Inherently Elastic Actuation for Soft Robotics

  • Chongjing Cao

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


Dielectric elastomer actuators (DEAs) are an emerging type of soft actuators that have the attractive features of large actuation strains, fast response speed, and high energy density.

Soft robotics is a rapidly growing research field that seeks solutions to reduce the complexity of safely interacting with environments. In contrast to rigid robots, soft robots utilize soft materials such as silicone rubber that can deform when interacting with unknown environments. The compliance nature of DEAs makes it ideal for soft robotics.

However, despite the development of DEAs for two decades, the inherent elasticity of the dielectric elastomer has been long overlooked as a resource for improved efficiency, higher work output and resonant operation. In this work, the author investigates the principles of utilising the elasticity in DEAs and develop high performing DEAs that uses these principles for soft and bioinspired robotics.

This study is built on a comprehensive electromechanical dynamic model of generalized cone DEAs and three different configuration variations: (i) circular and planar dielectric elastomer oscillator (DEO), (ii) double cone dielectric elastomer actuators (DCDEAs), (iii) magnetically coupled dielectric elastomer actuators (MCDEAs). For each configuration, its dynamic response is analysed in-depth and physical insights are drawn with the generalized cone DEA model and experiments.

Based on the cone DEA configuration, different elastic actuation principles for different dielectric elastomer materials are proposed. For Very-High-Bond (VHB) acrylic material with high viscoelasticity, the elastic actuation is demonstrated by recovery of elastic energy via a reduced the actuation duty ratio which enables the elastic energy stored in the membranes to contribute to work output. For silicone material with low viscosity, the elastic actuation is demonstrated by driving the DEA at its resonance which amplifies the stroke/power output.

By using the three DEA configurations, along with the two elastic actuation strategies, applications that demonstrate the feasibility and clear advantage of inherently elastic actuation and the use of DEAs in soft/bioinspired robotics are developed.

Based on large out-of-plane resonant actuation of the DEO, a monolithic electroadhesion (EA) – DEO soft gripper is developed to overcome the slow de-adhesion issue of conventional EA grippers. The performance of the EA-DEO gripper is tested against different lightweight and flexible materials. By using the resonant actuation of the DEO, the release speeds of the gripper are sped up from several minutes to 100s of milliseconds, which demonstrates at least two orders of magnitude of improvement.

A bioinspired robotic leg and flapping wing mechanism driven by the DCDEA elastic artificial muscles are presented which utilizes the elastic energy recovery principle and resonant actuation principle respectively. For the flapping wing mechanism, a peak flapping stroke of 31˚ at the resonant frequency of 30 Hz is reported, which far outperforms previously published DEA driven flappers.

Finally, the first DEA driven pneumatic pump by using the MCDEA developed in this work is proposed. A proof-of-concept prototype has a 40 mm diameter and 30 mm height. This pump design exhibits a peak pressure output of 30.5 mbar and flowrate of 0.9 SLPM at the resonance of the driving DEA with a low power consumption of 40 mW.

The inherently elastic actuation of DEAs demonstrated in this thesis shows the clear advantage of applying it for soft and bioinspired robotics applications.
Date of Award28 Nov 2019
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorAndrew T Conn (Supervisor), Stuart C Burgess (Supervisor), Arthur G Richards (Supervisor) & Shane P Windsor (Supervisor)


  • dielectric elastomers
  • soft robotics

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