Liquid crystal electrostatic voltmeter

A voltage sensor for sensing the voltage on a charged surface, said voltage sensor comprising:

• a pickup electrically coupled to a charged surface;
• a liquid crystal cell, electrically connected to said pickup and having a varying electro-optical response to signals therefrom;
• a light source directing light through said liquid crystal cell;
• a light detector detecting the intensity of light passed through said liquid crystal cell and providing an output indicative thereof, whereby electro-optically induced changes in said light passed through said liquid crystal cell are detected, representative of the voltage on said charged surface;
• and
• charging means for controllably directing a series of voltage pulses to said liquid crystal cell to periodically charge said liquid crystal cell to a selected bias condition.

A voltage sensor for sensing voltage on a charged surface, comprising:

• first and second liquid crystal cells, said liquid crystal cells comprising a high impedance electro-optic material;
• said first liquid crystal cell electrically connected to a charged surface having a voltage to be measured;
• said second liquid crystal cell electrically connected to a reference voltage;
• said first and second liquid crystal cells each provided with a light source for directing light through said liquid crystal cell, and a light detector for detecting light passed through said liquid crystal cell and providing a detector output signal indicative of detected light intensity;
• comparison means for comparing said detector output signals from each said light detector, and providing a comparison signal representative thereof as a corrected output signal;
• and
• charging means for controllably directing a series of voltage pulses to said liquid crystal cells to periodically charge said liquid crystal cells to a selected bias condition.

A method of operating a voltage sensor for sensing a voltage on a charged surface, said voltage sensor including a pickup electrically coupled to a charged surface in an electrophotographic device;

• high impedance electro-optic material electrically coupled to said pickup capacitor, whereby a voltage is created across said electro-optic material proportional to the voltage on said charged surface and causing electro-optical changes in said electro-optic material;
• a light source directing light through said electro-optic material;
• and light detecting means for detecting the intensity of light passed through said electro-optic material and generating a proportional signal therefrom, said method comprising the steps of:
• (a) removing said pickup from electrical coupling with said charged surface and applying a reference voltage to said pickup;
• (b) biasing said electro-optic material to a predetermined voltage level of a selected polarity;
• (c) removing said bias voltage from said electro-optic material and removing said reference voltage from said pick-up;
• (d) coupling said pickup to said charged surface;
• (e) sampling transmissivity of the electro-optic material for a selected period;
• (f) removing said pickup from electrical coupling with said charged surface and applying a reference voltage to said pickup;
• (g) biasing said electro-optic material to a predetermined voltage level of an opposite polarity;
• (h) removing said bias voltage from said high impedance electro-optic material and removing said reference voltage from said pickup;
• (i) coupling said pickup to said charged surface;
• (j) sampling transmissivity of the electro-optic material for a selected period;
• (k) repeating steps (a)-(j).

A method of operating a voltage sensor for sensing a voltage on a charged surface in an electrophotographic device, said voltage sensor including a pickup electrically coupled to a charged surface in an electrophotographic device;

• and electrically connected to a liquid crystal cell, whereby a voltage is created across said cell proportional to the voltage on said charged surface causing transmissivity variations in said liquid crystal cell;
• a light source directing light through said liquid crystal cell;
• and a light detector for detecting the intensity of light passed through said liquid crystal cell and generating a proportional signal therefrom, said method comprising the steps of:
• (a) removing said pickup from electrical coupling with said charged surface and applying a reference voltage to said pickup;
• (b) biasing said liquid crystal cell to a predetermined voltage level of a selected polarity;
• (c) removing said bias voltage from said liquid crystal cell and removing said reference voltage from said pickup;
• (d) coupling said pickup to said charged surface;
• (e) sampling transmissivity of the liquid crystal cell for a selected period;
• (f) removing said pickup from electrical coupling with said charged surface and applying said reference voltage to said pickup;
• (g) biasing said liquid crystal cell to said predetermined voltage level of an opposite polarity;
• (h) removing said bias voltage from said liquid crystal cell and removing said reference voltage from said pickup;
• (i) coupling said pickup to said charged surface;
• (j) sampling transmissivity of the liquid crystal cell for a selected period;
• (k) repeating steps (a)-(j) during operation of the electrophotographic device.

A method of operating a voltage sensor for sensing a voltage on a charged surface, said voltage sensor including a pickup electrically coupled to a charged surface;

• a first liquid crystal cell electrically coupled to said pickup, whereby a voltage is created across said liquid crystal cell proportional to the voltage on said charged surface causing electro-optical changes in said liquid crystal cell;
• a second liquid crystal cell electrically coupled to a reference voltage;
• each liquid crystal cell having a light source directing light through said cell for detection by a light detector which generates a proportional signal therefrom said method comprising the steps of:
• (a) removing said pickup from electrical coupling with said charged surface and applying said reference voltage to said pickup;
• (b) biasing said liquid crystal cells to a predetermined voltage level of a selected polarity;
• (c) removing said bias voltage from said liquid crystal cells and removing said reference voltage from said pickup;
• (d) coupling said pickup to said charged surface;
• (e) sampling transmissivity of the liquid crystal cells for a selected period;
• (f) comparing the outputs of each light detector to derive a corrected output signal;
• (g) removing said pickup from electrical coupling with said charged surface and applying said reference voltage to said pickup;
• (h) biasing said liquid crystal cells to a predetermined voltage level of an opposite polarity;
• (i) removing said bias voltage from liquid crystal cells and removing said reference voltage from said pickup;
• (j) coupling said pickup to said charged surface;
• (k) sampling transmissivity of the liquid crystal cells for a selected period;
• (l) comparing the outputs of each light detector to derive a corrected output signal;
• (m) repeating steps (a)-(l) during operation of the electrophotographic device.