High speed D.C. non-contacting electrostatic voltage follower

• US 4,878,017 A
• Filed: 10/28/1986
• Issued: 10/31/1989
• Est. Priority Date: 10/28/1986
• Status: Expired due to Term

• Current Assignee: Williams, Bruce T.
• Original Assignee: Williams, Bruce T.
• Inventors: Williams, Bruce T.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): BURNS, WILLIAM
• Application Number:
  US06/924,054
• Time in Patent Office: 1,099 Days
• Field of Search: 324/72.5, 324/172, 324/109, 324/123 R, 324/123 C, 324/457, 324/458, 324/130, 307/359, 307/264, 330/10, 330/100, 330/85, 328/167
• US Class Current: 324/109
• CPC Class Codes: G01R 15/165 measuring electrostatic potential, e.g. with electrostatic voltmeters or electrometers, when the design of the sensor is essential electrometers with passively moving electrodes G01R5/28; measuring electrostatic fields G01R29/12; measuring charge G01R29/24; measuring in circuits with high internal resistance G01R19/0023
• CPC Class Codes: G01R 29/12 Measuring electrostatic fields or voltage-potential

Abstract

A non-contacting electrostatic voltage follower employs a vibrating capacitance detector coupled to a measured surface containing both wideband electrostatic A.C. data and D.C. potentials. A detector amplifier having an inverting input is connected to the detector, while its output is connected to the input of a high bandwidth amplifier. A first feedback path is provided from the output of the high bandwidth amplifier to the non-inverting input of the detector amplifier which provides both a signal which is equal to and follows the wideband electrostatic A.C. potentials on the measured surface, and a signal, at the vibrating capacitance modulation frequency, representative of the D.C. potential of the measured surface. A second feedback path is provided including a demodulator and integrator to monitor the first feedback path to provide a D.C. correction signal to the high bandwidth amplifier to null the signal on the first feedback path which is representative of the vibrating capacitance modulation of the D.C. potential of the measured surface. As a result, the monitored signals on the first feedback path will be driven to, follow, and exactly match all wideband A.C. and D.C. electrostatic data on the measured surface. An electrostatic probe employs a vibrating transparent detector element and is disposed in a path of radiation whcih is directed onto a measured electrostatic surface.

Claims

1. A non-contacting electrostatic voltage follower comprising:

a) a single detector electrode sensitive to electrostatic quantities such as electrostatic fields, voltages or charges having continuous capacitive coupling to a surface;

b) means operatively associated with said electrode for varying the capacitive coupling between said electrode and said surface bearing an electrostatic quantity to which said electrode is capacitively coupled;

c) a detector amplifier having first and second inputs and an output, said amplifier having a high input impedance;

d) means for connecting said detector electrode to said first input of said detector amplifier;

e) a high bandwidth amplifier having an input and an output;

f) means for applying the output of said detector amplifier to the input of said high bandwidth amplifier; and

g) means for providing a first feedback path from said output of said high bandwidth amplifier to said second input of said detector amplifier in a manner producing at the output of said high bandwidth amplifier signals having dynamic components matching all dynamic components of electrostatic quantities on said surface;

h) whereby the magnitude and phase of said signals produced by said output of said high bandwidth amplifier provide information as to the magnitude and polarity of said electrostatic quantities borne by said surface.

2. An electrostatic voltage follower according to claim 1, wherein said means for varying the capacitive coupling comprises modulator means for mechanically vibrating said electrode to produce capacitance modulation.

3. An electrostatic voltage follower according to claim 2, further including means for connecting the output of said high bandwidth amplifier to said modulator means in a manner providing a reference for said modulator means.

4. An electrostatic voltage follower according to claim 1, wherein said means for applying the output of said detector amplifier to the input of said high bandwidth amplifier comprises resistance means.

