HP 1200C User's Manual
Contents
1. Predicted Distribution Fig 7 The measured media advance down the length of one page of a single machine is shown along with the predicted accuracy cal culated with measurements of the drive components as inputs to the model which is shown as a distribution The actual output of the model is the mean and standard deviation for a given machine standard deviation which were compared with the model First components were measured and used to build a set of machines Then the swath advance of the machines built with these components was measured Finally the statistics of the measured swath advance were compared with the results from the model with the measured components as inputs Fig 7 also shows the results of the simulation the distribution of swath advances predicted by the model As can be seen the measured advance fits within the predicted distribution and the mean and standard deviation of the pre dicted distribution agree well with the measured mean and standard deviation Thus we confirmed the accuracy of our fabrication processes the simulation model and the printers we manufacture Acknowledgments As with any large development effort there are innumerable people without whose contributions this product would not exist or function as smoothly as it does We would like to thank Gerold Firl Mike Green J oe Milkovits Tim Zantow and Tom Halpenny for their contributions to the media path We would like to thank Ron Kap2. issues were How do we design a low cost anti backlash device and How do we get the speed and torque that we need out of this low cost stepper motor To determine whether the permanent magnet stepper would work a theoretical model was developed that allows the simulation of the motion of a stepper motor from a standing February 1994 Hewlett Packard Journal 75 1600 7 1400 1200 Velocity steps s 0 0 005 0 01 0 015 0 02 0 025 Time seconds Fig 2 Simulated velocity profile of a failing motor 8 ohm 1600 7 1400 1200 Velocity steps s e 0 0 005 0 01 0 015 0 02 0 025 Time seconds Fig 3 Simulated velocity profile of a good 2 ohm motor 0 5 10 15 20 25 30 Time ms Fig 4 Simulated currents in a failing motor 15 7 0 5 10 15 20 25 30 Time ms Fig 5 Simulated currents in a stable running stepper motor start through high speed and back down to a stop see Stepper Motor Simulation Model on page 75 This model includes the motor system inertia inductance motor resis tance and friction It also dynamically calculates back EMFs and motor currents This simulation predicted that a motor with a resistance of one or two ohms could run significantly faster than previous Desk et motors Speeds of 2000 full steps per second 2500 r min were predicted as possible high end speeds where older permanent magnet motors had not been r
3. Media Path for a Small Low Cost Color Thermal Inkjet Printer The Desk et 1200C media path is heated for media independence requiring development of a new grit drive roller and pinch wheel combination A new stepper motor was developed to attain the target Speed and accuracy M edia flatteners and precise gearing with an antibacklash device contribute to accuracy by Damon W Broder David C Burney Shelley Moore and Stephen B Witte The media path is the part of a printer that moves media past the print cartridges stopping and accurately locating the media for printing The media path also constrains the media in a plane a fixed distance from the print cartridges A media path should advance media accurately and quickly be quiet be inexpensive hold the media flat and keep the media the correct distance from the print cartridges Fig 1 shows an overview of the media path of the HP Desk J et 1200C printer Media stacked in the input tray 1 is indi vidually picked by a media pick roller not shown and driven around the curved preheat zone 2 where it is pre conditioned moisture is driven off and the temperature is raised When the page reaches the pinch drive rollers 3 the main drive system 4 takes over from the pick roller drive not shown Once in the print zone 5 the media is heated further and ink is sprayed onto the page The heating soaking and drying causes the media to move out of its Paper Contr
4. ally any paper on the market at one point we were even printing on paper bags With such a wide range of media as the goal and the added challenge of fast throughput it became evident that the Desk et 1200C would need a heated media path to meet these goals with any kind of reasonable print quality The heated media path of the Deskj et 1200C see Fig 3 is composed of two main components the preheater and the main heater The preheater is a flexible polyimide heater that serves as the inner guide for the media As the media is February 1994 Hewlett Packard Journal 73 Writing Zone Preheater Fig 3 Heated media path fed up into the main heater and the writing zone it is wrapped around the preheater This contact allows the pre heater to precondition the media so that when the media reaches the writing zone it is much more dimensionally stable Much of the paper fiber shrinkage that occurs with heating happens before the paper reaches the writing zone The main heater consists of a Kanthal wire and a quartz tube Current drawn through the wire causes it to heat up and emit infrared radiation The radiation and convective heat from the bulb help evaporate the water from the ink in the writing zone This increase in the evaporation rate of the ink allows increased throughput and improved print quality over a much broader range of media Drive Roller Development The Desk et 1200C media advance is controlled by a high pr
