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1、Analysis of the effect of Integral Windup and Design of the Anti-Windup deviceAbstract: The windup phenomenon appears and results in performance degradation when the proportional-integral-derivative (PID) controller output is saturated. In this paper, two anti-windup methods based on the digital PID
2、 controller are represented, which are known as conditional Integration and Back-calculation & Tracking respectively, and an anti-windup device is designed based on the RST controller. According to the simulation results, all of the methods can reduce the degradation of the integral windup phenomeno
3、n effectively.1. IntroductionWhen the feedback controller surpasses the saturation (i.e. maximum) limits of the system actuator and is not capable of instantly responding to the changes in the control error. When this phenomenon, which is also known as windup occurs, the actuator constantly runs at
4、its saturation limit despite any output the system might have. This means that the system now runs with an open loop instead of a constant feedback loop.Further more, Integral windup also known as integrator windup1 or reset windup2，refers to the situation in a PID feedback controller where a large
5、change in setpoint occurs (say a positive change) and the integral terms accumulates a significant error during the rise (windup), thus overshooting and continuing to increase as this accumulated error is unwound (offset by errors in the other direction). The specific problem is the excess overshoot
6、ing.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
7、the selected output to the integral circuit of all controllers in the selection scheme so that a closed loop is maintained. These approaches may be classified into three categories: conditional integration, tracking back calculation, and limited integrator schemes. In conditional integration schemes
8、 35, the integral action is suspended when control input is saturated and only the PD control is activated. When the control input lies within the saturation limit, the PID control is effective, and a zero steady-state error can be guaranteed. The initial value of the integrator in PI control is pro
9、perly calculated by using the previous steady-state value of the integrator in 3. The difference between saturated and unsaturated control input signals is used to generate a feedback signal to act on integrator input for the tracking back calculation methods, 69. Transient performance such as overs
10、hoot depends heavily upon feedback gain of the control difference rather than PID gains. Both conditional integration and tracking back calculation are combined in 1012. For the limited integrator scheme 13, the integrator value is limited by feeding the control back with a high-gain dead zone to gu
11、arantee operation in the linear range. For saturation limits, this scheme is the same as the back calculation method.In this paper, two anti-windup methods based on the digital PID controller are represented, which are known as conditional Integration and Back-calculation & Tracking respectively, an
12、d an anti-windup device is designed based on the RST controller. According to the simulation results, all of the methods can reduce the degradation of the integral windup phenomenon effectively.2. Conditional Integration method used in Anti WindupThe basic ideal of the condition integration method i
13、s, turning off the integration when the control is far from steady state to avoid the increase of overshoot. And turn on the integration when the control signal gets close to the reference value in order to eliminate the steady error. Integral action is then only used when certain conditions are met
14、, otherwise the integral term is held constant, see the following detail steps:1) Set the threshold according to specified situations;2) Use PD controller when to decrease the overshoot;3) Use PID controller when to guarantee the accuracy of the control system.Which can be represented as(1)Where is
15、the switch coefficient of the integration term(2)A simulation is executed to prove the effectiveness of the condition integration anti windup method, and the Simulink structure is shown in Figure 1, where the transfer function is as simple as . Finally, the simulation result is given in Figure 2. Fi
16、gure 1. Simulink structure of the conditional integrationFigure 2. Comparison of the output signals with and without conditional integration Anti Windup3. Back-calculation and Tracking method used in Anti WindupThe basic idea of this method is that, we need to decrease the integral action instead of accumulating the integration term when the control signal gets into the saturation region. Therefore, we need to judge whet
