System for sensing the temperature of a printhead in an ink jet printer

Abstract

The invention describes a temperature sensing system which provides a digital signal representative of the temperature of an ink jet printhead during a predetermined time period. In one embodiment, a temperature controlled oscillator is formed on the printhead, the oscillator including a resistor whose resistance varies proportional to printhead temperature variations. An analog signal from the resistor representative of printhead temperature is converted by the oscillator into a digital output signal whose frequency varies with variations in the resistor and, therefore, with the printhead temperature. The oscillator outputs are counted in an electrical counter over a sampling period. The accumulated count is converted into a digital code representing the printhead temperature. This byte is available for comparison with predetermined values stored in a look-up table and is converted into electrical image signals sent to the printhead which are adjusted and compensate for changes in the volume of ink ejected by the printhead nozzles. In a preferred embodiment, the oscillator counter and synchronizing circuitry are formed on one chip located on the printhead. The printhead can be either a single partial width printhead located on a carriage which is moved through a print zone or a full width printhead. The temperature sensing circuit has a high signal to noise ratio resulting in a more accurate temperature measurement. Because of the noise immunity, the sensing can take place during a print operation.