Dual clutch application area and control

An adaptive clutch torque capacity control for an automotive vehicle clutch utilizing a dual area apply piston for engagement of the clutch plate for accurate control over a larger than normal torque range. The use of two piston areas allows one area to be controlled in a "coarse" fashion, while the other piston area, which is generally a small area, provides a "vernier" or fine adjustment control. The small area piston is controlled by an electronic closed loop for fine adjustment of any errors occurring in the "coarse" control.

BACKGROUND OF THE INVENTION 
In a conventional reciprocating piston, internal combustion engine, torque 
is near zero at low engine output speed and peaks somewhere in the middle 
of the engine operating range and then decreases as output speed further 
increases. On the other hand, a gas turbine engine has very high torque at 
low engine speeds. 
To provide a friction start clutch in a transmission for a high torque 
engine, such as a gas turbine engine, one would need a very wide range of 
torque capacity, and thus, a very high capacity clutch. A single large 
clutch piston would require a clutch piston apply area having sensitivity 
at a high level. If 100 psi were necessary to apply the clutch, a 1.0 psi 
variation in pressure would result in a large torque variation which would 
be objectionable at light throttle starts. Such an adjustment to provide 
adequate clutch apply pressure would not provide a controllable 
sensitivity. 
The present invention provides a novel clutch arrangement and control 
strategy for transmission starting in a high torque engine. 
SUMMARY OF THE INVENTION 
The present invention relates to a novel starting for a high torque engine 
utilizing a clutch and control strategy therefor wherein as the throttle 
for the engine provides an engine torque, a torque is also applied to the 
clutch to balance the engine torque and provide a desired engine speed. As 
torque is applied to the inlet clutch side, the engine is accelerating so 
that the clutch slips and additional torque is applied to the clutch until 
the input side and the output side of the clutch are synchronized as the 
torque is balance which is lock-up. A closed loop speed control acts to 
control the clutch torque while maintaining engine speed. 
The present invention also relates to a novel clutch arrangement and clutch 
control stategy for transmission starting in a high torque engine, such as 
a gas turbine engine for utilization in an automobile. The clutch 
arrangement comprises a single piston having a relatively small apply area 
and a separate relatively large apply area to actuate the pressure plate 
for engagement of the clutch plate. Each clutch piston area is defined by 
a hydraulic fluid chamber having a separate source of pressure from a 
control system. For low torque clutch apply, only the small piston area 
would be used controlled by a fast response system. For higher torque 
clutch apply, the large piston area would have pressure coarsely 
controlled to provide a torque capacity somewhat less than needed overall. 
The small piston area with its very fast and accurate control system would 
then supply the added torque capacity to produce the closely controlled 
net overall torque requirement. 
The present invention further relates to a novel clutch arrangement having 
a clutch control strategy wherein a fast response closed loop 
electro-hydraulic system controls the hydraulic pressure to the small 
piston apply area and a coarsely controlled hydraulic pressure for the 
large piston apply area. The small piston area would be controlled by the 
electronic closed loop to finely adjust any errors of the coarse control. 
Further objects are to provide a construction of maximum simplicity, 
efficiency, economy and ease of assembly and operation, and such further 
objects, advantages and capabilities as will later more fully appear and 
are inherently possessed thereby.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring more particularly to the disclosure in the drawings wherein is 
shown an illustrative embodiment of the present invention, FIG. 1 
discloses a friction clutch assembly 10 for use with a gas turbine or 
similar high torque engine in a vehicle, such as an automobile, for 
control of the torque transmitted from the engine to drive the wheels of 
the vehicle. This assembly provides a clutch application strategy where 
the torque capacity must be controlled to a very small percent of the 
overall clutch capacity. 
The clutch assembly includes a cover plate 11 mounted on a central pilot 12 
extending into the engine drive shaft 13 and a reaction plate 14 abuts the 
cover plate and is secured thereto by bolts 18. Grooves 15 are formed in 
the cover plate 11 to provide exhaust passages a will be later described. 
A flexplate 16 on a hub 17 is secured to the engine drive shaft 13 
parallel to and is also secure in spaced relation to the cover plate 11 by 
bolts 18. A clutch cover 19 is further secured to cover plate 11 to house 
the internal clutch structure. The cover includes an axially extending 
wall 21 terminating in a radial flange 22 secured to the periphery of the 
cover plate by bolts 18. The wall 21 also has a splined portion 23 
engaging the splined periphery 25 (FIG. 3) of an annular pressure plate 24 
for rotation together. 
