PRACTICAL PERFORMANCES OF HIGH-SPEED MEASUREMENT OF GEAR TRANSMISSIONERROR OR TORSIONAL VIBRATIONS WITH OPTICAL ENCODERS

We evaluated in this paper an improved technique for measuring gear tr ansmission error (GTE) at high speed, by using low pulse per revolutio n optical encoders. The originality of this technique lies in the fact that highly precise, completely digital measurements of torsional vib ration or transmission error (TE) at high speed are achievable by the use of low-price, basic optical components. The lengths of encoder pul ses are estimated with a high-frequency timer (100 MHz): thus, it appe ars that the theoretical precision of this device depends only on the angular speed of shafts, not on the number of pulses per revolution of the encoder. In practice, the intrinsic encoder accuracy (namely the grating or electronic signal processing precision) directly affects pr ecision measurements. Alternatively, the number of pulses per revoluti on of the encoder specifies the resolution. We examined the possibilit y of calibrating encoders through using a specific test rig. The deter mination of corrective data assigned to each grating leads to an insig nificant improvement of the precision measurement. The coherence from one revolution to another does not present any significant determinist ic component. The overall precision achieved is less than 0.03 second of are for each frequency of the power spectral density. This calibrat ion device only gives a good assessment of eccentricity induced by mec hanical mounting of optical discs on a shaft, compared with the direct measurement on grating discs. The correlation between the two measure ments is less than 3% of the magnitude of the relative eccentricity. T hus, the encoder technique seems to be a cheap and easy way to impleme nt transmission error measurement on real mechanical systems with high precision and sufficient reliability.