Three-dimensional Touch Probe
Using Three Fiber Optic Displacement sensors

Introduction

In recent years, coordinate measuring machines (CMMs) have been widely used for precision measurement in various fields. Recently, the need for sub-micron measurement has become particularly pressing because of the development of 3D micro-fabrication methods. To make sub-micron measurement practical, it is necessary to improve the sensitivity, pretravel, natural frequency and measuring force of the probe system. In a conventional CMM touch probe system, contact between the ball and the workpiece is detected by certain kinds of sensors, e.g., electrical contacts, acceleration sensors or force sensors. Most probe systems have employed a ball stylus consisting of a precise ball and a shank having a certain length. In these systems, there is some distance between the touching point and the sensors. In addition, for the measurement of deep holes or workpieces with complicated shapes, ball styli with long shanks have often been used. However, lengthening the shank increases the unsprung mass and decreases the natural frequency of the probe system. Consequently, the influence of a disturbance becomes serious. Moreover, considerable deflection of the shank increases the pretravel and uncertainty of a probe. In this study, displacement sensors installed into the shank measure the displacement and direction of the tip ball directly. The use of high-resolution fiber optic sensors as displacement sensors has the potential to improve the performance of a probe system. This paper describes the fundamentals and experimental results of the proposed system when one sensor or three sensors are used.



Fundamentals

Figure depicts the fundamentals of a 3D probe consisting of three displacement sensors and a tip ball. An elastic body connects the ball to the sensor tips. Light from the light source enters the illuminating fibers and then radiates to the surface of the ball. The reflected light enters the receiving fibers and then is detected by the photodetector. Consequently, the distance between each fiber tip and the ball surface can be determined by the output voltage of the detector. Thus, when the tip ball touches the workpiece, the three fiber optic displacement sensors measure the ball's displacement and direction directly. The proposed probe has a number of advantages over the conventional probe:
(1) because the fiber sensor has nanometer-order resolution and high thermal stability, accuracy should be considerably greater;
(2) the offset between the sensitive directions of the sensors and the tip ball is minimized, as the sensors are located in strict accordance with the Abbe principle;
(3) the three sensors measure not only the displacement of the tip ball but also the direction;
(4) the direction information compensates for probe lobing;
(5) because the only unsprung mass is that of the ball, the higher natural frequency improves the measurement speed and the resolution of the probe;
(6) the increment of the stylus length has little effect on the performance of the probe;
(7) there is no influence of friction except at the contact point between the tip ball and the workpiece;
(8) the probe system can be miniaturized.




A prototype using three displacement sensors



A prototype using three displacement sensors.
Tip ball diameter: 1/4" (6.35 mm)
Pretravel: less than 50 nm



This is Miniaturized three-dimensional touch trigger probe using optical fibre bundle.
Tip ball diameter: 1.2 mm
Diameter of stem: 0.8 mm
Measurement force : less than 0.9 mN in all directions

References

Papers

  1. Takaaki Oiwa and Toshio Tanaka: Miniaturized three-dimensional touch trigger probe using optical fiber bundle, Measurement Science and Technology, 16 (2005) pp.1574-1581.
  2. Takaaki OIWA and Hiroshi Nishitani: Three-Dimensional Touch Probe Based on Fiber Optic Displacement Sensors, Measurement Science and Technology, Vol. 15, No. 1(2004), pp84-90.

Proceedings

  1. Kimitaka Kitano, Takaaki Oiwa, and Junichi Asama: Three-dimensional touch trigger Probe Using Optical Fiber Bundle, The 4th International Conference on Manufacturing, Machine Design and Tribology, ICMDT2011, P-13, 187-188, 2011.04

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