Development of a touch trigger probe for micro/nano CMM Wei-Li Wang*a, Kuang-Chao Fana,b, Ye-Jin Chena and Ye-Tai Feia a School of Instrument Science and Opto-electric Engineering, Hefei University of Technology, Hefei, China 230009; b Dept. of Mechanical Engineering, National Taiwan University, Taipei, China 106 ABSTRACT Conventional Coordinate Measuring Machines (CMMs) restrict the possibilities for measuring micro mechanical products and noncontact optical measuring methods are not suitable for true three-dimensional measurements. The integrated combination of optical sensors and mechanical transducers employed in this research makes an ideal low cost and high precision touch trigger probe possible to measure miniature components. This touch probe is composed of a stylus with a fiber sphere (310µm in diameter) and a sensor integrated floating plate. The ball tip is fabricated using optical fiber with melting and solidification processes. The stylus is attached to a floating plate, which is connected to the probe housing via four micro wires. The shape and dimension of the floating plate, as well as the length and diameter of the micro wire, are determined according to the selected contact forces. When the probe tip is displaced the wires will perform elastic deformation and four mirrors mounted onto the four-legged plate will amplify the up/down displacement at each mirror position. These displacements can be detected by four corresponding laser focus probes. Experiments were carried out to test the unidirectional touch trigger repeatability. The standard deviation is less than 10nm. The probe structure and the experimental results were also analyzed and validated with finite element AnSYS software. Keywords: DVD pickup head, optical fiber probe, touch trigger probe, CMM
1. INTRODUCTION In the last decade, the increasing use of micro systems in industry together with an ever-increasing demand for higher measurement accuracy has led to the developments in the field of micro/nano dimensional metrology. Coordinate Measuring Machines (CMMs) as a versatile and widespread dimensional metrology tool can efficiently perform complex measurement tasks with a resolution about 0.1µm and repeatability about 0.3µm [1]. But they can’t perform ultrahigh precision three-dimensional surface measurements for further miniaturization and modularization in Microsystems technology because of the limitation of the probing system. Several research groups have started developing nano-CMMs that can measure three-dimensional parts in nanometer resolution [2-4]. Even with all the advancements in noncontact probing system and their abilities to accurately measure into the submicron range, these devices still cannot cope with the deep trenches, sidewalls and nozzles geometry measurements [5]. In order to measure the true three-dimensional geometry parameters of miniature components down to the size level below one millimeter， a touch trigger probe system is in most cases best suited. Pril [6] designed a probe based on silicon technology and the probe tip was manufactured by gluing a spherical ruby ball to a metal stem. Uncertainty of this probe is 20nm. Brand [7] et al. also developed a probe based on silicon technology. A commercial stylus with a ruby ball was used and attached to the membrane using expoxy adhesive. Piezo resistive strain gauges on the backsides of the membrane measure the 3D displacement of the probe tip with uncertainty 50nm. Peggs [8] developed a probe based on capacitive sensors. The ruby ball was also used as the probe tip. The measuring uncertainty of the probing system is 40nm. Different detecting techniques are employed in these notable probing systems, but the probe tip technologies are still limited to using commercial products. As the ball becomes smaller the sphericity and center offset of the ball to stem are difficult to be proportionally down scaled. The integrated combination of optical sensors, mechanical transducers and manufacturing of micro touch trigger probe in a single piece employed in the probe system is necessary to satisfy aforementioned requirements. This paper mainly focuses on an innovative touch trigger probe system. The application of focus probe, the design of the mechanical transducer and the fabrication of micro probe with quality spherical tip will be introduced. Experimental results will also be presented and proved by simulation. *
[email protected]; phone 86-551-2903823; fax 86-551-2903823
Fourth International Symposium on Precision Mechanical Measurements, edited by Yetai Fei, Kuang-Chao Fan, Rongsheng Lu, Proc. of SPIE Vol. 7130, 71300K · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.819556
Proc. of SPIE Vol. 7130 71300K-1 2008 SPIE Digital Library -- Subscriber Archive Copy
2. LASER FOCUS PROBE The sensor used in this touch trigger probe is based on laser focus probe which is reconfigured from DVD pickup head. Being a mass produced device, the DVD pickup head is cheap and it can be used outside the DVD player as a distance and angle measuring device. 2.1 Measurement principle The laser focus probe adopts astigmation principle [9], and its measurement principle can be expressed by Fig. 1. A light from a laser diode is primarily polarized by a grating plate. Having passed through a beam splitter and a quarter wave plate (mounted on the beam splitter), it is focused by an objective lens onto the object surface as a spot approximately 1 µm in diameter, about 2 mm from the object lens. The reflected beam signal is imaged onto a fourquadrant photo detector within the sensor by means of the quarter wave plate. The photodiode outputs are combined to give a focus error signal (FES) which is used to respond to the surface variation. At the focal plane the spot is a pure circle. When the object moves up or down away from the focal plane, the spot appears an elliptical shape in different orientations. The corresponding focus error signal provides an S-curve signal proportional to the object movement. The FES has a sharp transition from a maximum value to a minimum. During this transition a high sensitivity and hence a high resolution can be achieved.
