Stepping Over Virtual Obstacles with an Actuated Gait Orthosis

Mathias Wellner∗

Sensory-Motor Systems Laboratory, ETH Zurich

Joachim von Zitzewitz

Sensory-Motor Systems Laboratory, ETH Zurich

Alexander Duschau-Wicke

Hocoma AG, Volketswil

Sensory-Motor Systems Laboratory, ETH Zurich

Robert Riener

Sensory-Motor Systems Laboratory, ETH Zurich

SCI Center, Balgrist University Hospital, Zurich

ABSTRACT

The rehabilitation robot LOKOMAT has been developed at Univer-sity Hospital Balgrist to automate treadmill training of spinal cordinjury and stroke patients. Current rehabilitation training on thatrobot consists of moving the patient’s legs on predefined trajecto-ries. However, this kind of training is not challenging, as patientsare moved regardless of their efforts and do not see their advance-ment.

To enhance rehabilitation training with the LOKOMAT, a virtualreality setup was installed. It consists of a passive stereo projectionsystem (screen size 3 m×2 m), a Dolby 5.1 sound system and anelectric fan. With that setup an obstacle crossing scenario was im-plemented. The patients can see their advancement on the screen,as an animated figurine (avatar) moves along a path simultaneouslywith their own movements. Additionally they can hear sounds (e.g.environmental sounds, steps), feel the wind, and experience forcefeedback, provided by the orthosis, when hitting obstacles.

The objective of a first study on visual feedback was to inves-tigate which feedback suits best to perceive the obstacle distancesand heights correctly. To answer this question, 14 healthy subjectswalked in the actuated gait orthosis, received visual feedback andtried to avoid collisions with obstacles. Subjects could move freelywithin the gait orthosis and determine their own speed and steplength. They had to cross the obstacles independently, with hap-tic feedback, indicating obstacle hits. Results show that the sideview results in least obstacle hits and that 2D excels 3D display inthis respect.

Keywords: Rehabiliation, Gait, Obstacle Walking, Virtual Reality,Biofeedback

Index Terms: H.5.1 [Information Interfaces and Presentation(e.g., HCI)]: Multimedia Information Systems—Artificial, aug-mented and virtual realities

1 INTRODUCTION

For stroke and spinal cord injury patients, one method for restora-tion of gait is manual treadmill training [1]. This form of rehabilita-tion requires two or three physiotherapists, who move the patient’slegs in a sitting position. Due to the exhaustive character of thistherapy for the therapists the duration and number of training ses-sions are limited. To ameliorate said disadvantages and increasealso the accuracy of trained gait pattern, the actuated gait orthosisLOKOMAT was developed at University Hospital Balgrist [7].

The LOKOMAT is a bilateral robotic orhosis that can be used inconjunction with a dynamic body weight support system to con-trol a patient’s leg movement in the sagittal plane. Hip and kneejoints are actuated by drives, which are integrated into an exoskele-tal structure. In this structure, position and force sensors are in-

∗e-mail: wellner@mavt.ethz.ch

tegrated to provide input for the controller. In contrast to otherapproaches the LOKOMAT is a stationary device for robot-assistedtreadmill training.

In clinical use, the LOKOMAT is position-controlled. The pa-tients are moved regardless of their own efforts and do not see anyimmediate effects of their own contributions. Thus, training is nei-ther motivating nor challenging.

With the advance of computer processing capabilities virtual en-vironments accrete in many application areas, including rehabilita-tion [6, 8, 3]. Several research groups investigate the potential andrisks of using virtual reality techniques in rehabilitation [5, 2, 4].

To enhance treadmill training with the LOKOMAT, the orthosis iscombined with a virtual reality system. Our goal is to increase thepatients’ motivation, provide biofeedback for the patient to improvemotor function, and guide patients to perform special motor tasks.

2 METHODS

2.1 LOKOMAT and Haptic Feedback

For clinical use, the LOKOMAT is position-controlled. For research,additional modes exist, allowing the subject to move more freelywithin the orthosis. The so called Zero-Impedance-Mode aims atreducing the burden of the orthosis by compensating its friction andgravitational effects. It is therefore possible to move the orthosiswith relative ease.