A clutch plate 27 is secured to the radial flange 29 of a hub 28 received 
within a central opening in the reaction plate 14, the clutch plate having 
friction facings 31 secured to the periphery thereof and located between 
facing surfaces 32 and 33 of the reaction plate 14 and pressure plate 24, 
respectively. A Belleville spring 36 is pivotally mounted in a corner 34 
of the cover 19 and is retained therein by a snap ring 38 mounted in a 
groove 37 in the cover. The Belleville spring bears on an annular pressure 
ring 41 received in a complementary groove 39 formed in the surface of the 
pressure plate 24 opposite to surface 33. 
The clutch cover also includes a portion 43 extending radially from an 
inner hub 42 and an annular recessed portion 44 between the flange 43 and 
corner 34. A single, unitary piston 45 has an inner radial portion 
defining a small piston area 46 opposite flange 43, an intermediate 
shoulder 47 and an outer radial portion defining a large piston area 48 
received in the chamber 55 defined by recessed portion 44; the piston 
having an inner sealing ring 49 engaging the hub 42 and an intermediate 
ring 51 and outer ring 52 engaging the chamber 55. A fulcrum ridge or 
surface 53 on the outer radial piston portion engages the inner peripheral 
edge of the Belleville spring 36 extending radially inwardly of the 
pressure ring 41. 
The inner radial portion of the piston provides the small apply area 46 in 
a first chamber 54, and the outer radial portion provides the large apply 
area 48 in a second chamber 55. A clutch hub 56 having a radial flange 57 
fitting within a recess defined by the exterior surface of recessed 
portion 44 and abutting the flange 43 of hub 42 has a clutch hub shaft 58 
secured thereto and extending rearwardly towards the vehicle transmission 
(not shown). A stationary sleeve shaft 59 is sealingly received in the 
clutch hub shaft 58 through axially spaced sealing rings 61 to define an 
annular flow passage 62 communicating with angularly disposed ports 63 in 
the clutch cover hub 42 leading to the chamber 54. Also formed between the 
clutch hub 56 and the clutch hub shaft 58 is a flow passage 65 leading to 
one or more radial passages 66 formed between the hub flange 57 and radial 
flange 43 of the cover and communicating with passages 67 in the shoulder 
68 defining the recessed portion 44 and leading to the large chamber 55. A 
port 71 communicates with annular passage 62 and a second port 72 
communicates with passage 65, the ports being sealingly separated by three 
seal rings 69. 
A transmission input shaft 73 extends through the sleeve shaft 59 to 
terminate in a splined forward end 74 received in the splined opening 75 
in the clutch plate hub 28. An inner annular passage 76 is formed between 
the input shaft 73 and sleeve shaft 59 to communicate through radial ports 
77 with an outer annular passage 78 between the sleeve shaft 59 and clutch 
hub shaft 58, which in turn is connected to a source of clutch cooling 
oil. The input shaft 73 is rotatably mounted in the sleeve shaft 59 by 
bearings 79 through which cooling oil flows to the interior of the clutch. 
Once the oil has moved radially past the friction facings, it is returned 
to a sump through radial passages 15 to a central passage 81 in shaft 73 
and radial ports 82 to an annular clearance between shafts 59 and 73 to 
exit from openings rearwardly of the clutch hub shaft 58. 
FIGS. 4 and 5 disclose more detailed schematic arrangements of the 
hydraulic control system for the clutch piston and the electronic control 
system for the clutch. As shown in FIG. 4, the hydraulic system for clutch 
control includes a pair of electro-hydraulic control valves, which valves 
have two stages. The first or pilot stage is a high response solenoid 
on/off valve that operates in a pulse-width modulated (PWM) mode, and the 
second stage is a spool valve for actually controlling or regulating the 
pressure to either chamber of the clutch. FIG. 4 discloses the hydraulic 
system 85 which includes apply systems 87 and 91 coupled to the line 
pressure inlet 86 and branch lines 86a and 86b wherein the hydraulic 
pressures delivered to the two clutch chambers 54 and 55 are controlled 
with pulse-width-modulated (PWM) solenoid valves 88 and 92. The PWM duty 
cycle for each solenoid is calculated by an electronic control unit every 
five milliseconds. 
