A novel fibre-optic vibration sensor

Abstract

Analysis of the vibrational characteristics of materials or mechanical systems had the promise of providing useful information of their integrity. Acoustic monitoring of heavy machine bearings is a well-established technique for detecting the onset of wear [1]. Damage to materials [2] or structures [3] results in local stiffness reductions which may be detected by measuring their dynamic response. Vibration sensors could also be used to monitor the passage of traffic over bridges and roadways or for security applications such as detecting forced entry into buildings. Vibration sensing is most commonly performed by electromechanical devices such as piezoelectric, piezoresistive or capacitative accelerometers. Whilst this is reliable and tested technology it has a potential drawback. Susceptibility to electromagnetic interference limits application around strong electric fields and the need for either strongly shielded leads or a power supply for a built-in amplifier results in relatively heavy, bulky cables. If an extensive network of sensors is to be incorporated into a structure, this may prove to be a considerable problem, even more so if there is a premium on weight, as in aerospace structures. Optical fibre sensors have been shown to be suited for embedment in composites [4] and cementitious materials [5]. They have largely unaffected electromagnetic interference and in some cases they may be multiplexed to give a distributed sensing system. Optical fibre sensors tend to be more expensive than their conventional counterparts, mainly because the equipment needed to interrogate the sensor is often delicate and complex. To measure vibration, either a strain gauge-type device is attached intimately to the structure to measure surface acoustic waves, or a fibre-optic accelerometer is employed in place of a conventional one. The device presented in this latter modulates the intensity of light transmitted through it and thus requires only simple and cheap instrumentation. It is illustrated schematically in Fig. 1. The details of its construction and operation are as follows. Two 50=125 multimode optical fibres are cleaved and the protective polymer jacket of one of the fibres is stripped back a short length. The prepared fibres are then inserted into a precisiondrawn glass capillary, their end-faces separated by a few tens of microns, and fixed with epoxy resin adhesive. When the sensor is subjected to an acceleration prependicular to the axes of the optical fibres, the motion of the unsupported fibre lags behind. As it moves out of alignment, relative to the fixed fibre, the transmitted optical power decreases. The power variations are detected with a fastresponse photodiode and amplifier and sent to a personal computer for further processing. The response of the sensor to sinusoidal and transient excitation was investigated. First, the sensor was mounted on an electromagnetic coil which was driven by a signal generator and amplifier and excited over a range of frequencies from 100 to 10000 Hz. The sensors which provided the results presented here had resonant frequencies of 3 Hz and 6 Hz, but this value can be varied by changing the length of the cantilevered fibre or by choosing fibres of different diameters. To test the response to transients, sensors of 3 Hz resonant frequency were bonded to the surface of 70 3 70 mm 16-ply crossply carbon-fibre reinforced plastic (CFRP) with a cyanoacrylate adhesive. An impulse was provided by either dropping a 6.3 mm diameter steel ball onto the panel from a height of 25 cm, giving an impact energy of 2.5 mJ, or a standard pencil break test [6]. Examples of sensor signals from these events are given in Fig. 2. The impact of the steel ball, considerably the more energetic of the two events, gave a greater signal amplitude, as well as a characteristic shape different from that of the pencil break. Results were consistent within both series, with the pencil break showing a little more variability.