Multi-DOF Motion Control System Design and Realization Based on EtherCAT

Transcription

1 6 Sixth Iteratioal Coferece o Istrumetatio & Measuremet, Computer, Commuicatio ad Cotrol Multi-DOF Motio Cotrol System Desig ad Realizatio Based o EtherCAT Jie Ma Cotrol ad Simulatio Ceter Harbi Istitute of Techology Harbi, Chia majie@hit.edu.c Baoxiag Xig Cotrol ad Simulatio Ceter Harbi Istitute of Techology Harbi, Chia 6448@hit.edu.c Sogli Che Cotrol ad Simulatio Ceter Harbi Istitute of Techology Harbi, Chia sogli@hit.edu.c Abstract EtherCAT (Etheret for Cotrol Automatio Techology) is gaiig wide spread popularity i the automatio idustry as a real time field bus based o low cost. This paper describes desig ad realizatio for a multi degree of freedom (Multi-DOF) motio cotrol system icludig ie rotatio or rectiliear motios where EtherCAT real-time Etheret was selected as the commuicatio bus. The commuicatio model, master ad slave of EtherCAT is provided. The multi-loop cotroller desig for the sigle rotatio motio is described i detail. Fially, some experimet results idicates the effectiveess ad techical beefits applyig the desig ad realizatio methods i this paper. eywords EtherCAT; Motio Cotrol System; Multiple Loop; Parameter Desig I. INTRODUCTION With the rad developmet of sciece techology ad social demad, idustrial cotrol systems are develog i the directio of digitalizatio, itelliget ad etworked []. Ad the multi degree of freedom (Multi-DOF) motio cotrol system is expected to have the ability of highly sychroized actuatio ad below millisecod scale cotrol periodic []. Tycally, a cycle time less tha ms ad a sychroizatio time i micro-secod level is eeded. However, the traditioal Etheret techology has bee uable to meet the requiremets of the idustrial cotrol system for real-time performace ad the sychroizatio performace due to its o-predictable medium access mechaism. Besides, the classical motio cotrol system usually adopt the dedicated aalog liks for the coectio of the motio cotroller ad the drives, ad the D/A coverter is coected to the PC by ISA or PCI iterface, ad its structure is show i Fig. [3]. But the motio cotrol system based o aalog liks have a series of drawbacks: aalog liks are susceptible to exteral oise, besides, the complex ad bulky aalog cablig is a great limit for the trasmissio distace ad the scalability [4]. Thus, the aalog lik-based motio cotrol system is o loger proper for the implemetatio of Multi-DOF motio cotrol system [5]. I recet years, with the perfectio ad stadardizatio of the real-time Etheret for idustrial automatio, ad military machiery, the motio cotrol system based o real-time Etheret are gaiig icreasig iterest, particularly, the motio system based o EtherCAT [6]. EtherCAT, a ope Etheret protocol offerig very high real-time performace ad This research work is partially supported by the Natioal Natural Sciece Foudatio of Chia (Grat No. 6333, 64789). origially preseted by Beckhoff, has obtaied the widespread attetio recetly. The EtherCAT offers may advatages over the traditioal Etheret ad the aalog liks. Firstly, owig to the high trasmissio speed, the cycle time has bee sigificatly reduced. Secodly, the sychroized clock mechaism makes it possible to desig the highly sychroized motio cotrol system over umbers of drives. Thirdly, the low developmet cost ad high reliability ca greatly cotribute to the competitiveess of EtherCAT. Computer r e Cotroller D/A r Load Sigal Processig Board Permaet maget sychroous motor system Ecoder Fig.. Classical motio cotrol system structure Although the beefits of the EtherCAT-based motio cotrol system, however, the study i Chia is still i its ifacy. Referece [7] has proposed a master statio scheme based o DSP ad PowerPC, but the operatio ability ad storage space is limited ad it is difficult to upgrade ad exted the product. Referece [8] has put forward a kid of implemetatio method of the EtherCAT cotrol system based o STM3 processor, it takes millisecods for the EtherCAT message from sedig to receivig, which ca t satisfy the real-time requiremet of idustrial motio cotrol system. Ad referece [9] ad referece [] oly have preseted the scheme for a uiaxial or biaxial motio cotrol system. I view of the problems metioed above, this paper has preseted the implemetatio ad aalysis of the EtherCATbased Multi-DOF motio cotrol system. Ad this paper is orgaized as follows. Sectio II itroduces the backgroud to the EtherCAT. Sectio III presets the structure of EtherCAT- Based Multi-DOF motio cotrol system. Ad i sectio IV, the cocrete implemetatio scheme of the Multi-DOF motio cotrol system is clarified. Sectio V describes the multi-loop cotroller desig for the sigle rotatio motio i detail. Ad fially i sectio VI, some experimets have bee carried out ad the results have verified the effectiveess ad techical beefits applyig the desig ad realizatio methods i this paper /6 $3. 6 IEEE DOI.9/IMCCC

