# RFID for angle encoding

Can RFID be used as an alternative to e.g. non contact magnetic encoders? Suppose a 150 mm diameter steel disk submersed in seawater subject to violent movement. Is it possible to put e.g. 30 passive RFID tags with the specific angle information at equal distances on the circumferance and having them read by a ruggedized RFID reader? Does such a reader exist and can it prioritize tags based on distance when multiple tags are within range of the reader? Thanks for any help on this matter.

Some clarification, but I'm not allowed to give all details: we need to monitor de angle of a device that's constantly moving in and out of the seawater (depth max. 25 m). The device is suspended on a pin and rotated with a hydraulic motor. beneath the suspension pin is a bearing where we want to measure the angle of movement around the pin. We have a working system with inductive sensors, but it's very delicate and expensive to ruggedize for the harch environment. I have no experience with RFID and it was just an out of the box idea I wanted to look into and ask some expert opinion.

• I don't see any way to do this that would work reliably, but that doesn't mean that none exist. I'd go with some kind of optical encoder, though. – Hearth Oct 29 '18 at 15:50
• At what depth of water? Is the disk always normal to the reader? How salty? So many details you would need to know to pass an opinion. – Jack Creasey Oct 29 '18 at 15:54
• If the disk pinned in any way, such as a central pivot point? – analogsystemsrf Oct 29 '18 at 15:57
• +1 For @Felthry optical encoder. It can be transparent or reflective. – Oldfart Oct 29 '18 at 16:39
• what do you mean by violent movement? ..... is the disk spinning on an axis? ... if yes, then how is the axis oriented in relation to the disk? .... if it is not spinning, then describe the expected motion – jsotola Oct 29 '18 at 23:41

## 1 Answer

RFID would be silly for an application like this. All you need to do is put some teeth or slots on the disk that encode the position as a 5-bit Gray code, and use five variable-reluctance sensors (same kind used in car ABS sensors) to read out the code.

• Marking or slotting the disk and using optical sensors would also be an option, no? Is there any pitfall there that makes the magnetic sensors preferred? – Hearth Oct 29 '18 at 17:08
• @Felthry: Sea water frequently contains opaque contaminants. Variable reluctance sensors were selected for the harsh environment of automobile wheel wells for a reason. – Dave Tweed Oct 29 '18 at 17:11
• Good point. I had somehow misread the question to be about a partially-submerged wheel. – Hearth Oct 29 '18 at 17:36
• In most cases this would be excessive. One sensor and one toothed wheel with an index gap would be more than enough (two sensors if the wheel can change direction). Then some counters can take care of the encoding. – Edgar Brown Oct 29 '18 at 17:59