Weighing Technology

Weight is an often measured and controlled variable. Checkout stations at supermarkets weigh produce to determine price. Roads and bridges set vehicle weight limits. 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.

10/01/2004



To read an expanded version of this article click here.

Product Focus: Scales and Load Cells
Weight is an often measured and controlled variable. Checkout stations at supermarkets weigh produce to determine price. Roads and bridges set vehicle weight limits. 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 on the loading dock 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.

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. (See accompanying graph, 'Primary applications for industrial scales and load cells.')

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.'


Domain-impact and event-frequency risk assessments produce index numbers applicable to a risk-ranking model, thus prioritizing equipment, instrumentation, and control system risk-reducation opportunities.

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.

Another 'discrete manufacturing area' would be product differentiation. Examples include charcoal briquettes sorted 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.

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 end, included:

  • 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 to determine quantity on hand.

The expanded online version of this article includes a diagram, 'Types of weighing operations,' that shows the specific ranking and distribution of these operations by percent of respondents.

Electronics improve results

According to survey results, industrial weighing systems are now overwhelmingly electronic, whether they use strain-gauge elements, electronic transducers, or sensor technologies. 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.

The various load-receiving elements that respondents use may be reviewed in the graph, 'Types of industrial scales used as defined by load-receiving element,' included in the expanded online version of this article.

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.

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 products

For more manufacturers, visit www.controleng.com/buyersguide . For system integrators, www.controleng.com/integrators . Also visit company Web sites listed.

ONLINE

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Configurable OI


Jagextreme graphical operator interface (JXOI) provides visual representation of weighing operations so that users can rapidly and intuitively manage their processes. Display offers 256 background/foreground colors, numerous libraries of visual objects, and user-defined keys. Built-in Ethernet, PS2 keyboard port, and RS-232 serial port are said to allow for greater connectivity.

www.mt.com

Mettler Toledo

Compression load cells


LCKD Series compression load cells measure load ranges from 2.2 lb to 1,000 lb. Small devices feature stainless-steel construction and high-performance strain gages to ensure linearity and stability. Miniature electronics in the load cell cable perform temperature compensation.

www.omega.com

Omega Engineering Inc.

Module 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..

www.hardyinstruments.com

Hardy Instruments

Programmable device


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.

www.rlws.com

Rice Lake Weighing Systems

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. Rated capacity is 100,000 lb with a 300% overload protection factor.

www.fairbanks.com

Fairbanks Scales

High-resolution scales


Combics platform scales feature optional 316 stainless-steel base construction and ultra-sanitary electropolished weighing surfaces. Durable devices meet IP65 washdown standards. Users can specify both base and indicator for IP67 or NEMA4X protection. Painted steel or stainless-steel tread plate provides added safety. With extended weighing ranges of 5,000 lb or more, the scales are intended for use in pharmaceutical, chemical, and electronics industries. They can also be used in food and food processing.

www.sartorius.com

Sartorius GWT

Expanded load cell family


LD360 family of load cells now has capacities of 1,000 to 25,000 lb. Line includes high-value process application LD3xi and rugged LD3xiC. Both are intended for demanding use, including processing high-risk, high-cost materials in sanitary CIP/SIP applications. Load cells can be matched with the company's SVS 2000 controller, which features 'Quick-config' calibration and selectable speed and resolution parameters.

www.kistlermorse.com

Kistler Morse





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