Modeling of low severity soft-tissue injury due to unwanted collisions of a robot in collaborative settings is an important aspect to be treated in safe physical Human-Robot Interaction (pHRI). Up to now, safety evaluations for pHRI were mainly conducted by using safety criteria related with impact forces and head accelerations. These indicate severe injury in the robotics context and leave out low severity injury such as contusions and lacerations. However, for the design of an intrinsically safer robot arm, a reliable evaluation of the collision between a human and a robot that is based on skin injury criteria is essential. In this paper, we propose a novel human-robot collision model with and without covering, which is based on the impact stress distribution. The reliability of the proposed collision model is verified by a comparison with various cadaver experiments taken from existing biomechanical literature. Since the stress characteristics acting on the human head can be analyzed with this new collision model, the occurrence of certain soft-tissue injury can be estimated. Furthermore, the method serves for selecting the appropriate covering parameters, as e.g. elastic modulus and thickness, by evaluating the chosen skin injury indices.

In recent years, the potential for collision between humans and robots has drawn much attention since service robots are increasingly being used in the human environment. A safe robot arm can be achieved using either an active or passive compliance method. A passive compliance system composed of purely mechanical elements often provides faster and more reliable responses to dynamic collision than an active system involving sensors and actuators. Since positioning accuracy and collision safety of a robot arm are equally important, a robot arm should have very low stiffness when subjected to a collision force capable of causing human injury. Otherwise, it should maintain a very high stiffness. To implement these requirements, a novel safe joint mechanism (SJM-III) consisting of an inclined link, a slider with rollers, and linear springs is proposed. The SJM-III has the advantage of nonlinear stiffness, which can be achieved only with passive mechanical elements. Various analyses and experiments on static and dynamic collisions show high stiffness of the SJM-III against an external torque less than a predetermined threshold torque, with an abrupt drop in stiffness when the external torque exceeds this threshold. The safe joint mechanism enables a robot manipulator to guarantee positioning accuracy and collision safety, and which is simple to install between an actuator and a robot link without a significant change in the robot's design.

Collision safety between humans and robots has drawn much attention since service robots are increasingly being used in human environments. A safe robot arm based on passive compliance can usually provide faster and more reliable responses for dynamic collision than an active one involving sensors and actuators. Since both positioning accuracy and collision safety of the robot arm are equally important, a robot arm should have very low stiffness when subjected to a collision force greater than the injury tolerance, but should otherwise maintain very high stiffness. To implement these requirements, a novel safe joint mechanism (SJM-II) which has much smaller size and lighter weight than the previous model, is proposed in this research. The SJM-II has the advantage of nonlinear spring which is achieved using only passive mechanical elements such as linear springs and a double-slider mechanism. Various analyses and experiments on static and dynamic collisions show that stiffness of the SJM-II is kept very high against an external torque less than the predetermined threshold torque, but abruptly drops when the input torque exceeds this threshold, thereby guaranteeing positioning accuracy and collision safety. Furthermore, a robot arm with two SJM-IIs is verified to achieve collision safety in 2D space.

In recent years, collision safety has been one of the most important issues for service robots. To ensure collision safety assurance, a passive compliance method is preferred to an active one because it can provide faster and more reliable responses to dynamic collision. Since both positioning accuracy and collision safety are equally important, a robot arm should have very low stiffness when subjected to a collision force greater than the one causing human injury, but maintain very high stiffness otherwise. In order to realize these ideal features, a novel safe joint mechanism (SJM) composed of linear springs and a modified slider-crank mechanism is proposed in this paper. The SJM has the advantages of variable stiffness which can be achieved only by passive mechanical elements. Various experiments on static and dynamic collisions show the high stiffness of the SJM against an external force of less than the critical impact force, but an abrupt drop in the stiffness when the external force exceeds this critical force, which guarantees positioning accuracy and collision safety. Furthermore, the critical impact force can be set to any value depending on the application and the environment.

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.

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.

Accurate load sensing is increasingly important in performing various tasks involved with contacting the environment. Most research has been focused on improving the hardware of a force/torque sensor. Torque sensors for a manipulator suffer from crosstalk, which cannot be compensated even by calibration. Thus they require precise machining and complicated structure to minimize crosstalk, which often results in cost increase. This research proposes a new calibration method composed of two steps. By the primary calibration, the torque sensor output can be related to the joint torques. The secondary calibration, which is based on the virtual load, is conducted to compensate crosstalk of the torque sensor. The virtual load is obtained from the measured joint torques and the manipulator configuration. Using the proposed calibration method, the external load acting on the end-effector can be sensed accurately even with relatively low-quality torque sensors. Experimental results showed that the error in load sensing was significantly reduced by the proposed calibration method.