Note: Descriptions are shown in the official language in which they were submitted.
CA 02463408 2004-04-07
DRIVE SYSTEM FOR VEHICLES
Field of the Invention
The present invention relates generally to a drive system for vehicles,
especially for commercial vehicles such as agricultural or industrial
tractors,
comprising ~t least one first wheel that is driven by an associated axle or
sfingle-
wheel drive motor and at least one second wheel, in the drive train of which a
gearbox that can be switched between two speed transmission or multiplication
steps is arranged.
Backsaround of the Invention
For matching the drive power to different driving requirements, vehicle drive
systems have been suggested in which a separate drive is assigned to each
vehicle
axle or each wheel of the vehicle. EP-A-0 812 720 for example describes a
vehicle
with a drive system of the aforementioned kind. A generator driven by an
internal
combustion engine supplies the electric energy for electric motors. The wheels
of
the front axle of the vehicle are driven by an associated electric motor,
while the rear
wheels are driven electro-mechanically in that the internal combustion engine
drives
a drive shaft that is allocated to the rear axle of the vehicle. Each wheel of
the rear
axle is assigned a summing gear and an electric motor. The summing gear
combines the drive power of the drive shaft and the corresponding electric
motor and
passes it on to the appropriate rear wheel. A change-speed gearbox that can be
shifted between at least two transmission ratios is arranged between the
internal
combustion engine and the two summing gears. When switching the change-speed
gearbox, the tractive power to the rear wheels is interrupted. This can lead
to a
slowing of the vehicle and express itself in an unpleasant manner for the
operator in
the form of a shifting jolt. It has been found that even when using a change-
speed
gearbox that can be shifted under load the aforementioned disadvantages cannot
be
avoided.
CA 02463408 2004-04-07
Summary of th~ Invention
In view of the foregoing, it is an object of the invention to provide a drive
system of the aforementioned kind such that the problems described above are
overcome.
Anther object of the invention is the provision of a drive system of the
aforementioned kind wherein vehicle speed decreases and interfering shifting
jolts
are avoided when shifting a change-speed gearbox.
In general, a drive system for a vehicle, especially for a commercial vehicle
such as an agricultural or industrial tractor, the vehicle having at least one
first wheel
that is driven by an associated axle or single-wheel drive motor and at least
one
second wheel, in the drive train of which a gearbox that can be shifted
between at
least two speed transmission steps is arranged. To avoid vehicle speed drops
while
shifting the change-speed gearbox and to avoid interfering shifting jolts, it
is
suggested to provide a device for the detection of a shift command and a
control
unit, which in the presence of a shift command automatically applies a greater
load
at least on the drive motor driving the first wheel, controls the shifting
operation of
the change-speed gearbox of the second wheel and then lowers the load of the
drive
motor driving the first wheel.
The drive system comprises at least a first wheel that is driven by an
associated axle or single-wheel drive motor and at least one second wheel, in
the
drive train of which a gearbox that can be shifted between at least two speed
transmission stages is arranged. Pursuant to the invention a device for the
detection
of a shift command as well as a control unit are provided. The control unit
reacts to
a shift command by automatically applying a greater load at least on the drive
motor
driving the first wheel so that the first wheel yields greater torque. In a
time-related
connection the shift of the change-speed gearbox of the second wheel is
controlled.
In the final phase of the shifting process, the load of the drive motor that
drives the
2 _
CA 02463408 2004-04-07
first wheel is lowered again. Said motor can then generate for example again
its
original torque. With this shifting method no significant interruption or
decrease in
the tractive force occurs during the shifting process, which due to the
tractive
resistance could lead to the braking and deceleration of the vehicle. Rather,
the
decrease in power on the wheel that is to be shifted is compensated by a
corresponding increase in tractive force on the wheel that is not shifted and
continues to drive. The vehicle thus maintains its speed and unpleasant
shifting jolts
are avoided or at least weakened considerably.
