Design and Performance Analysis of a Micro-Displacement Worktable Based on Flexure Hinges

The flexure hinge is a kind of micro-displacement adjustment device with application prospects because of its high displacement resolution, positioning accuracy and repeatability. In this study, a micro-displacement worktable with four degrees of freedom (X→, Z→, X︵, Z︵) was designed. The micro-displacement worktable was composed of three different flexure hinges. The adjustment ranges and adjustment accuracy of flexure hinges in terms of their respective degrees were improved. The micro-displacement worktable performance was examined by FEA (Finite Element Method). The maximum displacement that was adjusted in X→ and Z→ was 1.67 µm and 1.74 µm. The maximum angle adjusted in the X︵ and Z︵ direction was 14.90° and 18.58°. A test platform was developed for micro-displacement worktable performance tests. The simulation results showed a good agreement with the experimental results.


Introduction
In the field of micro-manufacturing [1], precision measurement [2,3], medical instruments [4,5] and friction tester [6], the performance of the ultra-precision positioning table are more and more important. In recent years, the micro displacement mechanism based on the principle of flexure hinge has been widely developed. The mechanism has characteristics of compact structure, free of mechanical friction and high displacement resolution [7].
The original flexure hinge is a single structure that owns only one degree of freedom, and its adjustment range is limited. With the development of flexure hinges, it was gradually evolved from a single degree of freedom to multiple degrees of freedom. Paros [8] firstly introduced a theoretical approach to analyzing flexure hinges in 1965. Lobontiu not only discussed the rotation ability and motion accuracy of the right-angle flexure hinge but also proposed a theoretical analysis of multi-axis compatibility with the flexure hinge assembly process [9][10][11]. In addition, he proposed a flexure hinge calculation method of conical, curved surfaces, including elliptical and parabolic linear. Wu [12] improved the design formula of the flexibility or spring stiffness of rectangular single-axis hinges and right-circle hinges based on Paro's theory. Lin [13] recently designed and analyzed the performance of a hybrid flexure hinge. Pan [14] introduced the triple periodic minimum surface lattice structure and designed a new type of leaf flexible hinge. Kong [15] proposed a new type of conical V-shaped flexure hinges (CFHs). Wang [16] designed high-rate, low-stress, non-friction and non-bending flexure hinges for bridge compliant mechanisms. Qiu [17] proposed a variable pitch folding flexible hinge (PFFH) based on the theory and characteristics of coil spring. Li [18] proposed two generalized models for fast solving the flexibility and precision closed-form equations of multi-axis flexure hinges with small deflections.
With the improvement of the flexure hinge theory, more and more research has focused on flexure hinge assembly for multi-degree of freedom adjustments. Sun [19] and Li [20] designed a displacement-amplified gripper to achieve displacement adjustment of 2-DOF on the plane. Shao [21] presented a 2-DOF PFSM with a cross-axis decoupling capability. Qin [22] and Lin [23] designed 3-DOF flexure hinges that increase the rotation of the zaxis. A 3-DOF platform, which improved stability, was designed by Lee [24]. Zhao [25] designed a 4-DOF micro-displacement adjustment mechanism to achieve three directions of movement and one direction of rotation. Chen [26] described a 5-DOF alignment to compensate for alignment errors. With the continuous development of robot technology, flexure hinge structures of multiple degrees of freedom were used in the design of robots. Hesselbach [27] and Yun [28] designed a 6-DOF parallel robot using a flexure hinge. The 6-DOF platform improved structural stability and had better dynamic mechanical properties. Lu [29] designed a new three-dimensional ultrasonic vibration platform with adjustable characteristics based on T-shaped flexible hinges. Gräser [30] proposed a high-precision compliant 2-DOF micro-positioning platform with flexible hinges, which can realize the motion range of the X-Y axis ±10 mm. Gan [31] proposed a design of a 3-DOF bidirectional motion platform based on a Z-shaped flexible hinge and bridge mechanism. Gui [32] designed a three-degree-of-freedom flexible hinge micro-motion platform based on a new composite lever amplification mechanism, which has good motion decoupling and high precision. Flexure hinges play an increasingly important role in the precision instrument.
Finite element simulation plays an important role in the design and performance check of mechanical parts. Therefore more people are using FEA to design flexure hinges [33]. Cai [34] and Ivanov [35] used a simulation to analyze the mechanical properties of trapezoidal flexure hinges and rounded flexure hinges. Ma [36] presented a miniature piezoelectricdriven fatigue device with 3-DOF. Hwang [37] theoretically analyzed the working characteristics of flexure hinges and lever mechanisms. The safety of a piezoelectric ceramic actuator device based on a flexure hinge was checked by finite element simulation [38]. The flexibility of this kind of flexure hinge finger was greatly improved.
However, most of the reported flexure hinges have a small range of displacement, which cannot meet the application fields with higher accuracy requirements. For example, in the field of MEMS technology, when processing, positioning and assembling small parts, most need to adjust the position of small parts from multiple perspectives. In addition, flexure hinge structures that can adjust multiple degrees of freedom are most complicated, which has the problems of a large number of components; a heavy weight; long assembly time; and inter-mechanism friction, wear and lubrication. Thus, to solve the above problems, this paper designed a novel 4-DOF flexure hinge micro-displacement worktable, which is composed of three different flexure hinges. The worktable has the advantages of 4-DOF, high stiffness and good decoupling effect, and obtains a large range of micro-displacement adjustments to achieve high-precision displacement output. It can be applied to the field of precision machining, precision measurement, microelectronic manufacturing, and so on. Furthermore, the micro-displacement worktable has the advantages of simple structure, small volume, no mechanical friction, no gap and high motion sensitivity. The performance of the worktable was examined by finite element analysis. A test platform was developed for micro-displacement worktable performance tests. The micro-displacement worktable was tested by experiments, and the experimental results were compared with the simulation results. The experiment result shows that the flexure hinge has the potential for micromanufacturing and precision measurement, which helps to improve the accuracy and reliability of the micro-displacement worktable. The results of the micro-displacement table are compared with other similar designs, as shown in Table 1.
The workbench has a compact structure, large stroke and bidirectional movement, but it is relatively difficult to manufacture, has a high cost and it is difficult to control four actuators.

