Maths and me @ Colour loop system

An inline colour loop system is a powerful tool used in colour management during production processes. Year long ago, closed loop colour control systems were introduced for commercial web offset presses, these systems use colour bar and soon became standard equipment. The ink keys were adjusted so as to maintain proper colour characteristics of the patches in the colour bar. Controlling colour bars is relatively easy to understand, at least in principle. A measurement is made of a patch with a single colour of ink. If the colour is too weak, inking levels are increased. If the colour is too strong, inking levels are decreased. Gray balance control is a bit more complicated, but the strategy is similar. These control systems have two features that make the task easier. First, the colour bars are a known shape and size. Second, the colour bars are a known that do not change from one job to the next.

The advancement of technology has just recently made the next generation of colour control systems possible. These systems do not rely on colour bars, but rather control colour based on measurements of the printed images. For Colour Control in the Work (CCIW), the control system must be able to adapt to whatever is printed. Such a system must be able to decide how to adjust colour for any ink combination.

This CCIW is working with PID mathematical algorithmic function.

Source Wikipedia

What is PID ?

PID (Proportional-Integral-Derivative) control is a fundamental feedback control algorithm widely used in various engineering applications, including colour control in industries such as printing, textiles, and lighting. The application of PID control to colour management involves adjusting the control input to a system to achieve desired colour characteristics, thereby enhancing the consistency, quality, and accuracy of colour reproduction.

Fundamentals of Colour Control Using PID

In colour control, the primary goal is to maintain the desired colour properties, such as hue, saturation, and brightness, which are crucial for applications like colour printing, fabric dyeing, and LED lighting. PID control achieves this by providing a control signal that adjusts the system's input based on the error signal, which is the difference between the desired colour properties and the actual colour properties.

Proportional (P) Term: This term provides a control output proportional to the error signal. In colour control, it helps to quickly counteract deviations from the desired colour properties but might lead to oscillations if used alone.
Integral (I) Term: The integral term accumulates the error over time and provides a control output to eliminate steady-state errors. It ensures that the system maintains the desired colour properties over time, compensating for any persistent discrepancies.
Derivative (D) Term: The derivative term provides a control output based on the rate of change of the error. It helps to dampen the system's response, reducing oscillations and improving stability.

Challenges in PID Control for Colour

Tuning the PID parameters : Finding the optimal values for the P, I, and D gains is critical for effective colour control. Incorrect tuning can lead to poor performance, such as colour overshoot, oscillations, or slow response.
Complex Colour Dynamics: Colour control often involves complex dynamics due to the interactions of multiple colour components (e.g., RGB or CMYK) and the nonlinear nature of colour perception.
External Disturbances: Variations in ambient light, material properties, and other external factors can affect colour reproduction, impacting the performance of the PID controller.

PID control is a crucial technique in colour control, offering a straightforward yet effective way to manage colour properties in various systems. However, challenges such as parameter tuning, complex colour dynamics, and external disturbances necessitate advanced approaches like adaptive control, feedforward control, colour space transformations, and model predictive control to achieve optimal performance. By leveraging these techniques, engineers can design more robust and accurate colour control systems that enhance consistency, quality, and accuracy in colour reproduction across a wide range of applications.

AI in PID colour control workflow;

The use of AI in PID control has many advantages. Artificial Intelligence (AI) is an advanced technology that has revolutionized many industries, including control engineering. One of the most significant applications of AI in control engineering is in Proportional-Integral-Derivative (PID) control. PID is a widely used control algorithm that regulates a process by adjusting the control variables based on error feedback. Integrating AI with PID control can significantly enhance the system's performance, making it more efficient and effective

One of the primary advantages of AI in PID control is its ability to learn from data and adapt to changing conditions. AI algorithms can analyse large amounts of data, identify patterns, and predict the system's behaviour. As a result, an AI component can optimize the control variables and ensure that the system operates at peak efficiency. Additionally, AI can reduce the system's complexity by automating specific processes, thus improving the overall control performance.

However, there are also some challenges associated with the use of AI in PID control. One of the most significant challenges is the need for large amounts of data to train AI algorithms. This data must accurately represent the system's behavior to ensure that the AI can learn effectively.

