Weighing Technology Can Be Pretty Heavy Stuff

Determining the weight of a quantity of process media is a necessary part of many process applications. Weight can be used to infer the level of both liquids and bulk solids in hoppers and bins. Weighing is a necessary function in batch operations because recipes usually call for ingredients to be combined by weight.

03/01/2000


KEY WORDS

 

  • Process control &instrumentation

  • Process control systems

  • Weighing technology

  • Load cells

Sidebars:
Coal weighing: Making the best of a bad environment
Communication ensures quality concrete casting in the cold


Determining the weight of a quantity of process media is a necessary part of many process applications. Weight can be used to infer the level of both liquids and bulk solids in hoppers and bins. Weighing is a necessary function in batch operations because recipes usually call for ingredients to be combined by weight. Weighing may be used in custody transfer operations, especially when the media to be exchanged is very expensive or the amount to be transferred is very small. Finally, weighing can be used as part of a filling, packaging, or packing operation. These operations can very dynamic, requiring weight to be determined accurately as products pass over a sensor at high speeds.

According to David Thayer, senior applications engineer at Hardy Instruments (San Diego, Calif.), 'Weighing technology is broadly distributed over a wide 'food chain.' At the low end are just scales. These include consumer equipment such as bulk food and 'deli' devices, medical scales, and the like. The next step up includes truck, train, and airplane scales. These are sophisticated, heavy-duty devices that can be networked to a PC for recording and tabulating values measured. Single- and multilevel batch weight scales are next. Communication is key to their function because they are hooked to PLCs or other computing devices to open feed gates, start conveyors, etc. They can be networked for multi-ingredient applications. Weigh conveyors are in this class of equipment also.'

The business end

The vast majority-estimates put it at approximately 98%-of weighing applications depend on the strain gage load cell. This technology, which sprung from the 1930's development of the strain gage, served industrial applications for the better part of the 20th century. Some load cells have been in service so long that when time comes to replace one, the control engineer often finds the original device is undocumented or the original installer has long ago left the company. A mean time between failure of 10 years has a tendency to outlive filed paper documents and some single company career spans.

According to Sheldon Shepherd, product manager at Milltronics Inc. (Arlington, Tex.), longevity of load cells often allows users to update electronics only during a plant upgrade or expansion. Mr. Shepherd says, 'Load cells are often in excellent shape even with years of use. An electronics update may be all that is needed to bring communications capabilities into line with digital bus-based systems.'

Even though they are ubiquitous and long-lived, load cells do have limitations. Strain gage load cells work best between five and 1 million pounds because they offer no special advantage at very low weights. It can be difficult to get measurable deflections with only a few pounds. On the other hand, very heavy loads require very large load cells and the equipment needed to perform the task quickly becomes cumbersome.

The Advantage line of load sensors from Hardy Instruments offers improvements to this basic technology. The advent of highly reproducible manufacturing techniques, such as robotic assembly and advanced quality assurance, has allowed matching parameters of even competitively priced sensors. Off the shelf, they are calibrated in both mV/V/Ù and mV/V. Because of this, these load cells can be replaced in a weigh system without the need to recalibrate or corner adjust the system.

Use of high-pressure water washing, steam cleaning, and cleaning chemicals can be problematic for most transducers; load cells are no exception. Load cells, because of their function, are usually located at the base of a container assembly. They are often located outdoors 'in the weather.' Hermetic sealing is employed at the cable entry using a welded fitting. The gauged area is also hermetically sealed. Use of mechanical sealing avoids the problems of long-term sealant degradation.

Other features of Advantage load cells include increased safe and ultimate load capacities to help prevent overload damage, increased combined error (the combination of nonlinearly and hysteresis specified as a percent of rated output), and built-in conduit adapters for easy connection. The devices also feature grounding straps to protect them from high currents, which can be generated on a scale by static caused during a filling/emptying process or lightning strikes.