Primary control of the clutch is provided by the pressure delivered to the 
chamber 54 defining the small area 46 of the clutch piston 45. Under high 
throttle starting conditions, a bias pressure is applied to the chamber 55 
defining the large piston area 48. The pressure levels of the hydraulic 
fluid delivered to the large and small piston areas are controlled with 
two three-way spool valves 89 and 93 driven by the PWM solenoid valves 88 
and 92, respectively. Line pressure from inlet 86 passes through a 
pressure regulator 95 with the regulated pressure P.sub.REG being fed 
through branches 96 and 97 to the solenoid valves 88 and 92; a feedback 
loop 98 having an orifice 99 feeds back to the pressure regulator. The 
small clutch pressure P.sub.SC provided by the valve 89 from line pressure 
in line 86a is passed through line 101 to the port 71 leading into the 
small chamber 54, a pressure sensor 102 communicating with pressure line 
101. The small piston area control valve 89 is an open-center valve with 
no hydraulic feedback. The control software generating the PWM duty cycle 
for the solenoid valve 88 driving the small area control valve 89 includes 
feedback compensation of errors between the measured pressure in the small 
area of the piston and the pressure command from the pressure control 
strategy. 
The large piston area control valve 93 is a closed center valve with 
hydraulic feedback through loop 103 which may contain an orifice 104 for 
damping. The control software computes the PWM duty cycle for the solenoid 
valve 92 driving the control valve 93 based on the pressure command 
indicated by the pressure control strategy directly. An accumulator 105 
communicates with the line 97 between solenoid valve 92 and control valve 
93 with the pulse-width-modulated pilot pressure for the control valve 93 
minimizing the pressure ripple caused by the PWM signal. The large clutch 
pressure P.sub.LC provided by valve 93 from line pressure in line 86b 
passes through line 106 to the port 72 leading to chamber 55. 
The bias pressure for the large piston area is passed through a first-order 
lag circuit (not shown) before issuing the large area pressure command to 
avoid any sudden changes in clutch pressure. Both the bias set point and 
the lag frequency are a function of the throttle sensor voltage signal. 
The small area clutch torque command is equal to the desired clutch torque 
minus the expected torque generated as a result of the large piston area 
pressure command. This torque command is transformed into a small piston 
pressure command. The pressure command to the small piston area control 
valve is the sum of the pressure required to maintain the clutch touch-off 
position and the incremental pressure required on the small piston area to 
generate the commanded clutch torque. 
FIG. 5 is a block diagram of the control system for the clutch which is 
based on controlling the speed of the high torque engine or turbine for 
clutch control. A signal T.sub.E for the engine throttle setting is fed to 
a microcomputer 110 including an engine operating map or speed schedule 
111 containing data for engine stall at various throttle settings. The 
operating map provides a speed set point to a comparator 112 leading to a 
controller 113, both in the microcomputer 110, the controller providing a 
signal to the two-stage pressure valve 89. The pressure setting from this 
valve is provided to the small clutch piston area 46 through line 101 and 
port 71. The clutch engagement due to the pressure developed in chamber 54 
causes the engine speed to decrease and the change in engine speed 
provides a signal through a feedback loop 114 having a speed sensor 115. 
This signal is compared in comparator 112 with the stall speed set point, 
which in turn provides an altered output signal to the controller 113 to 
adjust the pressure valve 89. Also, a second feedback loop 116 having the 
pressure sensor 102 takes the pressure signal from the valve 89 and 
provides a secondary closed loop for a signal to the comparator. As the 
clutch is applied by the pressure from the control valve 89, the engine 
speed initially increases and then levels off while the vehicle speed 
increases in a linear fashion until it intersects the engine speed. This 
is clutch lock-up. 
Likewise the dual area pressure valve 93 is actuated once the small piston 
area 46 has completed clutch engagement. This provides pressure to the 
large piston chamber 55 upon an increase in engine speed by depressing the 
throttle. The large area 48 receives a pressure P.sub.LC from valve 93 
based on a large area pressure schedule 118, also a part of microcomputer 
110, and provides a coarse torque adjustment through line 106 and port 72 
to the large clutch area 48 for the clutch control once initial smooth 
engagement of the clutch has been achieved. For a heavy throttle, the 
controller will switch to the large piston area 48 in chamber 55 to give 
the clutch its full capacity as required. 
The small piston area 46 is the "vernier" area which overcomes the force of 
the Belleville spring 36, and the larger piston area 48 is the main apply 
area. This arrangement allows an accurate smooth takeoff from a vehicle 
stop and provides the capability for high torque with the large piston 
area.