5 Mistakes When Integrating Weighing Automation Devices

From METTLER TOLEDO

Editor’s Note: This article is adapted from the white paper, “12 Common Mistakes When Integrating Weighing Automation Devices.” Download the full paper at https://bit.ly/tj22mtwp to learn about the 12 frequent mistakes, including those related to using the zero function or integer weight value; keys to using automatic calibration; using a compliant automation interface; understanding the real meaning of “Legal for Trade;” and more.

Your assignment: integrate a weighing sensor or scale into a programmable logic controller (PLC) or distributed control system (DCS) for the next project. Sounds straightforward, right? Nevertheless, if you’re integrating weighing equipment for the first time, we can help you avoid some of the most common mistakes reported to our technical support groups around the world.

These tips will help you simplify your job and help your customers produce the highest quality products. And avoiding these common errors could save you substantial time, money and frayed nerves.

Let’s examine five common mistakes when integrating automated weighing devices and tips for avoiding them.

1. Mistake: You pick a scale that’s too large for the items you will weigh.

A large scale potentially could allow you to weigh products of various sizes while providing extra capacity, serving as a safety margin that prevents overloading.

However, you need to be careful. If you choose a scale that’s too large, you can no longer weigh very small items due to high measurement uncertainty — your measurements become a lot less accurate as you approach zero.

This uncertainty is a characteristic of any measuring device, and isn’t exclusive to weighing. Mechanical stops prevent damage to the sensors on many high-quality scales. Therefore, adding a safety factor just raises the risk of inaccurate results.

Tip: Compare your production tolerance to the scale’s measurement uncertainty when you’re specifying the scale.

2. Mistake: Your selected device doesn’t provide system status alerts or condition monitoring, or there are too many alerts.

In multisensor systems, broken load cells, bad junction boxes, and cables go unnoticed until your batch is tested in the lab. In a multicell system, a failed cell reduces the signal to the weight indicator, significantly reducing the weight available to the control system. The only practical way to discover a failed measuring chain is to test the scale before each batch.

However, with the right equipment, built-in condition monitoring and status alerts inform you that the scale is connected and all load cells are providing valuable weight data. Condition monitoring also tells you the optimal time to transfer the weight value for the best accuracy, such as when the scale is in motion.

Smart weight indicators and smart sensors include many condition-monitoring functions over the automation network that let you know in real time that your system is running properly.

Tip: Use condition monitoring capability and alarming function to catch problems early.

3. Mistake: You use an analog signal to transport your measurement to the control system.

Strain gauge scales deliver resolution up to 100,000 increments, and Electromagnetic Force Restoration sensors deliver resolution up to 210 million increments. A 16-bit 4-20ma signal is good for a maximum of 32,000 increments. When you factor out noise and uncertainty, you might only achieve 4,000 increments of readability. This means that the analog signal reduces your readability by at least 25 times and as much as 52,000 times.

Tip: Use an automation network to transport important weight data to achieve the highest accuracy.

4. Mistake: You trust the automatic calibration feature, but don’t confirm results with weights.

Many people are tricked by the idea of not using test weights. Automatic calibration features do not actually “test” the scale to confirm operation; they only adjust the scale to known default values for easy setup. Only test weights, or hydraulic calibration, prove that shipping hardware has been removed, that there is nothing blocking the scale, and that all sensors are transmitting accurate measurements.

Automatic calibration is a useful tool for quickly configuring your system, but when you really want to prove fit for purpose, it’s necessary to confirm your scale is accurate (true and repeatable). Routine calibration after initial calibration confirms that your scale continues to provide accurate measurements.

Tip: Physical calibration identifies broken components, cables or load cells.

5. Mistake: You use a noncompliant automation interface.

The Open Device Vendors Association (ODVA), for example, specifies and tests products to verify they’ll function well on EtherNet/IP™ networks. Compliance sets the basis for easy installation and consistent, trouble-free operation; not all manufacturers and equipment are compliant.

When you select a weighing device or use a gateway, make sure 1) it has a specific (not generic) conformance certificate, and 2) it meets your data throughput requirements.

Tip: Go to www.odva.org and confirm that a product conformance certificate is available. Make sure you have the EDS, AOP or other important drivers.

Scale and Network Components

These tips can help you avoid some of the most common mistakes we’ve seen across several weighing applications, and help make your project run a lot more smoothly. Remember to select the right scale and network components based on your production and automation requirements, calibrate by physical methods, review your control philosophy with an expert, and select modern products with condition monitoring and advanced filtering. 

Based in Switzerland with North American headquarters in Columbus, Ohio, METTLER TOLEDO is a Technology Partner in the Rockwell Automation PartnerNetwork™ ecosystem. The company provides weighing solutions that include industrial weigh modules, load cells and PLC connectivity devices that easily integrate weight into automated applications.

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