Handling moving weight

Because the load cell is not the right sensor for every application, other technologies have been successfully adapted. A case in point is a low weight, dynamic weighing application. The Northwood Panelboard Co. makes oriented strand board (OSB) in its Solway, Minn. plant. OSB is made using oriented wood chips bonded together and laminated in layers with resin to obtain its structural properties. The lightweight 'mats' needed to be weighed accurately 'on the fly' at about 100 ft/min. Ohmart/Vega Corp. (Cincinnati, O.) designed its W-4500 nuclear weigh scale specifically for this type of low-loading application (down to 0.5 lb/sq.ft).

The W-4500 nuclear weigh scale has no moving parts and uses radiation sources and a detector to measure product on a conveyor belt that passes between them. Individual gamma ray sources emit radiation, which is collimated (focused) into a curtain of radiation across the entire belt. The scale is 'zeroed' on an empty belt (any belt material is acceptable). Product on the belt attenuates the radiation, which means the detector measures less radiation.

The detector signal can be calibrated to either process loading (weight/length) or area density (weight/area). By adding a tachometer, the process rate (weight/time) can be calculated. Similarly, with the addition of an ultrasonic gauge, the bulk density of the material can be calculated. For engineered wood products, the primary benefit of the W-4500 is its ability to minimize board weight variation. This helps save product when bringing the line up or down, and also is essential in reducing wasted product from overweight boards. According to Kevin Carmichael, Ohmart/Vega's director of engineering, the W-4500 can operate as long as six months without a calibration adjustment.

In the OSB industry, chips and resin are the primary material costs. Typically, OSB lines that employ a scanning nuclear detector must set the control target weight 4-5% above the desired weight to consistently meet minimum board weight criteria. This large variance is due to the scanning gauge only measuring about 2% of the board and then predicting average board weight from that sample. At plants where the W-4500 was installed, its ability to measure all the wood and accurately report average board weight allowed operators to set the control target weight only 1% above the desired weight. This translates into wood and resin savings of 3-4% of the total material costs. 'At a typical plant, this can mean savings of as much as $500,000/year,' Mr. Carmichael says.


Quick and accurate

Although new in the 1980s, weigh cells based on the electromagnetic force compensation principle-also called magnetic force restoration (MFR)-have evolved into the standard for the European food and drug packaging industries. MFR weigh cells were developed to provide fast, high-resolution weight measurements with live-load capacities as low as 2 grams. In the process arena, the technology is intended for applications such as checkweighing, micro-ingredient filling, quality (SQC/SPC) control, and quality assurance validation. MFR devices feature inherent self-damping, high resolution (up to 1 million counts internal accuracy), variable dead-load capability, compact form factor, and protection against hostile environments (steam or water wash-down, etc.).

According to Dan Garlan, chief engineer at Kg Systems Inc. (East Hanover, N.J.), system integrators and manufacturers of custom automated weighing systems, MFR weigh cells have added value to many machines, especially in precision time-critical applications. 'Up to 400 units per minute can be weighed with short settling times-well below 200 msec,' Mr. Garlan says.

Improved static and dynamic performance of the MFR cell results from an electromagnetically produced counterforce (see accompanying diagram) that compensates for the applied load. The device's precision position control keeps the system stable. As soon as the slightest movement in the system is detected, a feedback circuit is initiated that runs current through the coil, returning the load to its original position. The current-proportional to the load weight-is transmitted through an internal A/D converter to the onboard microprocessor. The weighing system can be optimized in the user's environment by filter parameter and averaging configuration. This guarantees high precision with short measurement time. Readability of these types of load cells is as low as 0.0001 g. Compensated temperature ranges vary. Environmental protection is IP44 or IP65.

Getting systems to communicate

One of the major problems facing the control engineer responsible for integrating weighing parameters into a process is the difficulty in getting weigh sensors to 'talk' to the rest of the system. Mettler Toledo (Worthington, O.) has introduced MTConnections, a product said to eliminate the need for custom engineered interfaces by providing valuable process data through a standard open interface-such as dynamic data exchange, OLE for process control, open database connectivity, and ActiveX controls.

