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Theory of Operation
Overview
This topic covers the theory of operation of each subsystem within the printer:
s s s s s s
Functional block diagram Imaging Fusing Paper path Power supply Image processor
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Functional block diagram
The printer is made of eight major blocks:
s s s s s s s s
Imaging unit Laser scanner Toner cartridges Fuser Engine control board Power supplies Mechanicals Image processor board
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Imaging unit The imaging unit forms the heart of the printer. This customer-replaceable unit contains the photoconductive belt and accumulator belt on which imaging takes place. Laser scanner The laser scanner is the device that "writes" the image onto the imaging unit's photoconductive belt. Toner cartridges Four toner cartridges, black, yellow, cyan and magenta, contain the primary color toners. Each toner cartridge feature a voltage-biased developer that sequentially transfers that cartridge's toner onto the imaging unit's photoconductive belt, depending on where the laser exposed the belt. Subsequently, the toner is transferred to the imaging unit's accumulator belt from which it is transferred to a sheet of media.
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Toner cartridges
Image processor board
Laser scanner
Engine control boards Low V Power supply
Imaging unit Mechanicals
Hi V Power supply Fuser
Figure 5
Block diagram of the printer
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Fuser The fuser permanently bonds the toners to the media through a combination of heat and pressure. Engine control board Working as the "brains" of the print engine, the engine control board coordinates all printer functions. The engine control contains the microprocessor and ROM that control printer operations. Power supplies The low-voltage power supply provides regulated DC power source for the printer's electronics and motors. The high-voltage power supply provides high-voltage for the chargers and bias rollers. Mechanicals Mechanicals include the motors, gear trains and solenoids that drive the belts and rollers of the printer. Image processor board The image processor board converts the image data from the host computer into a raster format for the print engine. The image processor board also controls all messages displayed on the front panel.
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Laser imaging
Overview
The laser printer prints an image on paper Scorotron Erase using a technique called laser charger lamp electrophotography. The printer uses the electrographic process known as Discharged Photo conductive Area Development or "write black." In this belt cleaner process, a digitally modulated laser scans First bias transfer roller laterally across a negatively charged, rotating photoconductive belt. Where the Accumulator belt belt is exposed by the laser beam is where the image is written and toner is transferred. Multipurpose
tray pick roller Aligning +500 ~ rollers 700 v On-Off Supply roller Fuser Pick roller Intermediate rollers Waste toner bin Cleaning roller Second bias transfer roller Heated roller +500 ~ 2400 depending on media and humidity
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Laser scanner Photoconductive belt Toner developers Black Cyan Magenta Yellow Pre-transfer Lamp Accumulator Fuser roll belt cleaner Take-up roller Pressure roller
Transfer roller voltage plus400 v
Figure 6
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Laser printing process overview
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In the color print process, the photoconductive belt rotates past the laser scanner and the toner cartridges four times, once for each primary color and black. During each sucessive pass, the laser exposes the portions of belt that correspond to the primary color's component of the image. Toner is attracted to the laser-exposed portions of the belt. As the photoconductive belt rotates, it passes the accumulator belt. As the name implies, the accumulator belt accumulates or picks up each primary layer of toner from the photoconductive belt and holds it, layer upon layer, until it contains the entire image. At this point, a sheet of paper is advanced past the accumulator belt and the toner is transferred to the sheet of paper. The paper advances to the fuser, where heat and pressure permanently bond the toner to the paper. From the fuser, the paper is driven to the output tray. A photoconductive belt cleaning blade scrapes residual toner off the belt before the next primary color's toner is applied to the belt; this prevents each toner from contaminating the next color's layer. The cleaning blade is in constant contact with the belt. An accumulator belt cleaner scrapes residual toner off the accumulator belt; this prevent an image from one print contaminating or "ghosting" on the next print. The blade only comes in contact with the belt once the accumulated toner layers are transferred to the sheet of paper.
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Pre-exposure
The print process begins when the photoconductive belt passes by the preexposure lamp. The belt is moving at a speed of 96 mm-per-second for 600 dpi printing or 48 mm-per-second for 1200 dpi printing. The light of the pre-exposure lamp, which is a horizontal row of red LEDs, removes random negative charges from the photoconductive belt. Before pre-exposure, the surface of the belt varies from -500 volts to +50 volts. Following pre-exposure, the voltage level of the illuminated portion of the belt only varies from 0 to -50 volts. The pre-exposure lamp is sometimes called the erase lamp since it "erases" negative charges from the belt.
Pre-exposure lamp Light from the pre-exposure lamp's (erase lamp's) LEDs removes negative charges from the belt.
