The aim of this paper is to realize robust network motion control system. One of the most significant issues for network motion control realization is time delay during transmission. Obviously, most time delay systems are unstable. Additionally, time delay may cause control performance degradation in motion control system. In this paper, a modified disturbance observer with Smith predictor is proposed to compensate time delay. This modification realizes a robust two-degree-of-freedom control system ower network. The serial compensation of Smith predictor is utilized to compensate time delay. The validity of the proposed method is confirmed by the position control of linear motor over network experimentally. Experimental results show that the proposed control system has robustness against transmission delay fluctuation over network and external force.Cilj je ovoga članka realizacija sustava upravljanja gibanjem preko komunikacijske mreže. Jedan od najvećih problema u realizaciji sustava upravljanja gibanjem preko komunikacijske mreže jest kašnjenje u prijenosu podataka. To kašnjenje dovodi do narušavanja svojstava sustava upravljanja gibanjem, a može ga učiniti i nestabilnim. Za kompenzaciju kašnjenja u radu se predlaže modificirani rekonstruktor poremećaja zasnovan na Smithovu prediktoru. Predloženi rekonstruktor omogućuje izvedbu robusnog sustava upravljanja gibanjem s dva stupnja slobode. Valjanost predloženog sustava potvrđena je eksperimentalno na sustavu upravljanja pozicijom linearnog motora preko komunikacijske mreže. Dobiveni rezultati pokazuju robusno vladanje predloženog sustava u uvjetima promjenjivog kašnjenja prijenosa podataka preko mreže i pri djelovanju vanjske sile

Abdul Muis

Atsushi Kato

Kouhei Ohnishi

English

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ISSN 0005–1144ATKAAF 47(1–2), 5–10 (2006)Atsushi Kato, Abdul Muis, Kouhei OhnishiRobust Network Motion Control System Based on Disturbance ObserverUDK 621.398:621.313IFAC 4.3.2;5.8.6Original scientific paperThe aim of this paper is to realize robust network motion control system. One of the most significant issuesfor network motion control realization is time delay during transmission. Obviously, most time delay systems areunstable. Additionally, time delay may cause control performance degradation in motion control system. In thispaper, a modified disturbance observer with Smith predictor is proposed to compensate time delay. This modifi-cation realizes a robust two-degree-of-freedom control system ower network. The serial compensation of Smithpredictor is utilized to compensate time delay. The validity of the proposed method is confirmed by the positioncontrol of linear motor over network experimentally. Experimental results show that the proposed control systemhas robustness against transmission delay fluctuation over network and external force.Key words: motion control, network control systems, observers, linear motors1 INTRODUCTIONMotion control technology is very useful and es-sential for industrial application. Particularly, thistechnology is a key technology for mechatronicsand robotics. In addition, motion control realizesprecise positioning control and vibration control inmanufacturing system.Generally, the purpose of motion control is toimprove motion quality. Various motions, such aspositioning and pushing, are realized by motioncontrol. The criterion of motion quality is repre-sented as control stiffness that changes based onthe task reference. Basically, the realization of ro-bust control requires high control stiffness. How-ever, the robustness and control stiffness can be setby controling the acceleration independently. Hence,the control of acceleration will keep robustness andrealize various motion according to the task refe-rence [1].Most motion control system has been applied toclosed environment system such as industrial appli-cation. Recently, welfare and medical fields requiremotion control technology where open and remoteenvironment system such as mobility system andnetwork system applied [2]. The aim of this paperis to realize motion control system in open envi-ronment or remote environment system over net-work. In these