Integrating the printhead into the HP DeskJet printer - Hewlett-Packard - technical

Integrating the Printhead into the HP DeskJet Printer

THE HP DESKJET thermal inkjet printhead requires a higher level of support from the printer than earlier generations. There are more nozzles to drive, they have to be driven faster, and more electrical connections have to be made to the head. Smaller nozzles with fast-drying, plain-paper ink require protection to prevent the head from drying out and mechanisms to recover nozzles that have clogged. And like earlier disposable thermal inkjet printheads, a carriage is needed to move the printhead across the paper.

Several overall design constraints guided the design of the printhead support systems. Each element had to meet the longevity goals set for it, perform its task, be robotically assemblable, and be low in cost. To meet the last two goals our project attempted as much as possible to design the carriage mechanism for top-down assembly and minimum part count.

Head Drive Electronics

The printer circuitry to interface with the printhead is located on a printed circuit board mounted on the carriage mechanism. Two custom ICs on the board drive the printhead, with each IC driving half (Fig. 1). Each driver IC contains two 4-to-13-line decoders and has four address inputs, which are shared by both decoders in the IC, and two enable inputs, one for each decoder. The four address inputs indicate which of the thirteen outputs of each decoder will be turned on if its enable input is selected.

The decoder arhitecture only allows four dots to be fired at a time. It is necessary to minimize variations in the energy dissipated in the printhead thin-film resistors to maintain good print quality, so we did not want varying voltage drops across the commins between the power supply and the printhead when firing resistors. This requires that there be a separate common for each resistor when several are energized simultaneously. Fifty commons would have been impractical from a cabling standpoint, and one would have been impractical from a timing standpoint, so as a good compromise, four commons are provided. Since only four nozzles can be fired at a time, decoders are used to minimize the number of connections between the main board and the head driver board.

It takes approximately 120 microseconds to fire all 50 nozzles in letter-quality mode. The distance moved during this time could produce a noticeable skew in the column, so the nozzle positions in the head are skewed to compensate. The cycle time for dot firings is cut in half when printing in draft mode (at twice the normal speed) so that the skew compensation is correct at both speeds. The nozzles are fired in reverse order when printing backwards.

There are three other functions on the head driver board. The first of these is the continuity test circuit. This circuit monitors the voltage drop across resistors in series with the printhead commons to determine if current flows when a driver is activated. A comparator monitors the voltage drop across each series resistor, and its output goes low if the current through the printhead is high enough. The comparator's output is monitored by the microprocessor on the main board during self-test, and it will print out the numbers of any nozzles that have bad continuity. This allows the user to determine whether a nozzle failure is caused by a resistor failure in the printhead.

In addition to the fifty drive lines and four commons going to the printhead, there are two sense lines that are used for encoding a printhead ID. These lines are mask-programmable on the printhead, and they will be used in the future to sense different types of printheads that might be installed in a printer. These ID sense lines normally connect to two of the printhead drive lines so that when the drive line is pulled low, the sense line connected to it will also go low. If the sense line is open, however, it will not go low when the drive line is activated. In this way, the printer can detect whether the ID sense lines are open. The two ID sense lines are wire-ORed together and a single line is fed back to the microprocessor on the main board.

An optointerrupter mounted on the head driver board detects the presence of paper in the paper path. A vane with a window normally rests in a position where the infrared light from an LED in the optointerrupter shines through the window in the vane and is detected by a phototransistor. When paper is in the paper path, the vane moves up so that the light from the LED is blocked and the phototransistor is turned off. The output of the phototransistor is fed back to the main board, where it is detected by the microprocessor.

Printhead Energy Window Budget

The amount of energy dissipated in the thin-film resistors of the printhead is critical to both print quality and printhead reliability. If too little energy is delivered to the printhead, the print quality will be poor. On the other hand, if too much energy is delivered to the printhead, the printhead life will be reduced. There is a narrow window where good print quality is achieved without adversely impacting the printhead life.

There are several factors that can impact the amount of energy dissipated in the thin-film resistors when they are energized. The amount of energy dissipated in the resistors is: E = I.sup.2.R.sub.tf.t.sub.fire., where I = (V.sub.s - V.sub.sat.)/(R.sub.tf + R.sub.trace + R.sub.source + R.sub.cap.)

R.sub.tf is the printhead thin-film resistance, V.sub.s is the head supply voltage, V.sub.sat is the driver saturation voltage, R.sub.trace is the stray resistance on the head driver board, flex circuit, and printhead, R.sub.source is the resistors in series with the printhead commons, R.sub.cap is the effective series resistance of the head power supply output capacitor, and t.sub.fire is the time during which current flows through the printhead. Variations in any of these parameters will cause the energy dissipated in the printhead to vary. To complicate matters further, the energy required to produce good print quality (the turn-on energy) varies from printhead to printhead.

In addition to specifying tight tolerances on components, two specific measures have been taken to minimize parameter variations and their effects. The first step is incorporation of temperature compensation into the head driver ICs. The ICs are designed so that the temperature coefficient of the output saturation voltage tracks the temperature coefficient of the output storage time, so that as the saturation voltage increases, decreasing the dissipation in the drivers, the storage time also increases, increasing the dissipation. The temperature variations of these two parameters cancel each other, resulting in zero variation in energy with driver temperature.

The second step taken to minimize energy variation is the use of resistors in series with the printhead commons. As mentioned previously, source resistors are placed in series with the printhead commons to sense current flowing through the printhead. The principal reason for having these resistors, however, is to minimize the variation of applied energy with changes in printhead resistance. The change in energy dissipated in the printhead resistors when their resistance changes is at a minimum when the sum of all other resistance in the circuit is equal to the resistance of the printhead resistors. The sum of the other resistances is not quite equal to the printhead resistors, but having the source resistors helps reduce the change in printhead energy when the printhead resistance changes.

Monte Carlo Model

Normally, a consevative designer will use worst-case analysis to ensure that a design will perform satisfactorily under all conditions. However, selecting head drive circuit parameters to ensure that all printheads receive the minimum required energy under worst-case conditions would have resulted in a nominal energy high enough to shorten the life of the printhead. Since the variables affecting the energy delivered to the printhead are independent of each other, it was recognized that the probability of worst-case conditions occurring is very small. We decided to use Monte Carlo analysis to determine the nominal circuit values. Monte Carlo analysis uses a statistical approach to determine the distribution of a variable based on the distributions of its constituent parts.

The first step in Monte Carlo analysis is the creation of a model of the circuit. In our case, this model is the equation for printhead energy given in equations 1 and 2, along with equations based on the various printhead process parameters that affect turn-on energy. The second step in Monte Carlo analysis is determining the distributions of the variables in the model. In our analysis, a normal (Gaussian) distribution was assumed for all but two of the variables. These two variables are printhead resistance and printhead turn-on energy, which we test during the head assembly process. If a printhead is outside of preset limits on either of these parameters it is discarded, resulting in a normal distribution with the tails of the distribution cut off.

Once the model has been constructed and the distributions of the model variables are known, simulations can be run to determine the distribution of the variable of interest. We were interested in monitoring the distribution of the ratio of applied energy to turn-on energy, which is a key indicator for both print quality and reliability. The simulation involves "building" a printer and printhead by picking parameter values at random using their distributions, calculating the applied energy and turn-on energy using those values, and tracking the distribution of the ratio of these two energies. The two truncated distributions are easily simulated by checking the parameter values after they have been selected and before they are used in the model. If a parameter's value falls outside the allowable limits, that value is discarded and a new value is calculated. This process allowed us to set the nominal energy high enough that virtually all printheads and printers will work together, while keeping the nominal energy low enough to avoid impacting printhead life.

Printhead Interconnect

For the HP ThinkJet printer, HP developed a bumped flex interconnect technology that has proved very successful in practice. After examining other alternatives for the DeskJet printer, the design team settled on the same basic design expanded from 12 connections to 56. In the ThinkJet printer, the connections are made to gold-plated pads on the glass chip that contains the resistors that fire the drops of ink at the paper. For the DeskJet printer, a silicon chip with 56 of these connections would have been too large to be cost-effective. In addition, the large chip would have placed unacceptable constraints on paper path desing, because it has to lie almost in a plane with the nozzles, which have to be very close to the paper.

After careful analysis, a design employing gold pads on a tape automated bonding (TAB) flex circuit was executed (see article, page 55). The TAB circuit pads attach to the chip pads and the TAB traces fan out to the bottom of the printhead, where they are contacted by the bumped flex circuit on the carriage, which is shown in Fig. 2. Using this technology brings the pen cost to an acceptable level and frees paper path design considerably, because the interconnections, though actually larger in area, can be wrapped around to the bottom of the pen where they are out of the way.

Pen Maintenance

One of the major challenges the project team faced was making trade-offs associated with printhead maintenance. Since the head was being developed concurrently with the printer there were many unknowns. Constant contact with HP's Inkjet Components Operation (where the printhead designers reside) was maintained and proved invaluable to the creation of a successful product.

The product had five main objectives for printhead maintenance:

* Prevent the nozzles from drying out

* Keep paper dust off the nozzles while not printing

* Wipe off any paper dust that might accumulate during printing

* Provide a location for purging viscous plugs from the nozzles before beginning print

* Provide a method for the user to clear a dried-out or plugged head without taking the pen out of the printer. The service station that accomplishes these objectives is shown in Figs. 3 and 4.

