Electro-ionic soft actuators have the advantages of being possible to drive at low voltage and to operate in the air by using ionic liquid. However, they have a disadvantage of slow response. This paper proposes to accelerate electro-ionic soft actuators by feedforward control. Feedforward control can automatically generate online pulse-shaped input signals from any reference signals and is easy to implement. The experimental results show that the feedforward control can automatically generate control inputs for any references and achieve 2 times faster than the no-control case. In addition, it was found that the control performance was significantly degraded when a voltage limit was set to protect the actuator.
This paper discusses a time-domain analysis of the exact transfer function of Zhu’s multiphysics model of IPMC sensors. The authors recently derived the exact transfer function that describes the output voltage of Zhu’s IPMC sensor model. This paper shows that some time-domain constants such as the peak voltage and the steady-state voltage of a step response can be derived from the transfer function. It is shown that the values estimated from the exact transfer function agree with the numerical simulation in the time domain by COMSOL.
This study proposes a vibration excitation technique using a dielectric elastomer actuator (DEA) that can attach to a conductive structure based on an electro adhesion technique. Vibration responses of mechanical structures are experimentally analyzed to assess their mechanical characteristics. Conventionally, impulse hammers, heavy and rigid exciters, and lead zirconate titanate (PZT) actuators are applied to excite the vibration of the structure in a vibration experiment. However, the other vibration excitation technique should be applied for flexible structures with curved surface to evaluate their vibration response accurately without damages on them. Herein, a DEA can be applied to the vibration excitation taking advantage of its features of high flexibility, stretchability, and fast response. Conventional DEAs are attached to the target structures with adhesives degrading DEAs’ reusability. In this study, the electro adhesion technique, which can generate an attraction force were applied to the DEA. The proposed DEA can attach to the target without adhesives. The proposed DEA was fabricated by stacking layers that can generate the excitation force and that can generate the attraction force of the electro adhesion. Then, a vibration experiment for an aluminum pipe structure was conducted applying the DEA excitation. Finally, the effectiveness of the proposed vibration excitation technique was evaluated based on vibration responses of the target structure.
The quality of pear fruits is correlated with their firmness, which is assessed by a firmness index derived from the resonance frequency and mass. Postharvest pear fruits ripen during storage, which affects the firmness. A nondestructive measurement technique is necessary to predict fruit firmness without causing any damage. Thus, this study proposes a vibration experiment technique based on dielectric elastomer actuator (DEA) excitation to determine the resonance frequency of pear fruits without any damage. Therefore, DEAs can be attached directly on fruits with curved surfaces because of their stretchability, light weight, and responsiveness and can be used to transfer the excitation force effectively. For our experiments, thin laminated DEAs were fabricated to obtain sufficient vibration excitation force, and resonance frequencies of the pear fruits were confirmed. Subsequently, the firmness indices of each target fruit were calculated and assessed. Finally, the variations in firmness indices of pear fruits during storage were confirmed, and the effectiveness of the proposed technique was validated.
In recent years, polymer fiber actuators obtained by twisting polymer fibers have attracted much attention. These actuators actuate due to the reversible axial thermal contraction and radial thermal expansion of untwisted fibers. In this study, thermal contraction of the two polyamides, PA6 and PA610, fibers were investigated. The fiber length of both fibers changed reversibly in response to temperature change, but there was no initial load dependence on the amount of contraction. These results indicate that this thermal contraction is not due to the entropic elasticity effect seen in rubber. In addition, the thermal contraction was larger for PA610, which has a larger thermal expansion coefficient in the amorphous state. This suggests that the thermal expansion of the amorphous state was converted by its fiber structure into expansion in the diameter direction and contraction in the fiber axial direction [Kimura et al., Sens. Actuators B Chem., 2021].
Integral resonant control (IRC) is a vibration control method that measures the displacement of a flexible structure and positively feeds back the force generated by integrating the displacement. In the case of vibration control with IRC, the DC gain and the direct feedthrough term of the transfer function of a flexible structure have important roles for modeling and analysis. In the case of a piezoelectric bimorph beam, we experimentally found that the direct feedthrough term, which should have a positive value in the conventional model, has a negative value. This paper proposes a model in which the direct term is negative, considering the electrical coupling caused by the structure of a piezoelectric bimorph beam. In addition, the stability of the control system is shown based on the negative imaginariness, and the IRC is shown to be applicable to a control object with a negative direct feedthrough term.
