U.S. patent application number 10/927195 was filed with the patent office on 2006-03-02 for method and apparatus for braking and stopping vehicles having an electric drive.
Invention is credited to Dale Crombez, Vijay Garg, Raj Prakash, Peter Worrel.
Application Number | 20060047400 10/927195 |
Document ID | / |
Family ID | 34912842 |
Filed Date | 2006-03-02 |
United States Patent
Application |
20060047400 |
Kind Code |
A1 |
Prakash; Raj ; et
al. |
March 2, 2006 |
Method and apparatus for braking and stopping vehicles having an
electric drive
Abstract
A method and apparatus are provided for braking and stopping a
vehicle whose powertrain includes an electric drive. The electric
drive is used to generate braking torque which is used to
decelerate the vehicle down to a full stop. The braking torque is
achieved using any of several closed loop speed control systems.
The system can be used as a substitute for or as a supplement to
conventional friction bakes.
Inventors: |
Prakash; Raj; (Canton,
MI) ; Crombez; Dale; (Livonia, MI) ; Worrel;
Peter; (Troy, MI) ; Garg; Vijay; (Canton,
MI) |
Correspondence
Address: |
TUNG & ASSOCIATES
838 WEST LONG LAKE, SUITE 120
BLOOMFIELD HILLS
MI
48302
US
|
Family ID: |
34912842 |
Appl. No.: |
10/927195 |
Filed: |
August 25, 2004 |
Current U.S.
Class: |
701/70 |
Current CPC
Class: |
B60L 15/2009 20130101;
B60W 2540/12 20130101; B60L 7/14 20130101; B60W 10/08 20130101;
B60L 7/18 20130101; Y10S 903/947 20130101; B60T 13/586 20130101;
Y02T 10/645 20130101; Y02T 10/7275 20130101; Y02T 10/72 20130101;
B60W 2540/10 20130101; B60W 30/18127 20130101; Y02T 10/64
20130101 |
Class at
Publication: |
701/070 |
International
Class: |
G06G 7/76 20060101
G06G007/76 |
Claims
1. A method of braking and stopping a vehicle having at least one
traction wheel driven by an electric motor, comprising the steps
of: sensing a speed parameter related to the speed of the vehicle;
sensing a commanded braking rate; generating a motor control signal
to achieve a desired braking action; producing a negative torque
using the electric motor; applying a braking force to the traction
wheel using the negative torque produced by the electric motor;
and, controlling the amount of negative torque produced by the
electric motor using the motor control signal to achieve the
commanded braking rate.
2. The method of claim 1, wherein the motor control signal is a
torque command signal.
3. The method of claim 1, wherein, the motor control signal is a
speed command signal
4. The method of claim 1, wherein, the motor control signal is a
power command signal.
5. The method of claim 1, wherein, the motor control signal is a
force command signal.
6. The method of claim 1, wherein the motor control signal is
generated using the sensed speed parameter and the commanded
braking rate.
7. The method of claim 1, wherein the step of applying braking
force is continued until the vehicle is stopped.
8. The method of claim 1, wherein the step of generating the motor
control signal comprises: producing a torque command signal used to
control the motor until the vehicle decelerates to a preselected
speed, and producing a speed control signal used to control the
motor after the vehicle has decelerated to the preselected
speed.
9. The method of claim 1, wherein the step of applying braking
force is continued until the vehicle decelerates to a stop.
10. The method of claim 1, wherein the step of generating the motor
control signal comprises producing a torque control signal based on
the positions of the vehicle's brake and accelerator pedals.
11. The method of claim 1, wherein the step of generating the motor
control signal comprises: generating a negative torque control
signal used to control the amount of negative torque produced by
the electric motor until the sensed speed parameter reaches a
preselected value, and then, generating a speed control signal used
to control the speed of the electric motor after the sensed speed
reaches the preselected value.
12. The method of claim 1, wherein the step of sensing the speed
parameter comprises: sensing the speed of the motor, and sensing
the speed of at least one wheel of the vehicle.
13. The method of claim 1, wherein the step of sensing the
commanded braking rate comprises sensing a parameter related to the
operation of the brake pedal.
14. The method of claim 1, wherein the step of generating the motor
control signal comprises: generating a torque command signal,
generating a speed command signal, modifying the speed command
signal using the torque command signal.
