Closed loop control of a hydraulically controlled clutch

A control system (100) for a hydraulically controlled clutch (18) for coupling a source of rotary power (12) to a load (102) for accelerating the load from a first velocity to a second velocity within a time measured from a beginning of the acceleration and ending between first and second times measured from the beginning of acceleration with the load being variable during acceleration of the load from the first velocity to the second velocity in accordance with the invention includes a source of pressurized hydraulic fluid (38), a servo valve (42), having an inlet coupled to the source of pressurized fluid and an outlet coupled to the hydraulically controlled clutch, the servo valve being responsive to a control signal to vary the pressure of hydraulic fluid applied to the hydraulically controlled clutch to vary the coupling between the source of rotary power and an output shaft (22) which is coupled to the load, a speed sensor (112) for producing a speed signal proportional to the speed of the output shaft; a stored program (108) controlling a programmed acceleration of the load from the first velocity to the second velocity within a time interval measured from the beginning of the acceleration and ending between the first and second times; a controller (110), responsive to the stored program and the speed signal, for producing the control signal which is a function of a difference (E) between the velocity signal and a velocity controlled by the stored program.

DESCRIPTION 
1. Technical Field 
The present invention relates to hydraulically controlled clutches for 
coupling a source of rotary power to a load for accelerating the load from 
a first velocity to a second velocity within specified time limits 
measured from the beginning of the acceleration. More particularly, the 
present invention relates to hydraulically controlled clutches of the 
aforementioned type for use in auxiliary power units (APU) in airframes. 
2. Background Art 
FIG. 1 illustrates a block diagram of an APU 10 which was manufactured by 
the Assignee of the present invention. The APU 10 functioned to 
selectively couple output power from a turbine and gear reduction 
transmission 12 to a turbine propulsion engine 14 for purposes of starting 
the turbine propulsion engine. Coupling of rotary power from the output 16 
of the turbine and gear reduction transmission 12 is through hydraulically 
controlled clutch 18, which is controlled by an open loop control which 
specifies the acceleration of the turbine propulsion engine 14 in 
accordance with a velocity program 20. The output of the clutch 22 is 
applied to torque converter 24, which acts as a torque amplifier and 
reduces the magnitude and variation of load torque imposed on the clutch 
and functions to provide a limited degree of slippage to avoid high 
inertial shock caused by rapid engagement of the hydraulic clutch 18. The 
output 26 of the torque converter 24 is applied to gear box 28 having an 
output 30 which drives the turbine propulsion engine 14 at a velocity 
reduced from the velocity applied to the input 26. 
The oil supply circuit for the hydraulic clutch 18 is described as follows. 
Oil supply 32 supplied oil to pump 34 which provided pressurized oil on 
output 36 which is applied to relief valve 38 which regulates the output 
pressure of the pump 34 by feeding back oil to the inlet when the output 
pressure exceeds the rated pressure of the relief valve at which the 
feedback opens. The output 40 of the relief valve 38 is applied to servo 
valve 42 which opens under the velocity program 20 to control the pressure 
of oil applied to the hydraulically controlled clutch 18 on output 44. 
Variation of the oil pressure on output 44 controls the rate of slip 
between the output 16 of the turbine and gear reduction transmission 12 
and the output 22 of the clutch 18. When maximum oil pressure is applied 
to the hydraulically controlled clutch 18 on output 44, the output 16 of 
the turbine and gear reduction transmission 12 is locked to the output 22. 
As stated above, the torque converter 24 was necessary to reduce inertial 
shocks caused by the engagement of the plates 45 of the clutch 18. As a 
consequence of the velocity program 20 being an open loop control, 
non-linearities in the engagement of the plates of the hydraulic clutch 18 
as a function of the hydraulic pressure applied on output 44 cause the 
clutch to have a non-linear engagement characteristic as a function of the 
applied hydraulic pressure. The limited slip provided by the torque 
converter 24 cushions the application of torque impulses to the gear box 
28 which could lead to damaging of the teeth therein where there is a 
rapid change of torque as a consequence of a change in the magnitude of 
the velocity program during engagement of the hydraulic clutch 18. 
The torque load which the turbine propulsion engine 14 represents to the 
output 30 of the gearbox 28 is relatively linear as a function of 
acceleration of the rotor of the turbine from a stop condition to the 
speed at which it is to be driven for purposes of starting. 
