Flatness-Based Control of an Induction Machine Fed via Voltage Source Inverter - Concept, Control Design and Performance Analysis Joerg Dannehl and Friedrich W. Fuchs Power Electronics and Electrical Drives, Christian-Albrechts-University of Kiel Kaiserstr. 2, 24143 Kiel Germany jda@tf.uni-kiel.de, fwf@tf.uni-kiel.de Abstract— In this paper a rather new nonlinear control method based on differential flatness is applied to the induction machine fed by a voltage source converter. The control design is done by designing a feedforward using the flatness of the system to lead it to the reference trajectory. Due to nonidealities an additional stabilizing linear feedback controller is inserted. A speed and rotor flux control with inner current loops is designed and tuned for the induction machine. Simulation results are presented, analyzed and a comparison with the today’s standard control method for ac electrical drives, the field-oriented control, is carried out. Thereby the flatness-based control shows a good performance even though further opportunities for optimizations are left. I. I NTRODUCTION The induction machine fulfils many industrial requirements and today mostly sets up the standard for variable speed drives [1]. This drive has been investigated to a large extent. Nevertheless research still continues. The issue of controlling electrical drives is nowadays widely solved with the so-called field oriented control (FOC). It was introduced in [2] and has been thoroughly investigated meanwhile [3]. The basic idea is to use a coordinate system aligned to the rotor flux vector. It enables the asymptotic decoupled control of the rotor flux and motor torque or speed, respectively. The transformed system variables are controlled with PI-controllers whereas the control structure commonly consists of inner current and outer speed/torque and flux control loops. Due to the inherent nonlinearity of the machine model control design requires different time constants of the inner and outer loops. Furthermore decoupling between flux and speed is only achievable if the flux is kept constant. In recent years nonlinear control methods were applied to the speed and flux control of the induction machine. The assumptions of different time constants of the loops or a constant rotor flux are not necessary. The feedback linearization methods [4][5] use transformations in appropriate coordinates in which a nonlinear feedback is designed to exactly linearize the system and exactly decouple the control variables. Then an outer controller can be designed by linear methods. The passivity-based control [6][7] uses the property passivity to design a feedback control that guarantees stability. An iterative Lyapunov design method is called backstepping [8][9]. The design starts with a small subsystem which will be expanded stepwise till the control for the complete model is determined. Stability is proven in every iteration step. The sliding mode control [10] is a discrete control method. The control objectives are expressed in state space as a sliding surface. The control algorithm is designed to lead the system to it and keep it there asymptotic stable. A rather new control method uses the system property flatness [11] to design the control. In [12] and [13] the flatness was used to design a dynamic feedback linearizing scheme. Another approach is to design at first a feedforward control using the flatness to lead the system to the (in general time varying) reference trajectory, so the system may be linearized around it [14]. With focus on eliminating tracking errors a linear feedback control is inserted. Application of this flatness-based control (FBC) to the induction machine is presented in [15] and [16]. Based on the research in the field of modern control of electrical drives the FBC is expected to offer advantages over the other methods shown above concerning tracking behaviour, rejection of perturbations and robustness. Therefore