Integrated raster image processor and electro-photographic engine controller

This invention comprises a converged printer architecture for controlling both electro-photographic (EP) processing and raster image processing (RIP) of data for laser modulation over a video line. By converging EP process control with image processing control, significant advantages in cost, image quality, and output stability are achieved. A digital signal processor (DSP) serves as the central processing resource of the architecture. The DSP is well suited for the dual role of image processing and real-time feedback process control. The DSP can operate in open loop motor/motion control, print process control, and in closed loop feedback control including image processing adaptations for quality and stability enhancements.

TECHNICAL FIELD OF THE INVENTION

The technical field of this invention is print controllers.

BACKGROUND OF THE INVENTION

Conventional computer printers typically utilize laser or ink-jet technology to transfer a raster description of the text and images to be printed into a complete printed page. Such printers make use of printer controllers that normally employ a combination of RISC processors and ASIC devices or ASIC devices having embedded RISC processors. Printer controllers perform the raster image processing required to convert the page description language (PDL) statements and raster files sent to the printer into a form that the printer can act upon.

Typically within the printer controller, the RISC/ASIC devices are employed in a pipeline to perform first the task of interpreting printer control language/post script instructions/data and forming a display list equivalent. The second stage performs rendering of data from display lists and converts it to printer specific raster data. These tasks usually require a significant amount of memory within the printer. Such local memory for the printer controller pipeline is usually supplied by way of standard memory modules SIMM/DIMM configured to suit the printer controller application.

FIG. 1illustrates the prior art steps required to process the input data that a printer typically receives from a conventional personal computer (PC). The output from the PC normally is supplied by a printer driver101that prepares an output print file. This file includes a set of instructions and data in a page description language (PDL) or compressed bitmap format. These instructions and data may be transported to the printer via IEEE 1284 (Firewire) or Universal Serial Bus (USB) cabling or over a local area network and stored in an input buffer memory102.

The first computational step in the printer controller pipeline is interpretation103of the data. The display list from interpretation103includes a description of individual elements of graphics data or text data along with the position of these elements on the page. The display list may be in a banded or a non-banded format. In a banded format discrete bands are defined and formed as a part of the processing. After rendering, a number of these bands collectively form a full printer controller output page. In a non-banded format, each page is interpreted as a unit. After rendering, this unit forms an integral part of printer controller output.

The rendering pipeline stage104reduces the interpreted data of the display list to printer specific raster data. This process is sometimes called rasterization. The output of the rendering process is a bit map format in which discrete digitized dots (pixels) are generated to control the output device (e.g. ink jet pen, laser drum) with proportions of the colors cyan, yellow, magenta, and black. The rendering step is well suited to digital processing operations commonly used in digital signal processor devices. After rendering, the bit map data is stored in an output buffer memory stage105. This bit map data is sent as needed to the printer output mechanism106.

FIG. 2illustrates a high-level view of the full complement of printer pipeline functions of the prior art. The input data has a variety of sources, such as spooled jobs on disc201, parallel printer port202, Universal Serial Bus (USB) port203, Ethernet TCP/IP port204and IEEE 1284 (Firewire)205. Each data source has its specific data format. This data must be reduced to a common format for processing in the pipeline. Streams interface unit207adjusts the format of the input data as required. For example, data arrives in parallel form from parallel printer port202and is converted in streams interface unit207as necessary for uniform processing in later stages. Likewise, streams interface unit207often carries out format adjustments upon data from USB port203in queue coming from the host processor. Streams interface unit207sends data from USB port203through a serial to parallel interface which is accomplished by way of serial-in parallel-out register function. Ethernet TCP/IP port204allows clusters of computers connected by a local area network to share the same printer. FireWire port205allows world-wide access to a printer from any computer attached to the Internet.

Streams interface unit207sends data to the path that performs parallel interpretation of the composite postscript208, printer control language (PCL) page pipeline209or other PDL interpreter210. Page pipeline209re-assembles the results of the interpretation process into page format for page oriented processing before submitting page data to rendering unit212. Postscript interpreter208or printer control language (PCL) interpreter210may send banded format data directly to rendering unit212. Rendering unit212also performs compression, decompression or screening as required. PDL print controller to print engine controller interface unit225supplies data and control information to input/output interface elements213to drive paper path controller216, display/control unit214and video data unit215.

