Control of a robot manipulator in contact with the environment is usually conducted by the direct feedback control system using a force-torque sensor or the indirect impedance control scheme. Although these methods have been successfully applied to many applications, simultaneous control of force and position cannot be achieved. Furthermore, collision safety has been of primary concern in recent years with emergence of service robots in direct contact with humans. To cope with such problems, redundant actuation has been used to enhance the performance of a position/force controller. In this paper, the novel design of a double actuator unit (DAU) composed of double actuators and a planetary gear train is proposed to provide the capability of simultaneous control of position and force as well as the improved collision safety. Since one actuator controls position and the other actuator modulates stiffness, DAU can control the position and stiffness simultaneously at the same joint. The torque exerted on the joint can be estimated without an expensive torque/force sensor. DAU is capable of detecting dynamic collision by monitoring the speed of the stiffness modulator. Upon detection of dynamic collision, DAU immediately reduces its joint stiffness according to the collision magnitude, thus providing the optimum collision safety. It is shown from various experiments that DAU can provide good performance of position tracking, force estimation and collision safety.

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For tasks requiring robot-environment interaction, compliant motion is important to ensure stable contact and operational safety. The compliant actuators such as a series elastic actuator and a variable stiffness actuator (VSA) are expected to be one of the promising solutions to provide a compliant motion. In this paper, we propose a gripper actuated by the hybrid variable stiffness actuator developed in our previous work to show how the VSA can improve the ability of grasping. The gripper, which is equipped with two symmetric 4-bar linkages, can conduct the position and stiffness control simultaneously, and the grasping force can be calculated by the estimated torque of the HVSA. The HVSA-actuated gripper is able to grasp fragile objects with compliant motion and the relatively heavy object with stiff motion. Furthermore, the features of the variable stiffness provide operational safety by adjusting the stiffness according to the task. In this study, the relationships between the joint torque and the grasping force, and between the joint stiffness and the Cartesian stiffness are analyzed to control the grasping force. From a series of experiments, it is shown that the gripper can grasp fragile objects such as an egg and a wine glass, as well as relatively heavy object without any soft cover and force/torque sensor.

Abstract²Deburring is a machining process which involves much noise, vibration and dust, so it can be harmful to workers. Therefore, much research has been done to use a manipulator to perform deburring instead of human workers. The precise tracking of the contour of an arbitrary-shaped part is of major concern in robotic deburring. In this study, a tool path generation method based on the CAD model and the direct teaching method is proposed to minimize the position and orientation errors of the workpieces. Without knowledge of the position and orientation of the workpiece, which is often hard to obtain, the optimal deburring trajectory can be generated by matching the extracted tool path from the CAD model to the teaching points. Furthermore, impedance control is used to avoid applying excessive contact force. From a series of experiments on robotic deburring, the performance of the proposed algorithm is verified.

For tasks requiring robot-environment interaction, stiffness control is important to ensure both stable contact motion and collision safety. The variable stiffness approach has been used to address this problem. We propose a hybrid dual actuator unit (HDAU) which is a novel variable stiffness unit design. The HDAU is composed of a hybrid control module based on an adjustable moment arm mechanism and a drive module with two motors. By controlling the relative motion of gears in the hybrid control module, position and stiffness can be simultaneously controlled for the same joint. The HDAU provides a wide range of joint stiffness due to nonlinearity obtained from the adjustable moment arm. The joint stiffness can be kept constant independent of the passive deflection angle of the output shaft. Furthermore, stable interaction can also be achieved because the joint stiffness is indirectly adjusted by position control of the hybrid control module. The characteristics of the HDAU are analyzed in this study. We show by experiment that the HDAU can provide a wide range of stiffness variation and rapid response for stiffness change.

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For tasks that require robot-environment interaction, stiffness control is important to ensure stable contact motion and collision safety. The variable stiffness approach has been used to address this type of control. We propose a hybrid variable stiffness actuator (HVSA), which is a variable stiffness unit design. The proposed HVSA is composed of a hybrid control module based on an adjustable moment-arm mechanism, and a drive module with two motors. By controlling the relative motion of gears in the hybrid control module, position and stiffness of a joint can be simultaneously controlled. The HVSA provides a wide range of joint stiffness due to the nonlinearity provided by the adjustable moment arm. Furthermore, the rigid mode, which behaves as a conventional stiff joint, can be implemented to improve positioning accuracy when a robot handles a heavy object. In this paper, the mechanical design features and related analysis are explained. We show that the HVSA can provide a wide range of stiffness and rapid responses according to changes in the stiffness of a joint under varying loads by experiments. The effectiveness of the rigid mode is verified by some experiments on position tracking under high-load conditions. Index Terms-Adjustable moment arm, compliant joint, lever mechanism, variable stiffness actuator (VSA).