A tutorial overview on theory and design of offset-free MPC algorithms

Transcription

1 A tutorial overview on theory and design of offset-free MPC algorithms Gabriele Pannocchia Dept. of Civil and Industrial Engineering University of Pisa November 24, 2015

2 Introduction to offset-free MPC The role of feedback Feedback is central in control systems: In PID the tracking error control action e = r alone determines the In MPC an OCP is resolved at each time because a new state measurement is available Necessary to cope with disturbances and modeling errors How to achieve offset-free tracking with persistent errors? a matter of personal preference among different methods y 2

3 Introduction to offset-free MPC Different approaches to offset-free control PID control: tracking error is integrated LQG (extended): state/disturbance observer and compensator - C P Fr P IMC (but also early MPCs) disturbance evaluation and compensation Fd Fx Observer - Q P P - 3

4 Introduction to offset-free MPC Objectives Present an updated and comprehensive description of offset-free MPC algorithms for nonlinear (and linear) discrete-time systems Clarify the main concepts Show new results Highlight subtleties by means of challenging applications Outline of this talk 1. Offset-free Nonlinear MPC: principles, known and new results 2. Offset-free Economic MPC: idea, design, and results 3. Offset-free Linear MPC: new equivalence results 4. Application examples and conclusions 4

5 Part I: Offset-free Nonlinear MPC Pannocchia, Gabiccini, Artoni Offset-free MPC explained: novelties, subtleties, and applications 5

6 Offset-free NMPC: assumptions Plant x + p = f p (x p, u, w p ) y = h p (x p, v p ) Disturbances Nominal model x + = f (x, u) y = h(x) w := f p (x p, u, w p ) f (x, u), v := h p (x p, v p ) h(x) General assumptions and requirements Output y is measured at each time Assume that Disturbances (w,v) are bounded disturbances (w,v) are asymptotically constant Input and output constraints u 2 U, y 2 Y Offset-free tracking problem y c = r(y) Controlled variable: Design an output feedback MPC law u = apple(y) such that lim k!1 y c(k) =ȳ c 6

7 Offset-free NMPC: Augmented model and observability Offset-free MPC Plant Augmented observer Augmented model x + = F (x, d, u) d + = d y = H(x, d) Requirements and remarks Consistent with nominal dynamics F (x, 0, u) =f (x, u), H(x, 0) = h(x) Detectable (observable) Possible iff the nominal system is detectable (observable) Dimension of d is limited n d apple n y The disturbance state d follows an integral dynamics 7

8 Offset-free NMPC: Observer design Steady-state Kalman filter like framework Predictor ˆx k k 1 = F (ˆx k 1 k 1, ˆd k 1 k 1, u k 1 ) Prediction error e k = y k ŷ k k 1 ˆd k k 1 = ˆd k 1 k 1 ŷ k k 1 = H(ˆx k k 1, ˆd k k 1 ) Filtering ˆx k k =ˆx k k 1 + apple x (e k ) Assumption (Nominal observer) Functions (apple x, apple d ) This generally implies n d = p are continuous and satisfy apple x (0) = 0, apple d (e) =0, e =0 ˆd k k = ˆd k k 1 + apple d (e k ) The observer is nominally stable for the augmented model 8

9 Offset-free NMPC: Target and optimal control problems Offset-free MPC Target Calculation min `s(y, u) x,u,y subject to: x = F (x, ˆd k k, u) y = H(x, ˆd k k ) r(y) =ȳ c y 2 Y, u 2 U Plant Augmented observer Deviation variables x i := x i ũ i := u i x s (k) u s (k) min x,u Target and FHOCP can be merged together [LAAC08] NX 1 i=0 `( x i,ũ i )+V f ( x N ) subject to: x 0 =ˆx(k) x i+1 = F (x i, ˆd k k, u i ) H(x i, ˆd k k ) 2 Y, FHOCP u i 2 U x N 2 X f 9

10 Offset-free NMPC: main results Offset-free tracking property (MB02, PR03, MM12) Assume that the target and the FHOCP are feasible at all times, and the closed-loop system reaches an equilibrium (u 1, y 1 ) It follows that: r(y 1 )=ȳ c Sketch of proof. At equilibrium FHOCP solution (l(x,u) is PD) From the target calculation 10

