This invention relates to fluid level sensing and more particularly to ink tank level sensing.
Ink level detection in a printhead is required in printing systems where the main volume of liquid ink is stored in a reservoir away from the printhead. In order to perform full color printing, four kinds of inks, i.e., cyan ink, magenta ink, yellow ink and black ink, must be used. Accordingly, color printers may include four different fluid reservoirs, one reservoir for each type of ink. As the printhead consumes ink, the reservoirs periodically need to be refilled. Sensors are used to detect whether or not the printhead has adequate ink.
There are numerous methods by which liquid ink detection has previously been performed. Most of these methods rely on the electrical conductivity of the ink and use the ink to complete a “sensing” circuit. In these systems the reservoir containing the ink is frequently made of a conductive material and forms part of the circuit. A probe made of conductive material, either a metal protrusion insulated from the reservoir or a conductive pad on an insulated circuit board, is used as the sensor and the ink bridges the space between the probe and the reservoir to complete the circuit.
These sensing systems suffer from various shortcomings. For example, the systems typically have limited sensitivity leading to inaccuracies and some systems are unable to detect various inks, particularly those with low levels of conductivity.
Thus, printers having sensing systems with good sensitivity or that sense an ink level without relying on the conductive properties of the reservoir containing the fluid would be beneficial.