Sensors Go Systematic （5）

Published:2011/7/28 5:25:00 Author:Amy From:SeekIC

By Helmuth Lemme


However, it is also possible to use die actual fibre as the meas­uring element. Such "intrinsic” fibre-optic systems exploit the miniscule but still present effects of external physical quanti­ties on the propagation of light through the fibre. If the wave nature of the light is also taken into account, very high sensi­tivity and resolution can be achieved. A widely used technique involves writing a ’Bragg grating’ into the fibre. A Bragg grating is a periodic series of regions having an elevated refractive index, which is generated by means of UV irradiation. Light with a wavelength that exactly corresponds to twice the grat­ing spacing is reflected back to the optical source, while all other wavelengths pass through unaffected. If the fibre is stretched, light with a somewhat longer wavelength will be reflected, and this can be measured optically. This yields a highly sensitive elongation sensor that operates without elec­tricity, cannot be affected by extremely strong ambient electro­magnetic fields, and cannot itself interfere with anything else.

There are very many physical effects that can influence the propagation of light in a fibre. Quantities that can be meas­ured in this manner include elongation, tensile stress, vibra­tion, flow, filling level, pressure, angle of rotation, tempera­ture, ionizing radiation and many others. This makes it possi­ble to make a wide variety of measurements under difficult conditions, in situations where such measurements were pre­viously considered to be out of the question - such as deep inside the 1300-MW generator shown in Figure 6.



Passive RF sensors

With rapidly rotating machine parts, a sensor link via commu­tator rings is frequently not sufficiently reliable. In such situations, it is possible to use another sensor technology that does not require a local power source: surface acoustic wave chips. These components, which are made from piezoelectric crystals (such as lithium niobate), are used as bandpass filters in radio receivers. An RF alternating voltage applied to the input comb electrodes (typically with a frequency of several hundred mega­hertz to a few gigahertz) generates a shallow mechanical wave on the surface of the crystal. This wave propagates over the length of the crystal until it is reflected at the other end and returns back to the electrodes, where it again generates an alter­nating voltage that is slightly delayed from the excitation pulse. If a sensor (for example, a capacitive pressure sensor) is connected to a second set of electrodes at the far end of the crys­tal, the reflection of the waves will be affected by the sensor in a manner that depends on the pressure. The pressure can then be determined by evaluating the returned signal (Figure 7). If an aerial is connected to the input, the sensor can be queried using radio waves. This technology has been studied in the research departments of tyre manufacturers as a means of acquiring measurement data from rotating tyres (pressure, temperature and traction) without using a battery as a power source. For more on this subject, see the article "Wireless tyre-pressure monitoring’ elsewhere in this issue.

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