11 Offset-free NMPC: main results Asymptotic stability and offset-free control in state feedback If (y = x) choose the following augmented system (B d K d = I ) F (x, u, d) :=f (x, u)+b d d, H(x, d) :=x, apple x (e) =e, apple d (e) =K d e It follows that: lim x k ˆx k k 1 = 0, lim k!1 k!1 Remarks ˆd k k 1 = B 1 d 1.The closed-loop system is assumed to reach a stable equilibrium 2.Establish when this occurs is very hard (e.g., see [BFS13]) 3.How to design a nonlinear observer augmented system is a tricky task 4.When the state is measurable offset-free design and analysis is simpler w 11

12 Part II: Offset-free Economic MPC Pannocchia, Gabiccini, Artoni Offset-free MPC explained: novelties, subtleties, and applications 12

13 Economic MPC: a brief introduction Output feedback framework Economic MPC Plant State observer min x,u `e(h(x), u) subject to: x = f (x, u) u 2 U, h(x) 2 Y min x,u NX 1 i=0 `e(h(x i ), u i ) subject to: x 0 =ˆx(k) x i+1 = f (x i, u i ) u i 2 U, h(x i ) 2 Y (xs,us) is an asymptotically stable equilibrium in which the economic cost is minimized [DAR11, AAR12] x N = x s 13

14 Offset-Free Economic MPC: what do you mean? Plant x + = f (x, u)+w y = h(x)+v Best equilibrium min x p,ū,ȳ `e(ȳ,ū) subject to: x p = f ( x p,ū)+ w ȳ = h( x p )+ v ū 2 U, ȳ 2 Y Economic MPC Plant State observer Nominal EMPC Where do we go if we use nominal EMPC? Unclear Can we reach the best equilibrium? No with nominal EMPC 14

15 Offset-free Economic MPC: design Offset-free EMPC Plant Step 1. Augmented observer Evaluate (ˆx k k, ˆd k k ) Augmented observer Step 2. Current best equilibrium (x s (k), u s (k), y s (k)) := arg min x,u,y `e(y, u) subject to: x = F (x, ˆd k k, u), y = H(x, ˆd k k ) u 2 U apple 0, y 2 Y Step 3. FHOCP min x,u u i 2 U, NX 1 `e(h(x i, ˆd k k ), u i ) i=0 subject to: x 0 =ˆx k k x i+1 = F (x i, ˆd k k, u i ) H(x i, ˆd k k ) 2 Y x N = x s (k) 15

16 Offset-free Economic MPC: main result Convergence to the optimal equilibrium [PGA15] Assume that: Equilibrium problem (Step 2.) and the economic MPC problem (Step 3.) are feasible at all times The closed-loop system reaches an equilibrium with input and output (u 1, y 1 ) The achieved ultimate cost `(y 1, u 1 ) is the best achievable for the actual plant. The statement/proof in [PGA15] need a revision :( 16

17 Part III: Offset-free Linear MPC Pannocchia, Gabiccini, Artoni Offset-free MPC explained: novelties, subtleties, and applications 17

18 Offset-free LMPC: Disturbance model Nominal model f (x, u) :=Ax + Bu h(x) :=Cx, r(y) :=Dy Requirements (A,B) stabilizable, apple (C,A) detectable A I B rank DC 0 = n x + n r Disturbance model and observer The augmented system takes the form: F (x, u, d) :=Ax + Bu + B d d, H(x, d) :=Cx + C d d The linear observer is: apple x (e) :=K x e, apple d (e) :=K d e, Main results (design guidelines) [MB02,PR03] The augmented system is observable iff There exist valid (B d, C d ) iff n d apple n y If n d = n y then K d is invertible rank apple A I Bd C C d = n x + n d 18

19 A known alternative for LMPC: The velocity model A known alternative [PR01, W04, GAM08, BFS12-13] Extended model (state/control increments) x + = A x + B u y + = y + C x + = y + CA x + CB u Remarks on the observer x An observer is required to estimate (at least) Since y is measured, we can use (K y = I ) Main results (design guidelines) The velocity model is detectable if (A,C) is detectable Requirements (A,B) stabilizable, (C,A) detectable apple A I B rank C 0 = n x + n y The velocity model is stabilizable if (A,B) is stabilizable and the rank condition holds true: (K y = I ) (A K x CA) n y apple n u, i.e. a deadbeat observer A deadbeat observer is stable iff is stable 19