15. The method of claim 14, wherein; the step of sensing the speed
parameter comprises generating a motor speed signal representing
the speed of the electric motor, and the step of generating the
motor control signal comprises feeding back the motor speed signal
in a motor control feedback loop, and controlling the motor using
the feedback signal and the modified speed command signal.
16. A method of braking and stopping a vehicle powered by an
electric motor driving at least one traction wheel, comprising the
steps of: sensing a speed parameter related to the speed of the
vehicle; sensing a commanded braking rate; generating a commanded
motor speed signal; generating a torque limiting signal using the
commanded braking rate; generating a torque command signal using
the commanded motor speed signal and torque limiting signal;
controlling the motor using the torque command signal to produce
negative torque; and, applying a braking force to the traction
wheel using the negative torque.
17. The method of claim 16, wherein the step of sensing the speed
parameter comprises sensing the speed of the motor.
18. A method of claim 16, wherein the step of sensing the speed
parameter comprises sensing the speed of at least one wheel of the
vehicle.
19. The method of claim 16, wherein the step of sensing the
commanded braking rate comprises sensing a parameter related to the
operation of a brake pedal on the vehicle.
20. The method of claim 16, wherein the step of generating the
torque command signal comprises using the sensed speed parameter as
a feedback signal in a closed feedback loop, and performing dynamic
compensation of the torque command signal using the feedback
signal.
21. The method of claim 16, wherein the step of generating the
torque command signal comprises modifying the torque command signal
using the torque limiting signal.
22. The method of claim 16, wherein the torque limiting signal is
determined by at least one of the brake pedal or accelerator pedal
inputs.
23. The method of claim 16, further comprising the steps of:
controlling the motor using the torque command signal to produce
positive torque; and, applying a braking force to the traction
wheel using the positive torque.
24. A system for braking and stopping a vehicle powered by an
electric motor driving at least one traction wheel, comprising: a
torque command signal generator for generating a torque command
signal used to drive the motor and produce negative torque for
braking the traction wheel; and, a torque limiting signal generator
for converting brake pedal and accelerator pedal position values
into torque limiting signals used to modify the torque command
signal.
25. The system of claim 24, wherein the torque command signal
generator comprises a dynamic compensator for compensating for the
effects of changes in motor speed.
26. The system of claim 24, wherein the torque command signal
generator comprises a closed speed control loop.
27. The system of claim 26, wherein the torque limiting signal
generator includes a bipolar signal generator.
Description
FIELD OF THE INVENTION
[0001] This invention generally relates to vehicles with electric
drive systems, and deals more particularly with a method and
apparatus for braking and stopping the vehicle using the electric
drive.
BACKGROUND OF THE INVENTION
[0002] Many recent designs of electric powered and hybrid electric
powered vehicles employ a regenerative braking system in order to
increase operating efficiency. During a braking event, the electric
motor which normally drives one or more traction wheels is switched
to operate as an electrical generator. Using the momentum and
kinetic energy of the vehicle, the electric drive motor generates
electricity that may be used to recharge on-board energy storage
systems, such as batteries and ultra capacitors, power accessories,
or power auxiliary on-board systems.
[0003] Regenerative braking systems are particularly effective in
recovering energy during city driving, where driving patterns of
repeated acceleration and decelerations are common. Electric drive
vehicles employing regenerate braking typically utilize traditional
friction brakes, along with a vehicle control system that
coordinates the operation of the friction brakes and the
regenerative brake in order to provide adequate stopping ability
while making dual brake operations essentially transparent to the
driver. Normally, such a control system controls the electric motor
torque to perform regenerative braking until the vehicle
decelerates to a certain speed at which time the friction brakes
are gradually applied to bring the vehicle to a compete stop.
[0004] The dual braking strategy described above may not be optimum
for certain types of electric drive configurations, and may not be
appropriate for configurations where it is desirable to completely
avoid friction braking components. For example, a two axle vehicle
might be provided with friction brakes on the wheels of only one
axle; clearly it would be desirable to provide an electric means of
fully braking the axle not equipped with friction brakes. In some
configurations, it may be desirable to completely avoid the use of
friction brakes, thus necessitating the use of some electronic
means of achieving adequate braking. Even in those configurations
where all wheels are equipped with friction brakes, it may be
desirable to provide frictionless electric braking for each axle in
the event that the friction brakes are intentionally or
unintentionally disabled for any reason.
