Stack Actuator

Fig. 9. Structure of the 3-way microvalve using the piezoelectric stack actuator [26]...

Fig. 17. Structure of the micropump using piezoelectric stack actuator [6]...

Fig. 6.25 The multilayer stack actuator showing one strategy for electroding to avoid clamping stresses.

Because DEs are fabricated from conformable elastomers, they can be shaped into many actuator configurations over a wide range of dimensions. Most actuator designs use the area expansion of the DE film for actuation however, multilayer stacked actuators exist wherein actuation is through a reduction in thickness. Typical designs incorporate support structures to maintain prestrain in the films. 

Matysek M, Lotz P, Flittner K, Schlaak HF (2008) High-precision characterization of dielectric elastomer stack actuators and their material parameters. Proc SPIE 6927 692722... 

Kovacs G, During L (2009) Contractive tension force stack actuator based on soft dielectric EAP. Proc SPIE 7287 72870A... 

Consequently, the fabrication of a DE actuator (DEA) that can produce linear motion, called a multi-stacked actuator is presented in this chapter [Chuc et al. (2011)]. It is made of the aforementioned synthetic elastomer, which is a kind of DE as introduced in Chapter 6. The proposed actuator does not need any pre-strain to amplify the displacement and force. It just includes multiple, synthetic elastomer layers, where the electrode layers are connected in parallel. To improve the performance of the proposed actuator, the shape of the actuator is optimized to yield large deformations. Moreover, a high-voltage switching circuit is developed to drive the proposed actuator. In this circuit, the pulse-width-modulated proportional-integral-derivative (PWM-PID) feedback controller is incorporated. The performance of this controller is compared with that of a continuous PID controller via experiments. 

The multi-stacked actuator is designed to be directly driven by the Maxwell stress without any strain as mentioned above. Its fundamental principle of operation is shown in Fig. 7.1. When a voltage is applied between the two electrode layers, Maxwell stress is produced and thus, the dielectric elastomer is compressed along the axial direction. The compression of each layer results in the lateral expansion of the actuator because of the incompressibility of the polymer. Consequently, the deformation of the multi-stacked actuator is the summation of the deformations of individual layers and, thus, the total deformation is expressed as follows. 

Fig. 7.1 Operating principle of multi-stacked actuator. Reprinted from [Chuc et al. (2011)] with permission from IEEE, Copyright 2011.

A uncovered wire goes through each layer of the multi-stacked actuator as shown in Fig. 7.17. Each electrode layer is attached to this wire with the conductive epoxy and thus, all layers are connected in parallel. 

First, the deformations of the three prototypes of actuator were measured. As shown in Fig. 7.19, the performance of the trapezoidal actuator is better than those of the other actuators. These results are consistent with the analysis results. Consequently, the dimension of the free boundary of the actuator layer determines the deformation of the multi-stacked actuator. 

In the experiments, the force capability of the multi-stacked actuator is tested. The multi-stacked trapezoidal actuator of 20 g weight could operate when it is attached a total weight of 2 kg, as shown in Fig. 7.21. 

Based on the PW modulator, the PID controller is designed to control the multi-stacked actuator. The control system is illustrated in Fig. 7.24. The PID controller transforms the error between the command and the feedback signals from the sensor to the command for the PW modulator. The PW modulator transforms the output of the PID controller into a pulse sequence, which is applied to the multi-stacked actuator. 

Moreover, the hysteresis characteristics of the multi-stacked actuator was investigated to compare the performances of the PWM-PID control and continuous PID control, as shown in Fig. 7.29. The hysteresis of the multi-stacked actuator driven by the PWM-PID controller is smaller than that driven by the continuous PID controller. This can be explained by the discharging circuit of the high-voltage switching circuit. 

Finally, to evaluate the performance of the two controllers, the energy consumption of the actuator in a cycle was compared. In this work, the power consumed by both controllers to control the multi-stacked actuator in a cycle was calculated, as shown in Fig. 7.30. The average power consumption in a cycle can be calculated as shown in Table. 7.3. The experimental results show that the PWM-PID controller consumed less energy than the... 

The multi-stacked actuator that is introduced in Chapter 7 can generate linear motion like natural muscles. Consequently, it is necessary to transfer the linear motion into a rotational one. Therefore, a simple slider crank mechanism is used to convert the linear motion of the multi-stacked actuator to rotation. The Maxwell stress and the active elastic force of the actuator cause the piston to translate along a vertical axis. This action causes the link to rotate by an angle 0 as shown in Fig. 9.27. 

Here we determine the total output force and displacement of the link produced by the elastic force of the multi-stacked actuator. When the driving voltage is applied, the multistacked actuator is compressed by an amount x in length, the link L rotates an angle 6 calculated as follows ... 

Based on the proposed joint mechanism, a multi-jointed robotic finger utilizing a multi-stacked actuator is designed as shown in Fig. 9.34. The finger has two DOFs and the entire finger is covered with plastic material. The... 

Chaplya, P.M., Mitrovic, M., Carman, G.P., and Straub, F.K. (2006) Durability properties of piezoelectric stack actuators under combined electromechanical loading./. Appl. Phys., 100, 124111. 

Fig. 8.19. Piezoelectric stack actuator driven mechanisms for rotor blade leading...

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