An ink-drop sensor has been developed for use in ink-jet printers so that the function of the multinozzle printhead can be checked before printing starts or cyclically during printing. If the sensor detects that one or more nozzles have failed, the printhead can be restored to correct operation in a service station. This process, which is completely automatic and requires no intervention on the part of the user, increases the reliability of the ink-jet printer. The sensor principle utilizes the electrical conductivity of the ink. When ink droplets from any nozzle in the printhead are ejected onto comblike electrodes, conductive links are established between the prongs of these electrode combs, and changes in resistance can be measured at the sensor terminals. These changes in resistance are then converted in a signal-conditioning circuit into digital voltage signals. The author also discusses modified versions of the sensor suitable for special applications such as measuring the flight time of ink droplets and determining print position errors.
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