Electronically controlled clutch pedal dashpot driveline torque

An electronically controlled clutch pedal dashpot driveline torque limiter utilizes a solenoid to regulate the apply rate of a vehicle master clutch based on current vehicle operating conditions such as engine speed, transmission input speed, and clutch pedal release rate. The present invention allows greater control authority of master clutch apply rates to reduce or eliminate the incidence of engine stall and excessive slipping of the master clutch.

TECHNICAL FIELD 
The present invention relates to vehicular master clutches and, more 
particularly, to electronically controlled clutch dampers for limiting the 
application of torque to truck driveline components. 
BACKGROUND ART 
Driveline component manufacturers, especially in the trucking industry, are 
continually being urged to improve the durability and reliability of 
driveline components, such as driveshafts, axles and gears. Since the best 
solution is not always merely one of "beefing up" the individual 
components, various damping devices have been conceived and are well-known 
in the art. Clutch damping/torque limiting devices are illustrated in U.S. 
Pat. Nos. 4,668,207, 4,693,354, 4,888,539, 4,947,972 and 5,009,301. 
One problem of particular significance results when a vehicle operator 
overzealously engages the clutch by, for example, "side-stepping" the 
clutch. When a driver side-steps, the clutch pedal is depressed and a 
particular gear is selected. The engine is then revved up and the driver 
slides his foot sideways off of the clutch pedal, allowing the clutch 
pedal to snap back to the resting position. This results in a very rapid 
clutch engagement and application of torque to the driveline, inducing 
vibrations and oscillations into the driveline. The driveline components 
are seriously stressed and failure is hastened if the rate of torque 
application is faster than the natural frequency of the driveline. 
Specifically, unnecessary stress occurs when the driveline torque input 
has major harmonic content at frequencies at or above the lowest natural 
frequency of the driveline. Thus, if the rate of torque application is at 
or below the natural frequency, the driveline is not shocked. 
It is therefore desirable to prevent overzealous engagement of a clutch by 
controlling the rate of torque applied to the driveline in a reliable, 
durable and cost-effective way. 
In many applications, vehicles include electronically controlled 
powertrains, including an electronically controlled engine, transmission, 
and other auxiliary equipment, such as ABS, and the like. The use of 
electronic control modules (ECM's) provides coordinated control of various 
vehicle systems and subsystems to enhance overall vehicle control 
flexibility and capability. Electronically controlled systems typically 
provide more refined and accurate control than corresponding mechanical 
control systems. As more and more vehicles incorporate electronically 
controlled systems and subsystems, an increasing number of vehicle 
functions may be partially or fully automated and controlled by the ECM, 
such as shifting of the transmission. However, the initial cost and 
decreased fuel economy typical of a fully automated transmission result in 
continued demand for manually operated transmissions in some applications. 
For those applications requiring manipulation of a master clutch, it is 
desirable to electronically control the engagement rate of the master 
clutch to achieve the benefits of a mechanical clutch dashpot while 
providing more refined, context-sensitive control. 
The influx of partially or fully automatic transmissions has resulted in a 
number of drivers who are unfamiliar with operation of a conventional 
master clutch. Those who occasionally rent vehicles having a transmission 
requiring proper manipulation of the master clutch to operate, often cause 
significant wear on various driveline components due to their 
inexperience. A rapid clutch engagement may result in a shock to the 
driveline as explained above, or may stall the engine. Similarly, an 
excessively slow clutch engagement results in undesirable clutch slip and 
premature failure of the clutch friction material. Thus, it is desirable 
to provide a low-cost system and method for use with these applications to 
reduce unnecessary component wear and improve the driveability of the 
vehicle. 
DISCLOSURE OF THE INVENTION 
It is an object of the present invention to provide a system and method for 
preventing overzealous engagement of a clutch by controlling the rate of 
torque applied to the driveline. 
It is a further object of the present invention to provide an 
electronically controlled clutch pedal dashpot driveline torque limiter 
for preventing overzealous engagement of a clutch by controlling the rate 
of torque applied to the driveline. 
Another object of the present invention is to provide a relatively low-cost 
system which capitalizes on the availability of an ECU to provide 
context-sensitive limit control of driveline torque. 
A further object of the present invention is to provide a system and method 
for limiting application of driveline torque which are capable of reducing 
excessive clutch slip and the incidence of engine stall due to an 
inappropriate clutch engagement rate for the current operating conditions. 
