2Overall Course Objectives Develop the skills necessary to function as an industrial process control engineer.SkillsTuning loopsControl loop designControl loop troubleshootingCommand of the terminologyFundamental understandingProcess dynamicsFeedback control
3Reset 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).
4Reset WindupNote 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.
5Anti-Reset WindupWhen 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.
6Methods for Anti-Reset Windup Turn off the integral when a valve saturates or a control loop is not in use.Clamp the controller output to be greater than 0% and less than 100%.Apply internal reset feedbackApply external reset feedback
9Conventional PI Controller Therefore, internal reset feedback is equivalent to a conventional PI controller.It still has windup, but controller output can be clamped.
10External 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.
11Bumpless TransferWhen a control loop is turned on without bumpless transfer, the process can become unduly upset.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.
13Control Performance With and Without Bumpless Transfer
14Split Range Flow Control In certain applications, a single flow control loop cannot provide accurate flow metering over the full range of operation.Split range flow control uses two flow controllers (one with a small control valve and one with a large control valve) in parallel.At low flow rates, the large valve is closed and the small valve provides accurate flow control.At large flow rates, both valve are open.
22OverviewAll controllers that employ integral action should have anti-reset windup applied.Bumpless transfer provides a means for smooth startup of a control loop.When accurate metering of a flow over a very wide flow rate range is called for, use split range flow control.