Safety Supervisory Strategy for an Upper-Limb Rehabilitation Robot Based on Impedance Control

Transcription

1 Iteratioal Joural of Advaced Robotic Systems ARTICLE Safety Supervisory Strategy for a Upper-Limb Rehabilitatio Robot Based o Impedace Cotrol Regular Paper Lizheg Pa 1, Aiguo Sog 1,*, Guozheg Xu, Huiju Li 1, Hog Zeg 1 ad Baoguo Xu 1 1 School of Istrumet Sciece ad Egieerig, Southeast Uiversity, Najig, Chia College of Automatio, Najig Uiversity of Posts ad Telecommuicatios, Najig, Chia * Correspodig author a.g.sog@seu.edu.c Received 5 Dec 011; Accepted 5 Nov 01 DOI: / Pa et al.; licesee ITech. This is a ope access article distributed uder the terms of the Creative Commos Attributio Licese ( which permits urestricted use, distributio, ad reproductio i ay medium, provided the origial work is properly cited. Abstract User security is a importat cosideratio for robots that iteract with humas, especially for upper limb rehabilitatio robots, durig the use of which stroke patiets are ofte more susceptible to ijury. I this paper, a ovel safety supervisory cotrol method icorporatig fuzzy logic is proposed so as to guaratee the impaired limb s safety should a emergecy situatio occur ad the robustess of the upper limb rehabilitatio robot cotrol system. Firstly, a safety supervisory fuzzy cotroller (SSFC) was desiged based o the impaired limb s real time physical state by extractig ad recogizig the impaired limb s trackig movemet features. The, the proposed SSFC was used to automatically regulate the desired force either to accout for reasoable disturbace resultig from pose or positio chages or to respod i adequate time to a emergecy based o a evaluatio of the impaired limb s physical coditio. Fially, a positio based impedace cotroller was implemeted to achieve compliace betwee the robotic ed effector ad the impaired limb durig the robotassisted rehabilitatio traiig. The experimetal results show the effectiveess ad potetial of the proposed method for achievig safety ad robustess for the rehabilitatio robot. Keywords Rehabilitatio Robot, Safety ad Robustess, Fuzzy Logic, Impedace Cotrol 1. Itroductio Neurologic ijuries such as strokes ad spial cord ijuries (SCI) cause dysfuctio to the eural system ad motor fuctio, which geerally results i upper limb impairmet ad motio disabilities [1,, 3]. Covetioally, maual physical therapy is used to help patiets with upper limb impairmets regai fuctioal mobility [4]. I the last few years, there has bee a icreasig iterest i usig robotic devices to provide rehabilitatio therapy for eurologic ijuries [5, 6] ad most of the paradigms that have bee explored have bee cocerig physical iteractio durig movemet traiig [7, 8]. Due to the particular coditios of the participats, safety ad robustess are strogly addressed i cotroller desig, which has bee a critical issue for the rehabilitatio robot. I recet decades, may researchers have doe a lot of work o safety i hardware ad software desigs [9]. For Lizheg Pa, Aiguo Sog, Guozheg It J Xu, Adv Huiju Robotic Li, Sy, Hog 013, Zeg Vol. ad 10, Baoguo 17:013 Xu: Safety Supervisory Strategy for a Upper-Limb Rehabilitatio Robot Based o Impedace Cotrol 1

2 emergecy situatios, a emergecy stop butto ad some specialized mechaical structures have bee desiged. D.E. Barkaa desiged a quick release had attachmet device to deal with ay physical safetyrelated evets [10]. A ew reflex mechaism structure based o a istictive respose behaviour patter was adopted to esure o harm or miimal harm to the user [9]. I ref. [11], a robotic task practice system for the fuctioal rehabilitatio of arms was preseted which maximized operatioal safety by desigig a small overall workspace, ot strappig the patet i ad choosig oly a rotary movemet fuctioal tasks. I order to improve security durig fuctioal recovery therapy, some therapy robots have bee developed with peumatic actuators [1, 13]. Geerally, the role of the software is to moitor the distace betwee the user ad the robotic arm, to check for dagerous proximity betwee the payload ad other liks to the robot, ad to calculate the distace betwee the robot ad earby obstacles, which effectively prevets agaist collisio [14]. Furthermore, some parameters (such as the force, torque ad velocity) have limitatios placed upo them to esure cotrol safety [3, 15, 16]. I ref. [17], a motio cotrol method with a estimated dager idex was proposed to moitor the user s safety ad iitiate proper safety actios i case of dager. I additio, impedace cotrol has bee extesively adopted to achieve compliace ad security for huma machie iteractios, creatig a dyamic relatioship betwee the force ad positio [18]. All the above metioed safety based desigs, whether with hardware or software, are effective to some extet, but the hardware based methods are ot olie iheretly ad the software based methods, havig