A newly developed instrument, designed at Marshall Space Flight Center, uses the resistance to alternating current running through a device to measure the variances in capacitance. Engineers use this device to detect the presence or absence of dielectric material between the plates of a capacitor.

Previously, instruments were designed to measure one or more quantities (amplitude, frequency, or phase) that make up an alternating current waveform. However, these instruments were plagued by stray capacitance, solder traces and connectors.

This abbreviated schematic diagram shows the Major Functional Parts of the Signal-Conditioning Electronics for discrete capacitive sensors.

This device allows engineers to apply Ohm's law to make an impedance measurement of the sensor by knowing the voltage across the sensor and the current running through the sensor.

An inductive circuit is in parallel with the stray capacitance from the sensor cable, connectors, and solder traces on the printed circuit board within this device. This unique design puts the inductor and stray capacitance [including the cable capacitance at the end of a 300- to 400-ft (90- to 120-m) cable] in a parallel resonant circuit that completely nullifies the effects of stray capacitance. Because stray capacitances no longer have any affect on the measurement system, the circuit is said to be tune.

The signal conditioning electronics for this instrument are shown in the abbreviated schematic diagram below. The sensor is excited by a 1-V (rms) sine wave that is operated at 10 kHz. The signal is then sent through a coaxial cable to the capacitive sensor. (The coaxial cable shields the signal from any outside disturbance.) The signal at the sensor is again picked off from another coaxial cable to the right of the sensor and sent to the instrument in a series across the resistor. The current sense resistor prevents the current flow across the sensor from loading down the signal. The resulting signal is then fed to an amplifier.

The amplified signal is fed to a comparator circuit. If the voltage on the non-inverting terminal of the comparator exceeds the trip voltage setting on the inverting terminal, the output of the comparator goes high, turning the transistor ON, indicating that the capacitive plates are covered by the dielectric material. An OFF transistor reading indicates the sensor is no longer exposed to the dielectric material.

This work was done by Randal S. McNichol of Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category, or circle no. 143 on the TSP Order Card in this issue to receive a copy by mail ($5 charge). MFS-31195