Development of a fiber optic tiltmeter for static and kinematic applications

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April 2010



Engineering geodesists are accustomed to selecting appropriate sensors for often challenging and highly accurate measurements, but usually not to contributing directly to the sensor development. Knowledge in electronics, mechanical modeling, system theory and precision mechanics is required, which is beyond the basic education of geodesists. However, basic essentials of the geodetic education are instrumentation, signal processing and parameter estimation - also necessary requirements in sensor technology. Consequently, the attempt of combining these fields is made in this thesis.

The thesis is concerned with the development of a tiltmeter whose sensing element consists of a fiber optic cantilever. Tilt measurements are common observations in engineering geodesy, and the determination of the instantaneous inclination is a challenging task, especially in the kinematic case. Moreover, fiber optic sensor technology gains more and more importance due to the superior performance in harsh environments.

By reviewing existing sensors - conventional tiltmeters, fiber optic cantilever sensors and fiber optic tiltmeters - the straightforward and advantageous principle of using a bare fiber as a sensing element for tilt angles in two orthogonal axes is developed. Based on this review, a prototype development is described. The mechanical component of the tiltmeter, i.e., the sensor element itself, is supposed to act like a cantilever experiencing biaxial deflections. This is investigated in detail for the static and dynamic case to obtain a better understanding of what is happening inside the sensor development during operation. Experimental results excellently confirm the mathematical and mechanical models.

Light guided within the fiber can directly be used to monitor the deflections of the sensor element on a position detector, and these deflections are proportional to the tilt angles. The biaxial tilt angles are derived from the (quasi-) static acceleration component, which requires digital filtering techniques to extract the time-dependent positions of rest of the vibrating sensor element. Once these positions are determined, tilt angles can be calculated provided that the cantilever deflections and their relation to inclination angles are known. These relations are determined by two different calibration facilities and procedures, which were developed and set up in the laboratory of the Institute. The accuracy of the measured fiber deflections is in the range of 1 µm for deflections of ±4 mm after calibration of the position detector. Relating the fiber deflections to inclination angles yields an accuracy of the calibration results in the range of ±0.01° for ±30°. Static tests confirm a precision of 0.015° and an accuracy of 0.04° for the whole working range. This is sufficiently accurate for many applications, which is shown by several simple examples.

However, the determination of the instantaneous tilt angle in kinematic applications is limited by the problem of separating tilt and acceleration and by the unavoidable delay of the sensor output. For the described development, this delay is caused by the filtering process. A sensor enhancement is proposed which enables a more realistic estimation of the instantaneous tilt angle using a second but differently balanced sensor element and adaptive filtering. The principal applicability of the enhancement is confirmed by simulation results. Disturbing influences in certain directions can be reduced by arranging additional sensor elements in differently orientated axes. The combination of these individual cantilever observations then further improves the dynamic performance of the sensor.
EAN: 9783832290573
ISBN: 3832290575
Untertitel: Sprache: Englisch.
Verlag: Shaker Verlag
Erscheinungsdatum: April 2010
Seitenanzahl: 256 Seiten
Format: kartoniert
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