Photoconductive belt
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Figure 7 belt
Pre-exposing the photoconductive
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Electrostatic charging
The electrostatic potential of the belt is not uniform following pre-exposure. As the belt rotates, it passes a scorotron charger which bombards the belt with negative charges. The scorotron charger behaves somewhat like a vacuum tube: The grid of the charger, held at a potential of between -500 volts to -540 volts and coupled with the varying voltage potential on any discrete point on the belt's surface, determines how many electrons can flow from the corona wire onto that point of the belt's surface. The corona wire is charged to -6 kilovolts with a constant current of 400 ľA. The varying electron output from the scorotron, directly based on the varying charge of the belt surface, results in a uniform negative potential on the belt surface of -500 volts or -540 volts, depending on the selected dotsper-inch printing and ambient temperature.
Scorotron charger -6kv Grid -500 -540 v
The scorotron charger lays down a uniform negative charge on the belt surface.
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Figure 8 Electrostatic charging of the photoconductive belt
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Laser exposure
As the photoconductive belt rotates, the uniformly charged portion of the belt is exposed by the modulated laser beam. As the vertically moving belt passes in front of the horizontally scanning laser beam, negative charges on the belt surface are neutralized where exposed by the beam: this forms a latent image on the belt. The laser beam is turned off for "non-written" portions of the image. The power of the laser beam is varied from 0.4 mW to 0.5 mW depending on whether the printer is printing in 1200 dots-per-inch (dpi) mode or 600 dpi mode. Following laser exposure, the negative potential of the belt varies from -500 volts or -540 volts (unexposed) to -10 or -20 volts (fully exposed).
Laser scanner
The laser beam selectively neutralizes some of the negative charges on the belt surface.
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Figure 9 Laser exposure of the photoconductive belt
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The laser scanner
Lenses and mirrors in the laser scanner Collimator lens direct the beam at the photoconductive belt. The beam originates at a laser diode. The Horizontal beam is made parallel by the collimator lens sync mirror and is directed at the rotating polygonal mirror. The mirror rotates at a constant 23,000 revolutions per minute. This transforms the beam into a horizontally scanning beam, which is directed through a f- primary lens, which alters the beam's angular rotation motion into a constant horizontal motion. The beam then passes through a toric correction lens, which corrects the beam for any vertical misregistration. The beam then reflects off a mirror and passes through a window, where it scans across the rotating photoconductive belt. At the beginning of each of its PhotoConductive horizontal sweeps, the horizontal sync belt mirror deflects the laser beam to the horizontal sync sensor. This alerts the engine control board that the laser beam is Figure 10 The laser scanner beginning its horizontal sweep and that it can begin to modulate the signal for the data to be printed on that line of the image.
Laser diode
Rotating mirror f-0 primary lens Toric correction lens Horizontal sync sensor Mirror
Window
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Toner pickup (development)
As the photoconductive belt continues to -120 ~ -220 v rotate, it passes by one of the four toner Doctor blade Paddles cartridges. Each cartridge is selectively Cam driver Black cam-driven forward to bring its developer roller into direct contact with the belt when it is that cartridge's turn to transfer its toner Toner is transferred to the belt during the belt's four rotations Supply Cyan to the past the cartridges. The currently activated roller portion of toner cartridge's developer roller is charged the belt exposed by Magenta to a potential between -120 to -220 volts. the laser Toner is attracted to the exposed portions of beam. the belt in reverse proportion to the amount Yellow of negative charges left on the belt. Since Toner cartridges the charge on the exposed portions of the belt is about -20 volts, the greatest amount Pre-transfer The pre-transfer lamp of toner is transferred. The developer roller removes remaining lamp Photoconductive negative charges from rotates at 1.6 (600 dpi printing) or 2.13 belt the unexposed portions (1200 dpi printing) times the speed of the of the belt. 740-7-29 photoconductive belt to ensure a constant supply of toner. As the belt advances, it passes the pre-transfer lamp which, like the Figure 11 Toner pickup pre-exposure lamp, removes remaining negative charges from the unexposed portions of the belt.
Developer roller
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Inside each toner cartridge is a toner supply roller. which rotates in the opposite direction from the developer roller; this supplies a layer of toner onto the developer roller. The doctor blade smooths and evens out the toner on the developer roller. Also inside the toner cartridge are gear-driven paddles that churn the toner and keep it fluidized and moving toward the developer roller.
