High performance controller for variable displacement hydraulic motors

In a control (100) for a variable displacement hydraulic motor (12) having an electrically controlled hydraulic servo valve (14) for controlling the displacement of a hydraulic motor by varying position of a wobbler (28) in response to a servo valve control signal, a velocity transducer (72), coupled to a shaft (30) driven by the motor, for providing a velocity signal representative of the velocity of the shaft, and a wobbler position sensor (84), coupled to the wobbler, for producing a wobbler position signal indicative of the position of the wobbler, an improvement in accordance with the invention includes a controller (100) for producing the servo valve control signal as a function of a wobbler position command specifying a position of the wobbler wherein the wobbler position command is a function of at least one of the velocity signal representative of the velocity of the shaft and the wobbler position signal.

DESCRIPTION 
1. Technical Field 
The present invention relates to controls for variable displacement 
hydraulic motors. More particularly, the present invention relates to 
controls for variable displacement hydraulic motors used for powering 
actuators in airplanes having high resolution, stiffness, large load 
driving capability and speed of response. 
2. Background Art 
U.S. Pat. No. 4,487,109, which is assigned to the assignee of the present 
invention, discloses a control system for a variable displacement 
hydraulic motor. FIG. 1 illustrates the system disclosed in the '109 
patent including the basic proportional controls for a variable 
displacement hydraulic motor. The system is comprised of a variable 
displacement hydraulic motor (VDHM) 12 of conventional construction, and 
an electro-hydraulic servo valve (EHSV) 14 which functions in a manner to 
control the displacement of the hydraulic motor 12 in a manner described 
below and a servo control system 16 containing three proportional controls 
as described below. As illustrated, the VDHM 12 is an axial piston motor 
but it should be understood that the present invention is not limited to 
any form of hydraulic motor. The VDHM 12 has a plurality of pistons 18 
which are carried within a series of longitudinal bores within a cylinder 
block 20. Port plate 22 has a high pressure port 24 for receiving 
pressurized hydraulic fluid from a source (not illustrated) and an output 
port 26 which discharges hydraulic fluid for flow back to the source of 
hydraulic fluid. The flow of hydraulic fluid into the cylinders is 
controlled by the relative position of the port plate with respect to the 
rotating cylinder block 20. Wobbler 28 has a variable inclination with 
respect to drive shaft 30 which applies torque to a load 32. The angle of 
inclination of the wobbler 28 is varied by the axial position of the 
piston rod 34. The angular position of the wobbler 28 with respect to the 
drive shaft controls the displacement of the VDHM 12 with variations in 
displacement being produced by control of the axial position of piston rod 
34. 
The EHSV 14 is hydraulically coupled to piston 36 by means of hydraulic 
lines 38 and 40. Pressurized hydraulic fluid is applied to the EHSV 14 by 
hydraulic line 42 and is returned to the source of pressurized hydraulic 
fluid by hydraulic line 44. The EHSV 14 contains a movable valve 46 which 
is controlled by a servo valve control signal outputted from the servo 
controller 16. The servo valve control signal causes armature 48 to pivot 
about pivot point 50. 
The servo controller 16 has three separate control loops which each contain 
a proportional control of conventional construction. The first loop 52 is 
responsive to a position command 54 and a feedback signal 56 which is 
derived from a conventional position sensor 58 which is mechanically 
coupled to the drive shaft to detect its position. Summer 60 takes the 
difference between the position command and the feedback signal 56 
produced by the position sensor 58. A suitable operational amplifier 62 
having an amplification characteristic Kp amplifies the output signal from 
the position sensor. The amplification characteristic Kp is a function of 
the system in which the VDHM 12 is utilized including the load 32 being 
driven. The output from the operational amplifier 62 is applied to a 
suitable limiter 64 to prevent overdriving of the output velocity command 
68 produced by the limiter. The second loop 66 is a velocity control loop 
which is responsive to an input velocity command 68 and a feedback 
velocity signal 70. The velocity signal 70 is produced by a velocity 
transducer 72. Summer 74 produces an output signal which is equal to the 
difference between the input velocity command 68 and the feedback velocity 
signal 70. Operational amplifier 76 amplifies the output signal from 
summer 74. The gain of the operational amplifier 76 is a function of the 
VDHM 12 including the load 32 being driven. The output signal from the 
operational amplifier 76 is applied to a limiter 78 to prevent 
overdriving. The third loop 80 is responsive to an input wobbler position 
command 82 and a feedback signal 86 produced by wobbler position sensor 
84. Summer 86 produces an output signal 88 which is equal to the 
difference between the wobbler position command 82 and the feedback signal 
from the wobbler position sensor 84. The output signal 88 is amplified by 
operational amplifier 90 having a gain which is a function of the VDHM and 
the load 32 being driven to produce the servo valve control signal. The 
servo valve control signal 92 is amplified by current driver 94. 
