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- 1. Lecture: Anti-windup techniques Automatic Control 2 Anti-windup techniques Prof. Alberto Bemporad University of Trento Academic year 2010-2011 Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 1 / 17
- 2. Lecture: Anti-windup techniques Saturation effects Problem G(z) x(k) r(k) Kx(k)+Hr(k) linear controller u(k) saturation Wednesday, June 2, 2010 Most control systems are designed based on linear theory A linear controller is simple to implement and performance is good, as long as dynamics remain close to linear Nonlinear effects require care, such as actuator saturation (always present) Saturation phenomena, if neglected in the design phase, can lead to closed-loop instability, especially if the process is open-loop unstable Main reason: the control loop gets broken if saturation is not taken into account by the controller: u(k) = Kx(k) for some k [1] K.J. Åstrom, L. Rundqwist, “Integrator windup and how to avoid it”, Proc. American Control Conference, Vol. 2, pp. 1693-1698, 1989 Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 2 / 17
- 3. Lecture: Anti-windup techniques Saturation effects Example: AFTI-F16 aircraft Linearized model: ˙x = −0.0151 −60.5651 0 −32.174 −0.0001 −1.3411 0.9929 0 0.00018 43.2541 −0.86939 0 0 0 1 0 x + −2.516 −13.136 −0.1689 −0.2514 −17.251 −1.5766 0 0 u y = 0 1 0 0 0 0 0 1 x Inputs: elevator and ﬂaperon (=ﬂap+aileron) angles Outputs: attack and pitch angles Sampling time: Ts = 0.05 sec (+ ZOH) Constraints: ±25◦ on both angles Open-loop response: unstable open-loop poles: −7.6636,−0.0075 ± 0.0556j,5.4530 !"##$% &'$()*+%, -')., /0'$%+1 /0'$%+1 2+130*"#01)'4)50, 6$%*07)'4)50, 2)*$%)'4)50, !+'' 80*79:); Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 3 / 17
- 4. Lecture: Anti-windup techniques Saturation effects Example: AFTI-F16 aircraft LQR control + actuator saturation ±25◦ The system is unstable ! Actuator saturation cannot be neglected in the design of a good controller Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 4 / 17
- 5. Lecture: Anti-windup techniques Saturation effects Example: AFTI-F16 aircraft With a controller designed to handle saturation constraints: There are several techniques to handle input saturation, the most popular ones are anti-windup techniques Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 5 / 17
- 6. Lecture: Anti-windup techniques Saturation effects Saturation function Saturation can be deﬁned as the static nonlinearity sat(u) = umin if u < umin u if umin ≤ u ≤ umax umax if u > umax −2 −1 0 1 2 −1 −0.5 0 0.5 1 umin and umax are the minimum and maximum allowed actuation signals (example: ±12 V for a DC motor) If u is a vector with m components, the saturation function is deﬁned as the saturation of all its components (umin, umax ∈ m ) sat(u) = sat(u1) sat(u2) ... sat(um) Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 6 / 17
- 7. Lecture: Anti-windup techniques Anti-windup schemes: Ad hoc methods The windup problem reference output input To Workspace integ Saturation Process 1 s Integrator 1 s Gain 1 Derivative du/dt very simple process (=an integrator) controlled by a PID controller (KP = KI = KD = 1) under saturation −0.1 ≤ u ≤ 0.1 The output takes a long time to go steady-state The reason is the “windup” of the integrator contained in the PID controller, which keeps integrating the tracking error even if the input is saturating anti-windup schemes avoid such a windup effect Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 7 / 17
- 8. Lecture: Anti-windup techniques Anti-windup schemes: Ad hoc methods Anti-windup #1: incremental algorithm y(kT) r(KT) u(kT)incremental PID 1 1 − z−1 G(z) ∆u(kT) Sunday, May 16, 2010 It only applies to PID control laws implemented in incremental form u((k + 1)T) = u(kT) + ∆u(kT) where ∆u(kT) = u(kT) − u((k − 1)T) Integration is stopped if adding a new ∆u(kT) causes a violation of the saturation bound Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 8 / 17
- 9. Lecture: Anti-windup techniques Anti-windup schemes: Ad hoc methods Anti-windup #2: Back-calculation G(s) ! "e sTd Kp ! v yr u actuator !! ! PID ! ! 1 s 1 Tt !"Kp Ti anti-windup et Wednesday, June 2, 2010 The anti-windup scheme has no effect when the actuator is not saturating (et(t) = 0) The time constant Tt determines how quickly the integrator of the PID controller is reset If the actual output u(t) of the actuator in not measurable, we can use a mathematical model of the actuator. Example: et(t) = v(t) − sat(v(t)) Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 9 / 17
