**4.1 Simplified structure**

One of the significant shortcomings of inchworm type piezoelectric is the complex structure which brings trouble for the manufacture and control. **Figure 8** shows the structure of the proposed simplified piezoelectric actuator based on the parasitic movement of the flexure mechanism by Li et al. [26]. With the help of the parasitic movement of the flexure mechanism, only two piezoelectric elements are needed. It is mainly composed of the base, the slider, piezostack 1, piezo-stack 2, flexure mechanism 1, flexure mechanism 2, two wedge blocks, four micrometer knobs and eight screws. Piezo-stack 1 (AE0505D16, 5 × 5 × 20 mm, NEC/TOKIN CORPORATION) is inserted into the flexure mechanism 1 through the wedge block to push the linear slider. The assembly process of the piezoelectric stack 2 and the flexure mechanism 2 are the same. The highprecision four-micron knob (M6 from SHSIWI) is utilized to adjust the preloading force between the flexure mechanism and the slider. The slider is a commercial linear guide with high linearity produced by THK. The flexure mechanism is made of aluminum alloy AL7075 manufactured by WEDM. Screws are applied to stably assemble all components on the base. The overall size of the proposed stepping piezoelectric actuator is 100 mm × 60 mm × 18 mm.

**Figure 8.** *Structure of the proposed simplified piezoelectric actuator and motion principle [26].*

Two piezoelectric "legs" are required to alternately drive the slider, and this is why they are sometimes called "walking" type piezoelectric actuators. In addition, for traditional "walking" type piezoelectric actuator, in each piezoelectric "leg", at least two piezoelectric elements are required (one for flexure movement and one for longitudinal movement). The movement principle of the stepping piezoelectric actuator is the "circular movement" of the piezoelectric "legs". In short, each piezoelectric "leg" should achieve two movements in *x* and *y* directions.

In the proposed study by Li et al., the parasitic movement of the flexure mechanism is applied to simplify the entire system. Generally, the piezo-stack could only achieve the one motion in its longitudinal direction. Whereas, as shown in **Figure 8(b)**, with the aid of the asymmetrical flexure mechanism, the piezo-stack will generate an oblique upward force, which causes the motion displacement in both *x* and *y* directions. The parasitic movement *L*x in *x* direction is used to drive the linear movement of the slider. However, only one flexure mechanism cannot achieve walking motion, and at least two flexure mechanisms ("legs") are required. In addition, during the movement, the input square wave voltages *U*1 and *U*2 have the same magnitude but different phases. The experimental results display that the application of the parasitic motion of the flexure mechanism is able to simplify the inchworm type piezoelectric actuator. The stepping motion of the proposed actuator requires only two piezoelectric elements and two input signals. Additionally, performance of the proposed simplified piezoelectric actuator (stepping performance, speed performance, and load performance) has a certain relationship with the input voltage and frequency. Under the conditions of *U* = 100 V and *f* = 1 Hz, the maximum step displacement Δ*L* = 1.75 μm. Under the condition of *U* = 30 V and *f* = 1 Hz, the minimum step displacement Δ*L* = 0.18 μm. When *U* = 100 V, *f* = 20 Hz, the maximum movement speed *V*s = 39.78 μm. This study verifies the feasibility of design and simplification of inchworm type piezoelectric actuators with parasitic motion of flexure mechanisms, and provides a new idea for the research of piezoelectric actuators. Potential applications in optical engineering and cellular operating systems require more work.

#### **4.2 Simplified control**

For most of the inchworm type piezoelectric actuators, three input signals are necessary for one driving unit and two clamping units, which make the control system also complicated. In order to simplify the control system, Gao et al. proposed one novel piezoelectric inchworm actuator which uses a DC motor to drive the permanent magnet for alternate clamping, applies a laser beam sensor to detect the position of the permanent magnet and generates an excitation signal to drive the piezoelectric stack [27]. The actuator only needs a DC signal to drive and can adjust the frequency by changing the motor speed. The movement mechanism of the actuator is emphatically discussed, and the influence of the permanent magnet structure on the clamp is studied. The flexibility matrix method and COMSOL finite element software are used to simulate and analyze the flexure hinge. The driving signal for the piezoelectric stack is generated by self-sensing and automatically adapts to the frequency change, which simplifies the control signal of the inchworm actuator. The use of the magnetic clamping unit solves the serious friction and wear problems of the current clamping method of piezoelectric inchworm actuators. In addition, the driving unit and clamping unit of the proposed piezoelectric inchworm actuator are tested experimentally. The experimental results confirm the feasibility of the proposed scheme and obtained relevant optimized structural parameters.

