| 摘要 |
PURPOSE:To accurately detect abnormality of a position detection signal by processing an analog signal from a position detection sensor and detecting the abnormality. CONSTITUTION:An encoder 1 is provided with a light source 1, a scale provided with slits 3a, etc., at specific intervals, and two photodetectors 4a and 4b. The photodetectors 4a and 4b are arranged so that their output positions are 90 deg. different. Their outputs are squared by multipliers 5a and 5b and added together by an operational amplifier 6a. Namely, the output of the operational amplifier 6a is nearly constant because sin<2>theta+cos<2>theta=1, where sintheta is the output of the photodetector 4a and costheta is the output of the photodetector 4b. For the purpose, a comparing circuit part 7 decides this output to determine the abnormality unless the output is in a prescribed range. The decision on the abnormality is made not digitally, but on an analog basis, so the influence of an error in digitization is eliminated.
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