Monroe-electronics Electrostatic Fieldmeter - Static Monitor - model Manuale Utente Scaricare il pdf (Pagina 86)

APNE-0016

III. Principle of Operation

Refer to Figure III-1 for the following discussion.

The probe is placed to “view” the target surface, which is assumed to be charged. In this instance,

the gradient cap containing the aperture faces the target surface.

A sensitive electrode behind the aperture is vibrated perpendicular to the electric field by means of a

drive coil (vibrated toward and away from the target surface). An A.C. signal is induced onto the

sensitive electrode due to the motion of the vibrating electrode in the electric field, which is created

by the charges on the target surface. The modulation amplitude of the A.C. signal, relative to the

drive coil signal, is related to the polarity of the charge on the target surface.

This A.C. signal, conditioned by a preamplifier, filter, and signal amplifier, is fed into a phase-

sensitive demodulator. This signal from this demodulator feeds an integrating amplifier. A fraction of

the integrator’s output signal is fed back to the sensing electrode to null the signal from the external

electric field.

The voltage signal from the integrator is thus directly proportional to the field intensity at the sensing

electrode of the probe. The output signal from the integrator drives a meter for direct readout.

The fieldmeter gives a reading of the field intensity at any spacing. If the spacing is known, the

voltage on the target surface can be determined.

Figure III-1

Simplified Block Diagram for an Electrostatic Fieldmeter

Phase

Sensitive

Demodulator

Signal

Amplifier

Reference

Oscillator

Integrator

Voltmeter

High Input Impedence

Preamplifier

Signal

Reference

Feedback

Surface Under Test Gradient Cap (Probe Housing)

Sized Hole

(Aperature)

Vibrating

Electrode

Main Meter AssemblyProbe

Driving Coil

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Monroe-electronics Electrostatic Fieldmeter - Static Monitor - model Manuale Utente Pagina 86