The system of FIG. 1 has been modified to have an integrator in parallel 
with the proportional control provided by operational amplifier 76. U.S. 
Pat. application Ser. No. 298,751, entitled "Torque Velocity Control For 
Variable Displacement Motor", filed on Jan. 19, 1989, which is assigned to 
the assignee of the present invention discloses a variable displacement 
hydraulic motor control having a velocity control loop with an integrator 
in parallel with a proportional amplifier. The integral control provided 
by the addition of an integrator provides an accurate control of the motor 
speed permitting system operation at a maximum speed and increases the 
stiffness of control. 
However, the addition of an integrator to the system of FIG. 1 is not 
sufficient to provide for the performance required for a hydraulically 
powered actuator driven by a variable displacement motor for applications 
in actuators requiring performance characteristics of high resolution, 
stiffness, and speed of response in driving a load to a commanded position 
such as in high performance aircraft. A high bandwidth speed control loop 
is required to achieve the required performance. To achieve higher 
bandwidth it is necessary to increase the gain of both the proportional 
amplifier and the integrator in parallel with the proportional amplifier 
which may cause system instability and an oscillation. 
To increase the gain of the velocity control loop of the system of FIG. 1, 
motor acceleration feedback could be added. The disadvantage of this is 
that an additional acceleration sensor would be required with the 
attendant expense or, alternatively, a differentiator could be added to 
compute acceleration from the velocity sensed by the velocity transducer. 
However, it is known that a differentiator amplifies noise and may cause 
amplifier saturation or unwanted oscillations. 
FIG. 2 illustrates a block diagram of the transfer function of the system 
of FIG. 1 which has been modified to contain an integrator 110 in parallel 
with the proportional amplifier 76. Like reference numerals illustrate 
parts in FIGS. 1 and 2. Standard operator notation is used wherein s is 
the variable. The information within each block represents the transfer 
function of the element which the block identifies. K.sub.1 is the gain 
between a commanded position of the element being driven and the actual 
output position, K.sub.2 is the gain between the wobbler piston position 
and the motor velocity and K.sub.3 is the motor velocity sensor gain. The 
term "a" is a pole position of the wobbler position control and "b" is a 
pole position of the velocity control loop. V.sub.xp is a voltage 
corresponding to wobbler position. The transfer function of FIG. 2 which 
is a third order function of S is set forth below: 
##EQU1## 
DISCLOSURE OF INVENTION 
The present invention provides a control system for a variable displacement 
hydraulic motor for positioning a load at a commanded position with high 
resolution, stiffness, load moving capability and speed of response. The 
invention may be used as a control system for an actuator of primary and 
secondary flight controls in a high performance aircraft such as fighter 
aircraft. With the invention, the speed control loop for the variable 
displacement hydraulic motor is modified from the prior art of FIG. 1 to 
contain at least one proportional term not found in the prior art which 
simulates the feedback which would be obtained from motor acceleration 
feedback as described above without the attendant deficiencies of 
amplifying noise, causing amplifier saturation and unwanted oscillations. 
The proportional terms may be derived from feedback of the motor velocity 
and/or the position of the wobbler control piston or the sum thereof. In a 
preferred form of the invention, a sum is computed between the velocity 
signal and the position of the wobbler control piston which is subtracted 
from a signal which is a proportional and integral function of the 
difference between a velocity command and the feedback velocity signal to 
produce a wobbler position command. A signal which is a function of the 
difference between the wobbler position command and the position of the 
wobbler control piston is used for controlling the activation of the 
electrohydraulic servo valve. 