- 10. Lecture: Anti-windup techniques Anti-windup schemes: Ad hoc methods Anti-windup #2: Back-calculation reference output input To Workspace integ_aw Saturation Process 1 s Integrator 1 s Gain 1 Derivative du/dt anti-windup scheme for a PID controller (KP = KI = KD = 1, Tt = 1) Integrator windup is avoided thanks to back-calculation Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 10 / 17
- 11. Lecture: Anti-windup techniques Anti-windup schemes: Ad hoc methods Beneﬁts of the anti-windup scheme Let’s look at the difference between having and not having an anti-windup scheme 0 10 20 30 40 50 60 70 80 0 0.5 1 1.5 2 outputy(t) 0 10 20 30 40 50 60 70 80 −0.1 −0.05 0 0.05 0.1 inputu(t) 0 10 20 30 40 50 60 70 80 −5 0 5 integraltermi(t) time t Note that in case of windup we have strong output oscillations a longer time to reach the steady-state the peaks of control signal Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 11 / 17
- 12. Lecture: Anti-windup techniques Anti-windup schemes: Ad hoc methods Anti-windup #3: Conditional integration The PID control law is u(t) = Kp(br(t) − y(t)) + I(t) − KpTd dy(t) dt = Kpbr(t) − Kpyp(t) + I(t) where yp(t) = y(t) + Td dy(t) dt is the prediction of the output for time t + Td Consider the proportional band [yl(t),yh(t)] for yp(t) in which the corresponding u is not saturating yl(t) = br(t) + I(t) − umax Kp yh(t) = br(t) + I(t) − umin Kp where umin and umax are the saturation limits of the actuator The idea of conditional integration is to update the integral term I(t) only when yp is within the proportional band (for PI control simply set yp = y) An hysteresis effect may be included to prevent chattering Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 12 / 17
- 13. Lecture: Anti-windup techniques Anti-windup schemes: General methods Anti-windup #4: Observer approach The anti-windup method applies to dynamic compensators ˆx(k + 1) = Aˆx(k) + Bu(k) +L(y(k) − Cˆx(k)) u(k) = Kˆx(k) + Hr(k) by simply feeding the saturated input to the observer ˆx(k + 1) = Aˆx(k) + Bsat(v(k))) +L(y(k) − Cˆx(k)) v(k) = Kˆx(k) + Hr(k) u(k) = sat(v(k)) x(k) y(k) A,B u(k) C .1$1- -.1&%$1,+ !"#$%&'$()*+,'-..)/0).$12+$1&,# r(k) K x(k)ˆ++ H saturation v(k) Wednesday, June 2, 2010 Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 13 / 17
- 14. Lecture: Anti-windup techniques Anti-windup schemes: General methods Example The process to be controlled is obtained by exactly sampling ˙x = −1.364 0.4693 0.736 1.131 −1.08 −1.424 0.1945 −0.7132 0.0499 0.8704 −0.9675 −0.3388 −0.9333 0.8579 −0.5436 −0.9997 x + 0.05574 0 −0.04123 −1.128 u y = [−1.349 0 0.9535 0.1286 ]x with T = 0.5s The input u saturates between ±2 Control design by pole placement: poles in e−5T , e−5T , e(−2±j)T Observer design by pole placement: 4 poles in e−10T The dynamic compensator is ˆx(k + 1) = Aˆx(k) + Bsat(v(k)) + L(y(k) − Cˆx(k)) v(k) = Kˆx(k) + Hr(k) with K = 0.4718 −1.5344 −2.8253 2.1819 , L = 1.1821 1.1924 4.2054 −3.6554 , H = 1/(C(I − A − BK)−1 B) = 8.2668 Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 14 / 17
- 15. Lecture: Anti-windup techniques Anti-windup schemes: General methods Example (cont’d) Compare the results with anti-windup and without anti-windup: 0 5 10 15 20 25 30 35 40 45 50 −1 −0.5 0 0.5 1 1.5outputy(t) 0 5 10 15 20 25 30 35 40 45 50 −2 −1 0 1 2 inputu(t) time t Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 15 / 17
- 16. Lecture: Anti-windup techniques Anti-windup schemes: General methods Conclusions Conditional integration is easy to apply to many controllers, although it may not be immediate to ﬁnd the conditions to block integration and to avoid chattering Back-calculation only requires tuning one parameter, the time constant Tt. But it only applies to PID control The observer approach is very general and does not require tuning any additional parameter. It also applies immediately to MIMO (multi-input multi-output) systems Saturation effects can be included in an optimal control formulation. In this case the control action is decided by a constrained optimization algorithm, as in model predictive control (MPC) techniques Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 16 / 17
- 17. Lecture: Anti-windup techniques Anti-windup schemes: General methods English-Italian Vocabulary anti-windup scheme schema di anti-windup aileron alettone pitch beccheggio roll rollio yaw imbardata model predictive control controllo predittivo Translation is obvious otherwise. Prof. Alberto Bemporad (University of Trento) Automatic Control 2 Academic year 2010-2011 17 / 17

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