.
Lawn and garden tractors generally have no suspension system as such: the
tires of the wheels are soft enough to cope with such suspension motions as
are called for.
The tractors of Figs 1 and 2 have no common drive axle, no final drive gears,
and no differential to compensate for the wheels rotating a different speeds
when the tractor is being steered around comers. Instead, the tractor has a
motor speed control system, which will now be described.
Each wheel 20L,20R is provided with a respective speed sensor 40L,40R. This
speed sensor may take the form of a revolution-counter, separate from the
motor, or may take the form of a generator which rotates with the motor, and
which supplies a voltage or frequency signal indicative of the speed of the
wheel. Alternatively, the speeds of the DC motors can be derived from the
current fluctuations as the armatures pass the poles of the motors. In any
event, the speed sensor sends a signal indicative of the speed of the motor
(and hence of the wheel) to a controller 43. The signal may be pulses, the
frequency of which indicates motor speed, or the signal may be a voltage, the
steady level of which indicates motor speed, or the signal may be in any other
convenient form.
It is preferred that the speed is sensed from a parameter that is present in,
and can be measured in, the power wires going to the motor. In that way, no
separate wires are required for the sensor. Thus, it is preferred for the
sensor
to detect, for example, speed-proportional pulses in the current as the rotor
of
the motor sweeps the electrical poles: that can be done through the supply
wires to the motor. Probably the simplest way of sensing speed is to measure
the resistance of the field windings of the motor, which again can be done
through the motor supply wires, without special wires to a sensor.
CA 02162687 2005-O1-04
page s f l
In Fig l, turning the steering wheel 18 is e~~ective not only to swivel
the eteerable wheels but is effective also to change the status of a
steering potentiometer 45 mounted on the steering column. In Fig 2,
there is no steering wheel: instead, steering is effected entirely
electrically, by means of a joystick. Manipulation of the joystick
again is effective to change the status of the steering potentiometer
45.
The output of the steering potentiometer provides a reference signal
for comparing the speeds of the left and right motors. The designer of
the system seta in a formula or aZgor~,thm such that wk~en the steering
wheel is turned through a particular angle, the driving wheels are set
to rotate at speeds which differ by a particular differential speed.
That is to say, the wheel on the outside of the curve is constrained to
move at a faster' speed thaxl the wheel on the inside of the curve, and
the wheel speeds are const~razned to differ by a pre-determined amount.
In fig 1, this constraining of the wheels td rotate at different speeds
ensures that the non-steerable, driven, wheels follow the steered, noz~-
dxiven, wheels around a coxr~ex, just as if the tractor had been
equipped with a mechanical differential gear. In Fig 2, tlxere is no
eteerfng wheel, arid the differential speeds of rotation of the left and
right wheels is the sole factor which causes and controls the steering
motions of the tractor.
The behaviour of the Fig 1 tractor is not always as if the tractor had
been equipped with a meohanical differential gear. For example, when
one driven wheel is on ice or other low friction surface, and the other
wheel is on a normal friction surface, the differential speed control
system ensures that the wheel on ice does not speed up, as would happen
with a mechanical differential. The control system ensures that the
difference in wheel speeds always corresponds to the desired turning
radius, as set by the steering wheel or joystick, irrespective of the
friction grip of the tires to the surface.
The speed control. system automatically sets the power supplied to the
motor to the value necessary to maintain the speed as dictated by the
speed control potentiometer.
WO 94/26551 21 b 2 6 8 l PCT/CA94100232
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Similarly, if the tractor should encounter such an obstacle, for example, as
when one wheel passes over a tree root or the like, that wheel does not slow
down and then speed up; the control system momentarily feeds extra power
to the motor to drive the wheel over the obstruction without losing speed, and
then reduces the power to the motor as the obstruction passes, without the
motor gaining speed.
The speed of the tractor may be set by the driver by means of a separate
accelerator /brake control. The setting of this control defines the average of
the speeds of the left and right wheels; the steering potentiometer defines
whether the wheels are to rotate both at the same speeds, or at
proportionately different speeds. Alternatively, instead of one control for
vehicle speed and one for steering, the designer might provide two separate
wheel speed controls, one for each wheel, which the driver can operate
together or independently.