environments, one of the most signi-ficant issues for the realization of motion control istime delay on transmission. Obviosly, in closed-loopcontrol system,  most of time delay systems are un-stable. Additionaly, time dalay may cause controlperformance degradation. Particularly, a slight timedelay may lead to critical problem in motion con-trol.Recently, network control system has receivedtremendous interest in the last decade. Especially,teleoperation control system as an example of net-work control system has been utilized in practice[3, 4]. Here, data communication in network con-trol system realizes open and remote control sys-tem. However, data communication over networkcauses time delay, and data loss. Especially, closedcontrol system over network is unstable due to timedelay. Hence, the most considering technical issueof network control system is to keep its stability.Smith predictor [5] is a well known technique tocompensate time delay and to keep stability. Smithpredictor removes the impact of time delay fromclosed control system through the prediction ofcontrolled object’s output. However, it has beenshown earlier [6, 7] that Smith predictor could notmake the steady error to zero due to step distur-bance. To overcome this problem, a modified Smithpredictor has been suggested by Astron, et al. [8].A subsequent article by Zhang, et al. [9] has pro-posed a modified Smith predictor with two-degree--of-freedom control system where the disturbanceresponse is decoupled from the setpoint response.However, the application of these methods is usedonly for process control with time delay. It can notbe adjusted to motion control system directly.AUTOMATIKA 47(2006) 1–2, 5–10 5kato.qxd  30.5.2006  11:22  Page 5Generally, a disturbance observer is usually adop-ted to realize robust acceleration control system[10, 11]. Disturbance observer estimatees and com-pensates disturbance force by the comparing con-troller’s output and controlled object’s output. Di-sturbance observer is one of the two-degree-of-free-dom control system with rejection to disturbanceforce. However, it can not be applied directly totime delay system, since it requires an inverse mo-del of controlled objects.In this paper, disturbance observer based on net-work motion control system is proposed to controltime delay system such as network system. Here, adisturbance force is estimated by modified distur-bance observer based on minimum phase systemwithout time delay element. Hence, the modifieddisturbance observer realizes a rejection of distur-bance force. Moreover, in the proposed system, theserial compensation of Smith predictor to controlleris adapted to keep stability. By using this method,it is possible to realize the rejection of disturbanceforce while keeping the stability.To confirm the validity of the proposed controlsystem, the realization of motion control with net-work is performed experimently. Real-time datacommunication is applied over local area network.In the experiments, this control scheme is appliedto linear motor positioning control, and the effec-tiveness is clarified.2 NETWORK CONTROLLED OBJECT MODELIn this section, the network motion control sys-tem is described. Figure 1 shows the framework ofnetwork motion control system. The network repre-sented by local area network, links the controllerand the controlled object such as a linear motor.The controller’s output and the controlled object’soutput is transmitted over the network.Fm generated force by linear motor, NM thrust rod mass, kgx position, mia current input, AKt thrust constant, N/AThe representative of linear motor’s load forcesis friction force and external force. The linear mo-tor generates the thrust force proportionately ascurrent increases. Generally, the actuator current iacorresponds to the current reference due to thecurrent minor loop which is usually equipped with.The current reference value and position in-formation x are transmitted over network. However, time delay and lack of information such as packetloss exists due to network transmission. These in-formations can not be transmitted over network ac-curately. Hence, the network transmission modelsare represented as follows.