In carrying out the above objects, and other objects and features of the 
present invention, there is provided an electronically controlled clutch 
pedal dashpot for controlling the rate torque is applied to a vehicular 
driveline during coupling of an engine to the driveline. The clutch pedal 
dashpot is for use in a vehicle including an electronic control unit, a 
remote clutch pedal, and a clutch actuating mechanism for controlling 
torque coupling of the engine to the driveline, wherein the clutch pedal 
is displaced between a first position at which the clutch is engaged and a 
second position at which the clutch is disengaged. A pedal operated 
hydraulic master cylinder coupled to a slave cylinder is one example of 
this type of actuator. 
In one embodiment, the clutch pedal dashpot comprises a damper having at 
least one passage controlled by a solenoid to regulate the rate at which 
the clutch pedal returns to the first position from the second position 
during coupling so as to control the rate of clutch engagement and 
therefore the rate of torque applied to the driveline. 
In a preferred construction, the damper also includes a damper piston and 
the clutch pedal dashpot further comprises a damper piston stop member for 
limiting travel of the damper piston to only a portion of the possible 
travel during displacement of the clutch pedal from the first position to 
the second position. The clutch pedal dashpot also preferably comprises 
bias means for biasing the damper piston against the stop member during 
displacement of the clutch pedal from the first position to the second 
position. 
Also preferably, the damper piston sealingly cooperates with a damper 
piston housing to create a chamber, and the piston housing includes a 
check valve for allowing air to be drawn into the chamber as the pedal is 
displaced from the first position to the second position and for 
preventing or impeding air from being expelled from the chamber as the 
pedal is displaced from the second position to the first position. The 
piston housing preferably includes at least one passage controlled by a 
solenoid in communication with the electronic control unit for regulating 
the flow of air into the chamber as the pedal is displaced from the first 
position to the second position and expelled from the chamber as the pedal 
is displaced from the second position to the first position. 
A method for controlling the rate of torque application to the driveline 
during coupling is also provided. The method includes determining current 
operating conditions such as engine speed, transmission input speed, 
vehicle speed, transmission gear state, and attempted clutch engagement 
rate, and controlling the actual engagement rate based on at least one of 
the factors indicative of the current operating conditions. 
The advantages accruing to the present invention are numerous. For example, 
the present invention utilizes electronic control to limit the rate that 
engine torque is applied to a driveline even when the vehicle operator 
tries to overzealously engage the clutch. As a result, driveline 
components can be designed to withstand smaller torques and fewer 
driveline components will fail, resulting in a substantial cost savings. 
Electronic control also provides the capability to determine an 
appropriate clutch engagement rate for the current operating conditions, 
such as transmission gear ratio, engine speed, vehicle speed, and the 
like. Thus, excessive clutch slip and the incidence of engine stall can be 
reduced or eliminated. 
The above objects and other objects, features and advantages of the present 
invention will be readily appreciated by one of ordinary skill in the art 
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 
Referring now to FIG. 1, a graphical representation of a vehicle utilizing 
an electronically controlled clutch pedal dashpot driveline torque limiter 
according to the present invention is shown. FIG. 1 depicts a vehicle 10, 
such as a tractor of a tractor semi-trailer vehicle, having an 
electronically controlled engine E coupled to a compound transmission T 
via a clutch mechanism C. Although a vehicle such as depicted in FIG. 1 
represents one of the possible applications for the system and method of 
the present invention, it should be appreciated that the present invention 
transcends any particular type of vehicle employing an electronic control 
module to limit the engagement rate of a master clutch. 
In a preferred embodiment, transmission T is preferably a compound change 
gear or change speed transmission having a main section connected in 
series with an auxiliary section which includes an output shaft 12 coupled 
to a vehicle drive shaft 14. Vehicle 10 includes at least two axles such 
as a steer axle 16 and at least one drive axle, such as axles 18 and 20. 
Each axle supports corresponding wheels W having foundation or service 
brake components 22 which may be manually or automatically actuated 
depending upon the particular application and operating conditions. 
Service brake components 22 may include wheel speed sensors and 
electronically controlled pressure valves to effect control of the vehicle 
braking system as is known. 