[0005] Accordingly, a need exists in the art for a braking system
for vehicles with electric drive systems capable of providing
frictionless deceleration and braking of the vehicle to all speeds
down to and including zero speed, regardless of the configuration
of the vehicle's motor drive, axles and wheels. The present
invention is intended to satisfy this need.
SUMMARY OF THE INVENTION
[0006] A system is provided for decelerating and stopping a vehicle
equipped with an electric drive system without the need for
friction brakes, or with reduced need for friction brakes on at
least one wheel. Braking deceleration of the vehicle is achieved by
controlling the electric drive motor to produce negative torque
which is transmitted to the wheels, enabling deceleration down to
and including zero speed. To maintain the stopping position of the
vehicle on grade inclines, the electric drive motor is controlled
to produce a small, compensating amount of positive or negative
torque at zero speed, depending on the direction of the incline.
The system may also be used as a back-up braking system for
vehicles equipped with friction brakes, or to provide supplemental
braking on axle assemblies that are not equipped with friction
brakes.
[0007] One advantage of the invention is that the braking system
can be used with reduced need for conventional friction brakes.
Another advantage lies in the ability of the present braking system
to decelerate the vehicle down to and including zero speed, and
maintain the vehicle at a complete stop under various driving
conditions, such as on a grade, using the speed control loop of the
electric drive. A still further advantage of the invention is that
the need for conventional friction brakes may be completely
avoided.
[0008] In accordance with a first embodiment of the invention, a
method is provided for braking and stopping a vehicle having at
least one traction wheel driven by an electric motor. Braking and
stopping is achieved by sensing a speed parameter related to the
speed of the vehicle, sensing a commanded braking rate, generating
a motor control signal using the sensed speed parameter and
commanded braking rate, producing a negative torque using the
electrical drive motor, applying braking forces to the traction
wheels using the negative torque, and controlling the amount of
negative torque produced by the electric drive motor using the
motor control signal to achieve the commanded braking rate. The
motor control signal may include a torque command signal, a speed
command signal or a combination of these two signals. The torque
command signal can be used to control the motor until the vehicle
decelerates to a pre-selected speed, following which a speed
control signal is used for motor control. The motor control signal
is based on torque commands determined by the position of the
vehicle's brake and accelerator pedals. The sensed speed parameter
may include either the speed of the drive motor, the speed of at
least one wheel of the vehicle, or a combination of these sensed
speeds.
[0009] In accordance with a second embodiment of the invention, a
system is provided for braking and stopping a vehicle powered by an
electric motor driving at least one traction wheel. The system
includes a closed loop speed control loop whose speed command is a
zero speed signal. This closed loop system features modification of
its control signal (torque command signal for the electric drive)
by a bipolar torque limit signal-pair. The limit signal-pair is
directly derived from a torque command that is obtained by the
vehicle system controller, with the accelerator and brake pedals as
inputs. The torque command of the vehicle system controller may be
used for driving and deceleration at higher speeds, but the
torque-limited speed control loop is used for bringing the vehicle
to a stop.
