System and method for smooth transitions between engine mode controllers

A system and method for controlling an internal combustion engine using a controller to implement at least two control modes having corresponding first and second mode controllers with disparate control parameters include comparing output of the first and second mode controllers to generate an error, generating a correction value based on the error, and providing the correction value to one of the mode controllers to provide a smooth transition of control between the mode controllers. In one embodiment, the first controller is a torque controller which determines a desired air flow to achieve a desired torque and the second mode controller is an idle speed controller which determines a desired air flow to maintain a desired engine speed. The invention is advantageous in that it provides for smooth transitions between control modes, such as between idle mode and a normal driving mode, by harmonizing the outputs of the controllers. Drivability is improved by eliminating an aggressive and/or sluggish response to accelerator pedal position when transitioning between idle speed control and normal driving modes.

TECHNICAL FIELD 
The present invention relates to a system and method for providing smooth 
transitions between control strategies for internal combustion engines. 
BACKGROUND ART 
Control strategies for internal combustion engines have evolved from purely 
electromechanical strategies to increasingly more complex electronic or 
computer controlled strategies. Spark-ignited internal combustion engines 
have traditionally used air flow as the primary control parameter, 
controlled by a mechanical linkage between a throttle valve and an 
accelerator pedal. Fuel quantity and ignition timing, originally 
mechanically controlled, were migrated to electronic control to improve 
fuel economy, emissions, and overall engine performance. Electronic 
throttle control systems have been developed to further improve the 
authority of the engine controller resulting in even better engine 
performance. 
Electronic throttle control replaces the traditional mechanical linkage 
between the accelerator pedal and the throttle valve with an "electronic" 
linkage through the engine or powertrain controller. Because of this 
electrical or electronic linkage, this type of strategy is often referred 
to as a "drive by wire" system. A sensor is used to determine the position 
of the accelerator pedal which is input to the controller. The controller 
determines the required air flow and sends a signal to a servo motor which 
controls the opening of the throttle valve. Control strategies which 
imitate the mechanical throttle system by controlling the opening of the 
throttle valve based primarily on the position of the accelerator pedal 
position are often referred to as pedal follower systems. However, the 
ability of the controller to adjust the throttle valve position 
independently of the accelerator pedal position offers a number of 
potential advantages in terms of emissions, fuel economy, and overall 
performance. 
An engine control strategy typically has a number of operating modes, such 
as idle, cruise control, engine speed limiting, vehicle speed limiting, 
dashpot, normal driving, etc. The various control modes may or may not use 
the same or similar primary control parameters. Furthermore, modes of 
operation often use different control strategies, which may include 
open-loop and/or closed loop feedback/feedforward control strategies. 
Likewise, different strategies may utilize proportional, integral, and/or 
derivative control with control parameters tuned to particular 
applications or operating conditions. 
To provide optimal driving comfort and robust control of the engine under 
varying conditions, it is desirable to provide smooth transitions between 
control modes. In particular, it is desirable to provide smooth or 
seamless transitions between idle control mode, where the accelerator 
pedal is not depressed, and a normal driving mode where the accelerator 
pedal is depressed. 
SUMMARY OF INVENTION 
It is an object of the present invention to provide a system and method for 
transitioning between control modes of an internal combustion engine by 
harmonizing control values generated by each controller. 
A further object of the present invention is to provide a system and method 
for smoothly transitioning between an air flow-based idle speed control 
mode and a torque-based control driving mode for an internal combustion 
engine. 
In carrying out the above objects and other objects, features, and 
advantages of the present invention, a system and method for controlling 
an internal combustion engine using a controller to implement at least two 
control modes having corresponding first and second mode controllers with 
disparate control parameters include comparing output of the first and 
second mode controllers to generate an error, generating a correction 
value based on the error, and providing the correction value to one of the 
mode controllers to provide a smooth transition of control between the 
mode controllers. In one embodiment, the first controller is a torque 
controller which determines a desired air flow to achieve a desired torque 
and the second mode controller is an idle speed controller which 
determines a desired air flow to maintain a desired engine speed. 