Electromagnetic shunt damping is a sensorless vibration control method that uses a single voice coil motor (VCM) and may also be used for energy harvesting. If an amplifier is used to control the current of a VCM, the current control law has the same function as a shunt circuit. In this paper, we focus on a current-controlled VCM and propose a new method that analyzes not only the performance of conventional shunt vibration control but also the energy consumption and the servo performance of current. From a numerical example, it was clarified that the energy was consumed without being regenerated if the PI gain with the maximum damping effect was used. There is a trade-off between the damping effect and energy regeneration.
This paper discusses a simple characterization method for thin stacked dielectric elastomer actuators (DEAs), which is difficult to measure the mechanical impedance. We employ a method of suspension for characterizing the mechanical impedance. A weight is attached to a thin stacked DEA and then the DEA is electrically excited. The frequency response is obtained by measuring the vibration displacement. Consequently, we can estimate the mechanical impedance or viscoelasticity of the thin stacked DEA. From the experimental frequency response of electrical excitation, Young’s modulus and damping ratio were successfully estimated.
Fishing-line artificial muscles can exhibit various motions. Twisted Polymer Fiber (TPF) actuators, which are a class of fishing-line artificial muscle actuators, generate torsional motion by applying heat. In general, untwisted fiber contracts by heating. However, measuring the blocking thermal tensile force of a TPF, we have discovered that the tensile force decreased and fluctuated depending on the initial number of twists in the TPF. This suggests that a TPF expands depending on the initial number of twists. Furthermore, the tensile force does not decrease monotonically but fluctuates during heating.
This talk discusses analytical simplification of a multi-physics model for ionic polymer-metal composite (IPMC) sensors. Some methods including finite element method, and assumed mode method based on separation of variables are introduced. For the finite-element or the assumed-mode simulation, we have built in-house MATLAB programs. The obtained approximated models are represented by ordinary differential equations, and the computational cost is greatly reduced by the proposed simplifications. The magnitudes of the error in the approximated models for open-circuit voltage are kept acceptable level within at most 2%, although the computational time is greatly reduced to 1/100 to 1/1000.
A fishing-line artificial muscle actuator is typically tested under a constant weight load. This paper reports a new hysteresis phenomenon discovered by changing both load weight and temperature applied to a fishing-line artificial muscle actuator. Obviously the equilibrium position of an actuator changes by load weight. Interestingly, the equilibrium position also largely changes when the actuator is firstly heated and cooled just after exchanging the load weight. In this paper we call this phenomenon as temperature-dependent hysteresis. We have observed that the magnitude of the temperature-dependent hysteresis in the experiment reached the same level as the thermal contraction and was not negligible.
An Ionic Polymer-Metal Composite (IPMC) has characteristics as a sensor as well as an actuator. Zhu has recently proposed a multi-physical model representing sensor voltage of a deformed IPMC. This paper discusses approximation methods aiming at fast simulation or control system design. First, we linearize the nonlinear partial differential equations (PDEs) of Zhu’s model. Next, this paper considers two types of spatial discretization methods, Finite Difference Method and Finite Element Method. We have found that it is not necessary to use a large number of sample points or finite elements for simulating the sensor voltage.
This paper shows an experimental demonstration of a high efficiency step-down circuit for a dielectric elastomer generator. The step-down circuit, consisting of a surge arrester and a transformer, is classified as a passively-switched flyback converter and can transfer energy efficiently. In the experiments, the efficiency of the step-down circuit is nearly 100 times higher than that of a Zener diode, which is used for the simplest step-down conversion. Also, we discovered that the harvested power and the efficiency are improved better if the breakdown voltage of the surge arrester is selected to be higher. Finally, the wireless transmission of a microprocessor is demonstrated using the step-down circuit connected to a dielectric elastomer generator and a self-priming circuit.
KEYWORDS: Actuators, Polymers, Artificial muscles, Polymeric actuators, System identification, Control systems, Annealing, Control systems design, Modeling
This paper focuses on the torsional motion of a torsional type fishing-line artificial muscle actuator, so to speak, Twisted Polymer Fiber (TPF) actuator. TPFs are expected as limited rotation motors or limited angle motors for mechatronic applications. Aiming to construct a gray-box model for TPF actuators, this paper derives the first-order transfer function as the model from the applied electrical power to the generated torque of an actuator. The relation from the temperature to the generated torsional torque is simply assumed as a linear function of which coefficient is the torsional rigidity. In the experiment, the validity of the obtained model is evaluated, and then the blocked torque of the TPF actuator is controlled.
