Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
1999-08-13
2001-12-04
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S014000
Reexamination Certificate
active
06325481
ABSTRACT:
TECHNICAL FIELD
This invention relates to thermal printers, and more particularly to pen temperature control in thermal printers.
BACKGROUND
Computer technology is continually advancing, expanding the need for computers in the personal, business, and academic fields. As the need for computers has grown, so too has the need for various peripheral devices for use with computers, such as printers. One type of printer that can be used with computers is referred to as a “thermal” printer. A thermal printer uses a “pen” to apply small droplets of ink to paper to generate a printed version of data (whether it be text pictures, etc.). In a thermal printer, the amount of ink in each droplet is dependent on, among other factors, the temperature of the pen. When the pen is too hot, the droplets are too large, whereas when the pen is too cold, the droplets are too small. Thus, the temperature of the pen in a thermal printer should be regulated carefully in order to achieve an acceptable level of print quality.
The temperature of the pen in a thermal printer can be regulated by supplying energy to the pen in order to heat it, a process referred to as “pulse warming”. In order to determine when the pen should be heated, the current temperature of the pen is compared to the temperature the pen should be at for printing (the target temperature). The target temperature is typically hotter than the normal “room” temperature that the printer is located in. If the pen is too cool then pulse warming is used to heat it. If the pen is too hot, then the system waits for the pen to cool down.
In order to accurately compare the current pen temperature to a target temperature, an accurate target temperature must be available to the printer. Various circuitry can be included in a printer to identify a target temperature. However, due to variations in the fabrication process of the circuitry used in the printer, the actual target temperature identified by the circuitry may vary from printer to printer. Such differences in actual versus designed target temperature can affect the print quality of the printer and, if large enough, can actually prevent printing.
One solution to this problem is for each printer to use a channel of a digital to analog converter to generate indication of the target temperature. However, situations can arise where adding a digital to analog converter, or adding an additional channel to a digital to analog converter, is not a desirable option (e.g., there may be design time constraints or hardware cost constraints that make such an addition unattractive). Therefore, it would be beneficial to be able to accurately identify the target temperature without requiring such an additional digital to analog converter or converter channel.
The invention described below addresses these and other disadvantages of the prior art, providing improved pen temperature control in a thermal printer.
SUMMARY
The pen temperature control in a thermal printer is calibrated without requiring the use of an additional digital to analog converter or converter channel. A pen controller in the thermal printer generates a pulse width modulated (PWM) signal based on a reference voltage. The PWM signal is passed through a filter, resulting in a first voltage that varies with the duty cycle of the PWM signal. Additionally, a temperature sensing resistor on the pen generates a second voltage that varies with the temperature of the pen. A comparator is used to compare the first and second voltages to determine when the pen is at a proper temperature for printing.
The actual reference voltage in a printer may vary from its intended value due to variations in the fabrication process used to create the components of the printer. The printer is calibrated to account for such variations by changing the duty cycle of the PWM signal (thereby changing the first voltage) while the second voltage remains relatively constant. The comparator compares the first and second voltages and identifies when the duty cycle results in a first voltage that matches the second voltage. The pen controller can read, via an analog to digital converter channel, the value of the second voltage. Based on both the value of the second voltage at the point where it matches the first voltage, and the duty cycle at that point, the pen controller can calculate the actual reference voltage.
According to one aspect of the invention, an additional refinement or verification process is also used in determining the actual reference voltage. The duty cycle of the PWM signal is set to a new value that results in a first voltage higher than the second voltage. The pen is then warmed, resulting in an increase in the second voltage. At the point where the comparator determines that the first voltage again matches the second voltage, the second voltage and the new value of the duty cycle are used to calculate the actual reference voltage. This reference voltage may be used to verify the previously calculated reference voltage, or alternatively can be used in combination with the previously calculated reference voltage to determine the actual reference voltage.
REFERENCES:
patent: 4015192 (1977-03-01), Koyanagi
patent: 4675695 (1987-06-01), Samuel
patent: 5363134 (1994-11-01), Barbehenn et al.
patent: 6174038 (2001-06-01), Nakazawa et al.
Bliley Paul D.
Pietz Greg P
Thomsen Carl E.
Zarafshan Arezou
Barlow John
Hewlett--Packard Company
Stewart Jr. Charles W.
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