Torque limiters are a proven way to enhance the safety in robots. To further increase the safety, adjustable torque limits depending on the task and the joint configuration (joint angles, velocity, acceleration) would be preferable. Friction clutches can be used as adjustable torque limiters (ATL). In contact free motion the ATL can be set with torque limits higher than the required torque, thereby not influencing the position tracking performance. At an impact, the torque is intrinsically limited, enhancing the safety. Furthermore, depending on the implementation, friction clutches have another relevant property. They can have different torque limits for static and kinetic friction: When the static torque limit is exceeded (as it would be the case in an incidental contact situation), the clutch starts slipping, and the torque output automatically decreases, thereby reducing the forces in a quasi-static contact, as defined in ISO/TS 15066:2016. The current paper implements and profiles an ATL, which exhibits a kinetic torque limit of only 50.4% of the static torque limit at 10rpm. This ensures both an adjustable torque limit fitting to the task requirement and a lower but not zero torque after impact for enhanced safety. Impact experiments validate the safety benefits outlined above.

The proposed device consists of a gravity compensation mechanism to hold its own weight and a force limit element which is developed by ourselves that can ensure the safe interaction between it and the human operator. Since this developed element can be used to control force/torque, and the gravity compensation mechanism enables easy movement of the device, a force rendering device is built.Traditional force rendering devices usually use a force/torque sensor to measure interaction force/torque which increases the cost also introduces a delay caused by the control loop, the proposed device has intrinsic safety property due to its working principle and mechanical safety does not require a control loop to check if the interaction force exceeds the limit.The gravity compensation mechanism employs a spring and a rope to balance the weight according to different configurations of the device adaptively. A strong spring is required to hold the weight and this caused the rope to get stretched and the gravity compensation function can be degraded. To avoid this, a new type of rope that consists of Dyneema rope and another rope was made, experiments show that it can endure high tension while maintaining its length.In the end, the gravity compensation mechanism and the force/torque limit element together can achieve the force rendering device with safety property.

This research aims to achieve wide range force/torque feedback in 6 DoF for haptic rendering device in a virtual reality environment using previously developed Series Clutch Actuator as force/torque setting element. The actuators are embedded in the joints of a 6 DoF robot arm with gravity compensation mechanism to cancel the influence of the weight of the arm itself. Algorithms to control the force/torque output of each clutch actuator by the controller duty cycle is developed as well as the algorithm to map end-effector force/torque (which is the user side haptic feedback) to each joint torque. Virtual reality environment is built and fed to the user as the visual feedback, adding the force/torque feedback from the physical robot arm, the haptic rendering system is achieved. This system can also facilitate as a master-slave remote control system when the virtual reality environment is replaced by a remote slave device, and this can be beneficial in devastating environments.