Intrinsically Safe Robot Arm: Adjustable Static Balancing and Low Power Actuation

We present a design for a manipulator that is intrinsically mechanically safe, i.e. it can not cause pain (let alone damage) to a human being even if the control system has a failure. Based on the pressure pain thresholds for human skin, we derive a pinching safety constraint that limits the actuator torque, and an impact safety constraint that results in a trade-off between mass and velocity. To fulfill all constraints, the manipulator requires a spring balancing system that counteracts gravity in all configurations of the manipulator. This allows the use of extremely low-power DC motors (only 4.5 W). Thanks to the torque and speed limitations of these motors the manipulator is indeed intrinsically safe, yet still capable of moving a useful payload of 1.2 kg over a distance of 0.8 m in 1.5 s.