Identifying missing nozzles

US 2014/0085369 A1
Method of identifying defective nozzles in an inkjet printhead 
Zamtec Ltd

Patents previously led by silverbrook Research are now led and held by Zamtec Ltd, also known as Memjet IP holdings Ltd. The Memjet page array printhead has over 70,000 nozzles and it is important to be able to detect any nozzle failures so that any necessary corrections can be made.

Mar-14a The conventional way to carry out the detection of missing nozzles is to print a line of drops 101 from the nozzles. if a nozzle isn’t working then a blank space 104 is left. The spacing between nozzles 103 that are simultaneously printing is determined by the resolution of the scanning system that will evaluate the image. In addition spaces 102 are left between the ends of one array of nozzles and the next to discriminate between the line segments printed by different groups of nozzles.

There are several problems with this method. The pattern is very low density, with most of the printed test area empty, and is therefore inefficient. in addition the nozzles are being driven in an unrealistic way which may result in some artefacts not being recorded. For instance poor nozzle refill rates may only be apparent when adjacent nozzles are also generating drops, which will never occur with this test pattern. Also misdirected jets may lead to misinterpretation as to which nozzle’s behaviour is defective.

The solution is to use a more complex test pattern where the pixel pattern generated is encoded and then later decoded. A Hadamard matrix is used for this purpose. This is a square matrix with entries only either 1 and -1, and where the sum of each row or column (except for the first ones) is zero.

Mar-14b
Above an example pattern is shown. The nozzles across the array are divided into a number of cells. In addition a secondary scheme known as a Maximal length sequence or M-sequence is used. The values of these are shown above the pixels; 4 rows of pixels are shown. Note that every pixel sequence for a nozzle prints two drops, so the duty cycle is 50%.

Mar-14c
Here a section of a sequence is shown. After printing the image is scanned and analysed. If a nozzle isn’t working then no pixels are printed, as with the 4th nozzle from the left. If a nozzle is firing intermittently, such as nozzle 8 from the left, then this will also be detected.