Fundamental Research

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Transparent Actuation Systems for Recovering Natural Dynamics

To develop a transparent actuator system, we have investigated a cable-driven actuation system, a direct-driven high-power actuator, and an actuator module with a flexible transmission mechanism. The cable-driven actuation system was to remove the weight of the actuator from the distal body segments and to make the wearable part soft. The direct-driven actuator was to obtain large assistive forces with the minimal mechanical impedance by eliminating gears. In practice, the actuator module with a flexible transmission mechanism (i.e., a series elastic actuator (SEA)) was the most promising for assistive robots for incomplete paraplegic patients. The flexible transmission (i.e., a spring) isolated the actuator dynamics from the human body dynamics, which enabled the precise and robust control of the assistive force transferred through the flexible transmission. The mechanical design parameters, such as the spring constant, were obtained from optimal control theories by modeling the interaction force of the transmission as a feedback control law. The gears were designed such that the effective inertias were evenly distributed in a two-mass system representation, which was to maximize the open-loop frequency bandwidth. The overall actuation system was able to precisely generate the desired torque without any friction, which enabled realization of the natural dynamics of the overall robot system.

Structure of a cRSEA

Compact rotary series elastic actuator demonstration

Bio-inspired Mechanisms for Better Control Performance

To design an effective assistive mechanism, we investigated various mechanical components that can effectively support the body weight without disturbing the voluntary motions. In order to maximize the energy efficiency of locomotion, the human musculoskeletal system has been evolved to be actuated by both active (i.e., voluntarily contractile) muscles and passive elements (e.g., the ligament and the tendon), where the main role of the passive elements is absorbing and releasing energy exerted from the environment. Inspired by this principle, we have devised and utilized a controllable pneumatic cylinder in parallel with the transparent actuators at the robot joints. The controllable pneumatic cylinder behaves as a gas spring when the valve is closed, while it has a small friction when the valve is open. Therefore, the valve was controlled such that it supports the body weight by its compressibility during the stance phase and that it imposes the minimal resistance during the swing phase. The gas spring constant was determined based on optimal control theories by modeling the interaction force of the pneumatic cylinder as a linear state feedback control law. The use of the pneumatic cylinder effectively removed the bias in the control signal generated in practice, which implies that the burden of the actuation system was lowered.


Dual-layered Direct-driven Linear Actuator

Various actuation mechanisms are applicable to robotic leg systems, but the actuation system for the high-speed locomotion robots should satisfy good controllability, as well as high force (torque) capacity. Therefore, in our laboratory, we have developed an actuation system which adopts the principle of electric motors. For the light weight and improved mobility, the electric current is applied to the actuator via pairs of electrodes and brushes, which is often called the brushed direct-current (DC) motor mechanism. It should be noted that the brushed DC motors can be operated by pulse-width-modulation (PWM) signals amplified by a simple H-bridge amplification circuit. Therefore, the proposed actuation system does not require any heavy peripherals except for the H-bridge amplification circuit.

Specific Areas

Robotic Systems Control (RSC) Laboratory is one of the most active, innovative and productive university-laboratories in the area of Control Applications, Mechatronics, and Robotics. All of the members are motivated and passionate to become a world-leading researcher in this field.

The specific research topics, which the RSC Laboratory is dealing with, are introduced in the following figure. The figure also shows how Control Applications, Mechatronics, and Robotics research topics are distinguished and connected with each other.

Overall Research Flow

For the detailed research issues with specific applications, please click the Control Applications, Assistive Robotics, or Locomotive Robotics sub-menus. The research projects that the RSC Laboratory has been working on are listed at Projects.