The Development of an Displacement Sensor Embedded Voice Coil Motor Based Fast-Tool-Servo for the Machining of Micro-structure Zhijun Yang 1, a * , Youdun Bai 1, b, Sujuan Wang1, c , Xindu Chen1, d, Qiang Liu1, e, Ketian Li1, f, Han Wang1, g, and Xin Chen1, h 1
School of Electromechanical Engineering, Guangdong University of Technology, Guangzhou H.E.M.C, 510006, P.R.China a
[email protected] , b
[email protected],
[email protected] d
[email protected],e
[email protected], f
[email protected], g
[email protected], h
[email protected]
Keywords: Fast-Tool-Servo, Displacement Sensor Embedded, Voice Coil Motor
Abstract. The Fast Tool Servo (FTS) is widely used for the machining of micro-structures, especially for optical micro lens array. The working principle of FTS is that a voice coil motor or a piezoelectric (PZT) actuator is used as the driving elements, and the flexure hinges are developed as the guide mechanisms. In addition, an optical encoder is applied to measure the displacement. However, the existing design of FTS is too complicated and expensive. One reason is that the stroke of the FTS for the fabrication of optical micro lens array is only a few hundred micrometers, while its precision reaches to nanometric range, thus the optical encoder in not applicable. On the other hand, there exists sluggish and creep for piezoelectric materials, which makes the control of displacement difficult. This paper develops a displacement sensor embedded voice coil motor. In the design, the driving element is ampere force of the voice coil motor which is a linear ratio to the input current. Both the two covers are thin plates which serve as compliant mechanism by supporting the deformation at the Radial direction and provide linear stiffness in the axial direction. Therefore, the output displacement is proportional to the ampere force. The existence of external force affects the actual displacement, an embedded capacitor serves as displacement sensor will detects the real displacement, and the external force can be estimated by the current and measured displacement, which makes the motion control easy. At the same time, a multiphysics model of the developed FTS is built in this study by using finite element method and the displacement control under different cutting force is studied. The experimental results show that the developed FTS is efficient for achieving short stroke with high precision.
Introduction The Fast-Servo-Tool (FST) is widely used for micro-structure manufacturing especially for micro optical lens [1]. A flexure-based long-stroke fast tool servo for diamond turning is presented by Qiang Liu et.al[2]. In their work, a voice coil motor and a piezoelectric actuator are used as the driving elements, and two flexure hinges are developed as the guide mechanisms. However, there exists vertical displacement jump when the flexure hinges are driven by a voice coil motor or a piezoelectric actuator. Therefore, Zhijun Yang et.al [3] presented a new amplified structure allowing the horizontal motion while reducing the vertical displacement jump. The working principle is that, the piezoelectric actuator is apply to a beam which has two flexure hinges, one is linked to the frame, and the other is linked to tool holder which is situated through two parallel membranes. When the piezoelectric actuator deforms, the beam will rotate around the frame, and the displacement is amplified in the other end, causing the tool holder’s motion and the membranes are forced to bend, while the vertical motion is restrained by the membranes. As a result, the presented membrane based flexure structure is able to amplify the motion of piezoelectric actuator. In addition, the vibration
frequency of the membrane is easy to be adjusted by the preload force, it is important when the fast-tool servo is working at different frequency. The performance of the presented structure is analyzed using structural dynamics coupled with piezoelectric, and the parameters of the structure are optimized to remain linear relation between tool holder and the piezoelectric actuator, while the vertical jump is much smaller than the structure presented in reference [2]. However, there exists sluggish and creep for piezoelectric materials, which makes the control of displacement difficult. This paper develops a displacement sensor embedded voice coil motor. In the design, the driving element is ampere force of the voice coil motor which is a linear ratio to the input current. Both the two covers are thin plates which serve as compliant mechanism by supporting the deformation at the Radial direction and provide linear stiffness in the axial direction. Therefore, the output displacement is proportional to the ampere force. The existence of external force affects the actual displacement, an embedded capacitor serves as displacement sensor will detects the real displacement, and the external force can be estimated by the current and measured displacement, which makes the motion control easy. At the same time, a multiphysics model of the developed FTS is built in this study by using finite element method and the displacement control under different cutting force is studied. The experimental results show that the developed FTS is efficient for achieving short stroke with high precision. 1. Design Principle The presented device is based on a voice coil motor(including 4,6 and 7), which situated in a sleeve (5), and with the shaft pass two identified covers (3) through the center, and the covers fixed on the sleeve (5). There two electrodes, one is fixed onto the yoke (4) forms the Stationary Electrode(9), and the other one is fixed onto the Stand becomes a motion Electrode(10), when the shaft move along the axis, cause the changes of the distance of the two electrodes, resulting change in the capacitance between the two electrodes. As a result, the displacement of the shaft can be detected by the changes of the capacitance. Since the displacement of the FTS is only a few hundred micro meters, the magnetic field through coils remain constant, so the excited force should proportion to the input current, the cutting force can be estimated by the displacement difference.