In applications where the gearbox 28 is also driving at least one of a 
hydraulic pump and a generator a non-linear torque load is present during 
the acceleration of the rotor of the turbine propulsion engine 14 as a 
consequence of the energy drawn by a hydraulic pump being a non-linear 
function of its rotational velocity and further the load represented by a 
generator varying as a function of the electrical load being driven by the 
generator. Accordingly, the APU 10 of FIG. 1 functions satisfactorily for 
the purpose of starting the turbine propulsion engine 14 as a consequence 
of the torque converter 24 removing torque transients, and the relative 
torque represented by the rotor of the turbine propulsion engine 14 for 
purposes of starting being substantially less than if the gearbox 28 were 
also driving at least one of a hydraulic pump and a generator. 
U.S. Pat. No. 2,995,957 discloses a transmission control which utilizes a 
control loop to control pressure applied to a torque transmitter to 
maintain a predetermined engine speed profile during shifting of the 
transmission. 
U.S. Pat. No. 3,437,188 discloses a pneumatic control for varying the 
coupling of a constant speed motor to a variable speed output. The desired 
output speed is set by a speed set potentiometer. A control loop maintains 
the variable output speed in accordance with the speed set by the 
potentiometer. 
U.S. Pat. No. 3,822,770 discloses a system for regulating a hydraulically 
actuated clutch in a drive train by regulating the pressure of fluid 
applied to the clutch. Hydraulic feedback is utilized in the system. 
DISCLOSURE OF INVENTION 
The present invention is a control system for a hydraulically controlled 
clutch for coupling a source of rotary power to a load for accelerating 
the load from a first velocity to a second velocity within a time measured 
from a beginning of the acceleration and ending between first and second 
times measured from the beginning of the acceleration with the load being 
variable during the acceleration of the load from the first velocity to 
the second velocity. A preferred embodiment of the present invention is in 
an APU in which a hydraulically controlled clutch selectively couples a 
turbine and gear reduction transmission having an output rotating at high 
velocity, which may be variable in velocity, to an output of the clutch 
for driving a load which varies during the acceleration of the load from 
the first velocity to the second velocity with a closed loop velocity 
control being utilized to maintain the acceleration in accordance with a 
stored program for controlling a programmed acceleration of the load from 
the first velocity to the second velocity. In a preferred embodiment, the 
output of the clutch is coupled to a gearbox which drives at least one of 
a hydraulic pump and generator which represent a variable torque during 
the acceleration of the load from the first velocity to the second 
velocity as a consequence of a non-linear response of a hydraulic pump to 
a variable shaft input velocity and the electrical generator representing 
a load varying as a function of velocity. The hydraulic pump and/or 
generator may be used for starting of a turbine propulsion engine. A 
closed loop velocity control including a proportional and differential 
controller which is responsive to an error signal generated by the 
comparison of the actual shaft velocity of the output from the clutch and 
the desired velocity specified by the stored program generates a control 
signal for a servo valve which regulates the pressure of oil applied to 
the hydraulic clutch to precisely control the engagement of the clutch to 
cause the velocity of the output of the clutch to follow the stored 
program. Alternatively, a closed loop velocity control including a 
proportional, integral and differential controller, responsive to an error 
signal generated by the comparison of the actual shaft velocity of the 
output from the clutch and the velocity determined by integration of a 
stored acceleration, generates the control signal for the servo valve. The 
dynamic response characteristic of the hydraulic clutch as driven by a 
proportional and differential or a proportional and integral control 
provides fast closed loop response which enables the pressure of the oil 
applied to the hydraulic clutch to be modulated rapidly to counter any 
non-linearities in the velocity of the output of the clutch as a 
consequence of a variable load being driven and a non-linear friction 
response of the hydraulic clutch as a function of the velocity of the 
output of the clutch. 
