Radio Communication interferring with Vibration Transmitters

[kmg1983]

Has anyone experienced interference between hand held radios and vibration transmitters installed on rotating equipment?

At our facility, the vibration transmitters installed on several compressors inadvertently go into a state of alarm when exposed to a near radio signal. This causes shutdown of the equipment. What can be the cause and/or solution? I am not familiar with signal processing and interference such as this.

 

[Skogsgurra]

Not only radios. VFDs and phones also create that kind of problem.

The cheap (and sometimes effective) solution is to put a small HF capacitor (100 pF - 10 nF) across the transmitter pins and then a small resistor, 10 - 100 ohms*, in series with the two wires. Assuming a two-wire transmitter with 4-20 mA output and floating (no ground connection).

If that doesn't work, you need to ask someone with the right experience and equipment to jave a look at the problem. Many industries have banned transmitters from selected areas, where such problems exist.

*Be careful not to exceed compliance voltage.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[VEBill]

If the interference is getting into the wiring, then it's possible that it's a common mode issue (quite common in EMI issues). If so, then the solution is to shield the wiring.

 

[kmg1983]

What function will installing the capacitor and resistor in series do?

 

[Skogsgurra]

Radio frequency interference usually enters transducers via the "back door", i.e. through the connecting wires. They then usually get rectified in PN or NP junctions in the output transistors and the resulting DC current works as an extra base current in the output transistor so that the output leaves normal operating range and the receiving device indicates abnormal behaviour.

The capacitor across the output shunts the HF signal so it gets reduced. But, to do so, there needs to be some impedance in series with it. That is where the resistors come into play. Resistors and capacitor constitute an LP filter that attenuates signals coming from the outside. The resistors also attenuate HF signals entering in common mode so that effects due to parasitic capacitance from active circuitry to ground are minimized.

As I said, this is the first thing to try. If it doesn't work, then get a consultant and have her/him have a go on it.

If the wires are not already shielded, then the suggestion by VE1BLL is worth a try. Shielding can be tricky. Especialy where to ground the shield. Books have been written about that.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[VEBill]

Ferrite beads can be used in place of the resistors in some applications at some frequency ranges, sometimes.

Another option commonly employed is a big sign:

NO RADIO TRANSMITTERS OR MOBILES TO BE USED IN THIS AREA.