The invention is advantageous in that it provides for smooth transitions 
between control modes, such as between idle mode and a normal driving 
mode, by harmonizing the outputs of the controllers. Drivability is 
improved by eliminating an aggressive and/or sluggish response to 
accelerator pedal position when the transitioning to and from idle control 
mode. 
The above advantages and other advantages, objects, and features of the 
present invention, will be 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(S) FOR CARRYING OUT THE INVENTION 
FIG. 1 provides a block diagram illustrating operation of a system or 
method for providing smooth transitions between mode controllers according 
to the present invention. System 10 includes an internal combustion 
engine, indicated generally by reference numeral 12, in communication with 
a controller 14. Various sensors are provided to monitor engine operating 
conditions. Sensors may include a mass air flow sensor (MAF) 16 which 
monitors the air passing through intake 18. A throttle valve 20 regulates 
the air intake into engine 12 as well known in the art. A throttle 
position sensor (TPS) 22 provides an appropriate signal to controller 14 
to monitor the throttle angle or position of throttle valve 20. An 
appropriate actuator such as a mechanical or electronic accelerator pedal 
24 is used to determine the driver demand which, in turn, is used in the 
control of the position of throttle valve 20. 
In a preferred embodiment, system 10 is an electronic throttle control 
system which uses a pedal position sensor (PPS) 26 to provide a signal 
indicative of the position of an accelerator pedal 24. Controller 14 uses 
the pedal position sensor signal, along with various other signals 
indicative of current engine operating conditions, to control the position 
of throttle valve 20 via an appropriate servo motor or other actuator 23. 
Such electronic throttle control or "drive-by-wire" systems are well known 
in the art. 
Engine 12 may include various other sensors such as an engine speed sensor 
(RPM) 28, an engine temperature or coolant temperature sensor (TMP) 30, a 
manifold absolute pressure (MAP) sensor 32, a vehicle speed sensor (VSS) 
34, and the like. 
Processor 14 receives signals from the various sensors via input ports 36 
which may provide signal conditioning, conversion, and/or fault detection, 
as well known in the art. Input port 36 communicates with processor 38 via 
a data/control bus 40. Processor 38 implements control logic in the form 
of hardware and/or software instructions which may be stored in 
computer-readable media 42 to effect control of engine 12. 
Computer-readable media 42 may include various types of volatile and 
nonvolatile memory such as random-access memory (RAM) 44, read-only memory 
(ROM) 46, and keep-alive memory (KAM) 48. These "functional" 
classifications of memory may be implemented by one or more different 
physical devices such as PROMs, EPROMs, EEPROMs, flash memory, and the 
like, depending upon the particular application. 
In a preferred embodiment, processor 38 executes instructions stored in 
computer-readable media 42 to carry out a method for controlling engine 12 
using at least two mode controllers implemented in software and/or 
hardware to communicate with various actuators of engine 12 via output 
port 50. Actuators may control ignition timing or spark (SPK) 52, timing 
and metering of fuel 54, or position of throttle valve 20 to control air 
flow. Electronic control of air flow may also be performed using variable 
cam timing, for example. Preferably, controller 14 is used to implement at 
least two mode controllers which provide idle speed control and 
torque-based engine control depending upon the particular mode of 
operation of engine 12. 
FIG. 2 is a block diagram illustrating representative mode controllers for 
idle speed control and engine torque control according to the present 
invention. Idle speed controller 60 and engine torque controller, 
indicated generally by reference numeral 62, are preferably implemented 
within a powertrain control module or controller 14. However, the present 
invention is generally applicable to any control system having disparate 
mode controllers where control passes between mode controllers during 
operation. For example, the present invention could also be applied to a 
throttle angle/throttle follower based control system architecture where 
interpreted driver demand corresponds to a throttle valve position or 
angle. The present invention provides a trim value or correction value to 
the input of a first controller based on the difference in outputs of the 
first and second controllers to provide a smooth transition between 
controllers. Preferably, the correction value is generated by a third 
feedback controller 64 which is selectively activated to drive the 
difference or error between outputs of the first and second controllers 
toward zero. 
