Aiming to address the problem of the low amplification ratio of traditional microgrippers, a two-stage microgripper based on the principle of combined amplification was designed with a high amplification ratio and large displacement, using a simple and compact structure. The relationship between theoretical input variables and output variables were first calculated by a projection theorem. Secondly, the performance of the microgripper was analyzed by finite element analysis (FEA). Finally, the accuracy of the theoretical calculation and FEA was verified experimentally. The results show that the microgripper has high magnification and can be gripped in parallel, with self-adaptability for many irregular shaped micro objects. The actual magnification was 23.2×, which is greater than similar products.

Topics
Amplifiers, MEMS technology, Piezoelectric ceramics, Alloys, Machining, Mechanical stress, Micromanipulation, Piezoelectric materials, Smart materials, Finite-element analysis

  • A large-displacement microgripper was designed.

  • There is inevitable resistance from the second-grade amplification mechanism against the first-grade one

  • The microgripper designed in this paper improves the performance of the microgripper greatly.

Topics
Amplifiers, MEMS technology, Piezoelectric ceramics, Alloys, Machining, Mechanical stress, Micromanipulation, Piezoelectric materials, Smart materials, Finite-element analysis

In recent years, with the trend of miniaturization of microelectromechanical systems (MEMS) and other products, extensive research on micromanipulation and assembly technologies has been conducted.1–3 Compared with contact and non-contact micromanipulation technologies, such as optics, electrostatic, Bernoulli, ultrasound, and electromagnetics, the microgripper features are high precision, low cost and the ability to grasp objects of different shapes. The microgripper has been widely used in micro/nanotechnology, medical science, materials science, biology and tissue engineering.4,5 Therefore, in order to meet the requirements of micro-operation and micro-assembly, it is necessary to develop a new type of microgripper with high performance.

The key problems of micro-clamps are choosing the driving mode and how to increase displacement magnification. Common driving modes of the microgripper include piezoelectric,6,7 electrostatic,8,9 thermal,10,11 shape memory alloy (SMA),12,13 and electromagnetic.14,15 Compared with other driving modes, a piezoelectric drive has advantages of a fast response, high sensitivity and large output force.16 Increasing the magnification of the displacement is mainly accomplished by the amplifier. Commonly used amplifiers are a lever amplifier, bridge amplifier and rhombic amplifier. The single lever amplifier has advantages of a simple structure and easy realization. Single bridge and rhombic amplifiers have advantages of a high amplification rate and compact structure. The single lever amplifier, bridge amplifier, and rhombic amplifier are unipolar amplifiers. The amplification rate of the unipolar amplification mechanism is limited. A multipolar amplifier composed of several unipolar amplifiers can further enhance the amplification rate.17 The single-stage amplification microgrippers designed by Cui et al.18 and Zubir et al.19 based on the lever amplification principle can effectively improve the displacement amplification ratio, but the amplification ratio is low. Wang et al.20 designed a two-stage amplification microgripper based on the lever amplification principle; however, too many hinges increases the energy transfer process loss, and reduces the amplification rate. Zhang et al.21 designed a two-stage amplification microgripper based on the lever amplification principle that avoids the issue of too many hinges. The differential lever amplification mechanism is used to reduce the loss of the energy transmission. However, the driver is outside the mechanism and the structure is not compact enough. Ai and Xu22 designed a two-stage amplification microgripper as the principles of lever and triangle amplifications to avoid the problem of too many hinges and has a compact structure. However, parallel gripping of the lever amplifier was accomplished by adding a parallel four-bar mechanism, which does not conform to the simple structure of the lever amplifier.

In summary, the magnification of a multi-stage amplifier is usually greater than that of a single-stage amplifier. Under the condition of simple structure, compact structure, parallel gripping and piezoelectric driving, in order to obtain large amplification ratio, the microgripper adopts two-stage amplifier, the lever amplification mechanism on both sides of the microgripper adopts straight circular flexible hinge, which can effectively improve the amplification ratio, and the piezoelectric (PZT) actuator can achieve large displacement output under the action of small input displacement.