A rubber cap that seals to the orifice plate is provided to protect the nozzles. This cap prevents the nozzles from drying out and covers the nozzles while they are inactive, keeping them from being covered with paper dust. The cap is brought into contact with the printhead by a combination of carriage motion and ramps molded into the dc servo motor mount. The ramps raise the cap as the carriage goes to its parked position.

An elastomer tube attached to the underside of the cap provides a long diffusion path while venting the cap. The diffusion path maintains high humidity in the cap. The venting prevents the cap from acting as a bellows and blowing bubbles into the nozzles as the head comes to rest on the cap. The tube also serves as a primary component in the peristaltic used to service the printhead. If the nozzles are dried out (from sitting in a desk drawer uncapped, for instance) or a stubborn piece of paper dust is clogging nozzles, the user can actuate this pump to draw about 0.05 ml of ink from the head. This flushes the nozzles so the head can be returned to service.

The mechanism that performs the pump function with the tube is located near the end of the tube. The DeskJet pump employs only one roller rather than the traditional three that are usually used for peristalsis. This allows the tube to be vented to atmosphere when the pump is not operating without the need of a separate vent in the system. Ink from the pump is dumped into an absorber located in the bottom half of the printer, from which it evaporates.

A soft rubber wiper is built into the printer as a means of clearing from the nozzles any paper dust that might accumulate during printing. The paper dust is scraped into pockets on either side of the printhead. Thus, every time the printhead is changed, the accumulated paper dust is thrown out with the old printhead.

Since printhead service is an ancillary function of the printer, low cost was an absolute must. This was achieved by implementing a minimal-part-count passive desing. There are no motors dedicted to printhead maintenance. The pump is powered through a carriage-actuated transmission using the paper drive motor, and the cap is automatically engaged when the carriage comes home after printing. As much as possible, elements required for these functions are integrated with structural elements of the chassis. Thus, a peristaltic pump is molded into the main frame and the capping function is achieved with ramps molded into the dc servo motor mount.

The design team was aided in making this integration by HP ME Series 10, a computer-aided design tool from HP, which was effectively used to study interactions at an early phase of the project. The DeskJet mechanism team was one of the first sites in the corporation to begin using ME Series 10. (The plastic part on the cover of the May 1987 Hewlett-Packard Journal is the main chassis for the DeskJet printer.) The geometries of many of the parts in the product contain inputs from more than one negineer. Managing ongoing development of parts when so many are multifunctional and have several designers is made easier when ME Series 10 is available.

Carriage

The DeskJet printhead provides 300-dpi resolution. Taking advantage of this resolution requires a mechanism capable of locating the printhead accurately. Keeping track of tolerances was unusually important in the design of the printhead carriage, which is shown in Fig. 5.

The interconnect described above is contained in the carriage. Good connection between the printhead and the head drive circuit requires between three and five pounds of force. This load level is sufficient to deflect thin plastic parts more than can be tolerated, and here a problem was realized. The top-down manufacturability requirement allows little space for carriage alignment points. Carriage acceleration goals require relatively thin walls in the carriage structure for low mass. Thus, little plastic can be used for interconnect load support points. Since these points are also the head alignment points, stiffness is vital in whatever structure is used. To convince ourselves that it was indeed possible to satisfy these requirements, a linear static finite element model was created using HP-FE, and the carriage design is based on the results of that computer model. Several members of the team participated in the design of the carriage during various phases of its development and ME Series 10 was extremely useful in handing off the geometry from engineer to engineer with a minimum of documentation.

Carriage bearing life was also a concern. The DeskJet carriage employs molded-in-plastic bushings riding on a polished steel rail. This is both a cost-saving measure and a way to minimize carriage-to-shaft tolerances. The design is laid out to prevent undue bearing loads caused by cocking. The success of this design was proved early in the development cycle by an accelerated test bed, which ran ten carriages to over twice the projected printer life.

Helping the customer intuitively understand how to install a printhead was another design goal for the carriage. Several days of brainstorming and many failed designs led to the current implementation, which in one test was rated by users as being easier to use than the power switch. A funnel-shaped chute to drop the pen into, mating geometry between the printer and the printhead, ad color coding all play a role.

Acknowledgements

The head driver IC was designed by Plat Byrne and Dan Nguyen at the HP Santa Clara Technical Center, and Chuck Jarvie was responsible for procurement. Mark Lund developed the continuity test circuitry. Henry Flournoy and Bill Royce at the HP San Diego Division helped specify the head driver IC characteristics and developed the idea of using source resistors. Early carriage conceptualization was done by Dave Pinkernell. The DeskJet mechanism team is indebted to Brian King for his work on carriage development.

COPYRIGHT 1988 Hewlett Packard Company
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