Robotic Actuator Impedance

Robots perform repetitious and tedious tasks with great speed and precision in traditional manufacturing operations. In this setting where the environment is controlled and the tasks are repetitious, position controlled robots which trace predefine joint trajectories are optimal. However, in unstructured environments where little is known of the environment, force controlled robots that can comply with the surroundings are desirable. This is the case for legged robots which walking over rough terrain, robotic arms interacting with people, wearable performance haptic interfaces, enhancing exoskeletons, and other robotic applications.
A controllable actuator could be a perfect force source, outputting exactly the commanded force independent of load movement. All of force controllable actuators will have limitations which resulting in deviations from a perfect force source in the real world. These limitations include impedance, bandwidth and stiction. An actuator’s impedance is the additional force created at the output by load motion. Impedance is a function of the frequency of the load motion, typically increasing with frequency of load motion. A system of easily backdriveable is considered to have low impedance. Sticton describes the phenomenon of stick slip or sticky friction, which is [resent in most devices where mechanical components are in sliding contact. Stiction must be overcome by a breakaway force, which limits the smallest force the actuator can output. The bandwidth of an actuator is the frequency up to which forces can be accurately commanded. Bandwidth is affected by saturation of power elements, control system gain among other things, and mechanical stiffness. Impedance is zero, stiction is zero, and bandwidth is infinite. Muscle has extremely low impedance and stiction and moderate bandwidth and is the currently best known actuation technology that approaches a perfect force source.
Present day of technologies actuator have characteristics that have severely limited their use in force controlled applications. A geared electric motor has a high reflected inertia, a lot of stiction and is difficult to back drive. Hydraulic systems have high seal friction and are often impossible to back drive.

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