The tractor is equipped with a switch for changing between forward and
reverse motion.
In the Fig 1 tractor, manoeuvring is accomplished as in an ordinary front-
wheel-steering, mechanical-rear-axle-differential equipped vehicle. Thus, the
Fig 1 tractor "feels like" a normally-steered vehicle. The difference in
speeds
between the left and right rear wheels is in fixed correspondence with the
angle through which the steered front wheels are turned.
The Fig 2 tractor steers in what seems an unnatural manner, which is quite
unlike a conventional vehicle. On the other hand, the steering system of Fig 2
makes the tractor highly manoeuvrable around obstacles. Also, the tractor
can turn in very tight circles. Also, the tractor's manoeuvrability is not
dependent on the friction of the surface remaining constant.
In Fig 2, when the accelerator /brake control is set to move the tractor
forward
only very slowly, if at that time the steering is turned to a full turn
position, it
can happen that the difference in the speeds of the wheels, as dictated by the
steering potentiometer, makes one wheel turn in reverse. For example, if the
accelerator /brake control is set to a forward speed corresponding to a wheel
~"''"'' 94/26551 216 2 6 8 7 PCT/CA94/00232
7
speed of say 10 rpm, and if the steering control requires a speed differential
of
say 26 rpm, then the left wheel turns at 23 rpm forwards, and the right wheel
turns at 3 rpm in reverse. Equally, if the accelerator /brake control is set
to
zero, and the steering control is set to a full-turn position, the tractor
will spin
on its Yaw-axis.
If the Fig 2 tractor should become bogged down in a patch of slippery
ground, therefore, and the driver attempts different accelerator and steering
manoeuvres in order to extricate the tractor, it can happen that the tractor
comes out of the patch of slippery ground at an unpredictable angle.
However, the tractor never moves with an unpredictable speed, so there is
little safety problem associated with the Fig 2 steering system. With a little
practice, a driver soon becomes accustomed to the unusual steering
behaviour, and from then on welcomes the extra dimensions of
manoeuvrability it provides.
The Fig 2 steering system with a mechanical differential drive would be
impractical for a small tractor. The system can only be effectively controlled
if
the wheels are, or can be, constrained to move at pre-determined speeds.
It might be considered that the Fig 2 steering system could be done non-
electrically, by providing hydraulic drive motors, one to each wheel, and this
could theoretically be done; however, an hydraulic system would be
prohibitively expensive in the context of a lawn and garden tractor.
Fig 3 shows the cross-beam 34 on which the castors 32L,32R are mounted.
The beam is arranged for pivoting in the Roll-mode of the tractor about the
swivel pin 49. When the tractor passes over uneven ground, the beam axle
can follow such Roll-mode movement as may be required. The beam is
cranked so that the pin 49 lies close to the ground, whereby the castors
follow
the movement over a favourable portion of the beam's arc of swivel.
In Fig 1 and Fig 2, it will be noted that, in respect of each of the electric
drive
motors 27L,27R, the axis of the armature of the drive motor lies parallel to
the
axle 23L,23R of the corresponding wheel 20L,20R. The motor armature is not
co-axial with the wheel axle, however, but is offset therefrom.
WO 94/26551 216 2 b 8 7 PCTlCA94/00232
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The magnitude M of the offset between the motor armature axis and the wheel
axle is important. Of course, the two wheels 20L,20R must be co-axial. The
offset M should be large enough, in relation to the diameter D of the casings
of the motors, that the two motors 27L,27R can be mounted side-by-side on
the chassis: that is to say, the offset is large enough that the motors can be
mounted overlapped one behind the other, when viewed in the longitudinal
sense relative to the tractor.
If the chassis of the lawn and garden tractor were wide, ie wider than the
length of the two motors put together, then it would not matter whether the
motors were overlapped; but since, in a lawn and garden tractor, the chassis
is narrow compared with the length of the electric motors (that is one of the
characteristics of a lawn and garden tractor) it is very advantageous that the
offsets as described permit the overlapping arrangement.