In a control for a variable displacement hydraulic motor having an 
electrically controlled hydraulic servo valve for controlling the 
displacement of the hydraulic motor by varying a position of a wobbler in 
response to a servo valve control signal, a velocity transducer, coupled 
to the shaft driven by the motor, for producing a velocity signal 
indicative of the velocity of the shaft and a wobbler position sensor, 
coupled to the wobbler, for producing a wobbler position signal indicative 
of the position of the wobbler, an improvement in accordance with the 
present invention includes a controller for producing the servo valve 
control signal as a function of a wobbler position command specifying a 
position of the wobbler wherein the wobbler position command is a function 
of at least one of the velocity signal indicative of the velocity of the 
shaft and the wobbler position signal. The wobbler position command may be 
a function of the velocity signal indicative of the velocity of the shaft, 
a function of the wobbler position signal or a function of the sum of the 
velocity signal representative of the velocity of the shaft and the 
wobbler position signal. The controller further includes a limiter having 
an input coupled to a difference between the quantity equal to a velocity 
command, specifying a velocity of the motor, and a signal representative 
of the velocity of the motor and a signal which is a function of at least 
one of the velocity signal representative of the velocity of the shaft and 
the wobbler position signal for limiting the wobbler position command to 
produce a limiting function of the servo valve control signal. 
A method of controlling a variable displacement hydraulic motor having an 
electrically controlled servo valve for controlling the displacement of 
the hydraulic motor by varying position of a wobbler in response to a 
servo valve control signal, a velocity transducer, coupled to the shaft 
driven by the motor, for producing a velocity signal representative of the 
velocity of the shaft, and a wobbler position sensor, coupled to the 
wobbler, for producing a wobbler position signal indicative of the 
position of the wobbler in accordance with the invention includes 
generating a wobbler position command as a function the velocity signal 
indicative of the velocity of the shaft and the wobbler position signal 
specifying a position of the wobbler; and generating a servo valve control 
signal which is a function of a difference between the wobbler position 
command and the wobbler position signal. The wobbler position command may 
be of a function of a sum of the velocity signal indicative of the 
velocity of the shaft and the wobbler position signal. The method further 
includes limiting the wobbler position command to produce a limiting of 
the servo valve control signal. 
A control for a variable displacement hydraulic motor in accordance with 
the invention includes an electrically controlled hydraulic servo valve 
for controlling the displacement of the hydraulic motor by varying a 
position of a wobbler in response to a servo valve control signal; a 
velocity transducer, coupled to the shaft driven by the motor, for 
producing a velocity signal representative of the velocity of the shaft; a 
wobbler position sensor, coupled to the wobbler, for producing a wobbler 
position signal indicative of the position of the wobbler; and a 
controller for producing the servo valve control signal including a first 
summer which produces a first summer output signal that is a function of a 
difference between a commanded velocity signal and the velocity signal, a 
second summer which produces a second summer output signal that is a 
function of a difference between the first summer output signal and a 
signal proportional to at least one of the velocity signal indicative of 
the velocity of the shaft and the wobbler position signal and a third 
summer which produces a third summer output signal which is a function of 
a difference between the second summer output signal and the wobbler 
position signal with the servo valve control signal being a function of 
the third summer output signal. The second summer output signal may be a 
function of a difference between the first summer output signal and a 
signal proportional to the signal indicative of the velocity of the shaft, 
a signal proportional to the wobbler position signal or a sum of the 
velocity signal indicative of the velocity of the shaft and the wobbler 
position signal and the third summer output signal is proportional to the 
servo valve control signal. The invention further includes a limiter, 
coupled to the output of the second summer and to the third summer, for 
limiting the second summer output signal to produce a limiting function of 
the servo valve control signal. 
A control for a variable displacement hydraulic motor contained in a servo 
system for positioning a load driven by the motor at a commanded position 
specified by a position command in accordance with the invention includes 
a position control loop containing a first summer producing a velocity 
command which is a function of a first difference signal equal to a 
difference between the position command and a position of the load sensed 
by a position transducer; a velocity control loop containing a second 
summer producing a second difference signal which is a function of a 
difference between the velocity command and a velocity of the motor sensed 
by a velocity sensor, the second difference signal being applied to an 
integrator and to a proportional amplifier respectively producing outputs 
applied to a third summer to produce a signal which is equal to a 
difference between the sum of the second difference signal amplified by 
the proportional amplifier and integrated by the integrator and a signal 
proportional to at least one of the velocity signal indicative of the 
velocity of the motor and a wobbler position signal representative of a 
position of a wobbler to produce a wobbler position command; and a wobbler 
position control loop containing a valve for controlling displacement of 
the hydraulic motor by varying position of the wobbler in response to a 
valve control signal applied to the valve and fourth summer for producing 
an output signal equal to a difference between the wobbler position 
command and a wobbler position sensed by a wobbler position sensor coupled 
to the valve which is a function of the valve control signal. The third 
summer output signal may be a function of a difference between the sum of 
the second difference signal amplified by the proportional amplifier and 
integrated by the integrator and the sum of the velocity signal 
representative of the velocity of the motor and the wobbler position 
signal representative of the position of the wobbler. The invention 
further includes a limiter coupled to the output of the third summer, for 
limiting the wobbler position command which is applied to the wobbler 
position control loop.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 3 illustrates a control system of a variable displacement hydraulic 
motor in accordance with the present invention in which the load 32 is 
positioned at a commanded position. Like reference numerals identify like 
parts in FIGS. 1-3. The present invention is a modification of the 
velocity control loop of FIG. 1 with the remainder of the position control 
system being identical to FIG. 1. The position control system of FIG. 3 
has the high resolution, stiffness and speed of response necessary for 
controlling actuators in a high performance aircraft to move a load to a 
commanded position. The improvement of the characteristics of the velocity 
control loop provided by the present invention enables the stringent 
requirements of a positioning system for a higher performance actuator to 
be met. 
