TEMPERATURE COMPENSATED ELECTRONIC DISPLAY

• US 3,577,137 A
• Filed: 12/31/1968
• Issued: 05/04/1971
• Est. Priority Date: 12/31/1968
• Status: Expired due to Term

• Current Assignee: James Brennan Jr
• Original Assignee: James Brennan Jr
• Inventors: Brennan, James Jr.
• Application Number:
  US04/788,178
• Time in Patent Office: 854 Days
• US Class Current: 347/194
• CPC Class Codes: B41J 2/36 Print density control
• CPC Class Codes: B41J 2/365 by compensation for variation in temperature
• CPC Class Codes: G09F 9/30 in which the desired character or characters are formed by combining individual elements panels comprising a number of electrodes in a single cell controlling light arriving from an independent light source, e.g. electro-optical or magneto-optical cell, G02F1/00
• CPC Class Codes: G09G 3/16 by control of light from an independent source
• CPC Class Codes: H04N 3/14 by means of electrically scanned solid-state devices for picture generation H04N25/00

Abstract

A high speed electronic display system having a circuit for adjusting the power applied to a thermal display matrix during a printing cycle based upon the temperature of the print head. In one embodiment, the temperature of a matrix of air isolated semiconductor means is sensed by a diode diffused in an adjacently mounted integrated circuit immediately prior to the print cycle, and the power applied to drive the matrix during the print cycle is adjusted in accordance with the offset voltage across the diode. In another embodiment, the temperature of the matrix of elements is estimated using a temperature sensing element located on the heat sink and correcting the sensed temperature to compensate for the repetition rate of the print cycle, the number of elements activated during each print cycle, the rate at which heat is transferred to the heat sensing element, and the rate at which heat is dissipated from the heat sensing element. There is a single sensor for the thermal display matrix.

Claims

1. In an electronic display device, the combination of:

a character matrix of thermally separated elements each including a heating element and switching means for controlling current through the heating element;

circuit means for binary character data generation for operating said switching means in a Combination to heat the elements in a geometric pattern corresponding to the character represented by the character data; and

circuit means for sensing the temperature of the matrix and adjusting a supply voltage applied to the resistive heating elements in response to the sensed temperature whereby the elements will be heated to a temperature within a predetermined temperature range to achieve uniform display density.

2. The combination defined in claim 1 wherein the last mentioned circuit means comprises;

means for sensing the temperature of the matrix during one period and storing a voltage representative of the sensed temperature during a subsequent period; and

means for adjusting the voltage applied to the resistive heating element in response to the stored voltage.

3. The combination defined in claim 1 wherein the last mentioned means utilizes a PN junction to sense the temperature of the matrix.

4. The combination defined in claim 1 wherein the last mentioned means utilizes a temperature dependent resistance to sense the temperature of the matrix.

5. The combination defined in claim 1 wherein the last mentioned circuit means comprises:

an operational amplifier having noninverting and inverting inputs and an output connected to supply power to the resistive heating elements;

means for sampling and storing a voltage proportional to the temperature of the matrix during a sample period and storing the sampled voltage at one input of the amplifier during a print period; and

feedback circuit means connecting the output of the amplifier to the other input of the amplifier whereby the output voltage of the amplifier will be proportional to the stored voltage.

6. The combination defined in claim 5 wherein the feedback circuit means includes:

a feedback resistance connecting the output to said other input;

a first adjustable voltage supply for producing a first voltage equal to the stored voltage;

a first adjustable resistor connecting the other input to the first adjustable voltage supply; and

a second adjustable resistor connecting the other input to a voltage supply.

7. The combination defined in claim 6 further characterized by:

switch means for reducing the current required through the feedback resistance during the sample cycle to thereby reduce the output voltage.

