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Q: wireless sensors

Can anyone suggest some reading materials that I could use to understand how wireless sensors can externally excited, eliminating the need for batteries? A while back, a technician from a company that made a clutch used on a piece of machinery we make used this technique to get strain data from a shaft inside the clutch. I only got to talk to him for a minute but the method involved the use of radio signals to both excite the sensor and to retrieve the data.

This is way over my head electronically but I'd like to learn more.

Jon Juhlin Project Engineer JCI INC Eugene Oregon

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Jon Juhlin
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Naw. It's easy. The basic principle is used in RFID tags. The tag has an antenna and rectifier (detector) built in. The tag is "illuminated" by a nearby transmitter. The rectifier converts some of this RF to DC, which runs the RFID tag. There's usually a big cazapitor, but no battery.

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There are also tags that operate inductively (part of a transformer). Frequencies are usually 13.56MHz, 433MHz, and 915Mhz.

Data is sent back in one of two ways.

In a simplex system (tx and rx on the same frequency), the transmitter turns on and off for short periods. During the off times, the tag responds with data on the same frequency used to illuminate the tag. Most systems work this way.

In a duplex system, the tag belches data on the 2nd or 3rd harmonic, which can be received while the illuminating transmitter is on. These have the advantage of sending more data (no waiting for the xmitter to shut up).

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Jeff Liebermann

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