Cruise control governor using optimal droop selection logic

A cruise control governor which is able to dynamically define and switch between various goal droop curves in order to find the best goal droop curve for use with the current vehicle driving situation. For instance, the present invention will dynamically define and select different goal droop curves when the vehicle is lugging up a hill, coasting down a hill, cruising on level ground, preparing to crest a hill, or preparing to transition off of a downhill slope.

TECHNICAL FIELD OF THE INVENTION 
The present invention generally relates to cruise control governors and, 
more particularly to a cruise control governors using optimal droop 
selection logic. 
BACKGROUND OF THE INVENTION 
As is known in the art, a cruise control governor attempts to maintain a 
user-selected vehicle speed. Referring to FIG. 1(a), if the vehicle speed 
maintained by the cruise control governor is plotted as a function of 
time, it is apparent that the actual vehicle speed is not perfectly 
maintained at the cruise control set speed, because the controller can 
only attempt to maintain the desired set speed by measuring deviation of 
the actual speed from the set speed. The governor attempts to maintain a 
constant vehicle speed by controlling the amount of fuel which is provided 
to the engine, which is roughly proportional to the amount of torque that 
the engine will generate. FIG. 1(b) plots the engine torque vs. time which 
corresponds to the vehicle speed plot of FIG. 1(a). If the vehicle speed 
is plotted against engine torque, as in FIG. 2, a convenient paradigm is 
provided for visualizing the action of the cruise control governor. 
Viewing the cruise control governor from the perspective of FIG. 2 
indicates that the engine will produce whatever engine torque is required 
to maintain a constant vehicle speed. Since the torque that goes into the 
vehicle varies with the terrain, the torque generation from the engine 
must also vary in order to maintain a constant vehicle speed. 
Cruise control governors are devices that attempt to maintain a desired set 
speed condition by monitoring the system that they are trying to control. 
The cruise control governor monitors the road speed of the vehicle and 
reacts by changing the fuel command to the engine. For example, when the 
governor detects an underspeed condition, the governor increases the 
torque generation of the engine in order to increase the speed of the 
vehicle, thereby compensating for the undesirable underspeed situation. 
Thus, the governor is not capable of reacting until it recognizes that the 
vehicle has already deviated from the set speed. Once the vehicle has 
deviated from the set speed, it is too late for the governor to provide a 
perfect response, therefore the governor attempts to return the vehicle to 
the set speed as quickly as possible. Because the vehicle must deviate 
from the set speed before the governor reacts, it is impossible for the 
governor to provide a perfect response. This is why the plot of vehicle 
speed vs. time in FIG. 1(a) exhibits slight deviations both above and 
below the vehicle set speed. FIG. 3 is a process flow diagram which 
illustrates the interaction of the governor 22 with the vehicle/engine 
combination 24. The actual measured vehicle speed is subtracted from the 
desired set speed (which is set by the driver using the cab interface 20) 
in order to create a speed error signal. This speed error signal is input 
to the governor 22, which adjusts the fuel command signal to the 
vehicle/engine combination 24 in response thereto. 
The plot of engine torque vs. vehicle speed in FIG. 2 is referred to as a 
"droop" curve. Such a droop curve is realized because the controller is 
attempting to follow a goal droop curve. The controller adjusts its 
response, and thus the response of the engine, as a function of the 
current operating conditions of the vehicle and as a function of the goal 
droop curve. FIGS. 4a-f illustrate examples of various goal droop curves. 
The shape of the goal droop curve used with any particular controller 
depends upon the particular response that is desired from the controller. 
The ability for the controller to follow the goal droop curves depends upon 
the gain of the governor. The governor's gain is an indication of the 
aggressiveness of the controller. A high gain provides a very aggressive 
governor that will adjust engine torque generation rapidly in an attempt 
to follow the goal droop curve. However, aggressive gain governors also 
have a tendency to be unstable. In summary, the goal droop curves define 
where the controller attempts to maintain vehicle operation, and the 
governor gains define how aggressively the goal droop curves are followed. 
