Windup, integral

Since actuators are subject to saturation, the phenomenon of integral windup must be properly tackled namely, if the controller outputs a command beyond the... 

External reset is provided for the primary controller to prevent integral windup when the... 

What is integral windup of a controller, and what are its undesired consequences Does a P or PD controller exhibit integral windup Why ... 

Integral windup (see Windup, integral) Integrated process ... 

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

When a controller with integral action (PI or PID) sees an error signal for a long period of time, it integrates the error until it reaches a maximum (usually 100 percent of scale) or a minimum (usually 0 percent). This is called reset windup. 

A sustained error signal can occur for a number of reasons, but the use of override control is one major cause. If the main controller has integral action, it wfll windup when the override controller has control of the valve. And if the override controller is a PI controller, it will windup when the normal controller is setting the valve. So this reset windup problem must be recognized and solved. 

I Anti "reset windup (only integrate ettcr if op signal is between 0 and 1... 

In order to make sure that the pressure controller (PC-01) set point is changed slowly and in a stable manner, the valve position controller (VPC-02) is provided with integral action only, and its integral time is set to be about 10 times that of PC-01. In order to prevent reset windup when PC-01 is switched to manual or local control from cascade, the valve position controller is also provided with an external feedback signal off the pump speed. 

FLC system approach can be used to solve problems. Many applications of FLC are related to simple control algorithms such as the PID controller. In a natural way, nonlinearities and exceptions are included which are difficult to realize when using conventional controllers. In conventional control, many additional measures have to be included for the proper functioning of the controller anti-resist windup, proportional action, retarded integral action, etc. These enhancements of the simple PID controller are based on long-lasting experience and the interface of continuous control and discrete control. The fuzzy PID-like controller provides a natural way to applied controls. The fuzzy controller is described as a nonlinear mapping. 

Antireset windup can be implemented by simply not allowing the integral to accumulate when the manipulated variable is saturated. The manipulated variable is saturated when the control valve on the line supplying the manipulated variable is either closed or fully open. A saturated control valve can be identified when there is sustained offset between the maiupulated variable level... 

When an error e(t) persists for a long time, the value of the integral f e(t) dt increases significantly and may lead the output of a PI controller to its maximum allowable value. We say that the controller has been saturated and in physical terms it means that the valve is fully open or closed before the control action has been completed (i.e., before the error has been driven to zero). This situation is also known as reset windup. How would you handle such a situation You can consult Refs. 7 and 15. 

Reset windup problems can occur in override control structures when controllers have integral action. Large swings (bumps) in control valve position can result as control is transferred from one controller to another. One method for achieving bumpless transfer is the use of the Shinskey/Buckley control structure called external reset feedback. Unfortunately, this type of controller is not available in commercial dynamic simulators. 

---