limited parameters, lack flexibility with regards to cliical applicatio. However, most of the works are focused o collisio prevetio caused by exterior emergecy factors, ad few are cocered with sudde emergecies caused by iterior factors, such as a spasm or twitch. Moreover, durig rehabilitatio exercises, may iterior factors, such as pose or positio chages ad eve coughig, usually cause some disturbace to the rehabilitatio system. Thus, the desiged cotrol system should combie robustess ad safety, ad be able to effectively maage reasoable disturbaces ad sudde emergecies. I this work, we ivestigate safety ad robustess durig rehabilitatio exercises, assumig a traiig impairedlimb is the object of the exercises. A real time cotrol system based o a safety supervisory strategy is desiged, which maily ivolves evaluatig the physical state of the traiig impaired limb (PSTIL), a SSFC ad a positio based impedace cotroller. The proposed SSFC makes decisios to adapt the desired force i respose to reasoable disturbaces, or to quickly stop the rehabilitatio robot without doig harm to the traiig impaired limb whe it suffers from a sudde spasm or twitch. Cosiderig the essetial characteristics of the PSTIL, the developed cotrol system effectively combies robustess ad safety ad will perform a better rehabilitatio exercise.. Feature Extractio The upper limb rehabilitatio robot is a typical humamachie iteractio system ad usually the subject s affected limb comes ito physical cotact with the robotic ed effector i the course of the robot aided movemet treatmets. Durig the robot aided exercise, the traiig impaired limb is ot i a ideal static physical state, ad there are may ucertai factors which affect the PSTIL, e.g., pose or positio chages, sudde twitches, occasioal tremors ad eve coughig or other exteral disturbaces, which are partly subject to huma cotrol (aother uresolved cotroller) [15]. Therefore, the PSTIL is dyamically variable, which iflueces the performace of the robotic cotrol. It meas that pose or positio chages or sudde twitches made by the traiig impaired limb affect the performace of the positio velocity trackig. Thus, features of the positiovelocity trackig errors represet the PSTIL to a certai extet. The raw trackig error sigals with oise are ot directly used as iput iformatio to reflect the PSTIL. Therefore, certai features of the trackig errors are extracted i order to efficietly reflect the PSTIL. Amog various extracted features, e.g., mea absolute value, average rectified value, mea absolute value slope, root mea square, mea square error (MSE), value rage, ad slope sig chages, MSE value is selected, i this paper, as a feature of trackig errors that ca be extracted. The MSE value is widely used i applicatios for its good practicability ad effectiveess i real time cotrol. The commo MSE equatio is expressed as the followig: N 1 N ( x xi) (1) 1 i1 where N is the umber of sample data, x is the mea of th the whole data, ad x i is the value of i sample data. I order to obtai the features more effectively ad istataeously for practical applicatio, subsectio slidig mea square error (SMSE) is adopted i this paper, which is expressed as x 1 k ( xk xki) 1 i1 1 k xki i1 () It J Adv Robotic Sy, 013, Vol. 10, 17:013

3 where is the umber of sample data i the subsectio, th x k deotes the mea of k subsectio, xk i is the value of ( k i) th sample data, ad k is the correspodig th feature of the k sample data. 3. Cotrol System 3.1 Safety Supervisory Fuzzy Cotroller Fuzzy cotrol techology has bee developed to achieve huma like comprehesive judgmet. It is credited i various applicatios as a powerful tool providig robust approximatio for systems [19]. Fuzzy cotrol is well suited for rehabilitatio robots for several reasos. Firstly, the rehabilitatio robot system is essetially oliear, which is difficult to accurately portray with a model. Focusig o a oliear system is a outstadig applicatio for fuzzy cotrol; therefore it is a reasoable cotrol strategy for rehabilitatio robots. Secodly, the rehabilitatio robot is a real time cotrol system ad must be maaged usig a quick ad effective method of cotrol. Fuzzy cotrol well meets this requiremet. Safety ad robustess are the primary priciples i desigig robot aided euro rehabilitatio cotrol systems [0, 1]. We develop a SSFC to meet the requiremets from the essetial poit of the PSTIL, which are of course essetial to the mechaism. The desiged SSFC cosists of a iput, fuzzy cotroller ad a decisio makig sectio, as per Fig. 1, which ca automatically modify the desired force accordig to the PSTIL, or halt the robot i time if the user suffers from a sudde twitch. fuctio to represet the trackig error variatio ad k p ad kv are ratios. The correspodig output of the fuzzy cotroller is deoted by which is related to the evaluated PSTIL. Durig the