Impact of the Patch Effect on Displacement Measurements by Capacitive Transducers

• Applicant: Dr. Uwe Brand, since 8/2019
• Subject Area: Measurement Systems ; Metal-Cutting and Abrasive Manufacturing Engineering
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 318740291

Capacitive sensors are used in many applications to measure displacements over several micro-meters with resolution in the sub-nanometer range. However at small displacements non-linearity effects can introduce deviations. These effects are caused by the homogeneity of the electric field between the electrodes. However, there are other effects limiting the linearity, e.g. form errors and roughness from geometrical point of view and stray field due to inhomogeneity of the surface potential due to grains of different orientation or contamination.Between two electrodes, there is usually a difference in the work functions of the two electrodes which is in a range of some 100 mV. Due to the modulation of one or both electrodes induces a displacement current which can be minimized by applying a voltage which corresponds to the work function difference of the two electrodes. This is an average value over both electrodes' surfaces, because due to crystal orientation of single grains and contamination each real metal surface is not an equipotential surface. Real surfaces show a variation in the local work function, which leads to fluctuation of charge distribution. This is known as Patch-Effect. It is responsible inhomogeneity fields between two electrodes. This impact on the capacitive displacement measurements needs to be investigated.We will use special 1D and 2D pattern in the range of 10 ¿m to 500 ¿m for verifying distance-dependent effects. The electrode's local work function and its stability will be measured by Kelvin Probe microscopy. The pattern on the test electrodes will be produced with different materials and geometrical pattern. Since topography changes will introduce non-linearity, too, the pattern should not show large height differences, i.e. topography less or free, to avoid electric field effects from the edges. Special electrodes will be used, which allows to modulate the surface potential by applying a bias voltage to different fields. By modulation of this bias we can simulate the patch effect and reveals its influence on the displacement measurements of capacitive sensors. The overall aim of the project is a better understanding of the patch effect and its impact on displacement measurements.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Dr. Ludger Koenders, until 8/2019