Method and system for controlling the speed of a vehicle

A method and system for controlling the speed of a vehicle senses a position of an accelerator pedal. In a first embodiment, the accelerator pedal has a mechanical predetermined natural position, wherein when the position of the accelerator pedal is held constant at the predetermined natural position the control logic interprets the position of the pedal as a request for constant vehicle speed. However, at low vehicle speeds, the control logic interprets the position of the pedal as a request for constant vehicle speed when the position of the pedal is held constant at a position below the predetermined natural position and controls the speed of the vehicle according to a predetermined zero acceleration curve. In a second embodiment, constant speed control is assumed when the position of the pedal has not changed for a predetermined amount of time, regardless of the relative position of the pedal. Any torque offsets encountered during the constant speed mode is reduced and eventually eliminated when the constant speed mode is exited and the pedal position increases/decreases.

TECHNICAL FIELD 
This invention relates to methods and systems for controlling the speed of 
a vehicle based on a position of the accelerator pedal. 
BACKGROUND ART 
Current typical vehicle speed control systems utilize dashboard/steering 
wheel controls to regulate the speed of a vehicle at a level set by the 
driver without the driver having to maintain pressure on the accelerator 
pedal. The driver brings the car up to a desired speed and turns on the 
cruise control. Next, the driver programs that speed into the system by 
pressing a button. A sensor measures the speed at which the vehicle is 
traveling. Based on the difference, if any, between the actual speed of 
the vehicle and the desired speed set by the driver, a control logic 
controls a servo motor to set the throttle so that the speed of the engine 
is at the level needed to have the car travel at the desired speed. This 
type of cruise control, however, is difficult to use in traffic since 
frequent changes in the desired speed are required. 
In order to accommodate the driver's typical behavior in traffic, cruise 
control systems have been developed that control the speed of the vehicle 
based on a position of the accelerator pedal. For example, one known prior 
art control system disclosed in U.S. Pat. No. 4,615,409 provides a snap-in 
detention device at the junction point of an acceleration section and a 
deceleration section such that a foot can rest with a force slightly above 
the weight of the foot against the accelerator pedal while the speed of 
the motor vehicle is maintained constant. However, for low vehicle speeds 
(i.e., deceleration or starting from a stop position), the driver is 
required to push the accelerator pedal to a predefined "0" position to get 
the vehicle moving. 
Another known prior art speed control system that utilizes the position of 
the accelerator pedal to maintain the speed of the vehicle is disclosed in 
U.S. Pat. No. 4,541,052. In this system, the speed of the vehicle is 
controlled based on whether the accelerator pedal is an a transient 
operation mode or in a steady state operation mode. When steady state s 
indicated, the vehicle power output is brought into correspondence with a 
power command determined according to the difference between the vehicle 
velocity and a remembered velocity. This system, however, describes a 
linear relationship between output power and accelerator position which 
does not result in a normal feel for a driver used to existing vehicles 
which have a nonlinear relationship that varies with engine speed. 
DISCLOSURE OF THE INVENTION 
It is thus a general object of the present invention to provide a method 
and system for controlling the speed of a vehicle utilizing the 
accelerator pedal without disrupting the natural feel of the accelerator 
pedal and maintaining the relationship between the pedal position and 
vehicle performance at all vehicle speeds. 
In carrying out the above object and other objects, features, and 
advantages of the present invention, a method is provided for controlling 
the speed of a vehicle based on a position of the accelerator pedal. In a 
first embodiment, control logic is provided that is operative to control 
the speed of the vehicle at a constant speed when the position of the 
accelerator pedal is held at a position different from a predetermined 
natural position. In this embodiment, the method includes sensing a speed 
of the vehicle, sensing a position of the accelerator pedal, determining 
if the speed of the vehicle is less than a predetermined low speed 
threshold, and controlling the speed of the vehicle at a constant speed 
when the position of the pedal is held below the predetermined natural 
position and the speed of the vehicle is less than the predetermined low 
speed threshold. 
In the second embodiment, the method includes sensing a speed of the 
vehicle, sensing a position of the accelerator pedal, determining a first 
offset during a constant speed mode according to a predetermined torque 
curve, the speed of the vehicle, the position of the accelerator pedal, 
and a torque required to maintain the constant speed, determining a second 
offset based on the first offset and the position of the accelerator pedal 
when the vehicle has exited the constant speed mode, and controlling the 
speed of the vehicle during a non-constant speed mode based on the 
predetermined torque curve, the second offset, the speed of the vehicle 
and the position of the accelerator pedal. 
