Emerald Group Publishing Limited
Copyright © 2007, Emerald Group Publishing Limited
Level sensing and position measurement
Knowing the level and position of things is of primary importance for automated systems and for technological, industrial or environmental monitoring. As the market of proximity sensors is continuously increasing every year (Taylor, 2005), new measurement techniques are regularly proposed in order to take advantage of the new available technologies. It is true at any scale, from the nanometer (Smith and Seugling, 2006), to kilometers.
Although many level and position sensors still rely on relatively simple mechanisms, such as float or position switches, numerous different technologies are used to fit application specificities and requirements. The trend, which is obviously driven by the end-users, is to make cheaper and easy to use in order to reduce integration costs, and/or to provide more information at the same price. Remote sensors, that is to say sensors operating without direct contact with the object or level to detect, partly or completely integrated on silicon are then perfectly suited for that purpose.
In the family of remote sensor, near field sensors, which use electrostatics, magnetostatics or magnetic induction, are particularly interesting since they are cheap, sensitive, accurate, and robust. However, they are only sensitive to near objects, that is to say objects within a distance equivalent to the sensor size.
In the case of electrostatic sensors, which are generally used to detect the position of metallic or dielectric objects and the level of liquids, two electrodes sensors are usually designed though multi-electrode sensors can bring much more information (Lucas et al., 2006) for a quite low cost increase. For instance, images of the surrounding materials can be obtained (Jang et al., 2006) or the sensor can be rendered insensitive to spurious parameters, such as hygrometry drifts (Lucas et al., 2003). The recent dedicated integrated devices can balance the cost increase of multi- electrode sensors or reduce the one of the others. The easy integration of the whole sensor in a chip along with their low power consumption are others advantages of electrostatic sensors. In micro-electronic-mechanical-systems for instance, they are often used to detect the position of membranes, mirrors, cantilevers, and so on.
In the case of magnetostatic sensors, a small magnet is attached to the object to detect. That leads to low power sensors which are not sensitive to temperature variations, moisture, dust, or oil projections for instance. The development of new magnetic field sensors, based on Hall effect, giant magnetoresistance, or even quantum interference, gives magnetostatic sensors a very high sensitivity for a small size, and the possibility to be integrated in a chip (Schott et al., 2006). The developments of thin efficient magnetic material layers should also push magnetic induction sensors to that direction. The design of efficient coils is however yet to find.
In the family of remote sensor, far field sensors, which use electromagnetics (microwave), ultrasonics or optics, are also particularly interesting since they can be positioned rather far from the object to monitor resulting in a real integration cost reduction. Although they are less robust to rough environments than near field sensors, they can be easily protected. Specifically, for electromagnetic and optic sensors, the market of cellular phones and digital cameras has drastically reduced the cost of electronic devices such as high frequency amplifiers and mixers, image sensors, and high speed signal processors. Therefore, one part of the sensor complexity can be transferred to the software in order to reduce production cost and render the sensor more versatile.
In conclusion, future level and position sensors should be contactless. Near field sensors should be integrated in chips or provide improved capabilities as to offer accuracy at low price. At the same time the number of far field sensors based on image processing (visible and microwave) should increase to offer easy to use solutions.
Stéphane HoléLaboratoire des Instruments et Systèmes, Université Pierre et Marie Curie, Paris, France
1 Total Pole Airship expedition (2006-2008).2 See for instance Acam PS021 (December 2006) and Analog Device AD7142 Series (January 2007).
ReferencesJang, J.D., Lee, S.H., Kim, K.Y. and Choi, B.Y. (2006), “Modified iterative Landweber method in electrical capacitance tomography”, Meas. Sci. Technol., Vol. 17, pp. 1909-17.
Lucas, J., Bâtis, C., Holé, S., Ditchi, T., Launay, C., Da Silva, J., Dirand, H., Chabert, L. and Pajon, M. (2003), “Morphological capacitive sensors for air bag applications”, Sens. Rev., Vol. 23, pp. 345-51.
Lucas, J., Holé, S. and Bâtis, C. (2006), “Analytical capacitive sensor sensitivity distribution and applications”, Meas. Sci. Technol., Vol. 17, pp. 2467-78.
Schott, C., Racz, R. and Huber, S. (2006), “Novel analog magnetic angle sensor with linear output”, Sens. Actuator A-Phys., Vol. 132, pp. 165-70.
Smith, S.T. and Seugling, R.M. (2006), “Sensor and actuator considerations for precision, small machines”, Pre-cis. Eng., Vol. 30, pp. 245-64.
Taylor, J.K. (2005), North American Markets for Proximity and Photoelectric Sensors, Venture Development Corporation, Eugene.