2 II. ETHERCAT COMMUNICATION MODEL EtherCAT makes full use of the full-duplex characteristic of the Etheret. The EtherCAT commuicatio model is realized as a master/slave relatioship. Ad Fig. shows the operatio priciple for the orgaizatio of a tycal EtherCAT etwork cosistig of a master ad umbers of slaves. The master is resposible for iitiatig data trasferr to the slave odes. Ad the commuicatio process is elucidated as follows. At first, the master iitiates the packet frame ad sed it to the salves. Afterwards, the slave extract iput data from the frame ad isert output data ito the frame, ad the the frame is retrasmitted to the ext. Fially, the frame is trasferred back to the master util the last slave has processed the frame. Timig of data trasfers is cotrolled etirely by the master. For this reaso, the master s computig platform should be capable of determiistic, real-time performace to esure lowjitter cyclical trasfers. EtherCAT Header EtherCAT Master Slave Iput Data RX TX ESC proc I/O Termial Slave Data TX RX RX TX Slave i Iput Data ESC proc Servo Slave i Data TX RX RX TX Slave Iput Data ESC proc Slave Slave i Slave Fig.. EtherCAT commuicatio model Servo Slave Data III. MULTI-DOF MOTION CONTROL SYSTEM BASED ON ETHERCAT As we see, Fig. 3 gives out the whole structure diagram of the Multi-DOF motio cotrol system based o EtherCAT. EtherCAT Master Network Cable AD driver ( slave ) AD driver ( slave ).. AD driver ( slave 8 ) AD driver ( slave 9 ) Measure Agle Measure Agle Measure Agle Measure Agle Fig. 3. Multi-DOF motio cotrol system structure. The EtherCAT-based Multi-DOF motio cotrol system is composed of a master ad ie slaves. Accordig to Fig. 3, oe PC is regarded as the EtherCAT master i the system, improvig the operatio ability ad the storage capacity of the system. At the same time, the applicatio of the TwiCAT software with strog real-time kerel greatly ehaces the realtime performace of the system. Besides, ie AD drives are cosidered as the EtherCAT slaves. What s more, i the system, just etwork cables are eeded for the PC to exchage data with the AD drives by digital sigal. Thus, ot oly the trasmissio distace is much loger ad the trasmissio rate is faster, but also the wirig becomes simpler. TX RX IV. IMPLEMENTATION SCHEME A. The EtherCAT Master There are geerally two methods to realize the EtherCAT master: oe is based o the embedded processor, ad the other is based o PC []. For the embedded EtherCAT master, the commuicatio cotroller is embedded i the microprocessor, which limits the calculatig ability ad storage capacity of the EtherCAT master. Ad it is extremely difficult for the system to upgrade ad exted ew fuctios. However, for the PCbased EtherCAT master, just a etwork iterface card (NIC) is eeded. Ad the fuctios of the data lik layer ad the physical layer ca be easily realized through the NIC with Etheret commuicatio cotroller ad the physical data trasceiver. Furthermore, it ca take full advatages of the software ad hardware resources of the PC, sigificatly improvig the operatio ability ad storage space ad makig it easier to use high-level programmig laguage to develop the applicatio. Therefore, it is coveiet to exted the fuctios of the master ad provides coveiece for the implemetatio of Multi-DOF motio cotrol system. The fuctios of the EtherCAT master are achieved by the cofiguratio software TwiCAT. TwiCAT with excellet real-time kerel ca tur ay PC ito a real-time cotroller with NC cotrol, programig eviromet ad PLC system []. TwiCAT replaces the covetioal PLC, NC cotroller ad the operatig devices ad supports all EtherCAT features. Besides, it is resposible for iitiatig, sedig ad receivig the frames. Ad a EtherCAT frame ca be set with 486 bytes of data at most, so that oly oe or two frames are eeded i each commuicatio cycle to fiish the commuicatio for all the slave odes. The fuctio of EtherCAT master is show i Fig. 4, which maily icludes the followig aspects [3]: the TwiCAT aalyzes the XML descriptio file of the slave, the cofigures ad iitiates each slave ode accordig to the XML descriptio file; besides it maages the status of the slave, sets the mailbox parameters to realize the aperiodic data trasmissio ad processes the accidetal evets durig mailbox commuicatio; i additio, TwiCAT is resposible for process data commuicatio to realize the real-time data trasmissiocotrollig the equipmet ad moitorig the real-time status of slaves to respod i a timely maer. Slave Descriptio File (XML) TwiCAT System Cofiguratio Tool Process Data Descriptio File (XML) EtherCAT Master r Stadard Etheret MAC Cotrol Tasks Process Data Fig. 4. The fuctio block diagram of EtherCAT master 78