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Toner transfer to the accumulator belt
As the photoconductive belt rotates, it comes in contact with the accumulator belt, which rotates at the same speed. The first bias transfer roller, located under the accumulator belt at the contact point with the photoconductive belt, carries a charge that varies between +500 and +700 volts, based on the sensed temperature, humidity, and print speed. This strong potential attracts and holds the toner from the photoconductive belt to the accumulator belt. The accumulator belt rotates four times so that it can pick up or accumulate each of the four toner layers, one layer on top of the last. An accumulator belt homeposition sensor sync signal, generated from a timing mark on the accumulator belt, informs the engine control board that it is time to begin exposing the next toner layer's information onto the photoconductive belt. When that happens, the accumulator belt is rotated in the proper position to transfer the toner layer in proper registration with the previous layer(s).
Photoconductive belt cleaning blade Photoconductive belt The toner on the photo-conductive belt is attracted to the strong positive potential of the first bias transfer roller. Four layers of toner, latent images of cyan, magenta, yellow and black, build up on the accumulator belt
The pre-transfer charger gives the toner a uniform negative charge
+500~700v Accumulator belt home sensor First bias transfer roller Accumulator belt
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Figure 12 belt
Toner transfer to the accumulator
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Any toner remaining on the photoconductive belt after the transfer to the accumulator belt is scraped off by the photoconductive belt cleaning blade, which is always in contact with the belt. This leaves the photoconductive belt clean for the next layer of toner to be transferred from the toner cartridges.
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Paper picking
A sheet of paper (or transparency film) is picked when a pair of cam-shaped rollers, driven by the paper-feed motor, rotate and force a sheet of paper from the stack into a pair of intermediate rollers. The pick roller only rotates one revolution, which is enough to push the sheet of paper into the intermediate rollers. Alternately, depending on the user's selection, media may be picked from the Multi-purpose Tray. The Multipurpose Tray pick roller rotates to drive a sheet of media or an envelope into the aligning rollers. The intermediate rollers advance the sheet of paper to the aligning rollers. The paper is pushed against the clutch-driven aligning rollers slightly without them rotating, to create a slight buckle in the paper, which aligns the sheet of paper parallel to the paper path. At this point, the paper remains stationary (since the aligning roller's clutch is not yet energized) until the image is ready to be printed on the paper. The aligning sensor detects whether the sheet of paper arrived at the aligning rollers after being properly picked and traveling through the intermediate rollers.
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Multi-Purpose tray Pick Roller
Aligning roller
Aligning sensor
Intermediate rollers
Pick rollers
Path for paper from optional Lower Tray Assembly
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Figure 13
Paper picking
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A similar pick-and-feed operation takes place in the optional Lower Tray Assembly to pick and feed the sheet into the printer's intermediate rollers.
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Toner transfer to paper
Once all four layers of toner have been picked up on the accumulator belt, the aligning roller clutch is energized to advance a sheet of paper (which has already been picked) to the second bias transfer roller. (In the user guide, the second bias transfer roller is referred to as the transfer roller, a customer-replaceable consumable item.) The movement of the sheet of paper is timed so that the leading edge of the toner image on the accumulator belt meets with the paper after 5 mm of the leading edge of the paper has passed. A strong positive voltage in the second bias transfer roller, located under the sheet of paper, attracts the toner from the accumulator belt to the paper. The voltage of the transfer roller varies from +500 to 2400 volts, based on the ambient temperature, humidity, print speed and media being printed upon. The paper (or transparency film) advances at the same speed as the accumulator belt.
The strong positive potential of the second bias transfer roller attracts toner to the paper. The cleaning blade scrapes leftover traces of toner from the accumulator belt.
Accumulator belt cleaning blade Second bias transfer roller +500 ~ 2400 v Cleaning roller at transfer roller +400 v Waste toner bin
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Figure 14
Transferring toner to the paper
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As the toner is being transferred to the paper, the accumulator belt cleaning blade is activated. This blade scrapes any remaining traces of toner from the accumulator belt prior to the next image transfer of toner. At the second bias transfer roller, strong positive charge, equal to the voltage applied to the second bias transfer roller, +400 volts on the adjacent cleaning roller, attracts leftover toner particles from the transfer roller. A blade scrapes the toner off the cleaning roller into the toner waste bin.
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Fusing and exiting
As the paper receives the toner, it passes through the fuser. A heated roller on the top surface of the paper melts the toner on the paper. Sandwiched between a pressure roller underneath and the heated roller on top, the melted toner bonds into the paper. An oil supply keeps the heated roller lubricated so that the melted toner does not adhere to the roller. Following fusing, the paper advances to the output tray. When not printing, the heated roller is held at a temperature of 160o C (320oF). The heater roller is set to 165o C (329oF) for 600 dpi printing and 145o C (293oF) for 1200 dpi printing. For transparency film and other media, the fuser is set to 165oC (329oF) and run at half speed. The fuser exit sensor detects the sheet of paper as it leaves the fuser.