Because vehicle speed determines where on the goal droop curve the 
controller attempts to operate, environmental factors which affect the 
speed of the vehicle affect the performance of the controller. One such 
environmental factor is the grade of the road surface upon which the 
vehicle travels. Gradability is a concept that allows one to consider the 
relationship between vehicle speed, the grade of a hill, the full torque 
curve of the engine, aerodynamic drag, gearing and torque requirements. 
This concept utilizes a grade curve as illustrated in FIG. 5. The grade 
curve denotes the torque needed, at every speed, to remain at an 
equilibrium for a certain combination of hill grade, aerodynamic drag, and 
gearing selection. FIG. 6 shows some examples of how various hill grades 
affect the placement of the grade curve. Such grade curves are useful 
because they provide an easy means to determine if the vehicle is going to 
accelerate or decelerate. If, at the current vehicle speed, the grade 
curve is higher than the torque curve, then the vehicle will slow down to 
the point of intersection between the grade curve and the torque curve. 
If, at the current vehicle speed, the grade curve is lower than the torque 
curve, then the vehicle will accelerate to a vehicle speed where the grade 
curve and the torque curve intersect. FIG. 7 shows an example of such 
movement. 
When the vehicle goes over a hill, the grade varies depending upon where on 
the hill the vehicle is placed. FIG. 8 shows the various grades which are 
encountered by the vehicle on a symmetrical hill. As illustrated in FIG. 
9, the grade curve for a vehicle progressing to the top of a hill will 
move to the left as the maximum percent grade is reached, and then move 
back to the right as the grade is decreased back to zero. If the vehicle 
slows down at all before the crest of the hill, due to the higher torque 
requirements, then the vehicle will accelerate before the top of the hill 
because the grade curve moves to the right as the vehicle approaches the 
crest of the hill (0% grade). The exact location of the start of the 
acceleration will depend upon the shape and length of the hill, the rating 
of the engine, and the aerodynamics of the vehicle. 
Because most hills are relatively symmetrical and follow the model of FIG. 
8, acceleration of the vehicle as it nears the crest of the hill is 
undesirable due to the fact that the vehicle will accelerate automatically 
on the downside of the hill due to the negative grade. Conversely, a 
vehicle entering a valley will decelerate on the downside of the hill 
prior to its eventual automatic deceleration when it encounters the upside 
of the hill on the opposite side of the valley. When a vehicle accelerates 
prior to a point where the terrain will cause the vehicle to accelerate 
automatically, or when a vehicle decelerates prior to a point where the 
terrain will cause the vehicle to decelerate automatically, fuel is 
wasted. 
In the interest of increasing fuel economy of the vehicle, it is therefore 
desirable to design a controller which is able to recognize that the 
vehicle is cresting a hill or approaching the bottom of a valley and 
thereby alter the performance of the cruise control governor in order to 
obtain maximum fuel economy throughout the entire hill or valley event. 
The present invention is directed toward meeting these needs. 
SUMMARY OF THE INVENTION 
The present invention relates to a cruise control governor which is able to 
dynamically define and switch between various goal droop curves in order 
to find the best goal droop curve for use with the current vehicle driving 
situation. For instance, the present invention will dynamically define and 
select different goal droop curves when the vehicle is lugging up a hill, 
coasting down a hill, cruising on level ground, preparing to crest a hill, 
or preparing to transition off of a downhill slope. 
In one form of the invention a cruise control governor is disclosed which 
is operable to maintain a set speed of a vehicle by commanding fueling to 
an engine of the vehicle according to a plurality of goal droop curves, 
wherein at least one of the plurality of goal droop curves is dynamically 
defined during operaton of the vehicle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For the purposes of promoting an understanding of the principles of the 
invention, reference will now be made to the embodiment illustrated in the 
drawings and specific language will be used to describe the same. It will 
nevertheless be understood that no limitation of the scope of the 
invention is thereby intended, such alterations and further modifications 
in the illustrated device, and such further applications of the principles 
of the invention as illustrated therein being contemplated as would 
normally occur to one skilled in the art to which the invention relates. 
The present invention relates to a cruise control governor which is able to 
dynamically define and switch between various goal droop curves in order 
to find the best goal droop curve for use with the current vehicle driving 
situation. For instance, the present invention will dynamically define and 
select different goal droop curves when the vehicle is lugging up a hill, 
coasting down a hill, cruising on level ground, preparing to crest a hill, 
or preparing to transition off of a downhill slope. 
