Ionic Polymer Metal Composites (IPMC) modelling and control
Electroactive Polymers (EAPs) are materials that change their shape in response to electric stimulation. The properties of EAPs are rather similar to biological muscles in terms of force, strain and speed. EAPs have many appealing properties compared to traditional electromechanical robotic devices containing motors, gears and bearings. They are mechanically simpler, lightweight and easy to miniaturize, they are flexible and soft and with a high degree of freedom; their motion is also noiseless. There are many types of electroactive polymers with various properties and with a various type of reaction to electrical stimuli. By and large, EAP materials can be divided into two large groups, electronic and ionic polymers. Our research group is currently focused on ionic polymer materials, specifically ionic polymer metal composites (IPMC). Ionic polymers bend when electric stimulation is applied.
Opposite to electronic polymers they produce large displacement when stimulated and operate on low voltages. At the same time their response is relatively slow; they produce small actuation force and usually need an aquatic environment for operation. This research focuses on building EAP devices as well as methods for their control. Particularly, we are focusing at the following research problems:
- Design of novel EAP actuators.
- Development of position sensors for EAP actuators.
- Development of control methods of EAP actuators to achieve less energy consumption at large output force and torque.
- Development of feedback control methods for EAP actuators.
- Design of autonomous EAP actuators and devices.
State-of-the art of the research activities
The first years of our research has led to the following results:
- Building small-size fish-like underwater robot prototypes with IPMC fins. The fins are controlled in an open-loop. The goal of this research was to prove that IPMC actuators can be used to build complicated devices and the movements of IPMC actuators can be coordinated. Our ray-like swimming robot with 16 IPMC muscles (8 muscles on both pectoral fins) is to our knowledge the most complicated devices built with IPMC actuators. We replicated the undulating motion of the ray-like pectoral fins.
- A method for reduced energy consumption of EAP ionic actuators. We have shown that depending of the shape and size of the input signal the energy consumption of IPMC actuators can be reduced by 2.5 times. This allows us to build dedicated electronic circuits for driving IMPC actuators (artificial muscle drivers).
- A self-sensing IPMC sensor-actuator. We have developed an innovative design of a sensor-actuator pair that permits as to gain a feedback signal from an actuator. The signals of the self-sensing actuator are highly correlated and deterministic. We currently work on signal analysis of the actuator.
• Control of an IPMC muscle in a feedback loop. We have gained the preliminary results at controlling an inverted pendulum with an artificial muscle. We currently work on improving the control methods since the system is still very unstable. To our knowledge this is the first attempt to manipulate an object with an EAP actuator.
Future research
At present, a little is known about the behaviour of EAP materials and therefore their motion is hard to control. Therefore our main research objective is to model the EAP materials and find methods to control them. This also includes development of position, force and torque sensors since analogous devices for traditional electromechanical devices are often to large and heavy. To accelerate the achievement of these goals we are also looking for suitable applications of EAP devices where the advantages of these innovative materials can be exploited best. An example of this kind of application can be a soft and lightweight end-effector or manipulator with distributed sensing