Sunjai N. Shanmugam - electromechanical modelling and control of ionic electroactive polymer actuators

The future of soft robotic actuation systems with ionic electroactive polymer actuators is very promising especially in the microdomain for cutting edge applications such as micromanipulation systems, medical devices with higher dexterity, soft catheters with built-in actuation, bio-inspired robotics with bettermimicking properties and active compliant micromechanisms. However, still, there are significant challenges in understanding the transduction, developing simpler fabrication approaches and robust control schemes for real-time control. The work provides an electro-chemo-mechanical model with the emphasis of the charge storage mechanism in the ionic polymer actuator. The model is extended to design and structurally optimize the performance of the actuator through computer simulations before the fabrication process.

A simple additive manufacturing method is also presented for the actuator fabrication using low-cost syringe type printer that is a much more promising alternative to other known fabrication methods. The printing method allows to fabricate multipart and intricate patterns with high throughput production capability and also opens the opportunity to print a matrix array of identical actuators over a wide size scale.
In specific, the model addresses the background electrochemical kinetics responsible for mass and charge transport process specific to the type of electrode materials and also provides insights in understanding the influence of physical properties of electrodes such as porosity and tortuosity on the electrical conductivity and deformation of the actuator. In addition, the model can deduce mechanical deformation direction and rate of deformation dependence on the mobility of the ions.
For validation, simulations are conducted and the outcomes are compared with the experimental data. The results show that the developed model is able to well predict the behavior of the actuator, providing a comprehensive understanding of charge dynamics in the ionic polymer actuator. The high fidelity of the model provides the opportunity to design, simulate and optimize complex ionic polymer actuator patterns and soft robotic systems, and as a result, a soft parallel manipulation system with three degrees of motion capability is designed and simulated.
The developed system was manipulated by four independently controllable ionic polymer actuators and was capable of moving in 3 degrees of freedom, i.e. rotating around two axes (pitch and roll) and linear displacement along one axis. With satisfactory large displacements and rotation angles predicted from the model, fabrication of the device proceeds to the printing of the manipulation system. The results showed rotation angles of 11.36 degrees on two axes and 5.1 mm linear displacement along one axis with 0.27 s rise time.

The obtained results show a close match with the simulation outcomes representing high fidelity of the model. Also, the performance of the printed manipulator is better than the CDC based manipulator which is fabricated using the tedious spray painting technique. In 69 considering the larger linear and angular motion with three degrees of motion and high speed of response, the printed manipulator produces higher displacement and rotation angles compared to previous devices that are fabricated directly using commercially available materials. As a potential applications, a four-way optical switch and a microscope stage with feedback control using the printed CP actuator was demonstrated showing the capability of the parallel manipulation system. The four-way switch can optically trigger four light-dependent resistors in the xy-plane in an automated sequence, illustrating the ability of the monolithic flat printed actuators to manipulate in the xy-plane, which is a much simpler system as compared to two servo motors that are normally required for such applications.
For the microscope stage, model predictive control is developed for positioning and stabilizing the manipulator in the field of view under the microscope. It is shown that the MPC acts as a robust control scheme for deploying real-time applications that can foresee the constraints of the ionic actuators. The controller can also be extended to handle the uncertainties exhibited by the actuator due to changing environmental conditions.

It is shown experimentally that the developed controller was well capable of predicting and controlling the actuator for micromanipulation applications with precision control. Overall this work results in modelling, fabrication, and real-time control of ionic electroactive polymer actuators leading to the development of a low-cost, monolithic, flat, multi-DOF parallel manipulator for micromanipulation. The future direction of the work is to integrate a strain sensor as a part of the monolithic structure merged with printed electronics and onboard closedloop controller to control each of the actuators of the parallel manipulator to make a fully autonomous motion system.

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