1 Shaft; 2 Screw; 3 Cover; 4 Yoke; 5 Sleeve; 6 Coil; 7 Stand; 8 Permanent magnet; 9 Stationary Electrode; 10 Motion Electrode; Fig.1 The component schematic of the presented device 2. Modeling and simulation The presented structure is a typical metaphysics problem, the structure coupling with electric and magnet, the maximum excited force is 100N, and the require frequency is 100Hz. In the original
design, the thickness of the cover is 1mm. Using the finite element method, the displacement is 17.236μm, and the frequency of the first natural mode f is 1016Hz, the equivalent mass m can be calculated by k m= (1) (2πf ) 2 where k is the equivalent stiffness. However, the desired the amplitude is 100μm, and according to the basic differential equation of the bending thin plate Eh 3 ∇2 ∇2 w = p 12(1 − μ 2 )
(2)
where ▽represent the gradient operator, E, h, μ, w, and p are Young’s modulus, thickness, poisson ratio, deflection and pressure load, respectively. One can see that the deflection is proposition to cubic root of the thickness, so the desire thickness should be 0.5565mm. After redesign, the maximum deflection under 100N force load is 101.205μm, and the equivalent stiffness is 988094.3N/m, the frequency is 430Hz, so the equivalent mass is 0.135kg(shown in Table 1). Table 1 Maximum Thickness Stiffness Frequency Capacitance mass(kg) (mm) (N/m) (Hz) (μm) changes(nF) 17.236 3.22312E-09 1 5801716 1016 0.142367 5.37021E-09 0.5565 988094.3 430 0.135364 101.205 The output force of the voice coil motor is Fi = BLI (3) where B is the magnetic strength, L is the length of the coil, and I is the input current. The desire deflection is δ = Fi / k (4) When there exists cutting force Fc, the real deflection δ r = ( Fi − Fc ) / k (5) which can be detected by the capacitance, thus the cutting force can be calculated by Fc = Fi − kδ r (6) the cutting force estimate is very importing for the control of the high precision machining. The displacements of the presented device with difference thickness covers under same excitation are shown in Fig.2, and the relationship between capacitance changes and the deflection is shown in Fig.3. It can be seen that the deflection is linear to the excitation force, and the deflections is linear to the capacitance changes, and the deflections can be calculated according to the relation functions (also shown in Fig.) when the capacitances are detected.
Fig.2 The displacement with different cover thickness
Fig.3 The The relationship between the capacitance changes and deflections Summary In this paper , a membrane base voice coil motor served as a FTS, the excited force is linear to the input current, and the real deflection is detected by the embedded capacitance. The stiffness can be adjusted by changing covers with different thickness to obtain different stroke ranges. The real deflection can be measured by the internal capacitance, and the cutting force also can be estimated, the presented device with make the FTS more easy to control.
Acknowledgement This work is supported by The National Basic Research Program of China (2011CB013100-G)， Natural Science Foundation of China(U1134004, 50905033), Guangdong Innovative Research Tea m Program(201001G0104781202), National key technology support program (2012BAF12B10), Sp ecialized Research Fund for the Doctoral Program of Higher Education of China (20094420120001), Guangdong Province Science and technology plan(2010A090200017, 2012B011300067)
References [1] Sebastian Scheiding, Allen Y. Yi, Andreas Gebhardt, Lei Li, Stefan Risse, Ramona Eberhardt, Andreas Tunnermann. Freeform manufacturing of a micro optical lens array on a steep curved substrate by use of a voice coil fast tool servo [J]. Optics Express, 2011,19( 24 ) 23939 [2] Qiang Liu, Xiaoqin Zhou, Pengzi Xu, Qing Zou, Chao Lin. A flexure-based long-stroke fast tool servo for diamond turning[J]. Int J Adv Manuf Technol (2012) 59:859–867 [3] Zhijun Yang, Weibo Zhou, Xin Chen, Xindu Chen, Ketian Li. Modeling and optimal design of membrane based Fast -Tool-Servo for freeform manufacturing of micro optical lens array, Key Engineering Materials,2013, 552: 411-414
