ABB’s HPIR-R moisture sensor adds another high-speed moisture sensor to complement the successful HPIR-T sensor. HPIR-R measures paper surface moisture and temperature using infrared spectroscopy in some of the most challenging and extreme environments in the pulp and paper industry.

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_{Michael O’Hora ABB Pulp and Paper Dundalk, Ireland michael.ohora@ ie.abb.com

Håkan Hjalmers ABB Pulp and Paper Dundalk, Ireland hakan.hjalmers@ ie.abb.com}

Papermaking has a history that stretches back over thousands of years, but only in the past few decades have effective tools for paper quality evaluation become available. Even as late as the 1920s, companies relied upon the innate skills of operators who checked paper uniformity by sight, reel hardness with a stick and moisture content using the effect of static electricity on their arm hair.

The moisture content of paper was an early target for innovation and in the late 1920s, electrical conductivity techniques were employed to measure it. These primitive, “better than nothing” approaches gave a good indicator of moisture but provided very little in terms of trending, cross-sheet profiles or control capability.

The fundamentals of a paper production process require moisture as a key metric to see, firstly, the moisture content of the process and secondly, to combine with a total mass measurement to partition dry weight and water weight. Dry weight represents the weight/cost of feedstock applied; moisture/water weight the amount of drying (and associated energy costs) needed to achieve an optimal setpoint at key locations in the papermaking process.

Blink and you’ll miss it

Water and the organic materials within paper webs exhibit strong vibrational absorption bands across the electromagnetic spectrum’s infrared (IR) region. As a result, moisture concentration is typically estimated by differential absorption measurements in adjacent moisture and cellulose combination bands within the near-IR (1.7 to 2.2 μm). The IR approach has long been favored by moisture sensor developers – IR has a unique ability to measure from the lightest tissue to boards, from zero moisture up to the saturation limits of paper.

The signal processing used in ABB’s older generation and competitors’ versions of such IR moisture sensors exploits amplitude modulation and subsequent demodulation of the IR radiation to suppress any background phenomena that may influence measurements and overcome fundamental detector limitations. However, this modulation is usually accomplished by a mechanical IR beam chopper, which means, rather like a blinking eye, the system only catches measurements when the eye is open. This situation has disadvantages incompatible with many of today’s high-speed production processes. For example, the measurement speed is limited by the modulation frequency (the Nyquist limit) and the ability of the detector electronics to pre-filter and demodulate the signal.

ABB’s new HPIR-R sensor adopts a fundamentally different [1] and simpler approach to signal processing for IR moisture measurements. Instead of using amplitude modulation of the measurement beam, the instrument continuously delivers IR energy to the web process, ensuring that the instrument measures at the maximum potential speed (limited only by the photovoltaic detector characteristics) with the best signal-to-noise ratio and speed available in the market.

HPIR-R sensor design

The HPIR-R module contains a tungsten halogen lamp that provides constant and intense broadband visible light and an abundant and cheap source of IR →01-03. The module also includes a shutter that can be rapidly inserted in front of the lamp for a short time to block IR energy from reaching the web so that a normalization offset – a combination of electronic offset and background optical signal – can be established. So that no process data is lost, this background normalization is performed when the scanning instrument pauses to turn around after every five or more scans across the paper sheet. The shutter is also closed when the sensor is “parked” by the side of the sheet. During normal operation, the shutter is always open, allowing the high measurement rate that is HPIR-R’s defining characteristic.

HPIR-R’s optical components efficiently concentrate and guide the source lamp’s IR energy toward the paper surface through a window →04. The reflected energy is picked up by each of the three detectors (detecting a specific wavelength using an optical filter) and the light absorbed at each wavelength relative to a reference wavelength will be indicative of the concentrations of water and cellulose within the paper surface.

The detector consists of a fiber-optic multiplexer with a fiber-optic bundle (some 3,000 individual fibers) structured as a random network within the multiplexer to deliver input energy randomly to three termination ports. Fiber optics are a unique enabling technology for ABB infrared measurements. They are highly efficient, vibration- and temperature-insensitive, robust and allow a very simple, compact and lightweight device design. The flexible optical fibers allow optoelectronic components to be mounted in an optimal configuration for product packaging and temperature control. Other approaches involving mirrors, beam splitters and diffusing spheres are less space-efficient and susceptible to many environmental factors.

At each of the termination points, an IR bandpass filter is used to select the three near-IR wavelength bands of interest (reference, water and cellulose). Signals are then directed to three photodetectors to produce an electronic signal in proportion to the IR optical power incident on the photodetector. The photodetector signal is then conditioned by amplifiers set at an optimal gain to maximize digitizer resolution. Each detector channel has an equal view of the same spot on the process, allowing for continuous and simultaneous measurement at each wavelength. No backing tile on the other side of the sheet is needed.

All optoelectronic components are housed within a single air-cooled, temperature-controlled enclosure. HPIR-R is designed to operate in temperatures of up to 125 °C to measure moisture concentrations of 0 to 70 percent over a dry web weight range of anything from the lightest ply of tissue to the heaviest board. The measurement has an absolute accuracy of 0.25 percent and a resolution significantly better than 0.01 percent. This sensor is delivered as a single product type with a comprehensive factory calibration suitable for any paper type it will encounter, which makes the sensors fully interchangeable.

