Issue #0823/2 - Designed to improve the accuracy of gas gauge information presented to inkjet printer users, Hewlett-Packard has introduced a direct detect system of determining when the ink in a cartridge is exhausted. The system currently works only on the new HP364 pigment black cartridge, used in the new Photosmart D5460 printer (available August 2008 – ), at this stage.
A very worthwhile development, other manufacturers have used this method for nearly a decade, and perhaps longer. For instance, Sharp and Xerox used this technique in 2000. More recently, Canon’s PGI-5BK, CLI-6 and CLI-8 series all use optical sensing; as do Brother’s cartridges (optical mechanical).
Optical Sensor, Xerox M760All that is required is an optical sensor in the printer and provision to be made in the cartridge to serve up the relevant visual information to the sensor.
Most commonly, the cartridge has a prism of some sort moulded in the plastic of the cartridge body so that the presence of ink in the cartridge prevents light from reaching the optical sensor but, as soon as light reaches the sensor, indicating that there is no ink left in the prism area of the cartridge, the ink out message is triggered and the printer stops printing.
In its PGI-5BK and CLI-8 cartridge series, Canon has combined the use of the optical system with an on-cartridge chip that remembers that the cartridge has been emptied, thus preventing the user from simply refilling the tank and reinserting it.
In Brother’s case, a different system is used that incorporates a mechanical/optical device. The printer itself is still fitted with an optical sensor but the ink cartridge contains a mechanical lever arm with a ‘float’ at one end and a flag at the other end. In normal usage, the level of ink in the tank causes the float to force the flag down at the other end of the arm. Once the level of ink in the tank drops to a point where the float falls to the bottom of the tank, the flag rises, thus triggering the optical sensor.
Although having your printer suddenly stop printing because one cartridge is empty can be annoying, the benefit is that there is absolutely no doubt about the status of the cartridge and there is no danger of materials being wasted as a result of the ink running out half way through printing a document or photo.
Traditionally, Hewlett-Packard’s (and Lexmark’s) ink cartridges have relied on a drop-counting system that estimates the number of drops that the given quantity of ink can produce and matches that information against a count of drops fired for the documents printed. Relying on estimates, these systems are notoriously unreliable and cause frustration to users because of the inaccurate forewarning of an ink low status and wasted materials as cartridges run dry (especially if it is expensive photo paper!).
Because the ink itself is not being monitored, the ink low message is usually given well in advance of the cartridge being exhausted (often at least 20% and in Lexmark’s case, significantly more). Users can continue printing but there is no further alert to indicate that the ink is actually finished and, unless users discard the cartridge very early (thus wasting ink), the definitive ink-out comes only in the form of those failed prints – so both paper and time are wasted because the print job (or even multiple jobs) need resending with a fresh cartridge and fresh paper.
Another frustration for users is the knowledge that ink is being wasted because it is inaccessible within the cartridge. This applies to systems where drop counting is used but a safety mechanism is built-in to prevent the cartridge running dry and damaging the print head. With these cartridges, ink splashing around inside can often be heard when shaking a supposedly empty cartridge.
With most systems employing the optical detection method, it is possible to see that the cartridge really is empty and that there is no excessive ink wastage – an important customer relations factor!
Because the half of the cartridge that is adjacent to the cartridge/print head orifice is filled with foam, while the other half of the cartridge is an open tank, there is no danger of the print head being allowed to run dry when the sensor detects that there is no ink in the open side of the tank. There is always sufficient ink contained in the foam to protect the print head.
Brother’s system does not use foam but the physical design of the cartridge means that there is no danger of the print head running dry either.
Why Hewlett-Packard has implemented the system only on the pigment black cartridge for text printing is something of a mystery. However, the fact that the company has introduced a direct detect system at all is recognition of the fact that unsatisfactory prints are the result of an ink out system that relies on drop counting to estimate when the cartridge is running low (or rather, lack of ink out system), and that this is unsatisfactory for customers.
Hopefully, the direct detect system will be extended to include colour cartridges as well – after all, a colour cartridge running out mid-photo is far more wasteful and annoying than a black cartridge running out mid-text document! In our testing programmes, the drop-dead ink-out systems have proved to be more user-friendly by a very large margin than drop-counting, run dry systems.
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