Measuring Thickness of Dielectric Materials using MTI Instruments Accumeasure Capacitance Sensors - MTI Instruments - #1

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MTI Instruments, Inc. 325 Washington Avenue Extension Albany, NY 12205 PH: +1-518-218-2550 OR USA TOLL FREE: 1-800-342-2203 FX: +1-518-218-2506 sales@mtiinstruments.com www.mtiinstruments.com APPLICATION NOTE Measuring Thickness of Dielectric Materials using MTI Instruments Accumeasure Capacitance Sensors Introduction 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. Operation 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 amplifi er and supplied to the probe through low noise coaxial cable. The Accumeasure constant current amplifi er 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 amplifi er combination. A high precision buffer amplifi er 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. This effectively cancels all stray capacitances and permits the amplifi er to respond only to the capacitance between the face of the sensor probe and the target surface. This results in the Accumeasure amplifi er having a linear response to either gap changes, or dielectric material thickness changes. The Accumeasure equipment operates as a classic parallel plate capacitor system with the face of the probe being one of the plates, and the target or surface being measured as the other plate. As shown in Figure 1, a ground return path must be provided from the target back to the low side of the Accumeasure amplifi er in order to complete the current path. In many practical situations or applications, the target is already at ground potential and the ground return path is automatically provided through the power line grounding and the grounded power cords, but it is usually best to attach a grounding wire directly from the target to the ground return connector on the Accumeasure amplifi er. When measuring thickness of dielectric materials, a fi xed gap is established between the probe face and the grounded return plate. The dielectric material to be measured is placed in the fi xed gap, or placed in contact with the surface of the ground return plate. If the material to be measured is moving, it may be passed through the gap without contact. The MTI Instruments KD-CH-IIID, a precision calibration micrometer fi xture MTII appnote: accumeasure.pdf - Page 1 of 5 Figure 1