fuzzificatio ad defuzzificatio, the iputs ( p ad v ) ad output ( ) are scaled ito five fuzzy sets, amely, egative zero (NZ), zero (ZE), positive small (PS), positive middle (PM) ad positive big (PB), respectively. Triagle shaped membership fuctios are selected for ZE, PS ad PM, while trapezoidal membership fuctios are selected for NZ ad PB. As metioed above, the SSFC is developed with the aim of esurig that the cotrol system should be robust i case of reasoable disturbace ad ca avoid hurtig the subject i a emergecy situatio. With robustess ad safety i case of reasoable disturbace ad emergecy as their aim, the membership fuctios are desiged with irregular shapes, as per Fig., which ca well serve the proposed cotrol strategy accordig to the requiremets of the PSTIL. Each combiatio of mapped iputs activates oe cotrol actio accordig to the iferece rule table (Table 1). The, the value of is obtaied i the defuzzificatio process by usig the cetroid method: (a) Figure 1. Structure of the SSFC As metioed above, the features of the positio ad velocity trackig errors are used to represet the PSTIL. Therefore, the fuzzy cotroller is adopted with two iputs ad oe output type. I order to achieve good performaces uder coditios of reasoable disturbaces ad emergecies, the iputs of the cotroller should iclude the iformatio of the features ad trackig error variatios. The used positio ad velocity iformatio ( p ad v ) are expressed as iputs as the followig. (b) Figure. Iput output membership fuctios for SSFC. (a) Iput membership fuctios; (b) Output membership fuctios p fep kp f ( emax, emi) v fev kv f ( emax, emi) (3) where fep ad fev are the features of the positio ad velocity trackig errors respectively, f ( emax, emi) is a Figure 3. Cotrol surface map for SSFC Lizheg Pa, Aiguo Sog, Guozheg Xu, Huiju Li, Hog Zeg ad Baoguo Xu: Safety Supervisory Strategy for a Upper-Limb Rehabilitatio Robot Based o Impedace Cotrol 3

4 v p NZ ZE PS PM PB NZ NZ NZ ZE ZE PS ZE NZ ZE PS PS PM PS ZE PS PS PM PB PM PS PS PM PM PB PB PS PM PB PB PB Table 1. Fuzzy iferece rules for output i i1 i i1 where i idexes all the combiatios of activated cotrol actios from the rule table, is the elemet i the i physical uiverse of discourse of, ad u is the degree i of membership of. The overall cotrol surface map of i the itelliget supervisory cotroller is show i Fig. 3. It clearly demostrates that the output sharply icreases i a emergecy ad the desiged fuzzy cotroller maages the proposed strategy very well, differetiatig betwee a reasoable disturbace ad a emergecy. The decisio makig sectio maages the rehabilitatio traiig based o the evaluatios of the PSTIL. I this research, i respose to reasoable disturbace, the desired force is adapted with the followig equatio. i (4) f ( k) f ( k 1) f (5) d d e where fd( k ) ad fd( k 1) are the correspodig desired forces betwee the ed effector ad the impaired limb for th k ad ( k 1) th itervals, respectively, ad f e is the actual iteractive force betwee the robotic ed effector ad the impaired limb. Moreover, whe the traiig impaired limb suffers from a sudde spasm or twitch, the emergecy sigal is give, which is show as the followig. 0 s 1 where s is the emergecy sigal which deotes a emergecy whe it has a value of 1, otherwise it has a value of 0, ad is the threshold for a emergecy. 3. Rehabilitatio Robotic Cotrol System Upper limb rehabilitatio robots are used to assist those with eural ijuries to help improve their motio fuctio usig rehabilitatio traiigs. Oe of the major difficulties i carryig out rehabilitatio with robots is the cotroller desig. The cotrol system must be safe ad robust eough, give the particular coditios of the subjects. As metioed above, durig robot aided (6) rehabilitatio traiig, the PSTIL is dyamically variable. Therefore, suitable compliat behaviour betwee the robotic ed effector ad the impaired limb is oe of the primary requiremets i desigig the cotrol system, especially with regards to the physical cotact rehabilitatio robot. A well established framework for maagig this task is give by impedace cotrol [], which is a iheret model based approach. Accordig to the implemet mode of target impedace, there are two geeral approaches, amely, positio based impedace cotrol ad torque based impedace cotrol [3]. I cotrast to the torque based cotrol, the positiobased cotrol is more mature ad the performace is more stable [18, 4]. Thus i this paper, the positiobased impedace cotrol is adopted to desig the cotrol system. Impedace cotrol achieves compliat motio by regulatig the dyamic relatioship betwee the robotic ed effector positio/velocity ad cotact force. I this paper, the desired impedace equatio of the rehabilitatio robot ca be represeted by f M X B X K X d d d d f f (, f ) d e X X X, X X X, X X d d d X where Md, Bd, K d are desired iertia, dampig ad the