posted on October 31, 2014
When building robots that interact closely with humans, it is important to make the arms as light as possible. With heavy arms, soft and delicate interaction with people is not possible. Because motors are heavy, electric-powered robot arms with human-like arm speed pose challenges when interacting with people. Using multiple-stage gearboxes allows the use of lighter motors, but these gearboxes slow down the arm and increase the friction in the joints, leading to shaky, “robotic” motion.
To avoid this problem, we have developed a fluid-filled transmission that allows the electric motors for each joint to be located inside the body, instead of mounted in the arms. The transmission is much lighter than the electric motors it displaces, so the arms are lighter, and can move faster. In fact, most animatronic robots use pneumatic or hydraulic cylinders for exactly this reason: keeping the arms light. But consider how a hydraulic robot interacts with the environment: if the arm hits an obstacle, the fluid pressure rises, increasing the contact force rather than yielding to the contact; the control valves don’t move and the pump doesn’t spin backwards—we say that a hydraulic system is not backdrivable. If you blow into your garden hose, the valve will not turn backwards by itself. This makes hydraulic robot interaction with humans stiff and awkward. One solution is to add force sensing at every joint and pressure sensing in every hydraulic line. By feeding back this information, it is possible to create softer interactions, but the sensing and control demands lead to complicated designs, and because the controller must wait for sensor data before acting, achieving high speeds and soft interactions simultaneously is very challenging.
The following video shows a robot arm that we have built using our new backdrivable and low friction transmission. The arm is light and fast, and the very low transmission friction allows for delicate and precise movements. These properties allow for natural motion and compliant physical interaction with people. This robot arm was designed and built by Tianyao Chen, Peter Whitney, and Jessica Hodgins.
Full article: www.engineering.com/DesignerEdge/DesignerEdgeArticles/ArticleID/8831/A-Low-Friction-Passive-Fluid-Transmission-and-Fluid-Tendon-Soft-Actuator.aspx