The typical printer controller includes two circuit boards. The basic image processing and major hardware control is performed in PDL print controller board220. This typically contains RISC processor211and application specific integrated circuit (ASIC) print engine controller226and memory221. The second circuit board is print engine controller board222. Print engine controller board220includes input/output interface elements213, display/control unit214, video data unit215, and paper path controller216. PDL print controller to print engine controller interface225provides data and control exchange between PDL print controller board220and print engine controller board222.

Print engine controller board222encapsulates the printer mechanical and the electro-photographic (EP _processing environment. Print engine controller board222provides a consistent interface to PDL print controller board220. Because it is self contained, print engine controller board222constrains ability of PDL print controller board220to do EP process control and to dynamically adjust image quality. As part of the overall printer architecture, print engine controller board222is equivalent to an un-modifiable engine device driver.

Image density sensor224reads the density of a test pattern written to the organic photoconductor (OPC). Print engine controller board222makes voltage adjustments to produce a toner density on the organic photoconductor that corresponds to gray scale expected form the test pattern image. Prior art has controlled the printing EP process by using a one-shot calibration at power up and optionally a periodic calibration cycle over time. These calibrations use of the image density sensor224and sometimes use a relative humidity sensor to help determine the setting.

FIG. 3illustrates the construction of a typical laser printer. Protective casing301houses the laser energy source, a laser diode300. Video information302to be printed modulates the current to laser diode300. Rotating multi-faced mirror303deflects the laser beam within housing301, which then impinges on the photoconductor drum304. The charging roller305supplies a source of electrons to the photoconductor drum304creates a charge image of the video information on that surface. Toner development roller306transfers resin particles from the toner onto photoconductor drum304. Toner charge and toner level mechanism307maintains the toner charge and toner level. Transport rollers309and fuser roller310drive the intermediate transfer media (ITM)311. Intermediate transfer media311retrieves a toner charged version of the video information at interface325between photoconductor drum304and transfer roller315. Transfer belt charge management roller312, transfer belt temperature/charge regulator313, peltier cooler316, fuser temperature/charge regulator317, heated discharge AC corona318and color density detector319act to prepare the intermediate transfer media image for transfer to the paper surface at the fuser roller310. Paper rollers314complete the transport of the paper through the machine.FIG. 3illustrates the full set control tasks which must act in concert to assure the desired laser printer output quality. These include top of form detector320, paper property detector321, beam detector322, paper pickup323and paper jam detector324.

SUMMARY OF THE INVENTION

This invention comprises a converged architecture for controlling both the electro-photographic (EP) process and raster image processing (RIP) of data for laser modulation over a video line. By converging EP process control with image processing control, significant advantages in cost, image quality and output stability are achieved.

At the center of the architecture is a digital signal processor (DSP). The digital signal processor is well suited for the dual role of image processing and real-time feedback process control. The digital signal processor can operate in open loop motor/motion control, print process control, and in closed loop feedback architectures that include image processing adaptations for quality and stability enhancements.

The present invention describes an implementation of the full instrumentation electro-photographic process for a laser printer as a preferred embodiment. It uses sensor readings it obtains in a formula based model to track and adjust via actuator, motors, and voltage setting the toner chemistry and EP process. The sensor readings can be performed either in between normal imaging cycles of during those imaging cycles. This allows real-time adjustment of the printing process parameters to achieve best quality.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention proposes implementation of the full instrumentation of the electro-photographic process. It uses sensor readings it obtains in a formula based model to track and adjust via actuator, motors, and voltage setting the toner chemistry and electro-photographic process. The sensor readings can be performed either in between normal imaging cycles or during those imaging cycles. This allows real-time-adjustment of the printing process parameters to achieve best quality.

FIG. 4illustrates the digital signal processor centric converged architecture printer system of this invention. The major controller functions of page description language (PDL) controller and the print engine controller are partitioned on two circuit boards in prior art systems. Both these major controller functions are integrated into a single EP process control and feedback function411. The architecture ofFIG. 4includes: (1) a digital signal processor image processor408; (2) the embedded EP process control and feedback function411; and (3) a simple embedded RISC processor to manage input functions401to405and streams interfacing407. Chip integration technology available at present in digital signal processors or application specific integrated circuits (ASIC) allows several approaches. The majority of the required functions may be placed on a single silicon chip. Alternatively, the functions may be partitioned as necessary to meet specific cost goals.

This architecture takes advantage of the computational power of the digital signal processor, utilizing not only its image processing capability, but its capability to process real-time feedback control loops. In the compact integrated architecture, the digital signal processor may be made to operate not only in open loop motor/motion control, print process control tasks, but also in closed loop feedback-oriented tasks that include image processing adaptations for quality and stability enhancements.

