Heater control system for liquid crystal device

• US 4,119,842 A
• Filed: 06/17/1977
• Issued: 10/10/1978
• Est. Priority Date: 06/17/1977
• Status: Expired due to Term

• Current Assignee: General Motors Corporation (Motors Liquidation Co. GUC Trust)
• Original Assignee: General Motors Corporation (Motors Liquidation Co. GUC Trust)
• Inventors: Kaplit, Michael, Smith, George W., Hayden, Daniel B.
• Primary Examiner(s): Tokar, M.
• Application Number:
  US05/807,376
• Time in Patent Office: 480 Days
• Field of Search: 350/160 P, 350/160 LC, 250/201, 250/205
• US Class Current: 250/201.1
• CPC Class Codes: G02F 1/1309 Repairing; Testing
• CPC Class Codes: G02F 1/133382 Heating or cooling of liquid crystal cells other than for activation, e.g. circuits or arrangements for temperature control, stabilisation or uniform distribution over the cell

Abstract

A twisted nematic liquid crystal device contains a liquid crystal material having dielectric anisotropy Δε which is positive for certain values of temperature and frequency of an electrical signal applied across the material and negative for other values of these parameters. The device includes a heater and a test area (separate from the display area of the device) at which transmittance of light is monitored by a heater control circuit. A test frequency chosen to give Δε≈0 at the minimum desired operation temperature of the device is applied across the test area and the light transmittance is monitored to assure that the temperature is high enough. When the temperature is too low, the heater element is automatically energized to increase the temperature of the liquid crystal device.

Claims

1. A heater control system for a liquid crystal device of a type having a light transmission state dependent on the frequency of an electrical control signal applied thereto, the device having a first light transmission state when an electrical signal having a frequency below a crossover frequency is applied to the device, and otherwise having a second light transmission state, said crossover frequency increasing as the temperature of said device increases, and said device having a predetermined desired crossover frequency when the temperature of the device has a predetermined value, means for heating the device, means for applying to the device a test signal of a predetermined frequency less than said desired crossover frequency by a slight amount, means for detecting the light transmission state of said device when said test signal of said predetermined frequency is applied thereto, and a control circuit responsive to the light transmission state of the device for energizing the heating means when the detected light transmission state is indicative of a temperature of said device that is below the value that corresponds to said desired crossover frequency, thus assuring that said device will attain a temperature sufficient to respond to an applied control signal of frequency no greater than the frequency of the test signal.

2. A heater control system for a liquid crystal device of a type having a light transmission state dependent on the frequency of an electrical signal applied thereto, the device having a first light transmission state when an electrical signal having a frequency below a crossover frequency is applied to the device, a second light transmission state when the said electrical signal is not applied, and further having the second light transmission state when an electrical signal of frequency higher than the crossover frequency is applied to the device, the crossover frequency increasing as the temperature of said device increases, and said device having a predetermined desired crossover frequency at a predetermined desired temperature of the device, means for selectively applying to said device a control signal having a frequency much below the desired crossover frequency to thereby selectively control its light transmission state, means for heating the device, means for applying to the device a test signal of a frequency slightly less than said desired crossover frequency and greater than the control signal frequency, means for detecting the light transmission state of said device, and a control circuit responsive to the light transmission state of the device when said test signal is being applied thereto for energizing the heating means when the detected light transmission state indicates that the said device is below the said desired temperature that corresponds to said desired crossover frequency, thus assuring that said device will attain a temperature sufficient to respond to said control signal.

3. A heater control system for a liquid crystal device of a type having a light transmission state dependent on the frequency of an electrical signal applied thereto, the device having a first light transmission state when an electrical signal having a frequency below a crossover frequency is applied to the device, a second light transmission state when an electrical signal of frequency higher than the crossover frequency is applied to the device, the crossover frequency increasing as the temperature of said device increases, and said device having a predetermined desired crossover frequency at a predetermined desired temperature of the device, means for selectively applying to said device a control signal having alternatively a first frequency much below the desired crossover frequency and a second frequency above the desired crossover frequency to thereby selectively control its light transmission state, means for heating the device, means for applying to the device a test signal of a frequency slightly less than said desired crossover frequency and greater than the said first frequency, means for detecting the light transmission state of said device, and a control circuit responsive to the light transmission state of the device when said test signal is being applied thereto for energizing the heating means when the detected light transmission state indicates that the said device is below the said desired temperature that corresponds to said desired crossover frequency, thus assuring that said device will attain a temperature sufficient to respond to said first frequency of the control signal.

4. A heater control system for a liquid crystal display device of a type having a light transmission state dependent on the frequency of an electrical signal applied thereto, the device having a first light transmission state when an electrical signal having a frequency below a crossover frequency is applied to the device, and otherwise having a second light transmission state, the crossover frequency increasing as the temperature of said device increases, and said device having a predetermined desired crossover frequency at a predetermined desired temperature of the device, the device having a display area and a test area, means for selectively applying to the display area of said device a control signal having a frequency much below the desired crossover frequency to thereby selectively control its light transmission state, means for heating the device, means for applying to the test area of said device a test signal of a frequency slightly less than said desired crossover frequency and greater than the control signal frequency, means for detecting the light transmission state of the test area of said device, and a control circuit responsive to the detected light transmission state of the device when said test signal is being applied thereto for energizing the heating means when the detected light transmission state indicates that the said device is below the said desired temperature that corresponds to said desired crossover frequency, thus assuring that the display area of said device will be sufficiently warm to respond to said control signal.

5. A heater control system for a liquid crystal device of a type having a light transmission state dependent on the frequency of an electrical control signal applied thereto, the device having a first light transmission state when an electrical signal having a frequency below a crossover frequency is applied to the device, and otherwise having a second light transmission state, said crossover frequency increasing as the temperature of said device increases, and said device having a predetermined desired crossover frequency when the temperature of the device has a predetermined value, means for heating the device, means for applying to the device a test signal of a predetermined frequency less than said desired crossover frequency by a slight amount, means for detecting the light transmission state of said device when said test signal of said predetermined frequency is applied thereto including a light source on one side of the device and a photosensitive element on the other side of the device, and a control circuit connected to the photosensitive element responsive to the light transmission state of the device for energizing the heating means when the detected light transmission state is indicative of a temperature of said device that is below the value that corresponds to said desired crossover frequency, thus assuring that said device will attain a temperature sufficient to respond to an applied control signal of frequency no greater than the frequency of the test signal.

6. A heater control system for a liquid crystal device responsive to applied electrical signals to selectively permit and block light transmission respectively when the device is at a desired operating temperature, the device containing a liquid crystal material having a frequency and temperature varying dielectric anisotropy, the dielectric anisotropy being positive at a low frequency of applied signals to effect one state of light transmission and negative at a high frequency to effect another state of light transmission, the crossover frequency at which the dielectric anisotropy changes from positive to negative increasing as temperature increases,

means for heating the device,

means for applying to the device a test signal at a frequency intermediate the said low and high frequencies of the applied signals and corresponding to the crossover frequency at a desired operating temperature,

means for detecting the light transmission state of the device, and

a control circuit responsive to the light transmission state of the device upon application of the test signal for energizing the heating means when the detected light transmission state indicates that the liquid crystal material is in a negative dielectric anisotropic state to thereby raise the liquid crystal material to the selected normal operating temperature, whereupon the liquid crystal material assumes a positive dielectric anisotropy state at the frequency of the said test signal.