Variable impedance actuators: A review <s> Nonlinear mechanical interlink <s> For a robotic system that shares its workspace with humans and physically interacts with them, safety is of paramount importance. In order to build a safe system, safety has to be considered in both hardware and software (control). In this paper, we present the safe control of a two-degree-of-freedom planar manipulator actuated by Pleated Pneumatic Artificial Muscles. Owing to its low weight and inherent compliance, the system hardware has excellent safety characteristics. In traditional control methods, safety and good tracking are often impossible to combine. This is different in the case of Proxy-Based Sliding Mode Control (PSMC), a novel control method introduced by Kikuuwe and Fujimoto. PSMC combines responsive and accurate tracking during normal operation with smooth, slow and safe recovery from large position errors. It can also make the system behave compliantly to external disturbances. We present both task- and joint-space implementations of PSMC applied to the pneumatic manipulator, and compare their performance with PID control. Good tracking results are obtained, especially with the joint-space implementation. Safety is evaluated by means of the Head Injury Criterion and by the maximum interaction force in the case of collision. It is found that in spite of the hardware safety features, the system is unsafe when under PID control. PSMC, on the other hand, provides increased safety as well as good tracking. <s> BIB001 </s> Variable impedance actuators: A review <s> Nonlinear mechanical interlink <s> There is a growing interest in utilizing pneumatic artificial muscles (PAMs) as actuators for human-friendly robots. However, several performance drawbacks prevent the widespread use of PAMs. Although many approaches have been proposed to overcome the low control bandwidth of PAMs, some limitations of PAMs such as restricted workspace and torque capacity remain to be addressed. This paper analyzes the limitations of conventional circular pulley joints and subsequently proposes a design methodology to synthesize a pair of variable radius pulleys to improve joint torque capacity over a large workspace. Experimental results show that newly synthesized variable radius pulleys significantly improve position tracking performance in the enlarged workspace. <s> BIB002 </s> Variable impedance actuators: A review <s> Nonlinear mechanical interlink <s> Bringing mechanically compliant joints to robots is in the focus of interest world wide, especially in the humanoid robotics community. Variable Stiffness Joints (VSJ) promise to gain a high performing and robust robotic system. The presented DLR Floating Spring Joint (FSJ) is a VSJ module designed for the first 4 axes of the anthropomorphic DLR Hand Arm System. The DLR Hand Arm System aims to match the skills of its natural archetype. For this purpose, the joints have to be extremely compact to fit into the arm. At the same time they require a high power density in order to approximate the human arm skills. The new DLR FSJ is designed completely from an energy based point of view. This addresses not only energy efficient components and low friction design, but also that the potential energy of the spring is used as good as possible. A demonstration of robustness is given by the investigation of a blunt impact to the tip of the arm. <s> BIB003
By changing the properties of the mechanical interlink one can obtain different transmission ratios. In this category no devices have been reported in literature that use this as the primary principle to adapt the stiffness. However different connections principles exist to yield torque-deflection characteristics in function of the application (see Fig. 8 ). Mostly the compliant elements are connected to the joint using a pulley (see Fig. 8a ). PAMs for example have a limited joint motion due to the limited contraction ratio of the muscles and by choosing different connection principles the torque characteristic can be adjusted. In the biped Lucy  and the manipulator arm BIB001 a pull rod and leverage mechanism are implemented (see Fig. 8b ). By changing the angle and length of the lever arms the strong nonlinearity of the force-angle characteristic of the muscles is compensated in order to flatten the torque-angle characteristics of the joint. The knee rehabilitation robot Knexo uses a four-bar mechanism (see Fig. 8d ) in order to be able to encapsulate the desired torque and joint range requirements with a relative simple and compact mechanism. Another advantage is that the coupler of each four-bar linkage is easily equipped with two pairs of strain gauges for joint torque measurements. An exhaustive search approach is proposed by Beyl et al.  to solve this multi-objective optimization problem. Shin et al. BIB002 developed a design methodology to synthesize a pair of variable radius pulleys (see Fig. 8c ) that maintains high torque capacity while satisfying passive stiffness and workspace requirements. In the VS-joint BIB003 , Maccepa 2.0  and FSJ BIB003 a cam profile is used to have a desired torque-deflection characteristic.