8. The combination of claim 1 wherein the last mentioned circuit means comprises:

an operational amplifier having a noninverting input, an inverting input, and an output connected to supply power to the resistive heating elements;

first circuit means for establishing a first voltage at one input representative of the temperature of a temperature sensing element spaced from the matrix;

feedback circuit means for determining the gain of the amplifier including a feedback resistance connecting the output to the other input and a second resistance connecting the other input to a reference voltage;

second circuit means for increasing the current through the second resistance to compensate for the delay in the transfer of heat from the matrix to the temperature sensing element.

9. The combination of claim 8 wherein the second circuit means is responsive to the duration and rate of the period when printing, the duration of the period when not printing, and the number of resistive heating elements energized during the preceding print periods.

10. The combination of claim 9 wherein the second circuit means includes:

a capacitor having one plate connected to said one input of the amplifier;

a resistance connecting the other plate of the capacitor to the other input of the amplifier; and

circuit means connected to said other plate for charging the capacitor during each print period with a pulse the amplitude of which is related to the number of heating elements energized.

11. In a system for printing on a thermally sensitive recording medium, the combination of:

a supporting chip;

a matrix of semiconductor elements mounted on the chip in thermally separated relationship;

a heating element formed in each semiconductor element;

a transistor diffused into each element for switching current through the heating element;

means for applying power across the heating elements and the respective switching transistors, and means for varying the voltage to said transistors to conduct variable current through the respective heating elements and produce a character of predetermined density on the thermally sensitive recording medium.

12. The combination of claim 11 wherein the voltage is related to the temperature of the matrix preceding the application of power.

13. In an electronic display device, the combination of:

a matrix of thermally separated elements each including a resistive heating means, the matrix having an '"'"''"'"''"'"''"'"'on'"'"''"'"''"'"''"'"' cycle during which selected elements are '"'"''"'"''"'"''"'"'on'"'"''"'"''"'"''"'"' to produce a character and an '"'"''"'"''"'"''"'"'off'"'"''"'"''"'"''"'"' cycle during which all elements are '"'"''"'"''"'"''"'"'off'"'"''"'"''"'"''"'"';

means for sensing the temperature of the matrix during the '"'"''"'"''"'"''"'"'off'"'"''"'"''"'"''"'"' cycle; and

means for adjusting the power applied to the matrix during the '"'"''"'"''"'"''"'"'on'"'"''"'"''"'"''"'"' cycle in relation to the temperature of the matrix sensed during the '"'"''"'"''"'"''"'"'off'"'"''"'"''"'"''"'"' cycle to produce a predetermined temperature during the '"'"''"'"''"'"''"'"'on'"'"''"'"''"'"''"'"' cycle.

14. The method for printing electronically which comprises:

successively positioning a matrix of heating elements at a series of print positions of a thermally sensitive paper;

applying a supply voltage to the matrix at each position related to the temperature of the matrix after the print cycle at the preceding print position; and

switching the supply voltage across selected heating elements of the matrix to produce a desired character image at each print position.

15. The method of claim 14 wherein the supply voltage at each print position is related to the temperature of the matrix during the '"'"''"'"''"'"''"'"'off'"'"''"'"''"'"''"'"' period preceding the print period.

16. The method of claim 15 wherein the supply voltage is referenced to the voltage across a PN junction disposed in close proximity to the matrix.

17. The method of claim 16 wherein the supply voltage at each print position is related to the temperature of the matrix at some point in time in the past and to the rate at which power was applied to the matrix since that point in time.

18. An electronic display device comprising:

a matrix of thermally separated elements each including a heating element;

means for sensing the temperature of said matrix; and

means responsive to said temperature sensing means to control the temperature of said matrix of thermally separated elements.

19. An electronic display device comprising:

a matrix of thermally separated elements each including a heating element;

means for sensing the temperature of said matrix; and

means responsive to said temperature sensing means for adjusting the power applied to said matrix to produce a predetermined temperature at said matrix.

20. An electronic display device comprising:

a matrix of thermally separated elements each including a heating element;

means for sensing the temperature of said matrix; and

means responsive to said temperature sensing means to produce a predetermined temperature at said matrix.