This shifting and drive strategy pursuant to the invention can be applied in
various vehicle drive concepts in a beneficial manner. The inventive drive
concept for
example can be applied in a vehicle where the rear wheels are driven in the
conventional manner by an internal combustion engine and a mechanical gearbox
that can be switched between various transmission ratios or a hydrostatic-
mechanical gearbox. The front wheels by contrast are driven by separate drive
motors, wherein a drive motor can drive the two wheels of the front axle
jointly (axle
drive motor) or each front wheel comprises a separate drive motor (single-
wheel
drive motor). The drive motors preferably are electric machines, especially
electric
motors. When shifting a gearbox, the drive motor is or the drive motors are
driven
with increased power so that they compensate the interruption or reduction in
tractive power occurring on the rear wheels, and the vehicle can travel with
unchanged speed.
In another beneficial drive concept, both the rear wheels and the front
wheels are driven by axle drive motors or by single-wheel drive motors. With
regard
to the arrangement of shiftable gearboxes, several possibilities arise. Change-
speed
gearboxes can be arranged between drive motors of the front axle and the
associated front wheels or between drive motors of the rear axle and the
associated
rear wheels or both between the drive motors of the front axle and the
associated
front wheels and also between the drive motors of the rear axle and the
associated
rear wheels. When shifting the change-speed gearbox or the change-speed
3
CA 02463408 2004-04-07
gearboxes of an axle, the drive motor or the drive motors of the other axle
will
experience a greater load, i.e. will be operated at higher power consumption.
If to each of the four wheels of a vehicle a separate drive motor with a
subsequent shiftable gearbox is assigned, with the presence of a shift command
initially the change-speed gearboxes of a first axle can be shifted between
two
transmissiotl steps and during the shifting operation the drive motors of the
second
axle can be operated with increased drive power. Subsequently the change-speed
gearboxes of the second axle are shifted and during the shifting operation the
drive
motors of the first axle are operated with increased power.
The shifting operation can beneficially be conducted as follows: Initially the
torque that is required for driving the vehicle is split among all vehicle
wheels. When
for example during acceleration or deceleration of the vehicle its speed
approaches
a value that necessitates a shift into a higher or lower gear, the operator or
an
automatic shifting control device issues a shift command to the control
device. In
preparation of the shifting operation of the vehicle wheels of a first vehicle
axle, the
control device initiates a shift of the required torque to the vehicle wheels
of the
second vehicle axle by temporarily applying a greater load on the drive motors
of the
second vehicle axle, while the drive systems that are affected by the shifting
operation become torque-free so that the change-speed gearbox can be shifted.
Upon a completed shift, the necessary torque is shifted to the vehicle wheels
that
have already been shifted so that the wheels that have not been shifted yet
become
torque-free and can be shifted. After shifting the change-speed gearboxes of
all four
wheels, the torque that is required for driving the vehicle is again split
among all four
wheels.
In a vehicle with separate drive motors and change-speed gearboxes for
each wheel, a control device can also be beneficial, with which initially the
shiftable
gearboxes of two first diagonally opposing wheels are shifted simultaneously
with the
existence of a shift command, while the drive motors of the two other second
wheels
4
CA 02463408 2004-04-07
experience a greater load and are shifted simultaneously by the immediately
following gearboxes that are associated with the two diagonally opposing
wheels,
while the drive motors of the two i'Irst wheels experience a greater load. In
general,
however, an axle-specific shifting operation is preferable for driving
stability reasons.
With farming tractors, the change-speed gearbox for example can comprise
a first gear range that permits driving speeds of e.g. up to 27 kmlh and is
used
primarily during working operations. A second gear range permits driving
speeds
e.g. up to 65 km/h and is used when driving on roads. With such large jumps in
gear
ratios (1:2.4) normally interfering shifting jolts occur if during
acceleration or
deceleration of the vehicle a gear or range shift is performed by the operator
or by an
automatic gear change system. The shifting jolts are unpleasant especially
with
automatic transmissions because here they occur unexpectedly. These shifting
jolts
can be avoided by using a drive system pursuant to the invention.
