what the term “windiing-up” means in the Control systems language?
“Wind Up” refers to the effect exhibited by the Integral term of a PID controller. It can best be summed up by the Wikipaedia definition, below: Integral windup particularly occurs as a limitation of physical systems, compared with ideal systems, due to the ideal output being physically impossible (process saturation: the output of the process being limited at the top or bottom of its scale, making the error constant). For example the position of a valve cannot be any more open than fully open and also cannot be closed any more than fully closed. In this case integral windup can actually involve the integrator being turned off for periods of time until the response falls back into an acceptable range.
This usually occurs when the controller’s output can no longer affect the controlled variable, or if the controller is part of a selection scheme and it is selected right.
Integral windup was more of a problem in analog controllers. Within modern Distributed Control Systems and Programmable Logic Controllers, it is much easier to prevent integral windup by either limiting the controller output, or by using external reset feedback, which is a means of feeding back the selected output to the integral circuit of all controllers in the selection scheme so that a closed loop is maintained.
Since integration is fundamentally a process of multiplication followed by addition, the units of measurement are always the product (multiplication) of the function’s variables. In the case of reset (integral) control, we are multiplying controller error (the difference between PV and SP, usually expressed in percent) by time (usually expressed in minutes or seconds). Therefore the result will be an “error-time” product. In order for an integral controller to self-recover following windup, the error must switch signs and the error-time product accumulate to a sufficient value to cancel out the error-time product accumulated during the windup period.