20 The velocity model is not an alternative method Main result [P15] Consider the velocity form model, with a stable output deadbeat observer gain: K = apple K x I This is equivalent to using the following disturbance model and observer gains: B d = K x, C d = I CK x, K x = K x, K d = I some thoughts The velocity model is just a particular case of disturbance model. So, the question about which method is better makes no longer sense 20

21 Offset-free LMPC: Further equivalence results in state feedback case Some considerations When y = x, we can use a full deadbeat observer Disturbance model: rank(b d )=n, C d = 0, K x = I, K d = B 1 d Velocity model: K x = I, K y = I Equivalence result Full state disturbance model with deadbeat observer Consider the velocity form model with full deadbeat observer. This is equivalent to using the following disturbance model and observer gains: B d = I, C d = 0, K x = I, K d = I 21

22 Another alternative method State disturbance observer [T14] Introduced by Tatjewski [T14] to avoid state augmentation Choose an observer gain K such that is Hurwitz Define and compute a state disturbance and output bias Use the following prediction model in the FHOCP: It is the following disturbance model [P15]: 22

23 Yet another alternative method A different velocity algorithm [GAM08] It uses the following augmented system with velocity input: Both state components are estimated (at time k): It is the input disturbance model: 23

24 Part IV: Application examples and conclusions Pannocchia, Gabiccini, Artoni Offset-free MPC explained: novelties, subtleties, and applications 24

25 Application of offset-free NMPC A cart-pole system with obstacles m, J G q 2 pole obstacles (fixed walls) Nonlinear dynamics 2 ẋ 1 3 6ẋ 2 7 4ẋ 3 5 = ẋ x 3 x 4 a(x 2 )mls 2 J O x 2 4 mglc 2 a(x 2 )mls 2 mlx 2 4 c 2 M t g a(x 2) J O mlc u g G Final position q 1 cart u O Control task specifications Control force is constrained Reach final position stably and avoiding obstacles Unknown disturbance forces occur 25

26 Application of offset-free NMPC Showing the desirable behavior 26

27 Application of offset-free NMPC Cart-pole with obstacles: full state feedback NMPC0 uses no disturbance model vs NMPC1 uses state disturbance model Angle (rad) Forces (N) Displacement (m) Target value NMPC0 6 NMPC NMPC NMPC Time (sec) 0.3 Time (sec) NMPC0 NMPC Time (sec) Prediction errors Time (sec) Disturbance 50 NMPC0 NMPC Displacement (m) Angle (rad) Time (sec) 27

28 Application of offset-free NMPC NMPC0 Cart-pole with obstacles: full state feedback NMPC1 28

29 Application of offset-free NMPC Cart-pole with obstacles: output feedback ny = 2 NMPC2 uses a linear input disturbance model (nd = 1) vs NMPC3 uses a linear input/output disturbance model (nd = 2) Angle (rad) Forces (N) Displacement (m) Target value NMPC2 NMPC NMPC3 NMPC Time (sec) 0.3 Time (sec) NMPC2 NMPC Time (sec) Prediction errors Time (sec) Disturbance 50 NMPC2 NMPC Time (sec) Displacement (m) Angle (rad) Take Home Message Can use nd<ny in lucky cases So, use always nd=ny 29

30 Application of offset-free NMPC NMPC2 NMPC3 30

31 Application of offset-free EMPC Continuous Stirred Tank Reactor with exothermal reaction Mass and energy balances F 0 c 0 u 2 x 1 E ẋ 1 r 4ẋ x 3 k 0 exp( Rx 2 )x 1 H E = 4 C p k 0 exp( Rx 2 )x 1 + 2U 7 0 r C p (u 1 x 2 ) 5 ẋ 3 Coolant F 0 (T 0 x 2 ) r 2 )x 3 F 0 u 2 r 2 Control task specifications Measure (c, h) Enforce input and output constraints Economic cost `e(x, u) =x 1 = c 31

32 c (kmol/m 3 ) Application of offset-free EMPC CSTR: output feedback enmpc0 uses no disturbance model h (m) F (m 3 /min) Tc (K) Lower bound enmpc0 enmpc1 vs enmpc1 uses a linear input disturbance model (nd = 2) Time (min) Time (min) 0.74 Upper bound enmpc enmpc enmpc1 enmpc Time (min) Time (min) enmpc0 enmpc Time (min) enmpc0 enmpc Time (min) c (kmol/m 3 ) h (m) enmpc0 enmpc1 Take Home Message empc pushes towards constraints Offset-free output prediction is necessary to avoid instability 32

