Measuring Thickness of Dielectric Materials using MTI Accumeasure™ Capacitance Instrumentation

Although the MTI Instruments Accumeasure ™ capacitance based instrumentation is designed primarily to make noncontact measurements of position, displacement, vibration, or runout, it can also be used to make noncontact thickness measurements of dielectric1 materials. If the thickness is known or can be independently measured, the Accumeasure System can also be used to measure the dielectric constant of insulating materials. The Accumeasure System uses a constant current signal source at either 16kHz (Accumeasure 500 & 1500) or 100kHz (Accumeasure 5000) carrier frequency. The transducer (probe) is a passive element, in that it contains no active electronic circuitry. All of the active circuitry is contained within the Accumeasure electronic amplifier and supplied to the probe through low noise coaxial cable. The Accumeasure constant current amplifier circuitry supplies the control voltage required to keep the sensing current at a constant level over the rated displacement sensing range of the probe and amplifier combination. A high precision buffer amplifier is used to electrically drive the coaxial cable shield and the coaxial capacitance probe structure at the same amplitude and phase as the sensing signal.