Variable displacement hydraulic control with override

In the preferred form, the invention disclosed herein provides a single servo control valve to modulate the flow of control fluid to a servo mechanism of a variable displacement pump wherein the single valve is provided with both a manual primary input and an electro-hydraulic secondary input which modulates the primary input. The servo valve is spring biased toward a neutral position with the manual input displacement command being applied to such servo valve through a resilient linkage. Also applied to the servo valve is a hydraulic signal which is electrically controlled to modify the displacement of the servo valve relative to the yieldably applied manual input.

TECHNICAL FIELD 
This invention relates to a hydraulic servo control mechanism for a 
variable displacement hydraulic pump wherein a first manual input provides 
the primary setting of the servo control mechanism and a secondary 
electro-hydraulic input is utilized to modulate the manual input setting 
of the servo control mechanism. 
BACKGROUND ART 
It is well known in the art to utilize a hydraulic servo control mechanism 
to control the displacement of a variable displacement pump to modulate 
the drive of a hydraulic load. The hydraulic servo control mechanism 
usually comprises a manually controlled servo valve which modulates the 
flow of control fluid to a servo motor which is connected to the variable 
displacement pump in a manner to vary the displacement thereof. As per 
Hann U.S. Pat. No. 3,212,263 issued on Oct. 19, 1965, it is also known to 
utilize a resilient link to yieldably apply a manual input to the servo 
valve in a manner which prevents an excessive manual force from being 
applied to the servo valve. The resiliently applied manual input 
cooperates with a pump swash plate feedback linkage which senses the 
instantaneous displacement of the variable displacement pump to control 
flow through the servo valve to maintain the swash plate in a position 
proportional to the input command. The advantages of such system are fully 
described and disclosed in the aforesaid Hann patent. 
It is further known to utilize an electrical control signal to modulate the 
flow of control fluid to a servo motor which controls the displacement of 
the pump. As per Moon U.S. Pat. No. 3,365,886 issued Jan. 30, 1968, an 
electric override control signal is provided by a centrifugal governor on 
the output shaft of a variable displacement hydraulic transmission. The 
speed signal controls a solenoid valve which modulates the flow of control 
fluid. Such electrically controlled modulation of the control fluid is 
upstream and separate from the manually controlled displacement control 
valve. 
It is furthermore known to use electrically controlled pilot valves to 
modulate the flow of control fluid from a charge pump in a manner to 
axially position a servo valve which controls flow to a swash plate servo 
motor. Such a system is taught in Knapp et al. U.S. Pat. No. 3,901,031 
issued Aug. 26, 1975. This reference teaches electrically sensing the 
swash plate angle and utilizing an electronic circuit to provide a control 
signal proportional to such swash plate angle when compared to a reference 
electrical signal representative of a desired swash plate angle. Solenoid 
valves are modulated by such control signal to hydraulically position the 
servo valve. There is no manual input to the servo valve and thus there is 
no force balancing between the electro-hydraulic input and a manual 
primary command signal. 
DISCLOSURE OF THE INVENTION 
The primary feature of the invention disclosed herein is to provide a 
single relatively inexpensive servo control valve to modulate the flow of 
control fluid to a servo mechanism of a variable displacement pump wherein 
the single valve is provided with both a manual primary input and a 
secondary input which modulates the primary input. Preferably, the servo 
valve is spring biased toward a neutral position with a manual input 
displacement command being provided to such servo valve through a 
resilient linkage. Also applied to the servo valve is a hydraulic signal 
to modify the displacement of the servo valve relative to the manual 
input. 
It is thus an object of the present invention to provide a single servo 
valve to modify the displacement of a variable displacement pump in 
response to both a primary manual input and a secondary electro-hydraulic 
input which modulates the valve position relative to the manual input. 
The present invention, in one preferred form, senses a system parameter of 
a hydraulic transmission driven by the variable displacement pump to 
generate a control signal which in turn modulates the displacement of the 
variable displacement pump relative to an operator primary command. 
It is an object of another embodiment of the invention to provide a remote 
control station which provides an electro- hydraulic signal which is 
applied against a servo valve previously positioned by an operator primary 
command. 
Another object of the present invention is to obtain a proportional 
electro-hydraulic control of swash plate angle by balancing the forces 
applied to the swash plate servo valve including a centering spring, a 
primary manual input, swash plate feedback and the secondary 
electro-hydraulic input. 
