CHE 185 – PROCESS CONTROL AND DYNAMICS PID IMPLEMENTATION

Reset Windup for PID Controllers Windup results when the manipulated variable is not able to control to the setpoint resulting in sustained offset causing the integral of the error from setpoint to accumulate. When control returns, accumulated error causes an upset. Windup can occur when a control valve saturates or when a control loop is not being used (e.g., select control).

Windup example When deviations exceed maximum http://brettbeauregard.com/blog/2011/04/improving-the-beginner%E2%80%99s-pid-reset-windup/

Windup example Methods to counter – include limits http://brettbeauregard.com/blog/2011/04/improving-the-beginner%E2%80%99s-pid-reset-windup/

Windup example Methods to counter – include output limits http://brettbeauregard.com/blog/2011/04/improving-the-beginner%E2%80%99s-pid-reset-windup/

Reset Windup for PID Controllers Note that controller output saturates causing area “A” to accumulate by the integral action. After the disturbance returns to its normal level, the controller output remains saturated for a period of time causing an upset in y.

ANTI-WINDUP WINDUP OCCURS BECAUSE INTEGRAL CONTROL REMAINS IN EFFECT AFTER A PROCESS HAS REACHED SATURATION. ANTI-WINDUP IS INCLUDED TO PREVENT INTEGRAL CONTROL FROM CONTINUING TO OPERATE AFTER SATURATION HAS BEEN ACHIEVED. ANTI-WINDUP ALLOWS THE SYSTEM TO QUICKLY RETURN TO CONTROL CONDITIONS AT POINTS BELOW SATURATION

ANTI-WINDUP ANTI-WINDUP SHOULD BE INCLUDED IN ALL CONTROLLERS WHERE INTEGRAL IS EMPLOYED. WINDUP CAN BE A FACTOR IN SINGLE LOOP AND MORE COMPLICATED SYSTEMS

Anti-Reset Windup When the manipulated variable saturates, the integral is not allowed to accumulate. When control returns, the controller takes immediate action and the process returns smoothly to the setpoint.

ANTI-WINDUP ALTERNATE STRATEGIES CLAMPING - LIMITS THE CONTROLLER OUTPUT TO A SPECIFIC LEVEL INTERNAL RESET RESULTS IN PI CONTROL AND CAN BE USED WITH CLAMPING TO AVOID SATURATION EXTERNAL RESET USES OUTPUT FROM THE MANIPULATED VARIABLE TO AVOID SATURATION

Internal Reset Feedback

Conventional PI Controller Therefore, internal reset feedback is equivalent to a conventional PI controller. It still has windup, but controller output can be clamped.

External Reset Feedback An extension of internal reset feedback, therefore, it is equivalent to a conventional PI controller. When u saturates, windup will cease preventing windup. Less windup than clamping, but requires umeas.

BUMPLESS TRANSFER BUMPLESS TRANSFER OCCURS WHEN A SYSTEM IS SWITCHED FROM MANUAL TO AUTOMATIC CONTROL THE SYSTEM IS DESIGNED TO SET THE INITIAL OUTPUT EQUAL TO THE MANUAL VALUE THE SYSTEM IS THEN RAMPED UP/DOWN TO THE NEW TARGET VALUE. With bumpless transfer, an internal setpoint is used for the controller and the internal setpoint is ramped at a slow rate from the initial conditions to the actual desired setpoint to order to provide a smooth startup of a control loop.

BUMPLESS TRANSFER Control Performance With and Without Bumpless Transfer.

SPLIT RANGE CONTROL SPLIT RANGE IS USED WHEN THERE IS A NEED FOR MORE THAN ONE CONTROL RANGE THERE ARE TYPICALLY OVERLAPS IN THE RANGES OF OPERATION, BUT ONE ACTUATOR IS USED FOR HIGH FLOWS AND A SECOND IS USED FOR LOW FLOW RANGES USE OF PARALLEL FLOWS FOR HEATING AND COOLING IS ALSO SUGGESTED IN THE TEXT, BUT THIS IS NOT THE SAFEST FORM OF OPERATION BECAUSE IT CAN LEAD TO CONFLICT BETWEEN CONTROLLERS IN THE OVERLAP RANGE

SPLIT RANGE CONTROL Split Range Flow Controller

Example for Split Range Flow Control Uses parallel valves for naoh flow control

Example for Split Range Flow Control Titration Curve for a Strong Acid-Strong Base System Therefore, for accurate pH control for a wide range of flow rates for acid wastewater, a split range flow controller for the NaOH is required

PID FOR MULTIPLE INPUT/MULTIPLE OUTPUT (MIMO) SYTEMS COMPARISON BETWEEN CENTRALIZED AND DECENTRALIZED CONTROL DECENTRALIZED (MULTILOOP) CONTROL HAS SEVERAL SINGLE LOOP CONTROLS IN PARALLEL FOR A SINGLE PROCESS. REACTION SYSTEM EXAMPLE:

PID FOR MULTIPLE INPUT/MULTIPLE OUTPUT (MIMO) SYTEMS THE ALTERNATE TO THIS APPROACH IS CENTRALIZED (COORDINATED) CONTROL AND FOR THE SAME SYSTEM IT WOULD LOOK LIKE: