Active and Passive Control of an Exoskeleton with Cable Transmission for Hand Rehabilitation

Abstract

Abstract —This paper investigates the control algorithm of an exoskeleton for hand rehabilitation, which accomplishes both active and passive control mode. A double closed loop control structure is developed, which consists of position control loop and compensation control loop. The position controller is based on impedance control. The compensation controller is used for compensating the position error caused by deflection of the cable and sheath in the mechanical transmission. To realize the compensation, the spring model is used to represent the elasticity of the cable and sheath. With the proposed method, the maximum joint position error is about 1.5 degree, which satisfies the requirement in hand rehabilitation application. The experimental result demonstrates the validity of the propose method. Keywords- rehabilitative training; active control mode; passive control modes ； compensation controller I. I NTRODUCTION As we know, the normal motor capability of hand is crucial and important for human-being’s daily life. Hands, however, are apt to be injured in accident. And the rehabilitation is essential for the patients to recover after hand operation. Additionally, diseases, stoke for instance, can also result in the loss of hand function. In order to regain the motor capability, the hand rehabilitation is a fundamental therapeutic approach. The traditional rehabilitation approach is costly for patients and laborious for therapists. Recent research showed that hand rehabilitative training using mechatronic devices and virtual reality is possible and effective [1] and is attracting more research interests [2-8]. Dependent on different design and different application, some control algorithms are investigated [6-11]. Despite of the researches, there are still some problems to be investigated. The rehabilitation usually includes four modes, i.e. passive, active, assisted and resisted rehabilitation. So far, there is no solution which has covered all of the four control modes. Aimed at the hand rehabilitation, our research group developed a wearable exoskeleton for index finger rehabilitation [12]. In this paper, the control strategy of this exoskeleton is investigated to provide both active and passive control modes. A compensation control method is presented to reduce the position error due to deflection of the cable and sheath in the mechanical transmission. To do this, a double closed loop structure is developed, which is used for realizing the position control and error compensation, respectively. With the proposed method, the maximum joint position error is less than 1.5 degree, which satisfies the requirement in hand rehabilitation application. The remainder of the paper is organized as follows. Section 2 introduces the mechanical structure of the exoskeleton. Section 3 describes the control algorithm of active and passive control modes. Section 4 depicts experiment and the results. Section 5 gives conclusion and future work. II. M