Powershift transmissions can be used as change-speed gearboxes. It is
also advantageous to use a standard transmission in the drive train of which a
clutch
is arranged for interrupting the flow of power.
A beneficial embodiment of the invention that should be particularly
emphasized provides for electric machines or hydraulic motors to be installed
as the
axle or single-wheel drive motors. Electric or hydraulic single-wheel drive
motors are
described in the following also as wheel motors. Especially electric machines
offer
the advantage that they can be heavily overloaded for a short time without
being
damaged. The permissible consumed power and/or the maximum torque supplied
can for example be increased briefly by a factor of 2. It is thus possible to
perform
the inventive shifting operation even when a maximum drive power is demanded
from the vehicle. Even during such operations, a short increase in the drive
power of
the respectively affected electric machine is permissible. For example during
a
shifting process on one axle in a vehicle with four single-wheel drives the
wheel
motors of the other axle can assume, in addition to their existing tractive
power, a
CA 02463408 2004-04-07
portion of the tractive power or even the entire tractive power of the
currently shifted
axle (which can be free from drive power during the shifting operation).
The energy source for the electric machines is preferably an electric energy
storage unit that can be mounted on the vehicle and/or a generator that is
driven by
an internal combustion engine.
Preferably the electric machine and its selection are designed such that the
electric machine can be operated both as an electric motor, which drives the
associated wheel, or also as a generator, which slows the associated wheel
down.
When using a generator that supplies the electric energy for the electric
machines, preferably a converter and an intermediate circuit are arranged
downstream from it, permitting also a motor driven operation of the generator.
This
way it is possible to support the braking operation of the vehicle
electrically by
operating the axle or single-wheel drive motors with the generator. The
electric
energy they create is fed to the generator, which now functions like an
electric motor
and increases the speed of the internal combustion engine and thus supplies it
with
energy that decelerates the vehicle as braking energy ("electric brake").
As a supplemental or alternative °electric brake" also braking
resistance
systems can be used, which destroy the electric energy created by the electric
machines.
A preferred embodiment of the invention provides that the shiftable gearbox
comprises a planetary gearbox, particularly a powershift transmission or a
standard
transmission. It is also beneficial to arrange at least one speed-reducing
final drive
transmission, especially a planetary transmission, downstream from the axle or
the
single-wheel drive motor. This way the drive motors can be operated in
favorable
speed ranges.
6
CA 02463408 2004-04-07
For a compact design it is advantageous to arrange the single-wheel drive
motors within the wheel rim of the associated wheel. Likewise, a change-speed
gearbox that is arranged upstream from the drive motor andlor a wheel brake
and/or
a planetary gear reducing step that is arranged downstream from the drive
motor
can be arranged within the wheel rim or in the vicinity of the wheel rims.
To'acquaint persons skilled in the art most closely related to the present
invention, one preferred embodiment of the invention that illustrates the best
mode
now contemplated for putting the invention into practice is described herein
by and
with reference to, the annexed drawings that form a part of the specification.
The
exemplary embodiment is described in detail without attempting to show all of
the
various forms and modifications in which the invention might be embodied. As
such,
the embodiment shown and described herein is illustrative, and as will become
apparent to those skilled in the art, can be modified in numerous ways within
the
spirit and scope of the invention-the invention being measured by the appended
claims and not by the details of the specification.
Brief Descriation of the Drawings
For a complete understanding of the objects, techniques, and structure of
the invention reference should be made to the following detailed description
and
accompanying drawings, wherein:
Fig. 1 is a diagrammatic view of a vehicle drive system; and,
Fig. 2 is shifting arrangement for the electric components of a drive system
pursuant to Fig. 1.