[0010] These non-limiting features, as well as other advantages of
the present invention may be better understood by considering the
following details of a description of a preferred embodiment of the
present invention. In the course of this description, reference
will frequently be made to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is an exemplary block diagram of an electric drive
system for a vehicle;
[0012] FIG. 2 is a graph showing the brake torque as a function of
speed, produced in a vehicle equipped with the combination of
electric and friction braking systems;
[0013] FIG. 3 is a graph showing commanded torque and actual
electric drive braking torque as a function of speed, in a vehicle
equipped with the electric braking system, according to one
embodiment of the present invention;
[0014] FIG. 4 is a graph of the actual electric drive brake torque
as a function of speed generated in accordance with another
embodiment of the present invention;
[0015] FIG. 5 is a graph showing actual electric drive brake torque
as a function of speed, produced according to a further embodiment
of the present invention;
[0016] FIG. 6 is a block diagram of a system for braking and
stopping an electric drive vehicle, which includes torque limiting
with bipolar signals and a speed control loop, in accordance with
the present invention;
[0017] FIG. 7 is a block diagram of a system for obtaining averaged
motor speed; and,
[0018] FIG. 8 is a block diagram of a speed control loop having a
nested torque control system, used in the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0019] The invention relates to a method and apparatus for
decelerating, braking and stopping a vehicle equipped with an
electric drive system which includes an electric motor. A typical
electric drive system 10 is shown in FIG. 1. An electric motor 12
mounted on the vehicle's chassis has an output drive shaft 14 which
is connected through a differential gear-set 16 to a drive axle 18
carrying one or more traction wheels 20. Energy for powering the
motor 12 is derived from an on-board storage battery 22 which
provides DC power that is converted by an inverter 24 into AC power
used to drive the motor 12. Although an AC motor 12 has been
disclosed here, it should be noted that the present invention is
suitable for use with a variety of DC and poly-phased AC motors. A
vehicle control system 26 coordinates and controls the operation of
the energy storage and drive components, and manages system
functions such as charging, engine starting and stopping and
regenerative braking. The vehicle control system 26 may implement
any of a variety of known control strategies, using software
programs and input information derived from a variety of on-board
sensors 28, as well as accelerator pedal and brake pedal position
information 30. It should be noted here that although a drive
system 10 has been shown employing only a single motor 12, the
present invention may be used in drive systems employing multiple
electric motors, alternate fuel sources and hybrid configurations
employing at least one drive electric motor. Furthermore, the motor
12 may be in the form of a wheel motor that is incorporated
directly into one or more wheels on the vehicle. For sake of
convenience in the describing and claiming the invention, "negative
torque" applied to a drive wheel shall mean a torque that opposes
the motion of the vehicle, whereas a positive torque applied to the
wheel shall mean a torque that favors the vehicle's motion.
[0020] The vehicle control system 26 may deliver either a torque
command or a speed command to the motor 12, having a polarity and
magnitude that is based on the positions of the accelerator pedal
and the brake pedal 30. The torque command can be either positive
or negative in both drive and reverse "gear" selected as the
desired direction of travel; as is known in the art, a positive
command results in traction torque while a negative command results
in braking or deceleration torque. The details of generating both
torque and speed commands as a function of pedal positions depend
on the particular vehicle configuration and will be based on any of
various control strategies which are well known in the art. A
torque or speed command developed by the control system 26 is
delivered to the inverter 24, causing the motor 12 to produce
positive torque which is delivered by a driven axle 18 to traction
wheels 20. Based on the position of the accelerator and brake
pedals 30, the control system 26 switches the motor 12 to its
regenerative mode in which the motor 12 acts as an electrical
generator, converting the vehicle's kinetic energy into electrical
energy used to recharge the battery 22. During regenerative
braking, motor 12 produces a negative torque.
[0021] The relationship between the negative torque produced by
motor 12 and that produced by the vehicle's friction brakes is
better understood by reference to FIG. 2 which plots torque of the
motor 12 as well as friction brake torque as a function of vehicle
speed. The plot of FIG. 2 corresponds to a typical vehicle that
employs friction brakes on at least one wheel, in addition to
regenerative braking provided by at least one electric drive motor
on the vehicle. Different modes of braking torque occur over three
distinct regions respectively designated as Region 1, Region 2 and
Region 3. At higher vehicle speeds shown in Region 1, regenerative
braking results in an electric drive torque command 32 which
continues until the vehicle brakes to a speed at which friction
brakes are applied to produce friction brake torque 34 at the
beginning of Region 2. As the friction brake torque 34 increases,
the electric drive torque command 32 ramps down until Region 3 is
reached where the braking torque is entirely the result of the
friction brakes. In Region 3, friction brake torque 34 reaches a
constant value and the electric drive torque produced by the torque
command 32 is zero.
[0022] In accordance with the present invention, deceleration of
the vehicle down to and including zero speed (a complete stop) is
accomplished using negative torque produced by the motor 12,
without the use of braking torque supplied by friction brakes.