Vehicle 10 also includes various operator controls such as clutch pedal 24, 
accelerator pedal 26, brake pedal 28, and an operator interface, such as 
dashboard control console 30, which may include any of a number of output 
devices 32, such as lights, LED or LCD displays, alarms, buzzers, and the 
like. Dashboard control console 30 also includes various input devices 34, 
such as switches, potentiometers, push buttons, and the like. The vehicle 
control system includes an electronic control module such as engine 
control module (ECM) 40 and preferably includes an additional electronic 
control module for effecting control of transmission T, such as 
transmission control module (TCM) 42. Of course, engine and transmission 
control may be combined in a single electronic control module for some 
applications. The ECM 40 and TCM 42 communicate with a variety of sensors 
via inputs 44 and with numerous actuators via outputs 46. Sensors may 
include a steering angle sensor 48, wheel speed sensors (included in 
braking components 22), an electronic accelerator pedal sensor (APS) 50, a 
brake pedal sensor or switch 52, a clutch control/sensor 54, and an output 
speed sensor 56, among numerous others. Clutch control/sensor 54 may be 
manually operated, or may be partially or fully automated. In a preferred 
embodiment, clutch control/sensor 54 includes a clutch pedal position 
sensor and a solenoid to limit the clutch engagement rate as illustrated 
and described in detail with reference to FIGS. 2 and 5. 
It should be appreciated that the term "clutch pedal" as used herein is 
intended to be broadly construed, and the invention is not intended to be 
restricted merely to a device in a vehicle cab. One of ordinary skill will 
appreciate that, of course, the clutch pedal dashpot described in detail 
below could be positioned in numerous vehicle locations other than those 
specifically illustrated. 
Actuators may include a shift actuator 60 for automatically effecting a 
gear shift within transmission T, electronically controlled pressure 
valves (included in braking components 22), and an engine retarder 62 or a 
driveline retarder (not specifically illustrated). As is known, a retarder 
is a device utilized to supplement the foundation or service brakes when 
descending long grades and to prolong service brake life in high-frequency 
start and stop operation. Retarders may be categorized as engine brakes, 
exhaust brakes, hydraulic retarders and electric retarders. In a preferred 
embodiment, engine retarder 62 is an engine brake such as the well known 
Jacobs engine brake. This device converts a power producing diesel engine 
into a power absorbing air compressor. This is achieved by shutting off 
the fuel and hydraulically opening the exhaust valve as two or more 
pistons approach top dead center during the compression stroke. 
As also illustrated in FIG. 1, a diagnostics module 64 may be selectively 
connected to ECM 40 and preferably communicates status messages as defined 
by the SAE J1587 standard published by the Society of Automotive 
Engineers, the disclosure of which is hereby incorporated by reference in 
its entirety. These messages are also available to other system 
microprocessors during normal operation such as TCM 42 and include 
information such as current engine speed and torque, accelerator position, 
road speed, cruise control status, and cruise control set speed, among 
many others. It will be appreciated by one of ordinary skill in the art 
that the various connections between electronic controllers, sensors, and 
actuators may be changed to accommodate the particular requirements of a 
specific application without departing from the spirit or scope of the 
present invention. 
The ECM 40 and TCM 42 contain control logic rules which may be implemented 
in a variety of combinations of hardware circuitry components and 
programmed microprocessors to effect control of the various vehicle 
systems and subsystems including the clutch pedal dashpot driveline torque 
limiter of the present invention. Often, control functions are logically 
separated and have specific input parameters, control equations, and 
output parameters which may be unique or shared with other logical control 
functions and/or other system and subsystem controllers. 
Referring now to FIG. 2, there is shown a cross-section of an 
electronically controlled clutch pedal dashpot 54 of the present invention 
for limiting the rate torque is applied to a driveline of a vehicle, such 
as a heavy-duty truck as illustrated in FIG. 1. In the preferred 
embodiment, the clutch pedal dashpot 54 includes a master clutch housing 
112 and a dashpot assembly shown generally by reference numeral 114. As 
shown, the dashpot assembly 114 is fixedly attached to the master clutch 
housing 112 and a mounting wall 116, such as the firewall of the truck. As 
shown in the drawings and described in greater detail below, the dashpot 
assembly 114 is designed to receive a push rod 118, which is pivotally 
connected to a clutch pedal 24. 
