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«by Keisha Josephine Thomas A dissertation submitted to the faculty of The University of North Carolina at Charlotte in partial fulfillment of the ...»

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Keisha Josephine Thomas

A dissertation submitted to the faculty of

The University of North Carolina at Charlotte

in partial fulfillment of the requirements

for the degree of Doctor of Philosophy in

Electrical Engineering



Approved by:


Dr. Ivan Howitt ________________________________

Dr. Tom Weldon ________________________________

Dr. Yogendra Kakad ________________________________

Dr. Andrew Willis ________________________________

Dr. Xintao Wu ii © 2009 Keisha Josephine Thomas




KEISHA JOSEPHINE THOMAS. The development of the toner density sensor for closed-loop feedback laser printer calibration. (Under the direction of DR. IVAN L.

HOWITT) A new infrared (IR) sensor was developed for application in closed-loop feedback printer calibration as it relates to monochrome (black toner only) laser printers. The toner density IR sensor (TDS) was introduced in the early 1980’s; however, due to cost and limitation of technologies at the time, implementation was not accomplished until within

the past decade. Existing IR sensor designs do not discuss/address:

• EMI (electromagnetic interference) effects on the sensor due to EP (electrophotography) components

• Design considerations for environmental conditions

• Sensor response time as it affects printer process speed The toner density sensor (TDS) implemented in the Lexmark E series printer reduces these problems and eliminates the use of the current traditional “open-loop” (meaning feedback are parameters not directly affecting print darkness such as page count, toner level, etc.) calibration process where print darkness is adjusted using previously calculated and stored EP process parameters. The historical process does not have the ability to capture cartridge component variation and environmental changes which affect print darkness variation. The TDS captures real time data which is used to calculate EP process parameters for the adjustment of print darkness; as a result, greatly reducing variations uncontrolled by historical printer calibration. Specifically, the first and primary purpose of this research is to reduce print darkness variation using the TDS. The

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I would like to thank Dr. Ivan L. Howitt, for his steady encouragement and assistance through this process. He’s someone I look up to and try to model. He continued to guide me and steadily provided me with great guidance through professional and personal circumstances. I would also like to thank Dr. Yogendra Kakad for his guidance through my Master’s program and motivating my decision to pursue my Ph.D. Furthermore, I would like to thank my committee for allowing me the opportunity to share my interest and research with them. I extend an especially warm thanks to Lexmark International, Inc., for the opportunity to work and apply what I am learning towards a valuable Ph.D.

research application. This includes my managers, Ben Newman and Bob Tulenko, along with the rest of the Mono EP Cartridge Technology Development team, and other expert

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FIGURE 61: VREF of the TDS Under High Temperature and Humidity 98 Conditions with Moisture Sealant Applied to the PCB FIGURE 62: Competitive Darkness Comparison (Average & Variation ±4σ) 99

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AWGN additive white Gaussian noise BJT bipolar junction transistor CIE Commission Internationale de l’Eclairage (International Commission on Illumination) CMOS complimentary metal oxide semiconductor CMYK cyan, magenta, yellow, black COMP comparator

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Monochrome laser printers are known for the ability to produce high quality prints at fast speeds with little maintenance. Laser printing adopts the concept of electrophotography (EP) to produce robust prints. The EP process involves the transfer of surface charges to create electrostatic images. EP process parameters of the printer are utilized to control print darkness. As a result of part to part variation in EP components (discussed in chapter 2 section 2) along with environmental changes and device degradation affecting the printer cartridge, parameters controlling print darkness no longer maintain consistent darkness throughout the life of a cartridge. Consequently, there is a great deal of variation in print darkness. The desire is to produce an embedded sensing mechanism that captures real-time data utilized to adjust print darkness variation.

1.1 Goals of Printer Calibration Printer calibration is the process of maintaining consistent print darkness over the life of the cartridge. Printer calibration has been performed since the inception of printers.

The main goals of printer calibration are to:

• Maintain consistent print darkness through the life of a cartridge from the first print to the last print in room temperature. This is found by empirical testing in room temperature environment. Basic toner carries various shapes and

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print darkness must be the same when changing out a defective component or a worn component. Also mentioned previously, EP component part-to-part

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• Maintain print darkness through environmental changes surrounding the printer. As mentioned before, EP components are heavily dependent upon environmental conditions which affect print darkness. EP components are also affected by thermal conditions induced by the printer’s process speed. Today, high-end laser printers can operate faster than 60 pages per minute (PPM). Such speeds cause EP component characteristics to change.

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media have different elemental components, color, thickness, and size and shape, the print darkness also shifts. Even within each media type, there exist characteristic variations that affect print darkness.

The current print darkness calibration system tries to address these goals; however, the traditional system is not very effective in maintaining consistent print darkness.

1.2 The Current Printer Calibration Process and Challenges The current open-loop calibration process involves using EP process parameters to adjust print darkness. LUTs (look-up tables) are developed by empirically testing optimal values for each EP process parameter for every L* change in print darkness over cartridge life. L* is a CIE (Commission Internationale de l’Eclairage or International Commission on Illumination) [1] standard parameter used to measure change in lightness

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The LUT concept presents many problems:

1) The change of optimal values to adjust print darkness does not actually coincide with actual print darkness level. This is because there is no feedback relationship in the printer’s change of print darkness level.