3 B. The EtherCAT Slave The EtherCAT slave maily achieve data commuicatio ad cotrol the applicatios through ESC (EtherCAT Slave Cotroller) which is the core of the EtherCAT slave. The ESC defies the EtherCAT data-lik layer ad physical layer protocols, ad is used to extract data from the frame to save it to the storage area ad isert data i the iteral storage area ito the frame, realizig the data exchage betwee the EtherCAT master ad the slave. This paper regards the AD drive made by ollmorge as the slave. The AD drive with stadard EtherCAT iterface ad itegratig the fuctio of ESC, ca be coected to the PC or aother AD drive oly through a ece of etwork cable or optical fiber, istead of expesive basic compoets such as coectors or switch, so the AD drive ca easily achieve the fuctios of the EtherCAT slave. The AD drive ca achieve ot oly the data commuicatio fuctio but also the motio cotrol. There are three operatio modes: torque, velocity ad positio mode. To achieve the accurate positioig of the servo system, the drives are made to work i positio mode to realize the closed-loop cotrol. At this poit, the system is a cascade multi-loop cotrol system cosistig of curret loop, velocity loop ad positio loop. Moreover, i order to improve the trackig performace, feedforward cotroller is added. V. MULTI-LOOP CONTROLLER DESIGN This sectio, with sigle shaft as a example, itroduces the desig method of the cotrol parameters, icludig the cotrollers of the curret loop, velocity loop ad positio loop, feedforward ad coulomb frictio compesatio. What is metioed i the previous sectio is that the AD driver works i positio mode ad is resposible for achievig the closedloop cotrol i positio. Ad the actuator is a permaet maget sychroous motor, so the cotrol structure is as show i Fig. 5 [4], [5]. The meaig ad the values of the parameters i the Fig. 5 are eumerated i TABLE I. For the multi-loop cascade cotrol system, curret loop ad velocity loop are the ier loops, whose performace directly iflueces the whole precisio ad respose time of the servo system. The multi-loop cotrol system is desiged i tur from the ier loop to the outer, so that each loop is made stable, which ca guaratee that the whole cotrol system is stable. TABLE I. THE LIST OF PARAMETERS Symbol Meaig Value L Coil Iductace.47H R Coil Resistace 5 J Momet of Iertia kg m t Torque Costat 8.6 N m/ A e Couter Electromotive Force Factor 9.37 V / ( rad / s) v Speed Feedforward Gai a Acceleratio Feedforward Gai I f Coulomb Frictio Compesatio d / dt a SIGN I f d / dt v e * APR ASR ACR Ls R t Js s Fig. 5. The cotrol structure diagram of the system A. Curret Loop Desig What ca be see i figure 6 is that the couter electromotive force crosses the curret feedback with each other, makig it complex to desig the cotroller of curret loop. But the effect of couter electromotive force does t eed to be cosidered whe dyamic performace of the curret loop is the uppermost cosideratio, the the cotrolled plat of curret loop ca be equivalet to a first-order iertia lik. As a result, the trasfer fuctio of the cotrolled plat is defied as Gi () s LS R From the poit of view of dyamic requiremets, the actual system does ot expect a high overshoot, so proportio cotroller is selected for the curret loop. Ad the the opeloop trasfer fuctio of the curret loop ca be described by of which G LS R () iop s is the proportioal gai. With the purpose of icreasig the badwidth of curret loop, we select the sheared frequecy as ci Hz,the the proportioal gai ca be give by