In this paper, we propose a new method for tuning parameters in a negative capacitor circuit, which is utilized in the piezoelectric shunt damping. A stochastic numerical optimization method, called Covariance Matrix Adaptation-Evolution Strategy (CMA-ES), is selected to minimize the effect of disturbance to an experimental setup. In addition, an additional restriction is imposed on CMA-ES so that the internal stability of the closed-loop system is guaranteed. The effectiveness of the proposed method is demonstrated by numerical examples.
Ionic polymer-metal composites (IPMCs) generate electrical potential under deformation and can be used as sensors. Recently, Zhu et al. have proposed a sensor model which describes distribution of cations, water molecules and electrical potential under bending deformation. In this paper, we discuss a simplification of the multi-physical sensor model, which is represented by a set of nonlinear partial differential equations. The nonlinear partial differential equations are simplified and approximated into a set of linear ordinary differential equations, i.e., a state-space equation model. At the end, the simplified model is validated by comparing the simulation results with those of the partial differential equation model.
This paper discusses energy harvesting and its application using dielectric elastomer and self-priming circuit. With the self-priming circuit attached to the dielectric elastomer, the generated voltage increases exponentially according to the variation of the capacitance caused by applied deformation to the elastomer. Two-stage self-priming circuit is selected for optimal harvesting. The self-priming harvesting technique is able to increase the voltage of the dielectric elastomer from a few volts to kV order, however in this paper the generated voltage is limited up to 1kV in order to avoid the destruction of the dielectric elastomer. The ability of energy harvesting using dielectric elastomer and self-priming circuit is confirmed by both numerical simulation and experiments. In the experiment, the dielectric elastomer is deformed by an electric motor, and the harvested energy is stored to a charging capacitor through Zener diodes. A low-power microcomputer which has a radio transmitter is connected to the charging capacitor for the application example. The experimental results show that the temperature data can be transmitted only by the harvested energy. In addition, the efficiency of the energy harvesting is calculated by comparing the generated power with the charged power.
Recently, artificial muscles made of fishing lines or sewing threads, namely twisted and coiled polymer actuators (TCPAs), have been proposed by Haines et al. A TCPA contracts by applying heat and returns to its initial length by cooling. A TCPA can be driven by voltage if the TCPA is plated by metal or if conductive wire such as nichrome is wound around it. Compared with the conventional electroactive polymers, advantages of TCPAs are low cost, simple structure, large actuation strain, and large force. However, a big disadvantage of TCPAs is slow response due to heat transfer. The problem becomes apparent during cooling, although the response of heating can be improved by feedback control. This paper proposes a control method of switching heating and cooling. In the proposed method, a TCPA is cooled by an electric cooling fan. When the TCPA is heating, the cooling fan is stopped. In a previous report, the response speed can be improved by keeping cooling fan always on; however, unnecessary energy consumption is required even during heating. In the proposed method, energy consumption during heating does not increase and the response speed can be improved using fan only during cooling. The proposed control law is as follows. Firstly, the desired control input is determined by PI-D control with respect to the length of the actuator. Then, the control inputs to the heater and to the cooling fan are switched according to the sign of the PI-D controller output. The effectiveness of the proposed control method is demonstrated by comparing the cases with and without the cooling fan in the experiments.
KEYWORDS: Actuators, Polymers, Artificial muscles, Polymeric actuators, Electroactive polymers, Control systems design, Control systems, Data modeling, Feedback control, Digital signal processing, Annealing
Recently, fishing line artificial muscle has been developed and is paid much attention due to the properties such as large contraction, light weight and extremely low cost. Typical fishing line artificial muscle is made from Nylon thread and made by just twisting the polymer. In this paper, because of the structure of the actuator, such actuators may be named as coiled polymer actuators (CPAs). In this paper, a CPA is fabricated from commercial Nylon fishing line and Ni-Cr alloy (Nichrome) wire is wound around it. The CPA contracts by the Joule heat generated by applied voltage to the Nichrome wire. For designing the control system, a simple model is proposed. According to the physical principle of the actuator, two first-order transfer functions are introduced to represent the actuator model. One is a system from the input power to the temperature and the other is a system from the temperature to the deformation. From the system identification result, it is shown that the dominant dynamics is the system from the input power to the temperature. Using the developed model, position control of the voltage-driven CPA is discussed. Firstly, the static nonlinearity from the voltage to the power is eliminated. Then, a 2-DOF PID controller which includes an inversion-based feed forward controller and a PID controller are designed. In order to demonstrate the proposed controller, experimental verification is shown.