in this paper the FBC of the voltage-fed induction machine will be presented, analyzed and compared to the standard method of FOC. The preceding investigation is structured into five sections. Section II describes the system modelling. In section III the FOC and in section IV the FBC is presented and designed. The performance and characteristic features of the FBC are analyzed and compared to the FOC by simulations in section V. This paper is completed by a conclusion in section VI. II. I NDUCTION M ACHINE M ODEL In this section the model of a three-phase voltage source inverter-fed squirrel cage induction machine with an isolated neutral point will be described. It is assumed to be symmetric and to have a linear magnetic behaviour and constant resistances. Since the sum of the phase voltages equals zero, the system can be described by two-dimensional space vectors [17]. The following notation will be used for a space vector in an a/b-reference frame rotating with ωK : x→K = xa + jxb . The model parameters are specified in the appendix (see TABLE I). In the stationary reference frame (ωS = 0) the stator voltage vector can be expressed vsS = Rs isS + ψ̇sS → → (1) → whereas → isS and ψ→sS are the stator current and stator flux vectors. In the same way the rotor voltage vector can be expressed in the rotor fixed reference frame rotating with ωR vrR = Rr irR + ψ̇rR → → (2) → R whereas→irR and ψ are the rotor current and rotor flux vectors. →r The transformation in an arbitrary reference frame rotating with ωk yields [17]: vsk → vrk → = Rs isk + ψ̇sk +jpωk ψsk = → Rr irk → → + ψ̇rk → (3) → +jp(ωk − ωR ) ψrk → (4) the voltage-fed induction machine are θR and δ = ρ − p ωR [12]. Note that the derivative of δ is known as slip frequency. If the stator current dynamics are much faster than the speed and flux dynamics a fast inner current control loop can be designed using only equations (10) and (11) and assuming the speed and flux as constants [17]. For the outer speed and flux control design the stator currents are treated as new control inputs and the system behaviour is described by equations (8), (9), (12) and (13). This system of lower order is also flat with ψrd and θR as flat outputs [16]. The benefits of using the property flatness for the design of the control algorithms will be shown in the section about FBC. The voltage source inverter (VSC) is approximated as a continuously three phase variable voltage source modelled by a PT1 -lag element with TV SC = 1/(2 fp ) as time constant, whereas fp is the switching frequency. Moreover all state variables are assumed to be measurable, the flux may be measured indirectly. The flux vectors may be expressed: ψsk = Ls isk +M irk (5) ψrk = Lr irk +M isk (6) → → → → → → In the following the so-called d/q-reference frame which is aligned to the rotor flux vector will be used and the superscript will be omitted, i.e. ψr = ψrd + j0 = ψrS e−jρ → → (7) whereas ρ is the rotor flux angle in the stationary reference frame. Substituting the stator flux and rotor current vectors in (3) and (4) using (5) and (6) and introducing σ = 1 − (M 2 /(Ls Lr )), γ = (M 2 Rr /σLs L2r ) + (Rs /σLs ), α = (Rr /Lr ) and β = (M/σLs Lr ) gives: dψrd = −α(ψrd − M isd ) dt is dρ = pωR + αM q dt ψrd (8) (9) i2s disd vs = −γisd + αβψrd + pωR isq + αM q + d (10) dt ψrd σLs is is vs disq = −γisq −βpωR ψrd −pωR isd −αM q d + q (11) dt ψrd σLs The mechanical dynamics may be described by dωR Tl = µψrd isq − (12) dt J dθR = ωR (13) dt whereas µ = (3pM/2JLr ) and θR is the rotor angle and Tl the load torque. Equations (8)-(13) completely describe the induction machine dynamics. In [12] the voltage-fed induction machine was shown to be a flat system. Such systems are characterized by the fact that all system states and control inputs may be expressed as functions only of the flat outputs and model parameters. In other words the trajectory of the vector consisting of the flat output determines the state