4 B. Velocity Loop Cotroller Desig Whe desigig the cotroller of the velocity loop, the cotrolled plat is available by simplifyig the curret loop ad is give by GV () s sts ( ) where the gai ad the time costatti ca be respectively calculated usig t L TI J( R) R I order to realize the cotrol without static error, a itegral elemet must be cotaied i the speed cotroller ASR, so the ASR adopts PI cotroller, ad its trasfer fuctio is as followig: W ASR () s p I ( s) s where p ad are respectively the proportioal gai ad itegral gai. The the ope-loop trasfer fuctio of the velocity loop is preseted by i which N G Vop () s s is defied as follow N N ( s) ( T s) I p As we see, the velocity loop is a tycal type II system, so that the PI cotroller ca be desiged accordig to the egieerig desig method [6]. So as to esurig the performace of radity ad stability for velocity loop, the itermediate frequecy width is selected as h 5, the the gai N ad the factor T I satisfy the followig equatios: ht, I N h ht I Through uitig the equatios above, each parameter has bee calculated ad show as follow. 3.,.37, 58 p N C. Positio Loop Cotroller Desig Without cosiderig feedforward cotroller ad frictio compesatio, the cotrolled plat of positio loop is show as ( s) Gp () s sts ( s s ) N 3 I N N Due to the itegral elemet i the forward chael, the proportioal cotroller is regarded as the positio loop cotroller. At this poit, the system ca track the step sigal without static error. Defie W () s APR O the basis of Routh stability criterio, the stability coditio of the system ca be obtaied: p p O the premise of guarateeig the stability of the system, i order to improve the radity, we ca take p =8 Ad the the ope-loop bode diagram of positio loop is show i Fig. 6. From Fig. 6, the sheared frequecy ad phase margi are obviously clarified by 8 rad / s 85 ci So the system is stable. Magitude (db) Phase (deg) System: H Bode Diagram Frequecy (rad/s): 8 Magitude (db): -.93 System: H Frequecy (rad/s): 8 Phase (deg): Frequecy (rad/s) Fig. 6. The ope-loop frequecy characteristic of positio loop D. Feedforward Cotroller Desig Whe desigig the multi-loop cotroller, the rad trackig performace ca t be take care of much. If the trackig performace is eeded to further improve, the best way is to add the feedforward cotrol. The tycal cotrol system structure with feedforward cotrol is show i Fig. 7 [7]. The trasform the cotrol structure of AD driver ito form show i Fig. 7, amely s ( as v ps v p) Gs () p ( s) W () s p N( s) W () s 3 sts ( I s N sn) where W () s, W () s ad Gs () are the trasfer fuctios of feedback cotroller ad cotrolled plat as well as feedforward cotroller, respectively. 73

5 Gs () W () s W () s Fig. 7. The structure of the cotrol system with feedforward I the low frequecy, Gs () ad W () s satisfy the followig equatio: GsW () () s The the gais v ad a ca be calculated: v. a.379 E. Simulatio Accordig to the simulatio fuctio of Matlab, a simulatio block diagram is built. Firstly, the step respose idicator of the system is ispected. Ad the, the result is show i Fig. 8. It ca be see from Fig. 8 that there is o overshoot ad the respose time is about. secod, which presets that the system exhibits the good dyamic performace Step Respose Curve time/s Fig. 8. The simulatio step respose curve To test the trackig performace of the system, the sie sigal of which the frequecy is 3Hz is give. The iput sigal curve ad the output sigal curve of the system with or without feedforward cotrol are compared i Fig. 9. Iput ad without feed-forward - - Iput time/s Iput ad with feed-forward 3 Iput time/s As it is show i Fig. 9, the trackig performace of the system with feedforward cotroller has bee sigificatly improved. VI. EXPERIMENTS I this paper, the motio cotrol system we have built is as show i Fig., ad the the performace of the system has bee tested. Fig.. The actual system diagram To begi with, the step respose performace of the system is verified. The positio commad, degree, is give ad the velocity ad acceleratio are set to the maximum, the the step respose curve of the actual system is available ad is give i Fig The actual step respose curve time/ms Fig.. The step respose curve of the actual system Secodly, the experimet has bee doe to check out the real-time performace of the actual system. The EtherCAT frames that TwiCAT seds to the AD driver ad receives from the AD driver are captured by Wireshark as show i Fig.. Through the aalysis of the message i Fig., the cycle time of the actual system is about.5 millisecod ad the time of trasmissio delay is i the microsecod level, coformig to the requiremets of the real-time performace of the system. Fig. 9. The cotrast figure of the iput ad output sigal 73

