Method and apparatus for process control
First Claim
1. An apparatus for closed-loop control of a process performed with equipment that responds to a controlled variable signal to vary a first characteristic of that process, said apparatus having the improvement comprisingA. batch monitoring means for monitoring values of a first process characteristic of the process being performed and for generating a signal representative of an arithmetic mean of those monitored values and for generating a signal representative of a standard deviation of those monitored values,B. normalization means coupled to said batch monitoring means for responding to said mean value-representative signal and to said standard deviation-representative signal to generate an error signal representative of a normalized deviation of said monitored values,C. drift detection means coupled to said normalization means for responding to said error signal to detect a selected change in said first process characteristic, said drift detection means comprisingi) first accumulation means for generating a high-value accumulation signal representative of a summation of time-successive values of a difference between said error signal and a first slack value, andii) second accumulation means for generating a low-value accumulation signal representative of a summation of time-successive values of a difference between an additive inverse of said error signal and a second slack value, which may be independent of said first slack value,D. process control means coupled to said drift detection means for selectively generating a manipulated variable signal representative of a sum of a prior value of said manipulated variable signal and a scaled one of at least one of said high-value accumulation signal and said low-value accumulation signal, andE. closed-loop control means for applying said manipulated variable signal to the process-performing equipment to drive the controlled variable signal to a target value.
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Abstract
A statistical process control system provides asymmetrical nonlinear automatic closed-loop feedback control. The system has application, for example, in the control of equipment that responds to a controlled variable signal to vary a measurable characteristic of a process. The system uses an accumulated deviation of a measured subgroup means minus a target value, divided by the subgroup standard deviation, to modify the manipulated variable each time the accumulated value exceeds a decision interval above or below the target. The system permits the use of independent slack variables subtracted from the accumulated deviation to more closely model the underlying process. Likewise, the system permits the use of independent alarm values and variable gains to permit greater control of the process.
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Citations
39 Claims
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1. An apparatus for closed-loop control of a process performed with equipment that responds to a controlled variable signal to vary a first characteristic of that process, said apparatus having the improvement comprising
A. batch monitoring means for monitoring values of a first process characteristic of the process being performed and for generating a signal representative of an arithmetic mean of those monitored values and for generating a signal representative of a standard deviation of those monitored values, B. normalization means coupled to said batch monitoring means for responding to said mean value-representative signal and to said standard deviation-representative signal to generate an error signal representative of a normalized deviation of said monitored values, C. drift detection means coupled to said normalization means for responding to said error signal to detect a selected change in said first process characteristic, said drift detection means comprising i) first accumulation means for generating a high-value accumulation signal representative of a summation of time-successive values of a difference between said error signal and a first slack value, and ii) second accumulation means for generating a low-value accumulation signal representative of a summation of time-successive values of a difference between an additive inverse of said error signal and a second slack value, which may be independent of said first slack value, D. process control means coupled to said drift detection means for selectively generating a manipulated variable signal representative of a sum of a prior value of said manipulated variable signal and a scaled one of at least one of said high-value accumulation signal and said low-value accumulation signal, and E. closed-loop control means for applying said manipulated variable signal to the process-performing equipment to drive the controlled variable signal to a target value.
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20. A method for closed-loop control of a process performed on equipment that responds to a controlled variable signal to vary a first characteristic of that process, said method having the improvement comprising the steps of
A. monitoring values of a first process characteristic of the process being performed for generating a signal representative of an arithmetic mean of those monitored values and for generating a signal representative of a standard deviation of those monitored values, B. normalizing said standard deviation-representative signal to generate an error signal, C. responding to said error signal to detect a selected change in said plural time-successive values of said first process characteristic, said error-responding step including the steps of i) generating a high-value accumulation signal representative of a summation of time-successive values of differences between said error signal and a first slack value, and ii) generating a low-value accumulation signal representative of a summation of time-successive values of differences between an additive inverse of said error signal and a second slack value which may be independent of said first slack value, D. selectively generating a manipulated variable signal representative of a sum of a prior value of said manipulated variable signal and a scaled one of at least one of said high-value accumulation signal and said low-value accumulation signal, and E. applying said manipulated variable signal to the process performing equipment to drive the controlled variable signal toward a target value.
Specification