With continuing reference to FIG. 2, the dashpot assembly 114 preferably 
includes a substantially hollow piston housing 130 and a damper piston 132 
disposed within the housing. The damper piston 132 slides axially within 
the piston housing 130 between a rest position and a reset position based 
on the displacement of the clutch pedal 24 between a first, or resting 
position at which point the clutch is engaged and a second, or depressed, 
position at which point the clutch is disengaged. As shown, the damper 
piston 132 is generally hollow and preferably generally T-shaped so as to 
create a chamber 142, between the piston and the housing, within which a 
reset spring 138 is disposed. 
As best shown in FIG. 2, the piston housing 130 includes a check valve 
assembly shown generally by reference numeral 140. In the preferred 
embodiment, the check valve assembly 140 includes a one-way check valve 
and passage which allows air to be drawn into the chamber 142 as the 
clutch pedal 24 is displaced from the resting position to the depressed 
position (i.e. as the damper piston 130 is axially displaced toward the 
left). As shown, the check valve assembly 140 also prevents or impedes air 
from being expelled from the chamber 142 as the clutch pedal 120 returns 
to the resting position from the depressed position (i.e. as the damper 
piston 130 is axially displaced toward the right). Preferably, the check 
valve and passage are sized so as to restrict the flow of air drawn into 
the chamber 142 to a first rate. 
The piston housing 130 also preferably includes passages 144 and 146 in 
fluid communication with the chamber 142. As shown, the passage 144 
extends from the chamber 142 through the housing 130 and is in fluid 
communication with the atmosphere through an orifice. It should be noted 
that the passage 144 and the orifice allow air to be both drawn into and 
expelled from the chamber as the clutch pedal 24 is displaced between the 
resting position and the depressed position. Most preferably, the passage 
144 and orifice are sized so as to restrict the flow of air drawn into and 
expelled from the chamber to a second rate as the clutch pedal 24 is 
displaced between positions, the second rate being slower than the first 
rate at which air is allowed to be drawn into the chamber 142 via the 
check valve assembly 140. The piston housing 130 and damper piston 132 
preferably include O-rings 134 and 136, respectively, which provide a seal 
between the housing and piston. 
As also shown in FIG. 2, clutch pedal 24 includes a clutch pedal position 
sensor, indicated generally by reference numeral 181. In a preferred 
embodiment, clutch pedal position sensor 181 includes a potentiometer 
which has a resistive element 182 and a wiper arm 184. As is known, such 
an arrangement provides an indication of the relative clutch pedal 
position to the ECM 70 based on the voltage across wiper arm 184 and one 
of the terminal ends of resistive element 182. 
Clutch pedal dashpot 54 also includes a solenoid assembly 186 for 
controlling the air flow rate through passage 146 via movement of pin 188. 
A return spring 187 may be utilized to create a normally open or normally 
closed arrangement depending upon the particular application. In a 
normally closed arrangement, pin 188 normally blocks passage 146 so that 
air must be expelled through passage 144 and the orifice. Power may be 
applied to the solenoid to increase the rate of engagement of the clutch 
by retracting pin 188. If the solenoid should fail, a fault is logged in 
ECM 70 and the vehicle operator is alerted via dash console 30. The clutch 
apply rate is still limited by passage 144 and the orifice. 
In a normally open arrangement, pin 188 is normally retracted and passage 
144 is unrestricted. In this arrangement, passage 144 may be eliminated. 
If the solenoid should fail, a fault is logged in ECM 70 and the vehicle 
operator is alerted. However, little or no damping is provided since air 
may flow freely through passage 146. A normally open arrangement provides 
greater control authority since the engagement rate may be controlled 
between a maximum rate determine by the size of passage 146, and a 
nominally zero rate when pin 188 substantially blocks passage 146. Of 
course, the exhaust passage may be through the solenoid itself if desired. 
Referring now to FIG. 3, there is shown a cut-away of the clutch pedal 
dashpot shown in FIG. 2, illustrating the use of a lip seal 136' in place 
of the O-ring 136. The lip seal, as is known, folds back and forth between 
the housing 130' and the damper piston 132', allowing air to be 
transferred between the chamber 142 and the chamber 148 as the damper 
piston is translated axially within the housing. It should be appreciated 
that with the use of the lip seal 136', the check valve and/or the 
passages could be eliminated. It should also be appreciated that the 
clutch pedal dashpot could utilize different combinations of the lip seal, 
the check valve and the passages, so long as the desired result is 
achieved: that the damper piston be displaced from the rest position to 
the reset position when the clutch pedal is depressed at a rate which 
exceeds the rate at which the damper piston returns to the rest position 
when the clutch pedal is released. This slows the rate of clutch 
engagement and, therefore, controls the rate torque is applied to the 
driveline. 