In further carrying out the above object and other objects, features, and 
advantages of the present invention, a system is also provided for 
carrying out the steps of the above described methods. The system for the 
first embodiment includes a speed sensor for sensing a speed of the 
vehicle, a position sensor for sensing a position of the accelerator 
pedal, and control logic operative to determine if the position of the 
pedal is held constant for a predetermined amount of time, determine if 
the speed of the vehicle is less than a predetermined low speed threshold, 
and control the speed of the vehicle at a constant speed when the position 
of the pedal is held constant below the predetermined natural position and 
the speed of the vehicle is less than the predetermined low speed 
threshold. 
The system for the second embodiment includes a speed sensor for sensing a 
speed of the vehicle, a position sensor for sensing a position of the 
accelerator pedal and control logic operative to determine a first offset 
during the constant speed mode according to a predetermined torque curve, 
the speed of the vehicle, the position of the accelerator pedal and a 
torque required to maintain the constant speed, determine a second offset 
based on the first offset and the position of the accelerator pedal when 
the vehicle has exited the constant speed mode, and control the speed of 
the vehicle during the non-constant speed mode based on the predetermined 
torque curve, the second offset, the speed of the vehicle and the position 
of the accelerator pedal. 
The above object and other objects, features and advantages of the present 
invention are readily apparent from the following detailed description of 
the best mode for carrying out the invention when taken in connection with 
the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Turning now to FIG. 1, there is shown a schematic diagram of a system, 
denoted generally by reference numeral 10, including an internal 
combustion engine 12, an accelerator pedal 14 and a Powertrain Control 
Module (PCM) 16 which embody the principles of the present invention. The 
accelerator pedal 14 is used by the driver to control the vehicle speed. 
The accelerator pedal 14 is connected to a position sensor 18 such as, for 
example, a potentiometer, which generates a signal indicating the position 
of the accelerator pedal 14. 
The position signal is transmitted to the PCM 16 which contains 
predetermined control logic for controlling the speed and acceleration of 
the vehicle based on the position of the accelerator pedal 14 and various 
other inputs, as described below. The PCM 16 can be embodied by an 
electronically-programmable microprocessor, a microcontroller, an 
application-specific integrated circuit, or a like device to provide the 
predetermined control logic. 
The other inputs to the PCM 16 include a signal from a switch 20 in a brake 
system used to determine when the brake (not shown) is pressed, a signal 
corresponding to a speed of the vehicle from a speed sensor 22 disposed on 
a drive shaft 24, or alternatively, on a wheel (not shown) of the vehicle, 
a signal from the transmission 26 indicating the current gear of the 
transmission 26, a signal from the engine 12 indicating a temperature of 
the engine, and an air flow meter 28 coupled to a throttle plate 30 
indicating the position of the throttle plate 30 and the amount of air 
flow into the engine 12. In addition, inputs similar to conventional 
cruise control switches such as signals from, but not limited to, an ON 
switch 32, an OFF switch 34, an ACCELERATE switch 36, a DECELERATE switch 
38, and a SET switch 39 may be included to provide the conventional "foot 
off the pedal" speed control functionality that may be convenient for 
highway travel. 
The present invention, however, enables the driver of the vehicle to 
control or maintain the speed of the vehicle utilizing only the position 
of the accelerator pedal 14. In a first embodiment, the method and system 
provides a tactile feedback which allows the driver to consciously select 
constant speed operation. In a second embodiment, the method and system 
provides constant speed operation by automatically interpreting the intent 
of the driver in an unobtrusive manner. 
In the first embodiment, the accelerator pedal 14 has a mechanical 
predetermined "natural" position that allows the driver to easily position 
the pedal in a position that for most vehicle speeds is interpreted by the 
PCM 16 as a request to maintain the current vehicle speed. Pushing the 
pedal 14 beyond the natural position is interpreted as a request for a 
vehicle acceleration, while releasing the pedal 14 is interpreted as a 
request for deceleration. The rate of acceleration/deceleration varies as 
a function of the distance from the natural position. 
FIGS. 2a and 2b show simplified examples of two spring arrangements that 
provide a change in the pedal return force that can be easily felt by the 
driver of the vehicle. In FIG. 2a, two springs 40, 42 are utilized. Spring 
40 is in constant contact with the pedal 14 and provides a gentle force 
when the pedal 14 is between positions A and B, where position A 
corresponds to a relaxed position, i.e., 0% of pedal travel, of the pedal 
14 and position B corresponds to the natural position, e.g., 25% of pedal 
travel of the pedal 14. At position B, the pedal 14 contacts the second 
spring 42 which is held in place by a pre-load provided by stop 44. The 
stop 44 is fixably mounted to a housing of the pedal assembly at a 
position dependent upon the arrangement of the springs 40, 42. 
