This invention relates to multi-channel array droplet deposition apparatus and, more particularly, to a method of operating such apparatus of the kind comprising an array of parallel channels, respective nozzles communicating with said channels for ejection of droplets of liquid from the channels, droplet liquid supply means connected with the channels and electrically actuable means located in relation co said channels to impart energy pulses to respective selected channels for effecting droplet ejection from the nozzles of the channels selected. A particular case of droplet deposition apparatus of the kind sec forth is the, so-called, drop-on-demand ink jet printhead. The need exists to print ink dots in response to electronic print data at a high resolution, less than is readily resolved by the eye at a convenient reading distance. Many types of ink jet array have been proposed including U.S. Pat. No. 4,296,421 which operates on the thermal bubble jet principle and U.S. Pat. No. 4,584,590 which discloses one form of piezo-electric shear mode activated array. A further type of shear mode actuated array in which piezo-electric shear mode actuated channel dividing walls are employed is disclosed in U.S. Pat. Nos. 4,879,568 and 4,887,100 assigned to the assignee.
In the piezo-electric shared wall actuator array disclosed in U.S. Pat. No. 4,887,100 it is preferred that the-wall actuators are compliant, firstly because this leads to a higher linear density of channels and therefore assists to produce a high print resolution. A further reason is that the transduction of energy from the actuating voltage to pressure in. the ink channels and subsequently to the ejection of ink from the nozzle to form drops is most efficient when the walls are compliant. In this type of wall actuator it may accordingly be chosen in order to satisfy this condition that the value of: ##EQU1## is in the range 0.2&lt;K&lt;2. The operating state requiring maximum operating voltage occurs when all the odd (or even) numbered channels are actuated. The minimum value of this voltage occurs when K=0.5. It was also apparent that crosstalk between channels increases as the compliance increases. It is important that an ink droplet should be ejected only from those channels that are selected for printing and that pressure developed through crosstalk is maintained safely below the level that might cause a spurious drop to be ejected. In U.S. Pat. No. 4,887,100 (Col. 5, L40-50 and Col. 15, L15-23), it was indicated that there would generally be a limiting compliance where crosstalk would make operation impractical. However, a method was described therein by reference to FIG. 9 whereby crosstalk could be eliminated mechanically and operation could then take place without regard to the effect of compliance on crosstalk.
It was also recognised that crosstalk due to wall compliance could, in principle, be compensated by choosing an appropriate array of voltage values and a method of generating such voltage values is disclosed in U.S. Pat. No. 5,028,812.
The presence of crosstalk due to wall compliance in ink. jet printheads which are constructed with inactive walls between adjacent ink channels has not been reported in the literature. Such printheads include the thermal bubble jet and piezo-electric roof mode constructions. The absence of reports of crosstalk in these cases could be attributable to constructions in which the walls between adjacent channels are substantially rigid. In that case the channels are more widely separated than is necessary. After adopting the results of the present invention higher density array printheads substantially free of crosstalk can be constructed.
Compliant crosstalk, however, is disclosed in U.S. Pat. No. 4,381,515. This reference describes moreover a method of compensating for what is referred to as both positive and negative crosstalk, by introducing a network of compensating passive resistors. This proposal however is not applicable to the type of array disclosed in U.S. Pat. No. 4,887,100, since this array incorporates capacitors (representing the piezo-electric actuators), which are in parallel with the actuating signal lines. In U.S. Pat. No. 4,381,515, the actuators are in series with the signal lines. Nor is it relevant to arrays such as U.S. Pat. No. 4,296,421 where the actuating elements are resistive elements.