[0023] In accordance with one technique of the present invention,
the vehicle control system 26 delivers signals to the motor 12
commanding negative torque 32 as shown in FIG. 3, down to a
pre-selected speed where the negative torque is then ramped down to
zero. The actual electric drive torque produced by the commanded
torque signal 32 is designated by the numeral 36 and can be seen to
closely follow the commanded torque curve 32. Thus, using this
first technique, only torque control is used for decelerating and
stopping the vehicle. This technique is suitable for vehicle
operation on essentially level ground. If there is change in ground
elevation or grade resulting in an upward slope or downward slope
there could be some movement of the vehicle after coming to a near
stop. Thereafter, the system will react to the vehicle's speed and
effect deceleration but perfect holding at zero speed may not be
achieved with this technique if material ground (elevation or
grade) slope is present. Accordingly, it may be necessary in using
this technique to apply the vehicle's parking brakes, either
manually or automatically through the electronic controls, in order
to assure that the vehicle is held in a stationary position.
[0024] In accordance with another technique, the motor 12 is used
to produce negative torque down to a pre-selected speed using the
torque control mode previously described, following which motor 12
is switched to speed control mode in which the speed command is
zero or another command determined by the accelerator and brake
pedal position inputs 30. FIG. 4 is a graph of torque versus speed,
which illustrates the second technique more clearly. As can be seen
at higher speeds, using the torque control mode 56, the electric
drive actual torque 52 is relatively constant down to a
pre-selected speed where the control scheme is switched over to
speed control mode 58. Speed control results in the actual drive
torque 52 ramping down from a corner point 54 to zero speed where
the vehicle reaches a full stop. This technique provides adequate
position holding when the vehicle stops on a (ground
elevation/grade change) material grade slope, since at near zero
speed, a speed control loop used to implement the technique
generates enough torque to compensate for the slope.
[0025] If desired, the motor 12 can be operated in a speed control
mode throughout the deceleration and stopping procedure using a
zero speed command or other speed command that is based on the
position of the pedals 30. FIG. 5 is a graph showing the actual
electric drive torque 60 during the deceleration and stopping
procedure performed using only the speed control mode of operation.
It can be seen that the plot of the actual negative torque 60 is
more gradual in the reduction of torque as speed decreases.
Moreover, it can be seen that the actual torque 60 becomes slightly
positive at zero speed. This slightly positive torque at zero speed
corresponds to a situation where the vehicle is on a slightly
positive or upward grade incline. The slight amount of residual
positive torque maintains the vehicle in its stopped position, and
compensates for the incline. Similarly, if the vehicle comes to
rest on a downward grade incline, a small amount of negative torque
is applied at zero speed in order to maintain the position of the
vehicle on the incline.
[0026] In some applications and vehicle configurations it may not
be convenient to translate accelerator and brake pedal inputs 30
into a speed command. In order to address this possibility, a
further technique is provided in accordance with the present
invention which is illustrated in the block diagram of FIG. 6.
Accelerator and brake pedal positions 62, 64 are translated into
torque commands by a torque command generator 66; these torque
commands may be either negative or positive, depending upon the
vehicle's operating conditions. The torque commands are translated
into a bi-polar signal (two states--state 1 or state 2) by a
bi-polar signal generator 68 which is used as a bi-polar torque
limiter, with either positive or negative limit values 70, 72, to
further control the torque command signal 74 that is used to
control the electric drive 10.