Descritition of the Preferred Embodiment
in the figures equivalent parts and components were assigned the same
reference numbers. The vehicle drive system shown in Fig. 1 is provided for an
agricultural tractor and comprises a front axle 10 with front wheels 12 and a
rear axle
7
CA 02463408 2004-04-07
14 with rear wheels 16.
Each wheel 12 of the front axle 10 is driven by an associated electric motor
18. The output shaft 20 of each electric motor 18 is connected to the input
shaft of a
planetary gear-reducing transmission 22, which provides its output power to
the front
wheel 12 via a drive shaft 24, which comprises a cardan joint, as well as a
planetary
gear reduciing step 26. A mechanically actuated wheel brake 28 is integrated
into
the drive shaft 24. The cardan joints enable the steering of the front wheels
12. A
steering-angle sensor 30 detects the steering angle of the front wheels 12.
Each wheel 16 of the rear axle 14 is driven by an associated electric motor
32. Between the electric motor 32 and a change-speed gearbox 34 that can be
shifted between two transmission ratios a clutch 36 is arranged. The output
speed of
the change-speed gearbox 34 is further reduced in a planetary gear-reducing
step
38 and supplied to the associated rear wheel 16. Here as well a mechanically
actuated wheel brake 42 is integrated into the drive shaft 40, which runs
between the
change-speed gearbox 34 and the planetary gear-reducing step 38.
Via a drive shaft 46, an internal combustion engine 44 drives a generator 48,
which supplies the electric power for the electric motors 18, 32. The vehicle
speed is
detected by a radar sensor 50.
As can be seen in Fig. 2, the generator 48 is connected with a frequency
converter 54, which forms a generator intermediate circuit, via a cable 52. A
direct
current intermediate circuit 58 with an energy storage unit that is not shown
in detail
is arranged downstream from the frequency converter 54. The direct current
intermediate circuit 58 supplies via additional cables 60 the frequency
converters 62,
which are assigned to the individual electric motors 18, 32 and supply them
with
electric energy. The direct current intermediate circuit 58 moreover is
connected by
means of another cable 64 with a braking resistance disk 66, to which one or
more
cooled braking resistance units 68 are connected.
8 ._
CA 02463408 2004-04-07
An electric control unit 70, designed as a micro-controller, is provided,
which
is connected to a BUS system 72. Additionally the steering-angle sensor 30,
the
radar sensor 50 and a shift detection device 73 are connected to the BUS
system 72
so that the signals can be transmitted to the control unit 70 and be processed
there.
The shift detection device 73 can also be a switch, which is not shown and
could be
actuated by the operator, or a control unit, which generates a shift signal
automatically based on the driving conditions.
A plurality of additional electric sensors and input devices {not shown) can
be connected to the BUS system 72. For example the electric signals supplied
by
speed sensors 74 detecting the speed of the electric motors 18, 32 as well as
by
speed sensors detecting the wheel speeds, by temperature sensors recording the
temperature of the electric motors 18, 32, by position sensors detecting the
gas
pedal position and the brake pedal position, and by gear shifting sensors
detecting
the shifting signals for the planetary shift transmissions 34 can be fed into
the BUS
system 72 so that these signals as well can be recorded by the control unit 70
and
processed.
Moreover also an input device (not shown) can be provided, with which the
control unit can be programmed and which enables the input of vehicle-specific
data
such as wheel base, track width, diameters of the front and rear wheels, gear
ratios
of the transmissions, maximum permissible speeds for transmissions and
electric
motors and the like.
The BUS system is connected via a BUS system cable 76 with a micro-
controller 78 for the generator intermediate circuit 54, with the direct
current
intermediate circuit 58, with the micro-controllers 80 for the frequency
converters 62
of the electric motors 18, 32 and with the braking resistance disk 66 so that
these
can be selected by the control unit 70. By means of the BUS system cables 76,
the
micro-controllers 78, 80 and the direct cun-ent intermediate circuit 58 feed
electric
9
CA 02463408 2004-04-07
data with respect to current, potential and frequency to the control unit,
which
enables the calculation of torque, power and the like. The control unit 70
also
supplies electric control signals to the clutches 36 via the BUS system 72,
which is
not depicted in more detail.