Process data communications concepts are complex, and so many end-users expect problems when interfacing special-purpose weighing and measuring instrumentation. According to its developer, MTConnections will lower end-users' connectivity and maintenance costs and providealong with the freedom to select 'best in class' open technology process control solutions. And, with connectivity to any Mettler Toledo instrument or balance, end-users will not have to replace existing working equipment as the push for plant-wide integration of process weighing equipment and data.

For example, in pharmaceutical processing applications, certain individual components, such as colorants or active ingredients, must be pre-weighed. Most pre-weigh operations are manual or semi-manually controlled, where the operator must multitask between the process weighing instrument panel and hard-copy recipe card that provides the standard operating procedure. Many pre-weigh operations are cumbersome and subject to errors and risks that could jeopardize regulatory compliance, accuracy, quality, or overall performance.

To address this problem, Mettler Toledo has allied with Sequencia Corp. (Phoenix, Ariz.) to integrate Sequencia's batch automation software with Mettler Toledo's weighing systems for process industries, through the use of ActiveX controllers in MTConnections. This combined suite is bereportedly the first commercially available solution that allows end-users to integrate the manual activities of weighing and measuring with the benefits of batch automation. The software suite is intended for use in the pharmaceutical, food and beverage, and specialty chemical industries.

Communication among the various devices that make up a batching system can also be accomplished using proprietary logic and hardware. Often, these systems offer users simpler methods for managing the process because the software format is very specific to its use.

Rice Lake Weighing Systems (Rice Lake, Wis.) manufactures a family of Microsoft Windows-based formula batchers said to contain all logic and hardware necessary to create and control a batching process. Zero Language Controllers (ZLCs) require no programming language because their mouse-enabled software has the ability to 'walk' the user through the internal batch configuration process. The ZLC is available with two software versions: one can operate a single scale with up to 48 I/O points, the other can operate up to six scales with up to 288 I/O points. Besides handling unlimited recipe storage and attendant reports, it can communicate with hardware usually found in a batching setup including motors (blenders, gates, conveyors, etc.), instrumentation (scanners, flowmeters, etc.) and other control hardware.

'Both miniaturization and digital communication will have an impact on future weighing applications. Right now, some vendor-managed inventory installations use Internet-accessible liquid-level devices to keep track of and refill customers' inventories. This idea can be expanded into the bulk-solid arena by accessing the load cell in the same way. It is just a matter of time,' Milltronics' Mr. Shepherd says.

The need for accurate weighing technology is as old as recorded history. Very early on, the emergence of barter society often required goods, especially those thought to be scarce or precious, be weighed as part of the exchange. Even though weighing sensors and hardware have come a long way from the earliest beam balances, communication as part of the system is still evolving. Then again, if you look back at man's early history and consider what a verbal exchange might have sounded like during a heated bartering session, maybe communication has always challenged weighing applications.




Coal weighing: Making the best of a bad environment

Just outside Tuscaloosa, Alabama, reside some of the highest-grade U.S. coal mines. One of these is the Blue Creek Coal Mine No. 5 owned by Jim Walter Resources (JWR) Inc. (Greenwood, Ala.). It produces low-sulfur, medium-volatile coal for utility consumption and for export to metallurgical markets worldwide. At 2,200 ft deep, it is the deepest vertical shaft coal mine on the North American continent. In a shaft this deep, the process of removing, weighing, and transporting coal ore (65% coal and 35% rock) to the surface is particularly challenging.

