Modeling and control of robots with compliant actuation

Robotics has always contemplated the most perfect existing machine, Human being, dreaming to realize robots able to catch a small spark of this shining light. Robots that are able to emulate human reasoning, but also human motion capabilities, robots that are qualified to coexist and cooperate safely with humans, helping us in our everyday life. Compliance is one of the most fascinating characteristic of human joints, it gives the possibility to crash an aluminum can but in the same way to grab gently a little flower. Thanks to compliant joints human beings are able to do very stiff and accurate motions or moving softly reducing possible injuries in case of unforeseen collisions. Moreover it permits to store energy and release it quickly, for example for trowing objects or for jumping. Inspired by human joints, robotics research groups started to investigate deeply this characteristic, trying either to emulate or reproduce compliant behaviors with robot joints. These researches result in different views for obtaining compliant robot, they can be categorized in: passive compliance, where passive elements, i.e., springs, are arranged into the joint. In this way safety is improved, but the price is in term of performance; active compliance obtained by controlling rigid joints such as to emulate compliant behaviors. The principal disadvantage of this approach is that energy cannot be stored, and some safety issue cannot be completely solved; actively controlled passive compliance, where the characteristic of passive elements inside the device can be controlled in order to vary the joint compliance. The third approach seams to be the best thread-off between safety, capabilities and performance, and as a results of this research field new devices, which permit to control the compliance of the joint, called Variable Stiffness/Impedance Actuator (VSA / VIA), have been developed. These new actuators have been realized with different arrangement of motors and mechanisms with passive elements. In the first part of this dissertation an overview of compliant actuators developed by research centers and universities around the world is shown, then a categorization, which groups all existing compliant actuators in three different typologies, is presented. We characterize each compliant actuator typology, antagonistic variable stiffness actuator and serial variable stiffness actuator, by an equivalent arrangement of motors and flexible transmissions and we present the dynamic model associated to it. From a control point of view these new devices give the possibility to control simultaneously the link motion and the joint stiffness behavior. To take advantage of these capabilities new control methodology ere needed. In the second part of the Thesis we present the control algorithms for compliant joints we developed: i) A feedback linearization controller which permits to control the link position and the stiffness in a decoupled way. ii) A gravity cancellation methodology which solve problematic due to the presence of gravity allowing a simpler and more stable control of compliant joints. iii) A collision detector which estimates external forces using a residual-based methodology. iv) An online stiffness estimator needed to control compliant joints. All methods proposed are developed for each of the three typologies of compliant actuators, and their effectiveness is proved by simulations and experiments.