System for printhead pixel heat compensation

• US 6,249,299 B1
• Filed: 03/05/1999
• Issued: 06/19/2001
• Est. Priority Date: 03/06/1998
• Status: Expired due to Term

• Current Assignee: Codonics Incorporated
• Original Assignee: Codonics Incorporated
• Inventors: Tainer, Christopher Michael
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): FEGGINS, KRISTAL J
• Application Number:
  US09/262,988
• Time in Patent Office: 837 Days
• Field of Search: 347/191, 347/188, 400/120.11
• US Class Current: 347/191
• CPC Class Codes: B41J 2/35 providing current or voltage to the thermal head
• CPC Class Codes: B41J 2/365 by compensation for variation in temperature
• CPC Class Codes: B41J 2/375 Protection arrangements against overheating

Abstract

A thermal imaging system for compensating for the temperature dependent changes in thermal elements of a thermal printhead is disclosed. Specifically, the thermal imaging system generates a temperature or energy dependent profile of the resistances of each thermal element which makes up the thermal printhead. Based upon this profile, the imaging system estimates the resistances of thermal elements based upon a pixel density to be transferred to media. Based upon the estimated resistances, the imaging system adjusts the amount of energy to be applied to particular thermal elements for transferring a pixel having a density which more closely approximates the density of a corresponding pixel in a desired image.

Claims

1. A method of calibrating a thermal printhead to be incorporated into an imaging system for transferring images to media by applying power to the printhead, the printhead having a thermal element, the thermal element having a resistance and a pixel imaging surface, the method comprising:

measuring the resistance of the thermal element at a plurality of temperatures or energy levels to provide an associated plurality of resistance measurements; and

establishing or maintaining a temperature or energy dependent profile for the thermal element based upon the associated plurality of resistance measurements,

wherein the temperature or energy dependent resistance profile varies over at least a portion of an operational temperature or energy range of the thermal element.

2. The method of claim 1, the method further comprising pre-aging the thermal element by applying energy to the printhead to stabilize resistive material which provides the resistance prior to measuring the resistance of the thermal element.

3. The method of claim 1, said measuring including:

applying one of a first current and a first voltage to the thermal element to maintain the thermal element at a first temperature or energy level;

measuring the resistance of the thermal element at the first temperature or energy level to provide a first associated resistance measurement;

applying one of a second current and a second voltage to the thermal element to maintain the thermal element at a second temperature or energy level; and

measuring the resistance of the thermal element at the second temperature or energy level to provide a second associated resistance measurement and further wherein

said establishing or maintaining is based upon said first associated resistance measurement and said second associated resistance measurement.

4. The method of claim 1, wherein said measuring further includes:

applying a set voltage across the resistance of the thermal element; and

measuring a current through the resistance of the thermal element in response to the set voltage.

5. The method of claim 1, wherein said measuring further includes:

providing a set current through the resistance of the thermal element; and

measuring a voltage across the resistance of the thermal element in response to the set current.

6. The method of claim 1, the method further comprising applying a pulse width modulation signal to the thermal element to change the temperature or energy level of the thermal element.

7. The method of claim 1, wherein the thermal element has a capacitance, the method further comprising applying a capacitive discharge of the capacitance of the thermal element to the thermal element to change the temperature or energy level of the thermal element.

8. A method of transferring an image to media from a thermal printhead, the printhead having a thermal element, the thermal element having a resistance and a pixel imaging surface, the method comprising:

estimating a level of energy of the thermal element based upon a density of pixels in a desired image;

estimating the resistance associated with the thermal element based upon the estimated level of energy and a temperature or energy dependent resistance profile associated with the thermal element, the temperature or energy dependent resistance profile varying over at least a portion of an operational temperature or energy range of the thermal element; and

calculating an amount of energy to be applied to the thermal element based upon the estimated resistance.

9. The method of claim 8, the method further including applying the energy to the thermal element to transfer an image onto thermal reactive media.

10. The method of claim 8, the method further including applying the calculated energy to the thermal element to transfer dye to media as part of a dye diffusion process.

11. The method of claim 8, the method further including applying the calculated energy to the thermal element to transfer wax to media as part of a thermal wax transfer process.

12. The method of claim 8, wherein the printhead has a plurality of thermal elements, and wherein said plurality of thermal elements are formed in a row such that the individual pixel imaging surfaces form a linear imaging surface, the method further including:

translating a media surface over the printhead in a direction which is substantially perpendicular to the row of thermal elements;

selecting individual thermal elements at discrete time intervals to provide an image on the media at a desired intensity; and

sequentially calculating an amount of energy to be applied to each of the selected individual thermal elements at the time intervals based upon the associated estimated resistance and the desired intensity or energy level.