[0027] The speed control loop includes a dynamic compensator 38
which outputs a torque command signal 74 to the electric drive 10
after being subjected to limits 70, 72. The electric drive produces
a torque 50 and motor speed 48. The motor speed 48 is fed back in a
feedback loop 46 where it is compared at 40 with the motor speed
command (normally zero speed) and the error information is fed to
the dynamic compensator 38. The output of the dynamic compensator
38 is the torque command signal 74 which is subjected to the limits
70, 72 and hence may become limited. The resulting torque command
signal is the final torque command signal for the electric drive
10. One function of the speed control loop is to generate electric
drive torque command whose function is to reduce the speed of the
motor 12 to zero by closed loop control action. As previously
noted, due to the action of the speed control loop, the torque at
zero and near zero speeds will be positive (corresponding to the
traction) if there is a grade opposing the forward motion of the
vehicle, and it will be negative if there is a grade favoring the
forward motion of the vehicle. As can be seen in FIG. 6, the torque
control loop is nested within the speed control loop with the motor
speed 48 being fed back in loop 46 to be combined with the
commanded motor speed. The commanded motor speed is the desired
vehicle speed multiplied by an appropriate gear ratio related to
the gear-set 16 (FIG. 1). The initial condition of the dynamic
compensator 38 should be set to the value of the torque command
value existing at the moment preceding the transition from torque
control to speed control. It should be noted here that if the brake
pedal is not depressed a sufficient amount, the torque limiting
which is imposed will be small in magnitude and the electric drive
torque produced may not be sufficient to obtain zero motor speed
and vehicle speed. In this case, the vehicle operator will depress
the brake pedal a further amount in order to obtain zero speed. In
a similar manner, if the vehicle is being brought to a stop on a
steep slope, the operator may need to depress the brake
sufficiently and, possibly fully, in order to achieve and/or
maintain a complete vehicle stop. When the torque command as output
by the pedal interpreter changes sign, the speed control loop is
exited. It should be appreciated here that if the driver does not
depress the brake sufficiently and the above described torque
limiting function is not employed, the vehicle can be held on a
grade solely through the use of the speed control function. In
other words, if the torque command signal 74 of the closed loop
system of FIG. 6 is not subjected to the limits 70, 72, then the
system would be able to achieve and maintain zero speed on a grade
even though the brake is not depressed sufficiently.
[0028] It should be further noted that in each of the control
techniques described above, regeneration cannot take place at low
speeds, even though the sign of the electric drive torque is
negative. Due to certain fixed losses in the drive system, the
battery will be supplying power at speeds near zero, even though
the generated torque is negative. Moreover, due to the action of
the speed control loop, the torque at zero and near zero speeds
will be positive (corresponding to traction) if there is a grade
opposing the forward motion of the vehicle, and it will be negative
if there is a grade favoring the forward motion of the vehicle.
[0029] Depending upon the vehicle and electric drive configuration,
some small inaccuracies of the motor speed signal may occur at
vehicle speeds near zero. This may be caused in part by noise and
quantization effects due to the operation of motor speed encoders.
Thus, it may be desirable to improve the motor speed detection in
certain applications, and in this connection a technique is shown
in FIG. 7 for improving speed detection accuracy. A plurality of
wheel speed sensors, WSS#1-WSS#4 are used in combination with a
motor speed sensor 84 to arrive at a speed signal used for the
control process. The wheel speed sensor information is combined and
averaged at 76 and multiplied by a scale factor at 78 which is
related to the gear ratio between the motor 12 and wheels 20. The
averaged and scaled wheel speed information is added to the motor
speed 84 at 80 and then multiplied by a factor of 1/5 at block 82.
The resulting motor speed value having superior accuracy is used at
the feedback signal and loop 46 (FIG. 6).
[0030] FIG. 8 shows a simplified speed control loop with nested
torque control system. In this embodiment, the torque control loop
is a loop within the speed control loop, and the motor speed is
measured and used as the feedback signal. Commanded motor speed is
the desired vehicle speed multiplied by the appropriate gear ratio.
The same scheme given above and shown in FIG. 7 can be used for
obtaining motor speed in those embodiments of the invention wherein
the speed control loop shown in FIG. 8 is used.
[0031] From the foregoing description it is apparent that a novel
method is provided of braking and stopping a vehicle having at
least one traction wheel. The method includes the steps of sensing
a speed parameter related to the speed of the vehicle, sensing a
commanded brake rate, generating a motor control signal using the
sensed speed parameter and commanded braking rate, producing a
negative torque using the electric motor, applying a braking force
to the traction wheel using the negative torque, and controlling
the amount of negative torque produced by the electric motor using
the motor control, signals to achieve the commanded braking rate.
The motor control signal may be a power command signal or a force
command signal.
[0032] It is to be understood that the specific methods and
techniques which have been described are merely illustrative of one
application of the principles of the invention. For example, if the
motor torque capability is limited, the present method can be
utilized in combination with friction brakes that may or may not be
downsized. Moreover, the present invention does not require the
elimination of friction brakes on at least one wheel. Numerous
modifications may be made to the method and system as described
without departing from the true spirit and scope of the
invention.
* * * * *