The variable displacement hydraulic motor control system of FIG. 3 differs 
from the modification of the prior art system of FIG. 1 discussed above in 
that the wobbler position command which specifies a position of the 
wobbler under the control of piston 36 is a function of at least one of 
the velocity signal outputted by velocity sensor 72 which is 
representative of the velocity of the motor shaft and the wobbler position 
signal outputted by position sensor 84 and preferably is a function of the 
sum of the velocity signal representative of the velocity of the shaft and 
the wobbler position signal. Proportional amplifiers 102 and 104 
respectively having gains of K.sub.A1 and K.sub.A2 respectively couple the 
velocity signal representative of the velocity of the shaft of the motor 
12 and the wobbler position signal representative of the position of the 
control piston 36 of the wobbler to a summer 106 which combines the 
signals from the aforementioned sensors with the aforementioned gains. The 
resultant output signal from the summer 106 is applied to summer 108. The 
summer 74 produces a difference signal equal to the difference between a 
velocity command which is a commanded velocity outputted by the servo 
control and the velocity of the motor shaft. The output of the summer 74 
is amplified by a proportional amplifier 76 and is integrated by an 
integrator 110. The output signals from the proportional amplifier 76 and 
the integrator 110 are applied to summer 108. The summer 108 computes the 
difference between the sum of the outputs of the proportional amplifier 76 
and the integrator 110 and the output from the summer 106 as described 
above to generate the wobbler position command. The wobbler position 
command is limited by limiter 78 to limit its dynamic range. It should be 
noted that the functions performed by amplifiers 102 and 104 and summers 
106 and 108 may be integrated into a single summing amplifier with 
different gains. 
The variable displacement hydraulic motor control system of FIG. 3 has the 
advantage over the prior art of providing an increased gain and a stable 
response by simulating the effect of applying a term which is a function 
of motor acceleration as feedback in the velocity loop without the 
disadvantages of amplifying noise, causing amplifier saturation or 
unwanted oscillations. 
FIG. 4 illustrates a block diagram of the transfer function of the system 
of FIG. 3. Like reference numerals identify like parts in FIGS. 1-4. The 
transfer function is a third order function of the operator s, like that 
of the system of FIG. 2, with the proportional amplification terms 
introduced by the proportional amplifiers 102 and 104 with the gains 
respectively of K.sub.A1 and K.sub.A2 introducing additional control 
terms. 
The transfer function of FIG. 4 is set forth below: 
##EQU2## 
A comparison of the denominators of the transfer function represented by 
equations (1) and (2) reveals that the system of FIG. 3 has the same order 
as the system with the transfer function represented by equation (1) while 
providing two more adjustable parameters which are represented by the 
proportional amplifications K.sub.A1 and K.sub.A2 and provides more 
flexibility in controller design. The equivalence of the transfer 
functions represented by equations (1) and (2) may be demonstrated by 
setting the terms K.sub.A1 and K.sub.A2 to zero which reduces equation (4) 
to the same form as equation (3). 
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. For example, while the input signal to 
the summer 108 which is subtracted from the sum of the outputs from the 
proportional amplifier 76 and integrator 110 is preferably equal to the 
sum of the amplifiers 102 or 104, only a single one of the proportional 
terms from amplifiers 102 and 104 may be used. This configuration also 
provides the benefit of an additional proportional amplifier which may be 
used to simulate the benefits of a signal which is a function of motor 
acceleration without the detriments described above. It is intended that 
al such modifications fall within the scope of the appended claims.