Abstract:
A method and apparatus for controlling a valve/actuator plant by generating a plurality of independent correcting control signals each responsive to a plurality of respective input signals and at least one of these input signals being the set point signal. The plurality of independent correcting control signals includes a pneumatic flow signal. The correcting control signals may be derived from all linear control signals, all non-linear control signals, or a combination of linear control signals and non-linear control signals.

Description:
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&#39;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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures and in which: 
     FIG. 1 is a block diagram illustrating a valve controller with multiple-input-multiple-output control. 
     FIG. 2 is a block diagram illustrating the preferred embodiment of the present invention. 
     FIG. 3 is a block diagram illustrating an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a plurality of control inputs  10 , which include, but are not limited to, electronic control inputs, coupled to a valve controller  13  with a plurality of independent correcting control signals  15  to control a valve/actuator plant  17 . The plurality of independent correcting control signals  15 , which may be a pneumatic signal, is derived from either all linear control algorithms, all non-linear control algorithms, or a combination of linear and non-linear control algorithms. For example, correcting control signals  15 / 1  through  15 /n may be derived from either all linear control algorithms or all non-linear control algorithms. Moreover, correcting control signal  15 / 1  may be derived from a linear control algorithm and correcting control signals  15 / 2  through  15 /n may be derived from non-linear control algorithms, or correcting control signals  15 / 1  through  15 /n may be derived from any other combination of linear and non-linear control algorithms. 
     The plurality of control inputs  10  generate a plurality of correcting control signals  15  that are coupled to the valve/actuator plant  17 . The plurality of correcting control signals  15  are 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. For example, if correcting control signal  15 / 1  failed to operate, correcting control signals  15 / 2  through  15 /n should continue to function and control the valve/actuator plant  17 . The valve controller  13  may be, but is not limited to, an electropneumatic controller. 
     As mentioned above, the output of the valve controller  13  may include multiple independent correcting control signals  15  derived from all linear control algorithms, all non-linear control algorithms or a combination of linear and non-linear control algorithms. In one embodiment, as shown in FIG. 2, the valve controller  13  generates two independent correcting control signals including one derived from a linear control generator  20  and another from a non-linear control generator  23 . The linear control generator  20 , which is located in the valve controller  13 , includes a PD type controller  26  coupled to a signal amplifier  29 , such as a current to pressure (I/P) transducer, and a pneumatic amplifier  30 , such as a pressure relay. As described above, a P, PD, or PDI type control generates a linear correcting control signal from the plurality of control inputs  10 . The linear control generator  20  may be implemented by, but not limited to, the configuration used in a Type DVC5000 Digital Valve Controller, manufactured by Fisher Controls International, Inc. 
     The non-linear control generator  23  includes an “on-off-on” control  33  coupled to a signal amplifier  36  and solenoids  39 . The non-linear control generator  23  is implemented by, but not limited to, pulsing two pneumatic solenoids in an “on-off-on” type control  33 , which has a duty cycle proportional to an error signal. The signal amplifier  36  includes a current-to-pressure (I-/P) transducer that provides a pressure output in response to a current input. The solenoids  39  provide a pneumatic flow in response to the:pressure output of the signal amplifier  36 . One solenoid provides a pneumatic supply flow and the second solenoid provides an exhaust flow capability. 
     During a transition from one set point to another, the non-linear control generator  23  attempts to control a pressure rate  42 , which builds or decreases in an actuator  45  located in the valve/actuator plant  17 . The pressure rate  42 , used for the non-linear control generator  23 , may be either fixed or user-defined. As mentioned above, the non-linear control technique compensates for non-linearities such as, but not limited to, friction, dead band, and hysterisis, that are inherent in control valve/actuator plants. The non-linear control generator  23  may be implemented by a modified version of the Type DVC5000 Digital Valve Controller referred to above. 
     The correcting control signals  15  from the linear control generator  20  and the non-linear control generator  23  are pneumatically summed and integrated to pressure by the load volume of the actuator  45 . The pressure rate  42  from the actuator  45  is fed back to the non-linear control algorithm  23  on line  43 . An error signal  57  is fed back on line  47  from the valve/actuator plant  17  that adjusts both the linear and non-linear control generators  20 ,  23 , respectively. 
     In FIG. 2, the two correcting control signals, which are generated by the linear control generator  20  and the non-linear control generator  23 , are pneumatically summed and integrated to pressure which, in turn, provides the driving force for the valve/actuator plant  17 . The valve/actuator plant  17  includes the actuator  45  coupled to a valve  51 . A desired set point signal  54  enters the valve controller  13  to operate the control generators  20 ,  23 . In the linear control generator  20 , the PD controller  26  provides an electrical drive signal, which may be, but is not limited to, a current signal. The signal amplifier  29  includes a current-to-pressure (I/P) transducer that provides a pressure output in response to a current input from the PD controller  26 . The pneumatic amplifier  30  includes a pressure relay that provides a pneumatic flow to the actuator  45  in response to the pressure output from the signal amplifier  29 . 
     The non-linear control generator  23  is active only when the feedback output signal, which may be the error signal  57  from the actuator  45 , is outside a predetermined percent of a set point signal  54 . If the output signal is within the predetermined percentage of the set point signal  54 , then the non-linear control generator  23  shuts off and the linear control generator  20  continues to bring the output signal of the actuator  45  to within the desired set point signal  54 . 
     FIG. 3 illustrates an alternate embodiment of the present invention, which includes a plurality of independent correcting control signals that may be a combination of linear correcting control signals and non-linear correcting control signals. For example, the valve controller  13  includes, but is not limited to, a combination of linear control generators  20  with correcting control output signals  15 / 1  through  15 /n and non-linear correcting control generators  23  with output signals  15 / 1 ′ through  15 /n′. The correcting control signals consist of, but are not limited to, pneumatic flows and are summed and integrated to pressure by the valve/actuator plant  17 . 
     The foregoing detailed description has been given for clearance of understanding only, and no unnecessary limitations would be understood therefrom, as modifications will be obvious to those skilled in the art.