The drive system allows a vehicle in normal operation to be driven either by
all four electric motors 18, 32 that are supplied by the generator with
electr9c energy
or solely by the two electric motors 32 of the rear axle 14. If the vehicle is
accelerated or decelerated such that a gear change is required on the change-
speed
gearboxes 34 so as not to operate the electric motors with too high or too low
a
speed, then the power of the electric motors 32 of the rear axle 14 is reduced
and
the clutches 36 are opened with electric signals. Now a switch of the
planetary
switching transmission 34 can occur. Subsequently the clutches are again
closed by
corresponding electric control signals, and the electric power of the electric
motors
32 of the rear axle 14 is again increased.
In order to avoid an interruption in the tractive force during this shifting
process, the two electric motors 18 of the front axle 10 are selected
simultaneously
for the purpose of generating torque on the front wheels 12, which will
balance the
drop in tractive power occurring on the rear wheels 16. The control unit 70
hereby
can synchronize the selection of the electric motors 18, 32 such that when the
power
of the electric motors 32 of the rear axle 16 is reduced the power of the
electric
motors 18 of the front axle 12 is accordingly increasingly. After shifting the
planetary
shifting transmission 34 and closing the clutch 36, the power of the electric
motors
32 of the rear axle 14 is increased again and the power of the electric motors
12 of
the front axle 10 is lowered in the same degree.
In the case of a tractor with good ballast balance, about 30% of the available
tractive power is transmitted via the front wheels (15% per front wheel) and
about
70% via the rear wheels (35% per rear wheel) for nominal tractive power. If
the
power of the tractor is not utilized fully for towing because e.g. the maximum
possible
CA 02463408 2004-04-07
tractive power is not being run, the tractive power remains split between the
front
wheels and the rear wheels roughly at 30% to 70%. Accordingly also the lower
overall drive power of the electric motors is divided between front and back.
A
subsequent adjustment of the torque on the individual wheels occurs in such a
way
that, to the extent possible, roughly equal slip values are created for all 4
wheels.
This way different wheel loads and different friction coefficients between the
tires and
the ground hre taken into consideration optimally. Each wheel hereby maintains
the
greatest possible lateral stability force. This improves driving stability
considerably
and hence driving safety. The vehicle does not veer from the desired path.
To find out the exact slippage, a radar sensor 50 can be used to detect the
actual travel speed v. The slippage s can be calculated from the wheel
circumferential speed a and the actual travel speed v:
s=(u-v)lu.
In practice, however, it is not absolutely necessary to know the actual and
exact slippage value s for each wheel. When the speeds of the wheels while
driving
straight ahead and driving in curves correspond to the rolling condition
according to
Ackermann, equal slippage exists on all wheels. According to Ackermann's
condition, the wheels of a vehicle travel on circular tracks around a common
center.
From the vehicle geometry we know the wheel base, the track width of the axles
and
the scrub radius. According to Ackermann, when traveling in curves the desired
speeds and desired rpm values of the individual wheels can be exactly
calculated
with the electronic control unit 70 based on the curve paths, which can be
calculated,
of the individual wheels. The current steering angle that is required is
determined
with the steering-angle sensor 30. When traveling straight ahead, the wheels
should
have the same circumferential speed. This likewise guarantees the same
slippage
on all wheels.
