Tom Carolan

Profile measurement of optically rough surfaces by fiber-optic interferometry

An instrument for measurement of the profiles of optically rough surfaces is described. The sensor is a robust and compact Fizeau interferometer, in which one reflection is derived from the test surface. A laser-diode source and a photodetector communicate with the sensor through an optical fiber. The optical output is demodulated with a phase-stepping algorithm, achieved by frequency modulating the source. The measured horizontal resolution was 7 μm, and the noise-limited vertical resolution was 0.3 nm/Hz.

Acoustic emission monitoring of tool wear during the face milling of steels and aluminium alloys using a fibre optic sensor. Part 2: Frequency analysis

Abstract An investigation of the relationship between tool wear and the energy of acoustic emission (AE) produced during various face milling finishing operations is presented. A model detailing how the AE energy, quantified by the r.m.s. value, varies depending on the material and the detailed tool geometry formed by flank and crater wear is described. Validation of the model was achieved in a series of practical machining tests covering a range of materials and tool types which resulted in various different wear forms. In all these wear tests a non-contact fibre optic interferometer was employed for AE detection directly from the workpiece. This sensor makes absolute, calibrated measurements of AE, unlike conventional contacting piezoelectric AE transducers, which may suffer uncertainties due to their frequency response and variations in transmission path. The fibre optic instrument is thus advantageous for studying variations of AE energy with tool wear.

Detection of acoustic emission in cutting processes by fibre optic interferometry

Acoustic emission has previously been shown to be a useful technique in monitoring the state of wear in machine tools. The piezoelectric transducers conventionally used for acoustic emission detection are contacting devices with a limited bandwidth. Operationally, a non-contacting transducer of broader bandwidth would be preferred. The authors have accordingly developed a technique based on optical interferometry, using fibre optics, to produce a miniature and robust probe for detecting acoustic emission by measuring the small amplitude ( approximately 0.1 nm), high frequency (0.1-1 MHz) surface vibrations that it produces. They demonstrate the technique in measurements made probing both the workpiece and the rotating tool holder during face milling of mild steel.

Surface finish parameters as diagnostics of tool wear in face milling

Abstract We describe a systematic series of wear tests in which surface profiles were measured at intervals during face milling of En24 steel, an aluminium alloy and a stainless steel. Profiles were measured by a stylus instrument and, in the case of the aluminium alloy, by a novel optical fibre instrument mounted on the machine headstock. Spatial frequency analyses are presented that show the long wavelength amplitude increases with insert wear, with a similar tendency in the high spatial frequencies (above the feed per tooth). Variation in tool set affected insert wear rate and the evolution of surface profile. Insert chipping damage was seen to increase the low spatial frequency content. The Ra parameter correlated with insert wear only for single point cutting.

Fiber-optic Sagnac interferometer for noncontact structural monitoring in power plant applications.

The design of a noncontact fiber-optic sensor is described for the detection of acoustic emission for structural integrity monitoring in high-temperature power plant applications. The sensor is based on a Sagnac interferometer and produces an output proportional to target velocity, without the need for active phase stabilization. It is inherently insensitive to low-frequency perturbations of the instrument or the target and incorporates an environmentally insensitive downlead, which may be of arbitrary length. It is shown that the sensor is capable of meeting the specifications for structural integrity monitoring of high-temperature power plant components based on acoustic emission detection and has a velocity resolution of 50 nm s(-1) Hz(-1/2).

Extrinsic Michelson interferometric fibre optic sensor with bend insensitive downlead

Abstract A novel optical arrangement is described for an interferometric optical fibre sensor of the extrinsic type. Based on a Michelson interferometer, it combines a bend insensitive downlead with the availability of antiphase outputs without insertion loss, and provides isolation of the source.

Optical fiber interferometry for photoacoustic spectroscopy in liquids.

Optical fiber interferometry was used for measurement of the subnanometer surface and bulk displacements associated with photoacoustic pressure waves in methanol and water. The measurement system is both broadband and noncontacting, giving a pressure sensitivity of 0.1 Pa/ radicalHz.

Measurement of aspects of surface form using an optical differential height measurement technique

We describe a fibre interferometer for making non-contact differential height measurements of machined metal surfaces without removal from the machine tool. The system comprises a probe which produces two small laser spots ( diameter, 3 mm apart) on the surface, an optical fibre addressing lead and signal processing unit incorporating an optical source and detector. The probe is an extrinsic fibre optic Michelson interferometer whose arms terminate in the laser spots on the test surface, and the signal processing unit calculates the difference in local surface height between the two spots. Differential height profiles of machined aluminium surfaces and optical mirror surfaces are presented.

Surface features as indicators of tool chipping in single point face milling of aluminium

Abstract We describe a set of wear tests in which machined surfaces were produced by single point face milling of an aluminium alloy. The objectives were firstly to identify surface characteristics associated with tool chipping, and secondly to demonstrate that a fibre optic surface profiling probe capable of in-situ operation on the machine tool had the ability to provide the same wear information as a metrological interference microscope. Spectral energy of the surface profiles decreased significantly with insert damage in a spatial frequency band around the tooth passing frequency of the cutter and also at higher spatial frequencies.

Fiber optic Sagnac interferometer for on-line acoustic emission monitoring of high-temperature power plant structures

This paper details the design of a non-contact fiber optic sensor for the detection of acoustic emission for structural integrity monitoring in high temperature power plant applications. The sensor is based on a Sagnac interferometer and produces an output proportional to target velocity, without the need for active phase stabilization. It is inherently insensitive to low frequency perturbations of the instrument or the target and incorporates an environmentally insensitive downlead which may be of arbitrary length. It is shown that the sensor is capable of meeting the specifications for structural integrity monitoring of high temperature power plant components based on acoustic emission detection and has a velocity resolution of 50 nms-1Hz-1/2.