To move the pedal 14 beyond position B, the driver must first overcome the 
pre-load force and then move the pedal against the combined force from 
both springs 40, 42. With this arrangement, the force required to move the 
pedal 14 is shown in FIG. 3a. 
In FIG. 2b, the pedal 14 bears against a single spring 46. Between 
positions A and B, the entire length of the spring 46 is free to flex and 
provides a light force. At position B, the spring encounters a fixed stop 
48 mounted to the housing of the pedal assembly. As the pedal 14 is moved 
beyond position B, the force required to deflect the spring 46 increases 
rapidly since the spring 46 is now constrained to bend around the stop 48. 
The resulting change in force required to move the pedal 14, as shown in 
FIG. 3b, is easily detected by the driver and allows the driver to find 
the "natural" position. 
Turning now to FIG. 4, there is shown a graph illustrating how the PCM 16 
interprets the position of the pedal 14 as a request for vehicle 
acceleration as a function of speed. The curves a1-a5 represent lines of 
constant vehicle acceleration rates (the rate of acceleration increases 
from a1 to a5). Curves z, z+ and z- represent the pedal position/vehicle 
speed regions that are interpreted as a request for zero acceleration, 
i.e., constant vehicle speed. The distance between z+ and z- is provided 
to allow for small errors in the sensor 18 and other parts of the 
mechanical system. 
Curves d1 through d5 represent lines of constant negative acceleration 
(deceleration). Thus, as the driver continues to press the pedal 14, the 
vehicle will continue to accelerate according to the a curves until the 
driver allows the pedal 14 to fall back to the natural position. 
Similarly, letting up on the pedal 14 from controlled speed operation 
causes the vehicle to decelerate according to the d curves. 
At low vehicle speeds, the z curve deviates from the natural position of 
the pedal 14. This provides vehicle operation that is similar to the 
driver's current expectation, i.e., it is not necessary to move the pedal 
beyond the natural position to start the vehicle in motion. This also 
eliminates problems with mode transitions that would occur if the speed 
control mode were simply disabled at low vehicle speeds. For example, if 
the driver presses the pedal 14 at 10% of full travel, the vehicle 
accelerates at a rate defined by a1, as shown in FIG. 4. As the vehicle 
speed increases, the rate of acceleration is decreased until the vehicle 
speed reaches approximately 15 mph, i.e., the position of the pedal 14 
that corresponds to zero acceleration, as shown in FIG. 4. This speed is 
maintained until the driver either lets up on the pedal 14 or pushes the 
pedal 14. This effect would feel natural to the driver of the vehicle. 
At higher vehicle speeds, the z curve may, if desired, be made to deviate 
from the natural position to discourage sustained high speed vehicle 
operation. As shown in FIG. 4, the driver of the vehicle must hold the 
pedal 14 down beyond the natural position in order to maintain a constant 
vehicle speed according to the z curves. 
Once the desired rate of acceleration is determined, the PCM 16 commands a 
change in the powertrain output to achieve the desired acceleration and 
speed. The PCM 16 uses various actuators to control the powertrain output 
to proved the desired vehicle acceleration. An electronic throttle 
actuation device 28 is used to control the position of the throttle 30 
and, thus, airflow into the engine 12. The PCM 16 also controls other 
engine actuators such as fuel injectors (not shown) which control the 
quantity of fuel ingested by the engine 12, the system that controls the 
timing of the spark used for ignition, and other devices to insure 
efficient engine operation. In addition to controlling the states of the 
engine 12, the PCM 16 can also use actuators (not shown) in the 
transmission 26 that control the transmission gear ratio and torque 
converter clutches to provide efficient operation and adequate 
acceleration rates. 
Turning now to FIG. 5, there is shown a flow diagram illustrating the 
general sequence of steps associated with the method of the first 
embodiment of the present invention. Although the steps shown in FIG. 5 
are depicted sequentially, they can be implemented utilizing 
interrupt-driven programming strategies, object-oriented programming, or 
the like. In a preferred embodiment, the steps shown in FIG. 5 comprise a 
portion of a larger routine which performs other speed control functions. 
Upon initialization, the method begins with the step of determining whether 
the conventional cruise control system has been activated, as shown at 
conditional block 50. If so, the speed of the vehicle is controlled 
according to a set target speed, as shown at block 52. To maintain the 
target speed, the actual speed of the vehicle is compared with the target 
speed, as shown at conditional block 54. If there is no difference between 
the two speeds, there is no change in the powertrain output, as shown at 
block 56. 