11
CA 02463408 2004-04-07
Within the respective gears of the change-speed gearboxes 22, 34, the
travel speed is adjusted via the speeds of the electric motors 18, 32. The
necessary
torque is adjusted such that no distortion arises between the individual
wheels 12,
16. This has been accomplished when all wheels 12, 16 have the same slippage.
In
a driven wheel 12, 16 on which greater slippage is detected than the average
of all 4
wheels, the-control unit 70 lowers the driving torque. When the slippage of a
driven
wheel 12, 16 is lower than the average of all 4 wheels, the driving torque is
increased. This way all 4 wheels arrive at the same slippage. This method also
enables an unproblematic driving operation when the ground is such that
different
coefficients of adhesion or coefficients of drive power result between the
tires and
the ground. This way it is possible to pull all wheels evenly in accordance
with their
wheel load and the ground friction coefficients and that atl wheels end up
with the
greatest possible lateral stability force. A wheel with high slippage loses a
large
portion of the possible lateral stability force. In extreme cases too great a
slippage of
individual wheels can lead to a skidding of the vehicle from the track; it is
therefore
important to control the even and low slippage of all wheels. This task of
monitoring
the slip values of the individual wheels is performed by the control unit 70,
which is
part of the drive control system of the vehicle. With slight braking that
occurs only by
means of the electric motors 18, 32 of the individual wheel drives the braking
torque
values are also adjusted in an analog fashion to the same negative slippage.
Tractors travel on solid roads and on soft agricultural ground. Accordingly,
this results in different tractive forces that are to be transmitted and thus
in different
torque values on the individual wheels 12, 16. The torque can be determined
indirectly. The driver specifies a desired speed. The vehicle motor 44 must
overcome road resistance and the additionally desired drive power (e.g. on a
power
take-off shaft that is not shown). This results in the drive power required by
the
vehicle motor 44. The drive power values of the individual electric motors 18,
32
and/or their torque values are divided in accordance with the specified speed.
30%
of the drive power for the front axle 10 means 15% for one wheel motor 18 of
the
12 _
CA 02463408 2004-04-07
front axle 10. Accordingly 70% of the drive power are divided for the rear
axle 14,
meaning 35% for one wheel motor 32 of the rear axle 14. The electric motor 18,
32
are also referred to as wheel motors here.
If the road enables good transmission of the tractive force between the tires
and the ground, slippage between tires and ground should remain below about
5%.
However, viiith every tractive power generated by the tractor a certain amount
of
slippage arises between the tires and the ground. The control unit 70 monitors
the
individual wheel speeds either by means of wheel speed sensors 74 on the wheel
motors 18, 32 or it determines them based on the electric data of the electric
motors
18, 32. It limits deviations in the slip values of the individual wheels 12,
16 to a
permissible amount. Any slip value should not deviate more than e.g. a maximum
of
5% from its desired value. A desired slip value of 5% should therefore be
within the
limits of 4.75% and 5.25%. Splitting of the drive power and selection of the
wheel
motors 18, 32 are performed by the electronic control unit 70. It forwards the
necessary information with regard to the availability of current flow,
potential and
frequency to the frequency converters 62 that are assigned to the wheel motors
18,
32 for the purpose of fulfilling the required slippage conditions.
With a steering angle of the steering axle 10, the required drive rpm values
of the wheels 12, 16 can be established based on~the Ackermann condition. When
performing a calculation with the Ackermann condition, it can be predicted
based on
the geometric rolling circumference values on the individual circular tracks
what
speed rpm values as a function of the steering angle are required. When
traveling in
curves the front wheels 12 drive on a larger circle than the rear wheels 16
and must
accordingly be driven with an adjusted higher speed than when traveling
straight
ahead. The Ackermann condition provides the necessary driving rpm value for
each
wheel 12, 16.
If due to the quality of the road the tractive force between the tires and the
ground is no longer transmitted well, the slip value between tires and ground
can
13 -
CA 02463408 2004-04-07
exceed a value of for example 5%. The control unit 70 assumes the task of
limiting
deviations in the slip values of the individual wheels 12, 16 to a permissible
amount.
The required wheel torque is obtained as a product from the tractive power
of each wheel and its rolling radius. By means of the adjusted transmission
ratios,
the torque of each electric motor 18, 32 can be determined. Torque and
required
wheel speed result in the drive power of each wheel motor 18, 32.