2) Part to part variation is not considered

3) LUT’s are universal and does not represent characteristics of all cartridges under all varying environmental conditions.

4) The outcome is print quality not being optimized resulting in darker or lighter

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Several challenges are faced when trying to effectively perform printer calibration using the traditional process. Higher demand for producing faster speed printers is another one of the major challenges. The current printer calibration process maintain less error for slower print speeds; however, with increasing speeds and printer options such as duplexing, the current printer calibration process must be altered or a new calibration process must be introduced. This has become necessary to effectively calibrate print darkness for today’s features, speeds, and applications.

1.3 Goals of the Toner Density Sensor Design The toner density sensor (TDS) or (also termed) the toner patch sensor (TPS) measures toner developed on the PC (photoconductor) drum. Collected data from the TDS is used in an algorithm to adjust EP process parameters that control print darkness over the life of a cartridge.

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motors rotating electrophotographic components (discussed in the Electrophotography section chapter 2 section 2).

The first goal of the toner density sensor is to reduce current print darkness variation through the life of a given printer cartridge. The second goal of the TDS is to mitigate electromagnetic interference (EMI) affecting the TDS output signal during implementation. The new toner density sensor design meets these goals, functions well under environmental changes, maintains a fast response and requires little additional resource to implement.

The dissertation addresses the methods in which the toner density sensor was designed and tested to verify its performance while experiencing EMI and various environmental conditions. Also discussed is the reduction of print darkness variation using the TDS. Reduction of print darkness variation and EMI experienced by the TDS helps to provide consistent image quality over the life of every cartridge through the life of the printer.

1.4 Dissertation Structure The dissertation describes previous research, accomplishments, and current techniques to improve print quality which drives the design of the toner density sensor.

Included in this document are techniques proposed to improve print quality including utilization of real-time closed-loop feedback data to adjust EP component process parameters. The dissertation describes the solution of the toner density sensor and its function specifically towards the printer calibration process. Theory behind the design

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The dissertation’s background (in chapter 2 section 1) describes the history of laser printers. The electrophotograhy (EP) section (section 2) describes the science behind laser cartridge technology and halftoning (section 3) describes the process of converting a continuous toned image into a reproduced digital image. The color science section (chapter 2 section 4) highlights common aspects and metrics applied in printer technology. The printer calibration section (section 5) mentions issues causing print variation which introduce various calibration methods applied to printers. Section 6 of chapter 2 identifies past applications of sensors in laser printers and highlights differences with the proposed toner density sensor. The last section of chapter 2 (section 7) discusses band-gap engineering advancements and the great impact these recent advancements have made regarding the toner density sensor.

Chapter 3 discusses previously explored printer calibration techniques including the traditional open-loop calibration and the new closed-loop calibration process. These processes are then compared to the TDS design. Chapter 4 goes into the details of related work regarding an infrared (IR) sensor used for closed-loop feedback printer calibration.

It compares and analyzes the details. The TDS design is discussed in detail in chapter 5 with theoretical analysis. The design reduces the common problems of EMI and environmental changes. Chapter 6 is the implementation and interpretation where it describes how the sensor was positioned in the printer and it is actually applied for calibrating print darkness. Chapter 6 also discusses challenges in implementing the sensor and test results (including actual print samples taken for comparison of print

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The final Assessment of the toner density sensor performance is discussed in chapter

7. Challenges with implementation, test results, and comparison to previous printer calibration methods are summarized. The conclusion (chapter 8) summarizes the toner density sensor’s use in monochrome laser printers and the benefits which can greatly

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The background chapter describes the laser printer industry, electrophotography, halftoning, color science, printer calibration, previous sensor applications in printers, and advancements in optics. The sensor is a contribution to each of these areas and each area has opportunity for future applications to improve printing using the toner density sensor.

Each section elaborates each topic and finally links its relevance to the TDS.

2.1 The Laser Printing Industry Today’s industries and businesses use laser printers for their high-quality prints at fast speeds. The first laser printer was produced by Gary Starkweather from Xerox

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invoices and mailing labels. The first laser printer produced for computer compatibility was the Xerox Star 8010 running pages at 8 pages per minute (PPM). The printer costs

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compared with today. Over the next several years, laser printers evolved into widespread applications. Other companies such as Cannon, Hewlett-Packard (HP), Brother, Samsung, and IBM’s Printer Division (now Lexmark) came aboard in the laser printing business. By the late 1980’s, an estimated 300 million electronic prints were being generated per day by laser printers [4].

Currently, laser printers include color laser printers, monochrome (black toner only) laser printers, multi-function laser printers (MFP’s) and all-in-one (AIO) laser printers.

Within each laser printer type are low-end, mid-range, and high-end models (typically determined by maximum print speed). Laser printers are continuing to replace ink jet printers (home-use printers) as improvements and attempts are made to reduce the cost and size of laser printer components (such as cartridges and other maintenance items).

Furthermore, laser printers are increasingly displaying improvement in photographic and graphic prints (where ink jets are currently more favorable). All of these improvements are translating to increasing popularity of the laser printer for home use.

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