Another challenge is the need for specialized skills to develop and maintain AI algorithms. This can be a significant barrier for small organizations needing more resources to hire AI experts.

Here are a few potential ways AI could impact colour management in the printing business:

Automated Colour Correction: AI algorithms can analyse images and automatically adjust colours to meet specific standards. This can reduce the need for manual adjustments and result in more consistent and accurate colour reproduction.
Predictive Analysis: AI can analyze historical data and patterns to predict potential color variations and recommend adjustments before printing. This proactive approach can help avoid issues and save time in the production process.
Machine Learning for Calibration: Machine learning algorithms can learn from past calibration processes and optimize color settings for different printing materials and conditions. This can lead to more efficient and precise colour management.
Quality Control: AI can be employed for real-time quality control by identifying colour discrepancies during the printing process. Automated monitoring can help catch errors early on, reducing waste and improving overall print quality.

Customization and Personalization: AI can enable more sophisticated colour customization based on individual preferences. This can be particularly valuable in industries such as packaging and marketing, where unique and brand-specific colours are crucial.

As technology continues to advance, it’s likely that AI will play an increasingly important role in colour management, bringing benefits such as improved efficiency, consistency, and the ability to handle more complex colour demands easily and automatically. However, the pace and extent of these changes will depend on the industry’s readiness to adopt and integrate AI solutions.

Workflow of Colour Control In the Work:

The full closed-loop system consists of several sub-components:

File Conversion: Ensures colour data compatibility.
Ink Pre-setting: Sets ink parameters based on colour requirements.
Scanning: The inline spectrophotometer continuously scans colour.
Automated Closed-Loop: Detects colour drift and triggers adjustments.
Optimization: Fine-tunes colour settings for optimal results.

In summary, an inline colour loop system automates colour control, catches deviations, and ensures consistent colour quality throughout production processes. It’s a valuable tool for industries where accurate colour matters, such as printing, packaging, and textiles.

File Conversion :

The first step in a closed-loop workflow actually begins in the pre-press department during the plate making process. Here, in addition to outputting a high resolution file used for plate making, an additional, low resolution file is also created for use in the closed-loop system. This file can be output as a CIP3/CIP4 file or as multiple 1-Bit TIFF files. These low resolution files are then moved into the file conversion “hot folder” which is located on server that is also accessible to computers in the pressroom. The file conversion process is a fairly simple task that runs in the background and converts the low resolution ripped files into a format that the main software application can use. The conversion happens when the low resolution image file is saved into its’ queue or hot folder and it then merges the image data with press specific data like number of ink keys, maximum sheet size, etc. The resulting, converted, file is then saved into a different network location that is accessible to press-side PC’s. The nice thing about the file conversion software is that one license can perform this task for an entire company. For example, if you have a central pre-press location, you can create ink-pre-setting ready files for all of your press side PC’s (assuming they all have access to the network location).

[CIP3 is an acronym derived from: International Cooperation for Integration of prepress, press and post press. The ‘international cooperation’ is a not for profit organisation with members from a range of industry suppliers. The mission of the CIP3 organisation was to link (pre-set) data from prepress to the print and post-press production phases. The result of the CIP3 groups’ work was a file format known as ‘PPF’ (print production format). It’s been around a long time and manifests itself most commonly in the pre-setting of printing machines from prepress systems.]

2. Ink Pre-setting

First, the traditional ink pre-set method conventional ink pre-sets are typically pre-adjusted by the operator of the press table based on experience. Before starting the machine, the approximate ink amount of each ink zone is estimated according to the distribution of the printing plate or the image of the proof, as a pre-set value of the ink amount, and further adjusted according to the specific change of the printed matter after the booting. Obviously, this kind of ink-discharging method is relatively rough, and the precision is greatly different due to different human operating experience and subjective knowledge. When the pre-discharge method is used from the start-up to the official printing, the paper consumption is large, and the adjustment time is cumbersome, and the work intensity of the machine operator is correspondingly increased. Initial start-up waste increased, and we get good copies after >500~ 800 revolutions, then settling to target density by manual took more time from heavy ink to -> target or low ink to - >target.

