Measuring Water Surface Displacements

It is a very challenging task to measure displacements with nanometer accuracy and vibrations with amplitudes of few picometers on a wide variety of materials, as performing such measurements is beyond the scope of most commercially available measurement techniques.

This article discusses the measurement results performed on a liquid surface by matching the sensor head of attocube’s Industrial Displacement Sensor (IDS) to the material and application. The IDS has the unique feature of accommodating advanced applications, for example under extreme environmental conditions and in very constrained spaces. Sensor heads with a diameter smaller than 1 mm and capable of withstanding temperatures up to 400 °C were developed by attocube. In this article, the results of displacement measurements realized on a water surface are shown to demonstrate the flexibility of attocube’s interferometer system.

Attocube’s IDS3010 interferometric sensor can perform displacement measurements down to the picometer range. The comprehensive sensor head portfolio has been optimized for measurements on a variety of surfaces and target materials such as silicon, gold, plastic, ceramics, aluminum, copper, glass, steel, silver or even water. Surface reflectivity as well as the type of sensor head used affects working distances and angular tolerances. The possible working distance is immediately after the sensor head and can exceed a range of 30 meters, using a standard target, such as a retroreflector.

The ability to measure liquid surface movements holds potential to realize unique applications in a variety of industrial and scientific areas, for instance, detecting liquid levels in a hydrostatic leveling system, determining the liquid level in falling film reactors, and vibration measurements and analyzes of hydraulic systems [1].

Experimental and Results

The schematic setup for displacement measurements conducted on the water surface is illustrated in Figure 1. Apart from two focused sensor heads with a focal length of 40 mm (F 40 mm), the experimental setup consists of an aluminum mirror, a cup of water and an optical table. A working range of a few millimeters and angular tolerances of a few tenths of degrees were achieved by focusing on the water surface.

experimental setup

Figure 1. The figure shows a sketch of the experimental setup to perform simultaneous displacement measurements on the surface of water and on a mirror.

The plot in Figure 2 depicts the displacement measurement results. It can be seen that during the first 0.714 seconds, the water surface (blue curve) is only slightly wavering with 11.6 Hz compared to the stable position measurement of the mirror (red curve). The water surface oscillates with a maximum deflection of approximately ± 20 µm when the optical table is hit between the two targets using a hammer. The zoom reveals that the two measurement arms exhibit similar behaviors in the high frequency range for the first milliseconds following the excitation.

displacements measured on the side of the mirror after hitting the optical table with a hammer.

Figure 2. The blue curve shows the water surface and sensor head movements and the red curve represents the displacements measured on the side of the mirror after hitting the optical table with a hammer.

Conclusion

The ability of the interferometer sensor to monitor water surface movements is demonstrated in this experiment.

Reference

[1] Meier E, Limpach P, Geiger, Ingensand H, Steiger A, Licht H and Zwyssig R 2010 Journal of Applied Geodesy 4 2 91-102

This information has been sourced, reviewed and adapted from materials provided by Attocube Systems AG.

For more information on this source, please visit Attocube Systems AG.

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