Stiffness and Reference Position Transition Method to Control Compliant Actuators and Application to Drumming Robot Design

Abstract

Compliant actuators have been developed to ensure safety and motional diversity. The main characteristic of a compliant actuator is the inherent elasticity of the actuator. Compared to conventional stiff actuators, compliant actuators are equipped with elastic materials such as springs, oil, and air. The inherent elasticity provides a secure contact in human-robot interaction and enables the actuator to perform various motions.

The first type of the compliant actuators is the Series Elastic Actuator(SEA). The SEA has a structure in which elasticity is connected in series between the rotor and the link. This series elasticity can increase the safety of the actuator, but at the same time makes it difficult to control the actuator. The elasticity of actuator makes the system delayed, and the delayed system makes the control unstable.

For this reason, SEA has evolved into a Variable Stiffness Actuator(VSA). VSA consists of variable stiffness mechanism with series elasticity. Through the stiffness transition, the control bandwidth of the actuator can be adjusted. VSA not only broadens the application of compliant actuators but also is suitable for realizing human motion due to mechanical similarity with human musculoskeletal structure (antagonistic structure).

This thesis covers various topics of compliant actuators, including the development of elastic actuators, the development of control methods, and the application of control methods. The list of the research topics is shown as follows :

- Development of low stiffness Series Elastic Actuator (DSSAS). - Study on control algorithms of stablization for low stiffness compliant actuators. - Study on drum robot applications using proposed control algorithms.

The subject of the first study is low stiffness. Low stiffness is often used for tasks requiring high levels of safety, such as rehabilitation. In the first part of this paper, development of a novel low stiffness actuator which named Dual Spiral Spring Actuation System (DSSAS) is presented.

The subject of the second study is the control of compliant actuators. Compliant actuators generally lack control bandwidths than stiff actuators. In order to overcome this lack of control bandwidth, variable stiffness is used. Inspired by the movement of the human body, two methods have developed : Stiffness Transition Method (STM) and Reference Position Transition Method (RPTM). STM and RPTM have been successfully applied and its safety assessment has also been carried out.

The subject of the third study is the application. A drum-playing robot was implemented using the proposed control method, STM, and RPTM. STM and RPTM helped to stabilize the movement of the drum-playing robot and made it possible to implement special ways of playing such as staccato strokes and drum rolls.

This thesis follows the following sequence: Introduction, development of low stiffness series elastic actuator, development of stabilization methods of compliant actuators, application of stabilization method through drum robot, and conclusion.