Timing Critical Wireless System


I just need an advice on a possible wireless solution for my system.

My task is to collect responses from test-subjects in medical research studies. This is done with different response pads with push-buttons. Since the test subject is inside a scanner during the examination it would be practical to have battery driven response pads with a wireless transmission solution instead of cabling.

But my biggest problem with such a solution I guess is the timing issue because timing in these studies are often critical, and deviation in the delay from one response to the other more than a

10-100 ms caused by signal delay in the system is not accepted.

Does anyone have a suggestion on a ready to use solution for such a timing critical wireless system?

-Transmission length is maximum 7-10 meters (30 feet???) from sender to receiver

-This is inside a faradays cage, meaning little RF noise in the room, but for regulative reasons only the common used frequency bands (open) should be used.

-Up to 3-4 independent transmitters should be able to work with the receiver simultaneously

My idea is that some alarm systems probably are wireless with strict requirements, and not to forget high-end gaming devices that use a wirless solution (if that excist...)

I'm greatful for any suggestions or advice.

Stian SS

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Whats the scanner ?

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An RF solution shouldn't have too much trouble with the latency requirements. The Faraday cage and scanner make me think that avoiding interference with the equipment might be your biggest problem. Have you considered an IR link?

Paul Hovnanian     mailto:Paul@Hovnanian.com
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Paul Hovnanian P.E.

Again - what type of scanner?

The shielding strongly suggests an MRI scanner, in which case the problem will not just be rf interference with the scanner.

Two other problems will be:

1) The scanner transmits very high power pulses of rf whiich could have a bad effect on your circuit.

2) There will be a VERY powerful magnetic field which can easily turn seemingly harmless items such as batteries with steel casings into dangerous projectiles.

You would be better off using a fibre-optic system where the button interrupts the light path between two optical fibres. All electrical and electronic components could then be outside the shielded enclosure.

Alternatively, thin plastic tubes and a pneumatic bulb under each button and a pressure switch at the other end could work.


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Do you need to react quickly or merely know exactly when the pad was pressed? You can have the circuit that it connected to the button have an internal clock that it records at the instant of the press.

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Yes we are talking about an MRI scanner here, but to answer your questions:

1: The response pads will be at the subjects hands, meaning outside the MR-bore not not so much affected by the shifting gradients and RF transponders - but it's not unusual to have electronics like LCD projectors/displays etc. located insider the bore mounted to he RF transponder coil. 2: The RF interference of the scanner is in the frequency range 60 Mhz and 120 Mhz, if we're going for a solution outside the frequencies RF interference should not be a to big problem.

So do anyone have any experience with wireless system with minimum latency?

Thanks in advance!

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How about TV remote controls? There's lots of handheld controllers to choose from, the number of available buttons is adequate, and receivers for computer (or other) connection are common enough; see

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for more info.

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Possible but also maybe not suitable due to EMC and high magnetic field compatability. Proper engineering should recommend a technology compatible with any type of medical scanner. Since controlled and preferably low latency is requires reliable detection with sub-millisecond total response time. This leads to continuous transmission at 10,000 symbols per second through the "push button" and a sturdy, reliable, low required force mechanism no matter how done. After that, RF, IR, and visible light are all reasonable choices. Air (or other fluids) will not be fast enough. The fiber interrupter looks like the best bet, followed by (perhaps moving mirror) free path optic interrupter (no wires of any kind, but much harder to setup and has other usage problems).

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