A quasi-zero stiffness biasing mechanism for electrostatic bellow muscle zipping actuators

Electrostatic zipping actuators are electromechanical drives capable of large voltage-induced deformations, with promising applications in soft robotics and wearable mechatronics. To produce a stroke, certain zipping actuator topologies need to be preloaded by an external force, producing a deformation that is then counteracted by electrostatic forces. This study presents a concept for a quasi-zero stiffness (QZS) biasing mechanism for zipping actuators and demonstrates its integration in a family of contractile actuators known as electrostatic bellow muscles (EBMs). The mechanism uses preloaded buckling beams to provide a nearly constant preload force, which reopens the actuator after each actuation cycle without increasing stiffness in operation. Under large external tensile loads, the beams transition into a load-bearing state that limits additional extension, acting as mechanical constraints that protect the actuator from failure. In this work, a model and design guidelines for the QZS mechanism are developed, along with a prototype of a coupled EBM–QZS mechanism system in which the biasing mechanism adds less than 10% weight to the actuator mass. Tests demonstrated that the EBM-QZS system provides a nearly constant low-force stroke without compromising the EBM frequency response, while preserving structural integrity under external forces tens of times larger than the EBM’s weight.