Second, pre-inking technology based on CIP3/CIP4 standard; at present, CIP3/PPF or CIP4/JDF, which is mentioned by most manufacturers, is the transmission of pre-discharge data and the maintenance of print quality stability. In terms of auxiliary ink discharge, the most important thing is to help the printing operator to save the inconsistency and waste of manual ink discharge. The required ink discharge data is directly obtained from the pre-press data file, and then transferred to the printing after a simple conversion. The ink control station of the machine makes the adjustment of the pre-discharge, because such an auxiliary ink-discharging method greatly saves time and thus improves the efficiency. Initial start-up waste decreased, and we get good copies between 200 ~ 300 revolutions, then settling to target density by auto is done quickly.

Why press pre-setting is important

The advantages of press pre-setting ;

Press pre-setting provides information to the press about the ink coverage of a job. How much ink is required for each colour, and importantly where on the sheet the ink is. The ‘area coverage information’ is translated by the press into ink-key openings, which set each ink-key position to allow more or less ink through into the ink roller chain, eventually making contact with the plate, offset blanket and finally the paper.

The ink film thickness/ink density is important and allowing just the right amount of ink into the ink roller chain at the job start will deliver the desired densities to the sheet most quickly. This translates into reduced make-ready times and reduced waste sheets, which in turns translate to reduced costs.

Once press pre-setting has been implemented it is essential the ‘ink-key pre-setting curves’ on the press (also known as characteristic curves) are calibrated to convert the area coverage supplied in the CIP3/PPF file into the appropriate ink-key opening.

The pre-setting data includes ink coverage information along with administrative data such as job name, job number, sheet number, sheet side (front/back), and even customer name. It may also extend to production settings such as sheet size, ink name, and even colour bar location for press side colour measurement equipment. A job preview (thumbnail) is also common place. The appropriate ‘conversion’ for each colour (from a number provided by the pre-setting file into a physical press ink-key opening) will be different depending on target densities, paper type, ink type etc. It is therefore essential this is optimised on each press.

These high-end colour measurement systems regulate ink film thickness very precisely and will highlight (and attempt to correct) any discrepancies between target and actual densities immediately. The importance of pre-setting and optimisation cannot be over emphasised when it comes to on-press or near press colour measurement systems, such as MHI DIAMOND EYE system. The number of copies required to meet the target depends very much on the ‘starting position’ (250~500 cps) and in the absence of optimised ink-key pre-setting these ink regulation systems will not perform well, resulting in extended makereadies, follow-ups, waste sheets and much frustration

Already we have discussed image ratio and ink pre-set value calculation, tested with Excel worksheet in Newsprint Ink page with linear interpolation formula [ (y2-y1)/(x2-x1)*(x-x1) ]. The basic concept around pre-set ink screw opening value calculation from image ratio value generated by CIP3/PPF and sent to press console. This curve setting is custom setting can be adjust any time by press person, note that this is not target value setting for press quality control - just pre - opening ink screw value which help to reduce/eliminate (unskilled) human effort by minimizing initial waste.

https://sites.google.com/view/guhansk/newsprint-ink/ink-mileage#h.p_4X8APWPUW13v

3. Scanning Inline Spectrophotometer:

An inline spectrophotometer is a critical component of the system. It’s a device that measures colour directly on a moving production line. Unlike traditional spectrophotometers, which require manual sampling and measurement, an inline spectrophotometer operates continuously and in real-time. It allows you to monitor colour as products move through the production process, catching any colour drift immediately.

CCIW scanning methods;

Finding color bars in the work [Older methods]
CMYK measurement through RGB colour separation [MHI's Diamond Eye system]
CMYK measurement through RGB+I colour separation [here +I refers to InfraRed channel which is used to separate K colour ]
CIELAB optimization [QuardTech Accu-cam system and Q.I. IDS-3D]

The closed-loop colour control system integrates the inline spectrophotometer with other components to ensure precise colour consistency.

Here’s how spectrophotometer helps:

Real-time Monitoring: The inline spectrophotometer continuously measures colour.
Colour Drift Detection: If colour drifts beyond acceptable tolerances, the system detects it.
Immediate Correction feed back: The system triggers adjustments to bring colour back within tolerance by continuous monitoring.
Production Schedule Compliance: By catching colour issues early, you avoid production delays.
ROI: Investing in this technology saves time, reduces waste, and ensures consistent colour quality.

4. Automated Closed-Loop:

In general, closed loop systems work by looking for an error. The error is the difference between the target value and the actual value. The magnitude of the error or delta E is used to drive the system for correction. They usually function based on PID setting parameters gain as set point, some time they may be unstable when the gain is too large and this system will overshoot and fight itself.

CLC Closed-loop colour system or CCIW works with repeated feedback algorithm as said above, so from inker unit to paper substrate spreading of ink is controlled by this loop-system. The system match for set point parameters (target range) with help of sensor output then process (PID) the signal and adjust inker move quality closer to set point target.

Following are detailed information on how this closed loop system works;

Source https://patents.google.com/patent/US6477954B1/en

The amount of ink that is transferred is most important for the print quality. Too much ink in the ink train leads to smearing and blurring of the printed image (high profile). Too little ink leads to faint print (low profile) and uneven distribution of ink colour. Here high and low profile are individual ink key - zone aperture or opening that decide ink film thickness called Ink profile.

Different amounts of ink are required in various zones according to the image to be printed. In order to vary the ink feed laterally across the width of the inker, the ink supply leaving the ink fountain can be adjusted with ink keys. Each ink key thereby defines the ink supply for a respective zone (ink profile of n zone). Depending on the type of printing unit, there may be provided any number of ink zones across the width of the inker.

The inker unit consist of 1 ink fountain roller 2. ink duct or roller 3 oscillators 5 transfers roller 4. The ink vibrator roller 6. Form rollers 7. plate cylinder and 8. blanket cylinder, finally ink is spread onto the paper web substrate 9.

The inking system is divided into zones as said above which lead to equal sized segments throughout all of the inker paths. These segments are discrete elements that can be processed in digitized or discretized format. The number of ink keys at the ink fountain 1 of the exemplary embodiment described herein defines 'n' zones. A command for the printed ink film of each zone n is entered into a controller 10

The controller 10 defines the ink key openings according to its transfer function Gc. The closed-loop system for the pre-setting obtains its error signal from a measuring bar which measures the ink thickness of each of the six zones at the rubber blanket 8.

In a pre-inking operation, the closed-loop system obtains its error signal feedback from a measurement at the form roller 6.

The closed-loop block diagram illustrates above is a printing unit simulation with error signal feedback F(s). The commands for the printed ink films R(s) are entered via a summer 11. The signals are processed in a controller 12 for 10 the ink keys. The controller 12 in the exemplary embodiment is a proportional-integral (PID) controller with a transfer function Gc that includes a proportional gain Kp and an integrated term KI/p, where KI represents the integral gain as shown below.

The feedback of the system is defined by a transfer function H(s) that includes the coverage input with reference to each zone s. The coverage represents the desired zonal coverage determined by the plate scanner or a digital image setter file.

As will be seen from the following exemplary illustration, the controller system transfer function Gc defines the final output signal C(s)—the ink key setting—both after the initializing simulation and the final color control preset calculation. The transfer function H(s) represents the printing plant or printing unit that is being investigated. It may either be a simulation or an actual system. The output of the feedback 14 represents the actual ink film (correct ink profile) or optical density at the rubber blanket 8, or else the actually printed ink film. This sequence is monitored and adjustment to target is made throughout the production process in continuous feedback loop.

QuadTech technology - AccuCam CCIW;

Source: http://www.blog.wan-ifra.org/sites/default/files/field_article_file/%28E%29%20QuadTech_AccuCam.pdf

QuardTech AccuCam press colour control system

The image-based colour control system provides full control from production start to finish by automatically controlling the ink keys and the ink fountain roller. No control element is used for colour measurement in the printed image.