stiffess matrix, respectively, X d, X are the desired ad actual positio of the rehabilitatio maipulator, X d, X, X d, X are the correspodig velocities ad acceleratios. ad f e are the output of the fuzzy cotroller ad actual iteractive force, respectively. I the frequecy space, the equatio (7) ca be expressed as the followig. X( s) 1 f ( s) M s B s K d d d d Durig the rehabilitatio exercise, the subject is part of the dyamic system ad the PSTIL is also ucertai. Therefore, i this research, the cotrol system of the rehabilitatio robot is desiged by icorporatig the positio based impedace cotrol ad the proposed itelliget SSFC. The overall cotrol system block diagram is show i Fig. 4, which maily cosists of a impedace cotroller, a positio cotroller, the developed SSFC (icludig a feature geerator, a fuzzy cotroller ad decisio makig respose sectios). The impedace cotroller brigs about compliat behaviour betwee the robotic edeffector ad the impaired limb ad the the positio cotroller is applied to execute the positio trackig. I itelliget SSFC, the feature geerator reflects the PSTIL i real time ad the fuzzy cotroller supplies a output correspodig to the PSTIL to the respose sectios (7) (8) 4 It J Adv Robotic Sy, 013, Vol. 10, 17:013

5 which automatically adapt the desired force or halt the device usig the so desiged decisio makig mechaism. As aalyzed above, the desiged rehabilitatio robotic cotrol system is promises to be characterized by safety ad robustess. 4. Experimets 4.1 Simulatio Experimet Simulatio Model ad setup With regards to physical cotact durig the rehabilitatio traiig, the complete dyamics [, 5] of rigid serial lik robot maipulator with degree of freedom revolute joits ca be preseted i the joit space as: M( ) C(, ) G( ) (9) e where,, R represet positio, velocity ad acceleratio of the rehabilitatio robot i the joit space, respectively. M( ), C(, ) R represet symmetric positivedefiite iertia matrix ad the cetripetal Coriolis matrix, respectively. G( ) R represet gravitatioal terms., R represet the joit iput torque vector ad e the geeralized vector of joit torques exerted o the edeffector by the impaired limb, respectively. The existig rehabilitatio robot is usually desiged with revolute joits to carry out the movemet. I essece, the developed cotrol algorithm maages the movemet by cotrollig each joit rotatio. Moreover, most of the traiigs are executed with plae motios. Thus, i this research, i order to ivestigate the effectiveess of the proposed method, a two lik rehabilitatio robot model (see Fig. 5) is used ad several group simulatio experimets were carried out o it. The dyamics of the robot are give as: m l m l l l l m l m l l M( ) 1 1 ( 1 1 cos ) 1 cos ml ml1l cos ml ml1l si ml1l si C(, ) ml1l si, 1 0 mlg si( 1 ) ( m1 m ) l1g si1 G( ) mlg si( 1 ), I the simulatio experimets, the parameters for the rehabilitatio maipulator were selected as l 1 l 1m, ad m kg, m 1.0kg. The impedace cotrol parameters ( Md, Bd, K d ) were chose accordig to those i paper []. Parameter i SMSE was set at 10 ad the safety threshold was selected as 0.6. The domais of the iput output variables i the proposed safety supervisory fuzzy algorithm were defied as [ 0.,1.], [ 0.,1.4], [ 0.,1.0]. p v Figure 4. Cotrol system block diagram for rehabilitatio robot Lizheg Pa, Aiguo Sog, Guozheg Xu, Huiju Li, Hog Zeg ad Baoguo Xu: Safety Supervisory Strategy for a Upper-Limb Rehabilitatio Robot Based o Impedace Cotrol 5

6 To ivestigate the performace of the proposed method effectively, the plaed movemet rages of predefied trajectories might be larger tha those applied i cliics but this does ot affect the aalysis of cotrol performaces at all. Figure 5. A two lik rehabilitatio robot model 4.1. Predefied Trajectory ad Simulated Sigal Reach ad grasp skill traiig is critical for patiets i their attempts to retur to a reasoable quality of life [11, 6], because may daily livig activities ivolve the use of the upper extremities [4, 7]. The liear reachig exercise [15, 8] is largely used at the early stages of rehabilitatio, i order to reduce spasticity [8] ad prevet cotracture [9]. Circular movemet is also commoly used i cliic robot aided rehabilitatio traiig [15, 18]. I this work, we desiged the two typical movemet trajectories, amely, liear ad circular traiig trajectories. The liear movemet trajectory was predefied i the Y directio ad the legth was 0.8m with the startig positio: (1., 0.4) ad ed positio: (1., 1.) ad the cycle time was 16s (see Fig. 5). I Cartesia space, it is expressed as x 1. y t y 0.1t t 8 8 t 16 (10) The circular movemet trajectory was predetermied with the cetre poit: (1, 1) ad radius: 0.5m (see Fig. 5). I Cartesia space, it is expressed as Durig rehabilitatio exercises, the PSTIL geerally presets three mai types, these are ratioal state, the presece of reasoable