Returning now to FIG. 2, axial displacement of the damper piston 132 within 
the piston housing 130 is restricted by a first piston stop member, or 
key, 150 so that the position of the damper piston relative to the clutch 
pedal 24 is fixed. It should be appreciated that as the clutch pedal 24 is 
depressed, the damper piston 132 is displaced toward the left to the key 
150 by the reset spring 138. The speed with which the damper piston 132 
moves is governed not only by the spring constant of the spring 138, but 
also by the rate at which air is drawn into the chamber 142. 
With continuing reference to FIG. 2, the master clutch housing 112 includes 
an extended piston 156 disposed within the master clutch housing 112. As 
shown, the extended piston 156 preferably receives, and is fixedly 
connected to, the push rod 118, such that the extended piston moves 
axially within the clutch pedal dashpot 110 as the clutch pedal 24 is 
displaced between the resting position and the depressed position. The key 
150 is sized to allow the extended piston 156 to pass therethrough while 
still restricting axial displacement of the damper piston 132 as described 
above. Thus, the damper piston 132 and the extended piston 156 are coupled 
to each other for only a portion of the clutch pedal travel. As shown, the 
extended piston 156 may include an orifice, or vent, 152 which is in fluid 
communication with the vehicle cab, allowing clean air to be cycled in and 
out of the housing. 
A second piston stop, or key, 158 restricts displacement of the extended 
piston 156 in one direction and a preload spring 180 restricts axial 
displacement of the extended piston in the other direction. It should be 
appreciated that the preload spring 180 is preferably selected so as to 
provide an appropriate preload force on the extended piston 156 so as to 
overcome the force exerted on the damper piston 132 by the reset spring 38 
and return the clutch pedal 24 to the resting position once the clutch 
pedal is released by the vehicle operator. 
As shown in FIG. 2, a hydraulic fluid reservoir 176 provides hydraulic 
fluid 174 to the master clutch housing 112 through a pair of fluid make-up 
ports 170 and 172. O-ring 178 functions as a seal between the hydraulic 
fluid reservoir 176 and the master clutch housing 112. Additional sealing 
between the extended piston 156 and the housing 112 is provided by the 
O-rings 160 and 162. 
Referring now to FIG. 4, there is illustrated a hydraulically actuated, 
self-adjusting clutch mechanism shown generally by reference numeral 190. 
As is known, the clutch mechanism 190 is positioned between an engine E 
and a transmission T of a driveline and functions to couple the engine to 
the transmission. More specifically, the clutch mechanism 190 releasably 
couples the adjacent ends of an engine crankshaft 194 and a transmission 
input shaft 196. 
As best shown in FIG. 4, the clutch mechanism 190 includes a hydraulic 
piston 200 which is attached to a clutch release arm 206. The hydraulic 
piston 200 moves axially within a piston chamber 202 which is in fluid 
communication with the master clutch housing 212 shown in FIG. 2. The 
clutch release arm 206 is pivotally connected to a release bearing 
assembly 208 through a pivot ball 210. As hydraulic fluid from the master 
clutch housing 112 fills the piston chamber 202, the hydraulic piston 206 
is axially displaced. This causes the clutch release arm 206 to slide the 
release bearing assembly 208 along the transmission input shaft 196. As a 
result, pivoted release levers 212, which are coupled to the release 
bearing assembly 208, apply pressure to a pair of spring-loaded pressure 
plates 214. 
Positioned between the spring-loaded pressure plates 214 and a flywheel 
216, which is fixedly attached to the engine crankshaft 194 for rotation 
therewith, is a driven disc 218. The disc 218 is lined on both faces with 
friction material, shown generally by reference numeral 220. The disc 218 
is free to float coaxially between the pressure plates 214 and the 
flywheel 216, and is carried on a hub 222 splined onto the transmission 
input shaft 196. As is known, this arrangement has the advantage of, in 
effect, doubling torque capacity of the clutch and halving the temperature 
of the rubbing surface during progressive engagement, thereby increasing 
the life of the friction material 220. 
