Development, modeling and deflection analysis of hybrid micro actuator with integrated thermal and piezoelectric actuation
- First Online:
- Cite this article as:
- Pourrostami, H., Zohoor, H. & Kargarnovin, M.H. Meccanica (2013) 48: 1081. doi:10.1007/s11012-012-9653-z
- 372 Downloads
Micro actuators are irreplaceable part of motion control in minimized systems.
The current study presents an analytical model for a new Hybrid Thermo Piezoelectric micro actuator based on the combination of piezoelectric and thermal actuation mechanisms. The micro actuator structure is a double PZT cantilever beam consisting of two arms with different lengths. The presented micro actuator uses the structure of electrothermal micro actuator in which polysilicon material is replaced by PZT. Also the voltage and poling directions are considered in the lengthwise of PZT beams. As a result, the piezoelectric actuation mechanism is based on d33 strain coefficient.
The tip deflection of micro actuator is obtained using Timoshenko beam theory. Analytical results are compared with FEM results along with other reported results in the literature. The effects of geometrical parameters and PZT material constants on actuator tip deflection are studied to provide an efficient optimization of HTP micro actuator.
KeywordsHybrid Micro actuator Thermo piezoelectric Deflection Timoshenko beam theory
Nowadays, micro actuators have a key role in the development of minimized systems. There is an immense growth in evolution and design of micro actuators in the precision field of micromanipulation and micro assembly. A variety of applications have been envisaged for micro actuators including cell manipulation , micro assembly of optical MEMS devices  and flexible micro assembly stations . To fulfill the required tasks, the micro actuators need high performances such as high resolution, high accuracy, quick response time and high range of positioning.
Different types of actuation mechanisms can be classified into thermal/electrothermal, electrostatic, magnetic/electromagnetic, piezoelectric and other categories. Electrothermal and piezoelectric micro actuators are the most prized actuators due to their remarkable characteristics. They are widely used in variety of applications including micro gripper , scanning probe arrays , micro positioning applications , liquid drop ejectors , control valves  and acoustic control .
High positioning accuracy, rapid response and high mechanical power can be achieved using the piezoelectric micro actuators, in spite of low deformation range capability . Taking the thermal micro actuators into account, they exhibit advantages including large output force with reasonable displacement, easy fabrication process and operation at small drive voltages . Despite consuming a lot of power and showing slow response time, their disadvantages are far outweighed by remarkable advantages. Combination of these two classes of micro actuator as a hybrid micro actuator overcomes their limitations. Rakotondrabe and Ivan have attempted to integrate electrothermal and piezoelectric actuation mechanisms . They presented a new hybrid thermo piezoelectric micro actuator which utilized the thermal bimorph principle and piezocantilever micro actuator. The micro structure was a unimorph piezoelectric cantilever making up of a piezoelectric layer and a passive copper layer. While electrical voltage was used to control the piezoelectric actuation, a Peltier module (as an external source of heat generation) was applied to provide uniform temperature variation. By using an electrical field, the piezoelectric layer expanded/contracted and led to bending the cantilever. In addition, by varying the temperature, a secondary bending was occurred due to the different thermal expansions in both layers.
Although the actuator behavior under input voltage and temperature has been studied, the effects of PZT material constant have not been investigated yet. In fact, PZT material constants including piezoelectric stain and dielectric constant are strongly dependent on operation temperature. Therefore, temperature has a double effect by causing different thermal expansions and changing piezoelectric material constants.
In the current study, a new model of thermo piezoelectric micro actuator is proposed. The presented micro actuator uses the structure of electrothermal micro actuator in which polysilicon material is replaced by PZT. Also the piezoelectric actuation mechanism is based on d33 strain coefficient which means the voltage and poling directions are considered in the lengthwise of PZT beams.
The effects of geometrical and material constants on actuator tip deflection were studied to provide an efficient optimization of HTP micro actuator. Moreover, Timoshenko beam theory was employed to accurately model the interaction between different physical fields including mechanical piezoelectric and thermal.
The structure of the micro actuator consists of a pair of adjacent arms with different lengths and a small conjunction beam [13, 14]. The actuator has an integrated structure with no hinges or joints. Moreover, both arms and conjunction beam have same cross-sectional areas and poling direction.
In electrothermal micro actuators with the same structure, passing the electric current through the arms, led to an enhancement in temperature and expansion of the longer arm, as compared with the shorter arm, which is due to larger electrical resistance of the longer arm. As a result, different expansions cause the deflection of the actuator. This type of actuation mechanism occurs when the actuators are made up with low electrical resistivity materials such as polysilicon (10−4 to 10−3 Ω m) .
Contrarily to polysilicon, PZT materials have high electrical resistivity (more than 109 Ω m at room temperature). By applying voltage, low heat will be generated in the micro actuator. Although, it is possible to generate more heat by increasing input voltage, it can also depolarize or destroy the micro actuator. Therefore, in order to have any types of thermal deformation it is necessary to provide an external source for applying temperature. Considering micro size of actuator and the reported experimental work, a uniform temperature variation is applied on the surface of micro beams , which results in higher elongation of the longer arm than the shorter arm.
If the applied voltage direction is along the polarization direction in PZT beam, it expands, otherwise it contracts (extensional mode) .
