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Proceedings of the 34th Chinese Control Conference July 28-30, 2015, Hangzhou, China Controller Design for a Permanent Magnet Linear Synchronous Motor: a Singular Perturbation Method YANG Chunyu, HAN Yuchen, SHI Yuxiang, ZHOU Linna School of Information and Electrical Engineering, China University of Mining and Technology, Xuzhou 221116, P. R. China E-mail: chunyuyang@cumt.edu.cn, han19900206@sina.com, zs13060105@cumt.edu.cn, linnazhou@cumt.edu.cn Abstract: This paper considers the design problem of a three-closed-loop controller for a permanent magnet linear synchronous motor. Taking into account the multiple time-scale property of the system, the design is achieved in two stages. In the first one, the inner loop (current-loop) controller is designed based on the singular perturbation theory and pole assignment technique. The controller gains can be constructed by solving a set of well-defined linear matrix inequalities. In the second stage, with the ob- tained controller gains, a homotopy algorithm is formulated to design the velocity- and position-loop controllers simultaneously. Simulation is given to illustrate the proposed results. Key Words: Permanent magnet linear synchronous motor, Singularly Perturbed system, Pole assignment, Homotopy algorithm 1 Introduction Compared with rotary motors, permanent magnet lin- ear synchronous motors (PMLSMs) have many advantages, such as no backlash and less friction, high acceleration and high precision and no conversion device [1, 2]. Thus differ- ent PMLSMs have been used in numerous engineering ap- plications, for example, electromagnetic aircraft launch sys- tems (EMALS) [3], machine tools [4], maglev vehicles [5], and workshop transportation [6]. Controller design for PMLSMs has attracted much atten- tion and many advance control methodologies have been modified to control PMLSMs. In [7], an iterative learning variable structure controller was established and achieved the fast point-to-point motion. In [8], an H∞ robust con- trol method was proposed in term of linear matrix inequal- ities (LMIs). To deal with the state-dependent uncertainties and bounded nonlinearities, a nonlinear adaptive robust con- troller with saturated actuator authority was designed in [9]. A sliding mode controller basing on fuzzy neural network (RFNN) was proposed in [10]. The RFNN can estimate the real-time lumped uncertainty for the position control of PMLSMs. In [12], a discrete adaptive sliding-mode (DASM) controller was proposed, which can guarantee system stabil- ity for both regulation and tracking tasks. Although the above mentioned design methods have been shown to have certain advantages in some aspects, PID or PI control is the most popular scheme in practical applications and most of the commercial motion controllers adopt PID or PI control structure [13]–[15]. This paper will consider the design problem of the three-closed-loop controller shown in Fig. 1. The three-closed-loop control system consists of a current-loop, a velocity-loop and a position-loop. The cur- rent error and velocity error are regulated by PI controller separately and position error is regulated by a proportional controller. In this control system, there is not a differential controller because it usually amplifies the effect of external This work was supported by the National Natural Science Foundation of China (61374043), the Jiangsu Provincial Natural Science Foundation of China (BK20130205), the China Postdoctoral Science Foundation funded project (2013M530278, 2014T70558), the Fundamental Research Funds for the Central Universities (2013QNA50, 2013RC10, 2013RC12, 2013XK09) and the Natural Science Foundation of Liaoning Province (201202201). disturbances. PI controllers have been shown to be compe- tent to regulate the current and velocity. In position-loop, only a proportional controller is used since a PI controller may cause large overshoot. Thus most commercial motion controllers, for example, ACS motion controller, adopt such a structure. The aim of this paper is to propose an efficient method to construct the controller gains K1, K2, K3, K4, K5. First, the model of the PMLSM is recalled and the model for the three-closed-loop control system is established. There ex- ist some nonlinear terms on Ki in the closed-loop model, thus it is difficult to design the five gain matrices simultane- ously. Then, in view of the multiple time-scale property of the system, we separate the current-loop from the whole con- trol system, which leads to a linear singularly perturbed sys- tem with a standard state feedback controller. By using the singular perturbation theory and pole assignment technique, the current-loop PI controller design is reduced to solving a set of LMIs. With the obtained gain matrices, a homotopy algorithm is formulated to design the velocity- and position- loop controllers simultaneously. Finally, simulation is made to illustrate the proposed results. 2 System description 2.1 PMLSM model Under standard assumptions [11], the PMLSM model in d-q coordinates is as follows. did dt = − R Ld id + Lq Ld π τ viq + 1 Ld ud, diq dt = − R Lq iq − π τ v(Ldid + ψ) + 1 Lq uq, (1) (2) = dv dt 3nπ 2τ M (ψiq + (Ld − Lq)idiq) − 1 M (Fl + Fd), (3) ds dt = v, (4) 4453
Fig. 1: Three-closed-loop control system where ud, uq and id, iq are the d-axis and q-axis voltages and currents, respectively. Ld, Lq are the d-axis and q-axis primary inductors, respectively. R, ψ, τ, v, s, n are the phase winding resistance, winding inductance, pole pitch, linear velocity, disturbance, the number of pole pairs, respectively. Fl, Fd, M, B are the load force, edge effect force, mass of the carrier, viscous friction coefficient, respectively. In this paper, we assume that the d-axis and q-axis primary inductors are equal, namely, Ld = Lq = L. Thus, (1)-(3) can be written as did dt diq dt dv dt = − R L = − R L 3nπ 2τ M = 1 viq + L vid − ψ L id + π τ iq − π τ ψiq − 1 M (Fl + Fd). ud, v + π τ 1 L uq, (5) (6) (7) Equations (5) and (6) show that the electrical dynamics id and iq are coupled by nonlinear terms proportional to viq and vid, which bring difficulties to controller design. There are two strategies to alleviate this problem. One is based on the introduction of a new control voltage absorbing nonlinear term [11]. This paper will adopt another which is known as id = 0 control strategy [16, 17]. In this strategy, the actual line current id is assumed to be equal to its reference ∗ d = 0. Then we have a simplified model for command i PMLSMs ds dt dv dt diq dt = v, 3nπψ = 2τ M = − R L iq − 1 M iq − ψ π L τ (Fl + Fd), v + 1 L uq. (8) (9) (10) 2.2 Closed-loop control system model To construct closed-loop control system model, we intro- ∗ − v, ˙ηq = i q − iq. ∗ ∗ − ∗ − s, ev = v According to Fig. 1, we have es = s ∗ ∗ = K1es, i ∗ q = K2ev + K3ηv, u q = duce to new variables ˙ηv = v q − iq, v ∗ v, eq = i K4eq + K5ηq. Then the dynamics of the closed-loop system is governed 4454 by the following equation ⎡ ⎤ ⎥⎥⎥⎥⎦ = (A0 + B0K0) ⎢⎢⎢⎢⎣ ˙v ˙es ˙ηv ˙ηq L˙iq ⎤ ⎥⎥⎥⎥⎦ + ⎡ ⎢⎢⎢⎢⎣ ⎡ ⎢⎢⎢⎢⎣ v es ηv ηq iq − 1 M0 0 0 0 ⎤ ⎥⎥⎥⎥⎦ d0, (11) where ⎡ ⎣ K0 = ⎡ ⎢⎢⎢⎢⎣ ⎡ ⎢⎢⎢⎢⎣ A0 = B0 = ⎤ ⎥⎥⎥⎥⎦ , 0 −1 −1 0 − πψ τ 0 0 0 3πnψ 2τ M 0 0 0 0 0 0 0 0 0 0 0 −1 0 −R 0 0 ⎤ ⎥⎥⎥⎥⎦ , d0 = Fl + Fd, 0 0 0 0 0 0 1 0 0 0 1 0 0 0 1 K1 0 0 −K2 0 −K2K4 K1K2K4 K3K4 K5 −K4 0 K3 K1K2 0 0 ⎤ ⎦ . It can be seen that K0 contains nonlinear terms on K1, K2, K3, K4, K5. Thus it is difficult to calculate