5. cel out all of the error caused by manu facturing variation except the locations of the stator teeth which are formed out of the sheet metal case of the motor Used in this way the stepper motor can provide very fast and accurate moves and is well suited for driving the media in an inkjet printer The gearing system in the Deskj et 1200C is designed to opti mize the accuracy of the media drive A single reduction drive is used because there are fewer components in the gear train so this type of drive is more accurate However accuracy is not the only reason to use a single reduction The size and spacing of the drive rollers relative to the print cartridges and heater also pushed the design towards a single reduction gear train The Desk et 1200C relies on a heated media path to dry the ink as it is being printed To make room for the heater while maintaining control of the print media the drive rollers had to be much smaller than other inkjet printers had typically used By using smaller drive rollers we were able to use a single reduction gear train The spacing of the drive rollers was a critical issue in the design of the Deskj et 1200C The spacing is constrained by the size of the print cartridges and heater Thus we had to trade off room for the heater and print cartridges against control over the leading edge the star wheels hold the me dia down against the output drive roller and print quality in the bottom margins the main d
6. components Analytical tools to measure system and component performance were devel oped or enhanced by this project This greatly improved cor relation of print quality goals with measured performance and design specifications High volume processes at multiple suppliers for this assembly were characterized with proven process margins Supplier inspection processes were improved to fit part requirements and inspection metrics Media Motor and Antibacklash Device The goal for selecting the Deskjet 1200C media axis drive was to design a motor and gearing system that is simple and compact and has better speed and accuracy than we had ever achieved before Not only did this new gear drive system have to have improved performance but it also had to be less expensive One key element of the media drive system is the motor selection The HP Paintj et Paintj et XL and Paint et XL 300 printers use a hybrid stepper motor to drive gear trains that control two roller media drive systems These systems have relatively high inertia and are limited in speed They also have multiple idler gears for the purpose of transferring position control to a different location in the machine For the HP Deskj et 1200C printer a decision was made to use a double pinion approach to eliminate the idler gears For low cost we believed that a permanent magnet stepper combined with an antibacklash device would be as accurate as the hybrid stepper system The key
7. e media indepen dence but at the same time the heater and ink wreak havoc with the fibers that make up the paper The fibers predomi nantly cellulose swell with ink and the paper expands Then as the paper is heated and dried the fibers shrink and the media contracts None of the expansion and contraction is uniform from one side of the media to the other so the media tends to move out of plane To print with high quality the distance from the print nozzles to the print media must be accurately maintained We hold the media flat against a grill which covers the heater with three shims center left and right as shown in Fig 2 The fixed right shim con strains the right edge of the media and acts as the zero refer ence point for printing The adjustable left shim adjusts to A and A4 sizes while constraining the left edge of the media The center shim helps keep the media flat against the grill and helps keep the media from crashing into the print car tridge The center shim also allows printing very near the bottom edge of the media as close as 5 mm As the paper advances out of the print region we also hold it down with the star wheels These systems provide good control over the media keeping it constrained in a plane ata fixed distance from the print nozzles Heated Media Path The Deskj et 1200C was envisioned to support a broad range of media not just A and A4 sizes but also glossy paper transparency film and virtu
8. er second This motor was built and is now the motor being used in the Desk et 1200C printer The currents in a failing stepper motor are shown in Fig 4 and the currents in a stable running stepper motor are shown in Fig 5 In the failing stepper motor the currents do not even reach their normal operating levels before they are switched again The characteristic hook in the graph of these currents reflects reality very well for the case of insufficient voltage used to drive the motor Stephen B Witte Development Engineer San Diego Printer Division Linear Plant Differential Equations Moment of Inertia of Motor and Gears Acceleration and Velocity of Rotor Friction Position Response Torque Delivered Digital Filter Position and oo Fig 1 Block diagram of the theoretical stepper motor model Velocity Feedback Sample Time 25 Microseconds See top of next page for Figs 2 3 4 and 5 The term traction was used to define media slippage with the drive surface to avoid preconceptions of friction behavior Paper has a composite surface of fibers and fillers that cause its frictional characteristics to be nonlinear with normal forces beyond a relatively low surface shear stress The characterization efforts revealed interactions with printer components not previously considered Affected components were redesigned and the traction surface successfully devel oped in concert with these