Prior art cruise control governors employ a maximum of three goal droop 
curves as illustrated in FIG. 10. These goal droop curves are referred to 
as the top standard droop, the isochronous droop and the bottom standard 
droop. The vehicle cruise control set speed is also indicated in the graph 
of FIG. 10, this value being set by the driver when cruise control is 
activated. The cruise control governor attempts to maintain the vehicle's 
speed at the set speed, however the torque commanded of the engine is 
determined by trying to maintain vehicle performance on one of the goal 
droop curves. For example, assume that a vehicle is operating at point 30 
on the isochronous droop curve. As the vehicle encounters an uphill slope, 
the grade of the terrain will cause the vehicle speed to decrease and the 
cruise control governor will send more fuel to the engine in order to 
increase the torque output of the engine. Such action by the cruise 
control governor will move the operating point of the vehicle upwards 
along the isochronous droop curve to, for example, point 32. At point 32, 
the vehicle speed is the same as it was at point 30, however more torque 
is being produced by the engine in order to counteract the decelerating 
influence of the positive grade. As the grade of the hill continues to 
increase, more torque will be required of the engine in order to maintain 
the set speed. In the simplest cruise control governor, only the 
isochronous droop curve would be present and the governor would attempt to 
maintain the set speed by increasing the torque output of the engine until 
the operating point of the vehicle reached the intersection between the 
set speed and the full torque curve. At this point, the engine is at 
maximum torque and further deceleration of the vehicle caused by the 
positive slope grade will cause the operating point of the vehicle to 
simply move left along the full torque curve. However, many prior art 
cruise control governor systems recognize that as the engine operating 
point moves closer to the full torque curve, the fuel efficiency of the 
engine severely decreases. Therefore, many such systems do not maintain an 
isochronous droop curve all the way to the full torque curve, but rather 
insert a top standard droop which transitions between the isochronous 
droop and the full torque curve. Therefore, as the slope of the hill 
further slows the vehicle, the operating point of the vehicle will be 
shifted to move along the top standard droop curve such as to the point 
34. The top standard droop curve allows an increasingly lower vehicle 
speed to be tolerated by the cruise control governor as the operating 
point of the engine moves nearer to the full torque curve. 
When the vehicle reaches the portion of the hill where the percent grade 
tends to decrease towards the crest of the hill, the speed of the vehicle 
will automatically increase. The cruise control governor will then adjust 
the fueling to the engine in order to maintain the operating point of the 
vehicle on the top standard droop curve, however this point will be moving 
in a downward direction toward the isochronous droop curve. Conversely, 
operation of the vehicle will transition to the bottom standard droop 
curve as the vehicle accelerates past the set speed. 
Theoretically, at the crest of the hill the operating point of the vehicle 
will be on the isochronous droop curve and the set speed will be 
maintained as a steady state condition. However, in practice the goal 
droop curves of FIG. 10 produce a significant amount of vehicle speed 
overshoot beyond the set speed as the operating point of the engine 
transitions from the top standard droop to the isochronous droop. 
Furthermore, the droop curves of FIG. 10 do not account for the fact that 
the vehicle will shortly be entering the downward slope portion of the 
hill. This means that the acceleration applied toward the crest of the 
hill will contribute to a vehicle overspeed condition as the vehicle 
accelerates down the downhill side. All of these problems contribute to 
rather large deviations from the vehicle set speed and reduced overall 
fuel economy for the vehicle. 
In order to solve these problems, the cruise control governor of the 
present invention utilizes not only a top standard droop and a bottom 
standard droop depending upon whether the vehicle is experiencing high 
torque or low torque, but also a top dynamic droop and a bottom dynamic 
droop which do not have fixed positions on the torque/mph graph, but 
rather are defined dynamically in response to current vehicle operating 
conditions. These are illustrated in FIG. 11. 
Operation of the cruise control governor of the present invention according 
to the goal droop curves of FIG. 11. is as follows. Assume that a vehicle 
is operating at point 40 on the isochronous droop curve. As the vehicle 
encounters an uphill slope, the grade of the terrain will cause the 
vehicle speed to decrease and the cruise control governor will send more 
fuel to the engine in order to increase the torque output of the engine. 