trajectory and by that it determines the system behaviour. The flat outputs of III. F IELD -O RIENTED C ONTROL The principle of field orientation was already introduced in [2] and is nowadays the standard way of controlling electrical drives. As can be seen in (8) and (12) using the d/q-reference frame enables the decoupled control of ωR and ψrd by isq and isd , respectively, if a constant rotor flux is assumed. The basic control structure is shown in Fig. 1. Basically it consists of inner stator current control loops and outer speed and rotor flux PI-control loops. Assuming different time constants of the inner and outer loops they are designed separately. A. Current Control The current control design is done with (10) and (11) in which couplings between both current components can be seen. Therefore a decoupling network is used which gives vsd,Dec ∗ and vsq,Dec ∗ (neglecting the time constant TV SC ): i2sq ∗ (14) vsd,Dec = σLs −αβψrd − pωR isq − αM ψrd isq isd ∗ vsq,Dec = σLs βpωR ψrd + pωR isd + αM (15) ψrd In Fig. 1 the decoupling is combined with the transformation of the measured three-phase stator currents into the d/qreference frame for which in general informations about the speed and the rotor flux are necessary. Assuming perfect decoupling and using (14) with (10) and (15) with (11) yields the resulting current dynamics: vs disd = −γisd + d,c (16) dt σLs vs disq = −γisq + q,c (17) dt σLs These first order dynamics are controlled by PI-controllers, that is 1 vsd,c ∗ = kpd isd − i∗sd + isd − i∗sd dt (18) TId 1 ∗ ∗ vsq,c = kpq isq − isq + isq − i∗sq dt (19) TIq Smoothing * Filter ~ ` r,d ~ a R * `r,d * Speed & Flux Controller aR Current Controller * isd + - vsd,c * + * + - isq + - vsq,c vs,123 * + + vsd,Dec isq isd Fig. 1. VSC * + + * vsq,Dec * `r,d Induction Machine aR is,123 Coordinate transform +Decoupling Control structure of Field-Oriented control of an voltage-fed induction machine whereas the controller gains kpd , kpq , 1/TId and 1/TIq may be tuned by standard tuning methods [17] [18]. Here the technical optimum [17] is used and the VSC time constant taken into account to prevent unrealizable voltage references. See (34) in the appendix for the gains. B. Speed and Flux Control The speed and flux control is designed with (8) and (12) whereas the inner closed current loops are modelled as PT1 lag elements with time constants Tid and Tiq . Assuming ψrd = ψr∗d = const. both PI-controllers can be treated separately and are tuned with the symmetrical optimum [17]: 1 ∗ ∗ isd = kpψ ψrd − ψrd + ψrd − ψr∗d dt (20) TIψ 1 ∗ ∗ ∗ isq dt (21) = kpω ωR − ωR + ωR − ωR TIω For the controller gains see (35) in the appendix. According to [17] using first order filters with time contstants Tf,ψ = Tf,ω = 4 TV SC to smooth the references enhances the system behaviour. IV. F LATNESS - BASED C ONTROL The property of flatness [11] can effectively be used for designing control algorithms. In general the control structure consists of a feedforward and a feedback part [14]. Since flatness allows to formulate the control inputs as functions of only the flat outputs this can be used to design the feedforward even for nonlinear systems. Under ideal conditions the feedforward can track the (time varying) reference if it is smooth enough. Otherwise due to derivatives in the references and limitations in the control inputs tracking errors would appear. Even if the references are smooth enough deviations from perfect tracking will appear due to disturbances, model uncertainties and other perturbations. Therefore feedback is introduced. Since the system is near to the reference trajectory via feedforward, thus linearizable around it, the feedback can be designed with linear methods also for nonlinear systems [14]. Even though the voltage-fed induction machine is flat and the application of FBC to the complete, nonlinear model would be possible, here a cascade is used due to its simplicity and comparability to the FOC. The