Thus a primary object of the present invention is to provide an override 
control for a hydraulic system comprising a variable displacement 
hydraulic pump driving a motor, a hydraulic servo mechanism comprising a 
servo motor coupled to the pump to vary the displacement thereof and a 
servo valve having a movable valve element for controlling the flow of 
control fluid to the servo motor, a first manual input resiliently 
connected to the valve element to establish a primary control position for 
said valve element, an expandable fluid chamber device operatively 
connected to the valve element so as to move the valve element against the 
bias of the manual input when the expandable chamber is subjected to fluid 
flow, a pilot valve modifying the flow of control fluid to said expandable 
chamber, and a second input means applying a control signal to the pilot 
valve for modulating the control fluid bias on the servo valve to modify 
the resiliently applied manual input. 
Yet another object of the present invention is to provide an electric 
override control for a hydraulic system comprising a variable displacement 
hydraulic pump including a swash plate movable to modify the output flow 
of the pump, a motor hydraulically connected to the pump to be driven 
thereby, a hydraulic servo mechanism comprising a servo motor for 
positioning the swash plate and a servo valve having a movable valve 
element for controlling the flow of control fluid to the servo motor, a 
feedback linkage operatively connecting the swash plate and the movable 
valve element, a moveable manual control providing a primary input signal, 
a resilient linkage interconnecting the manual control and the feedback 
linkage, an expandable fluid chamber device operatively connected to the 
valve element and adapted to move the valve element when subjected to 
fluid flow, a pilot line connected to the expandable chamber, an 
electrically operated pilot valve in said pilot line adapted to modulate 
the flow of control fluid to the expandable chamber, and secondary input 
means providing an electric control signal to be applied to the pilot 
valve to permit the flow of control fluid to the expandable chamber in a 
manner to modulate the position of the movable valve element relative to a 
position established by the feedback linkage and the primary manual input.

BEST MODE FOR CARRYING OUT THE INVENTION 
While the present invention is susceptible of embodiment in many different 
forms, there is shown in the drawings and will herein be described in 
detail various embodiments of the invention with the understanding that 
the present disclosure is to be considered an exemplification of the 
principals of the invention and not intended to limit the invention to the 
embodiments illustrated. The scope of the invention will be pointed out in 
the appended claims. 
Referring to FIG. 1, a hydrostatic transmission 10 includes a reversible 
variable displacement axial piston pump 11 and a fixed displacement axial 
piston motor 12. The transmission is adapted to be driven by a prime mover 
or engine (not shown) through an input shaft 13 connected to the pump 11. 
The pump 11 is provided with an angularly positioned swash plate 14 which 
modifies both the amount of flow and the direction of flow of the pump 
output in a manner well known. The pump 11 is connected to the motor 12 by 
main loop fluid lines 15 and 16 in a manner to drive the motor 12. While 
the embodiment shown is a hydrostatic vehicle transmission, the motor 12 
may also be a cylinder motor such as a hydraulic ram. It is common in the 
transmissions of the type shown, that lines 15 and 16 are provided with 
high pressure relief valves 17 and a shuttle valve 18 connected to a 
pressure relief valve 19. 
Furthermore, as is common practice, the transmission is provided with a 
charge pump 20 which is driven by the input shaft 13. The output of the 
charge pump 20 is controlled by a charge pressure relief valve 22 and 
flows through check valves 24 and 26 to either line 15 or 16, depending 
upon which is at low pressure, to replenish lost hydraulic fluids to the 
transmission main loop. The output of the charge pump 20 is also directed 
to a control fluid line 28. 
The variable displacement pump 11 is provided with a servo mechanism which 
consists of a servo valve 30 and a servo motor consisting of dual servo 
cylinder devices 32 and 34 connected to the swash plate 14. The servo 
valve 30 controls both the direction and amount of flow from control fluid 
line 28 to the servo motor cylinders to vary the angular displacement of 
the pump swash plate 14. The servo valve 30 also controls the fluid 
communication between the servo motor cylinders and a drain line 29. 
Although two servo cylinders are taught, it is also known in the art to 
have a servo motor consisting of a single dual acting servo cylinder. 
