Photoelectric measuring ruler including fault detection apparatus
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United States of America Patent
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Jul 26, 1983
Grant Date -
N/A
app pub date -
Jan 19, 1981
filing date -
Jan 18, 1980
priority date (Note) -
Expired
status (Latency Note)
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Abstract
A measuring rule has a coilable steel measuring blade extensible through an opening in a housing and is marked along its length with regularly spaced dark areas separated by light areas. Two pairs of light sensors are located near the rule each associated with a light emitter. Each sensor is 180.degree. out of phase with the other sensor of the pair and 90.degree. out of phase with the other two sensors. The outputs of each pair of sensors are used to produce a difference signal. The difference signals are used by a counter to measure the magnitude and direction of movement of the blade. Each sensor output signal is divided by voltage divider to give a divided signal and each sensor output signal is compared with the complementary divided signal at the instant the difference signal of the other pair of sensors changes state and a fault signal is generated if its minimum is larger than the maximum of the complementary divided signal. The fault signal is generated when an abnormal deviation in a sensor output signal occurs even if the deviation is insufficient to cause a count error.
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