Such action by the cruise control governor will move the operating point 
of the vehicle upwards along the isochronous droop curve to, for example, 
point 42. At point 42, the vehicle speed is the same as it was at point 
40, however more torque is being produced by the engine in order to 
counteract the decelerating influence of the positive grade. As the slope 
of the hill further slows the vehicle, the operating point of the vehicle 
will be shifted to move along the top standard droop curve such as to the 
point 44. The top standard droop curve allows an increasingly lower 
vehicle speed to be tolerated by the cruise control governor as the 
operating point of the engine moves nearer to the full torque curve. 
Thus far, operation of the cruise control governor of the present invention 
according to the goal droop curves of FIG. 11 is identical to the prior 
art cruise control governor using the prior art goal droop curves of FIG. 
10. However, once the operating point of the vehicle reaches the full 
torque curve, continued deceleration of the vehicle on the increasing 
uphill slope will cause the operating point of the vehicle to move along 
the full torque curve to the left in FIG. 11. Such leftward movement will 
continue until the percent grade of the hill ceases to increase and begins 
to decrease in its transition toward the crest of the hill. At this point, 
the vehicle will accelerate and the operating point of the vehicle will 
begin to move to the right on the full torque curve. In the example 
illustrated in FIG. 11, this corresponds to the point 46. Once this 
acceleration is sensed by the cruise control governor, the top dynamic 
droop curve is defined as having an upper end point at the point 46 and a 
lower end point at a value calculated using a calibratable predetermined 
slope. As the vehicle travels up the hill, the continuously reducing slope 
toward the crest of the hill will continue to cause the vehicle to 
accelerate, forcing the operating point of the vehicle to be moved down 
the top dynamic droop curve past, for example, the point 48. Once the 
vehicle reaches the crest of the hill, the operating point of the vehicle 
will return to the isochronous droop curve. 
It will be appreciated by those skilled in the art that the top dynamic 
droop curve reduces the amount of fueling to the engine much quicker than 
the top standard droop curve of the prior art cruise control governors. By 
doing this, the cruise control governor of the present invention reduces 
overshoot of the set speed once the vehicle reaches the crest of the hill, 
thereby increasing the fuel economy of the vehicle. Because the top 
dynamic droop curve is determined dynamically for each hill encountered by 
the vehicle, it is optimized for the current driving situation and 
therefore optimizes the possible fuel economy for each different hill 
scenario. As the vehicle enters the downhill slope of the hill, the 
operating point of the vehicle moves down the bottom standard droop curve, 
along the zero torque axis and then back up the bottom dynamic droop curve 
to the isochronous droop curve as the vehicle comes to the end of the 
hill. The lowest point 50 of the bottom dynamic droop curve is dynamically 
defined as the point where the vehicle begins to decelerate at the zero 
torque condition. It will be further appreciated by those skilled in the 
art that movement along the goal droop curves of FIG. 11 begins with the 
bottom droop curves when the vehicle enters a valley and ends with the top 
droop curves as the vehicle climbs out of the valley. 
The cruise control governor of the present invention recognizes that a 
vehicle will accelerate after cresting a hill. Therefore, to take 
advantage of this acceleration, fueling should be reduced before the crest 
of the hill. This will reduce both the possibility of overspeeding and the 
amount of fuel consumed. Likewise, for a vehicle that has just completed a 
hill and is heading toward a zero percent grade at the bottom of the hill, 
fueling should be increased early in order to ensure that the vehicle does 
not slow to an underspeed condition. The underspeed condition results from 
the fact that the vehicle decelerates when the momentum added by the 
downhill grade is removed. The present invention implements these maxims 
by defining the dynamic droops whenever the vehicle is preparing to crest 
a hill or preparing to transition off of a downhill condition. The upper 
dynamic droop is used for cresting a hill and the lower dynamic droop is 
used for transitioning off of a downhill situation. The upper dynamic 
droop is based on the fact that when a vehicle prepares to crest a hill, 
the vehicle will accelerate, thus the cruise control governor uses 
acceleration to detect the crest of a hill. The bottom dynamic droop is 
based on the fact that a vehicle transitioning off of a downhill 
decelerates. Thus, the cruise control governor uses deceleration to detect 
the end of the hill. Once activated, the upper dynamic droop rolls the 
fueling off to the engine before the crest of the hill. Similarly, the 
bottom dynamic droop adds fueling to the engine before exiting the 
downhill slope of the hill. Even though the top and bottom dynamic droops 
remove or add fueling before the set speed is reached, the net effect on 
the vehicle is that the vehicle speed is more consistent. 
