Fluid handling – Processes – With control of flow by a condition or characteristic of a...
Reexamination Certificate
2001-01-25
2003-11-11
Buiz, Michael Powell (Department: 3753)
Fluid handling
Processes
With control of flow by a condition or characteristic of a...
C137S487500, C700S043000, C700S067000, C700S282000
Reexamination Certificate
active
06644332
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to valve controllers and more particularly to a method and apparatus for controlling a valve/actuator plant with multiple independent controller outputs to the valve/actuator plant.
BACKGROUND OF THE INVENTION
A variety of control algorithms may be used by valve controllers to control a valve/actuator plant. Typically, valve controllers use an error signal, which is the difference between the desired set point signal and the output feedback signal, to control the valve/actuator plant. The valve controller generates a correcting control output signal, specifically a pneumatic signal proportional to the error signal, through a control algorithm to drive the valve/actuator plant. The control output signal may be the result of either a linear or non-linear control algorithm. This output signal from the valve controller becomes an input signal to the valve/actuator plant and hereinafter will be referred to as a correcting control signal.
A correcting control signal is generated from a Proportional, a Proportional plus Derivative, or a Proportional plus Derivative plus Integral type linear control algorithm. The Proportional (P) type control algorithm generates a correcting control signal directly proportional to the error signal. The Proportional plus Derivative (PD) type control algorithm generates a correcting control signal that is the sum of a signal proportional to the error signal and a signal that is proportional to the rate of change of the error signal. The Proportional plus Derivative plus Integral (PID) type control algorithm generates a correcting control signal that is the sum of a signal proportional to the error signal, a signal that is proportional to the rate of change of the error signal, and a signal that is proportional to the integral of the error signal.
Non-linear control techniques may compensate for the non-linearities, such as friction, dead band, and hysterisis that are inherent in controlling a valve/actuator plant. One example of a non-linear control algorithm would be one that uses a pulse width modulation technique. This algorithm would provide an “on-off-on” correcting control signal which has a duty cycle that has some defined relationship to the error signal. With this “on-off-on” control algorithm there is a dead-band parameter-that defines the values of the error signal for when the correcting control signal is “off” or zero, and when the correcting control signal is “on” or equal to one.
The correcting control signals from both the linear and non-linear control algorithms can be converted to pneumatic correcting control signals that would be used to drive a valve/actuator plant to the desired set point position. This pneumatic correcting control signal consists of, but is not limited to, a pneumatic flow.
All electro-pneumatic valve controllers use either linear or non-linear control algorithms to provide pneumatic correcting control signals to the valve/actuator plant. Typically, a valve controller provides a single pneumatic correcting control signal to the valve/actuator plant. Accordingly, if the algorithm and/or mechanics used to generate the correcting control signal fails to operate, then the valve controller ceases to function and ceases to provide a pneumatic correcting control signal to the valve/actuator plant.
SUMMARY OF THE INVENTION
In accordance with the principles of the present invention, there is provided a method and apparatus for controlling a valve/actuator plant with multiple independent correcting control signals including correcting control signals derived from linear control signals, non-linear control signals or a combination of linear and non-linear control signals. The correcting control signals may be a pneumatic signal. The multiple correcting control signals operate independent of each other so if one correcting control signal fails to operate, the other correcting control signal or signals will continue to function independent of the failed correcting control signal and still be able to drive the valve/actuator plant.
In accordance with the invention, a plurality of independent correcting control signals are generated, each responsive to a plurality of input signals which include a set point signal. The correcting control signals to the valve/actuator plant are summed together by the pneumatic volume of the actuator providing a single controlled output from the valve/actuator in response to the plurality of independent correcting control signals. This single controlled output is the valve's mechanical travel.
The plurality of independent correcting control signals can be generated by only linear control signals, only non-linear control signals or a combination of linear control signals and non-linear control signals.
A significant advantage of the present invention is in being able to incorporate a plurality of independent linear and non-linear pneumatic correcting control signals to a valve/actuator plant and provide a single controlled output from the valve/actuator plant.
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PCT International Search Report issued in PCT/US02/01441, Sep. 9, 2002.
Buiz Michael Powell
Fisher Controls International Inc.
Krishnamurthy Ramesh
Marshall & Gerstein & Borun LLP
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