6 Fig.. The EtherCAT trasmissio message Fially, the classical motio cotrol system are compared with the motio cotrol system based o EtherCAT. For classical motio cotrol system, the positio commad is iput to the AD driver after the D/A coverter, ad for the system based o EtherCAT, it is directly trasferred through the etwork cable i the way of digital sigal. Uder the coditio that the cotroller parameters are the same ad the same positio commad is give, Fig. 3 shows the curret curve diagram of the two kids of systems. What ca be see from Fig. 4 is that the amplitude ad the frequecy of the oise mixed i the curret of the system based o EtherCAT is much smaller tha that of the classical system, provig that the performace of the system based o EtherCAT is more superior. Curret/A Curret/A.. The curret of the classical system time/ms The curret of the EtherCAT system time/ms Fig. 3. The curret curve cotrast figure VII. CONCLUSION This paper proposes ad realizes a Multi-DOF motio cotrol system based o EtherCAT, a kid of real-time Etheret fieldbus, ad with sigle shaft as a example, itroduces the desig method for the multi-loop cotroller. Compared with the classical motio cotrol system ad traditioal Etheret commuicatio cotrol system, the EtherCAT-based motio cotrol system completely uses digital commuicatio, havig fast trasfer rate, strog atiiterferece performace ad superior real-time performace. What s more, the wirig of the syst3em is very simple, which greatly simplifies the system cotrol equipmet, reduces the complexity ad the implemetatio costs of the cotrol system ad improves the scalable property. Thus, EtherCAT is cosiderably suitable for etworked multi-axis real-time cotrol systems ad is expected to have a promisig applicatio prospect. However, the system is still eeded to ameliorate, ad the research cotet icludes the followig several aspects: () Exploit the EtherCAT master which is suitable for Widows system, to facilitate higher real-time performace ad get rid of the depedece o TwiCAT, savig the cost of project. () Through the EtherCAT master developed myself, make the AD driver work i velocity mode or torque mode ad realize positio closed-loop cotrol by computer, to choose differet cotrol algorithm ad achieve better cotrol effect. (3) Put the distributio clock fuctio of EtherCAT to use, to achieve the sychroizatio cotrol betwee each servo axis of the system. REFERENCES [] Wu Ju, Desig ad Applicatios o DSP-Based Multi-Axes Motio Cotroller [D], Shaghai: Shaghai Jiao Tog Uiversity,. [] Sug, M., im,., Ji, H. W., & im, T.. A EtherCAT-based motor drive for high precisio motio systems. I IEEE Iteratioal Coferece o Idustrial Iformatics (INDIN) (pp ). [3] Xie XiagLi, The Study o EtherCAT Network ad Serve Motio Cotrol System [D], Dalia: Dalia Uiversity of Thechology, 8. [4] D. Yu, Y. Hu, X. W. Xu, Y. Huag, ad S. Du, A Ope CNC System Based o Compoet Techology, IEEE Tras. Automatio Sciece ad Egieerig, vol. 6, o. :3 3, Apr. 9. [5] Rog AiQiog, Desig of EtherCAT based multi-axis motio cotroller [D], Guagzhou: South Chia Uiversity of Techology, 3. [6] J. Jaspereite, M. Schumacher, ad. Weber, Limits of Icreasig the Performace of Idustrial Etheret Protocols, I Proc. of Emergig Techologies ad Factory Automatio, pp. 7 4, Sep. 7. [7] Li JiaFeg, Su JiTao, Desig ad Implemeatito Based o EhterCAT master of DSP ad PowrPC, Computer owledge ad Techology, 9(6): , 3. [8] Gu ChaoYua, Software Desig ad Implemetatio for Slave Equipmet of EtherCAT Idustrial Etheret [D], Harbi: Harbi Istitude of Techology,. [9] Xiu JiaZhu, Research oreal-time Cotrol Techology with EtherCAT Network [D], Dalia: Dalia Uiversity of Thechology,. [] Sog HaiYag, Study o Method of Disturbace Atteuatio for Fiveaxis Flight Simulatio Turtable With Coordiate-Shaft-Structure [D], Harbi: Harbi Istitute of Techology,. [] Qi Sha, Desig of the Multi-Axis Motio Cotrol System [D], Tiaji: Tiaji Uiversity, 8. [] ag, C., Pag, Y., Ma, C., & Li, C. (). Desig of EtherCAT slave module. I IEEE Iteratioal Coferece o Mechatroics ad Automatio: 6 64,. [3] Shi XiaoLei, Research o the Software of Ope CNC System Based o Beckhoff TwiCAT [D], Harbi: Harbi Istitute of Techology,. [4] ollmorge, kollmorge-akd-istallatio-maual [5] Mei XiaoRog, Bai GuiZhe, Zhag MaoRui. Automatic Cotrol Compoets ad Circuit, Beijig: Sciece Publisher,. [6] Rua Yi, Che BoShi, Automatic cotrol system of electric drive: motio cotrol system, Beijig: Mechaical Idustry Publisher,. [7] Pei Ru, Automatic Cotrol Theory, Harbi: The Publisher of Harbi Istitute of Techology, 73