In order to obtain a high degree of precision in determining the colour gamut, the Colour Control System with AccuCam works with a propietary 6-channel spectral sensor that divides the spectrum into six areas. AccuCam also uses the patented QuadTech technology for L*a*b*-based colour control. Greg Wuenstel, AccuCam Product Manager, says: “The AccuCam sensor measures the printed web and calculates the L*a*b* values for the entire printed image. The printed image is then compared with the L*a*b* values obtained from the prepress file as target values. The system calculates the necessary adjustments that must be carried out to the individual ink key zones.“ Once the desired colour setting has been achieved, it will be maintained throughout the production run. According to the manufacturer, the fact that the entire printed image is taken into account and not just individual image areas means that high degrees of precision and consistency are obtained.

The AccuCam colour control system is image-based, i.e. there is no need to include control marks or disturbing colour measurement patches in the page. QuadTech gives as 18 m/sec the maximum web speed at which the system can be used.

AccuCam is a joint development of QuadTech and Newsprinters in Knowsley, England

MHI's DIAMOND EYE for newspaper offset press

Source: https://www.mhi.co.jp/technology/review/pdf/e461/e461048.pdf

The MAX DIAMOND EYE system was designed for commercial web offset presses using MHI’s DIAMOND EYE technology, an in-line quality control system for newspaper offset presses that requires no colour bar, the first such system in the world. DIAMOND EYE determines target values based on digital prepress image data and the printing characteristics and properties (calibrated basic process colour data) of the customer’s offset press.

The system automatically controls the offset press from the start of the printing process by comparing the target values with the actual images on the paper scanned by an image sensor on the press. The advantage of this system is that it does not depend on the skill and experience of the operators to standardize the quality of the tint. Four ink colours (black, cyan, magenta, and yellow) are used for normal colour printing, and it is not possible to separate the colours using a general RGB three-light-source sensor in areas where black and other colours are printed on top of each other.

However, the DIAMOND EYE image sensor is capable of separating these four colours using a special sensor with multiple light sources. The actual image scanned by the image sensor is compared to the target values set from the digital prepress image data and corrected using automatic closed-loop density control. This process requires no color bars in the white space to determine the density, unlike conventional in-line closed-loop density controllers. This closed-loop density control can now be used in printing applications such as newspapers, which cannot go through the colour bar cutting process after printing.

The DIAMOND EYE for newspaper offset press has already been utilized by many customers, and customers have expected MHI to develop the system for commercial web offset press using these technologies since the release of DIAMOND EYE.

MHI's DE use RGB light sources to obtain white base light for calculating reflectance, after invention of white LED's now it is possible to re design the technology in simple way and reduce the size of scanner bar in smart way.

5. Optimization

Finally, the press is being controlled by a closed-loop system! Now, the final step is system optimization. This is the last step in the set-up process where our trained staff reviews the ink pre-set data, closed-loop scans, & the final ink key positions and they make adjustments to the system to improve or optimize its’ performance. This optimization process can usually take place after only a few jobs have been run through the system and the results typically change from 70% efficiency (on the first run) to about 90% efficiency on all future press runs. Then going forward, recommend that periodical test, reviews on quality issues from prepress to press workflow to improve print quality. Re-optimize your closed-loop system to accommodate for drifting changes in the press, paper, ink, plate curves, etc to be closely monitor.

Mostly all inline colour-loop spectrophotometers are image based scanning, fixing target value at beginning stage of installation will be different from current period requirement. So we have to optimize this workflow whenever it is needed. Testing with large target file format like 913 colour patch are used to fine tune the machine and fix/feed target density level to PID colour loop system. And this is not required for re-optimizing, simple test strip on regular workflow is better to the job.

@Pre-press level: We have control over 1. TVI adjustment, 2. profile adjustment TIC etc.. and 3. image ratio adjustment and more that affect colour control loop.

@ Press level: We have options to work calibrate/edit 1. ink pre-set curve, 2. LUT (look up table) minor adjustment according to paper substrate and 3. adjustment in target density value. Here 2 & 3 are automate workflow in control loop.

Set value in PID control are initial parameters that are fed to the program at installation process - called fixing 'Target value', this can't be edit at press level. But we can change the target value by adjusting target density adjustment provision given in press console in percentage level.

