Control of McKibben pneumatic muscles for a power-assist, lower-limb orthosis T.-J. Yeh * , Meng-Je Wu, Ting-Jiang Lu, Feng-Kuang Wu, Chih-Ren Huang Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, Taiwan article info Article history: Received 7 October 2009 Accepted 15 July 2010 Keywords: Pneumatic muscle Lower-limb orthosis Hysteresis Maxwell-slip model LTR control Bumpless switching abstract In this research, a power-assist, lower-limb orthosis is developed to help the elderly or people suffering sports injuries walk or climb stairs. In the pneumatic muscle used for actuation, it is found that hysteresis phenomenon exists during the inflation–deflation process and such a phenomenon deteriorates the con- trol performance. In order to eliminate the influence of hysteresis on the control system, a hysteresis model is constructed and used to devise an inverse control for feedforward compensation. The inverse control is combined with loop transfer recovery (LTR) feedback control to achieve better tracking perfor- mance. Moreover, bumpless switching compensators are also incorporated into the combined control system to ensure smooth switching between different phases of operation. To verify that the developed orthosis can effectively accomplish the assistive function, a human subject wearing the orthosis is asked to walk and to climb stairs. Experiments indicate that the orthosis is indeed helpful in assisting human locomotion. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The pneumatic muscle actuator, or so-called McKibben pneu- matic artificial muscle, was developed in the 1950s and 1960s for artificial limb research [1,2]. It consists of a rubber inner tube surrounded by a braided mesh shell. When the inner tube is pres- surized, it expands in a balloon-like manner but the expansion is constrained by the braided shell. As the volume of the inner tube increases with the increase of pressure, the pneumatic muscle shortens and/or produces tension if it is coupled to a mechanical load. The pneumatic muscles are well-known for their exceptionally high power and force to weight/volume ratios. Moreover, they are inherently compliant that can generate soft contact and thus have excellent safety potential. These attractive features make the pneu- matic muscle a promising actuation source for robotic exoskeleton or powered orthoses applications where not only safety for human interaction is needed but also lightweight actuation design is de- sired. For instance, Kobayashi et al. [3] developed a muscle suit for supporting manual worker. The muscle suit consists of a mechanical armor-type frame and pneumatic muscles. Zhang et al. [4] proposed a novel curved pneumatic-muscle-based rotary actuator for a wearable elbow exoskeleton. A powered ankle–foot orthosis (AFO) that uses artificial pneumatic muscles was devel- oped by Ferris et al. to produce active plantar flexor torque [5]. Extension of AFO to a knee–ankle–foot orthosis (KAFO), which uses pneumatic muscles to power ankle plantar flexion/dorsiflexion as well as knee extension/flexion, has also been considered in [6]. In [7], a 10 DOF lower limb exoskeleton with legs powered by pneu- matic muscles was constructed for force augmentation and active assistive walking training. It should be noted that the above-mentioned references mainly focus on the overall system design and performance evaluation. Sophisticated behavior of pneumatic muscles are not particularly incorporated into the controller design. Instead, the forces or tor- ques generated by pneumatic muscles therein are simply achieved either by regulating the air pressures to fixed settings [3] or according to the processed electromyography (EMG) [5,6], via sim- ple PID controllers [7], or black-box based fuzzy controllers [4]. In order to better understand the behavior of pneumatic muscles for not only control but also design purposes, several research efforts have been devoted to the modeling of such actuators. For instance, the model in [8] shows that pneumatic muscles exhibit a nonlinear force–displacement relationship. Moreover, it is demonstrated in [9,10] that the thread-on-thread friction acting inside the braided shell further induces hysteretic behavior to such a nonlinear model. In [11], Klute and Hannaford developed a model that pre- dicts the maximum number of life cycles of the pneumatic muscle actuator based on available uni-axial tensile properties of the actu- ator’s inner bladder. In [12], the same authors presented another model that includes a nonlinear, Mooney–Rivlin mathematical description of the actuator’s internal bladder. It is shown experi- mentally that the model provides more accurate prediction on the actuator’s output force. In [13], a phenomenological model 0957-4158/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechatronics.2010.07.004 * Corresponding author. Tel.: +886 35742922; fax: +886 35722840. E-mail address: tyeh@pme.nthu.edu.tw (T.-J. Yeh). Mechatronics 20 (2010) 686–697 Contents lists available at ScienceDirect Mechatronics journal homepage: www.elsevier.com/locate/mechatronics