disturbace ad the occurrece of a sudde emergecy. The iteractive force is the oly iformatio shared betwee the impaired limb ad robotic ed effector. Thus, i this research, iteractive forces, correspodig to the differet types of PSTIL, were used to ivestigate the cotrol performaces of the proposed strategy. Tremor is a ivolutary, somewhat rhythmic, muscle cotractio ad relaxatio ivolvig toad fro movemets (oscillatios or twitchig) of oe or more body parts [30]. Neurological disorders or coditios (such as multiple sclerosis, stroke, traumatic brai ijury ad a umber of eurodegeerative diseases that damage or destroy parts of the braistem or the cerebellum) are liable to evoke tremors. Geerally, the tremor is characterized by 3 to 8Hz oscillatios or twitchig [31]. Based o the characteristics of a tremor, we simulated iteractive forces which are meat to represet a reasoable disturbace ad a sudde emergecy, respectively. I order to ivestigate the cases thoroughly, three kids of iteractive force, amely, regular siusoid, sawtooth ad complex siusoid, were cosidered, which are show i Table Feature Extractio Results To demostrate the performace of the feature extractio with SMSE, three kids of experimets were carried out, amely, with S1, S ad S3, respectively. I each experimet, the commo iteractive force betwee the ed effector ad the impaired limb was set at 6N ad the reasoable ad emergecy disturbaces were applied durig the sectios 3~4s ad 5~6s, respectively, of the total executio time of 10s. The experimets were carried out usig a predefied circular trajectory ad the simulated forces were exerted i X directio. The performaces of feature extractio for positio ad velocity trackig errors i X directio were demostrated i Fig. 6. x 0.5sit 1 y 0.5si( t ) 1 (11) Sigal type symbol Reasoable disturbace Emergecy evet (sudde twitch or spasm) S1 si(10 t) 4si(10 t) S 4(5 t [5 t]) 8(5 t [5 t]) 4 S3 si(8 t) si(3 t) rad() 6si(8 t) 3si(3 t) rad() Table. Simulatio Sigals for the PSTIL 6 It J Adv Robotic Sy, 013, Vol. 10, 17:013

7 I Fig. 6, it ca be clearly see that the extracted features of positio/velocity trackig errors i X directio durig 3~4s, 5~6s ad other sectios are obviously differet ad the features uder emergecy (5~6s sectio) are remarkable, whether with S1, S or S3 disturbace. It idicates that the extracted features ca represet the differet PSTIL types, especially the emergecy. From further aalysis of Fig. 6, we see that the extracted features for differet kids of simulated states are cosistet, which verifies the effectiveess of the proposed SMSE method for practical applicatio. Comparig the feature durig 3~4s with the oe durig 5~6s, the emergecy ca be detected easily. I other words, the extracted feature is oly related to the upperlimb physical state to some extet Cotrol Performace Results As metioed i sectio 3., impedace cotrol has show to be promisig i achievig compliat behaviour; therefore may researchers have adopted ad developed this method for their cotrol strategies. I this paper, i order to esure robustess ad safety, a method has bee suggested which develops the itelliget SSFC (see Fig. 4) by combiig it with the traditioal positio based impedace cotrol [8, 4, 3]. To ivestigate the cotrol performaces of the two methods, amely, the proposed method ad traditioal positio based impedace cotrol, diverse experimets were plaed. The simulatios were carried out with predefied liear ad circular trajectories, respectively, ad two formal simulated sigals (S1 ad S3) were selected to test the cotrol performace for each trajectory movemet i order to verify the uiversal effectiveess of the proposed method. Durig the movemet of the circular trajectory, the method of exertig the simulated disturbaces of the differet PSTIL was the same as metioed i sectio Durig the movemet of the liear trajectory, the simulated disturbaces were exerted i Y directio. The overall compariso of the cotrol performaces of liear movemet is show i Figs. 7 ad 8, for S1 ad S3, respectively. By comparig the velocity trackig performaces i Y directio betwee the proposed ad traditioal positio based impedace cotrol, it is evidet i Fig. 7(b) ad 8(b) that, at the begiig, the proposed strategy presets a preferable iitial respose. Meawhile, the proposed method presets a better positio trackig performace tha the traditioal method, which is demostrated i Fig. 7(d) ad 8(d). Furthermore, it ca effectively detect a emergecy i real time, which represets a otable advatage with respect to safety. Emergecy sigals are supplied at 5.0s ad 5.19s accordig to the evaluated PSTIL, as show i Figs. 7(a) ad 8(a), respectively, which correspod to the imitative impaired limb coditio. (a) (b) Figure 6. Feature extractio uder differet kids of disturbaces. (a) S1; (b) S; (c) S3 (c) Lizheg Pa, Aiguo Sog, Guozheg Xu, Huiju Li, Hog Zeg ad Baoguo Xu: Safety Supervisory Strategy for a Upper-Limb Rehabilitatio Robot Based o Impedace Cotrol 7