If we set/adjust target density example - Cyan 95% then ~0.90d * 0.95 = ~0.855d or 105% then ~0.90d * 1.05 = ~0.945d. From above example ± 5% move density to ± ~0.05d - Adjusting target density value is also the part of optimization. This adjustment may be based on current setting / requirement / standard, based on machine performance or may be change in characteristics of raw materials (LUT adjustment) etc..

Based on external disturbance & feedback there may be changes in target value for quality output, so optimization in CCIW is an important task that has to be carried out frequently when necessary.

Complex Colour Dynamics and External Disturbances that affect CCIW workflow;

Is tonal value increase (TVI) affect CCIW workflow ?
Yes, image gain in printing will defiantly affect CCIW workflow as gain increase or decrease scanners like RGB or RGB+I output separation signal move wrong and CCIW algorithm interprets error, so there will be error feedback loop and chance to alter the quality to negative position by PID. For example more gain in Black (Key) colour may be mislead the program to alter other colours CMY in case of RGB to CMYK separation of scanner conflict inbuild program module.
Remedy; Checking TVI strips in normal workflow production frequently in all units and confirming deviations. And adjustment in TVI curve setting with proper value to achieve correct gain. Some time we may notice full page heavy advertisement and working performance of PID to manage the production, there may be difficult in controlling black key channel or CMY channel in real-time.
Can profile adjustment TIC, GCR, UCR colour dynamic factor affect CCIW?
Yes, Scanner bar colour separation in CCIW is completely different from colour editing in photoshop and altering/changing profile will disturb Auto colour correction in the CCIW workflow, like said above (1) initial setup PID automation is different from external disturbance in colour complexity .
Can we say Ink pre-setting is target value?
No, Ink pre-set is key opening at initial stage (make-ready) of printing that is based on image ratio given by pre-press PPF job file. This image ratio vs ink key opening setting is fully controlled by press. Fine tune of pre-setting will reduce only initial waste on production start, but this is not target value even they are based on image coverage this pre-set curve is adjustable at any time. By monitoring auto correction and comparing the pre-set value after good copies, this per-set curve can be optimize or fine tune to near target value. This work really reduce the time of auto correction to reach good copies quickly.
Uneven pre-set take more time for PID to settle down to target range, so it is must to check and optimize pre-setting curve at press console.
Can Duct rotation curve setting in press console affect?
Yes, while machine ascend or descend ink supply get changed due to duct rotation curve setting, while improper setting is made that will affect the target density and take time to settle down to target range (give more work to PID correction). So it is important to fine tune duct setting curve in press console.
What is ink key drive oscillation or fluctuating movement of ink key opening?
Improper PID programming error or external disturbance may cause over ink key drive movement and colour inconsistence. PID workflow has complicated algorithm threshold lock system and error monitor loop section that rectify ink key drive oscillation that has to be confirm.
What is target density in CCIW workflow?
In-line colour scanners (image-based) that depend on RGB or CIELAB channel separation converted to CMYK that has capacity to measure and compare image with originals. So there is no exact density value, but in range between setpoint ~ target (Example Cyan Target = ~0.90d, ± 0.01d). Older methods on CCIW catch colour bar and give density readings as we get traditional way.

What is LUT (look up table) in MHI machine ?

LUT is a set of data, applied based on verity of substrate with their coefficient value for each channel (CMYK), if there is minor adjustment in target value due to change in substrate whiteness this value correct the quality output. This LUT value play important roll in CCIW workflow, not only altering target based on paper colour they visualise the difference between paper verity too. Small value [Example -NP Aspex paper LUT value >> K -5, C -3, M -4, Y 2 ] brings big difference in CCIW workflow process.

New Target = Current target density + LUT value * (0.1/Pre-set value)

For example ;- Cyan LUT -3 with target ~0.90d gives = ~0.80d as result , and if LUT 4 >> ~0.90d + 4* (0.1/Pre-set value) = ~1.03d. From this example LUT >> for the New target density LUT negative integer will move down the density and positive increase the density.

This LUT values are created after setting/Fixing -> Sensor Adjusting Target density function. If target density is adjusted, then all LUT data has to be re-set to fresh. And also having different Target density between units has to be consider while applying LUT for the same, it behave differently as target differs between them.

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