Actuator’s muscle like properties could allow legged robots to achieve the performance approaching that of their biological counterparts. Some of the beneficial properties of muscle include its low impedance, low friction, high force fidelity, and good bandwidth. Series Elastic Actuators share the properties beneficial with muscle and are well suited for legged robots. These high quality controllable actuators of force allow the control system to exploit the natural dynamics of the robot, to distribute forces among the legs, and to provide an active suspension that is robust to rough terrain.
Most airplanes are designed to have wings so that they glide stably, requiring only a simple power source and simple control to fly, early locomotives used fly ball governors, a mechanical feedback device, to help maintain constant speed, satellites and rifle bullets spin to stabilize their trajectory. These machines were designed so that their natural dynamics allow minimal control effort. Animals have evolved into similar mechanisms that exploit natural dynamics. Birds have wings that allow gliding stably.
The natural dynamics can be exploited in the control of bipedal walking robots: the swing leg can swing freely once started; a kneecap can used to prevent the leg from inverting; and a compliant ankle can be used to naturally transfer the center of pressure along the foot and help in toe off. Each of the mechanisms helps to make the easier control to achieve and results in motion that is natural and smooth looking.
The actuators must show the extremely low impedance and friction to the system in order to exploit passive dynamics in a robot. The output impedance and inertia are high with traditional actuation systems such as hydraulics and highly geared motors. In contrast, Series Elastic Actuators show extremely low impedance and low friction and thus may be used in robots that exploit their natural dynamics.
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