FIG. 12 schematically illustrates operation of the cruise control governor 
of the present invention. The vertical axis illustrates engine torque, 
vehicle speed and altitude concurrently, while the horizontal axis 
represents time. At time 0, the vehicle is already on the upward slope of 
the hill and the vehicle speed has decreased from the set speed. This 
corresponds to a point on the full torque curve of FIG. 11 somewhere to 
the right of point 46. With the engine at full torque and the grade of the 
hill continuing increase, the vehicle speed continues to fall away from 
the set speed as the vehicle moves toward the crest of the hill. However 
at the point 52, the vehicle speed ceases to decrease and starts to 
increase. This is caused by a decrease in the percent grade of the hill as 
the vehicle nears the crest of the hill. The cruise control governor 
senses this vehicle acceleration and calculates the top dynamic droop 
curve of FIG. 11, placing operation of the vehicle now at point 46 in FIG. 
11. As the speed of the vehicle continues to increase toward the crest of 
the hill, the cruise control governor rapidly decreases the engine torque 
by attempting to follow the path of the top dynamic droop curve of FIG. 
11. As illustrated in FIG. 12, the vehicle does not reach the set speed at 
the crest of the hill, however the acceleration boost provided by the 
downside of the hill smoothly returns the vehicle to the set speed. 
Without the more rapid decrease in engine torque provided by following the 
top dynamic droop (i.e. if the cruise control governor had instead 
followed the top standard droop of the prior art) the vehicle speed would 
have been returned to the set speed near the crest of the hill and the 
vehicle speed would have severely overshot the set speed on the downward 
slope of the hill. Thus, the goal droop curves of FIG. 11 provide a more 
stable speed for the vehicle throughout the entire hill event, thereby 
increasing fuel economy. 
Since the dynamic droops of FIG. 11 must be maintained until the crest of 
the hill has passed, and only activated as a response to certain 
predetermined conditions, the preferred embodiment of the present 
invention utilizes a state machine to implement the decision logic. A 
state machine is a device that exhibits two main properties. First, once a 
state machine decides on an answer, only certain predetermined conditions 
can change the answer. Second, which of the predetermined conditions 
needed to change the state machine's answer depends upon what state the 
machine is currently in. The advantage of utilizing a state machine (as 
opposed to other logic schemes which could be used to implement the 
present invention) is that the decisions of the state machine are based 
upon what the state machine's previous decision was. 
FIG. 13 illustrates a simplified example of a state machine which 
implements an engine control system. The states of a state machine are 
containers that hold action. While in each state, the action is performed 
until the transition requirement to the next state has been met. Once the 
transition requirement has been met, the state machine jumps to the next 
state and performs the next action until the new transition rule has been 
met. For example, in the state machine of FIG. 13, the stopped state 54 
performs the action of waiting until the engine start button has been 
depressed. This is the transition rule for the stopped state 54. Once the 
start button has been depressed, the transition rule for the stopped state 
54 has been met and the state machine moves to the starting state 56. The 
starting state 56 performs the action of starting the engine and waiting 
for the engine speed to exceed 500 rpm (the transition rule). Once this 
transition rule has been met, the state machine moves to the running state 
58 which performs the action of controlling the engine during normal 
operation. The transition rule for the running state 58 requires the user 
to key off the engine, at which time the state machine transfers control 
back to the stopped state 54. 