In the embodiment illustrated in FIG. 2, idle speed controller 60 generates 
a desired air flow (DESMAF) based on a desired engine speed (RPMDES). 
Likewise, engine torque controller 62 generates a desired air flow 
(TQ.sub.-- DESMAF) based on a desired total engine torque (TQ.sub.-- 
ENG.sub.-- TOT). The outputs from idle speed controller 60 and torque 
controller 62 are switched or multiplexed based on the accelerator pedal 
position as represented by block 84. A status indicator (APP) indicates 
whether the accelerator pedal is fully released, partly depressed, or 
fully depressed. Idle speed controller 60 is activated or active when the 
APP flag indicates that the throttle pedal is fully released. Otherwise, 
engine torque controller 62 is active. Block 66 selects the larger value 
of the output from block 64 and idle speed controller 60. The resulting 
air flow is converted to a desired throttle position and used to control 
the 5 throttle valve. 
In one embodiment, idle speed controller 60 also includes a dashpot control 
mode to control the rate of engine deceleration whenever engine speed is 
significantly above the idle speed and the accelerator pedal is fully 
released. 
The desired air flow outputs from idle speed controller 60 and engine 
torque controller 62 are compared at block 68 to generate an error signal. 
In this embodiment, controller 64 is a proportional-integral (PI) 
controller which updates its output only when the APP status flag 
indicates that the accelerator pedal is not being depressed. Of course, 
any kind of feedback controller could be substituted for the PI controller 
shown in FIG. 2. Preferably, the controller drives the control output 
continuously to provide a zero steady state error and quickly responds to 
changes in the error signal without objectionable oscillation or 
overshoot. The output of the proportional block 70 and integral block 72 
is combined at block 74. This control output is then converted from units 
of air flow to a unitless load at block 76. In a preferred embodiment, 
this is accomplished by dividing by the number of cylinders per minute 
(engine speed times cylinders divided by 2), and then dividing by the 
standard temperature air charge per cylinder, which depends on the per 
cylinder displacement of the engine. The result from block 76 is 
multiplied by a load-to-engine torque normalizer at block 78 to convert 
the unitless quantity to a torque. The output of block 78 is multiplied by 
a final gain at block 80 to provide the necessary correction value based 
on the air mass error. Of course, the gain provided by block 80 could be 
incorporated into controller 64 or block 78, but is provided for ease of 
calibration and tuning. The resulting correction value from block 80 is 
combined with the engine torque request (TQ.sub.-- ENG.sub.-- LOAD) at 
block 82. 
FIGS. 3a and 3b provide a graphical representation of a jittery transition 
between mode controllers without the benefit of the present invention. 
FIG. 3a represents the requested engine torque 90 as a function of time. 
FIG. 3b represents the requested or desired air flow from the idle speed 
controller 92, the engine torque controller 94, and the resulting final 
torque 96 based on the active controller. At time t.sub.1, the accelerator 
pedal is fully released and the idle speed controller is active. As 
illustrated in FIG. 3b, the driver demanded air flow 94 is greater than 
the idle speed control air flow 92 which is collinear with the final air 
flow 96. The accelerator pedal begins to be depressed at tine t.sub.2. The 
active controller transitions from the idle speed controller to the engine 
torque controller resulting in jitter of the final commanded air flow 96. 
FIGS. 4a and 4b are graphs illustrating a sluggish or "dead pedal" 
transition between mode controllers without the benefit of the present 
invention. As illustrated in FIG. 4b, the air flow requested from the idle 
speed controller 92 exceeds the driver demanded air flow 94 at time 
t.sub.1 when the idle speed controller is active. At time t.sub.2, the 
accelerator pedal is depressed and the engine torque controller becomes 
the active controller. However, the air flow requested from the idle speed 
controller exceeds that of the engine torque controller, and therefore 
controls the final commanded air flow 96. As a result, the final commanded 
air flow remains at the same level and there is no increase in the 
resulting engine torque even though the accelerator pedal is being 
depressed. The final commanded air flow does not begin to actually 
increase until the accelerator pedal is depressed to a point represented 
as time t.sub.3 resulting in a "dead pedal" feel, i.e. no increase in 
engine torque in response to an increase in the accelerator pedal 
position. 