5. A non-contacting electrostatic voltage follower comprising:

a) a single detector electrode sensitive to electrostatic quantities such as electrostatic fields, voltages or charges;

having capacitive coupling to a surface;

b) modulation means operatively associated with said electrode for providing capacitance modulation of the capacitive coupling between said electrode and said surface bearing an electrostatic quantity to which said electrode is capacitively coupled;

c) a detector amplifier having first and second inputs and an output;

d) means for connecting said detector electrode to said first input of said detector amplifier;

e) a high bandwidth amplifier having an input and an output;

f) means for applying the output of said detector amplifier to the input of said high bandwidth amplifier;

g) means for providing a first feedback path from said output of said high bandwidth amplifier to said second input of said detector amplifier; and

h) means for providing a second feedback path from said first feedback path to said input of said high bandwidth amplifier to cause said high bandwidth amplifier to produce at the output thereof a d.c. value necessary to null any signals in said first feedback path at the frequency of said modulation means;

i) so that the signal in said first feedback path will follow and exactly match all a.c. and d.c. components over a wide frequency range of the electrostatic quantities on said surface.

6. An electrostatic voltage follower according to claim 5, further including means connected to said first feedback path for providing information as to the magnitude and polarity of said electrostatic quantity.

7. An electrostatic voltage follower according to claim 5, further including means for connecting said first feedback path to said modulation means to provide a reference for said modulation means.

8. An electrostatic voltage follower acording to claim 5, wherein said detector amplifier comprises a current summing amplifier wherein said first input is a summing input and said second input is a non-inverting input.

9. An electrostatic type follower according to claim 5, wherein said means for applying the output of said detector amplifier to the input of said high bandwidth amplifier comprises resistance means.

10. A non-contacting electrostatic voltage follower comprising:

a) a detector electrode sensitive to electrostatic quantities such as electrostatic fields, voltages or charges having capacitance coupling to a surface;

b) means operatively associated with said electrode for varying the capacitive coupling between said electrode and said surface bearing an electrostatic quantity to which said electrode is capacitively coupled;

c) a detector amplifier having first and second inputs and an output;

d) means for connecting said detector electrode to said first input of said detector amplifier;

e) a high bandwidth amplifier having an input and an output;

f) means for applying the output of said detector amplifier to the input of said high bandwidth amplifier;

g) means for providing a first feedback path from said output of said high bandwidth amplifier to said second input of said detector amplifier in a manner following and matching all dynamic components of electrostatic quantities on said surface whereby the magnitude and phase of said output of said high bandwidth amplifier provides information as to the magnitude and polarity of said electrostatic quantity;

h) means for providing a second feedback path from said first feedback path to said input of said high bandwidth amplifier to cause said amplifier to produce at the output thereof a d.c. value necessary to null any signals in said first feedback at the frequency of capacitance variation so that the d.c. voltage difference between said detector electrode and said surface is zero and the signal in said first feedback path will follow and exactly match all a.c. and d.c. components over a wide frequency range which are on said surface; and

i) said means for providing said second feedback path comprising an amplifier having an input and an output and connected as a voltage follower, voltage divider means connected between said first feedback path and said amplifier input, demodulator means being having an input connected to the output of said amplifier and having an output, means for applying to said demodulator means a reference signal relating to the rate at which said capacitive coupling is varied so that said demodulator means produces an output d.c. signal proportional to the amplitude of any signal in said first feedback path related to the frequency and phase at which said capacitive coupling is varied, an integrating amplifier having an input and an output, means for connecting the output of said demodulator means to the input of said integrating amplifier, and means for applying the output of said integrating amplifier to the input of said high bandwidth amplifier, so that said high bandwidth amplifier produces an output signal having a d.c. value necessary to null any signal in said first feedback path having a frequency equal to the rate at which said capacitive coupling is varied.

11. An electrostatic voltage follower according to claim 10, wherein said means for connecting the output of said demodulator means to the input of said integrating amplifier comprises resistance means.

