Capstan brake: Passive brake for tendon-driven mechanism

Abstract

Tendon-driven mechanisms are one of the most popular mechanisms for transmitting force and power from a distance. The energy efficiency of a tendon-driven system can be improved if it can maintain the actuation force while not moving, without mechanical work. This can be achieved by using a brake. A brake without an additional actuator is preferred for achieving compactness of the entire system. We present a novel passive brake mechanism - the capstan brake, which consists of a capstan, rollers, and one-way clutches. The friction between the capstan and tendon amplifies a small resisting force (originating from an inactivated motor) to gain enough brake force. Because no additional actuator is involved, no energy is consumed to generate the brake force. In addition, the one-way clutch enables the capstan to rotate in the winding direction. Therefore, the brake force is only exerted when needed, and the performance of the entire device does not decrease owing to the use of the capstan brake. The brake force of the system is limited to handling the safety issues. If the external load exceeds the maximum brake force, the brake becomes back-drivable again. The limit level can be controlled by changing the number of times the tendon is wound around the capstan. Another issue for a tendon-driven system is that the tendon escapes from a spooler when the tendon does not maintain proper tension. By implementing the rollers with the capstan brake, the tendon cannot escape from a spooler even though the tendon is not pulled by an external load in all actuating situations. The performance of the proposed brake mechanism was evaluated through several tests. The results showed that the capstan brake is suitable to be applied to the wearable robotic hand, which is one of the prospective applications of the brake. The force loss caused by the brake was negligible and the brake operated properly even though the tendon was not pulled by an external load in all actuating situations.