¯(3)(4)where T0 , T1 are time delay over network, and d0,d1 are information error due to lack of information.Consequently, the controlled object with networkis represented as Figure 2.0101sTref refa asTi e i dx e x d−−= −= −refairefai6 AUTOMATIKA 47(2006) 1–2, 5–10Robust Network Motion Control System... A. Kato, A. Muis, K. OhnishiFig. 1 Network motion control systemFig. 2 Network controlled object modelThe mechanical dynamics of a linear motor isexpressed as follows.(1)(2)whereFl load force, Nl mm t aM x F FF K i+ ==3 PROPOSED CONTROL SYSTEMA. Modified Disturbance observerAt first, this section describes the realization ofrobust motion control. Recently, a disturbance ob-server is used to realize motion control. The con-trol system based on disturbance observer is shownin Figure 3. Here, r is command input, u is thecontroller’s output, y is controlled object’s output,d′ is the load force, and ξ is the observation noise.Subscription »n« means nominal model. A low-passfilter is adopted as Q(s) to the reduce observationnoise. In this structure, disturbance observer esti-mates and compensates the disturbance force. Thedisturbance force including the difference betweenreal and nominal model is defined as follows.(5)1 1.nd d yP P⎛ ⎞′≡ − −⎜ ⎟⎝ ⎠kato.qxd  30.5.2006  11:22  Page 6In Figure 3, the command input response Gry(s)and sensitivity function S(s) are represented as fol-lows.(6)(7)(8)(9)Having disturbance observer, the command inputresponse and the disturbance response can be set in-dependently from each other. Hence, two-degree--of-freedom control system is realized by distur-bance observer. However, disturbance observer cannot be adapted to time delay system since it requi-res the inverse controlled object’s model.In this paper, a modified disturbance observer isproposed to adapt the network controlled object(Figure 2). The modified disturbance observer is re-presented as Figure 4. Here, the controlled object,the disturbance force and the observation noise aredefined as follows respectively.(10)(11)(12)1 11( ) .n nsT sTsTe d e e xξ− −−≡ + − −[0 0021( )( ) ( )ndis ttnsT sT reft tn a nd F K dKK e K e i M M s x− −≡ + +⎤+ − − −⎦2( )tnnnKP sM s=11( )( )( )( )( )( ).( ) ( )ryrynnG sG sS sP sP sQ sC s P s≡−=+¶¶1( )( )( )( ) ( )( ) ( )rynny sG sr sC s P sC s P s==+This paper assumes that the fluctuation of trans-mission delay over network (jitter) is included inthe disturbance force and in the observation noiseas well.In Figure 4, the command input response Gry(s)and sensitivity function S(s) are represented as fol-lows respectively.(13)(14)The modified disturbance observer also enablesthe command input response and sensitivity func-tion to be set independently. Therefore, it is possi-ble to reject disturbance force and realize robustacceleration control system. However, modified dis-turbance observer is likely to be unstable due totime delay. Therefore, the recovery method is de-scribed in the next section to compensate the trans-mission delay.B. The compensation of Smith predictorSmith predictor, a common method to compen-sate time delay, is shown in Figure 5. It can remo-ve the effect of time delay that causes instability.Hence, this paper considers to the modified distur-bance observer to recover the stability.Figure 6 shows the proposed control system. Theserial compensation of Smith predictor is adaptedto modified disturbance observer to keep its stabi-lity. In Figure 6, the command input responseGry(s) and sensitivity function S(s) are representedas follows respectively.11( )( ) .( ) ( )nnsTsTnQ s eS sC s P s e−−−=+01( ) ( )( )( ) ( )nsTnry sTnC s P s eG sC s P s e−−=+AUTOMATIKA 47(2006) 1–2, 5–10 7A. Kato, A. Muis, K. Ohnishi Robust Network Motion Control System...Fig. 3 Disturbance observerFig. 4 Modified disturbance observerkato.qxd  30.5.2006  11:22  Page 78 AUTOMATIKA 47(2006) 1–2, 5–10Robust Network Motion Control System... A. Kato, A. Muis, K. OhnishiFig. 5 Smith predictorFig. 