control structure used is shown in Fig. 2. A. Current Control As can be seen in Fig. 2 the current control consists also of a feedforward and a feedback. For the current control design ∗ and ψrd ≈ ψr∗d is assumed. At first the feedforward ωR ≈ ωR is determined using (10) and (11): i∗sq 2 di∗sd ∗ ∗ ∗ ∗ ∗ vsd,f = σLs +γisd −αβψrd −p ωR isq −αM ∗ (22) dt ψrd ∗ disq i∗s i∗s ∗ +γi∗sq+p ωR [βψr∗d+i∗sd ]+αM q ∗ d (23) vs∗q,f = σLs dt ψrd Combining (22) with (10) and (23) with (11) and neglecting TV SC yields the dynamics of the current tracking errors ∆isd = (isd − i∗sd ) and ∆isq = (isq − i∗sq ): vs d∆isd = −γ∆isd + d,c dt σLs vs d∆isq = −(γ + α)∆isq + q,c dt σLs (24) (25) These resulting dynamics for the tracking errors are controlled by PI-controllers: 1 ∆isd − ∆i∗sd dt (26) vs∗d,c= Vpd ∆isd − ∆i∗sd + τId 1 vs∗q,c= Vpq ∆isq − ∆i∗sq + ∆isq − ∆i∗sq dt (27) τIq whereas the controller gains Vpd , Vpq , 1/τId and 1/τIq are also tuned with technical optimum [17] and may be found in (36) in the appendix. Note that in contrast to FOC the controllers do not have to perform tracking tasks since ∆i∗sd = ∆i∗sq = 0 (tracking is done by feedforward). The controller gains should therefore be tunable to be more robust against perturbations as with FOC (not investigated here). As already mentioned the feedforward is only effective if the references are smooth enough. Thus the reference trajectory generation is very important for the FBC. In this paper besides a first order filter a rate limiter with different settings is * Current Feedforward ~ * r,d ~ a R Voltage Feedforward * isd,f * ` * i sq,f `r,d Reference Generation * isd,c + + - aR + * * isq,c + + - + ~* isd * isd + Reference ~ * Gener- * isq isq + ation Speed & Flux Controller vsd,c vsq,c - * vsd,f * vs,123 * + + `r,d Induction Machine VSC * + + Current Controller isq isd Fig. 2. vsq,f aR is,123 Coordinate transform Cascaded control structure of Flatness-based control of an voltage-fed induction machine investigated by simulations (see section V). Additionally, it would be desirable to include the limitations in the control inputs directly into the reference trajectory generation [19]. B. Speed and Flux Control For the design of the outer speed and flux loop the stator currents are treated as new control inputs that will be realized by the inner loops delayed by τid and τiq . From (8) and (12) the feedforward can be determined (load torque Tl unknown): 1 dψr∗d 1 + ψr∗d i∗sd,f = (28) M α dt ∗ 1 dωR (29) i∗sq,f = µψr∗d dt Assuming the system near to the reference trajectory, neglecting the delays caused by the inner current loops and putting together (28) with (8) and (29) with (12) the resulting dynamics of the tracking errors ∆ψrd = ψrd − ψr∗d and ∗ may be expressed: ∆ωR = ωR − ωR d∆ψrd = −α∆ψrd + αM i∗sd,c (30) dt Tl d∆ωR = − + µψr∗d i∗sq,c (31) dt J As feedback PI-controllers are used only to eliminate the tracking error caused by disturbances and other nonidealities which are also tuned with symmetrical optimum [17]: 1 ∗ ∗ isd,c= Vψ ∆ψrd − ∆ψsd + ∆ψrd − ∆ψs∗d dt (32) τψ 1 ∗ ∗ i∗sq,c= Vω ∆ωR − ∆ωR + dt (33) ∆ωR − ∆ωR τω See (37) in the appendix for the controller gains. Similar to the Flatness-based current control smooth references should be used and therefore a reference trajectory generation should be implemented. Here first order filter and rate limiter with different settings are investigated by the simulations (see section V). V. S IMULATIONS The analysis of the designed FOC and FBR has been carried out with simulations by means of Matlab/Simulink. The system model was implemented according to section II (see TABLE I for system data). To prevent stability problems caused by saturation effects and possible wind up of the controller integrators an anti-wind up mechanism [18] was used. Furthermore the stator current components were limited to the nominal stator current. If not stated different the inertia from TABLE I is used. A. Current Control In a first step the