The heart of the overall system is where the reciprocating feeders and large weigh cells are located-nearly a half-mile down at the bottom of the shaft. At this point, the environment is noisy, dirty, and dangerous. Moving ore from the conveyors to the feeders and into the weigh cells is fast and unforgiving. Audible warnings remind personnel to say out of harms way during this operation in which ore passes to the feeder and is then transported and dumped into the large weigh cells that sit atop the load cells. Each weigh cell weighs 25 tons when empty and 50 tons full. Automated skips (coal buckets) carry the ore to the surface.

At this point in the mine, everything is covered with a black crust and penetrating dampness hangs in the air. The system's load cells are also located here and expected to perform their job 24-hours a day, six days per week. They must accurately weigh out 25-ton loads for each of the skips that take turns carrying the ore to the surface. It does this for about 2 million net tons or 100,000 weighing cycles annually.

Problem and solution

Accuracy and durability of equipment have plagued the weighing operation over the years. The problem was not solved until the mine removed its third failed system and replaced it with a Siemens (Alpharetta, Ga.) Siwarex M weighing system. To ensure durability, the new system's load cells (standard Wheatstone bridge type) are hermetically sealed in stainless-steel casings that are impervious to the harsh conditions. The weigh system works as follows: As coal ore is deposited inside the weigh cell, a compensation arm inside the load cell triggers a phototransistor switch, activating an electromagnetic coil. This causes a corresponding voltage change across a connected load resistor. This voltage change is processed by a Siwarex M processing module interfaced to a PLC.

Configuration software from Siemens runs on standard PC platforms. Working with the data provided by the weigh scale module, the software provides easy setup of such functions as tare weight, gross weight, net weight, scale standstill messages, calibration, automatic zero offset, coarse/fine feed signals, limit value generation, tolerance monitoring, and filter parameters. The weigh scale modules allow calibration to be controlled with 20-bit sampling that provides an accuracy of 0.05% with 65,000-division resolution,

Once all the parameters are configured, they are stored in the Siemens Simatic 505 PLC. The weigh-scale modules snap on a standard DIN rail inside the control cabinet just below the PLC. There are two modules, one for each load cell. Should it be necessary, modules can be replaced without scale recalibration, because stored parameters can be downloaded from memory. According to Randy Watts, JWR's electronics engineering manager, 'The modules come standard with an RS-232 port for connecting to a printer, PC, or host. They also have a 20-mA connector for interfacing a third-party HMI that the control room uses to report activity transmitted from the trip stations and the weigh areas. Technicians can monitor the loads as they are being filled in the weigh cells. As a load setpoint is reached, the PLC signals the feeder to stop; the gate for input into the weigh cell is closed; and then a signal is sent to the weigh cell to open a door, dumping the weigh cell load into the trip unit.

'There are also status LEDs on the modules that indicate system errors, operation errors, calibration mode, 'scales standstill,' '0.25e zero,' and 'scales adjusted' status,'' Mr. Watts continues. The control room on the surface exchanges information with the control room at the weigh stations over Ethernet. This allows both technicians at the bottom of the shaft and at the surface to know what is happening in either area. 'Built in alarms and interlocks at each PLC provide trouble-free automated operation,' Mr. Watts says. After almost a full year of operation, the weigh system is operating accurately and with zero downtime. Since initial calibration and commissioning, no further recalibration or other adjustments have been necessary.

Communication ensures quality concrete casting in the cold

When A.I.C. (Anchorage, Alaska.) needed to cast concrete blocks on site at Prudhoe Bay, Alaska, it called on Weiser Engineering & Manufacturing Inc. (Menomonie, Wis.) to build the casting machine. The blocks are used to build circular sea walls that prevent wave erosion and stabilize the dredged-out mud 'islands' used to support drilling platforms in the nearby Beaufort Sea. The casting machine mixes batches of castable concrete from locally obtained aggregate, cement, and water. The resulting mixture is mold formed, cured, and shipped to the next drill site in need of a reinforcing 'ring.'