The servo valve 30 has a stationary valve body 36 which defines a valve 
bore 38. Located within the valve bore 38 is a movable servo valve element 
40 which may be axially displaced with respect to the valve bore 38 to 
control flow through the servo valve 30. Servo motor lines 42 and 44 
connect swash plate servo cylinders 32 and 34 respectively with the valve 
bore 38. Furthermore the control fluid line 28 and drain line 29 are also 
in communication with the valve bore 38. The servo valve element 40 is 
provided with two axially spaced apart lands 41 and 41' which prevent flow 
from control fluid line 28 and servo motor lines 42 and 44 when the valve 
element 40 is centered. When the valve element 40 is moved toward the 
right, land 41' uncovers the port where servo motor line 44 communicates 
with the valve bore 38 so that control fluid may pass from the centrally 
located control fluid line 28 to servo motor line 44 and thus pressurize 
the servo cylinder 34. At the same time, land 41 uncovers the port 
communicating servo motor line 42 to permit flow from servo cylinder 32 to 
drain line 29. Movement of the valve element 40 toward the left directs 
flow in the opposite direction to connect servo motor line 42 and servo 
cylinder 32 to control fluid line 28 while draining servo cylinder 34. To 
permit selective flow to drain from both servo cylinders 32 and 34, drain 
line 29 is ported to the valve bore 38 toward both the right and left ends 
thereof and outboard of the valve element lands 41 and 41'. It is the 
progressive axial positioning of the movable valve element 40 that 
modulatingly controls the fluid communication between the swash plate 
servo cylinders 32 and 34 and the fluid control line 28 or drain line 29 
in a manner well known. 
The servo valve 30 is biased toward a centered position preventing the flow 
from the control fluid line 28 to either of the servo cylinders by a 
centering spring mechanism 46 shown schematically in FIG. 1 and in greater 
detail in FIG. 2. The spring mechanism 46 consists of a single coil spring 
48 located between two washers 50 and 52, all located about a reduced 
diameter stem portion 54 of the movable valve element 40. The washer 50 
abuts the stepped-down portion between the valve stem 54 and the main 
portion of the valve element 40 while the washer 52 abuts a lock ring 56 
secured to the free end of the valve stem 54. 
An adjustment sleeve 58 with lock nut 59 is threadably located with respect 
to the valve body 36 and includes an internal flange 60. An adjustable 
plug 62 with threaded lock nut 63 is threadably located within the sleeve 
58 and has a peripherally located end wall 64. The threaded sleeve 58 and 
the threaded plug 62 furthermore provide a factory preset to adjustably 
locate the movable valve element 40 in a centered position and take up 
spring backlash. The internal flange 60 and the end wall 64 furthermore 
provide abutment stops for the spring washers 50 and 52 to permit 
compression of the centering spring 48 when the valve element 40 is moved 
toward the right and toward the left respectively. Thus when the valve 
element 40 is moved to the right, the lock ring 56 and washer 52 compress 
the coil spring 48 against flange 60. When the valve element 40 is moved 
toward the left, the stepped-down portion between the valve stem 54 and 
the main portion of the valve element 40 abut the washer 50 to compress 
the coil spring 48 against end wall 64. 
To establish a primary control position for the movable valve element 40, a 
manual input means is provided consisting of a handle 66 pivotably mounted 
by a pin 68 and connected to a resilient linkage means 70. Resilient link 
70 functions similarly to the centering spring mechanism 46 in that it 
yieldably applies a force in two directions but utilizes only a single 
spring means. The spring element for the resilient link 70 consists of a 
coiled torsion spring 72 with the coiled portion thereof surrounding a 
sleeve 74 so as to be angularly movable about pin 68 (as seen in FIG. 2). 
The coiled torsion spring has legs 76 and 78 which form a bifurcated 
connection with pins 80 and 82. Movement of the pin 80 is controlled by 
the manual control handle 66. In the preferred structure taught in FIGS. 2 
and 3, handle 66 and a cam 96 are secured to the pin 68 for common angular 
displacement. The pin 80 is connected to the cam 96. Movement of the cam 
96 imparts movement to pin 80 and thus to pin 82 through the bifurcated 
torsion spring 72. To provide better clarity, the relative position of 
pins 80 and 82 is reversed in FIG. 1 to that shown in FIG. 3. A lever 84 
is pivotably mounted on pin 68 and is eccentrically connected to a link 86 
by the pin 82. Thus, any angular displacement of pin 82 imparted by the 
bifurcated torsion spring 72 causes movement of link 86. Therefore, the 
movement of the handle 66 is imparted to link 86 in a manner that would 
prevent the transfer of excessive manual force. 