A control system for a hydraulically controlled clutch for coupling a 
source of rotary power to a load for accelerating the load from a first 
velocity to a second velocity within a time measured from a beginning of 
the acceleration and ending between first and second times measured from 
the beginning of the acceleration with the load being variable during the 
acceleration of the load from the first velocity to the second velocity in 
accordance with the invention includes a source of pressurized hydraulic 
fluid; a servo valve, having an inlet coupled to the source of pressurized 
fluid and an outlet coupled to the hydraulically controlled clutch, the 
servo valve being responsive to a control signal to vary the pressure of 
hydraulic fluid applied to the hydraulically controlled clutch to vary the 
coupling between the source of rotary power and an output shaft which is 
coupled to the load; a sensor for producing a velocity signal proportional 
to the speed of the output shaft; a stored program controlling a 
programmed acceleration of the load from the first velocity to the second 
velocity within a time interval measured from the beginning of the 
acceleration and ending between the first and second time limits; a 
controller, responsive to the stored program and the velocity signal for 
producing the control signal which is a function of a difference between 
the velocity signal and a desired velocity of the output shaft during 
acceleration from the first velocity to the second velocity produced by 
the stored program. The controller is a proportional and differential 
controller and the control signal is proportional to and a differential of 
the difference between the velocity signal and the desired velocity 
controlled by the program. Alternatively, the stored program may be a set 
acceleration for accelerating the load from the first value to the second 
value and the controller is a proportional integral and differential 
controller with the control signal being proportional to and differential 
of the difference between the velocity signal and an integral of the set 
acceleration. The stored program may be a series of velocity values which 
are read out synchronously during accelerating of the load from the first 
velocity to the second velocity. The load may comprise a gearbox driving 
at least one first load which is variable during the acceleration between 
the first and second velocities. The first load may be a hydraulic pump or 
a generator. Additionally, the gearbox may drive at least a first load and 
a second load which are variable during acceleration between the first and 
second velocities with the first load being a hydraulic pump and the 
second load being a generator. Finally, the source of rotary power may be 
variable in velocity.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 2 illustrates a block diagram of an APU 100 in accordance with the 
present invention. Like reference numerals identify like parts in FIGS. 1 
and 2 and will not be described herein in conjunction with FIG. 2 except 
to the extent necessary to understand the present invention. The APU 100 
of FIG. 2 has a performance specification which is much more stringent 
than that of the APU 10 of FIG. 1. In a commercial implementation of the 
APU 100 of FIG. 2, acceleration of the output 22 of the clutch 18 from 
stop to full velocity is to occur between 1.8 seconds and 2.2 seconds. If 
acceleration of the output 22 occurs for an interval longer than 2.5 
seconds, tests have shown that the life of the plates 45 in the 
hydraulically actuated clutch 18 is drastically limited in comparison when 
the acceleration of the output is made from stop to full velocity in no 
greater than 2.5 seconds. Additionally, acceleration of the output 22 to 
full velocity in less than 1.8 seconds is not permitted to avoid damaging 
the gearbox 102 as a consequence of loading the input with a torque higher 
than the rated torque of the teeth of the gear drive therein. Finally, the 
gearbox 102 is driving a load which is several times greater than the load 
of the turbine propulsion engine 22 of the system of FIG. 1 as a 
consequence of the presence of the torque converter 24. The load is at 
least one of a hydraulic pump 104 and an electrical generator 106. The 
hydraulic pump 104 has a characteristic such that the load torque required 
to drive it as a function of velocity from stop to full velocity is 
non-linear. The generator 106 has a characteristic caused by oil viscous 
drag in a cooling mechanism within the generator which varies as a 
function of velocity during the acceleration of the generator from stop to 
full velocity. As a consequence of at least the hydraulic pump 104 
representing a non-linear load to the gearbox 102 during acceleration from 
stop to full velocity, the load torque represented by the input of the 
gearbox which is driven by the output 22 of the clutch varies during the 
acceleration substantially from zero to full velocity. FIG. 3C illustrates 
a possible variation of the load torque represented by the gearbox 102 to 
the output 22 of the hydraulic clutch 18 with it being understood that 
other non-linear loads would be provided by the gearbox 102 depending upon 
individual characteristics of the hydraulic pump 104 and generator 106 
chosen and additionally whether output power from either or both of the 
hydraulic pump and generator was being drawn during the acceleration of 
the gearbox from stop to full velocity. Additionally, the coefficient of 
friction of the clutch varies dynamically during the acceleration of the 
output 22 from stop to full velocity as illustrated in FIG. 3B. The 
desired velocity control program 108 as illustrated in FIG. 3A varies 
linearly as a function of time between zero and full velocity over a time 
interval equal to T1. It should be understood that in the preferred 
embodiment of the present invention the time elapsed between zero and T1 
is midway between the lower time limit of 1.8 seconds and the upper time 
limit of 2.2 seconds. 