With continuing reference to FIG. 4, since the clutch mechanism 190 is 
self-adjusting, when the driven disc 218 wears, the pressure plate spring 
force from the springs 224 forces the pressure plates 214 to move to the 
left, carrying the release levers 212 and the release bearing assembly 208 
to the left. This in turn forces the hydraulic piston 200 deeper into the 
chamber 202, displacing hydraulic fluid therefrom to the fluid reservoir 
176 through the master clutch housing 112. Therefore, clutch pedal 24 
position is not significantly altered with clutch friction material wear. 
With combined reference to FIGS. 2 and 4, it should be appreciated that as 
the clutch pedal 24 is depressed, the extended piston 156 is displaced 
toward the left, compressing the preload spring 180. Depending on how fast 
the clutch pedal is depressed, the extended piston 156 may separate from 
the damper piston 132. With the clutch pedal 24 depressed and the extended 
piston 156 displaced to the left, the reset spring 138 expands and 
displaces the damper piston 132 to the left. The speed with which the 
damper piston 132 moves is governed not only by the spring constant of the 
spring 138, but also by the rate at which air is drawn into the chamber 
142. 
Hydraulic fluid is forced out of the master clutch housing 112 of FIG. 2 
and supplied to the clutch mechanism 190 of FIG. 4. Once the clutch pedal 
24 is released by the vehicle operator, the pedal begins to return to the 
resting position primarily due to the hydraulic fluid pressure created by 
the clutch pressure plate springs. Preload spring 180 is secondary to the 
hydraulic pressure on the piston. It is effective, after the make-up port 
172 is traversed, to push the piston to the stop 158. The fluid pressure 
forces the extended piston 156 back toward the right. As this occurs, 
fluid is pushed into the clutch housing 112 from the clutch mechanism 190. 
As the pressure plate springs 224 expand, the extended piston 156 travels 
through the key 148 and contacts the damper piston 132, displacing it to 
the right, compressing the reset spring 138 and forcing air to be expelled 
from the chamber 142 through the passage 144 and/or passage 146. 
As is known, every portion of clutch pedal travel does not directly affect 
clutch engagement and disengagement. For example, the first portion of 
clutch pedal travel does not result in disengagement of the clutch due to 
internal clearances of clutch mechanism components. In the preferred 
embodiment, the dashpot 110 is designed such that the clutch begins to 
engage (i.e. the clutch begins to couple the engine to the driveline) when 
the extended piston first contacts the damper piston 132. This is commonly 
referred to as the "touch point" or the "point of incipient engagement." 
The rate of further clutch engagement, i.e. the rate at which torque is 
applied to the driveline, is then controlled via the reset spring 138 and 
the passages 144 and 146 (controlled by solenoid 186), all of which 
function to slow the rate at which the clutch fully engages by slowing the 
rate at which the clutch pedal is allowed to return to the resting 
position. As a result, the time rate of change of torque to the driveline 
is controlled, regardless of how zealously the driver attempts to engage 
the clutch. 
Referring now to FIG. 5, there is shown a cross-section of an alternative 
embodiment, shown generally by reference numeral 240, of an electronically 
controlled clutch pedal dashpot driveline torque limiter according to the 
present invention. In this embodiment, the dashpot 240 includes a master 
clutch housing 242 and a dashpot assembly shown generally by reference 
numeral 244. As shown, the dashpot assembly 244 is fixedly attached to the 
master clutch housing 242 so as to facilitate cooperation with the clutch 
pedal 246 as shown. It should be noted that the master clutch housing 242 
may be functionally similar to the master clutch housing 112 described 
above with reference to the embodiment shown in FIG. 2. It should be noted 
that alternatively, the connection between the clutch pedal and the clutch 
could be a self-adjusting cable system well known in the art. Regardless 
of the alternative utilized, the clutch engagement positions are 
preferably fixed relative to the clutch pedal attachment points. 
