Stable contact and safe responses to the collision have been studied to develop interactive robots such as service and collaborative robots. Stable and safe interactions are usually achieved through the inherent compliance of a motion controller with external torque estimation. However, a fixed control gain would sacrifice either compliance or position tracking performance. Additionally, external torque estimation is susceptible to model errors. In this study, a novel variable stiffness control approach is proposed to achieve a high position tracking performance in free motion and compliant behavior in the contact state. For this purpose, a precise estimation of the external torque and control gains that change based on the external torque are required. To estimate the external torque precisely, a collision detecting learning algorithm that uses long short-term memory (LSTM) is adopted. Although this method uses only proprioceptive sensors, its torque estimation capability is comparable to that of methods that use additional sensors. Then, the stiffness of a motion controller is adjusted based on the external torque in the stable region. Moreover, by adopting the Operational Space Formulation considering joint elasticity for a motion controller, high position tracking performance can be achieved with only proprioceptive sensors. The performance of the proposed method was validated through comparative experiments with two degrees of freedom (DoF) manipulator.

In this study, a mechanism that realizes a novel structural form of the harmonic reducer is introduced. Conventional robots often use various mechanical reducers owing to low torque and high-speed characteristics of electric motors. Among them, harmonic reducers are frequently used because of their compact size and backlash-free precision. The plate harmonic reducer which uses the same topological geometry and reducing mechanism as the conventional harmonic reducer is a novel type of strain gear that changes its shape to a plate form for axial deformation. It has unique differences in terms of axial thickness, torsional stiffness, and efficiency due to its morphological characteristics. This study introduces and analyzes the reducing principle of the plate harmonic reducer and describes the methodological solutions for realization. Finally, the theoretical performance improvement and operating feasibility of the plate harmonic reducer are analyzed using finite element method and a 3D-printed prototype model.

Torque controlled robots have the capability of implementing compliant behavior with back-drivability. In practice, however, joint elasticity often prevents an accurate position tracking performance of a robot. In particular, humanoid robots are influenced more by elasticity because of a long kinematic chain between the feet and hands. In this paper, we present a new inverse dynamics based control approach for torque controlled humanoid robots with joint elasticity. When formulating the operational space control framework, feedback control consists of only motor-related parts with measured motor angle values, and the link dynamics is compensated by the feedforward terms. The experiment results of the proposed approach show a noticeable improvement in the position tracking performance in 6-DoF manipulator. Finally, the proposed method was applied to a torque controlled biped robot for walking. Both stiff motion control of the CoM and compliant motion control of the foot were simultaneously achieved, demonstrating the advantage of the torque controlled robot.