Radio frequency interference (RFI) noise on the plant floor can cause electrical interference that often affects the function of proximity sensors. This RF noise is caused by the use of variable frequency drives, stepper motors and high powered communication devices, such as two-way radios, pagers, some cellphones, etc. The energy/frequency transmitted from these devices can cause an inductive proximity sensor to switch its output without a target present.
This RF noise (which, like light, can propagate in a vacuum) radiates from its sources and reaches the electronics of nearby sensors. This RF noise can be suppressed by on-board filters in the noise emitting devices, or by designing sensors to be tolerant to a level of noise so that their operation is not affected.
The International Electrotechnical Commission's (IEC) 60947-5-2 standard for tolerance of radiated emissions in a proximity switch is 3 V/m [volts per metre] or greater over a frequency range of 80 MHz to 1 GHz. At this rate, a 5 W walkie-talkie could be within 1,8 to 3 metres from the sensor and cause a false trip. Keeping in mind that a 5 W walkie-talkie can produce approximately 150 to 200 V/m at the antenna. Subsequently, a device with higher emissions would trigger the sensor at further distances. This level of susceptibility is not acceptable on the factory floor, as people using these devices are often near sensors. It is therefore important to either remove this influence on sensors, or to maintain a very low level of interference - to permit distances typically less than 50 mm.
To address this, manufacturers have developed sensors to withstand varying degrees of electromagnetic interference (EMI). Tests are conducted to measure an electronic product's tolerance to RF noise, in accordance with IEC 61000-4-3, for 1 V/m, 3 V/m or 10 V/m. In these instances, the same 5 W walkie-talkie could be within a few centimetres of a sensor that is tolerant to 10 V/m without causing a false trip. Some sensors are even capable of performing at 15 V/m in which case the walkie-talkie could be virtually touching the sensor.
This phenomenon affects inductive proximity sensors, and can also have a varying influence on capacitive, IR, laser sensors and ultrasonic sensors.
Sensors that have been developed for RFI tolerance have incorporated special inner circuitry, on-board filters and shielding to reduce their susceptibility to RF noise. Sensor manufacturers continually work to keep all noise influences to a minimum. This is increasingly difficult, as the sources for these emissions are being developed with more and more power output to get clearer signals at longer ranges. The picture shows some of Turck's RFI tolerant sensors.
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