6 Proposed control system4 EXPERIMENT RESULTSThe proposed control system is applied to net-work system to confirm the validity of the proposedcontrol system. In the experiments, two PCs (Pen-tium 4: 2.8 GHz) are used at 0.1 ms sampling time.The overview of experiment system is shown in Fi-gure 7. The network links the controller and a line-ar motor as the controlled object. The linear motoris shown in Figure 8. Table 1 shows the linear mo-tor parameters. The linear motor is controlled bythe motor driver. Its position is estimated by theposition encoder with 0.1 µm resolution.Fig. 7 Network linear motor motion control system for experimentsTable 1 Linear motor parametersKt thrust constant, N/A 22.0M mass, kg 0.2Fig. 8 Experimental systemIt is well-known that hard real time system is re-quired to realize motion control. RT Linux whichis the hard real time extension for Linux is utilizedto realize hard real time system in this experimentsystem. Here, network motion control system invol-ves real time data communication. In this paper,the real time communication network with RT-Soc-ket is utilized. RT-Socket is a real time communica-tion package included in RT Linux and is based onUDP/IP protocol. Thus, it is possible to realize realtime network system. The performance of the realtime communication based on RT-socket was evalu-ated by [12]. The real time network system of thisexperiment system transmits and receives transmis-sion data at 0.1 ms sampling time respectively.(15)(16)where, Tn is nominal round trip time, and theround trip time means total transmission delaywhich is expressed as T = T0 + T1.Hence, the proposed control system is a two-de-gree-of-freedom control system. The characteristicof the command input response is determined byC(s) regardless of sensitivity function and time de-lay. Where C(s) can be set at a desirable commandinput response.The rejection of disturbance is expressed as sen-sitivity function. Hence, the rejection of disturbanceis determined by round trip time and Q(s). SinceQ(s) is low pass filter,(17)Consequently, the proposed control system willmake the steady error to zero due to step distur-bance. Additionally, the compensation with Smithpredictor removes the impact of time delay fromclosed control system through the prediction ofcontrolled object’s output. Hence, the compensa-tion can make it stable.00lim ( ) .sS s→→1 1 11( ) ( )( )( ( ) )( )( ) ( )n nsT sTnnC s P s e Q s eS sC s P s− −+ − −=+01( ) ( )( )( ) ( )sTnrynC s P sG s eC s P s−=+kato.qxd  30.5.2006  11:22  Page 8AUTOMATIKA 47(2006) 1–2, 5–10 9A. Kato, A. Muis, K. Ohnishi Robust Network Motion Control System...In the experiments, PD control system is adap-ted to C(s) which determines the command inputresponse of the proposed system. In the PD con-trol system, a one order low pass filter is adoptedto the approximate differentiation as noise reduc-tion. A second order low pass filter is adopted asQ(s). Table 2 shows control parameters of the pro-posed control system.(18)(19)2( )( ) .vp vvddsgC s K Ks ggQ ss g= ++⎛ ⎞= ⎜ ⎟+⎝ ⎠Table 2 Controller parametersPosition gain Kp, rad/s 1440Velocity gain Kv, rad/s 120Differential gain gv, rad/s 100Observer gain gd, rad/s 100Nominal time delay Tn, ms 10sion delay which includes time delay, jitter, andpacket loss. 2000 packets were transmitted fromcontrolled object’s PC and the controller’s PC re-ceived 629 packets for 0.2 s. Here, 68.6 % packetwas lost. Moreover, Figure 9(b) shows round triptime and jitter. However, the response convergedto the step command in Figure 9(a). The steadyerror is 0.05 mm and the settling time is 0.135 ms. The position response is shown in Figure 10(a),where the external force is applied to the linearmotor in position control. Applied external force isdenoted in Figure 10(a). Figure 10(b) shows trans-mission delay with jitter. The control parameters isthe same as the previous experiment. Figure 10 re-veals that proposed control system has the robust-ness to external force.Fig. 9 Experimental result of step response under jitterFig. 