Corona Charging and Current Measurement Using Phi-Type Corona Electrodes

In order to develop a noncontact surface resistivity measurement technique, the corona charging of a test material with simultaneous measurement of an induction current caused by a traveling surface charge was investigated using phi-type electrodes. The phi-type electrode consists of a high-voltage needle electrode that penetrates a circular hole of a grounded planar electrode. The phi-type electrode is positioned above an electrically isolated test material. The purpose of the electrode design is to supply static charge to the test surface, to produce a ground potential close to the charged test material, and then to measure the induction current or the surface potential caused by the propagated surface charge. Test materials with surface resistivities from 10
⁶
to 10
¹²
Ω/□ were prepared by coating conductive polymer layers onto polyvinyl chloride disks. Two setups were prepared for higher surface resistivity (Model H) and lower surface resistivity (Model L). For Model H, a surface voltmeter was used to measure the slow propagation of surface charge. The rise time of the surface potential increased linearly with the surface resistivity from 3 × 10
⁹
to 1 × 10
¹²
Ω/□. The Model L had two induction probes to measure the fast propagation of surface charge. The rate of the total induction charges was a function of the surface resistivity from 3 × 10
⁶
to 3 × 10
⁹
Ω/□. Experimental results obtained from both the H and L models agreed with the predicted results. The phi-type electrode was verified as effective for noncontact surface charge measurements.