Weighing Technology (Product Focus: Scales and Load Cells)
Dick Johnson -- Control Engineering, 10/12/2004
To read the original print article click here.
Weight is an often measured and controlled variable. Checkout stations at supermarkets weigh produce to determine price. Roads and bridges set vehicle weight limits. Most food-stuffs, whether boxed or canned, are marked with net weight. Airlines monitor luggage weight. Railroad cars, cement trucks, and over-the-road bulk haulers are marked with a load limit. Industrial feedstock and most raw materials are sold by weight. So are precious metals and gemstones.
Body weight has become a serious health issue, spawning many products and services to help control it. Yet, the basic instrument to monitor weight remains the same: the lowly scale. Whether at home or at the health club, in the doctor’s office, or on the loading dock, it too often is a low-tech device. However, it is effective for the task at hand, and today incorporates advanced electronics that make weighing tasks more accurate and reliable.
New research: many applications
In its first look at trends in the industrial scale/load cell user marketplace,
Control Engineering and Reed Research Group — both part of Reed Business
Information (www.reedbusiness.com)—surveyed subscribers by e-mail and
over the Internet, asking them about their preferences in industrial scales/load
cells and related issues.
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| Weighing and load cell technologies have a variety of applications, with the largest category (35%) falling into both continuous and batch processing operations. |
The survey indicates that weighing and load cell technologies find use in a variety of applications. Survey returns revealed 35% of respondents used them in both continuous and batch processing. Batch processing operations accounted for 22.8% of all use. Scales and load cells have also found their way into discrete product manufacturing (17.1%) and testing (14.6%) applications. Continuous processing only (7.3%) and Other (nondefined) operations (3.3%) accounted for the remaining responses. The accompanying graph, “Primary applications for industrial scales and load cells,” illustrates the distribution pattern.
Defining use of weighing technology in continuous and batch operations is simpler than in discrete manufacturing or testing applications. Whether ingredients are chemically combined or simply mixed, using available weighing technologies to determine and record ingredient proportions is relatively straightforward.
According to Ted Kopczynski, process weighing product manager at Hardy Instruments, use of scale technology in process and batching operations has increased over the years. Use in control systems has changed as well. “Hardy is seeing a big trend in manufacturers moving to more automation using common communication networks, with more of the process’ data being available over a company's intranet,” says Kopczynski. “We are also seeing comparable growth in scale use for technology for discrete manufacturing and testing operations. More manufacturers want to get results automatically into a database.”
Unlike weighing technology in process/batching operations, discrete product manufacturing involves individual unit production (or testing) and usually low volumes and high complexity. An example would be an injection-molding machine in which the extracted part is weighed to ensure no material is left in the mold and determine that the part had been successfully made—a form of check weighing.
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| Besides basic weighing tasks (50.4%), various other types of operations were cited in the survey, including calculation by weight (39.2%), force measurement (26.4%), and web tension measurement (4.8%). |
Another “discrete manufacturing area” would be product differentiation. Examples include charcoal briquettes sorted by “brand name” or “house brand,” according to crush strength; ultimate strength and hardness of concrete test cores determined; and failure strength of finished products, like chain or rope checked. All these use variations of the universal testing machine, essentially a mechanical or hydraulic press incorporating scale technology [usually a load cell or strategically placed strain gage(s)] to record loads on test specimens.
Weighing for this and that
Industrial weighing technology varies greatly. Not all users are interested
in weight as the final value to be controlled and/or recorded. Of actual operations
reported by respondents, only four fell into that category:
- Basic weighing;
- Batching
- Pre-weighing or filling by weight; and
- Inspection by weight (check weighing).
Other operations, which apply weighing technology to a different needs included:
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| Survey respondents named many types of load-receiving elements. Bench types were listed by more than half (58%), followed by floor or warehouse (52.9%), portable (43.7%), and hopper or tank (31.9%). |
- Force measurement, a variable often sought as part of dynamometer testing; and
- Web-tension measurement, a critical control variable in printing, papermaking, and metal fabrication.
