For a long time, robots were seen as rigid machines driven by sturdy motors. This raised worries concerning the safety of people interacting with them. One option to make robots safer is to equip them with compliant actuators that can adapt to external forces, such as a human getting in the way. Note that most natural systems also rely on compliant actuators such as muscles that can store energy, thereby making them more efficient for tasks such as running or hopping.

Building on the potential of safe and energy efficient actuators, Vanderborght et al. propose a new type of actuator called MACCEPA 2.0 (Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator). As seen in the figure below, when the position of the profile disk (heart shape) is changed by a servomotor or the joint is bent, this causes the tendon that is guided over the profile to pull on the spring. To counteract the pulling force, a torque will be generated that depends on the shape of the profile. To change the compliance of the actuator, simply replace the profile by another shape. Similar to what happens in human legs, the stiffness of the actuator increases with joint flexion.

Working principle of the MACCEPA 2.0. Top: Bent position (generating torque). Middle: At equilibrium position (not generating torque). Bottom: Preloaded spring caused by rotating profile.

The actuator was demonstrated on the 1DOF hopping robot Chobino1D shown below. The spring is preloaded by turning the profile using a servomotor before releasing the tension for the jump. Using MACCEPA 2.0, the robot was able to jump much higher than a robot with a stiff actuator.