For a service robot requiring physical human-robot interaction, stable contact motion and collision safety are very important. To accomplish these functions, we propose a novel design for a parallel-type variable stiffness actuator (PVSA). The stiffness and position of a joint can be controlled simultaneously using the PVSA based on an antagonistic actuation inspired by the musculoskeletal system. The PVSA consists of a dual-cam follower mechanism, which acts like a human muscle, and a drive module with two motors. Each cam placed inside the dual cam-follower mechanism has two types of cam profile to provide a wide range of stiffness variation and collision safety. The use of the PVSA enables position and stiffness control to occur simultaneously. Furthermore, joint stiffness instantly decreases when the PVSA is subject to a high torque exceeding a pre-determined value, thereby improving collision safety. Experiments showed that the PVSA provides effective levels of variable stiffness and collision safety.
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.
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