the con- troller gains. To alleviate this issue, we decompose the de- sign into two successive steps in view of the multi-time scale property of the control system. We will first design current- loop controller, then design position- and velocity-loop con- trollers simultaneously. To design the current-loop PI controller, we assume that ∗ q and the velocity are constant. Then the current command i according to Fig. 1 and equation (10), the dynamics of cur- rent error eq is governed by the following equation = (A1 + B1F ) d1, (12) ˙ηq L ˙eq where A1 = 0 1 0 −R ηq eq 0 −1 0 1 + , d1 = Ri . , B1 = F = K5 K4 ∗ q + πψ τ v,
With known K4, K5 and letting es = K1es, equation (11) ⎡ can be transformed into ⎡ ⎤ ⎤ ⎤ ⎥⎥⎥⎥⎦ = (A2 + B2G) ⎢⎢⎢⎢⎣ ⎢⎢⎢⎢⎣ ˙v ˙es ˙ηv ˙ηq L˙iq ⎡ ⎢⎢⎢⎢⎣ ⎥⎥⎥⎥⎦ + v es ηv ηq iq ⎥⎥⎥⎥⎦ d2, − 1 M0 0 0 0 ⎤ ⎥⎥⎥⎥⎦ , where ⎡ ⎢⎢⎢⎢⎣ A2 = B2 = G = 0 0 0 0 1 0 0 0 0 0 K5 −R − K4 3πnψ 2τ M 0 0 −1 τ ⎤ 0 0 0 0 0 0 −1 0 − πψ ⎡ 0 0 0 −1 0 0 1 0 0 K4 ⎢⎢⎢⎢⎣ −K2 K2 K3 K1 0 0 ⎥⎥⎥⎥⎦ , d2 = Fl + Fd, . 0 0 0 0 Assume that d1 and d2 are constant. Then system (12) and system (13) are linear time-invariant systems subject to con- stant disturbances. If the closed-loop systems are designed to be asymptotically stable, they have unique equilibrium point. Specifically, the equilibrium point of (12) must sat- isfy 0 = ˙ηq = eq and the equilibrium point of (13) should satisfy 0 = ˙ηv = −v + ¯es and 0 = ˙¯es = −K1v, which imply ¯es = 0, that is, es = 0. It can be seen that asymptotic stability of the closed-loop systems guarantees the conver- gence of the tracking errors eq and es, which is independent of the disturbances. Therefore, the tracking PI controller de- signs can be reduced to stabilization controller designs by omitting the disturbances. 3 Controllers design Taking into account the fact that L is very small, system (12) and system (13) can be rewritten as the following sin- gularly perturbed system. E(ε) ˙x(t) = (A + BK)x(t) +Bd, (14) where E(ε) = diag{I, εI}, ε = L represents the singular perturbation parameter. According to the discussion in the previous section, the tracking PI controller designs can be reduced to stabilization controller designs by omitting the disturbances. Thus we assume d = 0 in the sequel. To enhance the performance of the control system, pole constraints are considered in the controller design. To do this, we recall the following definition and lemma. DEFINITION 1 [19] A subset D of the complex plane C is called an LMI region if there exist a symmetric matrix L ∈ Rd×d and a matrix M ∈ Rd×d such that D = {z = x + jy ∈ C : fD(z) < 0}, (15) where the characteristic function fD(z) is given as follows: (16) fD(z) = L + M z + M T z. LEMMA 1 [19] Given a dynamic system ˙x(t) = Ax(t) and an LMI region D, the system is D−stable, i.e., Λ(A) ∈ D if there exists a matrix X ∈ Rn×n with X = X T > 0 such that (13) L ⊗ X + M ⊗ (AX) +M T ⊗ (AX)T < 0, where Λ(A) is the set of eigenvalues of A and ⊗ denotes the Kronecker product of the matrices. 3.1 PI Controller for current-loop According to equation (12), the PI Controller for current- loop is a standard state feedback controller and can be de- signed by the following theorem. THEOREM 1 Given an LMI stability region D, if there exist matrices Zk(k = 1, 2, 3) with Zk = Z T k (k = 1, 2) and F satisfying the following LMIs Z1 > 0, Z1 + εZ3 εZ3 εZ T 3 εZ2 > 0, S1 < 0, S1 + εS2 < 0, where V1 = U1 = Z1 0 0 0 0 Z1 Z3 Z2 , V2 = , U2 = 0 Z T 3 Z3 Z2 0 Z T 3 0 0 (17) (18) (19) (20) , , S1 = L ⊗ V1 + M ⊗ (AU1) +M T ⊗ (AU1)T +M ⊗ (B2F ) +M T ⊗ (B2F )T , S2 = L ⊗ V2 + M ⊗ (AU2) +M T ⊗ (AU2)T , and ⊗ denotes the Kronecker product of the matrices, then, the poles of the closed-loop system with K = F (U1 + εU2)−1 are all within the given LMI region D. Proof: Assume that the LMIs in this theorem are feasible. Then by simple calculation, we have E(ε)Z(ε) = V1 + εV2, and S1 + εS2 = L ⊗ V1 + M ⊗ (AU1) +M T ⊗ (AU1)T +M ⊗ (B2F ) +M T ⊗ (B2F )T +ε(L ⊗ V2 + M ⊗ (AU2) +M T ⊗ (AU2)T ) = L ⊗ (V1 + εV2) +M ⊗ [A(U1 + εU2) +B 2F ] +M T ⊗ [A(U1 + εU2) +B 2F ]T = L ⊗ [E(ε)Z(ε)] + M ⊗ [(A + B2K(ε))Z(ε)] +M T ⊗ [(A + B2K(ε))Z(ε)]T . (21) 4455