Design of Flexure Hinge Mechanisms
The micro-displacement working platform can improve micro-cutting precision, so it is suitable for micromachining and precision measurement. The micro-displacement working platform is shown in Figure 1. The platform includes flexure hinges A, B and C. The flexure hinge A consists of a parallel four-bar mechanism, which can realize horizontal and vertical displacement adjustment. The flexure hinges B and C can achieve angular displacement adjustment. The most widely used flexure hinge in this micro-displacement working is corner-filleted. The structure diagram of the corner-filleted is shown in Figure 2a. The force condition of the flexure hinge is shown in Figure 2b.  When the left end is subjected to the load F y , the displacement at the free end (x = 0, y = 0) is expressed as follows.
When the left end is subjected to the bending moment M Z , the displacement at the free end (x = 0, y = 0) is expressed as follows.
When the left end is subjected to the load F X , the displacement at the free end (x = 0, y = 0) is expressed as follows.
The total displacement of the flexure hinge is shown in Equation (6): When the displacement of both ends of the flexure hinge is not zero, the overall stiffness of the flexure hinge under combined deformation is as shown in Equations (7) and (8). The displacement range of the platform can be calculated easily by using the angular displacement matrix.
When input → X and → Z direction displacement of hinge A at the point A and B occurs in Figure 3a, the displacement of the → X and → Z direction is adjusted. When input Y displacement of flexure hinge B at point C in Figure 4a and flexure hinge C at point D in Figure 5a occurs, the displacements of the X and Z direction are adjusted. When flexure hinges A, B and C are assembled, the linear adjustment of hinge A and angular displacement adjustment of hinge B and C can be combined. The micro-displacement can achieve 4-DOF micro-displacement adjustment in precision machining confidential measurement.
As shown in Figure 4, the input Y-direction displacement of the flexure hinge B at point C is applied at the marked position, and the output response surface can realize angular displacement output in the → X direction. The output angle of → X direction is θ 1 .
As shown in Figure 5, the input Y-direction displacement of the flexure hinge C at point D is applied at the marked position, and the output response surface can realize the angular displacement output in the Z direction. The output angle of Z direction is θ 2 .

