Probe provides high sensitivity for coordinate-measuring machines

Probes for conventional coordinate-measuring machines (CMMs) are generally bulky and use large forces, limiting their application for measuring small parts. To address this shortcoming, we have investigated a novel integrated optical probe design based on a fiber Bragg grating (FBG), and find that it can make 3D measurements over the range of hundreds of µm to cm with a resolution of several nanometers.

Figure 1 shows the proposed fiber probe. One end of the probe stem is fixed by a holder and the other comprises a fused spherical tip that touches the surface being measured. A uniform 5mm-long FBG, which acts as a strain sensor, is fabricated within the stem as near as is practical to the ball tip. Laser light of wavelength λ enters the probe, and light at the Bragg wavelength λ_B is reflected and subsequently detected. Figure 2 shows a schematic of the optical-fiber interferometric detection system.

The touch signal is produced and detected by exploiting the grating's high strain sensitivity along the probe stem direction and the resulting wavelength shift, which is proportional to the applied strain, according to photoelastic theory.When the probe tip touches a surface and applies a force is in the axial direction (along the probe stem), a uniform compression strain is imposed and the reflected Bragg wavelength signal shifts to a shorter wavelength.

Under lateral loading, a bending deflection results, causing the uniform FBG to effectively become a linear chirped grating. The broadened spectrum arises because small lateral loadings elongate the grating side under tension, shorten the other side under compression, and leave a central axis that is perpendicular to the loading force unchanged and unstrained. The Bragg wavelength shifts to a longer wavelength when the grating elongates, and to a shorter one when it shortens. The net effect is a broadening of the spectral reflection, which will have a bandwidth determined by the maximum strain within the FBG.

To verify this analytical model, we simulated the strain distribution along the spherical probe tip with axial and lateral loading using the latest version of the popular finite-element package ANSYS. Given a tip diameter of 360µm and a fiber stem diameter of 125µm, axially applying a force of 0.02mN to the tip compresses the 5mm long stem by 0.1nm. A total compression of 5nm occurs over the whole probe, including the ball. Along the optical fiber stem, the strain is uniform.

With a lateral loading force of 0.1µN, the spherical tip is displaced by 5nm. The simulated strain distribution in this case agrees with the theoretical predictions. Figure 3 shows that, as expected, the maximum strain in the fiber probe occurs at the top, where the probe stem is fixed by the holder. Accordingly, the maximum strain on the FBG is adjacent to the holder. The strain on the top layer near the core position is 2.7nanostrain (2.7ne), and 0.6ne is detectable.¹ Strain resolution of 5nm is possible with an interferometer interrogation system.²

Using a narrow-band laser-diode source and an external cavity formed between it and the FBG, a wavelength shift of 0.3nm was observed through a standard grating spectrometer. Figure 4 shows the wavelength shift that results when a voltage is applied to a 10mm piezoelectric transducer to stretch the 5mm-long probe stem, yielding a 0.96µm FBG stretch. Using this arrangement, a 3nm stretch resolution was easily observed.³

We have proposed a novel fiber Bragg grating embedded optical fiber probe for miniaturized 3D CMM application. We have analyzed its strain distribution and the sensitivity, and demonstrated the probe's high sensitivity.