Denote FD = L ⊗ X(ε) +M ⊗ E +M T ⊗ E −1(ε)(A + B2K(ε))X(ε) −1(ε) × (A + B2K(ε))X(ε)T , (22) where X(ε) is a matrix with appropriate dimensions. It is easy to show that [I ⊗ E(ε)] × FD × [I ⊗ E(ε)] = L ⊗ [E(ε)X(ε)E(ε)] +M ⊗ [(A + B2K(ε))X(ε)E(ε)] +M T ⊗ [E(ε)X(ε)(A + B2K(ε))T ]. (23) Let X(ε) = Z(ε)E −1(ε). LMIs (17), (18) imply that E(ε)Z(ε) = Z T (ε)E(ε) > 0, which shows X(ε) = X T (ε) > 0. Substituting X(ε) = Z(ε)E −1(ε) into (23) gives [I ⊗ E(ε)] × FD × [I ⊗ E(ε)] = L ⊗ [E(ε)Z(ε)] +M ⊗ [(A + B2K(ε))Z(ε)] +M T ⊗ [Z T (ε)(A + B2K(ε))T ]. (24) (25) From (20), (23) and (25), it follows that [I ⊗ E(ε)] × FD × [I ⊗ E(ε)] < 0, which shows that FD < 0. (26) By Lemma 1, inequalities (24) and (26) imply that the poles of the closed-loop system with K = F (U1 + εU2)−1 are all within the given LMI region D. This completes the proof. REMARK 1 LMIs (17) and (19) seems redundant. But they are significant to guarantee the whole LMI conditions in Theorem 1 to be well-defined for any singular perturbation parameter ε. REMARK 2 Theorem 1 can be used directly to design PI controller for the current-loop. And a suitable LMI region D will leads to satisfactory control performance. 3.2 PI Controllers for velocity- and position-loop It can seen from (13), the controllers for the velocity- and position-loop are not a standard state feedback controllers since the gain matrix is of specific structure, which make the design complex. By Theorem 1, we have the following theorem. THEOREM 2 Given an LMI stability region D, if there exist matrices G and Zk(k = 1, 2, 3) with Zk = Z T k (k = 1, 2) satisfying the following inequalities Z1 > 0, Z1 + εZ3 εZ3 εZ T 3 εZ2 > 0, (27) (28) S1 < 0, S1 + εS2 < 0, (29) (30) , , 0 Z T 3 Z3 Z2 0 Z T 3 0 0 where V1 = Z1 0 0 0 , V2 = 0 Z1 Z3 Z2 , U2 = U1 = S1 = L ⊗ V1 + M ⊗ [(A + BG)U1] +M T ⊗ [(A + BG)U1]T , S2 = L ⊗ V2 + M ⊗ [(A + BG)U2] +M T ⊗ [(A + BG)U2]T , then, the poles of the closed-loop system with K = G are all within the given LMI region D. To solve the inequalities (27)-(30), we first construct a controller without this constraint by using Theorem 1, then perform a homotopy algorithm to obtain the controller with the specific structure −K2 K2 K3 K1 0 0 . 0 0 0 0 G = We rewrite the inequalities (27)-(30) into a compact form F (G, Z1, Z1, Z3) < 0. (31) Following the line [20], we define the following matrix function L (G, Z1, Z2, Z3, λ) = F ((1 − λ)G0 + λG, Z1, Z2, Z3), (32) where λ is a real number varying from 0 to 1, G0 is the full- state-feedback gain which can be obtained by Theorem 1, and G is the feedback gain with the specific structure. The term (1− λ)G0 + λG in (32) defines a homotopy interpolat- ing a full-state feedback controller and a desired state feed- back controller, and our problem of finding a solution to (31) is embedded in the family of problems L (G, Z1, Z2, Z3, λ) < 0, λ ∈ [0, 1]. (33) Now we are ready to formulate the homotopy algorithm. Algorithm Step 1 Solve the LMIs in Theorem 1 and obtain the full state feedback gain G0, and matrices Z10, Z20, Z30. If the LMIs are not feasible, this method fail to design a controller. Step 2 Set k = 0, Gk = 0, and Z1k = Z10, Z2k = Z20, Z3k = Z30. Step 3 Set k = k + 1, λk = k/N. Compute a set so- lutions Gk of L (G, Z1, Z2, Z3, λk) < 0, if it is feasible, goto Step 4; if it is not feasible, compute a common solution Z1k, Z2k, Z3k of L (Gk−1, Z1, Z2, Z3, λk) < 0, if it is fea- sible, goto Step 4, if it is not feasible, set N = 2N and goto Step 4. Step 4 If N > Nmax, where Nmax is a prescribed upper bound, then the algorithm ends without feasible solution, else if k < Nmax, goto Step 3, and if k = N, the obtained GN , Z1N , Z2N , Z3N are the feasible solutions. 4456
