Electrostatic Capstan Clutch to Reduce Actuation SWaP

Roboticists have demonstrated that large mobile robotic platforms are possible. However, we still lack highly dynamic autonomous mobile systems with reasonable operating times. Reducing actuation size, weight, and power is the most effective way to improve the speed, dexterity, and operating time of current platforms. For example, current legged robots consume considerable power just to stand still. In addition, conventional, fully actuated robots employ one actuator per joint, which tends to be heavy, particularly for high degree-of-freedom (DoF) designs. By using clutches that generate high holding torques with low power consumption, one motor can be used for multiple outputs, known as mechanical multiplexing. Clutches accomplish this by routing mechanical power through a system, letting multiple outputs benefit from sharing resources. Clutches can also act as a braking mechanism, locking joints to block unwanted motion from consuming power. Through the synergistic combination of the capstan effect and electrostatic adhesion, we produced the highest reported braking force per unit area (specific shear stress) for electrostatic clutches at 31.3 N/cm^2. This is a 49% increase compared to the previous highest.<br/><br/>Electrostatic clutches are based on the attractive force from an applied voltage between two electrodes separated by a dielectric. In a typical device, one electrode is adhered to the dielectric, while the other acts as the braking surface between itself and the open face of the dielectric when a voltage is applied. Electrostatic clutches can be implemented in a light, thin, and low power fashion and have the added benefit of electrical rather than mechanical control over the braking force. Our design also utilizes the capstan effect. Capstan-based clutches are especially useful devices that take a small input tension and exponentially scale the output tension by wrapping a flexible line around a shaft. Generally, the capstan effect is used in marine or industrial applications where a human operator can hold entire ships or large equipment in place with little input tension. The combination of electrostatic and capstan effect is accomplished by adding a dielectric to the shaft and controlling the electrostatic adhesive force between the shaft and line wrapped around the shaft. The addition of electrostatic adhesion produces a second term in the capstan equation that is a function of geometry, material properties, and applied voltage. Our device delivered a holding torque of 7.1 Nm on a 25.4 mm diameter output shaft using only 500 V and consuming 2.5 mW/cm^2. The clutch elements weighted 20.2 g. We found that due to the exponential nature of the capstan effect, such devices improve holding torque and power efficiency as the number of wraps increases.<br/><br/>In previous work, electrostatic clutches demonstrated mechanical multiplexing by operating a 10 DoF tentacle robot using a single servo. Clutches in the joints rapidly controlled the power and motion between segments. Locked joints transferred power via tendons running alongside the robot. Unlocked joints were then manipulated via these tendons. Even though the robot only had one motor, it was still capable of highly dexterous tasks such as grasping and manipulating two objects.<br/><br/>Electrostatic clutches will allow for novel robot configurations unencumbered by the need for numerous motors that are typically the heaviest and most expensive parts of a robotic platform. Mechanical multiplexing gives an actuator the multi-functionality to provide mechanical power to different elements of a robot. Incorporating electrostatic adhesive clutches into robot platforms will increase control and mobility while also reducing power consumption.