9. essure nip concept drive roller and pinch wheel located near the left and right media margins Fig 4 As mentioned earlier this gives a clear advantage in space conservation and design simplicity over other inkjet products using large diameter drive rollers and low pressure nip concepts 74 February 1994 Hewlett Packard J ournal A traction surface designed to tolerate thermal shock witha low thermal expansion needed to be developed The low cost high quality advance mechanism goals also required a reliable manufacturing process that would produce 100 in specification parts The traction surface characteristics were initially defined from a customer satisfaction viewpoint The combination of nip pressure and drive roller roughness could not mar transparencies or leave tracks in plain paper The traction surface had to push the media with no slippage Banding had to be controlled much more tightly than before to meet print quality expectations This meant that the trac tion surface had to advance the media consistently a con stant distance for a given arc of rotation that is the pitch diameter had to be as tightly controlled as technology would permit The printer s performance could not be adjusted to compensate for poor control of this specification in manu facturing Print quality swath banding has a 1 1 correlation with this assembly The cost had to be low and the process compatible with high volume manufacturing The ma
10. h drive rollers 3 the main drive system 4 takes over from the pick roller drive not shown Once in the print zone 5 the media is heated further and ink is sprayed onto the page The heating soaking and drying causes the media to move out of its plane but the media control shims 6 help hold it flat for bet ter print quality The page is then incrementally advanced and printed upon until the entire page has been printed Finally the page is fed out into the output tray 7 and the process is ready to repeat G Print Zone Media Direction Q Output Tray e Move media very quickly through the print area while main taining high placement accuracy for good graphics print quality e Keep the price low The parts of the media path Fig 1 that drive and meter the media include the stepper motor the drive pinions the drive gears the antibacklash device the shaft bushings the ad justable plate the drive rollers and the pinch rollers While driving the media these components work together to en sure fast accurate movement of the print media which in turn provides the best possible throughput The stepper motor provides the fastest response available in an inexpensive motor Although stepper motors aren t al ways the most accurate type of motor the accuracy of the system can be optimized by designing the system so that the motor always moves in multiples of four half steps Moves of four half steps can
11. imulations described in the accompanying article originated from a classic control systems point of view The model includes simple position and velocity feedback control algorithms Fig 1 is a block diagram of the model A nonlinear block in the model allows the nonlinear characteristics of the stepper to be included This block makes the stepper torque output match the desired torque for a given error signal The resultant nonlinear system was linearized by running it at a very high sample rate of 40 000 samples per second At this sample rate speeds positions and currents are changing very slowly Hence linear control systems analysis is valid The model was used to compute velocity profiles with several different motor resistances inductances and so on The lowest resistance motor available in a permanent magnet motor at this time was an 8 ohm motor Fig 2 shows that the Nonlinear Stepper Simulator Error i Reference Torque Coil Resistance and Inductance PETE Position Desired Current Level Maximum Holding Torque Maximum Back EMF Feedback Generator Optimum Switching Time to Match Desired Torque Half Stepping and Full Stepping Modes simulated velocity clearly does not reach the desired 1400 steps per second This motor fails and stalls in the real system In Fig 3 the simulated velocity of a 2 ohm motor is shown In this successful run it appears that the motor has no trouble reaching the desired 1400 steps p
12. lan for his valuable assis tance in the early characterization and tooling development Frank Nasworthy for his expertise in designing the pinch wheel Steve Witte for his assistance with the hardware con nectivity and programming skills Damon Broder for his overall support of the paper advance system J eff Wilson and Dennis Culver for their design and fabricating assis tance in tooling design and our staff of lab technicians Tau Ngo Nhuong Nguyen Robert Schmidt Damon J ohnson and Danny Zepeda for their many long hours of testing Thanks also to Steve Gaddi for his support of paper advance testing HP UX is based on and is compatible with UNIX System Laboratories UNIX operating system It also complies with X Open s XPG3 POSIX 1003 1 and SVID2 interface specifications UNIX is a registered trademark of UNIX System Laboratories Inc in the U S A and other countries X Open is a trademark of X Open Company Limited in the UK and other countries
13. ol Shims Preheat Zone K bm Ue g lt m U M alll Pinch Drive Ue Rollers Main Drive System 72 February 1994 Hewlett Packard J ournal Input Tray plane but the media control shims 6 help hold it flat for better print quality The page is then incrementally advanced and printed upon until the entire page has been printed Finally the page is fed out into the output tray 7 and the process is ready to repeat Design Approach For the office printer market the Desk et 1200C is designed to support a wide variety of plain papers to be HP Laser et printer compatible to print text very quickly to print high quality graphics and to be cost competitive These character istics forced the design team to face the following challenges e Constrain plain papers flat even though plain paper tends to cockle and curl in various directions when ink is sprayed on and heated e Print to 50 dot row margins for Laser et compatibility even though such small margins allow very little control over media flatness Fig 1 Overview of the media path of the HP Deskj et 1200C printer Media stacked in the input tray 1 is individually picked by a media pick roller not shown and driven around the curved preheat zone 2 where it is pre conditioned moisture is driven off and the temperature is raised When the page reaches the pinc