An ionic polymer material can generate electrical potential and function as a bio-sensor under a non-uniform deformation. Ionic polymer-metal composite (IPMC) is a typical flexible ionic polymer sensor material. A multi-physical sensing model is presented at first based on the same physical equations in the physical model for IPMC actuator we obtained before. Under an applied bending deformation, water and cation migrate to the direction of outside electrode immediately. Redistribution of cations causes an electrical potential difference between two electrodes. The cation migration is strongly restrained by the generated electrical potential. And the migrated cations will move back to the inner electrode under the concentration diffusion effect and lead to a relaxation of electrical potential. In the whole sensing process, transport and redistribution of charge and mass are revealed along the thickness direction by numerical analysis. The sensing process is a revised physical process of the actuation, however, the transport properties are quite different from those of the later. And the effective dielectric constant of IPMC, which is related to the morphology of the electrode-ionic polymer interface, is proved to have little relation with the sensing amplitude. All the conclusions are significant for ionic polymer sensing material design.
KEYWORDS: Electromagnetism, Transducers, Sensors, Actuators, Resistance, Digital signal processing, Vibration control, Amplifiers, Data modeling, Magnetism
In this paper, a new sensor-less parameter estimation method is proposed for electromagnetic shunt damping. The purpose is to estimate parameters of an electromagnetic transducer and a vibrating structure. The frequency domain measurements of an electrical admittance are only supposed to be available but any other sensor measurements are not; therefore, the estimation problem is nontrivial. Two types of numerical optimization, a linear optimization to select an initial seed and a nonlinear optimization to determine a final estimate, are presented. The effectiveness of the method is demonstrated by vibration control experiments as well as parameter estimation experiments.
This paper studies the stability problem for a piezoelectric shunt damping system with a simple negative capacitor circuit. A key issue is to consider perturbations of parasitic series and parallel-leakage resistances in a piezoelectric element. Then, it is a bit surprising that the perturbed system becomes unstable, in particular, due to the effect of the parasitic leakage resistance. This instability phenomenon is analytically proved based on the Routh-Hurwitz stability criterion and is also demonstrated by numerical simulations. This paper then illustrates robustification of a negative capacitor circuit by inserting a negative resistance in parallel with the negative capacitor in the simple negative capacitor circuit. This robustification is also demonstrated by a numerical simulation.
In practical applications, stress-relaxation phenomenon is not preferable feature of IPMC (ionic polymer-metal
composite) actuators. In this study, we propose a control method using two (or more) IPMCs in order to reduce
the stress-relaxation phenomenon. In the experiment, the force generated by two IPMC strips is measured by a
force sensor. The proposed control signal consists of a small fluctuating signal which is oscillating independently
of the command, in addition to a simple feedback controller with a feedforward term. We have found that the
time to reach the limit voltage became more than twice if the fluctuating signal was added.
This paper considers driving an Ionic Polymer-Metal Composite (IPMC) actuator using a Pulse Width Modulation
(PWM) amplifier, in order to shed light on the characteristics of PWM driving of IPMCs. Generally, it is
said that an efficiency of a PWM amplifier is higher than that of a linear amplifier. However, high current flows
across the IPMC in the use of a PWM amplifier, and it is supposed to become the power consumption high. We
solve this problem by putting an inductor between the PWM amplifier and the IPMC. The simulation and the
experiment results demonstrate the effectiveness of the proposed method.
This paper discusses a simple robust PID (proportional, integral and derivative) tuning method for force control
of ionic polymer-metal composite actuators. The model is represented by a transfer function which consists
of an electrical part and an electro-mechanical part. The uncertainty is represented by an interval polynomial
set of the closed-loop characteristic equations. Using Kharitonov's theorem, we show that just one Kharitonov
polynomial stability is necessary and sufficient for satisfying the robust stability of the system. The PID gain is
determined by pole placement of the derived Kharitonov polynomial. Experimental results show the effectiveness
of the PID force control achieved by the proposed method.
