Measurement of surface potential of non-uniformly charged insulating materials using a non-contact electrostatic voltmeter

Non-contact measurements performed with the auto-compensated electrostatic induction probe of an electrostatic voltmeter are often employed to characterize the charging state of insulating materials, such as films or non-woven media. The aim of the paper is double: (i) describe a simple method for assessing the resolution of such a probe; (ii) discuss the significance that can be attributed to the value displayed by the electrostatic voltmeter when the charge is non-uniformly distributed at the surface of the bodies examined by the probe and no constant surface potential can be defined. The measurements were performed for some simple experimental models that simulate surface potential non-uniformity of constant-charge insulating bodies. By examining the different surface potential distribution curves that were obtained for each of the situations under study, it is possible to estimate the resolution of the probe. A commercial computer program based on the superficial charge simulation method was employed for the numerical analysis of the electric field in a simplified 2-D representation of the geometric system formed by the probe, the experimental model, and the grounded plate. The potential that anneals the electric field at the surface of the probe (to simulate its principle of operation) was calculated by an iterative method. A good agreement was found between the results of the numerical simulations and the experimental data. The errors that can be made in the interpretation of the results of surface potential measurements are illustrated by the case of a non-uniformly-charged non-woven polypropylene fabric.