The reverser allows the prints to exit to the exit tray printed Reverser Using heat and pressure, the fuser bonds the toner to the paper
Reverser rollers
Take-up Fuser roll roller
Output tray full sensor
Supply roller Heated roller Fuser Assembly Paper exit sensor Fuser entrance sensor
Pressure roller
Output tray
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Figure 15
Fusing the toner to the paper
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Depending on printer driver instructions, the paper may go directly to the output tray, printed side up. Or, if instructed, the sheet of paper may be routed to the reverser, which drives the sheet up a narrow channel and then reverses direction, and routes the sheet of paper to the output tray face-down. This is appropriate for a collated series of prints that need to remain in first-to-last order. The paper exit sensor detects the sheet of paper as it enters the exit rollers. The reverser can also redirect the sheet of media to the installed auto-duplex unit installed in the main tray slot.
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Auto-duplexing
The auto-duplex unit, replacing the top tray, gives the printer the ability to print on both sides of a sheet of paper. After printing on one side of the paper, a solenoid-activated gate directs the print up into the reverser. The reverser, using another solenoidactivated gate, then directs the print (printed side up) into the auto-duplex unit. In the auto-duplex unit, an angled roller, geared to the paper path gear drive train, pulls the print into the duplex unit, aligns it to the paper path and positions it at the auto-duplex pick roller. To printing on the second side, the print engine picks the print from the auto-duplex unit much as if it was the top tray. The print is processed through the paper path like an ordinary print. The auto-duplex unit is driven by a gear train from the print engine. An electric clutch in the auto-duplex unit energizes to transmit drive power to the angled roller and to the auto-duplex unit pick roller. A pick sensor detects if the print was correctly picked from the auto-duplex unit.
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Auto duplex pick sensor
Reverser
Intermediate rollers Alignment rollers Fuser rollers
Exit tray
Angle rollers
Auto duplex unit
Pick rollers Paper tray
Paper tray
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Figure 16
Auto-duplexing
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Print modes
The printer features four print modes: Presentation, Fast Color, Standard, and Premium. Standard, Presentation and Fast Color mode are performed using 600 x 600 dpi printing. Premium mode is 1200 x 1200 dpi printing. If no motion were involved, the laser dot produced would be an ovoid shape, measuring about 50 microns wide by 55 microns tall. In practice, to print a dot, the sweeping laser beam turns on long enough to create a roughly circular dot. In Standard, Presentation and Fast Color 600 dpi (horizontal and vertical) printing mode, the laser pulses often enough to produce 600 dots or pixels per inch. The area allowed for a pixel is 42 microns square. Toner particles attracted to the laser dot measure about 8 microns in size.
42 55 600 dpi printing 50 0.5 mW
70% duty cycle of laser beam produces fully saturated dots 21 1200 dpi printing 0.4 mW
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Figure 17
Print modes and printing dots
Premium mode 1200 dpi (horizontal and vertical) printing is achieved by cutting in half the speed of the photoconductive and accumulator belts and the paper transport. In addition, the laser energy is lowered to 0.4 mW and the laser pulse rate is doubled. Controlling the amount of horizontal and vertical overlap of the dots results in even finer grayscaling.
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Printer color correction
Manually selecting color density. The color density of each primary color can be selectively altered by the user. Color adjustments can be made manually using front panel Calibrate Colors menu. This involves printing a reference page, comparing the Reference Page to sample colors on the color calibration page of the printer's user guide and entering value changes through the front panels. This process is described in the Phaser 740 Color Printer User Guide as well as in the topic, "Manually setting color corrections" in the Phaser 740 Service Quick Reference Guide. The changes are sent to the image processor board, which makes calculations and then downloads a color correction table to the print engine control board. The color correction table indicates the exact laser beam pulse duty cycle to the desired colors.
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Low voltage power supply
The low-voltage power supply generates four DC voltages:
s
+5 VDC for the image processor and the engine control logic. Fuses F352 and F353 are provided for short protection. +24 VDC for the motors, high-voltage board and the cams, clutches and solenoids. Short protection is provided by fuse F351. Input AC fusing is provided by F201 and F203 (110 VAC units), and by F202 (220 VAC units, not pictured). Fusing for the fuser heater is provide by F201 for 110 VAC units and by F202 for 220 VAC units (not pictured).
F 201 Filter Zero cross switch Fuser halogen lamp F 351 +24VC GND Filter
AC
s
s
s
F 203 Energy star mode control DC-DC converter F 353
+5V F 353 +5V GND
Control IC
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Figure 18
The power supply
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