14. ps the plastic gears pushed against the motor mounting plate Motor Pinions One of the most difficult challenges on the HP Desk et 1200C printer was to achieve an overall accuracy goal that was as good as the Paint et XL300 over twice the distance of that printer It turns out that one of the key com ponents in the Desk et 1200C mechanism is the quality of the motor pinions We chose to hob the pinions instead of molding them because their relatively small size makes hob bing relatively low in cost Also hobbing is substantially more accurate Gear accuracy is commonly measured in the industry and the key measurement is called total composite error or TCE This measurement is very similar to runout and essentially behaves the same way as runout However while it is easy to measure runout on a motor driven smooth shaft it is difficult to measure the TCE of a pinion after it has been mounted on the motor shaft especially with a step per motor To measure the TCE of a pinion after it has been mounted on the motor shaft it is necessary to turn the motor very smoothly and slowly at only a few r min So first we designed the stepper motor which is by nature very oscilla tory to go extremely fast and then we tried to drive it at very slow speeds We were only able to make the motor move at slow speeds by using two function generators con strained in a phase locked loop to be 90 degrees out of phase then connecting their amplified outp
15. rive roller is more accurate than the output drive roller As the space between the roll ers increases there is a longer span of media at the top of the page to control with the star wheels and a longer space at the page bottom where the media is driven by the output roller A major problem in reducing the shaft spacing was reducing the size of the drive roller Drive rollers in inkjet printers have typically been rather large elastomer coated shafts 40 to 70 mm in diameter To shrink the roller to a size we could use lt 20 mm we had to find a different process one that was new to inkjet printing We decided to try the grit drive system used in several HP plotters which consists of a grit coated metal drive wheel and an elastomer pinch wheel By using a grit system we could minimize manufacturing errors associated with the size and shape of elastomer rollers and reduce the size to one we could use Of course we then faced many problems adapting the grit drive system to a heated media path system Right Shim Left Shim Fig 2 A grill and three shims hold the media flat in the print zone The media path must also hold the print media flat The en vironment in the print region is somewhat hostile to paper The ink soaks the paper and the heater boils the water out of the paper and dries it out The Desk et 1200C is intended to print on any office paper with nearly equal print quality The heater and the ink design help achiev
16. terials selected had to survive with no degradation of their properties within the thermal operating environment of the printer A fundamental design goal for the Deskjet 1200C drive roller assembly was to develop a roller surface that would not slip on any media type This was approached from a mechanical friction and traction point of view Lab tooling adequate to characterize the traction surface had to be developed quickly The reality of our schedule required high risk decisions with data lagging by several months Concurrent development was started for lab tools proto types and metrics simultaneously We had to allow the de velopment of the drive roller assembly to slip out of phase with the rest of the project and get convergence by the time of the production build Fig 4 Drive roller and pinch wheel 1 Nip area drive roller and pinch wheel one of two in the printer 2 Pinch wheel The pinch wheel rolls on the media above the drive roller to apply normal loading of the media into the traction surface of the drive roller 3 Drive roller Rotary motion of the drive train is con verted to linear media motion The surface must not slip or leave tracks in the media 4 Pitch di ameter is twice the radius from the drive axle center of rotation to the contact zone between the media and the traction surfaces Stepper Motor Simulation Model The model used to simulate a permanent magnet stepper motor for the s
17. un much faster then 600 to 1000 steps per second A motor vendor was located that would be willing to wind the motor coils with a heavy gauge wire and prototype motors were assembled These motors performed at the predicted speeds and astounded the motor R amp D engineers themselves They previously had not built permanent mag net steppers with this kind of speed capability This vendor developed manufacturing processes to produce these low resistance coils in production and ultimately won the con tract to provide HP with these motors for the Deskjet 1200C printer Fig 5 shows the complete motor gears and backlash system 76 February 1994 Hewlett Packard J ournal Fig 5 The complete motor gears and backlash system Reducing Stepper Noise Once we had the speed that we needed we had several more hurdles to pass It was neces sary to control the motor and make it start and stop pre cisely without the noises that normally come from stepper motors When the first Desk et 1200C lab prototypes were built people were very concerned about the stepper noise Hardware was developed that allows the dynamic response of both the motor and the gear driven shaft to be measured This hardware was combined with personal computer based software that makes the process of smoothing out the step profile of the motor relatively easy Because the designers had been clever enough to make it easy to download step times to the printer it
