**1. Introduction**

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The fields of Robotics and Automation have been experiencing a boom the last few years. According to the "International Federation of Robotics" (IFR) in 2011, 140, 000 robot units have been sold. This made 2011 the year with the most robot sales ever [52, 55]. Experts predict a worldwide population of 1.3 million industrial robots until 2014. The prognoses for the field of service robotics are also very promising [53, 54, 56]. The growing number of service robots changes the general requirements for those robots. Whereas industrial robots are optimized regarding precision, repeatability, and reliability, requirements for service robots are different, due to more human-robot interaction (HRI) and operation in unstructured environments. This means these robots need to integrate new concepts in terms of adaptivity, safety, and universality, necessitating change in the characteristics of the actuation systems and the structures.

Safety strategies can be divided into pre-collision and post-collision strategies [75].

Post-collision strategies traditionally refer to the fields of measuring and control. The important questions in these fields are how human injury and robot damage can be minimized after a collision has occurred. Existing standards regarding robot safety [59, 60] are currently revised based on the most recent studies in the area of post-collision safety [12, 43–45].

Pre-Collision safety strategies have been discussed over the last 30 years [49, 67, 91, 93] including their limitations. More recent works discuss the fact that since the robots are operating in a human-centered environment, this must inform the design of the robots. Kathib et al. [68] describe how a robot should be designed to establish autonomous tasks as well as human guided tasks. Haegele et al. [46] determine that robots should either have a broad sensory infrastructure to limit forces and moments through measuring them, or should be inherently compliant. In 2006 Alami et al. [4] published a novel design paradigm for robots: **"design robots that are intrinsically safe and control them to deliver performance"**. In summary the requirements for a high degree of pre-collision safety are:

©2012 Gaiser et al.), licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


These requirements are obvious in the field of service robotics since any interaction with animals, technology, or humans will be safer by implementing them. Operating industrial robots usually requires a strict division between the working area of the robot and the working area of the human, since industrial robots normally do not fulfill these requirements. However, there are many tasks that could be accomplished resulting in higher quality or more efficiency, with closer human-robot interaction.

A lower inertia of the robot allows faster operating speed. At the same time lower inertia reduces the impact in case of a collision. The application of composite materials can achieve this while maintaining stiffness and precision of the robot. An impressive example is the DLR "Lightweight Robotic Arm III" (LWR III) [50]. Another example presenting a robot for the manipulation of small masses is described in [75].

When looking at compliant actuation systems for robots it becomes clear that there is currently much effort to add compliance to conventional drives by adding elastic elements to the drive chain. This concept is referred to as series elastic actuation and has been carried out in many various forms [24, 121, 138, 140]. Other drives with more or less inherent compliance are piezo-drives, shape-memory-actuators (SMA), electrorheological drives, and polymeric actuators. Fluidic actuators are a well-suited actuation principle for compliant actuation. Whereas pneumatic actuators are already compliant because of the compressibility of gases, hydraulic actuators need the integration of compliant membrane structures in order to achieve compliance. This group of actuators is referred to as "Flexible Fluidic Actuators".

In addition to its drive elements a robot consists of structural elements connecting the drive elements. So independently from the drive system a robot can exhibit compliant characteristics via the integration of compliant structural elements.