Photodiode IC Laser diode icer Dicdc Polarization beam splitter Grating (irating
Rectangular prism
Collimation lens
1/4λ Plate P1
surface est Test Surface
Cylindrical tube
P2
Fig. 1.Principle of laser focus probe
Reflector
Objective lens Fig. 2. Reconfigured DVD pick-up head
2.2 Reconfigured DVD pick-up head Normal DVD pickup head will occupy bigger space if more than one of them is employed in probe system. The reconfigured DVD pickup head suitable for touch trigger probe is shown in Fig. 2. A rectangular prism is added to shift the laser beam and the objective lens is moved down and isolated via a cylindrical tube. With such a change, the DVD pickup head can be integrated into a multi-sensor probe system with miniature size. Four DVD pickup heads as displacement sensors are employed in this probing system and the sensitivity will increase fourfold.
3. FABRICATION OF FIBER SPHERE The probe tip must be spherical with diameter ranging from 500µm to 100µm, or less. It is normally made by gluing a micro-ball on a micro tungsten wire. The concentricity of the wire to the ball is a problem in assembly that will cause measuring error because the probe radius has to be compensated. A technique of fabricating monolithic probe stylus with melting and solidification processes of a thin glass fiber to form a micro-sphere tip has developed in this research. 3.1 Fabrication process The single mode (SM) glass fiber with 125µm diameter is selected to manufacture the ball tip. Glass fiber has sufficient mechanical strength to perform contact measurements. It can be easily fused within an appropriate heating field. The fiber tip absorbs the arc discharging power and melts instantaneously. Due to the surface tension, the melting part of the fiber starts to form a spherical tip gradually during solidification. The FITEL S199S model single mode fiber fusion
Proc. of SPIE Vol. 7130 71300K-2
splicer was the basic apparatus employed in this fabrication, as shown in Fig. 3. Arc check function is the pre-splicing procedure during which the discharging arc and fuse the end face but not splice one SM fiber to the other. With proper selection of the process parameters in the splicer the end face of the glass fiber can be melted and formed to micro-sphere due to the surface tension phenomenon. An XYZ linear stage and a rotary stage are added into the experimental system to precisely position the fiber. V-groove V-groove
-Y-Z Stage X-Y-Z Stage
totatorStage stage Rotator
Fiber Fiber Fixture fixture
Fiber feeding structure
SM Fiber Fiber
Electrode
Fiber FiberHolder holder
Fiber splicer
Fig. 3. Experiment setup to fabricating fiber tip 3.2 Optimization with Taguchi method By setting parameters of arc power, cleaning arc power offset and cleaning cycle, the micro ball tip will be growing. A good spherical probe for precision measurement requires three critical characteristics: uniform ball diameter, good roundness, and small center offset of the ball from stylus. Without a scientific experiment design, time is consuming and the center offset due to gravity during the melting process can’t be compensated well. Additionally, the results are not repeatable. Therefore, the Taguchi method was adopted to optimize the fabrication. Parameter design is a key step in Taguchi method to find the optimal factors which affect final product quality [10]. In this experiment, the fusing times, arc power, cleaning arc power offset and cleaning time are the factors needed to design. According to Taguchi method, proper orthogonal array is selected. Experiments are carried out with the parameters in the orthogonal array. Based on the experimental results, signal/noise ratios for diameter, roundness and center offset are analyzed. Analysis of variance (ANOVA) is also carried out to investigate the effects of the parameters. Therefore the optimal manufacturing parameters combination is obtained for getting smaller probe diameter, smaller probe roundness and smaller center offset. With the optimal parameters combination, a series of confirmation experiments were carried out. The image of one of the probes viewed at four angular positions with respect to the fiber is shown in Fig. 4. The corresponding measured results are summarized in Table. 1.