Finally, scales can be used for comparative operations. These include:
- Calculation by weight—determining bulk volume or level using bin dimensions and net weight; and
- Counting by weight—using a scale to compare weight of a single inventory item with the total weight of all similar items on hand to determine quantity on hand. The inventory taker can do the math, but, more commonly, calculations are performed by a “counting scale.”
The “Types of weighing operations” diagram shows the ranking and
distribution of these operations.
Electronics improve results
According to survey results, industrial weighing systems are now overwhelmingly electronic, whether they use strain-gauge elements, electronic transducers, or sensor technolo-gies. Increasing use of electronics began with improvements in load cell technology. In the late 1960s, mechanical systems first started to decline in popularity.
Terry Mackin, process weighing business leader for Mettler Toledo recalls that conversion of older mechanical scales to electronic-based weighing technology got into full swing in the mid-1970s. The change required replacing mechanical links to the load-receiving elements with load cells to harness the advantages of the new technology. (Review various load-receiving elements that respondents use in the accompanying graph, “Types of industrial scales used as defined by load receiving element.”)
Reasons to convert mechanical weighing systems remain compelling. Electronic scales are more accurate, repeatable, and less susceptible to errors caused by environmental factors, such as vibration and shock.
“New mechanical systems account for less than 1% of our business at this point,” says Mackin. Growth of electronic weighing is warranted, he goes on. New electronic scales and scale systems are dependable and less expensive to integrate into new and existing automation systems, much easier to maintain, and provide real-time readings that can be easily integrated into a company’s business system. “In many applications, the scale is the ‘cash register.’ Keeping track of material inventory and exchange in real time and error free is essential,” he adds.
Weighing technology continues to advance. Even the simplest scales have shifted to electronics. Developments in battery technology, electronics packaging, and miniaturization that have helped drive industrial scale development are available to let you know just as accurately and repeatably at home if your diet is still on track.
Weighing technology products
For more manufacturers, visit www.controleng.com/buyersguide. For system integrators,
www.controleng.integrators. Also visit company Web sites listed.
Configurable operator interface
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Mettler Toledo
Versatile, rugged truck scale
70X10 Titan truck scale comes in above-ground permanent scale and portable unit
versions. Both share the same features and benefits. Steel deck truck scale
features rocker column load cells as load receiving elements. Accurate and durable
load cells are hermetically sealed with glass-to-metal welds impervious to moisture.
Entire cell is jacketed in stainless steel for protection against rodents and
fraying. Rated capacity is 100,000 lb with a 300% overload protection factor.
All incorporate Fairbanks’ Intalogix technology, which uses a digital
signal more than a million times stronger than conventional signals to eliminate
voltage spikes and surges from lightning strikes.
Fairbanks Scales
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Module optimized for controllers
HI 1769-WS single-channel PLC-style weigh scale module is optimized for use
with Rockwell Automation controllers. It reads and conditions weight and diagnostic
data supplied by strain gauge load sensors or scale load cells and communicates
over an integrated backplane to Allen-Bradley CompactLogix or MicroLogix 1500
processor. Device limits installation costs because it needs no external standalone
scale instrument and wired communications link to the PLC. Module includes manufacturer’s
core technologies, including WaverSaver, which eliminates vibration effects
and provides a stable weight display, C2 electronic calibration capability,
and Integrated Technician system diagnostics and trouble-shooting techniques.
Hardy Instruments
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Programmable indicator/controller
920i programmable indicator/controller is designed for multi-scale, data acquisition,
batching, and industrial automation applications. It features a large LCD that
can be custom configured with weight characters in sizes up to 1.2 in., active
bar graphs, status messages and icons, operator prompts, and multiple soft key
functions. Modular design provides open connectivity with Allen-Bradley remote
I/O and DeviceNet, Profibus DP, and Ethernet systems. Unit allows up to four
scale displays with legal-for-trade information, or up to 32 weight-only displays
that can include individual scales and any combination of totalized scales.
A truck in/out program stores up to 1,000 identification numbers. System can
accommodate a custom database program for storing information, such as inventory
records, mix formula, and shipping information.
Rice Lake Weighing Systems
Compression load cells
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Omega Engineering Inc.
High-resolution scales
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Sartorius GWT
Expanded load cell family
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Kistler Morse
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