R. Van Ham

Variable impedance actuators: A review

Variable Impedance Actuators (VIA) have received increasing attention in recent years as many novel applications involving interactions with an unknown and dynamic environment including humans require actuators with dynamics that are not well-achieved by classical stiff actuators. This paper presents an overview of the different VIAs developed and proposes a classification based on the principles through which the variable stiffness and damping are achieved. The main classes are active impedance by control, inherent compliance and damping actuators, inertial actuators, and combinations of them, which are then further divided into subclasses. This classification allows for designers of new devices to orientate and take inspiration and users of VIAs to be guided in the design and implementation process for their targeted application.

Pleated pneumatic artificial muscles: actuators for automation and robotics

Reports on a type of pneumatic artificial muscles (PAMs) that was developed at the Vrije Universiteit Brussel, Department of Mechanical Engineering. Its distinguishing feature is its pleated design. Due to this, it has a very high contraction force and an equally high travel. The weight of these pleated PAMs is very low: a muscle of only 60 gr can pull up to 3500 N and contract by 42%. Furthermore, dry friction and associated hysteresis, typical of many other designs, is avoided by the folding-unfolding action. This significantly simplifies position control using these actuators. Although the force-displacement characteristics of our actuators are non-linear, they can be effectively controlled using basic linear PI techniques. Another advantage of these actuators is their inherent and controllable compliance, making them ideally suited for walking/running machines or whenever delicate tasks, e.g. handling fragile objects, have to be performed. In view of all characteristics pleated PAMs are very well suited for automation and robotic applications.

Proxy-Based Sliding Mode Control of a Manipulator Actuated by Pleated Pneumatic Artificial Muscles

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.

Step Length and Velocity Control of a Dynamic Bipedal Walking Robot With Adaptable Compliant Joints

Controlled passive walking is an approach that extends the passive walking by adapting the compliance of the joints. Natural motions can be chosen in order to obtain a controllable and energy-efficient walking motion. In this paper, actuators with online adaptable compliance are used based on the concept of controlled passive walking, to obtain adjustable step length and velocity during dynamic bipedal walking. We designed and constructed a bipedal walking robot Veronica which is actuated by the MACCEPA actuators, in which the compliance and equilibrium position can be controlled independently. In addition, a 2-D seven-link bipedal model for simulated walking of Veronica is built to analyze the relation between joint compliance and walking characteristics. Experimental results show that effective walking transitions between different walking speeds and step lengths are realized in both simulations and physical robot experiments.

MACCEPA: the mechanically adjustable compliance and controllable equilibrium position actuator for 'controlled passive walking'

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

An exoskeleton for gait rehabilitation: Prototype design and control principle

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.

Estimating robot end-effector force from noisy actuator torque measurements

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.

Pleated pneumatic artificial muscles: compliant robotic actuators

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.

Modeling Hysteresis in Pleated Pneumatic Artificial Muscles

Estimating the force exerted by a pneumatic muscle actuator by measuring its gauge pressure is challenging since hysteresis is almost always present. This paper investigates the hysteresis phenomenon in pleated pneumatic artificial muscles, which is found to be largely independent of gauge pressure. A Preisach based hysteresis model that can cope with the specific shape of the force-contraction characteristic of pneumatic muscles is proposed, and its results are presented.

Variable impedance actuators: Moving the robots of tomorrow

Most of todays 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.