0°
90°
180°
270°
Fig. 4. Image of one fiber probe
It is obvious that the parameters design based on Taguchi method does work to reduce the droop of the probe tip and the bending of the fiber. The roundness error and the center offset of the probes both can be controlled to within 1µm with repeated fabrications under the same condition. Optical fiber probe with diameter close to 310µm has been achieved. The fiber probe can be used in the touch trigger probe with the fiber stylus inserted into a needle and glued to the suspension mechanism.
Proc. of SPIE Vol. 7130 71300K-3
Table 1. Measurement results of the tip at various rotational angles (in µm) Dimension
0°
90°
180°
270°
314.07
313.84
314.05
313.89
Roundness
0.54
0.82
0.37
0.61
Center offset
0.37
0.96
0.28
0.72
Diameter
4. MECHANICAL STRUCTURE The mechanical structure of the probe consists of the probe housing and suspension mechanism. The probe house itself is kept as simple as possible and the function of it is the interfacing of the probe to the micro-CMM and the shielding of the four reconfigured DVD pickup heads with 90° distributed. Each of them can be fine adjusted vertically to ensure the focus spot on the reflect mirror. The novel suspension mechanism is composed of a four-legged float plate connected to the probe housing via four micro wires. The stylus with the fiber sphere is fixed in the middle of the float plate and four reflect lenses are glued onto the four legs, as shown in Fig. 5. The mechanism dimensions and properties are analyzed with finite element method. 4.1 Suspension mechanism The main task of the suspension is to give the probe a stable rest position and orientation relative to the probe house while enabling moving of the probe tip in three orthogonal directions. The shape and dimension of the floating plate, as well as the length and diameter of the micro wire, are determined according to the selected contact forces. The probing force due to displacement of the tip from its rest position should be less than 1mN. The molybdenum wire with 0.2mm in diameter is employed as the micro wire because of low elastic coefficient. The length of the stylus is 4.5mm with 0.2mm in diameter. The diameter of fiber sphere is 310µm. Reflect mirror
Float plate Micro wire
Stylus with probe tip
Contact force
Fig. 5. Mechanical structure of suspension
Fig. 6. Deformation analysis with AnSYS
4.2 Finite element analysis The suspension mechanism is symmetric in horizontal XY plane. When the same contact force is applied to the probe tip in XY plane from different direction, the probe tip is expected to perform the same displacement that could decrease the error compensation after the measurement and improve measuring uncertainty. The finite element analysis software AnSYS was used to simulate the suspension structure to improve such a property. The model was meshed with small element SOLD 186. Fixed the degrees of freedom (DOF) at the end of the four micro wires and applying contact force of 40µN on the fiber tip in horizontal XY plane, the displacement of the fiber tip and deformation of the suspension mechanism can be analyzed with AnSYS, as shown in Fig. 6. The model is simulated every 15º and there are 24 point have been analyzed. The displacement variations of the probe tip at these points are plotted in Fig. 7. The average displacement is about 234nm and the biggest difference is 5nm. The result proves that the suspension mechanism has the similar property along 360º directions in XY plane, which means the pre-travel variation of the probe will be small in the real measurement if the probe tip is triggered by the same contact force with same velocity. So the future error compensation function is easy to achieve to improve the measurement uncertainty.