13. The method of claim 12, the method further including sequentially applying the calculated energy to the selected thermal elements at the time intervals.

14. The method of claim 8, wherein the printhead has a plurality of thermal elements, each thermal element having a resistance and a pixel imaging surface, and the method further including selecting the thermal element from among the plurality of thermal elements based upon a digital representation of a desired image.

15. An imaging system for transferring an image to media from a thermal printhead, the printhead having a thermal element, the thermal element having a resistance and a pixel imaging surface, the imaging system comprising:

logic for estimating one of an energy level and a temperature to be applied to the thermal element;

logic for estimating the resistance associated with the thermal element based upon the estimated temperature or energy level and a temperature or energy dependent resistance profile associated with the thermal element, the temperature or energy dependent resistance profile varying over at least a portion of an operational temperature or energy range of the thermal element; and

logic for calculating an amount of energy to be applied to the thermal element based upon the estimated resistance.

16. The imaging system of claim 15, the imaging system further including a circuit configured to apply the calculated energy to the selected thermal element to transfer an image onto thermal reactive media.

17. The imaging system of claim 15, the imaging system further including a circuit configured to apply the calculated energy to the selected thermal element to transfer dye to media as part of a dye diffusion process.

18. The imaging system of claim 15, the imaging system further including a circuit configured to apply the calculated energy to the selected thermal element to transfer wax to media as part of a thermal wax transfer process.

19. The imaging system of claim 15, wherein the printhead has a plurality of thermal elements, and wherein said plurality of thermal elements are formed in a row such that the individual pixel imaging surfaces form a linear imaging surface, the imaging system further including:

a media feed configured to translate a media surface over the printhead in a direction which is substantially perpendicular to the row of pixels;

logic configured to select individual thermal elements at discrete time intervals to provide an image on the media at a desired intensity; and

logic configured to sequentially calculate an amount of energy to be applied to each of the selected individual thermal elements at the time intervals based upon the associated estimated resistance, the desired intensity and the estimated temperature or energy level of the selected thermal element.

20. The imaging system of claim 19, the imaging system further including logic for sequentially applying the calculated energy to the selected thermal elements at the time intervals.

21. The imaging system of claim 15, wherein the printhead has a plurality of thermal elements, each thermal element having a resistance and a pixel imaging surface, and the imaging system further including logic configured to select the thermal element from among the plurality of thermal elements based upon a digital representation of a desired image.

22. The imaging system of claim 15, the imaging system further comprising a circuit for applying a pulse width modulation signal to the thermal element to change the temperature or energy level of the thermal element.

23. The imaging system of claim 15, wherein the thermal element has a capacitance and further wherein the temperature or energy level of the thermal element is changed by discharging a current from the capacitance through the resistance of the thermal element.

24. A system for calibrating a thermal printhead to be incorporated into an imaging system for transferring images to media by applying power to the printhead, the printhead having a thermal element, the thermal element having a resistance and a pixel imaging surface, the system comprising:

a measurement circuit configured to measure the resistance of the thermal element at a plurality of temperatures or energy levels to provide an associated plurality of resistance measurements; and

logic establishing or maintaining a temperature or energy dependent resistance profile for the thermal element based upon the associated plurality of resistance measurements,

wherein the temperature or energy dependent resistance profile varies over at least a portion of an operational temperature or energy range of the thermal element.

25. The system of claim 24, the system further comprising a circuit configured to pre-age the thermal element by applying energy to the printhead to stabilize resistive material which provides the resistance.

26. The system of claim 24, the measurement circuit including:

a circuit configured to apply one of a first current and a first voltage to the thermal element to maintain the thermal element at a first temperature or energy level;

a circuit configured to measure the resistance of the thermal element at the first temperature or energy level to provide a first associated resistance measurement;

a circuit configured to apply one of a second current and a second voltage to the thermal element to maintain the thermal element at a second temperature or energy level; and

a circuit configured to measure the resistance of the thermal element at the second temperature or energy level to provide a second associated resistance measurement, and further wherein

said temperature or energy dependent resistance profile is based upon said first associated resistance measurement and said second associated resistance measurement.

27. The system of claim 24, wherein the measurement circuit further includes:

a circuit configured to apply a set voltage across the resistance of the thermal element; and

a circuit configured to measure a current through the resistance of the thermal element in response to the set voltage.

28. The system of claim 24 wherein the measurement circuit further includes:

a circuit configured to provide a set current through the resistance of the thermal element; and

a circuit configured to measure a voltage across the resistance of the thermal element in response to the set current.