High-performance actuators are required for numerous novel applications such as human-robot assistive devices. The torque-to-weight ratio and energy efficiency of current actuation technology is often too low, which limits the performance of novel robots. Therefore, we developed a Series-Parallel Elastic Actuator (SPEA) which enables variable recruitment of parallel springs and variable load cancellation. Finding suitable intermittent mechanisms for the SPEA is however still challenging. This paper reports on the innovative design of an intermittent self-closing mechanism for a MACCEPA-based SPEA that can deliver bi-directional output torque and variable stiffness, while minimizing friction levels. Experiments on a one-layer intermittent self-closing mechanism are conducted to validate the working principle and the proposed model. A demonstrator of the MACCEPA-based SPEA with intermittent self-closing mechanism is presented and the experiments validate the modeled output torque and lowered motor torque for different stiffness settings.

Most of today's robots have rigid structures and actuators requiring complex software control algorithms and sophisticated sensor systems in order to behave in a compliant and safe way adapted to contact with unknown environments and humans. By studying and constructing variable impedance actuators and their control, we contribute to the development of actuation units which can match the intrinsic safety, motion performance and energy efficiency of biological systems and in particular the human. As such, this may lead to a new generation of robots that can co-exist and co-operate with people and get closer to the human manipulation and locomotion performance than is possible with current robots.

This paper discusses two ways to estimate the interaction force at the end-effector of a robot. The first approach that is presented combines filtered dynamic equations with a recursive least squares estimation algorithm to provide a smoothened force signal, which is useful in the (common) case of noisy torque measurements. The second approach, which uses a generalized momentum based disturbance observer, is mainly discussed to compare it to the first approach. Although very different in appearance, it is shown that a close connection exists between both approaches. Simulation results for both algorithms are shown, and experimental results derived from a sensorless admittance controller that was implemented using the algorithms are presented.

The MACCEPA (Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator) is an electric actuator of which the compliance and equilibrium position are fully independently controllable and both are set by a dedicated servomotor. In this paper an improvement of the actuator is proposed where the torque-angle curve and consequently the stiffness-angle curve can be modified by choosing an appropriate shape of a profile disk, which replaces the lever arm of the former design. The actuator has a large joint angle, torque and stiffness range and these properties can be made beneficial for safe human robot interaction and the construction of energy efficient walking, hopping and running robots. The ability to store and release energy is shown by simulations on a 1DOF hopping robot. Its hopping height is much higher compared to a configuration in which the same motor is used in a traditional stiff setup. The stiffness of the actuator has a stiffening characteristic so the leg stiffness resembles more a linear stiffness as found in humans.

Research in robotic gait rehabilitation still faces many challenges regarding ankle assistance, body weight support and human-robot interaction. This paper reports on the development, focusing on these challenges, of a gait rehabilitation exoskeleton powered by pleated pneumatic artificial muscles. The first prototype is intended as a platform for the evaluation of design and control concepts. The mechanical design procedure is explained with the emphasis on optimization. A proxy-based sliding mode control approach is proposed and evaluated by means of simulation. Simulation results indicate good tracking performance and safe system behavior, encouraging experimental validation on the prototype.

Kikuuwe and Fujimoto have introduced proxy-based sliding mode control. It combines responsive and accurate tracking during normal operation with smooth, slow recovery from large position errors that can sometimes occur after abnormal events. The method can be seen as an extension to both conventional PID control and sliding mode control. In this paper, proxy-based sliding mode control is used to control a 2-DOF planar manipulator actuated by pleated pneumatic artificial muscles (PPAMs). The principal advantage of this control method is increased safety for people interacting with the manipulator. Two different forms of proxy-based sliding mode control were implemented on the system, and their performance was experimentally evaluated. Both forms performed very well with respect to safety. Good tracking was also obtained, especially with the second form.

In this paper a novel rotational actuator with adaptable compliance is presented. First the importance of adaptable compliance for bipedal walking is explained, and then a number of comparable designs are given with their possible drawbacks. The MACCEPA concept and design is described in detail. The formula to calculate the generated torque is derived. It is shown, depending on the design parameters, that the torque is a quasi linear function with respect to the angle between equilibrium position and actual position. Also the change of the pre-tension has a quasi linear effect on the torque. Another advantage is that the actuator can be built with standard components, e.g. electrical servo motors. Experiments show the independent control of equilibrium position and compliance. Finally, the concept of controlled passive walking is explained, which is a combination of the control strategies of active and passive walking robots. Controlled passive walking requires actuators with adaptable compliance, preferably where the control of equilibrium position and compliance are independent

In the biped Lucy pleated pneumatic artificial muscles are used instead of electrical motors to power the joints, because in an antagonistic set-up both the torque and the compliance are controllable. The muscles have also a high power to weight ratio and they can reduce impact effects. Interesting characteristics that can be exploited for legged robots. In this paper a control strategy is discussed where a torque control unit tracks a predefined trajectory and a compliance controller is used to reduce control efforts and energy consumption by fitting the compliance of the actuator to the natural compliance of the desired trajectory. The first part of this paper focusses on the torque control unit for the biped. The proposed control architecture consists of the joint trajectory generator and the joint trajectory tracking controller. The trajectory generator calculates trajectories represented by polynomials based on objective locomotion parameters, which are average forward speed, step length, step height and intermediate foot lift. The joint trajectory tracking controller is divided in three parts: a computed torque module, a delta-p unit and a bang-bang pressure controller. Results of the incorporation of this control architecture in the real biped Lucy are given. Several essential graphs showing tracking performance and pressure regulation are given and the effectiveness of the control algorithm is discussed. A second part of the paper focusses on the compliance controller which is experimentally tested on a one DOF pendulum. A mathematical formulation to exploit the natural dynamics with respect to different walking patterns for this purpose is explained. The experimental results show the effectiveness and importance of the adaptation strategy

This paper reports on the control structure of the pneumatic biped Lucy. The robot is actuated with pleated pneumatic artificial muscles, which have interesting characteristics that can be exploited for legged machines. They have a high power to weight ratio, an adaptable compliance and they can reduce impact effects. The discussion of the control architecture focusses on the joint trajectory generator and the tracking controller which is divided in four parts: a computed torque module, an inverse delta-p unit, a local PI controller and a bang-bang pressure controller. The control design is divided into single support and double support where specifically the computed torque differs for these two phases. A full hybrid dynamic simulation model is used to evaluate the control architecture of the biped. This simulator combines the dynamical behaviour of the robot with the thermodynamical effects that take place in the muscle-valves system. The observed hardware limitations of the real robot and expected model errors are taken into account in order to give a realistic qualitative evaluation of the control performance and to test the robustness. Finally the first results of the incorporation of this control architecture in the real biped Lucy are given.

Pleated pneumatic artificial muscles (PPAMs), developed at the Vrije Universiteit Brussel, Department of Mechanical Engineering, are used as robotic actuators. Their distinguishing feature is their pleated design, as a result of which their contraction forces and maximum displacement are very high compared to other pneumatic artificial muscles. The PPAM design, operation and characteristics are presented. A rotative joint actuator, made of two antagonistically coupled PPAMs, is discussed to demonstrate their suitability for robotics. It has several properties that are similar to those of skeletal joint actuators. Positioning tasks are seen to be performed very accurately using a simple PI control. Furthermore, the antagonistic actuator can easily be made to have a soft or careful touch, contributing greatly to a safe robot operation. In view of all the characteristics PPAMs are very well suited for automation and robotic applications.