1. INTEGRATOR (or RESET) WINDUP
Prof. Ing. Michele MICCIO
Dip. Ingegneria Industriale (Università di Salerno)
Prodal Scarl (Fisciano)
rev. 2.6 of April 9, 2019
 see also §13.2 at pag. 247
Stephanopoulos, “Chemical process control: an Introduction to theory and practice”

2. A new operating problem
Reset windup refers to the situation in a PID feedback controller where a large change in setpoint occurs (say a positive change) and the integral terms accumulates a significant error during the rise (windup)
Reset Windup may happen:
during start-up
during large set point changes
if the proportional gain is too small
Reset Windup

3. FEEDBACK LOOP BLOCK DIAGRAM
ideal case 
adapted from:
Ch. 1 “Fundamental Principles of Process Control” in Cooper D. (2008), "Practical Process Control using Loop-Pro Software", PDF textbook
Reset Windup

4. FEEDBACK LOOP BLOCK DIAGRAM
actual case 
PROCESS
SENSOR
PID Controller
ySP(t)
ε(t)
o(t)
m(t)
d(t)
ym(t)
y(t) + -
saturation block 
in addition to the final control element, saturation may also affect the controller output or the actuator
In practice, all final control elements or actuators have physical limitations:
a control valve cannot be more than fully open or fully closed, a motor has limited speed, etc.
Reset Windup

5. Saturation block
Controller output
ideal case 
actual case 

6. The valve as a final control element
Saturation block
The valve as a final control element

7.  a situation referred to as integral windup or reset windup.
PI Controller error + −
If the error cannot be eliminated within a reasonable period, the integral term can become so large that the controller or the final control element is saturated:
 a situation referred to as integral windup or reset windup.
Reset Windup

8. Windup
y(t)
ySP(t)
ε(t)
Integrator windup is illustrated through a simulation of a system where the control signal (e.g., manipulated variable) has a saturation at a level of ±0.1 followed by a linear system with the transfer function:
saturation
CO or MV
The net effect is that there is a large overshoot.
The “integrator output” begins to decrease when the error passes through zero
Reset Windup

9. Anti-Reset Windup  Methods for Anti-Reset Windup
Initialising the controller integral to a desired value
Clamp the controller output to be greater than 0% and less than 100%
Conditional integration: Turn off the integral function when the actuator saturates or a control loop is not in use.
Back-calculation: Back-calculating the integral term to constrain the process output within feasible bounds
Alternative:
Increasing the setpoint in a suitable ramp
rather than a step 
Modern controllers will not suffer from reset windup. It is an old problem that has long been solved !
Reset Windup

10.  http://aer.ual.es/ilm/pidwindup.php
Windup Demo 
Interactive Learning Modules Project
 PIDWindup.app
by Matthias Bauerdick
South Westphalia University
Reset Windup

11. Windup Demo Level control
Reset Windup

12. Windup Demo ↑
The discharge flow rate has a saturation value of 1.5 m3/s corresponding to a fully open flow control valve.
The red and blue curves correspond to a controller with and without anti-windup.
Reset Windup