Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Patent
1996-11-06
1999-07-27
Westin, Edward P.
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
250214A, 250214AG, 25023113, 73655, B64C 2700, G01V 800, G01D 526
Patent
active
059294310
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to an optical tracker system.
BACKGROUND OF THE INVENTION
Optical tracker systems are used for tracking a rotating body such as a fan or rotor blade in order to measure the displacement thereof. Optical tracker systems are well known. Among other things, optical tracker systems are used to detect the displacements of helicopter rotors and may be embodied in portable maintenance or test equipment or in a more sophisticated health and usage monitoring system (HUMS). HUMS apparatus is mounted permanently in a helicopter and the optical tracker system associated therewith is used continuously to measure rotor blade displacement during all flight operations of the helicopter. Consequently, a greater performance is required of the optical tracker system since it must function in all normal operating environmental and flight conditions and not just the controlled conditions of a special purpose test flight.
Known optical tracker systems are described in for example U.S. Pat. No. 2,960,908 (Willits et al.), issued on Nov. 22, 1960, U.S. Pat. No. 3,002,420 (Willits et al.), issued on Oct. 3, 1961, U.S. Pat. No. 4,812,643 (Talbot), issued on Mar. 14, 1989, and European Patent Application No. 0 208 399 (Stewart Hughes Limited), priority date of May 16, 1985. The systems described in these publications include an optical sensor comprising a lens and usually at least two light sensing devices such as photodiodes. The lens defines a field of view and as a blade passes through the field of view an image of the blade is formed on the light sensors and is detectable thereby. As the blade passes through the field of view the blade image moves across the photodiodes which produce respective output signals that vary in time and represent the passage of the blade. The time variations in the output signals from the photodetectors can be used to determine the displacement of the rotor blade.
The image formed by the lens comprises the blade and the background and there must be sufficient contrast in the image between the blade and the background in order to be able to distinguish between the two. Optical tracking devices may be used in flight during the day or at night. During the day the background will usually be bright daytime sky and adequate contrast can be achieved by arranging for the portion of the blade that passes through the field of view of the sensor to be much darker than the background. At night, the background will usually be dark night sky and adequate contrast can be achieved by illuminating the blade or a reflective patch mounted to the blade as the blade passes through the field of view of the sensor.
Signals from the sensing device are input to signal processing circuitry where they are analyzed in order to determine the relative position of the blades as they rotate. The processing circuit may be a dedicated electronic circuit or it may be sophisticated computer based circuitry.
While known systems work adequately there is nevertheless a problem associated with various imperfections in the optical sensor which result in stray light arriving at the photodiodes along different optical paths within the optical sensor.
Light scattering within the optical sensor can reduce significantly the contrast between the background image and the blade image. Scattered light is created because of various imperfections in the optical sensor. A small component of light is reflected back out through the entrance of the optical sensor but the majority of the light remains in the optical sensor and must be absorbed in order to avoid the aforementioned reduction in contrast.
One way in which the reflected light can be absorbed is by way of a field or aperture stop. It is not feasible to make a field or aperture stop completely absorbent of reflected light and therefore in practice some portion of the reflected light will remain causing a reduction in the contrast of the image.
The effects of scattered light can be further minimized by ensuring the cleanliness of all optical compone
REFERENCES:
patent: 2960908 (1960-11-01), Willits et al.
patent: 3002420 (1961-10-01), Willits et al.
patent: 3023317 (1962-02-01), Willits et al.
patent: 3938762 (1976-02-01), Murphy
patent: 4529874 (1985-07-01), Zierhut
patent: 4617569 (1986-10-01), Lotoquart
patent: 4812643 (1989-03-01), Talbot
patent: 5249470 (1993-10-01), Hadley et al.
patent: 5253531 (1993-10-01), Walker et al.
Lee John R
Ostroff Irwin
Pfeifle Erwin
Stewart Hughes Limited
Westin Edward P.
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