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Theory of OperationThe operating principle of the opto-probe is extremely simple. Figure 1 illustrates its function and contrasts it with the performance of a conventional magnetic pickup derived ROTOR signal. An infrared light emitting diode (IRED) is mounted in one of the two bore holes on parallel axis in the end of the probe. The IRED output, by means of an integral lens, is directed upon a black and white striped target. In case of a turbine-generator rotor, the optical target is affixed around the shaft's circumference at any convenient location and rotates at synchronous speed, ws, revolutions per second. The incident normal infrared light is alternately reflected from the white stripes and absorbed by the black stripes. Target rotation therefore produces a pulse train of reflected infrared light, some of which strikes a photo-transistor mounted in a bore hole adjacent to the IRED hole. Signal conditioning electronics are also contained in the probe. The output is a + or - 13 volt square wave whose frequency is: fprobe= Where:
The opto-probe design is simple and rugged. It has been tested extensively under laboratory and field conditions. In all respects, it has proven either equivalent or superior to comparable toothed wheel/magnetic pickup input. Some of the advantages of optoprobe rotor speed sensing over toothed wheels follow:
Figure 1 also indicates the typical frequency ranges one might expect for either toothed wheel or opto-probe signal inputs. Typical magnetic pickup signals may approach 100 volts zero to peak at a frequency of 240 to 6,000 Hz contingent upon the number of wheel teeth and the machine's nominal rotational speed. Opto-probe frequency may vary over a similar range but its output amplitude is independent of speed. Typically, an output frequency of 600 Hz, or higher is easily obtained. A 10-stripe black & white stripe pair target on a 3,600 RPM (60 rev/sec) machine produces 600 Hz, for instance.
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