The organic photoconductor (OPC) surface of photoconductor drum304inFIG. 3is a specially coated cylindrical surface which is charged with electricity from charging roller305. Laser diode300writes an image to photoconductor drum304by selectively removing the electric charge as it scans over the drum. The toner charge and toner level mechanism307adjusts the toner charge so the correct amount of toner will be attracted to the image on photoconductor drum304.

The intermediate transfer media (ITM)311is given a voltage to attract and keep the toner deposited on photoconductor drum304. Once all color planes have been collected they can be transferred to paper in a single pass.

Fuser roller310permanently bonds the toner image to the paper by heating the toner and paper as the paper passes between fusing rollers. Returning toFIG. 4, additional sensors are used to align the top of drum where the EP images are placed with the top edge of the paper so that both are in proper registration.

The optical control panel409provides a graphical user interface (GUI) by which the PDL program communicates with the user. Optical control panel409displays print status during printing, printer state and error conditions. It guides the user through maintenance such as paper and toner replacement. Optical control panel409supplies display/data412to the engine control board (not shown). Optical control panel409receives control button actuation signals413from the engine control panel.

Engine data interface410provides the low level autonomous control of the printing process. Engine data interface410receives pixel clock414, page synchronization signal417and frame synchronization signal419. Engine data interface410generates video data418for control of laser diode300(FIG. 3). Engine data interface410thus provides synchronization of video with paper movement, movement of photoconductor drum304in response to commands sent from the RIP/PDL application.

Electro-photographic processor control and feedback function411interfaces with roller functions, sensors, the intermediate transfer media (ITM), fuser functions and laser functions. Roller functions include an input charging voltage sensor433and outputs to charging roller motor431, charging roller high voltage (HV)432, organic photoconductor (OPC) roller motor434, developer roller motor435and developer roller high voltage (HV)436. Sensor inputs include toner charge level sensor437, toner level sensor438, relative humidity (RH) and temperature sensor439, media position sensor440and cleaning blade sensor441. Intermediate transfer media (ITM) functions include inputs intermediate transfer media (ITM) voltage sensor445, media voltage level446, color level sensors447, organic photoconductor (OPC) discharge corona voltage448, paper property sensors449and image density sensors450. Intermediate transfer media (ITM) functions include outputs intermediate transfer media (ITM) high voltage442, intermediate transfer media (ITM) roller motors443and intermediate transfer media (ITM) discharge corona444. Fuser functions include input fuser temperature sensor454and outputs fuser motor451, fuser heater voltage452and fuser high voltage453. Laser functions include inputs scan line start sensor458and top of form registration sensor459and outputs laser scanner motor455, laser diode drive456and laser modulator457. These permit delivery of consistent quality images over all operating conditions and ranges as well as for the life time of the print engine hardware. The first print ever made on the printer will have quality consistent with the next print. This process includes major hardware component replacement in addition to replacement of consumable components. This high level of quality will thereby extend from machine to machine of the same model.

The new architecture uses feedback loops to adjust electro-photographic process parameters for changes in toner charging, media properties, and motion artifacts. Closed loop control employs sensors of:

5. average developed mass per page,

From this sensor data, developer and transfer biases, fuser temperature and laser modulation adjustments are made.

Open loop sensors in the paper path and transfer fuser elements are used to control process set points. Sensing of transferred optical density of each color plane coupled with color gamut control points are used to adjust color management parameters and screening.

Some examples of uses for the combined raster image processing and electro-photographic control follow. This is by no means an exhaustive list.

This invention enables optimization of the spin-up and stabilization time. This reduces the first-page-out time by using sensors scan line start signal458and top-of-form registration signal459illustrated inFIG. 4. Scan line start and top-of-form registration are not related to laser spin-up and stabilization time. Spin-up and stabilization time is minimized by controlling the motor driving the spinning mirror (303) in the laser scanner. The nature of the feedback mechanism to control the rate of spin up and spin down and keep the rotation constant is similar to the strategies used to control any spinning motion such as found in digital signal processing applications in washing machines and dryers. Scan line start signal458is used to time the data being feed to the laser modulator. As the laser beam hits the photoconductor at the side print margin, this signal starts modulation of the laser beam for that scan line.

Top-of-form registration signal458tells the system the paper location within the paper path and establishes the top print margin. Both of these signals can be are typical of application specific integrated circuit (ASIC) driven solutions but the programmability of the digital signal processor in this application allows more flexibility for manufactures and end-users to change the timing as desired in new the applications. The important issue is not that other ASIC based methods could be employed but that the digital signal processor system control gives the print process architect new programmable ways to control the system, more dynamically using a broader range of control parameters, to establish print process control.

