Human-robot collaboration, despite its numerous advantages, is possible only if user safety is guaranteed at all times. Indeed, it would be unacceptable to place a human in the proximity of a dangerous machine. Even if it is possible to improve safety by placing sensors on the robot to detect and avoid dangerous situations, it is preferable to design intrinsically safe robots which are physically unable to hurt people.


    A possible approach to achieve this objective consists in reducing the inertia of the manipulator and the power of its actuators. Using lighter materials and optimising the mechanical design are effective methods to lower a robot's weight. However, it is difficult to reduce inertia beyond a certain limit if the load that must be lifted by the robot is itself significant.

    Static balancing consists in eliminating the efforts that must be exerted by the actuators to support the manipulator's weight at rest. As a result, less powerful and thus safer actuators can be used. Adding a counterweight is a simple means to achieve static balancing. However, if it is added on the manipulator, the total inertia is increased which is counterproductive. To avoid this problem, the counterweight can be deported on the base and be linked to the manipulator by a passive transmission, as illustrated on Figure 1. As a result, the additional inertia is only reflected when the robot is moving vertically.

    Fig. 1: Deported balancing principle using hydraulic cylinders.

    The deportation principle can also be applied to actuators. By placing them on the manipulator's base, the inertia of the mobile components is reduced and the required power is minimized. This technique has been succesfully implemented using a cable (for vertical motion) and belts, as can be seen on Figures 2 and 3. The counterweight being placed on a lever, it is possible to displace it to balance various loads. The obtained prototype requires an input power similar to a hairdryer (for a payload of 100kg!), which attests the effectiveness of the approach which consists in placing the counterweights and actuators on the base.

    Fig. 2: Balancing system and belt routings to deport the actuators of an assist device prototype.
    Fig. 3: Photo of the assist device prototype.