A novel artificial muscle actuator called as twisted and coiled polymer actuator can be easily fabricated by commercially available Nylon fibers. It can be thermally activated and has remarkable properties such as large deformation and flexibility. The actuator using conductive Nylon fibers can be activated by Joule heating and easily controlled; however, it is reported that dynamics exhibit nonlinear property due to a heat transfer and a thermomechanical property. In addition, it is reported that dynamic characteristics change from several conditions such as load, ambient temperature and the fabrication method. These finding suggested that the actuator may not able to achieve high accuracy control compared to electrical motors. Therefore, it is desirable to construct controllers that have good robustness and high tracking performance. PID controls with nonlinear compensator have been applied for the twisted and coiled polymer actuator. The PID controller should be adjusted for good robustness and high tracking performance depending on external disturbance, load weight fluctuation or reference signal. In this paper, a control system with the disturbance observer is applied to solve these problems. The actuator model is identified by using input-output data and a control system is designed to compensate the external disturbance based on the actuator model. The validity of the applied method is investigated through numerical simulations.
An ionic polymer-metal composite (IPMC) actuator is one of polymer-based soft actuators. It is produced by chemically plating gold or platinum on both surface of a perfluorosulfonic acid membrane which is known as an ion-exchange membrane. It is able to be activated by a simple driving circuit and generate a large deformation under a low applied voltage (0.5-3 V). However, individual difference and characteristics changes from environmental conditions should be considered for realizing a stable or precise control. To solve these problems, we applied a stochastic ON/OFF controller to an integrated IPMC actuator with parallel connections. The controller consists of a central controller and distributed controllers. The central controller broadcasts a control signal such as an error signal to distributed controllers uniformly. The distributed controllers switch the ON/OFF states based on the broadcasted signal stochastically. The central controller dose not measure the states of each IPMC actuator, and the control signals is calculated by using the output signal of the integrated actuator and reference signal. The validity of the applied method was investigated through numerical simulations and experiments.
In developments of robots, bio-mimetics is attracting attention, which is a technology for the design of the structure and function inspired from biological system. There are a lot of examples of bio-mimetics in robotics such as legged robots, flapping robots, insect-type robots, fish-type robots. In this study, we focus on the motion of earthworm and aim to develop a peristaltic mobile robot. The earthworm is a slender animal moving in soil. It has a segmented body, and each segment can be shorted and lengthened by muscular actions. It can move forward by traveling expanding motions of each segment backward. By mimicking the structure and motion of the earthworm, we can construct a robot with high locomotive performance against an irregular ground or a narrow space. In this paper, to investigate the motion analytically, a dynamical model is introduced, which consist of a series-connected multi-mass model. Simple periodic patterns which mimic the motions of earthworms are applied in an open-loop fashion, and the moving patterns are verified through numerical simulations. Furthermore, to generate efficient motion of the robot, a particle swarm optimization algorithm, one of the meta-heuristic optimization, is applied. The optimized results are investigated by comparing to simple periodic patterns.
Ionic polymer-metal composite (IPMC) is one of the electro-active polymer materials which respond to electric stimuli
with shape change. IPMC actuators can be activated with simple driving circuit and common control approach; however,
dynamic characteristics change from environmental conditions such as the temperature or humidity. The output force of
IPMC is very small, and the stress relaxation exists depending on the type of the counter-ions in the electrolyte.
Therefore, it is desirable to construct robust controllers and connection of multiple actuator units to obtain stable and
large output force. In this study, we apply a control method for cellular actuators to solve above problems. The cellular
actuator is a concept of the actuators which consist of multiple actuator units. The actuator units connect in parallel or
series, and each unit is controlled by distributed controllers, which are switched ON/OFF state stochastically depending
on the broadcast error signal which is generated in the central controller. In this paper, we verify the control performance
of the cellular actuator method through numerical simulations. In the simulations, we assume that the one hundred units
of IPMC connected in parallel, the output force is controlled to the desired value. The control performance is
investigated in the case of some mixed ratio of units whose counter-ions are Sodium (Na) ion or Tetraethylammonium
(TEA). As a result of simulation, it was confirmed that the tracking performance is improved by combining the fast
response actuator units of Na ions and the large output actuator units of TEA ions.
This paper discusses a model of IPMC sensors and the characteristics of the frequency responses. There are
two different methods of measurements, the current sensing and the voltage sensing, which exhibit completely
different frequency responses each other. A simple model based on Onsager's equation is shown in order to
explain the experimental results of the current sensing. The voltage sensing model is derived by the equivalent
transform of the voltage and the current sources. In contrast to the constant gain of the charge response, the
characteristics of the voltage response are directly related to the impedance dynamics. In the experiments, the
frequency responses of the charge/current sensing and the voltage sensing for two species of counter ion are
measured. The ratio of the obtained frequency responses and the measured impedance are also compared to
validate the voltage sensing model. Though the theoretical prediction of the sensor coefficient does not match
the experimental one, the structure of the model agrees with the experimental data.
We are developing an artificial muscle linear actuator using ionic
polymer-metal composites (IPMC) which is an electro-active polymer that bends in response to electric stimuli and the goal of our study is to apply the actuator to robotic applications especially to a biped walking robot. In this paper, we will describe the structure of the actuator and an empirical model of the actuator which has two inputs and one output, and whose parameters are identified from input-output data. Based on the empirical model, we demonstrate walking simulations of a small-sized biped walking robot. In the numerical simulation we assume that the developed actuators are connected both in series and in parallel to a joint of the walking robot so that the actuators supply enough torque to the robot and that they are stretched and compressed enough. It is shown throughout the simulation that the biped walking robot with the actuators can walk on a level ground with a period synchronized with a period of input signal.