Fig. 1 is the structure of the microgripper. The size of the microgripper is 39.68 mm × 23.6 mm × 5 mm. The microgripper mainly consists of a straight circular flexure hinge, straight flexible hinge, grasping jaws, lever amplifier, rhombic amplifier, fixing holes, and preload bolt. The mechanism adopts a left-right symmetrical structure, which can balance the stress inside the structure and reduce the error caused by deformation.

Fig. 1.
Fig. 1. Structural diagram of the microgripper.
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Structural diagram of the microgripper.

Fig. 1.
Fig. 1. Structural diagram of the microgripper.
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Structural diagram of the microgripper.

Close modal

After fixing the microgripper, the power supply is turned on. When the piezoelectric ceramics are electrified, the rhombic amplifier is driven upward. Under the action of bidirectional symmetrical force, the lever amplifier on both sides is pulled inward and parallel to complete the gripping action of the grasping jaws.

Based on a pseudo-rigid-body model, the flexible hinge is regarded as a movable hinge with torsional spring, and the connecting rod is regarded as a rigid member. The right half of the microgripper is analyzed, and its equivalent model is shown in Fig. 2, where i (i = A, B, …, i) represents the rotation center of the flexible hinge, and Din and Dout represent the input displacement of the stack PZT ceramic actuator (SPCA) and the output displacement of the grasping jaws, respectively.

Fig. 2.
Fig. 2. Pseudo-rigid-body model of the microgripper.
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Pseudo-rigid-body model of the microgripper.

Fig. 2.
Fig. 2. Pseudo-rigid-body model of the microgripper.
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Pseudo-rigid-body model of the microgripper.

Close modal

As shown in Fig. 2, A-B-C-D-E-F is equivalent to a six-bar slider mechanism in which the initial angular positions of linkage devices AB, BE, CD, and FE are θ1, θ2, θ3 and θ4, respectively.

(1)

The length of the connecting rod is Lx (x = AB, BC, …, AF). Angle β is the angle between the connecting rod BC and BE. Therefore, the following geometric and kinematic relationships are true:

(2)

For Eqs. (1) and (2) in the derivation of time:

(3)
(4)

where β=arctan2llBC⁠.

If the real part and imaginary part of Eqs. (3) and (4) are equal, the following relationship exists:

(5)
{lABω1⁡sin(θ1−θ4)+lBEω2⁡sin(θ2−θ4)=0lABω1⁡sin(θ1−θ2)+lEFω4⁡sin(θ4−θ2)=0lABω1⁡sin⁡θ1+lBCω2⁡sin⁡θ2+lCDω3⁡sin⁡θ3=0

Thus:

(6)

The displacement magnification can be expressed as:

(7)

where vo and vi represent the input and output speed, respectively.

The velocities of vo and vi are expressed as:

(8)
(9)

By substituting Eqs. (8) and (9) into Eq. (7),

Ramp=2ω4llABω1⁡cos⁡θ1+lBCω2⁡cos⁡θ2+lCDω3⁡cos⁡θ3

Thus:

(10)

The selection parameters of the microgripper are as follows: the material of the microgripper was 7075 aluminum alloy with an elastic modulus E = 71 GPa, Poisson’s ratio ν = 0.33, yield strength σ = 455 MPa and density ρ = 2810 kg/m3.

A displacement of 20 μm was applied at the input end. The displacement nephogram and parallel characteristic diagram of the microgripper are shown in Fig. 3(a). The output displacement of the microgripper was 235.36 μm and the displacement amplification was 23.536 times. As shown in Fig. 3(b), the maximum stress of the mechanism was 246.86 MPa, which is less than the yield strength of the material (455 MPa). Therefore, the product can be used safely.

Fig. 3.
Fig. 3. Strain and displacement analysis diagram of the microgripper.
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Strain and displacement analysis diagram of the microgripper.

Fig. 3.
Fig. 3. Strain and displacement analysis diagram of the microgripper.
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Strain and displacement analysis diagram of the microgripper.

Close modal

The relationship between the input displacement of piezoelectric ceramics and the gripping force of the grasping jaw is shown in Fig. 4 when the microgripper is used to grip micro-parts with diameters of 500 μm, 400 μm, and 300 μm. There is a linear relationship between the input displacement and gripping force, which shows that the performance of the microgripper is stable, and a smooth transition is achieved from the grasping jaw closure to the gripping process of the micro-parts. The output force of the grasping jaw is 2 N under an input force of 20 μm when the 500 μm micro-axis was gripped.