10 Experimental result of applied external forceFinally, robustness to parameter mismatch is in-vestigated. It is assumed that Mn = M/3.0 in this ex-periment. The position response is shown in theFigure 11. Figure 11(a) shows that the position re-sponse is converged to the command in spite ofthe degradation of control performance. Hence, itis revealed that proposed control system has therobustness to parameter mismatch.Figure 9 shows the experimental result of stepresponse. It describes the robustness to transmis-kato.qxd  30.5.2006  11:22  Page 9AUTHORS’ ADDRESSESAtsushi KatoAbdul MuisKouhei OhnishiKeio University, Yokohama, Japane-mail: atsushi@sum.sd.keio.ac.jpmuis@sum.sd.keio.ac.jpohnishi@sd.keio.ac.jpReceived: 2006-03-3010 AUTOMATIKA 47(2006) 1–2, 5–10Robust Network Motion Control System... A. Kato, A. Muis, K. Ohnishi5 CONCLUSIONThis paper presents network motion controls sys-tem. The technical issue to realize this system istime delay on transmission. Hence, this paper pro-posed modified disturbance observer with Smithpredictor to compensate time delay. The proposedmethod is two-degree-of-freedom control system.The command input response and the disturbanceresponse can be set independently from each other.In addition, the robust stability of the proposedcontrol system is confirmed experimentally. The ex-perimental results show the robustness against trans-Fig. 11 Experimental result of step response under mass mismatchmission delay, which includes jitter and packet loss,external force and the mismatch of linear motormodel. Consequently, this paper shows a method torealize network motion control system.REFERENCES[1] K. Ohnishi, M. Shibata, T. Murakami, Motion Control forAdvanced Mechtronics, IEEE/ASME Trans. on Mechatro-nics, 1(1), pp 125–247, 1996.[2] M. Morisawa, K. Ohnishi, Motion Control Taking Environ-mental Information into Account. EPE J. 12(4), pp 37–41,2002.[3] R. J. Abderson, M.W. Spong, Bilateral Control of Teleope-rators with Time Delay. IEEE Trans. on Aut. Cont., 34(5),pp 494–501, 1989.[4] Y. Yokokohji, T. Imaida, T. Yoshikawa, Bilateral Teleope-ration under Time-varying Communication Delay. Proc.IEEE/RSJ Int. Conf. on Inteleigent Robotics and System,pp 1854–1859, 1999.[5] O. J. Smith, A Controller to Overcome Dead-time. ISA. J.6(2), pp 28–33, 1959.[6] K. Watanabe, M. Ito, A Process-model Control for LinearSystem with Delay. IEEE Trans. on Aut. Cont., 26(6), pp1261–1269, 1981.[7] N. Abe, N. Yamanaka, Smith Predictor Control and Inter-nal Model Control – a Tutorial. SICE 2003 Annual Conf.Vol. 2, pp 1383–1387, 2003.[8] K. J. Astrom, C. C. Hang, B. C. Lim, A New Smith Predictorfor Controlling a Process with an Integrator and LongDead-time. IEEE Tran. on Auto. Cont. 39(2), pp 343–345,1994.[9] W. Zhang, Y. Sun, X. Xu. Two Degree-of-freedom SmithPredictor for Processes with Time Delay. Automatica.34(10), pp 1279–1282, 1998.[10] T. Umeno, Y. Hori, Robust Speed Control of dc Servomo-tors Using Modern Two Degrees-of-freedom ControllerDesign. IEEE Trans. on Ind. Electron., 38(5), pp 363–368,1991.[11] Y. Fujimoto, A. Kawamura, Robust Servo-system Based onTwo-degree-of-freedom Control with Sliding Mode. IEEETrans. on Ind. Electron., 42(3), pp 272–280, 1995.[12] A. Muis, A. Kato, T. Tsuji, K. Ohnishi, RTLinux Based Real--time Control Cooperation through Internet Protocol.MechRobo 2004., pp 117–122, 2004.Robusni sustav upravljanja gibanjem preko komunikacijske mre`e zasnovan na rekonstrukciji poreme}aja. Ciljje ovoga ~lanka realizacija sustava upravljanja gibanjem preko komunikacijske mre`e. Jedan od najve}ih problemau realizaciji sustava upravljanja gibanjem preko komunikacijske mre`e jest ka{njenje u prijenosu podataka. Toka{njenje dovodi do naru{avanja svojstava sustava upravljanja gibanjem, a mo`e ga u~initi i nestabilnim. Za kom-penzaciju ka{njenja u radu se predla`e modificirani rekonstruktor poreme}aja zasnovan na Smithovu prediktoru.Predlo`eni rekonstruktor omogu}uje izvedbu robusnog sustava upravljanja gibanjem s dva stupnja slobode. Valja-nost predlo`enog sustava potvr|ena je eksperimentalno na sustavu upravljanja pozicijom linearnog motora prekokomunikacijske mre`e. Dobiveni rezultati pokazuju robusno vladanje predlo`enog sustava u uvjetima promjenjivogka{njenja prijenosa podataka preko mre`e i pri djelovanju vanjske sile.Klju~ne rije~i: upravljanje gibanjem, sustavi upravljanja preko mre`e, rekonstrukcija signala, linearni motorikato.qxd  30.5.2006  11:22  Page 10

Robusni sustav upravljanja gibanjem preko komunikacijske mreže zasnovan na rekonstrukciji poremećaja

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