As a consequence of the non-linearities described above which are present 
during the acceleration of the gearbox 102 from zero to full velocity, 
substantial variation in actual acceleration from the desired linear 
acceleration of FIG. 3A would occur without the velocity control loop 
provided by controller 110 as described below. A velocity sensor 112 
produces a signal which is proportional to the velocity of the output 22 
of hydraulic clutch 18. The velocity signal produced by the velocity 
sensor 112 is converted from analog to digital by A to D converter 114. A 
clutch timing switch 116 produces an output signal at time 0 of the 
velocity program illustrated in FIG. 3A. The signal produced by the clutch 
timing switch 116 synchronizes the digital velocity program 108 which may 
be in the form of a constant acceleration or a series of velocity values 
which are read out synchronously during the accelerating of the gearbox 
102 from stop to full velocity in accordance with the velocity program of 
FIG. 3A. Summer 118 produces an error signal proportional to the 
difference of the actual velocity signal produced by the velocity sensor 
112 and the desired velocity at the time of comparison. The error signal 
is applied to a digital proportional and differential control 120 of 
conventional design. Alternatively, a set acceleration may be applied to a 
PID control 120 from the program 108 which integrates the acceleration to 
produce the desired velocity program. The PID control 120 produces an 
error signal equivalent to that produced by summer 118 and proportionately 
and differentially amplifies the error signal of the servo control signal 
122. The dotted lines indicate connections when the aforementioned set 
acceleration and PID control is utilized. Digital control 120 provides a 
servo-control signal 122 which is proportional to the error signal and a 
differential thereof to provide the necessary high frequency response to 
modulate the pressure applied on control line 44 to the hydraulically 
controlled clutch 18 to cause the velocity of the output 22 of the 
hydraulic clutch to follow the desired velocity program of FIG. 3A. The 
servo-control signal is converted from digital to analog by digital to 
analog converter 124 prior to application to servo valve 42. It should be 
understood that the desired velocity program of FIG. 3A of accelerating 
the gearbox 102 which drives non-linear loads represented by the hydraulic 
pump 104 and the generator 106 during the acceleration from stop to full 
velocity could not be achieved without the combination of the closed loop 
control and hydraulic servo system. Otherwise, the various perturbations 
in the system would cause the actual acceleration of the gearbox 102 to 
vary outside of the lower and upper limits described above. 
FIG. 4 illustrates a flowchart of the operation of the clutch control of 
the present invention. Operation proceeds from starting point 200 to 
initialization of the system at point 202. At point 204 a determination is 
made if the clutch is to be engaged. If the answer is "yes" at point 204, 
servo valve 42 is opened at point 206. At decision point 208 a 
determination is made if the hydraulic system of the clutch 18 is full of 
fluid. At point 208 when the answer is "yes" the clutch timing switch 116 
starts the acceleration cycle during which velocity sensor 112 is 
interrogated at point 210 by controller 110 At point 212 the error E is 
determined. At point 214, the PD or PID control 120 amplifies the error 
signal E to generate the servo control signal 122. The servo control 
signal 122 is applied to servo valve 42 at point 216. At decision point 
218 a determination is made if the final velocity of FIG. 3A is reached. 
If the answer is "no", the operation loops back to point 210. If the 
answer is "yes" at decision point 218, the program proceeds to point 220 
where the clutch 18 is fully engaged. At point 222 a decision is made 
whether to disengage the clutch. If the answer is "no" at point 222 the 
operation loops back to point 220. If the answer is "yes" at point 222, 
operation proceeds to point 224 where the servo valve is shut off and 
operation loops back to point 202. It should be understood that other 
operational sequences are within the scope of the invention. 
While the preferred embodiment of the present invention is an APU utilized 
in an airframe, it should be understood that other implementations of the 
invention are possible where it is necessary to drive a non-linear load 
between first and second velocities with a velocity program having 
characteristics which do not permit substantial variation from the desired 
velocity program. 
While the invention has been described in terms of its preferred 
embodiment, it should be understood that numerous modifications may be 
made thereto without departing from the spirit and scope of the invention 
as defined in the appended claims. It is intended that all such 
modifications fall within the scope of the appended claims.