Fig. 4.
Fig. 4. The relationship between the output displacement of SPCA and grasping force.
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The relationship between the output displacement of SPCA and grasping force.

Fig. 4.
Fig. 4. The relationship between the output displacement of SPCA and grasping force.
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The relationship between the output displacement of SPCA and grasping force.

Close modal

The finite element modal analysis of the microgripper was performed to check the resonant modal frequency and shape of the microgripper. The first six resonance modes are shown in Fig. 5, and the corresponding resonance frequencies are shown in Table 1. The first resonance mode at 576.86 Hz was caused by the translation of two parallel quadrilateral mechanisms in the same direction. The second mode at 622.66 Hz was generated by the translation of two parallelograms in the opposite direction, which corresponds to the closing/opening mode of the gripper tip. The third to fifth modes were caused by the out-of-plane deformation of the structure, while the sixth mode was caused by the in-plane bending deformation of the one gripper arm.

Fig. 5.
Fig. 5. FEA simulation results of the first six mode shapes for the microgripper.
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FEA simulation results of the first six mode shapes for the microgripper.

Fig. 5.
Fig. 5. FEA simulation results of the first six mode shapes for the microgripper.
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FEA simulation results of the first six mode shapes for the microgripper.

Close modal
Table 1.

Frequencies corresponding to sixth-order modes.

Mode1Mode2Mode3Mode4Mode5Mode6
576.86 622.66 1756.4 1770.9 4122.5 4563.1 
Mode1Mode2Mode3Mode4Mode5Mode6
576.86 622.66 1756.4 1770.9 4122.5 4563.1 
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The physical model of the microgripper is shown in Fig. 6. The material of the microgripper was 7075-T6 (SN) aluminum alloy. The microgripper was processed by a WEDM machine tool. After the processing was completed, the micro-gripper is drilled and polished. The experimental equipment includes a HPV-1C 0300 A0300 piezoelectric ceramic driving power supply, micro-sodium positioning worktable, PZT (Suzhou Mat, Inc. SZBS150/5 × 5/20, open-loop travel 20 μm) driving micro-positioning stage, a high resolution capacitive displacement sensor (BJZD’s MA-0.5) and a data acquisition card (NI’s PCI-6221), a 24 V DC regulated power supply WP100-D-G, host computer, and display. In order to eliminate the external interference as much as possible, all of the devices were installed on the high performance vibration isolation platform.

Fig. 6.
Fig. 6. Physical model of the microgripper.
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Physical model of the microgripper.

Fig. 6.
Fig. 6. Physical model of the microgripper.
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Physical model of the microgripper.

Close modal

4.2.1. Performance analysis

A series of experiments were performed to verify the performance of the microgripper in order to further test the performance of the microgripper. The experimental device is shown in Fig. 7. Fig. 8 is the working principle of the output displacement measurement.

Fig. 7.
Fig. 7. Experimental setup of the microgripper.
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Experimental setup of the microgripper.

Fig. 7.
Fig. 7. Experimental setup of the microgripper.
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Experimental setup of the microgripper.

Close modal
Fig. 8.
Fig. 8. Working principle diagram of the experimental device.
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Working principle diagram of the experimental device.

Fig. 8.
Fig. 8. Working principle diagram of the experimental device.
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Working principle diagram of the experimental device.

Close modal

Fig. 9 shows the relationship between the output displacement of SPCA and the applied voltage of the piezoelectric ceramic actuator from 0 to 100 V. The results confirm the nonlinearity and hysteresis of the piezoelectric materials. When the input voltage was 100 V, the maximum output displacement was 17.585 μm.

Fig. 9.
Fig. 9. Output displacement of SPCA versus the applied voltage.
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Output displacement of SPCA versus the applied voltage.

Fig. 9.
Fig. 9. Output displacement of SPCA versus the applied voltage.
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Output displacement of SPCA versus the applied voltage.