18. unted on the motor shaft With this measurement ability we were able to work with the vendor and resolve tricky problems such as damag ing the pinions and bending the motor shafts while mounting the gears on the shafts The resultant print quality of the HP Desk et 1200C printer is good enough that few swath ad vance accuracy problems have occurred in production Media Drive Accuracy The media advance accuracy is an important metric of the performance of the media drive system As media moves through the printing region it is stopped printed on and then advanced to the next printing location The distance it moves each time depends upon the print mode and what is being printed Print quality for both text and graphics de pends on advance accuracy However graphics print quality is more dependent on advance accuracy because there is no blank space in which to hide the advance error Every ad vance made during graphics printing has the potential to show a print quality error whereas during text printing it is possible to avoid splitting text between advances and thereby avoid showing advance error In the early stages of development the design team needed an estimate of the drive accuracy of the printer A mathe matical model was constructed that simulates an advance made by the worst case components in the worst possible orientation This model provided a good guide to what sort of tolerances we needed in our manufacturing processes b
19. ut was too conservative to simulate what we expected most advances to look like The next model built was a Monte Carlo model that chooses components from simulated dis tributions and orients the parts randomly much like a manu facturing process When this model is run for a large number of cases a good approximation of the expected mean and standard deviation of an advance is produced Once the mean and standard deviation are known process control of advances is possible The goal of the modeling effort was to simulate advances and establish tolerance limits on advance accuracy This goal was achieved and the process limits were confirmed by measuring drive components building machines and measuring their swath advances The Deskj et 1200C project was able to use tools previously developed for media advance measurement for other print ers The best tool we found is an optical vision system which when given a specific plot measures a series of advances and reports the data For our purposes predominantly graphics print quality measuring 32 nozzle advances pro vides all the information we need about the drive system The 32 nozzle advance data can easily be extrapolated to the other advance distances we are interested in Fig 7 shows media advance measurements down the length of a page This data was processed to find the mean and 78 February 1994 Hewlett Packard J ournal Measured Paper Acivance Distance inches
20. uts to the phases of the motor Effectively this became a sine wave drive for the stepper and it did successfully drive the permanent magnet 1 Anti Backlash Spring The Function of the Anti Backlash Spring Pa Motor M oves Forward x Motor Slows Down but Still Moving Forward Anti Backlash Spring Anti Backlash Not Working Spring Working A x A z Motor backs up slightly and stops Teeth stay tightly meshed because the backlash device pulls the big gear backw ards against the small gear Motor backs up slightly and stops Teeth have become unmeshed because the big gear continued to move forw ard w hile the motor was backing up This is a failure Fig 6 Controlling the backlash The large drawing at the top shows the function of the antibacklash device The purpose of the backlash sheet metal spring is to keep the teeth of the gears meshed tightly even if the motor backs up slightly If the gears were to become un meshed the resultant accuracy error would be twice as much as all other error sources combined The middle drawings show the motor moving forward and the motor slowing down but still moving for ward The two drawings on the bottom show the backlash spring working left and the backlash spring not working right February 1994 Hewlett Packard Journal 77 stepper at slow speeds For the first time we could measure pinion quality after the gears had been mo
21. was possible to try out different com binations of step times quickly and observe the response of the motor and drive shaft visually in addition to hearing it The dynamic response was successfully smoothed out and the system became much quieter At the same time we were also anticipating problems with overshoot Controlling Backlash Fig 6 shows the function of the anti backlash device The purpose of this sheet metal spring is to keep the teeth of the gears meshed tightly even if the motor backs up slightly If the gears were to become unmeshed the resultant accuracy error would be twice as much as all other error sources combined The Desk et 1200C media axis completes a 1 3 inch swath advance in under 58 milliseconds For comparison the Paint et XL300 completes a 1 6 inch swath advance in about 200 milliseconds Hence it was expected that it would be difficult or impossible to prevent gear backlash However we came up with a way to reduce this backlash The idea is to use a piece of sheet metal steel that pinches the gears adding friction so that when the motor stops and backs up the gears follow it backwards This steel antibacklash device has three springs built into one component The first spring applies a controlled pinch force on the gear The second spring is stretched forward every time the motor moves for ward and provides the restoring force or antibacklash func tion The third spring provides a thrust load that kee
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