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Capacitive Ink Level Detector

IP.com Disclosure Number: IPCOM000080994D
Original Publication Date: 1974-Mar-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 3 page(s) / 38K

Publishing Venue

IBM

Related People

Phillips, DW: AUTHOR

Abstract

A low-ink level in an ink jet printer reservoir is sensed without directly contacting the ink. The ink serves as one plate of a variable capacitor; variable in that as the ink level falls, the plate area decreases and hence, the capacitance decreases. A typical previous capacitive level detector used parallel plate capacitors with an ink column between. Since the ink was at ground potential, when the ink was high it blocked the passage on any signal. When the ink fell from between the plates a capacitor was formed. This capacitance was extremely small because of the thick air dielectric necessitated by the ink column space. A very high-frequency, high-level input signal was needed because of the small capacitance.

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Capacitive Ink Level Detector

A low-ink level in an ink jet printer reservoir is sensed without directly contacting the ink. The ink serves as one plate of a variable capacitor; variable in that as the ink level falls, the plate area decreases and hence, the capacitance decreases. A typical previous capacitive level detector used parallel plate capacitors with an ink column between. Since the ink was at ground potential, when the ink was high it blocked the passage on any signal. When the ink fell from between the plates a capacitor was formed. This capacitance was extremely small because of the thick air dielectric necessitated by the ink column space. A very high-frequency, high-level input signal was needed because of the small capacitance.

In the present case, the dielectric is the bottle wall itself and the bottle material has a dielectric constant which is three to four times that of air; hence, a much larger capacitance is formed. The present detector can easily work with a relatively low-level signal such as a 5 volt, 100 kHz input signal. The structures include a plastic blow-molded reservoir bottle 1 that is sealed in so far as level detection circuitry is concerned, except that the ink 4 is maintained at a ground potential. Bottle 1 is disposed of once its ink supply is exhausted, and a fresh one inserted.

Two conductive plates 2 and 3 are placed against the outside of the bottle so that plate 2 is located higher than plate 3. Upper plate 2 is also made larger than lower plate 3. The plates can be plated or painted onto the bottle, or they can be separate from the bottle and come into intimate contact with it when it is inserted into the machine. Each of the plates 2 and 3 serves as one of the plates of two capacitors. The bottle wall is the dielectric and the ink 4, which is grounded, acts as t...