Process servo control system including error signal limitation to prevent degraded response

Claim 2

A process control apparatus comprising: means for measuring a controlled variable of the process and providing a first signal indicative of the measured value, means for providing a second signal indicative of the set point for said controlled variable, means for comparing said first and second signals to produce an error signal in accordance with the difference therebetween, means for producing a control signal from said error signal so that said control signal has a proportional and reset component, means operable when said control signal exceeds a predetermined maximum value to modify said error signal to which said control signal producing means responds so that said control signal does not exceed said maximum value, and means responsive to said control signal for controlling a manipulated variable of said process so as to tend to bring said measured value of the controlled variable to equality with said set point, said last-named means including an integrating amplifier and a control means for changing the manipulated variable until its value corresponds with the output of said integrating amplifier.

Claim 3

A process control apparatus comprising: means for measuring a controlled variable of the process and providing a first signal indicative of the measured value, means for providing a second signal indicative of the said set point for said controlled variable, means for comparing said first and second signal to produce an error signal in accordance with the difference therebetween, means for producing a control signal from said error signal so that said control signal has a proportional and reset component, means operable when said control signal exceeds a predetermined maximum value to modify said error signal to which said control signal producing means responds so that said control signal does not exceed said maximum value, said last-named means including parallel connected oppositely poled Zener diodes for establishing the maximum values for said control signal and a high gain amplifier for producing a signal to modify said error signal until the control signal is maintained below said maximum value, and means responsive to said control signal for controlling a manipulated variable of said process so as to tend to bring said measured value of the controlled variable to equality with said set point, said last-named means being effective to operate limit switches which selectively short an appropriate one of said Zener diodes when said manipulated variable has been altered to a limit condition, whereby the control signals are reduced to zero for error signals calling for a change in the manipulated variable in the limited direction.

Claim 4

A process control apparatus comprising: means for measuring a controlled variable of the process and providing a first signal indicative of the measured value, means for providing a second signal indicative of the said set point for said controlled variable, means for comparing said first and second signal to produce an error signal in accordance with the difference therebetween, means for producing a control signal from said error signal so that said control signal has a proportional and reset component, means operable when said control signal exceeds a predetermined maximum value to modify said error signal to which said control signal producing means responds so that said control signal does not exceed said maximum value, said last-named means including parallel connected oppositely poled Zener diodes for establishing the maximum values for said control signal and a high gain amplifier for producing a signal to modify said error signal until the control signal is maintained below said maximum value, and means responsive to said control signal for controlling a manipulated variable of said process so as to tend to bring said measured value of the controlled variable to equality with said set point, said last-named means being effective to operate limit switches which selectively short an appropriate one of said Zener diodes when said manipulated variable has been altered to a limit condition, whereby the control signals are reduced to zero for error signals calling for a change in the manipulated variable in the limited direction.

Claim 5

In a process control system, the method for controlling a manipulated variable so as to tend to bring the measured control variable to its set point without a degradation of the control of said manipulated variable due to inability of the system to produce changes in the manipulated variable as called for by the control system, comprising the steps of: estaBlishing a signal representing said measured control variable; establishing a set point signal representing the desired value of said control ; comparing said set point signal and said signal representing said measured control variable to produce an error signal; producing a control signal in accordance with said error signal, said control signal including a proportional and reset component; producing in accordance with said control signal an effective error signal of magnitude such that a corresponding control signal will not exceed the predetermined value which will cause the control response to be degraded, and modifying said manipulated variable in accordance with said control signal by the steps of: a. producing at regular intervals pulses of duration proportional to the magnitude of said control signal; b. accumulating said pulses to produce a signal indicative of the desired state of the control element which controls said manipulated variable; c. producing a signal indicative of the actual state of said final control element, and d. comparing said signal indicative of the desired state with said signal indicative of the actual state and producing in accordance with said difference a signal for controlling the said final control element until said difference is reduced to zero.

Claim 6

In a process control system, the method for controlling a manipulated variable so as to tend to bring the measured control variable to its set point without a degradation of the control of said manipulated variable due to inability of the system to produce changes in the manipulated variable as called for by the control system, comprising the steps of: estaBlishing a signal representing said measured control variable; establishing a set point signal representing the desired value of said control ; comparing said set point signal and said signal representing said measured control variable to produce an error signal; producing a control signal in accordance with said error signal, said control signal including a proportional and reset component; producing in accordance with said control signal an effective error signal of magnitude such that a corresponding control signal will not exceed the predetermined value which will cause the control response to be degraded, and modifying said manipulated variable in accordance with said control signal by the steps of: a. producing at regular intervals pulses of duration proportional to the magnitude of said control signal; b. accumulating said pulses to produce a signal indicative of the desired state of the control element which controls said manipulated variable; c. producing a signal indicative of the actual state of said final control element, and d. comparing said signal indicative of the desired state with said signal indicative of the actual state and producing in accordance with said difference a signal for controlling the said final control element until said difference is reduced to zero.

Claim 7

In a process control apparatus responsive to an error signal for modifying the manipulated variable of the process so as to tend to bring the associated controlled variable to its set point, means for preventing a degradation of the control due to a limitation in the response of one of the components of said apparatus to changing error signals comprising: control means for producing a control signal representative of the change in the manipulated variable called for by said control apparatus in response to said error signal, said control means being operative to calculate a proportional response part of the control signal in accordance with the time derivative of the effective error signal; limit means for establishing a maximum value for said control signal which will avoid said degradation; and correcting means operable when said control signal exceeds said maximum value to change the effective error signal to its said control responds by an amount sufficient to bring said control signal to a value not exceeding said maximum.

Claim 8

In a process control apparatus responsive to an error signal for modifying the manipulated variable of the process so as to tend to bring the associated controlled variable to its set point, means for preventing a degradation of the control due to a limitation in the response of one of the components of said apparatus to changing error signals comprising: control means for producing a control signal representative of the change in the manipulated variable called for by said control apparatus in response to said error signal, said control means being operative to calculate a proportional response part of the control signal in accordance with the time derivative of the effective error signal; limit means for establishing a maximum value for said control signal which will avoid said degradation; and correcting means operable when said control signal exceeds said maximum value to change the effective error signal to its said control responds by an amount sufficient to bring said control signal to a value not exceeding said maximum.