The double reduction gearboxes 25L,25R are arranged so that the wheel axle
and the motor armature are parallel. Such a gear arrangement involves a
relatively modest expense, as compared with, for instance, the expense of a
gearbox that had to accommodate a crown bevel gear to provide an angle
difference between the armature and the axle.
It is recognised that the large offset M allows the wheels 20L,20R to be co-
axial, but allows also the physical bulk of the motors to be accommodated in
the relatively narrow chassis. The combined motor/gearbox unit as illustrated
is available as a proprietary unit, of excellent economy and service
reliability.
The road wheels are mounted directly on the gearbox output shaft, and so
sturdiness is important. If the drive system had to be designed around, say, a
chain drive, the limitations of that would be much more restrictive on the
designer.
It is one of the features of the invention is that the two motors are each
driven
at pre-determined speeds. When the tractor is moving in a straight line, power
is supplied to the left and right motors so as to keep both of them rotating
at
the same speed, irrespective of variations in the ground surface etc. When
the tractor is moving around a curve, power is supplied to the left and right
1~'~ 94/26551 21 b 2 6 8 7
YCT/CA94/00232
9
motors so as to keep them rotating at some pre-determined ratio of speeds.
When more power is required to keep the motor turning at the desired rpm --
when, for example, one tractor wheel encounters an obstacle -- so the power
supplied to the motor increases. Cut-outs are included in the controller to
prevent the extra current from rising so high as to cause damage. However,
the cut-outs are time-dependent: an electric motor can withstand considerable
current for a short period, and can withstand very high current for periods of
a
few seconds.
The control system as described makes use of regenerative braking. A small
electric tractor has the characteristic that a large proportion of its weight
is
taken by batteries. Therefore, when one or both of the wheels is over-running
its respective motor, the inertia of the tractor can be considerable, even
though the tractor moves at hardly more than walking pace.
The system of driving the wheels of the tractor as described herein is
practically useful for small tractors in the power range of 10 to 20 HP,
preferably 15 or 16 HP, as those ratings are applied to gasoline engine-
powered tractors. Above that size, the tractor is physically too large and too
powerful to be powered, economically and practically, by a rechargeable
battery pack. Below that size, the tractor is too small to be practical. (It
will
be understood that these figures can change, as the power output available
from batteries is constantly being improved.)
As mentioned, the above mentioned power ratings are equivalent to the
ratings generally given to gasoline powered tractors. The motors actually
used in the tractors are of roughly the same size as electric motors rated for
continuous use at 0.8 HP. As mentioned, the motors have substantial
capability to be supplied with very high current for short periods. The
designer will generally set the voltage to be used on the tractor at 36 or 48
volts, and may arrange for short-lived current of 100 amps or even more at
that voltage.
WO 94/26551 216 2 ~ ~ ~ PCT/CA94/00232
The tractor may be used with different attachments, such as a grass cutter, a
snow plough, a snow blower, a tippable scoop or wheelbarrow for carrying
and dumping loads, etc.
Another useful item that can be included on the tractor is the type of
implement known as a WeedEater (TM) in which a plastic wire spins at high
speed. A normal mains-voltage model can be used if a suitable voltage
inverter is provided on the tractor. It is very simple and convenient also to
provide a rack or hook for the WeedEater on the tractor. In his task of
cutting
a large area of grass, therefore, the driver can break the task up into
periods
of sitting on the tractor and periods of using the WeedEater, which breaks up
both tasks, and takes away much of the "chore" character of both tasks.
The motor controls include two-quadrant choppers capable of accelerating
and decelerating the motors, making the tractor highly responsive to operator
control. Current and voltage feedback circuits improve the response of the
unit to asymmetric variations in drive-wheel torque. The capacity for high
peak
current enable rapid motor acceleration. The chopper is a high-efficiency
power semi-conductor device which, in combination with a motor and free-
wheeling diode, behaves like a variable output voltage DC-to-DC converter.
