Capacitive measuring system with automatic calibration

Abstract

In a device for continuously monitoring the characteristics of a moving filament, such as the denier of an extended synthetic yarn, by passing the filament through a capacitive sensor to develop an electrical signal representing an absolute measurement of the filament with reference to a prescribed datum, the problem of measurement signal drift arising from contamination of the capacitive sensor is obviated by developing compensating signals to be combined with the filament measurement signal. The compensating signals are digitally formed and stored, thereby eliminating drift in the compensating signals themselves. The compensating signals are developed in an auto-calibration circuit, including an auto-zero circuit and an auto-gain circuit, which receives the measurement signal from the capacitive sensor. While the sensor is vacant, the auto-zero circuit digitally counts clock pulses to generate a digital output, converts the digital output into an analog signal varying with the digital count, detects a prescribed comparison between the analog signal and the input measurement signal, and stops the clock pulse count at a zero compensating value when the comparison is detected. Then, the auto-gain circuit applies an unbalanced drive to the sensor, digitally counts clock pulses to generate a digital output, and varies the measurement signal gain with the digital output. The circuit detects a prescribed comparison between the gain-adjusted measurement signal and a standardized output signal, and stops the clock pulse count at a gain compensating value when the prescribed comparison is detected.