12. A non-contacting electrostatic voltage follower comprising:

a) a detector electrode sensitive to electrostatic quantities such as electrostatic fields, voltages or charges having capacitive coupling to a surface;

b) modulation means operatively associated with said electrode for providing capacitance modulation of the capacitive coupling between said electrode and said surface bearing an electrostatic quantity to which said electrode is capacitively coupled;

c) a detector amplifier having first and second inputs and an output;

d) means for connecting said detector electrode to said first input of said detector amplifier;

e) a high bandwidth amplifier having an input and an output;

f) means for applying the output of said detector amplifier to the input of said high bandwidth amplifier;

g) means for providing a first feedback path from said output of said high bandwidth amplifier to said second input of said detector amplifier;

h) means for providing a second feedback path from said first feedback path to said input of said high bandwidth amplifier to cause said high bandwidth amplifier to produce at the output thereof a d.c. value necessary to null any signals in said first feedback path at the frequency of said modulation means so that the signal in said first feedback path will follow and exactly match all a.c. and d.c. components over a wide frequency range of the electrostatic quantities on said surface; and

j) said means for providing said second feedback path comprising an amplifier having an input and an output and connected as a voltage follower, voltage divider means connected between said first feedback path and said amplifier input, demodulator means having an input connected to the output of said amplifier and an output, means for connecting said modulation means to a reference input of said demodulator means, an integrating amplifier having an input and an output, means for connecting the output of said demodulator means to the input of said integrating amplifier, and means for applying the output of said integrating amplifier to the input of said high bandwidth amplifier, so that said high bandwidth amplifier produces an output signal having a d.c. value necessary to null any signal in said first feedback path having a frequency equal to that of said modulation means.

13. An electrostatic voltage follower according to claim 12, wherein said means for connecting the output of said demodulator means to the input of said integrating amplifier comprises resistance means.

14. A non-contacting electrostatic voltage follower comprising:

a) a single detector electrode sensitive to electrostatic quantities such as electrostatic fields, voltages or charges having capacitive coupling to a surface;

b) means operatively associated with said electrode for varying the capacitive coupling between said electrode and said surface bearing an electrostatic quantity to which said electrode is capacitively coupled;

c) a detector amplifier having first and second inputs and an output, said detector amplifier comprising a current summing amplifier wherein said first input is a summing input and said second input is a non-inverting input;

d) means for connecting said detector electrode to said first input of said detector amplifier;

e) a high bandwidth amplifier having an input and an output;

f) means for applying the output of said detector amplifier to the input of said high bandwidth amplifier; and

g) means for providing a first feedback path from said output of said high bandwidth amplifier to said second input of said detector amplifier in a manner producing at the output of said high bandwidth amplifier signals having dynamic components matching all dynamic components of electrostatic quantities on said surface;

h) whereby the magnitude and phase of said signals produced by said output of said high bandwidth amplifier provide information as to the magnitude and polarity of said electrostatic quantities borne by said surface.

15. A non-contacting electrostatic voltage follower comprising:

a) a single detector electrode sensitive to electrostatic quantities such as electrostatic fields, voltages or charges having capacitive coupling to a surface;

b) means operatively associated with said electrode for varying the capacitive coupling between said electrode and said surface bearing an electrostatic quantity to which said electrode is capacitively coupled;

c) a detector amplifier having first and second inputs and an output;

d) means for connecting said detector electrode to said first input of said detector amplifier;

e) a high bandwidth amplifier having first and second inputs and an output;

f) means for applying the output of said detector amplifier to the input of said high bandwidth amplifier;

g) means for providing a first feedback path from said output of said high bandwidth amplifier to said second input of said detector amplifier in a manner producing at the output of said high bandwidth amplifier signals having dynamic components matching all dynamic components of electrostatic quantities on said surface; and

h) means for providing a second feedback path from said first feedback path to said input of said high bandwidth amplifier to cause said amplifier to produce at the output thereof a d.c. value necessary to null any signals in said first feedback path at the frequency of capacitance variation so that the d.c. voltage difference between said detector electrode and said surface is zero and the signal in said first feedback path will follow and exactly match all a.c. and d.c. components over a wide frequency range which are on said surface;

i) whereby the magnitude and phase of said signals produced by said output of said high bandwidth amplifier provide information as to the magnitude and polarity of said elctrostatic quantities borne by said surface.