Abstract—Safe interaction between rehabilitation robots and patients requires the development of dedicated technology. Inherently compliant actuators, such as artificial muscles (McKibben actuators) can help achieve this goal. This paper presents McKibben actuators with an initial length that is invariant with pressure. They also feature improved range of stiffness and increase range of extension. An application of the developed actuators in an anthropomorphic robotic arm presented. A stiffness control scheme was implemented that allows it to safely shake hands with humans. I. INTRODUCTION OMMONLY, in industrial settings (e.g. spot welding robots in a car assembly line), humans are shielded from robots. In robot-aided rehabilitation, robots are intentionally put in contact with humans. This is true for therapy robotics, where clinicians are supported by robotic devices to relieve their hard physical work, such as the Lokomat [1], Lopes [2] and the MIT Manus [3]. It also holds for socially interactive robotics (e.g. [4]), where presently most of these do not physically interact with humans, but where it is foreseeable that they will be extended in this direction, for instance to touch a patient, shake hands, pet, or similar. Clearly, the design approach needed in these areas is different from the one that works well in industry. The observation that precise positioning accuracy and stiffness in trajectory control can be (or must be) much less in rehabilitation applications than in industrial applications, gives the designer of rehabilitation robots great latitude in conceiving new technology dedicated to inherently safe human-machine interaction. Inherent safeness of a robotic device can be defines as the device being harmless even if it goes out of control. One approach to achieve this is by (1) statically balancing the robot and (2) applying soft control [5]. Static balancing is the realization of continuous equilibrium, regardless of the configuration of the robot, without the use of friction [6]. Thus, gravity is eliminated, and zero actuation is required to maintain any position. In systems that move relatively slowly, acceleration torques are low and low power actuators can be selected. Safe control can be realized by incorporating softness in the actuators themselves rather than in their control, a concept related to impedance control Manuscript received February 18, 2007. Johan Surentu and Just L. Herder (contact author) are with Delft University of Technology, Department of Biomechanical Engineering, Mekelweg 2, 2628CD, Delft, The Netherlands, tel: +31-15-2784713, fax: +31-15-2784717, email: j.l.herder@tudelft.nl . Gabriëlle Tuijthof is with the Academic Medical Center, Department of Orthopedics, Amsterdam, The Netherlands. [7]. Among inherently soft actuators there are artificial muscles or McKibben muscles. They have been used in a number of applications, both in a research environment (e.g. [8], [9]) and in commercially available products [10]. They work well (e.g. specific power of 10 W/g, [11]-[13]) but suffer from some imperfections. One is that typically not only their stiffness depends on the feed pressure but also their initial length. This paper proposes an improvement in artificial muscles, namely a design where the initial length is invariant with feed pressure. An extended model will be presented, prototypes that were manufactured and tested will be described, and a preliminary impression of their application in a robotic arm will be given. II. METHODS AND MATERIALS A. Description of McKibben muscles Pneumatic artificial muscles (McKibben actuators) consist of a flexible hose that is constrained in radial extension by a braiding out of woven nylon fibers (Fig. 1). The muscle can be pressurized, which changes its force-length relationship and usually also its length (Fig. 2). Chou and Hannaford [14] derived a quadratic characteristic, but in practice the force-length relationship is fairly linear, although significant hysteresis is present. Stiffness is linearly dependent on pressure, therefore a McKibben muscle can be regarded an adjustable spring [15]. The tubing is often out of latex. This provides a good behavior, but poor durability. In order to improve this, a silicone tubing is proposed. However, a problem is that the silicone tubing is much stiffer that a latex one, which results in a strong influence of the pressure on the initial length of Optimized artificial muscles for an inherently safe robotic arm Johan Surentu, Gabriëlle J.M. Tuijthof, Just L. Herder, Member, IEEE C Fig. 1. McKibben pneumatic artificial muscle actuator. From top to bottom: braiding of woven fibers, tubing, and assembled actuator.