Much of the equipment found in factories creates radiofrequency and electromagnetic interference (RFI/EMI). The list of usual suspects includes variablefrequencydrives, stepper motors, high-powered transmitters for AM and FM radio, twoway radios, paging systems, cell phones, and electrical-power contactors. All these devices transmit energy that may trigger sensor outputs on or off.
One way to control electrical noise is with filters and snubbers. But it is not always practical to eliminate RFI/EMI with these measures. And they do nothing to prevent "roving interference" as created by a walkietalkie. A better method is to design sensors so they tolerate a sufficiently high level of electrical noise.
The International Electrotechnical Commission (IEC) sets tolerance levels in its 60947-5-2 standard for how much radiated emissions proximity switches must withstand. The standard says a proximity switch must withstand a 3-V/m field over 80 to 1,000 MHz. At that level a 5-W walkie-talkie 10 ft from the sensor may cause a false trip. Of course, more-powerful sources with stronger emissions can trigger sensors from farther away. The level called out in the standard is not acceptable on the plant floor. Handheld two-way radios radiate stronger signals and may well do so within sensor range. Yet not every sensor will see high-level RFI/EMI. It would be unnecessarily costly to equip every sensor in a plant for worst-case emissions.
Addressing this reality, manufacturers develop different sensors to handle varying degrees of electromagnetic interference. Tests conducted in accordance with IEC 61000-4-3 measure sensor tolerance to RF noise at field strengths of 1, 3, and 10 V/m. The same 5-W walkie-talkie can now sit a few inches away from a sensor designed to tolerate 10 V/m without causing a false trip. Sensors capable of handling 15 V/m work just fractions of an inch away from a walkie-talkie.
Special internal circuitry including onboard filters and possibly Zenerdiode voltage clamps let these sensors work in the presence of high noise. An RFI/EMI shield gives additional protection.
Turck Inc. (turck.com) provided information for this article.