Byeong Sang Kim

A Serial-Type Dual Actuator Unit With Planetary Gear Train: Basic Design and Applications

Control of a robot manipulator in contact with the environment is usually conducted by a direct feedback control system using a force-torque sensor or an indirect impedance control scheme. Although these methods have been successfully applied to many applications, simultaneous control of force and position cannot be achieved. To cope with such problems, this paper proposes a novel design of a dual actuator unit (DAU) composed of two actuators and a planetary gear train to provide the capability of simultaneous control of position and stiffness. Since one actuator controls position and the other actuator modulates stiffness, the DAU can control the position and stiffness simultaneously at the same joint. Both the torque exerted on the joint and the stiffness of the environment can be estimated without an expensive force sensor. Various experiments demonstrate that the DAU can provide good performance for position tracking, force estimation, and environment estimation.

Double Actuator Unit with Planetary Gear Train for a Safe Manipulator

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.

Design and Control of a Variable Stiffness Actuator Based on Adjustable Moment Arm

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.

Safe Link Mechanism based on Passive Compliance for Safe Human-Robot Collision

A safe robot arm can be achieved by either passive or active compliance. The passive compliance systems composed of purely mechanical elements often provide faster and more reliable responses for dynamic collision than the active ones involving sensors and actuators. Since both positioning accuracy and collision safety are important, a robot arm should exhibit very low stiffness when subjected to the collision force greater than the one causing injury to humans, but maintain very high stiffness otherwise. To implement these requirements, a novel safe link mechanism (SLM), which consists of linear springs, a double-slider mechanism and shock absorbing modules, is proposed in this research. The main contribution of SLM lies in its variable stiffness capability implemented only by passive mechanical elements. Various experiments for static and dynamic collision show that the stiffness of SLM is kept very high for the external force less than the critical impact force, but it drops abruptly as the external force exceeds the critical force, thus guaranteeing the collision safety. Furthermore, the critical impact force can be set to any value depending on the applications.

Development of a Braille Display using Piezoelectric Linear Motors

The refreshable Braille displays for visually impaired people should be portable and capable of real-time display. To this end, the actuators used to move Braille dots should be small and lightweight. So far various actuators such as solenoids, piezoelectric materials, electroactive polymers have been suggested, but these actuators have not been very successful for various reasons. In this research, piezoelectric linear motors were adopted for this purpose. These actuators can provides the response time fast enough to offer real-time display, and are small and lightweight enough to be employed for a portable device. In this paper, the details of the proposed Braille cell consisting of six piezoelectric linear motors are discussed. The electrical circuit to drive the motors is also introduced. Various tests have been conducted for seven visually impaired people. These tests for the prototype Braille display system show that it can deliver the Braille information which can be well recognized by visually impaired people

Autonomous stair climbing algorithm for a small four-tracked robot

In outdoor environments, mobility, adaptability and reliability of a robot are more important than its speed and precise trajectory. From a practical point of view, tracked robots have an advantage over wheeled robots in outdoor applications. The tracked robot is frequently operated by using the remote controller, but the remote operation is not effective for all cases. To overcome some complex obstacles such as rocks or stairs, the information related to the robot posture is required. However, the sensor information is not intuitive to the user to control the robot. In this research, a multi-active crawler robot (MACbot) was developed and the autonomous stair climbing algorithm was implemented to deal with those problems. Various experiments show that the MACbot can climb the stair autonomously.

Object grasping using a 1 DOF variable stiffness gripper actuated by a hybrid variable stiffness actuator

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.

Preliminary experiments on robotic assembly using a hybrid-type variable stiffness actuator

Precision robotic assembly requires compliant motion to avoid jamming or wedging. To achieve compliant motion, impedance control and a passive compliance device, such as a RCC (remote center compliance) device, have been used in robotic assembly. However, impedance control cannot provide low impedance for the high-frequency range, and load capacity and allowable misalignment of the RCC device are limited. To cope with these problems, we propose to use a variable stiffness actuator in robotic assembly. The 3-DOF manipulator including two HVSAs was developed, and the experiments on robotic assembly were carried out. Two HVSAs provide low impedance to compensate for the lateral and angular errors between the assembly parts. To show the advantages of the HVSA-actuated manipulator over the force-controlled manipulator, we conducted comparison experiments on robotic assembly with the 6-DOF robot manipulator which was controlled by an impedance controller. A series of experiments show that the HVSA-actuated manipulator is more beneficial to execute the tasks requiring both fast motion for high efficiency and low impedance for operational safety.

Impedance-Control Based Peg-in-Hole Assembly with a 6 DOF Manipulator

The maximum accuracy of position control by using an industrial robot is about , whereas the maximum tolerated imprecision in the position of precision parts is about several tens of micrometers. Therefore, it is very difficult to assemble parts by position control only. Moreover, in the case of precision assembly, jamming or wedging can easily occur because of small position/orientation errors, which may damage the parts to be assembled. To overcome these problems, we investigated a force control scheme that provides proper motion in response to the contact force. In this study, we constructed a force control system that can be easily implemented in a position-controlled manipulator. Impedance control by using an admittance filter was adopted to perform stable contact tasks. It is shown that the precision parts can be assembled well by adopting impedance control and blind search methods.

Tool path generation based on matching between teaching points and CAD model for robotic deburring

Deburring 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.