HPIR-R is field repairable, which, coupled with its insensitivity to dust and disturbances, enables high uptime and lower total cost of ownership than similar solutions. Customers can be located deep in a forest close to raw materials and energy or adjacent to city centers, so field repairability is crucial. Most repair steps are part swaps requiring only hex wrenches and a few minutes. Furthermore, as scanners are embedded deep in the paper machine, ABB outlines a preventative maintenance program (every two years) that can be synchronized with machine downtime.

Compensating for extraneous IR effects

The hot web process will also emit broadband IR energy. The irradiance and spectral characteristics of this emission will depend on web temperature. To “remove” this effect from the main IR signal, HPIR-R has an integrated auxiliary IR pyrometer that provides a simultaneous, separate web IR-emission measurement. This reading is also used to compensate the moisture measurement for other background IR effects. In other words, HPIR-R gives the papermaker the bonus of a simultaneous temperature measurement, providing additional insight into their paper surface characteristics.

Fastest moisture measurement on the market

HPIR-R produces up to 5,000 fully integrated, statistically independent moisture measurements per second. This rate is unprecedented and unmatched in the industry. For comparison, HPIR-R delivers 83 times more (100 times more in 50 Hz systems) data points than its 1973 equivalent, with a spatial measurement resolution of just 6 mm – the same dimension as the fingertip measurement of the early days of papermaking.

Part of the ABB Ability™ Quality Management System, the HPIR-R IP-format output is fully digital, which ensures signal integrity. The torrent of data generated can immediately be sent through the mill’s fiber-optic backbone to an edge server, quality management applications and the cloud. The high-speed measurements are accompanied by a rich set of diagnostic data that is ideal for analytics to inform onsite or corporate-level decision-making. HPIR-R is applicable to all ABB Network Platform scanner types, ready to be used to measure publication grades, graphic papers, board, tissue, packaging, coated products, colored products, specialist and recycled grades.

Cost and energy savings

The unprecedented speed and accuracy of the moisture measurements delivered by HPIR-R’s patented technology lead to optimized drying, decreased steam usage, significant energy savings in dryer sections and reduced carbon footprint. For instance, changing the steam pressure to create a moisture change of just 1 percent can equate to $ 400,000 in annual energy savings. The small spot of IR energy used in the sensor provides excellent streak resolution and accurate edge-to-edge sheet measurement, even in the most challenging environments →05.

While the sensor can be placed anywhere, the highest impact location is before a size press, where better precision and accuracy allow operators to raise the moisture target, giving optimized starch/sizing pickup and reduced drying energy demand →06. Another high-value location is after the wet press section, where the sensor enables operators to adjust press loadings to improve the moisture profile and increase sheet dryness going into the dryers. In addition to energy savings, this fine-tuning also gives higher sheet strength in this sensitive area of the machine, meaning improved runnability and sheet break recovery.

Confidence to raise moisture targets and tighten tolerance bands

With the fastest moisture measurement available today, HPIR-R provides the precise, edge-to-edge measurement that gives operators the confidence to raise moisture targets throughout the process, thus improving profiles and decreasing energy usage. Further, because the exact moisture levels are known at all times, a tighter moisture target zone can be applied. This improvement immediately translates to significant energy cost savings as it eliminates over-drying some areas of the sheet in the effort to make sure all areas are within their moisture specifications. Moreover, the papermaker can get to on-specification product faster, helping to increase production tonnage, save energy and reduce quality losses.

The data benefits of not blinking an eye

Beyond the basics of moisture control, HPIR-R’s unblinking eye enables the global shift whereby companies increasingly adopt Industry 4.0 working practices and the Industrial Internet of Things (IIoT) to serve up all-important operational data. Modern information-based systems thrive on sharing and analyzing data so quality levels can be constantly gauged and compared historically against variations in shift patterns, operator capabilities, seasonal temperature ranges and raw materials, to name but a few.

ABB’s HPIR-R moisture sensor adds another high-speed moisture sensor to complement the successful HPIR-T sensor. HPIR-R measures paper surface moisture and temperature using infrared spectroscopy, bringing spectroscopy and high-performance measurement to some of the most challenging and extreme environments in the process industries.

Through the rising steam, recycled residue, vibration and high temperatures of a modern paper machine, HPIR-R peers at the surface of the running paper in the infrared to reveal those invisible “colors” of chemistry. Millimeter by millimeter as it traverses the paper width, HPIR-R builds up data of the surface moisture and surface temperature of the paper at 5,000 measurements each second. Moisture data, combined with basis weight sensor data, also reveals the dry mass of the process.

These data reveal the minute variations of the paper during production, assembled as a digital fingerprint feeding ABB control algorithms with the information needed to reduce the variability of the process and rapidly and efficiently achieve and maintain the desired setpoint. In such a high-speed and energy-intensive process, minute variations accumulate very quickly, so fast detection and response is critical. Control of moisture has been and always will be fundamental to the efficiency and sustainability of paper production.

Transforming ABB’s sensors for an era of sustainability and digitization is the challenge. HPIR-R meets this challenge with a compelling, digitally native solution that draws on the best of ABB’s long experience and combines it with modern technology to make a product that will address the needs of the papermaking industry in the coming decades.

_{Reference

[1] S.P. Sturm et al., “Method and apparatus for on-line web property measurement,” U.S. patent, US8148690, April 3, 2012.}

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Reflecting on paper moisture