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 Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Learn how to create value with re-use; gain productivity with lean automation and connectivity, and optimize panel design and construction.
Go deep: Automation tackles offshore oil challenges; Ethernet advice; Wireless robotics; Product exclusives; Digital edition exclusives
Lost in the gray scale? How to get effective HMIs; Best practices: Integrate old and new wireless systems; Smart software, networks; Service provider certifications
Fixing PID: Part 2: Tweaking controller strategy; Machine safety networks; Salary survey and career advice; Smart I/O architecture; Product exclusives
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Look at the basics of industrial wireless technologies, wireless concepts, wireless standards, and wireless best practices with Daniel E. Capano of Diversified Technical Services Inc.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.