8 Figs. 9 ad 10 show the compared cotrol performaces for circular movemet, for S1 ad S3, respectively. The velocity trackig errors i X directio for S1 ad S3 are show i Fig. 9(b) ad 10(b). The same coclusio is draw as previously: that the suggested strategy presets a better iitial respose tha the traditioal method. Figs. 9(a) ad 10(a) show the detectig performaces of the proposed method ad the emergecy was detected at 5.01s ad 5.17s for S1 ad S3, respectively. The edeffector trackig performaces i Fig. 9(d) ad 10(d) reveal that the proposed strategy also provides better positio trackig capability for circular movemet. To come to a overall appraisal of the proposed ad traditioal methods, the results were further aalyzed with three idices, amely, overshoot (the maximum absolute error durig the adjustig procedure), the sum of the absolute error (SAE) ad maximum deviatio (MD, the maximum absolute error durig the 3~5s). Here, the recordig data from 0~5s was selected to ivestigate the performace because the proposed strategy halted the robot i real time due to a simulated emergecy. Two cases (positio ad velocity trackig errors) were studied to demostrate the cotrol performaces. The aalyses of the results of cotrol performaces are show i Tables 3 ad 4, for S1 ad S3, respectively. Accordig to the results, the method proposed with smaller overshoot provides better stability tha the traditioal method. Meawhile, the results idicate that the ovel cotrol system with proposed method outperforms the traditioal cotrol system o trajectory trackig. Most importatly, the proposed method ca effectively detect emergecies ad works more safely ad robustly. Trajectory type Liear Circular Method Positio trackig error (m) Velocity trackig error (m/s) Overshoot SAE MD Overshoot SAE MD Traditioal Proposed Traditioal Proposed Table 3. Cotrol performace comparisos for S1. Durig the circular movemet, the positio trackig error is defied alog the radial directio ad the aalyzed velocity is i X directio. The sample frequecy is set at 1kHz. Trajectory type Liear Circular Method Positio trackig error (m) Velocity trackig error (m/s) Overshoot SAE MD Overshoot SAE MD Traditioal Proposed Traditioal Proposed Table 4. Cotrol performace comparisos for S3. Durig the circular movemet, the positio trackig error is defied alog the radial directio ad the aalyzed velocity is i X directio. The sample frequecy is set at 1kHz. (a) (c) (b) Figure 7. Cotrol performaces of liear movemet ad emergecy detectio: S1. (a) Emergecy sigal; (b) Velocity trackig compariso; (c) Actual force with S1; (d) Trajectory trackig compariso (d) 8 It J Adv Robotic Sy, 013, Vol. 10, 17:013

9 (a) (c) (b) Figure 8. Cotrol performaces of liear movemet ad emergecy detectio: S3. (a) Emergecy sigal; (b) Velocity trackig compariso; (c) Actual force with S3; (d) Trajectory trackig compariso (d) (a) (c) (b) (d) Figure 9. Cotrol performaces of circular movemet ad emergecy detectio: S1. (a) Emergecy sigal; (b) Velocity trackig compariso; (c) Actual force with S1; (d) Trajectory trackig compariso (a) (c) (b) Figure 10. Cotrol performaces of circular movemet ad emergecy detectio: S3. (a) Emergecy sigal; (b) Velocity trackig compariso; (c) Actual force with S3; (d) Trajectory trackig compariso (d) Lizheg Pa, Aiguo Sog, Guozheg Xu, Huiju Li, Hog Zeg ad Baoguo Xu: Safety Supervisory Strategy for a Upper-Limb Rehabilitatio Robot Based o Impedace Cotrol 9

10 4. Real Experimet 4..1 Experimet setup Additioally, the proposed cotrol strategy was further verified o a WAM rehabilitatio robot. The WAM rehabilitatio system maily cosists of the Barrett WAM Arm, a 3 D force sesor [33], a arm supported device, ad a exteral PC, as show i Fig.11. The stadard WAM Arm is a four degree of freedom (DOF) highly dexterous, aturally back drivable maipulator, which stretches the impaired limb for movemet exercises. The improved 3 D force sesor was desiged ad istalled o the ed effector to record the iteractive force durig the exercises ad the arm supported device was desiged to support the forearm of the stroke participat. The proposed cotrol strategy was developed o a exteral PC with Ubutu Liux system. More detailed iformatio o WAM rehabilitatio systems is give i ref. [0]. emergecy sigal was also recorded whe the rehabilitatio system was ruig with the proposed method. The experimetal results are show i Fig.1. The subject simulated reasoable disturbace ad emergecy at 8~10s ad 6.5~8.5s, respectively. I Fig.1, comparig the positio ad velocity trackig errors of two cotrol methods, it ca be cocluded that the proposed method presets a better trackig performace, which is strikigly similar to the result of simulatio experimets. The proposed method supplied a emergecy sigal to halt