Motors: expansible chamber type – Differential
Reexamination Certificate
1999-04-30
2001-08-07
Nguyen, Hoang (Department: 3748)
Motors: expansible chamber type
Differential
C091S433000
Reexamination Certificate
active
06269733
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to servo valves for operating hydraulic actuators, and, more specifically, to using asymmetric nonlinear hydraulic force (pressure) feedback in a servo system that controls a bidirectional hydraulic actuator to compensate for force asymmetries related to the actuator, such as unequal force gain on oppositely directed strokes, unequal loading forces in oppositely directed strokes, or the driving of a nonlinear load.
2. Background
Servo mechanisms are used to move aircraft control surfaces, to position machine tools, to move robotic manipulators, to simulate earthquakes, to test noise, vibration, and harshness characteristics of vehicles, as energy sources for geophysical exploration, to grind eyeglass lenses, to move flight simulator cabins, to provide tactile feedback to joy sticks, to control pavement breaking machines, and for many other applications. Precision servo hydraulic actuators are typically controlled by servovalves. Non-precision actuators are typically controlled by similar but less expensive “proportional” valves. The following discussion will specifically address servovalves, but may apply equally well to proportional valves. Hereinafter, the term “servovalve” should be taken to include the so-called proportional valves.
In a typical form, a servovalve includes a spool valve which receives a flow of fluid from a source through one or more pressurized inlet ports, and whose position along its axis is controlled by variable volumes of fluid in two differential control chambers. These control chambers receive pressurized fluid from a prior stage of the servovalve. A difference in hydraulic force between the two control chambers tends to accelerate the spool toward the chamber with lower hydraulic force. Two differential outlet ports are provided in a four-way valve for delivering pressurized fluid to either one of the opposite chambers of a linear actuator that includes a bidirectional piston mounted in a dual-chamber cylinder. The valve further includes at least one fluid return port that communicates with a fluid reservoir.
A conventional spool valve includes a spool having spaced-apart lands which is mounted for linear motion in opposite directions within a bore in a valve body. With the spool shifted to a first position, one or more pressurized fluid inlet ports are placed into hydraulic communication through a first outlet port with a first chamber of the actuator. Simultaneously, one or more low pressure fluid return ports are placed into hydraulic communication through a second outlet port with the opposite chamber of the actuator, thereby tending to move the actuator piston in one direction to change the position of a load which may be mechanically coupled to the actuator. Commonly, the instantaneous displacements of both the spool and the load are monitored by a Linear Variable Differential Transformer (LVDT) of any well-known type, or a Hall-effect position transducer, or a magnetostrictive transducer.
The actuator piston is urged to move in an opposite direction by shifting the servovalve spool in the opposite direction whereby one or more pressurized fluid inlet ports are placed into hydraulic communication through the second outlet port with the second chamber of the actuator. Simultaneously, one or more low pressure fluid return ports are placed into hydraulic communication through the first outlet port with the first chamber of the actuator, tending to move the actuator piston and connected load in the opposite direction.
The control law used in conventional flow control servovalves is to ideally make hydraulic fluid flow rate proportional to an input signal. The polarity of the input signal determines the direction in which the servovalve spool will move, while the magnitude of the input signal determines the velocity and displacement of spool movement. The magnitude of spool displacement from the center “null” position determines the magnitude of hydraulic fluid flow through the outlet ports. In the ideal case, which ignores flow restrictions and loading effects, flow from the outlet ports is proportional to the input signal.
The servo system may include a single stage wherein the spool valve is mechanically or electrically directly shifted from one position to another to control operation of an actuator. Alternatively, the servo system may employ two stages in which the first stage is a torque motor which in turn controls the positioning of the valve spool of a second stage, which directly controls the actuator. To produce or control large dynamic loads, a third stage of amplification may be added as is well known. Thus in a multistage servovalve, the output stage is driven directly by one or more previous stages.
In hydraulic servo systems, the velocity of the load is a function of the fluid flow rate. By Newton's second law, the actuator applies force to the load and the load applies equal and opposite force to the actuator. The load force may be directly calculated by multiplying the hydraulic pressure on each side of the actuator piston by the piston area exposed to that pressure force, and taking the difference of the two products, i.e., Actuator Force=Load Force=[(Pressure1×Area1)−(Pressure2×Area2)]. For a typical control problem this equation may be simplified to: Hydraulic Force is proportional to [(Pressure1−Pressure2) ×Area Ratio]. This is the differential pressure times the piston area ratio. When the piston area exposed to hydraulic pressure is the same on both sides, the Area Ratio is 1. Otherwise, the potential actuator force is asymmetric, resulting in unequal force gain on oppositely directed strokes.
Since the hydraulic pressure on each side of an actuator piston is in direct fluid communication with its respective output port of a servovalve, actuator pressure may be measured either at the actuator or at the servovalve. The measurement site may be selected for convenience.
In some applications, whether the actuator piston Area Ratio is 1 or another value, force asymmetry might be caused by an asymmetric loading of the actuator. For example, an actuator which is oriented to move in a vertical axis and which supports a heavy load has load asymmetry caused by the force of gravity on the load. The gravitational force on the load increases pressure on the bottom side of the actuator piston in the quiescent state when the load is supported by hydraulic fluid. The quiescent state may be envisioned as a hydraulic lift which supports a load. An illustration of this situation is a machine which supports and shakes an automobile for noise, vibration and harshness (NVH) testing.
In other applications, force asymmetry may occur due to the actuator driving a nonlinear load, such as a linear to rotary translation stage where the mechanical advantage changes with actuator extension, or where the load is a nonlinearly compressible material, such as limestone.
The above described asymmetries tend to cause undesirable nonlinear distortion in an actuator's dynamic output. They tend to cause significant even-order harmonic distortion, and also odd order harmonic distortion. There is thus a need for a servo control system capable of compensating for force asymmetries related to a force servo actuator. The present invention fills this need through a servovalve with asymmetric nonlinear differential hydraulic force feedback.
3. Discussion of Related Art:
A number of servo systems have been suggested wherein negative pressure feedback is returned from the output of the power stage of a servovalve to an earlier stage.
P.F. Hayner, in U.S. Pat. No. 3,260,273, issued Jul. 12, 1966, entitled Motor Valve having Differential Pressure Feedback, teaches a pressure control hydraulic servovalve wherein a pilot valve positions a control valve member to control the application of fluid under pressure through an outlet in the control valve to provide an output differential pressure across an output actuator
Fellers Snider Blankenship Bailey & Tippens, P.C.
Nguyen Hoang
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