The batch mixing operation of the casting machine uses a Scale-Tron Inc. (Lachine, Quebec, Canada) batching controller. The controller provides a touchscreen-based graphic user interface and communication capability that allows the load cells to 'talk' to the system's PLCs, moisture sensors (aggregate and finished mixture), and final control elements required to automate the complete mixing/casting operation. The machine is located inside an industrial tent that withstands ambient temperatures of -40 °F common during the Arctic winter. The simplicity of the Scale-Tron controller has allowed it to operate dependably since its installation in June 1997. The touchscreen controls are said to be simple, reliable, and easy-to-use.

Communication ensures quality concrete casting in the cold

When A.I.C. (Anchorage, Alaska.) needed to cast concrete blocks on site at Prudhoe Bay, Alaska, it called on Weiser Engineering & Manufacturing Inc. (Menomonie, Wis.) to build the casting machine. The blocks are used to build circular sea walls that prevent wave erosion and stabilize the dredged-out mud 'islands' used to support drilling platforms in the nearby Beaufort Sea. The casting machine mixes batches of castable concrete from locally obtained aggregate, cement, and water. The resulting mixture is mold formed, cured, and shipped to the next drill site in need of a reinforcing 'ring.'

The batch mixing operation of the casting machine uses a Scale-Tron Inc. (Lachine, Quebec, Canada) batching controller. The controller provides a touchscreen-based graphic user interface and communication capability that allows the load cells to 'talk' to the system's PLCs, moisture sensors (aggregate and finished mixture), and final control elements required to automate the complete mixing/casting operation. The machine is located inside an industrial tent that withstands ambient temperatures of -40 °F common during the Arctic winter. The simplicity of the Scale-Tron controller has allowed it to operate dependably since its installation in June 1997. The touchscreen controls are said to be simple, reliable, and easy-to-use.

Communication ensures quality concrete casting in the cold

When A.I.C. (Anchorage, Alaska.) needed to cast concrete blocks on site at Prudhoe Bay, Alaska, it called on Weiser Engineering & Manufacturing Inc. (Menomonie, Wis.) to build the casting machine. The blocks are used to build circular sea walls that prevent wave erosion and stabilize the dredged-out mud 'islands' used to support drilling platforms in the nearby Beaufort Sea. The casting machine mixes batches of castable concrete from locally obtained aggregate, cement, and water. The resulting mixture is mold formed, cured, and shipped to the next drill site in need of a reinforcing 'ring.'

The batch mixing operation of the casting machine uses a Scale-Tron Inc. (Lachine, Quebec, Canada) batching controller. The controller provides a touchscreen-based graphic user interface and communication capability that allows the load cells to 'talk' to the system's PLCs, moisture sensors (aggregate and finished mixture), and final control elements required to automate the complete mixing/casting operation. The machine is located inside an industrial tent that withstands ambient temperatures of -40 °F common during the Arctic winter. The simplicity of the Scale-Tron controller has allowed it to operate dependably since its installation in June 1997. The touchscreen controls are said to be simple, reliable, and easy-to-use.

Communication ensures quality concrete casting in the cold

When A.I.C. (Anchorage, Alaska.) needed to cast concrete blocks on site at Prudhoe Bay, Alaska, it called on Weiser Engineering & Manufacturing Inc. (Menomonie, Wis.) to build the casting machine. The blocks are used to build circular sea walls that prevent wave erosion and stabilize the dredged-out mud 'islands' used to support drilling platforms in the nearby Beaufort Sea. The casting machine mixes batches of castable concrete from locally obtained aggregate, cement, and water. The resulting mixture is mold formed, cured, and shipped to the next drill site in need of a reinforcing 'ring.'

The batch mixing operation of the casting machine uses a Scale-Tron Inc. (Lachine, Quebec, Canada) batching controller. The controller provides a touchscreen-based graphic user interface and communication capability that allows the load cells to 'talk' to the system's PLCs, moisture sensors (aggregate and finished mixture), and final control elements required to automate the complete mixing/casting operation. The machine is located inside an industrial tent that withstands ambient temperatures of -40 °F common during the Arctic winter. The simplicity of the Scale-Tron controller has allowed it to operate dependably since its installation in June 1997. The touchscreen controls are said to be simple, reliable, and easy-to-use.