4 Simulation In this section, the proposed methods are illustrated. The parameters of the PMLSM are as follows: R = 4.7Ω, L = 1.5mH, τ = 0.015m, M = 2.2kg, n = 1, ψ = 0.4297wb. Then the system matrices of (12) are as follows Choose an LMI region D with A1 = , B1 = L = 0 1 0 −4.7 −1000 −1 +sin 70◦ −cos70◦ 0 0 −1 0 −110 , cos70◦ 1 +sin 70◦ . , , M = Solving the LMIs in Theorem 1 gives K5 K4 = 9886.1 40.3 which serves as the current-loop controller gains. ⎤ Thus the system matrices of (13) are as follows ⎡ A2 = ⎢⎢⎢⎢⎣ 0 0 0 0 −1 1 0 0 −90 0 ⎡ 0 0 0 0 0 ⎥⎥⎥⎥⎦ , 0 0 0 0 61 0 0 −1 9886.1 −44.7 ⎤ 0 0 0 1 40.3 Choose an LMI region D with B2 = L = ⎥⎥⎥⎥⎦ . 0 −1 0 0 0 ⎢⎢⎢⎢⎣ −3000 , −1 +sin 89◦ cos89◦ −cos89◦ 1 +sin 89◦ −K2 K2 K3 −395 395 2813.4 0 0 0 0 0 0 0 −10 K1 0 0 0 144 0 0 0 0 , M = = By using the homotopy algorithm with N = 100, we have . (34) Now K1, K2, K3, K4 and K5 are all obtained. The sim- ulations are given in Fig. 2 and Fig. 3. It can be seen that system reach steady state in 0.05 seconds. When the sys- tem is subject to 50kg load, the steady state error is about 2μm. Furthermore, Fig. 2 shows that a larger load can lead to larger steady state error. Our future work will consider this problem. 5 Conclusion In this paper, we have investigated the controller design problem for PMLSMs. A two-stage design method bas- ing on the singular perturbation theory and pole assignment technique was proposed. The current-loop controller was designed in the first stage by solving a set of well-defined LMIs. Then the velocity- and position-loop controllers were designed simultaneously by a homotopy algorithm. Simula- tion has shown the effectiveness of the proposed methods. Fig. 2: The position tracking performance by step signal Fig. 3: The steady state errors by step signal References [1] D. A. Bristow, and A. G. Alleyne, A high precision motion con- trol system with application to microscale robotic deposition, IEEE Trans. on Control Systems Technology, 16(5): 1075– 1082, 2008. [2] I. Rotariu, M. Steinbuch, and R. Ellenbroek, Adaptive itera- tive learning control for high precision motion systems, IEEE Trans. on Control Systems Technology, 16(5): 1075–1082, 2008. [3] J. R. Quesada, and J. F. Charpentier, Finite difference study of unconventional structures of permanent-magnet linear ma- chines for electromagnetic aircraft launch system, IEEE Trans. on Magnetics, 41(1): 478–83, 2005. [4] X. Li, R. Du, B. Denkena, and J. Imiela, Tool breakage moni- toring using motor current signals for machine tools with linear motors, IEEE Trans. on Industrial Electronics, 52(5): 1403– 1408, 2005. [5] J. F. Hoburg, Modeling maglev passenger compartment static magnetic fields from linear Halbach permanent-magnet arrays, IEEE Trans. on Magnetics, 40(1): 59–64, 2004. [6] K. Yoshida, S. Nakata, and S. Koga, Orthogonally switching motion control of PMLSM vehicle in mass-reduced mode, in Proceedings of 6th International Conference on Electrical Ma- chines and Systems, 2003: 469–472. [7] J. Wu, and H. Ding, Iterative learning variable structure con- troller for high-speed and high-precision point-to-point mo- tion, Robotics and Computer Integrated Manufacturing, 24(3): 384–391, 2008. [8] G. Chen, Q. Fang, and J. Li, H∞ robust controller design of permanent magent linear synchronous motor based on LMI, Mechanical and Electrical Engineering Magazine, 28(6): 704- 4457
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