Swash plate 14 is provided with a feedback linkage 88 consisting of a link 
90 connected to the swash plate 14 and a link 92 connected to the servo 
valve element 40 by means of a pin 94. The manual input is yieldably 
applied to the feedback link 92 by resilient linkage 70 and link 86. The 
manual input through the yieldable link 70 and the swash plate feedback 
linkage 88 provide a mechanical primary input to the servo valve movable 
element 40 which axially positions such valve element 40 against the bias 
of the dual acting centering spring mechanism 46. When a manual input is 
applied to the handle 66, this imparts an axial motion to the valve 
element 40 which initiates fluid flow from the control fluid line 28 to 
one or the other of the servo cylinder lines 42 or 44 and thus to the 
servo cylinder 32 or 34 to cause angular movement of the swash plate 14. 
Such angular movement imparts a corresponding movement to the feedback 
linkage 88 which further axially positions the movable valve element 40 
relative to the manual input to maintain sufficient flow to the servo 
cylinders 32 or 34 to maintain the swash plate 14 in an angular position 
corresponding to the manual input. This displacement balancing between the 
resiliently applied manual input and the swash plate feedback to position 
a servo valve element is taught in Hann U.S. Pat. No. 3,212,263 as 
previously mentioned in the Background Art. 
As seen in FIG. 2, the cam 96 having notches 98 is secured to handle pivot 
pin 68 by means of a pin 100 so that rotational movement of the handle 66 
is also imparted to the cam 96. Cooperating with the cam 96 is a detent 
mechanism 102 biased by spring 104. The detent mechanism 102 engages the 
cam notches 98 to maintain the handle 66 in a preselected position. While 
the cam notches as shown in FIG. 1 provide a neutral position, one forward 
position and one reverse position, a plurality of angular positions may be 
selected by providing a plurality of notches 98. Furthermore it is 
contemplated that a friction mechanism could also be utilized instead of 
the spring detent mechanism to maintain the handle in a preselected 
position. 
The control described so far provides constant displacement of the axial 
piston pump 11 relative to a manual input. It is sometimes desirous to 
further modify the displacement of the pump to modify pump output flow. 
One such example is when pump 11 and motor 12 form a hydraulic 
transmission for propelling a vehicle. On level terrain and with constant 
load, a given position of handle 66 will maintain constant vehicle speed. 
However, when the vehicle encounters an incline or other increase in 
vehicle load, the speed of the transmission will be reduced. To compensate 
this reduction in speed, the motor output shaft 106 is provided with a 
tachometer consisting of a magnetic wheel 108 and an electronic pick-up 
110. Many suitable types of tachometers are well known. One type 
contemplated will provide an electrical output proportional to the 
rotational speed of shaft 106 in the form of a sine wave represented by 
wave form 112. This sine wave is fed to a frequency to voltage converter 
114 and then to a summing device 116 which compares an actual speed signal 
generated by the tachometer with a reference speed signal represented by 
reference voltage generator 118. The output from the summer 116 is then 
supplied to a control filter circuitry 120 and voltage to duty-cycle 
converter 122 to generate an electrical control signal represented by 
square wave 124 as is well known in the electronic control circuit art. 
The control signal 124 is used as a secondary input to modify the primary 
or mechanical input to servo valve 30 described above. In order to apply 
the secondary control signal without requiring a separate complicated 
control device, a servo valve pilot line 126 connects the control fluid 
line 28 with an expandable chamber device 128 adapted to act on the servo 
valve movable element 40 in a manner to cause axial movement thereof 
against the bias of the centering spring mechanism 46 and any mechanical 
input to the valve element 40. As seen in FIG. 2, both the control fluid 
line 28 and pilot line 126 join the servo valve bore 38 at a central 
location so they are always in fluid communication. Located in the pilot 
line 126 is an electrically controlled pilot valve 130. 
The pilot valve 130 in one form of the invention is a two position valve 
operated by a solenoid 132. The solenoid 132 is connected to the electric 
control circuitry by an electrical connector 133. In this form, the pilot 
valve 130 either permits full flow or prohibits flow in the pilot line 126 
to the expandable chamber device 128. The pilot valve 130 includes a pin 
134 which seats in the bore 136 of valve seat 138 due to the influence of 
the solenoid spring (not shown) when no current is provided to the 
solenoid 132. When current is provided, the pin 134 is lifted from the 
bore 136 to permit flow from pilot line 126 to the expandable chamber 
device 128 through a line 138. The frequency and/or the pulse width 
duration of the square wave 124 (depending on the type control chosen) 
thus controls the amount of flow through the pilot line 126 in proportion 
to the speed output signal generated by the tachometer pick-up 110. The 
electronic control circuit may be selectively actuated by a manual control 
switch 125. 
