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Mechanical behavior of a bi-stable negative-rate bias spring system

When a high electric field is applied across a dielectric electro-active polymer, the stiffness, in the in-plane direction, decreases. This change in stiffness can be used to generate linear actuation in the out-of-plane direction if the dielectric electro-active polymer (DEAP) is subject to a suitable bias force. This bias force is commonly provided by a linear spring, but a recent research work suggests the use of so-called negative-rate bias springs (NBS) to increase the achievable stroke. These systems are geometrically non-linear systems with bi-stable mechanical characteristics with a negative stiffness between equilibrium points that can be efficiently matched to the DEAP load/deformation behavior. This paper provides an overview of current work performed using NBS and discusses why NBS provide more displacement output. In addition, this paper presents a simple analytical model and a FE simulation of a negative-rate bias spring (NBS). The simple model is introduced to explain the non-linear snap-through behavior of the bi-stable NBS. FE simulated results obtained with bi-stable buckled beam are examined and compared with a linear spring model. The beam buckling and stiffness are identified to be analogous to the simple linear spring model's pre-compression and spring stiffness. These parameters can be used to tune a mechanism to appropriately match with a DEAP film. Future work includes NBS mechanism design and their coupling with circular DEAPs.