How to get accurate basis weight measurements when the QCS is offline

Previously, when the QCS was unavailable, the sheet weight on the paper machine was unmeasurable. Now Virtual Measurement techniques solve this dilemma.

Today’s producers of quality grades must meet stringent customer demands for uniform paper, both in the machine-direction (MD) and cross-direction (CD). That’s why accurate measurement and control of basis weight are crucial in meeting end-customer quality specifications.

But operators are left in the dark when a Quality Control System (QCS) goes offline for a sheet break or start-up event. And that means operators can’t ensure paper meets specifications until the scanner is back on the sheet without additional inputs. 


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What are Virtual Measurements?

Virtual Measurements, also known as soft sensors, are data models generated by machine learning technologies that use key process variable data points to produce inferred calculations of paper properties. By adding an online Virtual Measurement component to traditional monitoring and testing methods, papermakers can get valid data-based estimates of pulp and paper properties – which for various reasons may not always be directly measurable in real-time or with a physical sensor.

Weight Virtual Measurement gives faster start-ups and reduced rejects

Weight Virtual Measurement allows inferred calculations to estimate weight measurements when the physical measuring equipment on the QCS is either off-line or not scanning.

This information is especially valuable for the machine crew during a sheet break, since it helps them get the sheet weight back on target faster and therefore minimizes both:

  • Total minutes of downtime
  • Total tons of off-spec paper produced.

In the sheet break example below (Figure 1) we compare and visualize what occurs when the QCS is off-line and how the virtual measurement (the red line) steps in to indicate what the estimated sheet weight is, in the absence of the QCS weight scanner. Operators can adjust inputs such as adjusting the machine speed to keep the weight on-target even before the QCS comes back online. 

Figure 1

Estimated Weight Virtual Measurement displayed

Figure 2

The main screen (Figure 2) below gives mill personnel real-time data for the weight measurement over a configurable time range. The estimated Weight Virtual Measurement value, in this case 43.56, is shown at the top left, and is updated as frequently as every 5 seconds. This weight Virtual Measurement value is based on various upstream process inputs from stock prep, the approach flow system, and the wet end. The key process inputs used here are:

  • Stock flow rate
  • Consistency of the stock
  • Machine wire speed

From these values a very accurate inferred Weight Virtual Measurement can be obtained.

Figure 2 also shows trendlines for both the Weight Virtual Measurement and the QCS weight measurement on the left side. The green line is the scanner measurement, while the blue line represents the virtual measurement, and the red vertical shaded area indicates the sheet break duration.


Continuous self-correlation for best accuracy

The status of the Virtual Measurement is shown in the block immediately above the trendlines. During normal operation, this box will remain gray and read ‘Normal’. This indicates that the virtual measurement is receiving valid inputs to calculate the inferred weight, and also that a valid scanner measurement of weight is available. 

During this normal operation, while the scanner is on-sheet and measuring, the virtual measurement continuously correlates itself to the scanner measurements – to improve accuracy during the critical times when the scanner is unavailable.

If there is a lack of data for the online calculation, or the input signals are invalid, an error message such as “out of range” appears.

When the machine starts up, so do the input signals, and the virtual measurement can pull data to calculate the inferred weight. The status changes to “estimating”, turning a bright blue to draw attention.

This would be an indication that the QCS scanner is offline. It’s also when the Weight Virtual Measurement is most valuable, becoming the stand-in for the now missing physical measurement of weight.

Did you know?

It is still considered downtime when the machine is running but the sheet is not threaded to the reel. If the paper stays on target, the sheet is easier to rethread, reducing downtime.

Operators can stabilize weight before scanner is back in action

By leveraging Weight Virtual Measurements before the scanner is back online, the PM operators can take step-change actions, such as manipulating the stock flow, to reach the target weight. This sharply cuts the time needed to get back on spec, reducing the impact and cost of a sheet break.

Weight Virtual Measurement can help you get faster start-ups, lower rejects and improved profitability.

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