the rehabilitatio exercise at 7.48s whe the impaired limb was sufferig from a emergecy, which demostrates remarkable superiority with regards to safety. The data from the first exercise sessio was further aalyzed to preset the cotrol performaces of the proposed ad traditioal methods. Note that the data of the secod sessio is exclusive, because the proposed method halted the rehabilitatio exercise due to a emergecy to protect the impaired limb. Two idices, the maximum of the absolute error (MAE) ad the sum of absolute error (SAE) of the trajectory trackig errors, were adopted to evaluate positio ad velocity trackig performaces of two cotrollers. The outcomes are preseted i Table 5. The aalyzed results also show that the proposed method rus rehabilitatio traiig with a better trackig performace tha the traditioal method i the presece of reasoable disturbace. Figure 11. WAM rehabilitatio system 4.. Experimet scheme A shoulder extesio/flexio, horizotal movemet trajectory was predefied, whose rage, expressed i the WAM Arm s world coordiates, was deed as 0.45 rad i exio ad 0.45 rad i extesio (XOY plae), with 18.1s for each sessio. A healthy adult male voluteer was istructed to deliberately simulate the differet PSTIL durig the rehabilitatio exercise. The voluteer was required to simulate each type of physical state (reasoable disturbace ad sudde twitch) as much as possible with the same actio ad at the same positio. I order to avoid ufairly ifluecig the experimet results, the voluteer was firstly asked to do extesive simulatig exercises, moreover, each experimet was carried out five times. The evetual outcome was derived from the average of the 5 results Cotrol Performace To demostrate the superiority of the proposed strategy, the proposed ad traditioal cotrol methods were used o the WAM rehabilitatio system, respectively. The positio ad velocity trackig errors were recorded durig the experimetal exercise ad at the same time the Figure 1. Cotrol Performace compariso Cotrol performaces Methods Proposed Traditioal Positio trackig (rad) MAE SAE Velocity trackig (rad/s) MAE SAE Table 5. Cotrol performace compariso of proposed ad traditioal methods 10 It J Adv Robotic Sy, 013, Vol. 10, 17:013

11 5. Coclusios System security ad the robustess of the cotroller are the critical cosideratios whe desigig a rehabilitatio robot. I this paper, we preset a ovel safety supervisory cotrol strategy to meet the demads that the system be robust whe faced with reasoable disturbace ad safe eough i emergecies. The proposed method is highly suitable for the real time safety moitor. Regardig the traiig impaired limb itself, the features of the positio ad velocity trackig errors were extracted to evaluate the physical state of the limb i real time. A safety supervisory fuzzy cotroller was developed to achieve systematic safety ad robustess. The fuzzy cotroller automatically adapts the desired force betwee the ed effector ad impaired limb to esure robustess i the evet of reasoable disturbace. Furthermore, the fuzzy cotroller ca effectively detect a emergecy such as a sudde spasm or twitch etc, ad halt the robot quickly eough so as to avoid causig the impaired limb further harm durig the emergecy. Meawhile, a positio based impedace cotroller has bee desiged to achieve iteractive compliace durig the rehabilitatio traiigs. The diverse simulatio ad real experimetal results have verified the effectiveess ad practicability of the proposed method to achieve security ad robustess for a upper limb rehabilitatio robot. 6. Ackowledgemets The authors appreciate the help of all their colleagues i the Remote Measurig ad Cotrol Laboratory, who made valuable cotributios to this work. This work was supported by the Natioal Natural Sciece Foudatio of Chia (No , No , No ), the Natural Sciece Foudatio of JiagSu Provice (BK010063), the Techology Project Foudatio of JiagSu Provice (BE01740), the Idustrial Techology Project Foudatio of the ChagZhou Govermet (CE010085), ad the Ph.D. Program Foudatio of the Miistry of Educatio of Chia (No ). The authors would also like to thak the aoymous reviewers for their very useful commets. 