| 摘要 |
Highly sensitive yarn monitors which work on the basis of electrostatic phenomena can, in particular instances of use, have considerable susceptibility to faults as a result of the pick-up of mains hum. It is proposed to apply the measuring signal (US) overlaid by a hum voltage (UB) to the sensing input of a "sample-and-hold" switching stage and to connect its control input to a monostable multivibrator which is excited by the mains voltage or by the mains rectifier to emit rectangular pulses synchronous with the mains frequency. At the same time, in the "sample-and-hold" switching stage, the noise-like signal (UM) caused by the running yarn and disturbed by hum pick-ups is sensed synchronously with the mains frequency and the respective sensed value is stored until the next sensed value, so that a "staircase-like" output signal (UA) is obtained. The picked-up hum voltage (UB) generates a direct voltage, whereas the noise-like measuring signal generates a staircase-like alternating voltage. By means of a high pass, the direct voltage obtained from the hum pick-up can be blocked off by the alternating voltage, thereby eliminating the disturbing effect of the hum pick-up.
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