Close modal

However, the nonlinearity of the piezoelectric material does not affect the linear relationship between the input and output of the microgripper. Fig. 10 demonstrates the tip displacements of the microgripper jaw vary with the input displacement obtained by different approaches. The input displacement of the microgripper has a linear relationship with the tip displacement of the single jaw, demonstrating the microgripper has stable performance. The theoretical magnification is 25.268 times and the FEA magnification is 23.536 times; the error is mainly due to the theoretical calculation of the flexible hinge as a movable hinge with torsion spring and the connecting rod as a rigid member. In actual motion, the rigid bar will also undergo minor deformation. The experimental results show that the actual magnification of the microgripper is 23.2 times, which is mainly due to the processing error, vibration and noise of the equipment during the experiment.

Fig. 10.
Fig. 10. Tip displacement versus the input displacement of SPCA.
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Tip displacement versus the input displacement of SPCA.

Fig. 10.
Fig. 10. Tip displacement versus the input displacement of SPCA.
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Tip displacement versus the input displacement of SPCA.

Close modal

In order to verify the performance of the microgripper in the actual clamping process, several grasping experiments were performed. Fig. 11 shows the microgripper gripping plastic plate, metal wires and balls. The results show that the microgripper has the characteristics of parallel gripping, self-adaptability to different shapes of objects, and can successfully grip a variety of micro-parts without causing damage.

Fig. 11.
Fig. 11. Grasping manipulation for different shape and size micro objects: (a) 200 μm plastic plate, (b) 150 μm metal wire, and (c) metal ball with diameter of 250 μm.
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Grasping manipulation for different shape and size micro objects: (a) 200 μm plastic plate, (b) 150 μm metal wire, and (c) metal ball with diameter of 250 μm.

Fig. 11.
Fig. 11. Grasping manipulation for different shape and size micro objects: (a) 200 μm plastic plate, (b) 150 μm metal wire, and (c) metal ball with diameter of 250 μm.
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Grasping manipulation for different shape and size micro objects: (a) 200 μm plastic plate, (b) 150 μm metal wire, and (c) metal ball with diameter of 250 μm.

Close modal

4.2.2. Performance comparison

The magnification of our microgripper versus similar devices18–22 are shown in Table 2. The magnification of the microgripper proposed in this paper reached 23.2 times, which is greater than other microgrippers.

Table 2.

Comparison of the parameters with similar microgrippers.

MicrogripperMagnification
Ref. 18  6.88 
Ref. 19  3.68 
Ref. 20  16 
Ref. 21  6.1 
Ref. 22  12.6 
Proposed in this paper 23.2 
MicrogripperMagnification
Ref. 18  6.88 
Ref. 19  3.68 
Ref. 20  16 
Ref. 21  6.1 
Ref. 22  12.6 
Proposed in this paper 23.2 
View Large

A high-power piezo-driven microgripper based on the principle of combined amplification was designed. By establishing a pseudo-rigid body model, the relationship between theoretical input variables and output variables was obtained. The output force of the grasping jaw was 2 N when the input displacement was 20 μm and a 500 μm micro-axis was gripped. The error between the theoretical value and simulation value of the input displacement and output displacement was 6.85%, and 8.18% between the theoretical and experimental value. The displacement amplification ratios of three different methods are very close, which proves the correctness of the theoretical analysis and finite element analysis. The correctness of the theoretical value and simulation value was verified. The actual magnification of the microgripper was 23.2 times. The microgripper successfully grasped micro-parts of different shapes. The design of the microgripper provides a useful reference for further research on a high-rate microgripper.

This work was supported by Department of Education of Liaoning Province Project (L2017LQN024).

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 Xiaodong Chen received the B.Eng. degree in mechanical manufacturing and automation from the Northwest University of Technology, Xian, China, in 2017. He is currently working toward the M.Sc. degree in Liaoning University of Petroleum and Chemical Technology. His current research interests include flexure mechanisms and MEMS.

 Zilong Deng received the B.Eng. degree in mechanical manufacturing and automation from the Liaoning University of Technology, Shenyang, China, in 1993, and the M.Sc. degree in mechanical manufacturing and automation from Jilin University, Changchun, in 2000.

He has been a Professor at the School of Mechanical Engineering, Liaoning University of Petroleum and Chemical Technology, since 2013. His current research interests include Mechatronics and flexure mechanisms.

Copyright © 2019 Tianjin University. Publishing Service by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
2019
Tianjin University