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Engineering Process Control and SPC

# Engineering Process Control and SPC - Engineering Process...

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11/10/2008 1 Engineering Process Control and SPC Group 1 Outline EPC concept and Integral control Example of Integra Control The Adjustment Chart Combining SPC and EPC

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11/10/2008 2 SPC Indentify assignable causes through control charts Permanent process improvement or reduction in variability Based on Two Assumptions: it is possible to bring the process into a state of statistical control(only stable random variation around the process target) Process spc once the process is in this in control state, it will tend to stay there for a relatively long period of time without continual ongoing adjustment. Engineering Process Control In continuous process such as chemical processes, di t ff t t despite our efforts to remove the assignable causes, the process may still have a tendency to drift away from the target. Process compensation and regulation, in which some manipulatable process Process EPC variables is adjusted in real time in order to minimize the difference between the output and the target
11/10/2008 3 A simple Adjustment Schema: Integral control t x 1 + y EPC gx T y = 1 t (1) A model with feedback control: t t + 1 + t y T t x g :Manipulatable variable in period t : output in period t+1 :target value :Constant process gain (2) A model without feed back control: e disturbanc a is N where N T y t t t 1 1 1 + + + = EWMA disturbance model: ( ) EWMA the for factor weighting the is and t period time at error prediction the is N N e where e N N N N N t t t t t t t t t 1 0 1 < = + = + = = λ ) ) ) ) ) In this model, the process is uncontrolled, and the process output will be drifting away from the target.

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11/10/2008 4 (3) Combining (1) and (2), get the adjusted process at time t: t t t gx N T y + = + + 1 1 Aim: set Xt(setpoint) to cancle out the disturbance at period t+1 In order to calculate setpoint, estimate the disturbance: 1 1 1 + + + = t t t N N e ) So that ) t t t t gx N e T y + + = + + + 1 1 1 We got the setpoint: g N x t t 1 + = ) As a result of the adjustment: 1 1 + + = t t e T y () + + = = = t t t t
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Engineering Process Control and SPC - Engineering Process...

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