The movable valve element 40 is provided with a radial bore 140 which 
communicates with an axial bore 142. Bores 140 and 142 communicate with 
line 138 leading from the pilot valve 130 to permit flow of control fluid 
from the pilot line 126 to the expandable chamber device 128. The 
expandable chamber device 128 consists of a chamber 144 defined by plug 
62, the inside of the adjustable sleeve 58 and the end of the valve 
element 40. When control fluid is allowed to pass from control fluid line 
28 and pilot line 126 by the opening of the valve 130, pressure is 
generated in the fluid chamber 144 which biases the movable valve element 
40 toward the right against any forces applied by the centering spring 
mechanism 46 and mechanical forces applied by the swash plate feedback 
mechanism and manual input previously described. 
The valve element 40 is also provided with a second radial bore in the form 
of a restricted orifice 146. The restricted orifice 146 connects chamber 
144 by means of axial bore 142 to valve bore 38 which is in communication 
with drain line 29. The restricted orifice 146 is of such size to permit 
flow from the chamber 144 to drain when there is no flow from pilot line 
126 and thus remove fluid pressure from the chamber 144 in a manner to 
permit the movable valve element 40 to move toward the left in accordance 
with other forces applied thereto. The restricted orifice 146 however is 
of such reduced size as to allow pressure build up in chamber 144 from 
flow in pilot line 126 when the valve 130 is opened. 
The electrical control signal as represented by the square wave 124 causes 
a rapid pulsing of the solenoid 132 and thus a rapid opening and closing 
of the valve 130 in a manner which modulates flow through a pilot line 126 
proportional to the speed signal generated by the tachometer pick-up 110. 
In another embodiment of the preferred form of the invention as shown in 
partial sectional view FIG. 2A, the solenoid control valve 130 is replaced 
by an electronically controlled electric force motor control valve 130', 
of the type commercially supplied by the Fema Corporation. In this 
embodiment, the valve stem 134' is axially positioned by the force motor 
132' proportional to the voltage applied to the force motor 132'. The 
axial distance between the valve stem 134' and the bore 136' in the valve 
seat 138' controls the amount of flow through the bore 136'. This is 
utilized to modulate the flow through the pilot line 126 in a manner 
similar to the control of the solenoid control valve 130. With the 
solenoid control valve, a pulsating electrical control signal is provided 
as represented by the square wave 124. With the electric force motor 
control valve 130', a steady voltage output is generated by the voltage to 
duty-cycle converter 122 proportional to actual speed signal generated by 
the tachometer pick-up 110. This steady signal then modulates the axial 
positioning of the valve stem 132' to modulate the flow of control fluid 
through the bore 136'. 
Another use for the electric override control of the present invention is 
shown in an embodiment taught in partial schematic view FIG. 1A. This 
embodiment, rather than using an electric signal generated by a system 
parameter such as transmission output speed, uses an electric remote 
control station to generate the electric control signal. In a product 
application such as a cement mixer having a drum driven by the hydrostatic 
transmission, a vehicle operator provides a manual input to determine a 
preset drum speed in a manner as discussed above. At a remote station, a 
manually controlled electric rheostat 150 is provided which when used in 
conjunction with a voltage source 152 provides an input voltage signal to 
duty-cycle converter 122. The converter 122 converts the voltage signal 
into an electrical control signal such as the square wave 124 to control 
the solenoid 132 which modulates the pilot valve 130 in a manner similar 
to the speed control of FIG. 1. If it is chosen with this embodiment to 
use an electric force motor 132' such as taught in FIG. 2a, the output of 
the voltage to duty-cycle converter 122 would be a steady voltage signal 
proportional to the voltage input from the manually controlled rheostat 
150. In this manner, the solenoid or electric force motor may be utilized 
to modulate the hydraulic bias on the servo valve 40 proportional to a 
voltage signal generated at a manually controlled remote station and thus 
provide the secondary input or electric override. 
The manual input and swash plate feedback mechanism described above, 
provides a displacement balancing system for controlling the position of 
the servo valve element 40. The override control system of the present 
invention, by providing a force which acts directly on the servo valve 
element 40, provides a force balancing system when used in conjunction 
with a yieldably applied manual input. The override control, by modulating 
the flow through pilot valve 130, generates a pressure within the chamber 
144 of the expandable chamber device 128. This pressure provides a force 
on the left end of the movable valve element 40 which biases the valve 
element 40 to the right against any previously supplied manual primary 
input. Yieldable linkage 70 permits the rightward movement of the valve 
element 40 due to the pressure generated in chamber 144 even though the 
manual input is fixed. Furthermore, the yieldable linkage 70, due to the 
spring 72, provides a reactive force which counterbalances the force 
generated by the expandable chamber device 128. 