Proc. of SPIE Vol. 7130 71300K-4
270° 240°
255°
225°
285°
238
300° 315°
233
210°
330°
228
195° 180°
223
345°
218
0°
165°
15°
150°
30°
135°
45° 120°
105°
75°
60°
90°
Fig. 7. Displacement variation of probe tip (in nm)
Fig. 8. Prototype of the touch trigger probe system
5. EXPERIMENT OF THE PROBE SYSTEM The suspension mechanism with the stylus is fixed under the probing house with four reconfigured DVD pickup heads mounted in. The prototype of the probe system is shown in Fig. 8. The unidirectional touch trigger repeatability is an important factor to evaluate the precision of the probe system. Experiments are carried out. Based on the Mico/CMM developed in Lab. [11], as shown in Fig. 9, the probe system is installed on the Z-ram and XY table with the gauge block is driven to contact the probe with small impact force. Given a small trigger voltage in the program, when the gauge block contacts the probe tip, the stylus with the suspension mechanism will transfer the horizontal displacement to tilt the float plate, resulting the up and down displacement of the mirror. Then, the reflective mirror leaves away from the focus plane and the reflected laser beam will be changed its shape. When the voltage variation is up to the trigger voltage, the control program will stop the table and record the current displacement of the table. Repeating this program for ten times, the displacements are listed in the Table. 2. The standard deviation is calculated and the result is 8.9nm.
Micro-CMM
Probe system Precision xy-stage
Fig. 9. Experiment setup for unidirectional touch trigger repeatability Table. 2. Unidirectional touch-trigger repeatability experiment results
Displacement (µm)
1
2
3
4
5
6
7
8
9
10
Standard deviation
6.227
6.225
6.218
6.225
6.245
6.232
6.219
6.220
6.229
6.240
8.9 nm
Proc. of SPIE Vol. 7130 71300K-5
6. CONCLUSION This paper describes the needs of touch trigger probe for mico/nano measurements of micro products. This innovative touch trigger probe combines the mechanical floating mechanism and optical detectors, resulting high resolution with low cost. A novel suspension mechanism is proposed to increase its sensitivity and the deformation of the structure is simulated by finite element method. A fine fiber probe tip is developed to substitute the traditional ruby ball and the fabrication processing is optimized with Taguchi method, which guarantees both the roundness and the offset of the ball with less than 1µm. The probe has the symmetric property in horizontal XY plane and the displacement deviation along 360º is only 5nm. Experimental results show that the standard deviation of unidirectional touch trigger repeatability is 8.9nm. Such a touch trigger probe can measure side walls in nanometer resolution with micro Newton contact force.
ACKNOWLEDGEMENT The work reported forms part of a research program funded by the Natural Science Foundation of China (50420120134).
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
K. C. Fan, C. L, Chu, J. I. Mou, Development of a low-cost autofocusing probe for profile measurement, Measurement Science and Technology, 12: 2137-2146, 2001. T. A. M. Ruijl, Ultra precision coordinate measuring machine: design, calibration and error compensation, ISBN 90-6464-287-7, PhD, Thesis, TUD. Van Seggelen, J.K. Rosielle, P.C.J.N, Schellekens, P.H.J., Design of a 3-D CMM with elastically guided z-axis and x, y axis with less than 2mm Abbe offset, Proc. Of EUSPEN, Eindhoven, the Netherlands, pp.29-32, 2002 K. Enami, et al. Development of nano-probe system using optical sensing. In IMEKO-XV World Congress, 1999 S. Cao, U. Brand, et al. Recent developments in dimensional metrology for Microsystems components. Microsystem Technologies, 8, pp. 3-6, 2002. W.O. Pril, Development of high precision mechanical probes for coordinate measuring machines, ISBN 90-3862654-1, PhD. Thesis, TUE. 2002. U. Brand, et al, Development of a special CMM for dimensional metrology on microsystem components, ASPE, 15th Annual Meeting in Scottsdale, 22th-27th, October 2000. G. N. Peggs, A.J. Lewis, Design for a compact high-accuracy CMM, Annals of the CIRP, 48, pp. 417-420, 1999. K. C. Fan, C. Y. Lin, and L. H. Shyu, Development of a low cost focusing probe for profile measurement, J. Meas. Sci. Tchnol., 11(1), pp. 1-7, 2000. G. Taguchi, Taguchi on robust technology development, New York, ASME press. 1993. K. C. Fan, C. L. Liu, P. T. Wu, et al. The structure design of a micro precision CMM with Abbe principle, Proc. Of the 35th International MATADOR conference, pp. 297-300. July, 2007, Taiwan.
Proc. of SPIE Vol. 7130 71300K-6
