System for detecting defective thermal printhead elements

• US 4,595,935 A
• Filed: 08/14/1984
• Issued: 06/17/1986
• Est. Priority Date: 08/14/1984
• Status: Expired due to Term

• Current Assignee: NCR Canada Limited (NCR Voyix Corporation)
• Original Assignee: NCR Canada Limited (NCR Voyix Corporation)
• Inventors: Connell, Brian P., Brooks, Ralf M., Vyas, Arvindkumar C.
• Primary Examiner(s): Goldberg, E. A.
• Assistant Examiner(s): EVANS, ARTHUR G
• Application Number:
  US06/640,894
• Time in Patent Office: 672 Days
• Field of Search: 400/72, 400/54, 400/120, 346/76 R, 346/76 L, 346/76 PH, 346/139 C, 346/154, 219/216 PH, 219/482, 219/490, 219/492, 219/497-499, 219/506, 324/62-64
• US Class Current: 347/171
• 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 7/96 Means checking correctness of setting

Abstract

A system and method are disclosed for automatically detecting any defective elements in a linear array of thermal printhead elements of a thermal printer and for automatically correcting for at least one defective thermal printhead element. In a preferred embodiment of the invention an ON/OFF line from a microprocessor is turned OFF to turn off a voltage regulator, thereby preventing the thermal printhead elements from printing. With the voltage regulator OFF, a diode is forward biased to enable a voltage divider to be formed between a sensing resistor and any one of the thermal printhead elements that is activated at any given time. When a subsequent sense voltage (or resistance measurement) of an element exceeds the value of the initial reference sense voltage for that by a predetermined amount, that element is detected as a defective element. A software subroutine, in the microprocessor is then utilized to change the position of serial data bits carrying data to be printed such that the detected defective thermal element is not utilized during a normal printing operation.

Claims

1. In a thermal printer including a linear array of thermal elements, an apparatus comprising:

means for producing during a first mode of operation serial character data having a preselected number of character positons selectively associated with said thermal elements and for producing during a second mode of operation serial test data;

means coupled to said thermal elements being selectively responsive to said serial test data for developing for each of said thermal elements an associated test signal during each second mode of operation;

means for selectively comparing each test signal for each said thermal element against an associated predetermined reference signal in order to automatically generate an associated failure signal when a defective thermal element is detected during a second mode of operation; and

means responsive to a failure signal associated with a defective thermal element for automatically causing said producing means to change the character positions associated with said thermal elements so that said defective thermal element is located between adjacent character positions.

4. The apparatus of claim 1 wherein:

said comparing means utilizes a fixed predetermined signal as said predetermined reference signal to compare against each test signal of each of said thermal elements.

2. In a thermal printer including a linear array of thermal elements, an apparatus comprising:

means for producing during a first mode of operation serial character data having a preselected number of character positions selectively associated with said thermal elements and for producing during a second mode of operation serial test data;

means coupled to said thermal elements being selectively responsive to said serial test data for developing for each of said thermal elements as associated test signal during each second mode of operation;

means for selectively comparing each test signal for each said thermal element against an associated predetermined reference value in order to generate an associated faiure signal when a defective thermal element is detected during a second mode of operation; and

means for storing the initial test signals respectively derived by said developing means for said thermal elements during an initial second mode of operation, each said initial test signal derived for an associated thermal element being said associated predetermined reference value for said associated thermal element.

3. The apparatus of claim 2 wherein:

said storing means is a non-volatile memory.

5. In a thermal printer including a linear array of thermal elements, an apparatus comprising:

means for producing during a first mode of operation serial character data having a preselected number of character positions selectively associated with said thermal elements and for producing during a second mode of operation serial test data;

means coupled to said thermal elements being selectively responsive to said serial test data for developing for each of said thermal elements an associated test signal during each second mode of operation, said developing means including;

means for removing a first voltage from said thermal elements to prevent said thermal elements from printing during the second mode of operation;

voltage divider means responsive to said serial test data and to the removal of said first voltage for outputting a second voltage corresponding to the resistance of said thermal element being selected to be tested; and

means for converting said second voltage to a test signal associated with said thermal element being selected to be tested; and

means for selectively comparing each test signal for each said thermal element against an associated predetermined reference value in order to generate an associated failure signal when a defective thermal element is detected during a second mode of operation.

6. In a thermal printer including a linear array of thermal elements, an apparatus comprising:

processing means for producing during a first mode of operation serial character data having a preselected number of character positions selectively associated with said thermal elements and for producing during a second mode of operation a control signal and test data;

means being responsive to said serial character data for selectively applying signals corresponding to said serial character data to said thermal elements during the first mode of operation and for selectively applying signals corresponding to said test data to said thermal elements during the second mode of operation;

means being responsive to the absence of said control signal for enabling said thermal elements to be selectively energized in accordance with said serial character data and being responsive to the presence of said control signals for enabling said thermal elements to be selectively measured during each second mode of operation; and

means coupled to said thermal elements for developing for each of said thermal elements as associated test signal during each second mode of operation;

said processing means including means for selectively comparing each test signal for each of said thermal elements against an associted predetermined reference signal in order to automatically generate an associated failure signal when a defective thermal element is detected, and program means responsive to said failure signal associated with said defective thermal element for automatically causing the character positions of the serial character data to be changed so that said defective thermal element is located between that changed character positions of the serial character data.

