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The Windrock wireless encoder link can be rendered inoperative if nearby intentional radio sources are operating on or very near the frequency used by the wireless encoder link. Windrock’s wireless encoder link uses the fixed frequency of 921.37 MHz which is in the ISM radio band. The ISM band is commonly used for industrial communication.
We have encountered several instances of SCADA or instrumentation radios that operate in the ISM band which includes 100 channels from 902 to 928 MHz. The symptom of this is no or sporadic engine speed/crankshaft location information when attempting to use the wireless encoder link. Most of the ISM band SCADA radios operate in a spread-spectrum or frequency hopping scheme in which the radio moves randomly from channel to channel throughout the ISM band. When the radio hops to or near the 921 MHz frequency used by the encoder link, the analyzer locks onto the wrong radio signal and looses speed information from the machine. When the SCADA radio is very close, it can completely block out the encoder radio signal.
On some SCADA radios, especially the GE MDS 900 radio, it is easy to program the radio to exclude the frequency used by Windrock. In the MDS radio, the ISM band is divided into 10 zones. It happens that zone 8 include the channels from 920.224 to 922.4395 MHz, which includes the frequency used by Windrock. The user-interface on this radio allows a zone or several zones to be excluded from use. It does not decrease the performance of the SCADA radio to skip a single zone or up to three zones.
We have seen cases where a SCADA radio has completely rendered the wireless encoder link useless . By programming the SCADA radio to skip the appropriate frequency zone, the problem was corrected.
If a user is encountering difficulty with the wireless encoder link, an easy way to determine if an intentional radio source is the problem is to turn off the encoder transmitter and observe the LED next to the receiver antennae. If the LED is flashing sporadically with the encoder transmitter off, then an intentional radio interference source is very likely.
The surest method of detecting interference is with the use of an RF spectrum analyzer. Set the analyzer to sweep the frequency range from 902 to 928 MHz (the complete ISM band). One should see a strong signal at 921.37 MHz when the encoder transmitter is on. One can also easily spot intentional radio transmissions that are fixed or frequency hopping.
EBF – 4/6/2011
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