FIGS. 5a and 5b provide graphs illustrating a smooth transition between 
mode controllers according to the present invention. FIG. 5a illustrates 
operation of the correction value according to the present invention. The 
correction value, represented generally by line 100, is added to the input 
to the engine torque controller, represented by line 102. The resulting 
requested torque is represented by line 104. Unlike the examples 
illustrated in FIGS. 3 and 4, the total requested torque shows a smooth 
transition when the final commanded air flow transitions from the idle 
speed controller to the engine torque controller. As represented in FIG. 
5b, air flow requested by the idle speed controller, represented by line 
92, exceeds the air flow requested by the engine torque controller, 
represented by line 94, prior to time t.sub.2. During this time, the 
correction value feedback controller generates a correction value 100 
which is added to the input of the engine torque controller to increase 
the requested air flow 94. As a result, the air flows requested by the 
idle speed controller and the engine torque controller are approximately 
equal at time t.sub.2. As such, when the accelerator pedal is depressed at 
time t.sub.3, a smooth, seamless transition between mode controllers 
results. 
In a preferred embodiment, the correction value is preferably added to the 
input of the engine torque controller. In addition to providing a 
filtering effect, this technique provides a correction that represents an 
actual torque. This is advantageous in that the engine torque controller 
assumes that the requested torque is the total engine load for the purpose 
of calibration of various other control parameters including spark, EGR, 
and pumping losses which will result. If the idle air flow were simply 
added to the engine torque requested air flow, the resulting load would be 
higher than expected by the torque-to-load calculation, resulting in 
unsatisfactory performance. Providing the correction value to the input of 
the engine torque controller provides a more robust control of engine 
torque and smooth transitions between the idle/dashpot controller and the 
engine torque controller. 
Referring now to FIG. 6, a flowchart illustrating control logic for 
providing smooth transitions between mode controllers in a system or 
method according to the present invention is shown. One of ordinary skill 
in the art will recognize that the control logic may be implemented in 
software, hardware, or a combination of software and hardware. Likewise, 
various processing strategies may be utilized without departing from the 
spirit or scope of the present invention. For example, most real-time 
control strategies utilize event-driven or interrupt-driven processing. As 
such, the sequence of operations illustrated is not necessarily required 
to accomplish the advantages of the present invention, and is provided for 
ease of illustration only. Likewise, various steps may be performed in 
parallel or by dedicated electric or electronic circuits. 
Block 110 represents determination of the accelerator pedal position for an 
electronic throttle control application. The accelerator pedal position 
may be used by block 112 to determine which controller is active. Of 
course, various other inputs may also be utilized to determine the active 
mode controller, such as the status of the cruise control or various other 
engine operating parameters. When the first controller is active as 
determined by block 112, an initial value for the correction term is 
retrieved from storage as indicated by block 114. The outputs from the 
first and second controllers are compared to generate an error signal as 
represented by block 116. The error signal is used to generate a 
correction value which is preferably feedback-controlled to reduce the 
error toward zero as represented by block 118. The correction value is 
converted to the proper parameters or units as indicated by block 120. The 
correction value may also be normalized, if desired, as described in 
greater detail above. In a preferred embodiment, block 120 converts an air 
flow error to a correction value in units of torque. The correction value 
is then provided to one of the controllers as represented by block 122. 
If the first controller is not active as indicated by block 112, then the 
previously generated correction value, if any, is stored for future 
retrieval as represented by block 124. This step is performed in a 
preferred embodiment to prevent excessive integrator wind-up in the PI 
feedback controller. Depending upon the particular feedback controller, if 
any, this step may not be necessary. 
While the best mode for carrying out the invention has 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.