7. In a thermal printer including a linear array of thermal elements, an apparatus comprising:

processing means for producing during a first mode of operation serial character data having a preselected number of character positions selectively associated with said thermal elements and for producing during a second mode of operation a control signal and test data;

means being responsive to said serial character data for selectively applying signals corresponding to said serial character data to said thermal elements during the first mode of operation and for selectively applying signals corresponding to said test data to said thermal elements during the second mode of operation;

means being responsive to the absence of said control signal for enabling said thermal elements to be selectively energized in accordance with said serial character data and being responsive to the presence of said control signals for enabling said thermal elements to be selectively measured during each second mode of operation; and

means coupled to said thermal elements for developing for each of said thermal elements an associated test signal during each second mode of operation;

said processing means including means for selectively comparing each test signal for each of said thermal elements against an associated predetermined reference value in order to generate an associated failure signal when a defective thermal element is detected, and program means responsive to said failure signal associated with said defective thermal element for causing the character positions of the serial character of data to be changed so that said defective thermal element is located between the changed character positions of the serial character data, and wherein said associated predetermined reference value is the initial test signal derived for each of said thermal elements and said comparing means selectively compares the initial test signals of said thermal elements with associated subsequent test signals of said thermal elements and generates a failure signal for a defective thermal element when one of the subsequent test signals associated with said defective thermal element exceeds the amplitude of the initial test signal associated with said defective thermal element by a preselected amount.

8. In a therma printer including a linear array of thermal elements, an apparatus comprising:

processing means for producing during a first mode of operation serial character data having a preselected number of character positions selectively associated with said thermal elements and for producing during a second mode of operation a control signal and test data;

means being responsive to said serial character data for selectively applying signals corresponding to said serial character data to said thermal elements during the first mode of operation and for selectively applying signals corresponding to said test data to said thermal elements during the second mode of operation;

means being responsive to the absence of said control signal for enabling said thermal elements to be selectively energized in accordance with said serial character data and being responsive to the presence of said control signals for enabling said thermal elements to be selectively measured during each second mode of operation; and

means coupled to said thermal elements for developing for each of said thermal elements an associated test signal during each second mode of operation;

9. said processing means including means for selectively comparing each test signal for each or said thermal elements against an associated predetermined reference value in order to generate an associated failure signal when a defective thermal element is detected, and program means responsive to said failure signal associated with said defective thermal element for causing the character positions of the serial character data to be changed so that said defective thermal element is located between the changed character positions of the serial character data, and wherein said associated predetermined reference value is a fixed predetermined value and said comparing means selectively compares each test value of each of said thermal elements with said fixed predetermined value and generates a failure signal for a defective thermal element when one of the subsequent test signals associated with said defective thermal element exceeds the amplitude of the fixed predetermined value.

10. In a thermal printer system including a linear array of thermal elements and storage circuitry having character positions associated with said thermal elements, an apparatus comprising:

means for selectively developing for each of said thermal elements an associated initial reference signal and an associated test signal during each of a sequence of test periods;

means responsive to the associated initial reference signal for each test signal for each of said thermal elements for automatically generating an associated first signal when a defective thermal element is detected; and

means responsive to said first signal associated with said defective thermal element for automatically changing the character positions associated with said thermal elements so that said defective thermal element is located between character positions.

11. In a thermal printer system having a plurality of storage circuits for serially storing zero and one state bits of character data, a plurality of heater elements, and a plurality of circuits selectively enabled by a common strobe pulse and by each one state bit from the plurality of storage circuits for selectively enabling the plurality of heater elements in a thermal printhead to thermally print during an operational mode;

an apparatus for automatically detecting and compensating for at least one defective heater element, said apparatus comprising;

processor means for generating during an operational mode of operation serial data comprised of groups of character data bits separated by a preselected number of zero state bits and for generating during a test mode of operation a control signal and a one state test bit to be serially shifted through said plurality of storage circuits; and

first means responsive to the initial said control signal for producing and storing in said processor means an initial test signal for each of said plurality of heater elements as the initial one state bit is shifted through said plurality of storage circuits during the initial test mode of operation, said first means being responsive to each subsequent control signal for producing a subsequent test signal for each of said plurality of heater elements as the associated subsequent test bit is shifted through said plurality of storage circuits during each subsequent test mode of operation;

said processor means receiving said test signals and including second means for producing a failure signal when a subsequent test signal for one of said heater elements deviates from the associated initial test signal for said one of said heater elements by a preselected amount, and third means responsive to said failure signal for positioning the serial data during an operational mode such that said one of said heater elements is not utilized in subsequent thermal printing.

12. A method for automatically detecting any defective thermal element in a linear array of thermal elements in a thermal printer, said method comprising the steps of:

15. The method of claim 12 wherein said determining step comprises the step of:

selecting a fixed predetermined signal to be commonly used as the associated reference signal for each thermal element.

13. determining an associated reference signal for each thermal element;

storing the associated reference signal for each thermal element;

measuring a test signal for each thermal element during each of a sequence of test modes of operation;

comparing each test signal for each thermal element with its associated reference signal during each test mode of operation;

detecting a defective thermal element when a test signal for that defective thermal element deviates by a preselected amount from the associated reference signal for that thermal element during said comparing step; and

automatically changing the position of serial data bits carrying data to be printed during a normal thermal printing operation such that the detected defective thermal element is not utilized during the normal printing operation.

14. A method for automatically detecting any defective thermal element in a linear array of thermal elements in a thermal printer, said method comprising the steps of:

determining an associated reference signal for each thermal element, said determining step comprising the step of measuring an associated initial test signal for each thermal element during an initial test mode of operation in order to determine the associated reference signal for each thermal element;

storing the associated reference signal for each thermal element;

measuring a test signal for each thermal element during each of a sequence of test modes of operation;

comparing each test signal for each thermal element with its associated reference signal during each test mode of operation; and

detecting a defective thermal element when a test signal for that defective thermal element deviates by a preselected amount from the associated reference signal for that thermal element during said comparing step.