Soft robotics. The motivation behind electrostatic actuation
As robots become increasingly pervasive in our daily lives and workspaces, their shapes and interaction mechanisms progressively need to resemble those of living beings. In this regard, soft robotics is leading the way in creating compliant robots that can safely interact with their surroundings. Here soft actuators, often called „artificial muscles “, play a crucial role. In addition to being soft, these actuators must be easily controllable and efficient.
The effectiveness of artificial muscles is defined by our ability to produce, control, and apply the required actuation stimulus, whether it is light, heat, electricity, or other, and by the ability of these actuators to efficiently respond to it. For example, thermo-responsive polymers are confined by the slow dynamics of heat transfer to applications with low actuation frequencies. Magnetostrictive polymers, which respond to external magnetic fields, require inefficient and bulky field generators. Direct electrical excitation, emerges as a readily accessible, easily controllable, and widely applicable driving method for soft robots.
What are the Electrostatic multilayer systems?
Some promising highly performant designs of electrostatic actuators, called electrostatic multilayer systems (EMS), employ a displaceable dielectric (oil, air), sandwiched between insulated and flexible electrodes (Fig. 1 a). By applying voltage, charges on opposite electrodes attract and squeeze out the fluid, allowing the actuators to contract, just like biological muscles. This principle allows designing configurations that function over vast frequency and force ranges, in various scales. Their low weight, quite operation, large energy per weight, and sustainable and recyclable nature, boosts EMSs beyond soft robotics and artificial muscles, forming the basis of numerous other applications, such as grippers, optical systems, haptic devices, airborne robots, and energy harvesters.
Limitations of the EMSs
In previous works on the topic, scientists stumbled upon a heavily detrimental behavior of their EMSs prototypes: the actuation force drastically when these are driven with constant voltage. As we uncover in this work, this happens because both the insulating polymer films and especially the dielectric fluid have finite conductivity. As a result, under constant voltage conditions, the charges redistribute between different interfaces, altering the electric fields within the stacks and modifying the force. This effect, also known as Maxwell-Wagner-Sillars polarization, renders continuous quasi-static actuation (holding the force) under a constant voltage unfeasible (Fig. 1b, red curve and Fig. 2a top).
The commonly used, “artificial” remedy is rapidly swapping the polarity of voltage from positive to negative, to cyclically redistribute the charges and achieve nearly steady forces. However, this method entails large power losses with drastic reduction of energy efficiency and force oscillations that are unacceptable for high precision applications.
Our theoretical and experimental framework
Our team has formulated a theoretical framework to explain and predict how the electrostatic zipping force in EMSs evolves in time. Our model describes the response to high voltage signals for any stack of dielectrics, which is determined by their thicknesses and material properties (technically, the permittivities and conductivities). Our findings show that interfacial charging can be prevented by employing dielectric materials with matched properties (Fig. 1b, blue curves). We found that a practical way of achieving that is by using electrically leaky polymer films, whose properties would match those of typical dielectric oils, that are typically quite leaky. We validated this framework with an experimental test bench capable of measuring the electrostatic force variation for a wide range of dielectrics combinations and employed it as a tool for materials screening. This effort allowed identifying pairs of suitable dielectrics and delivered materials solutions that can retain constant actuation force, such as a biodegradable polyester (BP) film matched with Envirotemp FR3, an insulating natural ester oil (Fig. 2a, bottom).
Low voltage actuation
The framework inspired our team to venture further and tackle the challenge of reducing the operating voltages in EMS to below 1kV. After tuning the material parameters in our model, we discovered that P(VDF-TrFe-CTFE), a commercially available ultrathin high permittivity terpolymer, approximately matching with paraffin oil, not only allows us to achieve constant force actuation, but also to significantly reduce the voltage. We proved this with an artificial muscle demonstrator, which demands over 2 kV to operate with conventional dielectrics and actuates at just 700 V in the same conditions using the terpolymer.
Addressing the energy efficiency of our approach, our team developed a methodology to assess the power losses both in conventional methods, as well as in ours. We realized that there are primarily two mechanisms behind the losses, energetic losses due to polarization, dominant in the case of high frequency polarity inversion, and losses due to conductivity, more significant in the constant voltage case. Additionally, in the case of polarity inversion, there are losses incurred during the discharging phases of the EMSs. Although the polymers we propose incur higher conductivity losses due to their leaky nature, remarkably, compared to conventional methods, the overall power consumption in this case is up to 1000 times (Fig. 2b).
The material combinations that we found allowed building state-of-the-art artificial muscles for robotics applications, in addition to tunable lenses that can change their focal length, and soft tactile actuators able to develop continuous haptic feedback.
Our experimental materials screening techniques inspired by the theoretical framework, provide predictive tools for future material and design innovations. These findings may enable a wide range of EMS based actuators, energy harvesters and transducers with energy and power efficiencies well beyond conventional technologies.