FBC is analyzed with different reference generation types and settings. The current control simulations are performed with constant speed (ωR = 153, 20rad/s). Fig. 3 shows step responses with FBC using different first order reference smoothing filters (τf,q ) and with FOC also using different smoothing filters (Tf,q ). Both control schemes behave similar if the references are either very fast or slow. In the first case the feedforward terms of FBC containing derivatives are too high (for a short time) and therefore limited due to inverter voltage limitations and by that are less effective. In the second case these terms are small and the feedback controller is also able to track the smoothed reference. But in between the feedforward is effectively contributing to the tracking since no limitations appear. Analytical investigations on the determination of the reference generation should further enhance the FBC. In the following τf,q = TV SC and Tf,q = 0 are used. The FBC and FOC with the selected filter settings are compared with FBC using a rate limiter as reference generation block that transforms a step into a ramp for example. Fig. 4 shows the comparison whereas the max. slope of the rate limiter was tuned to give the fastest response. The faster responses to a step in i∗sq of FBC (with both filters) becomes apparent, but at the same time slightly higher osscillations occur. The coupling between the d- and q-components of the stator current are somewhat higher with FOC. Altogether, both control methods yield comparable results in current control. In the following only the flatness-based current control with first order reference smoothing will be investigated. B. Speed and Flux Control The design of the outer loops was performed with the approximation of the inner loops as P T1 -lag elements (FOC: FBC FBC 21 FOC 820 820 800 800 18 17 Reference τf,q = 0 16 τf,q = TVSC ωR/min−1 isq/A 19 τf,q = 4 TVSC 15 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 ωR/min−1 20 780 780 760 760 740 740 reference Tiq = 0.2 ms Tiq = 0.3 ms Tiq = 0.4 ms 2 FOC 0 21 2 4 6 8 10 0 2 4 6 8 10 20 30 30 20 20 17 Reference Tf,q = 0 16 T 10 10 f,q =T VSC Tf,q = 4 TVSC 15 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 t/ms Fig. 3. Current control with different first order reference filters isq/A 18 isq/A isq/A 19 0 0 −10 −10 −20 −20 −30 0 2 4 6 8 −30 10 Tiq = 0.2 ms Tiq = 0.3 ms Tiq = 0.4 ms 0 2 4 t/ms 21 11 Reference FBC (first order) FBC (rate limiter) FOC 20 10.9 19 10.8 i /A 17 sd isq/A 18 6 8 10 t/ms Fig. 5. Step response to a filtered speed reference step using different approximations of the inner current loops (Tf,ω = 4 Tiq ; τf,ω = 4 τiq ) 10.7 16 10.6 15 FBC 14 3 0 600 200 0 800 800 3 FBC (first order) FBC (rate limiter) FOC 400 usd/V 780 760 reference τ =2τ 740 τf,ω = 3 τiq f,ω 720 iq τf,ω = 4 τiq 200 0 1 2 3 780 760 reference Tf,ω = 2 Tiq 740 Tf,ω = 3 Tiq Tf,ω = 4 Tiq 720 4 0 1 2 3 4 0 2 3 −200 0 1 2 3 30 30 20 20 10 10 t/ms Fig. 4. Current control: FBC (first order reference filter τf,q = TV SC ), FBC (rate limiter, max. slope: 30A/1ms) and FOC without filter isq/A t/ms 0 τf,ω = 2 τiq −10 τ f,ω −20 −30 iq τf,ω = 4 τiq 0 1 2 t/ms 3 T −20 Tf,ω = 3 Tiq −30 4 f,ω =2T iq Tf,ω = 4 Tiq 0 1 2 t/ms 3 4 Fig. 6. Step response to a speed reference step using different smoothing filters (J = 100 kg cm2 ; Tiq = 0.3 ms ; τiq = 0.2 ms) 820 800 800 780 760 reference τ =2τ 740 τf,ω = 3 τiq 720 sq FOC 820 f,ω iq τf,ω = 4 τiq 0 1 2 3 780 760 reference Tf,ω = 2 Tiq 740 Tf,ω = 3 Tiq Tf,ω = 4 Tiq 720 4 30 30 20 20 10 10 isq/A ωR/min−1 FBC i /A Tid and Tiq ; FBC: τid and τiq ). Fig. 5 shows the response to a speed reference step, filtered with Tf,ω = 4 Tiq (FOC) and τf,ω = 4 τiq (FBC), respectively, using different approximations. If the inner loop is modelled to fast there arise considerable oscillations in the speed controlled with FOC while the FBC still works properly. In the following analysis Tiq = 0.3 ms and τiq = 0.2 ms is