Referring now to FIG. 14, a preferred embodiment state machine is 
illustrated which implements the goal droop curves of FIG. 11. The state 
machine of FIG. 14 is indicated generally at 60. State 1 of the state 
machine 60 implements the isochronous droop curve of FIG. 11. State 1 may 
transition to state 3, which implements the top standard droop curve of 
FIG. 11, whenever fueling to the engine is determined to be greater than a 
predetermined amount. This predetermined amount is referred to in FIG. 14 
as the upper standard droop offset. Conversely, the state machine 60 can 
transition between state 3 and state 1 whenever fueling to the engine is 
less than the upper standard droop offset. State 5 of state machine 60 
corresponds to operation of the vehicle on the full torque curve of FIG. 
11 (i.e. the maximum fueling curve). The transition rule from state 3 to 
state 5 simply requires that the engine torque be at a maximum value. 
Conversely, the transition rule from state 5 to state 3 requires that the 
engine torque be less than the maximum value. State 7 of state machine 60 
implements the top dynamic droop curve of FIG. 11. The transition rule 
from state 5 to state 7 requires that the vehicle be accelerating. The 
transition from state 5 to state 7 is indicated as point 46 in FIG. 11. 
The transition rule from the top dynamic droop curve of state 7 back to 
the isochronous droop curve of state 1 requires that the vehicle be 
accelerating less than a predetermined threshold amount while the vehicle 
jerk (the rate of change of acceleration) is negative or if the vehicle 
speed is greater than the set speed. It will be appreciated by those 
skilled in the art that movement on the top dynamic droop curve is 
unidirectional. In other words, it is not possible to transition from 
state 7 to state 5 or from state 1 to state 7. The top dynamic droop curve 
is traversed only from state 5 to state 7 to state 1. The operation of 
states 2, 4 and 6 of state machine 60, as well as the transition rules 
therebetween, are directly analogous to the operation of states 3, 5 and 
7. 
The integration of the state machine 60 with the cruise control governor 62 
is illustrated schematically in FIG. 15. A cruise control cab interface 64 
determines the set speed at which the driver wishes to maintain the 
vehicle. This cruise reference speed is input to the cruise control 
governor 62 which determines the percent of fueling to command of the 
engine/vehicle 66. The actual speed of the vehicle 66 is measured and 
subtracted from the cruise reference speed in order to determine the 
amount of speed error. This much of the cruise control operation is 
identical to the prior art devices, as illustrated in FIG. 3. However, the 
state machine 60 of the present invention provides a correction factor to 
the speed error before it is transmitted to the cruise control governor 
62. The state machine 60 determines this correction factor based upon 
inputs of the engine fueling level and the vehicle acceleration and 
vehicle jerk. Using the curves of FIG. 11 and the transition rules 
illustrated in FIG. 14, the state machine 60 determines the correction 
factor to be applied to the speed error prior to transmission of the 
cruise control governor 62. 
As illustrated in FIG. 16, when transitioning from state 5 to state 7, the 
state machine 60 recalculates the location of the top dynamic droop. The 
top dynamic droop is moved to the exact location on the full torque curve, 
at which the vehicle began to accelerate. Thus, larger hills will lug back 
further and result in top dynamic droop curves that are further to the 
left than those generated by small hills. The reason that the dynamic 
droops are moved is to better match the terrain and to fully optimize the 
vehicle operation. Similarly, the location of the bottom dynamic droop is 
recalculated when the state machine 60 transitions from state 4 to state 
6. 
As illustrated in FIG. 16, the upper dynamic droop is moved automatically 
in order to account for the severity of the hill. The reason for moving 
the dynamic droops is that larger hills have more potential for speed 
increase as the hill is crested. Thus, to overcome this speed increase, 
the dynamic droop is more aggressive. Additionally, the top dynamic droop 
is limited in its operating region to a calibratable lower limit. An 
analogous movement and upper limit applies to the bottom dynamic droop. 
Furthermore, the state machine 60 turns off the dynamic droops while the 
vehicle is resuming to the set speed due to a driver initiated resume 
event, when the vehicle is not operating in top gear, or immediately 
following a driver-initiated set event. 
While the invention has been illustrated and described in detail in the 
drawings and foregoing description, the same is to be considered as 
illustrative and not restrictive in character, it being understood that 
only the preferred embodiment has been shown and described and that all 
changes and modifications that come within the spirit of the invention are 
desired to be protected.