But moving the actuated gait orthosis alone would not be suf-ficient for self-controlled movement as long as treadmill speed isconstant. Allowing subjects to intuitively walk with their own speedwas a major improvement on the way to cooperative control. Theoperation principle of treadmill speed adaptation is to measure theforce, generated by the subject in movement direction (Fap) andcontrol the treadmill speed (vT ) according to the Newtonian law

vT =

∫ t

t0

Fap

mdt, (1)

where m is the subject’s body mass.

The last building block is that of haptic feedback. During over-ground walking through an environment with obstacles, collisionsmay occur. To approach realistic collision behaviour, the leg move-ment must be impeded by the LOKOMAT whenever the virtual wallof an obstacle is touched. The approach chosen in this setup was toproduce a reverse force Fx when the subject’s toes penetrate an ob-stacle. In technical terms, this behaviour resembles a linear spring-damper system (with spring constant K and damping coefficient B)and can be noted as

Fx =

−K∆x−B|vx| for ∆x > 0 and vx > 0−K∆x for ∆x > 0 and vx < 0

0 for ∆x < 0, (2)

where ∆x represents the penetration depth of the foot tip, K thespring constant (500 N/m), B the damping coefficient (20 Ns/m), andvx the x-velocity of the foot tip. Note that damping is activatedonly for inward penetration. This is necessary as damping in both

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Figure 1: Virtual reality setup, scheme

directions leads to force discontinuities when moving out of thevirtual wall. With Fx the respective hip and knee torques of the gaitorthosis can be determined.

2.2 Virtual Reality Setup

Figure 1 shows the setup for creating immersive environments. Twoprojectors create three-dimensional graphics with linear polariza-tion. This technology has the disadvantage of malfunctioning whenthe viewer tilts his head too much. But this is not likely to happenin our setup since the head orientation is relatively fixed. Also theweight and cost of glasses was taken into consideration, favoringpassive linear stereo. The screen size is 3 m×2 m, due to the de-mand for a high level of immersion at a viewing distance of 1.2 mfrom the subject in the LOKOMAT. Sound is provided by a Dolby5.1 system, placed around the subject.

2.3 Obstacle Scenario

For the purpose of creating a virtual environment in the contextof gait rehabilitation, an obstacle scenario has been the first choice.The reason for that decision was that in the long monotonous courseof a rehabilitation session obstacles provide alternation and forcethe patients to increase their muscle efforts.

The visual part of the scenario was implemented with a 3Dgraphics engine (Coin3D). This tool was used to animate the pathwith obstacles, the surrounding, the background canvas, and a fig-urine which moves in the same way as the subject in the LOKOMAT.The created scene can be seen in Figure 2.

2.4 Experimental Protocol

To determine the quality of visual feedback, 14 healthy subjectswalked with the LOKOMAT in Zero-Impedance-Mode (see section2.1 and received force feedback when hitting obstacles. Goal pa-rameters were relative obstacle hits, self-chosen walking speed andobstacle clearance.

3 RESULTS

The results for obstacle hit percentage are displayed in Table 1. Themajority of values lies in the range of 20%-60%. Noticable is alsoa large variation which makes it necessary to check the data forsignificant differences with analysis of variance (ANOVA). As twodimensions (perspective, 2D/3D) of data were chosen, a two-wayANOVA was performed. The resulting p-values are p1 = 0.012 forperspectives and p2 = 0.033 for 2D/3D.

Figure 2: Avatar with beach environment

Behind Side Ego

3D 0.40 (0.19) 0.31 (0.23) 0.44 (0.16)2D 0.30 (0.20) 0.28 (0.15) 0.42 (0.16)

Table 1: Median and (standard deviation) for relative obstacle hits

4 CONCLUSION

This paper showed the successful creation of a virtual environmentfor the actuated gait orthosis LOKOMAT. Patients with locomotordysfunctions at various stages of rehabilitation will be able to crossobstacles and see their movement in a virtual environment.

Results showed that the side perspective caused least obstaclehits and that 2D excels 3D in obstacle hit percentage. The othergoal parameters did not show significant differences in their me-dian values what made conclusions more difficult. Altogether, wecould gain some important information on our initially chosen per-spectives and on experimental design.

ACKNOWLEDGEMENTS

The author wishes to thank Eldin Smajic, Thomas Thuring, Huubvan Hedel, Lars Lunenburger, Matthias Harders and Markus Wirz.

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