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.

Purpose -Safety is an important issue when manipulators operate in an environment where humans are present, such as the agriculture industry. An intrinsically safe mechanical system guarantees human safety when electronics or controls fail. However, industry also demands a certain operating velocity. A low inertia is the most important aspect to combine safety with a useful operating velocity, because this will limit the amount of kinetic or potential energy in the system and the required actuation forces. Low-actuation forces limit the amount of static contact pressure between manipulator and human, a requirement for intrinsic safety. Low energy means that less contact force is required to put the manipulator to a stop in collision, an additional requirement. The goal of this paper is to find the maximum industrially applicable, manipulator mass for which intrinsic mechanical safety is guaranteed. Design/methodology/approach -Observing existing and proposed manipulators in agriculture results in a required cycle time of 0.9 s, trajectory of 0.8 m and payload of 2 kg. Three important trade-offs applying to the manipulator are identified. The first is between maximum velocity and acceleration, using cycle time and trajectory. The second is between maximum acceleration and mass, based on a measure for pain in contact pressure. The third is between maximum velocity and mass, using a collision model and the contact pressure during collision. Findings -Combining all three trade-offs results in an allowable arm effective inertia of 5.1 kg. Taking payload into account and converting to a realistic mass distribution results in a total mass of 9.3 kg. Compared to existing manipulators, both mass and payload are ambitious but realistic for the future development of an intrinsically safe manipulator. Research limitations/implications -Accuracy in positioning is not taken into account. Originality/value -This paper combines safety criteria on maximum energy and maximum static pressure, while also taking industrial applicable operating velocity into account.