By varying the effective output voltage of this converter, the operator can
vary
the speed of the DC motor. A "one-quadrant" chopper is capable of
accelerating a DC motor in one direction only: a four-quadrant chopper is
capable of accelerating and decelerating a DC motor in both directions.
The system provides that the speed of the wheels is determined purely by the
electrical settings of the controls. The motors draw current as required, to
maintain speed. This results in smooth progressive operation, with no jerks.
Fig 4 shows the electrical arrangement of the tractor, in diagrammatic form.
The batteries 50 of the *.ractor may be recharged by plugging into a mains-or
solar-powered charger 52, or as available. The tractor includes means 54 for
hitching and unhitching accessories such as a grass-cutter, snowblower, or
the tike.
~"""~' 94/26551 216 2 6 8 7 PCT/CA94100232
11
The tractor may have a steering yoke 56 to control left/right steering, a
separate forward/reverse switch to control direction, and a separate speed
lever 58 to control speed. Alternatively, all three of these functions can be
provided by a single joystick. The single joystick may be set up so that
fore/aft motion of the joystick controls both direction and speed, and so that
left/right motion of the joystick controls steering.
The joystick on the tractor may be duplicated by (or even replaced by) a
remote-control joystick, which operates the tractor by radio or other
transmissions. The electrical control system can be arranged to require only a
non-active potentiometer output from the joystick, which can easily be
duplicated by radio control.
The control of power to the motors is done by chopping the current at
programmed intervals, and at a programmed frequency. Power is supplied to
the motor therefore via what is, in effect, a DC-to-DC converter 60. The
relationship between frequency and on/off chopping ratio is controlled by the
microprocessor 63 so as to maximise efficiency of the drive system, ie to keep
the drive current at a minimum for a particular setting of the joystick.
The speed control system as described may also be used to control a four-
wheel drive tractor. A single joystick (on board or remote) can even be used
in that case also. The control system simply determines at what speed each
wheel should be running, given the inputs from the joystick, and regulates the
current to the motors accordingly. Preferably, mechanically-separate motors
are provided, one to each wheel.
When all four wheels are driven, provision may also advantageously be made
for the tractor to be steered and driven in crab-fashion: that is to say, in
which
the back wheels and the front wheels are both turned (or both made to rotate
at differential speeds) in order to make the tractor move sideways. Crab
steering can be useful when operating the tractor on the side of a hill, for
example.
In place of the proprietary motor/gearbox unit as illustrated, the drive unit
may
be a high-torque motor without a gearbox, or a stepper motor, with or without
2~626aT
i2
a gearbox. A stepper motor of course is very readily
controlled as to its speed of rotation.
When the tractor encounters a hill, theoretically no change of
the joystick position is needed to make the tractor go uphill.
However, people naturally feel the need to increase the
throttle setting when going up a hill. To improve the ''feel"
of the drive system, therefore, a little bias may be added in
the controls such that an equal resistance to both wheels
(indicating a hill) requires a slight extra pressure on the
joystick.
The drive motors preferably should be of the type which can
exert a high torque at low or zerc speed. The need for the
tractor to have energy-absorbing brakes can be reduced in that
case. (The tractor should have at least a parking brake, for
safety reasons . )
The drive system as described is not suitable for road-going
vehicles, for which there are safety requirements for a
mechanical connection between the steering wheel and the road
wheels. The system is most suited to a small tractor, wherein
the chassis of the tractor is relatively narrow as compared
with the size of the drive motors, and wherein there is little
requirement for suspension. The drive system as described
also has limitations in that, without drive transmission
shafts, gears, etc, there is a practical maximum to the level
of electrical power that can be conveniently fed to the
wheels, whereby the system would not be appropriate for a
vehicle larger than a lawn/ garden tractor. Another type of
small vehicle wherein steering control can be allowed to be
independent of mechanical connections is a motorized wheel-
chair, and the invention maY be applied tp that.
Examples oz the background art which can be regarded as useful
for the understanding, searching, and examination of the
invention are cited in the annexed international search
rspart .
~PtD~D MEET
