Sang Duck Lee

Novel collision detection index based on joint torque sensors for a redundant manipulator

Human-robot collision has drawn increasing attention in recent years and collision safety can be improved by successfully detecting collisions between a human and a robot. For a manipulator working in human environments, collisions usually occur at the manipulator body while the robot performs a contact task using its end-effector to interact with the environment. Therefore, both collision force and the force on the end-effector contribute to the external torques which can be estimated from the robot dynamics and the joint torques measured by the joint torque sensors, which means whether or not a collision has occurred cannot be reliably determined using this estimation. In this study, we propose a novel collision detection index to detect collisions independently of the end-effector force of a redundant manipulator equipped with joint torque sensors. Using the null space projection of a redundant manipulator, the collision detection index can be expressed as a function of the torque generated by a collision and the manipulator configuration. The proposed index is verified through various simulations. Simulation results show that collisions can be reliably detected regardless of the presence of the end-effector forces even in situations with external torques contaminated by substantial error.

Sensorless collision detection for safe human-robot collaboration

As there have been many attempts for human-robot collaborations, various solutions to collision detection have been proposed in order to deal with safety issues. However, existing methods for collision detection include the usage of skin sensors or joint torque sensors, and cannot be applied to robots without these sensors such as industrial manipulators. In this study we propose a sensorless collision detection method. The proposed method detects the collision between robots and humans by identifying the external torques applied to the robot. Without using extra sensors, we observed the joint friction model and motor current. We have formulated the friction model for the robot by using the dynamics of the robot and the observer based on the generalized momentum. In addition, formulation of the friction model and the identification scheme did not include any use of extra sensors. The performance of the proposed collision detection method was evaluated using a 7 DOF manipulator. The experimental results show that collision can be reliably detected without any extra sensors for any type of robot arm.

Human–robot collision model with effective mass and manipulability for design of a spatial manipulator

As the use of service robots becomes more popular, many solutions to ensure human safety during human–robot collision have been proposed. In this paper, we address one of the most fundamental solutions to design an inherently safe robot manipulator. A collision model is developed to evaluate the collision safety of any spatial manipulator. Most collision studies have focused on collision analysis and safety evaluation, but not on the use of evaluation results to design a safer robot arm. Therefore, we propose a collision model that relates design parameters to collision safety by adopting effective mass and manipulability. The model was then simplified with several assumptions. Furthermore, experimental results from biomechanical literature were employed to describe a human–robot collision. The major advantage of this collision model is that it can be used to systemically determine the design parameters of a robot arm.

Torque control based sensorless hand guiding for direct robot teaching

In recent years, much research has been done to develop various direct teaching schemes, in which an operator directly teaches a robot by hand guiding instead of using a teach pendant. However, conventional direct teaching methods are usually sensor based, thus leading to an expensive solution. To deal with this problem, this study proposes a sensorless hand guiding method based on torque control. The dynamic model of a robot along with the motor current and friction model is used to determine the users intention to move the end-effector of a robot instead of directly sensing the external force by the user. A friction torque observer is employed for the friction model identification. The proposed method was experimentally verified using a 6 DOF robot. The experimental results show that the proposed sensorless hand guiding scheme is quite useful for practical direct teaching although the sensitivity is slightly lower than the sensor-based approach.

Design of a 6-DOF collaborative robot arm with counterbalance mechanisms

Most collaborative robots use high-power motors for a good weight-to-payload ratio, thus leading to not only an increase in manufacturing cost but also possibility of injury at a collision between a human and a robot. To maintain high-performance with low-power driving units, a spring-based counterbalance mechanism (CBM) and a robot arm based on these CBMs were developed in our previous study. In this study, a 6-DOF collaborative robot equipped with a multi-DOF CBM is proposed. A double parallelogram linkage and a slider-crank mechanism are employed for a compact and durable design of a multi-DOF CBM. Unlike the previous prototypes in which some portions of CBMs were protruded out of the robot body due to their large volume, the proposed CBMs can be embedded inside the robot links. The performance of the developed CBM and collaborative robot were verified based on simulations using dynamic simulation software. Simulation results show that the proposed CBMs can effectively reduce the joint torques required to operate the robot. This reduction in the torque enables low-power motors to be used in a collaborative robot, thus significantly improving collision safety and energy efficiency.

5 DOF Industrial Robot Arm for Safe Human-Robot Collaboration

Collision safety is very important when it comes to human-robot collaboration. However, most of the industrial robots are equipped with large capacity motors for high performance, which on the contrary, increases risk of injuries caused by collision between humans and robots. For this reason, collaboration with common industrial robots has been restricted unless the robots are isolated from workers. To address this problem, in this study we propose a safe collaborative robot arm equipped with the counterbalance mechanism (CBM) and sensorless collision detection method. Furthermore, the sensorless collision detection scheme includes the reaction scheme in case of collisions without the use of any expensive sensors. The performance of the proposed CBM and collision detection method was evaluated based on simulations and experiments, respectively. The simulation and experimental results show that it is possible to operate the robot even with only small capacity motors while maintaining its performance, and collision can be reliably detected without any extra sensors for any type of robot manipulator.

Collision detection of humanoid robot arm under model uncertainties for handling of unknown object

In recent years human-robot collision safety has received considerable attention. Thus, various collision detection algorithms have been proposed to ensure human-robot collision safety, and these algorithms are usually model-based. However, the dynamic model of a robot arm is uncertain or unknown in cases where the arm performs a task with various objects or tools. In this paper, we propose a collision detection method for a robot arm that changes its tools or pick up various objects. For this purpose, a novel collision detection index, which is decoupled from the inevitable external force generated by the object being handled by the robot, is developed. The proposed index is verified through various simulations using a 7-DOF robot arm, and the corresponding results show that regardless of the object that is being handled, it is possible to detect collisions.

Collision detection for safe human-robot cooperation of a redundant manipulator

Human-robot cooperation have a potential risk of collision between a human and a robot, and collision detection is one of the most practical solutions to ensure human safety during this cooperation. However, collisions during human-robot cooperation cannot be reliably detected using conventional collision detection methods because collision forces and intended interaction forces cannot be distinguished from each other. In this study, we propose a collision detection method that can be used to distinguish collisions from intended task force at the end-effector. To this end, we developed a collision detection index that is decoupled from the intended external force applied for human-robot cooperation. The proposed method was verified through several experiments, and experimental results show that collisions can be detected regardless of the presence of physical contact between humans and robots for cooperation.

Guideline for determination of link length of a 3 DOF planar manipulator for human-robot collision safety

In recent years, collision safety between a human and a robot has been increasingly important along with the spread of service robots. Much research has been conducted on how to design a safe manipulator, but it is not applicable to real manipulators yet. To deal with this problem, we propose a guideline to design a safe manipulator, which is composed of two phases, the safety evaluation and design parameter adjustment. To evaluate collision safety, we adopt the effective mass and manipulability and simplify the spatial collision between a human and a multi-DOF manipulator. Moreover, the experimental results from the biomechanical literature are employed for more realistic evaluation of collision safety. The design parameters and proper adjustment strategy can be established based on the safety evaluation results, and the design parameters of a manipulator can be modified to satisfy the collision safety and design requirement. The proposed guideline is implemented in the design of a 3 DOF safe manipulator, and it is shown that this guideline can be used to design a multi-DOF safe manipulator.