Toward the construction of the unified model of ionic polymer actuators, this paper discusses the system modeling
with the electro-stress diffusion coupling theory. The theory can explain the differences of the relaxation
phenomenon of polymer electrolytes with respect to the various counter ion species in the polymer. In addition
to the mechanical system which employs a simple beam model, the electrical system and the electro-mechanical
coupling systems are also represented by partial differential equations. The electrical system is modeled based on
the non-uniform distributed circuit which represents the electrode roughness. The electro-mechanical coupling
system is derived from the electro-stress diffusion coupling theory. The overall system is represented by a statespace
equation with a feedback structure. The comparisons between the simulation result and the experimental
result show the validity of the model.
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.
From the observation of the measured frequency response, the electrical impedance of IPMC has the characteristics
of a distributed parameter system. Especially in the case of TEA ion, we found that the frequency response
cannot be approximated by a simple ideal capacitor or even by low-order transfer functions. In this study, we
discuss a black-box circuit modeling of the electrical system of IPMC from the point of view of the frequency
response. We employ some models whose transfer functions are not rational. One of such models is a distributed
circuit (transmission line). Another is a black-box circuit model with a distributed parameter element (constant
phase element). Both transfer functions consist of square root of 's'. In the experiment, the electrical impedance
of an IPMC (gold plated Nafion) is measured under some conditions such as electrode clamp sizes and two
cation species, Na ion and TEA ion. From the result, we found that the electrode clamp condition less affects
the measured impedance. However, we observed that the impedance highly depends on the cation species. From
the experimental frequency response, the parameters of the model are identified. Larger resistance and smaller
capacitive element are identified in TEA case than those in Na case. The identified parameters are consistent
with the physical intuition that TEA ion movement is slower than Na ion.
It is found from the locomotion of snake-like underwater robot using Ionic Polymer-Metal Composite (IPMC) as its actuator that, although we specify the same amplitude of driven voltages to each segmented IPMC unit, the resultant bending amplitudes along the body's progressive waves change from small to large toward the robot's tail. To analyze this phenomenon, which is also observed in locomotions of slender fishes, we discuss the modeling and analysis of bending motions of IPMC actuators using the Euler-Bernoulli beam theory. Eigenfunction expansion technique is used to solve the model of a partial differential equation. The envelope curve can be drawn by the obtained solution, and simulation results reappear the same phenomenon.
Deflection of the real robot is measured by video camera and laser beam.
Experimental results verifies the validity of the proposed model.
Parameter identification is also performed with measured data.
Ionic Polymer-Metal Composites (IPMCs) are soft actuators, generally referred to as "artificial muscles" which are made out of high polymer gel films of perfluorosulfonic acid chemically plated with gold. These composites bend by applying a low voltage between electrodes on both sides. The actuator is soft and works in water. It bends silently, responds quickly and has a long life. In our previous work, snake-like swimming robots and a 3DOF 2-D manipulator have been developed. In this research we have investigated the bending response of an IPMC artificial muscle in high-pressure water environments, with future applications in deep-sea actuators and robots. The artificial muscles have an advantage over electric motors because they do not need sealing from water, which is difficult in high-pressure water environments. Bending responses of artificial muscles were measured at three different pressure levels, 30MPa, 70MPa and 100MPa. The maximum pressure, 100MPa is the same pressure as the deepest ocean on earth, (10,000m.) From experiments, there was found to be almost no difference with that at normal water pressure of 1Pa. We present detailed results of responses of these artificial muscles including current responses and videos of bending motion with respect to combinations of several different input voltages, frequencies and wave patterns.
The essential motion of the ionic polymer-metal composite (IPMC) is bending, therefore some mechanisms are expected to transform from the bending to other required motions. Motivated by the motion of a spiral spring, we discovered that the bending of the ionic polymer could be directly transformed to the limited angle rotation. We introduce the model of the rotary actuator, which consists of mechanical, electrical and electromechanical dynamics. The motion of the rotary actuator is demonstrated in the experiment. The stationary properties are measured and the parameters of the dynamical model are identified, which are also validated by experiments.
Piezoelectric shunt damping is a method to reduce structural vibration by shunting a piezoelectric element with an electrical circuit. This paper proposes a method to identify the physical parameters that are required to design piezoelectric shunt damping systems. Employing self-sensing actuator methodology, the proposed method requires no additional sensor or actuator other than a simple bridge circuit. The parameters can be identified from a transfer function simply by linear least squares method. To demonstrate the validity of the proposed method, the shunt damping of the steel plate with the piezoelectric ceramic is performed based on the identified parameters.
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