Finite Element Analysis
In order to investigate the maximum output displacement of flexure hinges and examine the performance of flexure hinges, the FEM (Finite Element Method) model of flexure hinges was established. AISI 1045 steel was selected as the material of the model. The physical properties of AISI 1045 steel are shown in Table 2 (http://www.matweb.com, accessed on 10 January 2022) [39]. The flexure hinge was meshed by triangular mesh with a mesh size of 0.1 mm. The meshes are refined at flexure hinges to guarantee simulation accuracy. The input displacement points and fixed points of the flexure hinge A, B and C are shown in Figures 3-5, respectively. Then flexure hinges A, B and C were assembled in the FEM model.  Figure 6 illustrates the corresponding stress distribution acting on flexure hinges during the movement. As shown in Figure 6, the stress concentration occurs at flexure hinges. The stress of flexure hinges must be less than the allowable stress (407.7 MPa) of the material. Figure 7 shows the relationship between the stress of flexure hinges and the input displacement. From this Figure 7, it can be seen that the maximum input displacement of → X and → Z direction of the flexure hinge A is 8 µm. The maximum input displacement of hinge B and C is 12 µm and 18 µm, respectively. Flexure hinges work safely when the input displacement is within the specified range of displacement. If the input displacement is beyond the displacement range, flexure hinges may be damaged.  The output displacements of three flexure hinges are calculated using FEA. When the stress of the danger points is less than allowable stress, it can be seen input displacement and output displacement have linear relationships. The results are shown in Figures 8 and 9.  Table 3 summarizes all simulation data of the flexure hinge.

Experiment Tests
Experimental tests are carried out to investigate flexure hinges input displacement and adjustable range. The test platform is shown in Figure 10. The displacement of the flexure hinge is at the micrometer level, so the tiny vibration can cause a serious error in the experimental results. A vibration-isolated optical platform (ZDT18-10) was used as the base of the test platform. The amplitude of this floating platform was 0.002 mm. The vibrationisolated optical platform can isolate the outside disturbance from the experiments. A linear motion actuator whose positioning accuracy is 0.05 µm was used as a loading device. The displacements on the output terminals are recorded via a laser displacement sensor (LK-H080). The flexure hinge was fixed on the fixture, flexure hinges were loaded displacement by the linear motion actuator, and the test point of the laser displacement sensor was placed on the corresponding output surface for output displacement data collection.

Experimental Results
Flexure hinges were tested on the test platform. The experimental results are shown in Figures 11-13. There are linear parts and nonlinear parts in the input-output curves. The material of the flexure hinge deformed plastically when the nonlinear part occurred. Therefore, the flexure hinge should work below the critical transition displacement between the linear part and the nonlinear part.   Figure 12a,b, using Equation (9) to calculate the output angle.
The experimental results of the flexure hinge B and hinge C are shown in Figure 12. There is an obvious relationship between input displacement and output angle when the input displacement is lower than 12 µm and 18 µm, respectively. The maximum angle adjusted in the X and Z is 15.1 • and 22.3 • , respectively. The ratio of input/output angle of hinge B and hinge C is 1.3774 and 1.2649, respectively. The simulation results show a good agreement with the simulated results.
The flexure hinge assembly was tested on the experimental platform. The experimental results for the direction of  Figure 13b. The maximum angle in the X and Z is 14.90 • and 18.58 • , respectively. The ratio of input/output angle of the hinge C is 1.3258 and 1.0643, respectively. Table 4 summarized all experimental data of flexure hinge and compared the experimental results with simulation results.

Conclusions
This paper introduced the design, finite element simulation test and experimental analysis of a new 4-DOF micro-displacement worktable. The 4-DOF micro-displacement worktable consists of three flexure hinges, which are simple and have good stability. The theoretical analysis of the designed flexure hinge was carried out, and the performance of the flexure hinge was tested by finite element simulation. A flexure hinge structure was developed by using electro-discharge wire cutting. A test platform was built to test the performance of the flexure hinge. The maximum output displacement in → X and → Z directions are 1.67 µm and 1.74 µm. The maximum output angle that can be adjusted in the X and Z direction is 14.90 • and 18.58 • . The maximum output error of the experimental and the simulated is 6.0%, 2.0%, 1.7% and 3.4%. The simulation results show a good agreement with the experimental results. This micro-displacement worktable can be used in the fields of precision machining, precision measurement, microelectronic manufacturing, etc.