If the actual speed of the vehicle is less than the target speed, the 
powertrain output is increased, as shown at block 58. As discussed above, 
this can be accomplished by controlling the throttle position, fuel flow, 
spark timing, transmission gear ratio, etc., to match the target speed via 
known closed loop control techniques. Similarly, if the actual speed of 
the vehicle exceeds the target speed, then the powertrain output is 
decreased, as shown at block 60. 
Returning to conditional block 50, if conventional cruise control has not 
been activated, the method proceeds to determine a target acceleration 
according to the predetermined acceleration curves illustrated in FIG. 4, 
as shown at block 62. Next, a determination is made as to whether the 
target acceleration is positive, negative, or zero, as shown at 
conditional block 64. 
If the target acceleration rate is positive, i.e., the driver has pressed 
the accelerator pedal 14 beyond the Z-curve, the target speed is increased 
according to the position of the pedal 14 and the corresponding 
acceleration curve, as shown at block 66. The method then proceeds to 
control the powertrain output as discussed above. 
If the target acceleration rate is negative, i.e., the driver has let up on 
the pedal 14, the target speed is decreased according to the position of 
the pedal 14 and the corresponding deceleration curve, as shown at block 
68. As with the positive acceleration rate target, the method proceeds to 
control the powertrain output as discussed above. 
If, on the other hand, the target acceleration rate is zero, i.e., the 
driver has positioned the pedal 14 at a point on the Z-curve, a 
determination is first made as to whether this is the initiation of 
constant speed mode, as shown at conditional block 70. Keep in mind that 
this constant position does not necessarily have to be at the mechanical 
predetermined natural position. For lower vehicle speeds, this position 
decreases from the natural position. At higher vehicle speeds, this 
position increases from the natural position. If this is the first 
initiation of constant speed mode, the target speed is set equal to the 
current vehicle speed, as shown at block 72. If not, the there is no 
change to the target speed, as shown at block 74. The method proceeds to 
control the powertrain output as discussed above. 
Returning to the discussion of the second embodiment mentioned above, the 
desire for constant speed operation is determined by observing the 
behavior of the pedal 14 and subsequent vehicle response. If the driver is 
holding the pedal at a constant position, and the vehicle speed has been 
nearly constant for some period of time, the PCM 16 then decides that the 
driver intends to maintain that speed. The PCM 16 maintains the vehicle 
speed until the driver changes the position of the pedal 14. The change in 
the position of the pedal 14 is interpreted as a request to accelerate or 
decelerate at a rate related to the magnitude of the change in the 
position of the pedal 14. 
Turning now to FIG. 6, there is shown a graph illustrating the desired 
relationship between the position of the pedal 14 and the powertrain 
output torque for various vehicle speeds. Note that the maximum powertrain 
torque is lower at higher speeds. This is due to the required changes in 
transmission gear ratio. Also, this curve may be modified for factors such 
as barometric pressure, which would limit the torque available from the 
powertrain. 
The operation of the present invention will now be discussed with reference 
to FIGS. 7a-7b. FIG. 7a represents a potential scenario where the speed 
control mode is entered at point A; the curve X,A,Y is the pedal 
position/torque curve at 40 mph from FIG. 6. If the torque required to 
maintain the desired speed is reduced to point B (e.g., the vehicle is 
going downhill) and the driver decides to accelerate and move the pedal 
14, the relationship between the pedal position and the torque will now be 
represented by the curve X,B,C,Y. 
If the pedal is moved to point C, it is clear that the torque offset (C'-C) 
is smaller than the original offset (A-B) and now the curve X,D,C,Y in 
FIG. 7b describes the pedal/torque relationship. Additional changes to the 
pedal position (e.g., to point D) further reduces the offset and defines 
new pedal/torque relationships which will converge back to the original 
curve. 
During normal operation the desired powertrain output torque is calculated 
as a function of pedal position and vehicle speed, as shown in FIG. 6, and 
other modifiers such as barometric pressure (measured or estimated). The 
vehicle operation is very similar to what the driver has become accustomed 
to today with current vehicles. However, if it is determined that the 
vehicle is operating at a nearly constant speed and the driver is holding 
the pedal at a constant position, the system 10 will assume that the 
driver wishes to drive at a constant speed. The system 10 will then enter 
a closed loop speed control mode. 
During operation in the closed loop speed control mode, the system 10 
modifies the torque output of the powertrain by changing the throttle 
position, fuel flow, transmission gear if applicable, etc., to maintain 
the vehicle speed. Consequently, the actual powertrain torque at this 
constant pedal position may deviate from the torque derived from FIG. 6. 