used. The influences of the smoothing filter time contants on the speed tracking behaviour are shown in Fig. 6 and Fig. 7 using different inertias. In Fig. 6 the behaviour of FBC is only slightly better than that of FOC. Using a machine with a reduced inertia (Fig. 7) shows that the FBC slightly outperforms the FOC under this conditions. It gives a higher dynamic behaviour. Fig. 8 shows the behaviour of FBC and FOC responding to speed reference steps (Tf,ω = 2 Tiq ; τf,ω = 3 τiq ) and load torque steps. At t = 10 ms a load torque Tl = Tn /2 is applied to the rotor and t = 20 ms the load torque is reduced to Tl = 0 Nm. Both control schemes properly eliminate the speed tracking error. =3τ 0 −10 R 1 ω /min−1 0 sq usq/V 2 600 400 −200 1 820 R 2 820 ω /min−1 1 ωR/min−1 0 i /A 13 FOC 10.5 0 τf,ω = 2 τiq −10 τ f,ω −20 −30 =3τ iq τf,ω = 4 τiq 0 1 2 t/ms 3 4 0 1 2 3 4 0 −10 T −20 Tf,ω = 3 Tiq −30 f,ω =2T iq Tf,ω = 4 Tiq 0 1 2 t/ms 3 4 VI. C ONCLUSION In this paper a rather new nonlinear control method called flatness-based control was applied to the induction machine Fig. 7. Step response to a speed reference step using different smoothing filters and reduced inertia (J = 50 kg cm2 ; Tiq = 0.3 ms ; τiq = 0.2 ms) Parameter Stator resistance Rs Rotor resistance Rr Mutual inductance M Stator inductance Ls Rotor inductance Lr Pole pair p Mechanical power Ps Stator phase voltage Vs Torque Tn Stator frequency ωs Rotor speed ωR Power factor cos(φ) Inertia J Switching frequency fp Voltage limit 850 R ω /min−1 800 750 reference FBC FOC 700 0 5 10 15 0 5 10 15 20 25 30 35 25 30 35 30 20 sq i /A 10 0 −10 −20 −30 FBC FOC 20 t/ms Fig. 8. Response to speed reference steps and load torque steps (from t = 10 ms till t = 20 ms: Tl = Tn /2; ) fed by a voltage source converter. Due its simplicity and comparability to the conventional field-oriented control a cascaded control structure was used. Here the flatness-based control was used for the inner current and outer flux and speed loops. The control design and tuning was presented. A simulation study concerning the influence of different control subsystems and parameters was carried out. The field-oriented control was included for comparison issues. The flatness-based control was shown to be always competitive to the conventional field-oriented control, actually outperforms it slightly for special cases like small inertias. Designing a flatness-based control without using the cascade structure is possible due to the flatness of the complete machine model and could further enhance the system behaviour. Especially for small inertias this could be a promising possibility. Further investigations should be done on designing the reference trajectory generation. To exploit the possibilities the control input limmitations should be directly taken into account. Additionly, a different tuning of the feedback parts could offer advantages in perturbation rejection and robustness. A PPENDIX Controller gains (FOC): = kpq = TIq kpd TId kpψ = 1+(α Tid )2 α2 M Tid ; kpω = = = TIψ = 1 2Tiq µ ; σLs 2TV SC 2 TV SC γσ Ls 1+(α Tid )2 kpψ (1+α Tid )3 (34) 4Tid 2 TIω = 8Tiq µ (35) Controller gains (FBC): Vpd = Vpq = τId = Vψ = 2 TV SC γσ Ls 1+(α τid )2 α2 M τid ; Vω = 1 2τiq µ ; σLs 2TV SC ; τIq = 2 TV SC (γ+α)σ Ls 1 + (α τid )2 τψ = 4τid Vψ (1 + α τid )3 2 τω = 8τiq µ (36) (37) Value 0.399 Ω 0.45 Ω 0.1585 H 0.1616 H 0.1676 H 2 22 kW (rated) 230 V (rated) 143 Nm (rated) 2π 50Hz(rated) 153,20 rad/s (rated) 0,869 (rated) 100 kg cm2 5 kHz 565 V TABLE I N OTATION AND S YSTEM DATA R EFERENCES [1] Blaabjerg, F. and Thoegersen, P., “Adjustable speed drives - future challenges and applications,” in International Power Electronics and Motion Control Conference, vol. 1, 2004, pp. 36–45. [2] Blaschke, F., “Das Verfahren der Feldorientierung zur Regelung der Drehfeldmaschine,” Dissertation, TU Braunschweig, 1974. [3] Leonhard, W., Control of electrical drives, 3rd ed. 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