Conventionally, tendon-driven manipulators implement some force-based controller using either tension feedback or dynamic models of the actuator. The force control allows the system to maintain proper tensions on the tendons. In some cases, whether it is due to the lack of tension feedback or actuator torque control, a purely position-based controller is needed. This work compares three position controllers for tendon-driven manipulators that implement a nested actuator position controller. A new controller is introduced that achieves the best overall performance with regards to speed, accuracy, and transient behavior. To compensate for the lack of tension control, the controller nominally maintains the internal tension on the tendons through a range-space constraint on the actuator positions. These control laws are validated experimentally on the Robonaut-2 humanoid hand.

NASA and General Motors have developed the second generation Robonaut, Robonaut 2 or R2, and it is scheduled to arrive on the International Space Station in early 2011 and undergo initial testing by mid-year. This state of the art, dexterous, anthropomorphic robotic torso has significant technical improvements over its predecessor making it a far more valuable tool for astronauts. Upgrades include: increased force sensing, greater range of motion, higher bandwidth, and improved dexterity. R2's integrated mechatronic design results in a more compact and robust distributed control system with a fraction of the wiring of the original Robonaut. Modularity is prevalent throughout the hardware and software along with innovative and layered approaches for sensing and control. The most important aspects of the Robonaut philosophy are clearly present in this latest model's ability to allow comfortable human interaction and in its design to perform significant work using the same hardware and interfaces used by people. The following describes the mechanisms, integrated electronics, control strategies, and user interface that make R2 a promising addition to the Space Station and other environments where humanoid robots can assist people.