In one example of operation wherein the transmission 10 is used to provide 
the propulsion for a vehicle, the vehicle operator moves the handle 66 
toward the right so as to provide a manual primary input into the control. 
This clockwise movement of the handle 66 imparts right-hand movement to 
the link 86 through clockwise movement of pin 80, the bifurcated resilient 
spring 72 and pin 82. This imparts right-hand movement to the valve 
element 40 to permit control fluid to flow from line 28 through the servo 
valve 30 to servo motor line 44 and servo cylinder 34 to cause clockwise 
rotation of the swash plate 14. This angular movement of the swash plate 
14 increases the displacement of the pump 11 to generate forward vehicle 
propulsion. The clockwise movement of the swash plate 14 also imparts 
counterclockwise movement to the swash plate feedback linkage 88. This 
pivots link 92 counterclockwise around its connection to link 86 (held 
stationary by the detented manual input) to move pin 94 and thus servo 
valve element 40 toward the left to counter the previous right-hand 
movement of the servo valve element 40. When the swash plate 14 has 
reached an angular displacement proportional to the manual input, the 
servo valve element 40 will again be centered due to the cooperation 
between the manual input and the motion of the feedback linkage 88 to 
prevent further flow to the servo cylinder 34. This displacement balance 
continues to modulate flow through the servo valve 30 to maintain the 
angular position of the swash plate 14 relative to manual input until an 
outside force is further applied to the servo valve element 40. 
The override control of the present invention provides a further or 
secondary input to the servo valve 30. Assuming the modification taught in 
FIG. 1 is chosen, as vehicle speed slows due to an incline or other load, 
the output speed signal 112 generated by the tachometer will be reduced. 
This is compared to reference voltage 118 which is proportional to desired 
vehicle speed. The control circuitry provides a control signal to the 
solenoid valve 132 which opens pilot valve 130 to cause flow from pilot 
line 126 to the expandable chamber device 128. This flow generates a 
right-hand force on the valve element 40 biasing it toward the right 
against the resiliently applied manual input from the handle 66, resilient 
link 70 and the feedback linkage 88. The right- hand movement of servo 
valve element 40 generated by the secondary input causes further flow of 
control fluid from line 28 to servo motor line 44 and thus servo cylinder 
34. This increases clockwise rotation to the swash plate 14 to increase 
the displacement of pump 11 and thus increase the forward propulsion drive 
of the vehicle. It is noted that the force generated by the secondary 
input acts against the yieldably applied force of the primary input. Since 
both the primary manual input and the hydraulically applied secondary 
input act on the servo valve element 40, these forces are balanced in a 
nature not permitted by a separate manual control and override control. 
When the vehicle has reached the desired speed determined by the reference 
voltage 118, the input to solenoid 132 reduces the modulated flow through 
pilot valve 130 to a point which balances the flow through the restricted 
orifice 136 to drain line 29 thus stablizing the secondary input to the 
servo valve element 40. If an overspeed condition occurs, the tachometer 
generated control signal reduces the input to the pilot valve 130 which 
reduces flow to the expandable chamber device 128 to a point below that 
flow permitted by the restricted orifice 136. This allows the expandable 
chamber device 128 to drain permitting the servo valve element 40 to move 
toward the left due to the resiliently applied manual input. 
The above described operation works in a similar manner whether the pilot 
valve is a solenoid control valve 130 as depicted in FIGS. 1 and 2 or an 
electric force motor valve 130' as depicted in FIG. 2A. Furthermore the 
same fluid flow and force balancing described above is generated by a 
control signal generated by a remote station 150 as depicted in FIG. 1A 
which may utilize either the solenoid control valve 130 or the electric 
force motor control valve 130'. It is of course contemplated by the 
present invention to use a system parameter to generate a control signal 
which reduces pump displacement or provides reverse operation. It is 
furthermore contemplated by the present invention to have two secondary 
inputs applied to the servo valve element 40 by duplicating the pilot 
control loop 126 and 130 and adding a second expandable chamber device 128 
to the right side of the servo control valve element 40. Thus a positive 
control force can be applied to both sides of the servo valve, one for an 
underspeed condition and one for an overspeed condition, in addition to 
manual input. The balancing of forces supplied by the yieldably applied 
manual primary input and hydraulically applied secondary input to a single 
servo valve provides a particularly effective electric override control 
and thus meets the objectives of the present invention.