33 c (kmol/m 3 ) T (K) h (m) Tc (K) F (m 3 /min) Application of offset-free LMPC CSTR: full state feedback, tracking cost LMPC0 uses a partial state disturbance model (nd = 2) Target value LMPC0 LMPC Time (min) LMPC LMPC Time (min) Target value LMPC0 LMPC Time (min) LMPC LMPC Time (min) LMPC0 LMPC Time (min) vs c (kmol/m 3 ) h (m) LMPC0 LMPC1 uses a full state disturbance model (nd = 3) Time (min) LMPC LMPC Time (min) Take Home Message Offset-free LMPC can be effective on nonlinear systems if correctly designed Again the choice nd<ny is not safe! 33

34 Conclusions Described the latest advances in the design of offset-free MPC algorithms A self-contained summary of the available results for nonlinear MPC: use of disturbance models and observers extended the existing asymptotic convergence results Extended the concept of offset-free estimation to the design of economic MPC for systems with persistent errors/disturbances. Linear MPC: a commonly known method based on the velocity form model is a particular case of disturbance model, and not an alternative route to offset-free tracking Challenging examples of nonlinear processes (controlled by NMPC, LMPC, and EMPC) highlight the significance of the presented results emphasize specific subtleties related to the number of used disturbances and to the process dynamics, which may result in an incorrect design Some open problems Effective nonlinear disturbance modeling coupled with nonlinear (MHE) observers More general results for economic MPC Robust stability questions Time varying systems (tracking and economic MPC) 34

35 Acknowledgments Prof. Moritz Diehl for the kind invitation Dr. Marco Gabiccini, Dr. Alessio Artoni, and Mr. Mirco Lupi for help on the talk preparation Prof. James Rawlings and Mr. Dauglas Allan UW, Madison) for useful discussion on economic MPC 35

36 References [AAR12] Angeli, D., Amrit, R., and Rawlings, J.B. (2012). On Average Performance and Stability of Economic Model Predictive Control, IEEE Trans. Auto. Contr., 57(7), [BFS12] Betti, G., Farina, M., and Scattolini, R. (2012). An MPC algorithm for offset-free tracking of constant reference signals. Proceedings of 51st IEEE Conference on Decision and Control, Maui, Hawaii, USA. [BFS13] Betti, G., Farina, M., and Scattolini, R. (2013). A robust MPC algorithm for offset-free tracking of constant reference signals. IEEE Trans. Auto. Contr., 58(9), [DAR11] Diehl, M., Amrit, R., and Rawlings, J.B. (2011). A Lyapunov function for economic optimizing model predictive control. IEEE Trans. Auto. Contr., 56(3), [GAM08] González, A.H., Adam, E.J., and Marchetti, J.L. (2008). Conditions for offset elimination in state space receding horizon controllers: A tutorial analysis. Chem. Engng. and Proc.: Process Intensification, 47(12), [LAAC08] Limon, D., Alvarado, I., Alamo, T., and Camacho, E.F. (2008). MPC for tracking piece-wise constant references for constrained linear systems. Automatica, 44, [MB02] Muske, K.R. and Badgwell, T.A. (2002). Disturbance modeling for offset-free linear model predictive control. J. Proc. Cont., 12, [MM12] Morari, M. and Maeder, U. (2012). Nonlinear offset-free model predictive control. Automatica, 48, [P15] Pannocchia, G. (2015). Offset-free tracking MPC: A tutorial review and comparison of different formulations. In Proceedings of European Control Conference (ECC15) [PGA15] Pannocchia, G., Gabiccini, M., and Artoni, A. (2015). Offset-free MPC explained: novelties, subtleties, and applications. Invited semi-plenary paper in Proceedings of Nonlinear Model Predictive Control (NMPC15). [PR01] Pannocchia, G. and Rawlings, J.B. (2001). The velocity algorithm LQR: a survey. Technical Report , TWMCC, Department of Chemical Engineering, University of Wisconsin-Madison. [PR03] Pannocchia, G. and Rawlings, J.B. (2003). Disturbance models for offset-free model predictive control. AIChE J., 49, [W04] Wang, L. (2004). A tutorial on model predictive control: Using a linear velocity-form model,. Dev. Chem. Eng. Mineral Process., 12(5/6),