In order to achieve more deflection in presented micro actuator, the applied voltage direction must be along the polarization direction in long arm and in the opposite direction in short arm. In this case, the long arm expands not only due to temperature change but also due to piezoelectric actuation whereas in the short arm thermal effect increases the length but piezoelectric actuation decreases the length.
The actuator can be considered in the state of plane stress because the beam’s width and heights are much shorter than the beam’s length. Furthermore, it is assumed to deflect only laterally and no external mechanical load is applied on the actuator.
2.1 Beam model and field approximations
2.2 Constitutive relations
3 Solution method
The actuator under consideration is a statically indeterminate structure with the degree of the indeterminacy of 3. Each support can be released and replaced by two forces and one moment components. The three redundant systems of forces (X1, X2 and X3) are shown in Fig. 3(b).
4 Results and discussion
In this section, the derived governing equations are solved for beam’s tip deflection. Primarily, the obtained analytical results are compared with the results of FEM model along with the reported results in the literature [12, 13]. Then, the effects of geometrical parameters and PZT material constants on actuator tip deflection are studied to provide an efficient optimization of HTP micro actuator.
Geometrical data for actuator
Length of the long arm, L1 (μm)
Length of the gap, Lg (μm)
Width of beams, w (μm)
Height of beams, h (μm)
Stiffness matrix component, c11 (GPa)
Stiffness matrix component, c33 (GPa)
Stiffness matrix component, c12 (GPa)
Stiffness matrix component, c13 (GPa)
Piezoelectric Transverse strain constant, d31 (C/N)
Piezoelectric Extensional strain constant, d33 (C/N)
Piezoelectric strain constant, d15 (C/N)
Expansion coefficient, α (1/°C)
Dielectric constant, ϵ11 (C/V m)
Dielectric constant, ϵ33 (C/V m)
In current analysis, PZT-5H is used because it owns the largest piezoelectric coefficients at room temperature among other PZT ceramics; however it has the low Curie point temperature .
Curie point is a temperature which if a piezoelectric element is heated to it, the element becomes completely depolarized and loses its piezoelectric effect. Therefore a piezoelectric element can function for long period without marked depolarization only at temperature below the Curie point. For PZT-5H, its low curie point (170 °C) limits the working temperature range. It is considered that the working temperature range for safe function is between 25 °C (room temperature) and 125 °C.
Several researches have been done about temperature dependence of PZT material properties which are shown that for PZT-5H: (i) piezoelectric strain constants d31 and d33 increase by increasing temperature while d15 almost remains constant. (ii) Piezoelectric dielectric constants ϵ11 and ϵ33 increase by increasing temperature. (iii) For mentioned temperature range the elastic constants do not change remarkably by increasing temperature [19, 20].
It is seen that the electrical field component in z direction (Ez) has no tangible effect in actuator tip deflection.
4.2 Geometry effects
As it is expressed by Huang and Lee , the obtained results show that the deflection of the actuator is independent of the beam height and the beam width if the dimensions of arms cross section do not change along the arms length. However, it is shown by FEM that reduction of beam height would increase the deflection a little.
4.3 Material property effects
Based on derived formulations, it is clear that the actuator tip deflection will be a function of material properties of d33, d31, d15, ϵ11 and ϵ33. In general, all of these material properties for PZT materials are function of temperature . Based on foregoing discussion on Sect. 4.2, the best choices for the arm lengths will be L1=1000 μm and L2=500 μm.
As it seen in Figs. 9(a), (b) and (c), a nonlinear variation trend exists when piezoelectric strain constants increase. By increasing the piezoelectric Extensional strain constant d33, deflection decreases although by increasing the absolute value of d31, actuator tip deflection increases in a nonlinear fashion. Also as it is seen there is no tangible variation in deflection when d15 increases. The obtained results show that among PZT strain constants, d33 is the most sensitive strain constant that deflection depends strongly on it.
Based on the presented results on Figs. 9 and 10, it becomes clear that the highly sensitive material properties, will have significant influence on the deformation behavior of structures made of this materials. On the other hand, since in presented model the voltage and PZT polarization directions are in x direction and no voltage is applied in z direction, based on constitutive governing equation we can come to conclusion that in scale of micrometer there is a negligible shear strain in micro actuator and those parameters that cause shear strain (ϵ11, d15 and Ez) have no tangible effect on tip deflection.
By increasing the applied voltage and temperature gradient, the actuator tip deflection was linearly increased.
The actuator tip deflection was decreased upon increasing the gap length.
The maximum deflection of HTP micro actuator can be obtained when the arms length ratio is at the range of 0.4<L2/L1<0.5.
Increasing the piezoelectric transverse strain constant (d31) and piezoelectric dielectric constant (ϵ33) gave rise to a nonlinear increasing trend in actuator tip deflection.
Increasing the piezoelectric extensional strain constant (d33) decreased the actuator tip deflection.
Increasing or decreasing the piezoelectric strain constant (d15) and piezoelectric dielectric constant (ϵ11), had no effect in actuator tip deflection.
Temperature affected the deflection of hybrid actuator by causing different thermal expansions in arms and increasing the PZT material constants.
To achieve more deflection, PZTs with lower extensional strain constant (d33) and higher piezoelectric dielectric constant (ϵ33) were optimized for presented model of HTP micro actuator.