Tutorial: Running down interference to improve device performance

EMI and RFI can ruin your sensor data, and trying to trace sources can drive you nuts. There are solutions.

05/21/2009


In past issues we’ve talked about problems with ground loops and cross talk , and how these can ruin sensor data. Electromagnetic and radio frequency interference (EMI and RFI) can also cause your sensor data to go nuts, and finding its sources can make you do the same. The two types of interference can present similar symptoms, but the nature of their sources and solutions are not the same. Trying to define the differences between the two is a little vague and many people use the two terms interchangeably for all practical purposes.

EMI is broadband electrical noise that typically emanates from wiring and equipment. In any situation where there is heavy inductive load switching, electric motors, and particularly complex devices like VFDs (variable frequency drives), you’ll get EMI. In the same way it hurts your radio reception, noise is transferred to your sensors through inductive and capacitive coupling, often between wires. You can get it from having data cables too close to ac wiring in cable trays, for example, which means laying a thermocouple extension wire next to a 440 Vac line is asking for trouble.

RFI is higher frequency, and generally the tougher problem to fix. While EMI usually moves from cable to cable, RFI can be picked up all sorts of ways. It often comes from radio transmitters such as walkie-talkies, cell phones, CB radios, etc. (A driver triggering a CB radio from his truck while driving past my house nearly blew out my stereo speakers.) You’ve probably heard the characteristic beeping on the phone caused by someone’s Blackberry trying to exchange data. There are anecdotes of radios on barges sending systems haywire as they pass plants on the shore.

The thing that makes such interference hard to eliminate is that it can be intermittent. For example, a welder working in the plant makes the temperature sensors on a vessel change, but then they recover when the welding stops. The potential for damage to the process depends on the EMI severity and how well you’ve deployed suppression techniques. Problems that run all the time, more or less, can be tracked down and fixed. The TC wire and ac cable in the same tray are an example.

Many suppression techniques for both kinds of interference exist:

  • Keeping critical cables separated;

  • Use of twisted pair and shielded cables;

  • Filtering capacitors at strategic points;

  • Careful grounding where appropriate;

  • Use of signal isolators;

  • Appropriate case and enclosure designs;

  • Selection of devices that are designed to reject interference; and

  • Many other possibilities.

Going into depth in this context is not practical, but there are application notes from several sources that can help you analyze and solve your interference problems. Here are three links to get you started:

Acromag
Dataforth
Moore Industries

These companies can suggest specific practices and offer devices to make the task easier. Also, Control Engineering has offered more extensive discussions on related topics:

Drive EMI: How to control high-frequency noise of adjustable speed drives (ASDs)

Silence of the Drives

Who Puts the 'Industrial’ in Ethernet?

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
Register here to select your choice of free eNewsletters .





No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
Inside IIoT: How technology, strategy can improve your operation; Dry media or web scrubber?; Six steps to design a PM program
World-class manufacturing: A recipe for success: Finding the right mix for a salad dressing line; 2015 Salary Survey: Manufacturing slump dims enthusiasm
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming
Managing automation upgrades, retrofits; Making technical, business sense; Ensuring network cyber security
Designing generator systems; Using online commissioning tools; Selective coordination best practices

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
click me