Communication ensures quality concrete casting in the cold

When A.I.C. (Anchorage, Alaska.) needed to cast concrete blocks on site at Prudhoe Bay, Alaska, it called on Weiser Engineering & Manufacturing Inc. (Menomonie, Wis.) to build the casting machine. The blocks are used to build circular sea walls that prevent wave erosion and stabilize the dredged-out mud 'islands' used to support drilling platforms in the nearby Beaufort Sea. The casting machine mixes batches of castable concrete from locally obtained aggregate, cement, and water. The resulting mixture is mold formed, cured, and shipped to the next drill site in need of a reinforcing 'ring.'

The batch mixing operation of the casting machine uses a Scale-Tron Inc. (Lachine, Quebec, Canada) batching controller. The controller provides a touchscreen-based graphic user interface and communication capability that allows the load cells to 'talk' to the system's PLCs, moisture sensors (aggregate and finished mixture), and final control elements required to automate the complete mixing/casting operation. The machine is located inside an industrial tent that withstands ambient temperatures of -40 °F common during the Arctic winter. The simplicity of the Scale-Tron controller has allowed it to operate dependably since its installation in June 1997. The touchscreen controls are said to be simple, reliable, and easy-to-use.

Communication ensures quality concrete casting in the cold

When A.I.C. (Anchorage, Alaska.) needed to cast concrete blocks on site at Prudhoe Bay, Alaska, it called on Weiser Engineering & Manufacturing Inc. (Menomonie, Wis.) to build the casting machine. The blocks are used to build circular sea walls that prevent wave erosion and stabilize the dredged-out mud 'islands' used to support drilling platforms in the nearby Beaufort Sea. The casting machine mixes batches of castable concrete from locally obtained aggregate, cement, and water. The resulting mixture is mold formed, cured, and shipped to the next drill site in need of a reinforcing 'ring.'

The batch mixing operation of the casting machine uses a Scale-Tron Inc. (Lachine, Quebec, Canada) batching controller. The controller provides a touchscreen-based graphic user interface and communication capability that allows the load cells to 'talk' to the system's PLCs, moisture sensors (aggregate and finished mixture), and final control elements required to automate the complete mixing/casting operation. The machine is located inside an industrial tent that withstands ambient temperatures of -40 °F common during the Arctic winter. The simplicity of the Scale-Tron controller has allowed it to operate dependably since its installation in June 1997. The touchscreen controls are said to be simple, reliable, and easy-to-use.

Communication ensures quality concrete casting in the cold

When A.I.C. (Anchorage, Alaska.) needed to cast concrete blocks on site at Prudhoe Bay, Alaska, it called on Weiser Engineering & Manufacturing Inc. (Menomonie, Wis.) to build the casting machine. The blocks are used to build circular sea walls that prevent wave erosion and stabilize the dredged-out mud 'islands' used to support drilling platforms in the nearby Beaufort Sea. The casting machine mixes batches of castable concrete from locally obtained aggregate, cement, and water. The resulting mixture is mold formed, cured, and shipped to the next drill site in need of a reinforcing 'ring.'

The batch mixing operation of the casting machine uses a Scale-Tron Inc. (Lachine, Quebec, Canada) batching controller. The controller provides a touchscreen-based graphic user interface and communication capability that allows the load cells to 'talk' to the system's PLCs, moisture sensors (aggregate and finished mixture), and final control elements required to automate the complete mixing/casting operation. The machine is located inside an industrial tent that withstands ambient temperatures of -40 °F common during the Arctic winter. The simplicity of the Scale-Tron controller has allowed it to operate dependably since its installation in June 1997. The touchscreen controls are said to be simple, reliable, and easy-to-use.



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