New Slimline Non-Contact Rotary Sensor. ASM is introducing the PRAS20, a new non-contact, rotary position sensor in a slim 6.0mm low-profile package. This absolute, angular position sensor features a full 360-degree measurement range in a cost-effective package. It yields a 0.5-4.5 V ratiometric, analogue output that is available either with a complete 360-degree measurement range or with smaller ranges in 15-degree increments. This wear-free design precisely measures angular displacement with an external permanent magnet Circuitry for the sensor is encapsulated in a glass-fibre reinforced thermoplastic that provides excellent shock and vibration resistance ratings of 100 'g'/11ms shock, and 20 'g', 10Hz-2 kHz, 10 cycles vibration. The economical, compact and ultra-thin design of the PRAS20 makes it suitable for numerous OEM motion control and process automation applications ...

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Shukuji Asakura, Akimitsu Ishihara, Frequency Output Type Motion Sensor Using Self-excited Potential Oscillation, Sensors and Actuators, A 90, 2001, pp.111-117.

Abstract: A new method for measuring the velocity of a moving fluid around an electrode was proposed. The method measured the frequency of a self-excited oscillation of an iron electrode immersed in H3PO4-H2O2 solutions. The flow in the neighborhood of the electrode, on which transport processes depended, was presumed to change the oscillation frequency. The frequency is proportional to the concentration of the hydrogen peroxide, [H2O2], at the phosphoric acid concentration of 0.5 mol/dm3. The moving velocity of the electrode against the static solution could be determinated. In the same sense, the rotation speed of the electrode and the electrode tilt, which affected natural convection around an electrode, could be measured.

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Akimitsu Ishihara and Shukuji Asakura,  A Possible Motion Sensor with Frequency Output, In Proceedings of the 18th Chemical Sensor Symposium, April 3-5, 1994, Tohoku University, Japan, Vol.10, Supplement A.
Abstract - Electrochemical oscillations have often been observed in iron immersed in the solution containing phosphoric acid and hydrogen peroxide. This oscillation has been interpreted by the cross linkage between electrochemical reactions and mass transport processes in the vicinity of an electrode. Therefore, the oscillation frequency is expected to reflect the flow rate around the electrode. A rod of carbon steel S50C was mounted in rotating disc apparatus. The intersection of the rod rotated concentrically. In the mixture of 0.5M phospheric acid and 1.5M hydrogen peroxide the highly stable electrochemical oscillation appeared. The amplitude of potential change was about 600mVp-p. The oscillating frequency was a sensitive function of rotation rate in the range of 100 rpm to 2000. It is very possible to develop the motion sensors whose output is frequency by using this phenomenon.

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K.-M.H. Lenssen, D.J. Adelerhof, H.J. Gassen, A.E.T. Kuiper, G.H.J. Somers and J.B.A. van Zon, Robust Giant Magnetoresistance Sensors,  In Proceedings of the 13th European Conference on Solid-State Transducers (EUROSENSORS XIII), The Hague, The Netherlands, 12-15 September, 1999, pp.589-596.

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P. Rako, Automotive Hall-effect rotary-position sensor uses novel flux-concentrator technology, EDN, 10/18/2007