The desired travel speed results in the wheel speed required for it. From the
driving and acceleration resistance the torque values of the electric motors
18, 32
can be calculated. Torque and speed, respectively, result in the required
drive
power values. The overall required drive power is split among the 4 wheels in
accordance with the specified power distribution of 15% for each front wheel
and
35% for each rear wheel. Subsequent adjustment of the torque values for each
wheel drive occurs in accordance with the specification of same slippage
values for
all wheels 12, 16, i.e. subsequent adjustment of the wheel speeds and/or
speeds of
the electric motors 18, 32 in accordance with the Ackermann condition. The
control
unit 70 assumes this function. The wheel load distribution can change
drastically in
some applications, e.g. with a fully loaded front-end loader shovel and when
backing
up a steep slope. In this case, a relatively higher power is demanded from the
electric motors 18 of the front wheels 12. To this end, generally a brief
overload of
these electric motors 18 due to a higher release power can be permitted to the
extent that the driver desires it. Due to the subsequent adjustment to the
same
slippage on all wheels 12, 16 no distortion in the drive occurs and the
greatest
possible lateral stability force is maintained. A high lateral stability force
is important
in the case of slippery ground conditions and locations on slopes to prevent
the
vehicle from skidding off the travel path.
A wheel is torque-free when no tension and no current is generated by the
converter 62 and forwarded to the wheel motor 18, 32 and when the wheel 12, 16
is
not used to drive the electric motor 18, 32 (generator operation), i.e. when
electric
14
CA 02463408 2004-04-07
power is neither fed to the electric motor 18, 32 nor obtained from it. A
relatively low
torque due to frictional forces caused by bearing friction and gear friction
losses,
however, can still be present.
To protect the wheel drives from overload, temperature sensors are
provided in the electric components (electric motors). They feed temperature
signals
to the contrbl unit 70. In case of impermissible heating of the electric
motors 18, 32,
the applied tension and current is lowered to a permissible amount with the
help of
the control unit 70. Generally these current and tension values correspond to
those
for a maximum permissible permanent load. Thus an impermissible increase in
temperature normally leads to a decrease in the vehicle's travel speed,
however
even with a particularly high overload it generally does not lead to a
stopping of the
vehicle. The entire behavior of the vehicle is designed such that the highest
required
tractive force values in accordance with the state of the art are also
achieved with
the drive system pursuant to the invention.
Even if; when traveling downhill at the highest speed, the operator initiates
further acceleration by actuating the gas pedal, the control unit 70
automatically
lowers the drive power down to an automatic braking operation via the four
electric
motors 18, 32, which then operate as generators. The excess power is supplied
to
the generator 48, then operating as a motor, for driving the internal
combustion
engine 44 until it has reached its maximum permissible speed. Further excess
power can be destroyed in braking resistance units 68 andlor be stored in the
vehicle
battery, if necessary. This prevents an impermissible overspeed of the
individual
wheel motors 18, 32 effectively. Moreover the operator can be made aware of
overspeed situations with suitable acoustic or visual warning signals.
Thus it can be seen that the objects of the invention have been satisfied by
the structure presented above. While in accordance with the patent statutes,
only
the best mode and preferred embodiment of the invention has been presented and
described in detail, it is not intended to be exhaustive or to limit the
invention to the
CA 02463408 2004-04-07
precise form disclosed. Obvious modifications or variations are possible in
light of
the above teachings. The embodiment was chosen and described to provide the
best illustration of the principles of the invention and its practical
application to
thereby enable one of ordinary skill in the art to utilize the invention in
various
embodiments and with various mod~cations as are suited to the particular use
contemplated. All such modifications and variations are within the scope of
the
invention as determined by the appended claims when interpreted in accordance
with the breadth to which they are fairly and legally entitled.
16