7. Refereces [1] America Heart Associatio: Heart ad Stroke Statistical Update (011). Available: [] B.G. Xu, S. Peg, A.G. Sog, Robot Aided Upper Limb Rehabilitatio Based o Motor Imagery EEG, It J Adv Robotic Sy, vol. 8, o. 4, pp , 011. [3] P.R. Culmer, A.E. Jackso, S. Makower, A Cotrol Strategy for Upper Limb Robotic Rehabilitatio With a Dual Robot System, IEEE/ASME Trasactios o Mechatroics, vol. 15, o. 4, pp , 010. [4] K. Kiguchi, M.H. Rahma, M. Sasaki, K. Teramoto, ʺDevelopmet of a 3DOF mobile exoskeleto robot for huma upper limb motio assist,ʺ Robotics ad Autoomous Systems, vol. 58, pp , 008. [5] D.J. Reikesmeyer, J.L. Emke, S.C. Cramer, How to retrai movemet after eurologic ijury: a computatioal ratioale for icorporatig robot (or therapist) assistace, Proceedigs of the 5th Aual Iteratioal Coferece of the IEEE, pp , 003. [6] R. Rieer, T. Nef, G. Colombo, Robot aided eurorehabilitatio of the upper extremities, Med Biol Eg Comput, vol. 43, o. 1, pp. 10, 005. [7] P.S. Lum, G. Uswatte, E. Taub, A telerehabilitatio approach to delivery of costrait iduced movemet therapy, Rehabil Res, vol. 43, o. 3, pp , 006. [8] M.C. Laura, J. David, Review of cotrol strategies for robotic movemet traiig after eurologic ijury, Joural of Neuro Egieerig ad Rehabilitatio, 6:0, 009. [9] N. Tejima, D. Stefaov, Fail Safe Compoets for Rehabilitatio Robots A Reflex Mechaism ad Fail Safe Force Sesor, Proceedigs of the 9th Iteratioal Coferece o Rehabilitatio Robotics, Chicago, USA, 005. [10] D. Barkaa, J. Das, F. Wag, Icorporatig verbal feedback ito a robot assisted rehabilitatio system, Robotica, vol. 9, o. 3, pp , 011. [11] C. Youggeu, J. Gordo, K. Duckho, A Adaptive Automated Robotic Task Practice System for Rehabilitatio of Arm Fuctios After Stroke, IEEE Trasactios o Robotics, vol. 5, o. 3, pp , 009. [1] R. Morales, J.B. Fracisco, G.A. Nicolás, Peumatic robotic systems for upper limb rehabilitatio, Med Biol Eg Comput, vol. 49, o. 10, pp , 011. [13] K. Kirihara, N. Saga, N. Saito, Desig ad cotrol of a upper limb rehabilitatio support device for disabled people usig a peumatic cylider Idustrial Robot a Iteratioal, vol. 37, o. 4, pp , 010. [14] F. Fusco, R. Gallerii, Robotic Arm: the Problem of Prevetig Collisios The 6th ESA Workshop o Advaced Space Techologies for Robotics ad Automatio, ESTEC, Noordwijk, 000. [15] M.S. Ju, C.C.K. Li, D.H. Li, Ig Shiou Hwag, A Rehabilitatio Robot with Force Positio Hybrid Fuzzy Cotroller: Hybrid Fuzzy Cotrol of Rehabilitatio Robot, IEEE Trasactios o Neural Systems ad Rehabilitatio Egieerig, vol. 13, o. 3, pp , 005. [16] N. Tejima, K. Araki, Rehabilitatio robot risk reductio by three dimesioal force limitatio ad reflex mechaism, Proceedigs of the ICORR, 003. [17] P. Aslam, R. Jeha, Safe physical huma robot iteractio of mobility assistace robots: evaluatio idex ad cotrol, Robotica, vol. 9, pp , Lizheg Pa, Aiguo Sog, Guozheg Xu, Huiju Li, Hog Zeg ad Baoguo Xu: Safety Supervisory Strategy for a Upper-Limb Rehabilitatio Robot Based o Impedace Cotrol 11

12 [18] H.I. Krebs, B.T. Volpe, N. Hoga, Robotic applicatios i euromotor rehabilitatio, Robotica, vol. 1, o. 3, pp. 3 11, 003. [19] H. Chaoui, P. Sicard, Adaptive Fuzzy Logic Cotrol of Permaet Maget Sychroous Machies With Noliear Frictio, IEEE Trasactios o Idustrial Electroics, vol. 59, o., pp , 01. [0] G.Z. Xu, A.G. Sog, H.J. Li, Cotrol System Desig for a Upper Limb Rehabilitatio Robot, Advaced Robotics, vol.5, pp. 9 51, 011. [1] P. Beyl, K. Kaepe, S. Duerick, Safe ad Compliat Guidace by a Powered Kee Exoskeleto for Robot Assisted Rehabilitatio of Gait, Advaced Robotics, vol. 5, o. 5, pp , 011. [] N. Hoga, Impedace Cotrol: A approach to maipulatio: Part 1 theory, Part Implemetatio, ad Part 3 Applicatio, ASME J. of Dyamic Systems, Measuremet ad Cotrol, vol.107, pp. 1 4, [3] D. Lawrece, M.M. Dever, Impedace cotrol stability properties i commo implemetatios, IEEE It Cof o Robot ad Auto, vol., pp , [4] Y. Yag, L. Wag, L.X. Zhag, Arm Rehabilitatio Robot Impedace Cotrol ad Experimetatio, i Proceedigs of the ROBIO, 006. [5] H.S. Liu, K.R. Hao, X.B. Lai, Fuzzy Saturated Output Feedback Trackig Cotrol for Robot Maipulators: A Sigular Perturbatio Theory Based Approach, It J Adv Robotic Sy, vol. 8, o. 4, pp , 011. [6] J.H. Carr, R.B. Shepherd, Stroke Rehabilitatio Guidelies for Exercise ad Traiig to Optimize Motor Skill, Lodo, U.K.: Butterworth/Heiema, 003. [7] W.J. Coster, S.M. Haley, P.L. Adres, Reig the coceptual basis for rehabilitatio outcome measuremet: Persoal care ad istrumetal activities domai, Med. Care, vol. 4, pp , 004. [8] L.E Kah, M.L Zygma, W.Z. Rymer, Robotassisted reachig exercise promotes arm movemet recovery i chroic hemiparetic stroke: a radomized cotrolled pilot study, Joural of NeuroEgieerig ad Rehabilitatio, vol. 3, o.1, 006. [9] P.E. Williams, Use of itermittet stretch i the prevetio of serial sarcomere loss i immobilised muscle, A Rheum Dis, vol. 49, o.5, pp , [30] G. Cooper, R. Roditzky, The may forms of tremor: Precise classificatio guides selectio of therapy, Postgrad Med, vol. 108, 000. [31] B. Hellwig, P. Mud, A logitudial study of tremor frequecies i Parkiso s disease ad essetial tremor, Cliical Neurophysiology, vol. 10, o., pp , 009. [3] S. Hussai, S.Q. Xie, G.Y. Liu, Robot assisted treadmill traiig: Mechaisms ad traiig strategies, Med Eg ad Phys, vol. 33, o. 5, pp , 011. [33] A.G. Sog, J. Wu, G. Qi, W.Y. Huag, A ovel selfdecoupled four degree of freedom wrist force/torque sesor, Measuremet, vol. 40, o. 9,pp , It J Adv Robotic Sy, 013, Vol. 10, 17:013