The overall structure of the proposed actuator, as shown in **Figure 9**, is mainly composed of a sensing unit, a driving unit and a clamping unit. As shown

### *Principle, Design and Future of Inchworm Type Piezoelectric Actuators DOI: http://dx.doi.org/10.5772/intechopen.96411*

in **Figure 9(a)**, the clamping unit is mainly composed of a DC motor, a motor base, a permanent magnet after magnetization (RPM, red), a permanent magnet before magnetization (NRPM, blue), bearings and a bearing housing. As shown in **Figure 9(b)**, the sensing unit includes a cam, a laser beam sensor (OLS) and a bracket. The driving unit includes a flexible hinge mechanism with integrated piezoelectric stack (AE0505D16, NEC/TOKIN CORPORATION), a wedge-shaped adjusting mechanism (built-in a pair of wedges and a pre-tightening bolt) and a slider, as shown in **Figure 9(c)**. The designed slider can slide in the sliding groove of the flexible hinge mechanism. Two clamping modules and cams are fixed at the end of the output shaft of the DC motor, and each clamping module is assembled by a radially polarized permanent magnet RPM and a non-radially polarized permanent magnet NRPM. The piezoelectric stack is preloaded by the wedge-shaped adjusting mechanism and nested in the installation slot of the flexible hinge mechanism. The laser beam sensor is supported by two brackets and generates an excitation signal by detecting the position of the cam. In addition, the support block, the DC motor and the bearing are assembled on the base with eight bolts.

The proposed inchworm actuator by Gao et al. utilizes a DC motor to drive the permanent magnet for rotating to achieve alternate clamping. The actuator does not need to input the driving voltage signal of the piezoelectric stack. It only senses the position of the permanent magnet through the laser beam sensor, and generates an excitation signal to drive the piezoelectric stack to achieve precise linear displacement output. Its working principle is shown in **Figure 10**. Work performance of

**Figure 9.**

*Structure of the actuator by Gao et al.: (a) sensing unit; (b) sensing unit; (c) driving unit [27].*

**Figure 10.**

*Working principle of the inchworm piezoelectric actuator with simplified control system by Gao et al. [27].*

**Figure 11.** *Miniaturized inchworm type actuators: (a) by Risaku et al. [28]; (b) by Mehmet et al. [29].*

the proposed actuator was studied carefully. For the important component of the driving unit, the "Z" type flexure hinge, the flexibility matrix method is used to perform theoretical calculations. The error between the simulation results and the theoretical calculation results is about 2.13%, indicating the accuracy of the calculation; for the magnetic clamping unit, when the clamping distance is 1 mm, the magnetic clamping unit has better clamping capability. The experimental results show that the actuator has a good linear displacement. When *U*e = 150 V and *f* = 40 Hz, its maximum movement speed is 481.43 μm/s, and the maximum load is *m* = 950 g.

#### **4.3 Other directions**

The inchworm type piezoelectric drive device can not only obtain large output stroke, but also ensure high output accuracy and load-carrying capacity, which is favored by many scholars. The research of Inchworm piezoelectric driving device has its own characteristics at home and abroad, which provides a favorable technical basis for the development and application of piezoelectric precision drive technology. Besides the above future directions, the existing inchworm piezoelectric actuator is still in the stage of empirical design and test, lacking of relevant theoretical model guidance, and there are problems of empirical design and repeated attempts. Therefore, it is necessary to establish the dynamic model of the inchworm piezoelectric actuator to guide the design and research of the inchworm piezoelectric drive device. In addition, the miniaturization is always the hot point for piezoelectric actuators which could leads to the real application in many research and industrial fields.

Risaku et al. have developed a large stroke and high precision inchworm actuator [28] (**Figure 11**). With the combination of piezoelectric and electrostatic motion principles, the displacement accuracy of each step reaches tens of nanometers, which can be called ultra-high precision. The displacement accuracy is 59 nm/cycle, but the maximum travel distance is only 600 μm, which needs to be improved.

In order to solve the shortcomings that most inchworm type piezoelectric actuators require larger input voltage, Mehmet et al. took the lead in developing a new type of low voltage, largestroke, and large output inchworm actuator based on the micro-electromechanical systems (MEMS) [29]. It mainly applies the principle of electrostatic motion. Through the amplification of the flexible hinge, it achieves a total displacement of ±18 μm and an output force of ±30 μN at a low voltage of 7 V; a displacement of ±35 μm can be achieved at a voltage of 16 V, ±110 μN output force.

### **5. Conclusion**

The inchworm movement is a high-precision driving method that imitates the movement form of the inchworm in nature to realize the stepping movement of itself or the holding object. Inchworm movement is a kind of stepping movement, *Principle, Design and Future of Inchworm Type Piezoelectric Actuators DOI: http://dx.doi.org/10.5772/intechopen.96411*

which is different from other continuous movement. Its movement can be regarded as a combination of movement and stop in time, but it is also a continuous movement from the perspective of the overall effect of the movement. Inchworm motion can easily achieve large-stroke step-by-step linear motion. Although scholars in various countries have conducted a number of research work on inchworm-type piezoelectric driving devices, most of their research content is linear inchworm driving devices, which involve rotation. There are few reports on the inchworm actuator, and the existing inchworm-type piezoelectric actuator has complex structure control, lacks relevant theoretical model guidance, and has problems of empirical design and repeated attempts. In the future, there is still a lot of work to be solved for the inchworm piezoelectric actuator to promote the real practical use of the inchworm piezoelectric actuator.