If the driver changes the pedal position (or presses on the brake), the 
closed loop speed control mode is exited. At this point, it is desirable 
to smoothly reduce the torque offset that may have resulted from the 
closed loop operation. This invention does this by reducing the offset as 
the pedal is moved such that the offset would be reduced to zero at each 
extreme of the pedal travel. By reducing the offset with each pedal 
movement, the system will, in time, reduce the offset to nearly zero even 
if the pedal is never moved to either end of its pedal movement range as 
discussed with reference to FIGS. 7a-7b. 
Turning now to FIGS. 8a-8b, there are shown flow diagrams illustrating the 
general sequence of steps associated with the method of the second 
embodiment of the present invention. Although the steps shown in FIGS. 
8a-8b are depicted sequentially, they too can be implemented utilizing 
interrupt-driven programming strategies, object-oriented programming, or 
the like. In a preferred embodiment, the steps shown in FIGS. 8a-8b also 
comprise a portion of a larger routine which performs other speed control 
functions. 
Upon initialization, the method begins with the step of determining whether 
the conventional cruise control system has been activated, as shown at 
conditional block 80. If so, the required torque is determined, as shown 
at block 82. The required torque is then controlled, as shown at block 84. 
If conventional cruise control has not been activated, a determination is 
made as to whether the vehicle is in constant speed mode, as shown at 
conditional block 86. If the pedal 14 has been in the same position for 
the predetermined period of time, e.g., approximately five seconds, then 
constant speed mode is entered and the method proceeds to determine if the 
pedal 14 has moved any or the brake (not shown) has been pressed, as shown 
at conditional block 88. If there has been no movement by the pedal 14 or 
the brake, the system 10 remains in constant speed mode and proceeds to 
closed loop vehicle speed control, as shown at block 90 and 92, 
respectively. 
Once in the closed loop vehicle speed control mode, a comparison is made 
between the actual vehicle speed and the target speed, as shown at 
conditional block 94. If the two speeds are the same, the current torque 
is maintained and controlled by the PCM 16, as shown at blocks 96 and 84, 
respectively. If the actual speed is either greater than or less than the 
target speed, the torque is decreased or increased, as shown at blocks 98 
and 100, respectively. Thus, the actual torque at the constant pedal 
position may deviate from the torque value derived from the predetermined 
torque curve shown in FIG. 6. 
Returning to conditional block 88, if the pedal 14 has moved or the brake 
has been pressed, the constant speed mode is exited, as shown at block 
102. At block 104, an offset value is determined based on the difference 
between the current torque value and the torque value derived from the 
torque curve in FIG. 6. The torque delivered by the powertrain is updated 
based on the torque value from the predetermined torque curve plus the 
offset, as shown at block 106. This offset insures that there is no abrupt 
transition between constant speed control and manual 
acceleration/deceleration. 
Returning to conditional block 86, after the vehicle has initially exited 
from the constant speed mode, following block 106, a determination is made 
as to whether the vehicle is at near zero acceleration, as shown at 
conditional block 108, or whether the position of the pedal is not 
constant, as shown at conditional block 109. If the vehicle is not at near 
zero acceleration or if so, but the pedal position is not constant, a 
determination is then made as to whether the vehicle is accelerating or 
decelerating following the constant speed mode, as shown at conditional 
block 110. If the vehicle is accelerating, i.e., the position of the pedal 
14 is greater than the previous position of the pedal 14, then an 
acceleration offset is determined based on the previous offset value, as 
shown at block 112, according to the following: 
EQU Acceleration Offset=((100-pedal position)/(100-previous pedal 
position))*Previous Offset. 
If the vehicle is decelerating, a deceleration offset is determined based 
on the previous offset value, as shown at block 114, according to the 
following: 
EQU Deceleration Offset=(current pedal position/previous pedal 
position)*Previous Offset. 
The torque delivered by the powertrain is based on the torque curve from 
FIG. 6 and the new acceleration or deceleration torque, as shown at block 
106. Thus, the offset resulting from the constant speed mode is 
continuously reduced so as to eventually eliminate the offset. If the 
vehicle had not initially been in constant speed mode, the offset would 
have been zero and the torque to be delivered by the powertrain would be 
equal to the torque derived from the torque curve in FIG. 6. 
If the vehicle is at near zero acceleration, conditional block 108, and the 
pedal position is constant, conditional block 109, then constant speed 
mode is activated and the target speed is set at the current vehicle 
speed, as shown at block 116. At this point, the method proceeds to 
operate in closed loop speed control, as shown at block 92. 
While the best modes for carrying out the invention have been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.