The dashpot 240 of FIG. 5 preferably includes a bellows 250. The clutch 
pedal 246 cooperates with the bellows 250 through a pad 252 to expand and 
contract the bellows 250 as the vehicle operator displaces the clutch 
pedal between a resting position wherein the clutch is engaged and a 
depressed position wherein the clutch is disengaged. In the embodiment 
shown, air flow into and out of the bellows 250 is governed by a check 
valve 254 and at least one passage 256. In this embodiment, passage 256 
exhausts to atmosphere through solenoid 260. As in the previous 
embodiment, the check valve 254 is a one-way check valve which allows air 
to be drawn into the bellows 250 as the clutch pedal 246 is displaced from 
the resting position to the depressed position, and which prevents or 
impedes air from being expelled from the bellows as the clutch pedal 
returns to the resting position from the depressed position. Preferably, 
the check valve 254 is sized so as restrict the flow of air drawn into the 
bellows 250 to a first rate. Also preferably, clutch pedal 246 includes a 
clutch pedal position sensor (not specifically illustrated), such as 
position indicator 181 of FIG. 2. 
As also shown in FIG. 5, the passage 256 provides a fluid communication 
between the bellows 250 and the atmosphere through solenoid 260. By 
controlling the position of pin 258 via the current supplied to solenoid 
260 from ECM 70 via output 46, the air flow rate through passage 256 is 
regulated. It should be noted that the passage 256 may allow air to be 
both drawn into and expelled from the bellows 250 depending upon the 
position of pin 258 as the clutch pedal 246 is displaced between the 
resting position and the depressed position. 
As described above, every portion of clutch pedal travel does not directly 
affect clutch engagement and disengagement. In this embodiment, therefore, 
the dashpot is designed such that the clutch begins to engage (i.e. the 
clutch begins to couple the engine to the driveline) when the clutch pedal 
246 begins to engage the bellows 250. The rate of further clutch 
engagement, i.e. the rate at which torque is applied to the driveline, is 
then controlled via the passage 256 by solenoid 260 which functions to 
slow the rate at which the clutch fully engages by slowing the rate at 
which air is expelled from the bellows, thereby slowing the rate at which 
the clutch pedal is allowed to return to the resting position. As a 
result, the rate of torque application to the driveline is controlled 
regardless of how zealously the driver attempts to engage the clutch. It 
should be appreciated that the embodiment shown illustrates just one way 
to achieve the desired result: to slow the rate of clutch engagement and, 
therefore, control the rate torque is applied to the driveline. 
Referring now to FIGS. 6 and 7, flow charts are shown to illustrate the 
control logic of a system and method for controlling the engagement rate 
of a master clutch according to the present invention. The control logic 
is preferably executed by a programmed microprocessor within ECM 70 or TCM 
72, but may be performed by a dedicated clutch electronic control unit 
utilizing various combinations of electric and electronic circuitry and/or 
programmed microprocessor(s). Block 300 represents determination of the 
current operating conditions of the vehicle to facilitate a 
context-sensitive control strategy. Block 302 then controls or limits the 
master clutch engagement based on the current operating conditions. 
FIG. 7 provides a more detailed description of the various operating 
parameters used to control the master clutch engagement to prevent 
excessive slipping of the clutch and reduce or eliminate the incidence of 
engine stall due to an improper engagement rate for the current engine 
speed. Block 310 determines the current engine speed which may be obtained 
from an appropriate sensor and broadcast by ECM 70 to various other system 
and subsystem controllers. Block 312 determines the transmission input 
speed while block 314 determines the transmission output speed as 
indicated by output shaft sensor 56 of FIG. 1. Block 316 determines the 
clutch pedal engagement rate by monitoring the clutch pedal position 
sensor. This is the rate requested by the operator by releasing the clutch 
pedal and may be equal to or different from the actual master clutch 
engagement rate as determined by the electronically controlled clutch 
pedal dashpot of the present invention. 
Block 318 of FIG. 7 may control or limit the master clutch engagement rate 
by controlling an associated solenoid, or control the engine to avoid 
engine stall based on the current operating conditions as determined by 
steps 310 through 316. Of course, the current vehicle operating condition 
may be indicated by various other sensors without departing from the 
spirit or scope of the present invention. Control of the vehicle engine 
may include requesting an increase or decrease in engine speed prior to 
allowing engagement of the master clutch by opening or closing an 
appropriate passage with a solenoid as disclosed above. 
It is understood, of course, that while the forms of the invention herein 
shown and described constitute the preferred embodiments of the invention, 
they are not intended to illustrate all possible forms thereof. It will 
also be understood that the words used are words of description rather 
than limitation, and that various changes may be made without departing 
from the spirit and scope of the invention as recited by the following 
claims.