This invention enables control of paper feed by changing the rate and/or force of feeding mechanism based on paper smoothness sensor readings or motion resistance readings. This control is initiated by using paper property sensor321illustrated inFIG. 3. Paper property sensor321produces paper property signal449illustrated inFIG. 4. The important issue is the ability of the digital signal processor to use a motion feed back loop to dynamically adjust pressure on feed nip and the rate of rotation of feed nip to insure the paper gets feed properly.

The ability to handle the feeding of a wide range of paper grades reliably is an important challenge on all laser print systems. Such control increases reliability and extends the range of paper grades that a given print system can feed reliably.

This invention permits adjusting the charging AC/DC voltages to stabilize the contrast potential on photoconductor drum304as it changes with age and printing environment. This control employs input from charging voltage sensor signal433and adjusts charging roller high voltage434. The difference in the level of charge on photoconductor drum304in unexposed areas versus the level of charge in laser-exposed areas is the known as the contrast potential. Several factors affect the difference and it changes over time and with temperature and humidity. Once again digital signal processing software control of this parameter coupled with the toner charging parameters allows the print process architect new ways to develop algorithms to stabilize toner development. In an application specific integrated circuit (ASIC) based controller system this is possible, but is not be practical given the complex relationships and calculation required to control these design elements. The digital signal processor programmability and fast real time feedback loops are a special advantage.

Adjust the pre-rotation in developer and AC/DC development voltage based on tribo-charging sensor feedback at the charging roller305inFIG. 3. This could also be done using readings from relative humidity (RH) and temperature sensors439. Temperature and relative humidity inputs will feed back and form the basis of calculations for controlling toner development in conjunction with the inputs in 3 above.

The pre-rotation in the developer is another start-up operation. The faster the charge is established and stabilized the faster the print process can begin. Controlling the rate of pre-rotation and time of the pre-rotation based upon measured toner charge will speed up first page out.

Adjust the AC/DC voltages and fuser roller pressure based upon media roughness and dielectric thickness sensor measurements. The main issue here is that the DSP can control the pressure in the fuser nip and the AC/DC waveform applied to the nip. The direct current flowing in the transfer nip is controlled in existing print process but not the waveform coupled with the pressure. Controlling the waveform and the pressure controls the direct current but will also alter the electric field in the nip and in the pre- and post- nip regions. This control will reduce toner explosions and create sharper developed print edges. Fast-complicated calculation, well suited to the DSP control system, can be performed based upon several feedback-input signals.

Change fuser roller AC/DC voltages and heater current based upon media heat capacity and thickness sensor measurements. This can be done within a page to keep constant temperature across the page, top to bottom of page, as well as page to page with adjusted AC/DC voltages and heater current applied at outputs451,452and453. The issue is that feedback control of the temperature profiles and electric field in the fuser nip can be achieved. Digital signal processor408allows more dynamic and flexible control of these parameters, insuring better fusing, less toner offset, over a wider range of paper stock. The parameters in the fuser nip can be set and/or calculated base upon paper properties at the page level or changed within the page based upon area sensor feedback across and down the page. This is also true of fuser and paper feed properties.

Keep a constant force on photoconductor cleaning blade based upon resistance to motion of the photoconductor. Input sensing is done at cleaning blade sensor441. As environmental properties and wear change the surface of the photoconductor, the toner adhesion to the surface of the photoconductor will also change. To effectively clean the surface of the photoconductor pressure on the cleaning blade will also have to change. Using the parameters measure above, coupled with total rotation count, algorithms can be developed to adjust the pressure and insure more effect cleaning without excessive photoconductor surface wear.

Keep photoconductor rotation constant within a page thus reducing banding effects. Output is applied to organic photoconductor (OPC) roller motor434. Prior art for control of banding do use digital signal processor feedback control. They are included here as other examples of digital signal processor based print process control. The idea is an extension of motor and motion control using encoder feed back to insure constant motion and minimize the jitter induced by gear trains.

Banding can be reduced by adjusting laser modulation based upon encoder readings from photoconductor drum. Output is applied at laser scanner motor drive455. The idea here is to change the exposure level by changing laser modulation scan line by scan line based upon feedback from photoconductor drum motion sensors. If the motion will bring two scan lines to close to each other, over development would occur. By reducing the exposure of one of the scan, line over exposure would not occur and perceivable dark line or bands would not appear.