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I am creating a set of 50+ remote imu sensors. They will all be in a small distance from one another. Less that 3 meters. Each remote board will have a on board sd card for logging the imu data and a led that flashes when the imu begins its sampling. My remote board needs to be small, around 25mmX25mm all up. But the smaller the better!!!

The problem is that i cant figure out a reliable solution to send sync wireless from one master unit to these remote imu sensors. These sensors all need to be in sync so the data logged is valid and usable. The sync pulse will be between 24Hz and 120Hz and generated from an external source.

I have been looking into BLE because of the very small modules available with build in antennas and that i can talk directly to the imu. But i can only have up to 8 slaves connected to a master. I could use the broadcasting features and broadcast message to all modules that are listening, but im just not sure.

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  • \$\begingroup\$ Wireless is but reliable at all. Are you bound to wireless or can you use wired links, far more reliable? \$\endgroup\$ – user59864 Jan 26 '16 at 8:42
  • \$\begingroup\$ I am currently bound to using wireless. I want to see how far I can take it. See if I can actually get it to work. I might be able to sync each sensor(jam them) at the beginning via a cable. I wonder how long I could then keep 50+ detached sensors in sync before they start to drift. \$\endgroup\$ – colyton Jan 26 '16 at 10:38
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By the sound of it the IMU units don't need to transmit back, they just log locally?

Use one 433MHz transmitter, and a reciever on each device. These are small and cheap. Transmit a continuous stream of timestamps. Each will need a checksum, preferably bit error recovery (reed-solomon), and framing/sync bits around the data.

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  • \$\begingroup\$ Yea that is correct. I don't need bidirectional communication because each sensor will be logging data locally. Do you have an example of a good transmitter and receiver chip? This sounds like a great simple solution. Thanks \$\endgroup\$ – colyton Jan 26 '16 at 20:11
  • \$\begingroup\$ A random modular example: dx.com/p/… \$\endgroup\$ – pjc50 Jan 26 '16 at 21:27
  • \$\begingroup\$ The problem now would be that the receiver chip is too big. Thanks for you help. \$\endgroup\$ – colyton Jan 26 '16 at 22:27
  • \$\begingroup\$ It's an example, but antennas have a certain minimum effective size .. it's possible that some of the Nordic Semi chips operating in 2.4Ghz are better. \$\endgroup\$ – pjc50 Jan 26 '16 at 22:35
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Use broadcast.

As each IMU is logging its data, ensure that data includes a timestamp from its internal counter.

Even though all these counters will drift slightly relative to each other, they will be close enough that a missing timestamp in the logged data from any single unit will be easily detectable - unless all 50 units fail to receive a broadcast tick.

This way, while you can't guarantee complete integrity you can effectively guarantee to detect any missed events in the logged data.

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  • \$\begingroup\$ Are you suggesting BLE broadcasting or another type? Thanks. \$\endgroup\$ – colyton Jan 26 '16 at 20:15
  • \$\begingroup\$ Either. pjc50's 433MHz is one alternative. \$\endgroup\$ – Brian Drummond Jan 26 '16 at 20:20
  • \$\begingroup\$ Have you ever uses the used anything like this? atmel.com/devices/ATSAMR21G18A.aspx \$\endgroup\$ – colyton Jan 26 '16 at 20:38
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What you are asking is a real challenge using a wireless connection across a multihop network and you would have to implement your own protocol to try and do it. There are many research papers on alternative solutions to the time synchronization issue. You will probably find that the internal clocks on each device will not be terribly accurate and a regular resynchronization is necessary.

An alternative would be to use an out of band signal. For example could you have line of sight and use a flashing LED signal that all devices can see to synchronize them. Other solutions would be to use an ultrasound signal broadcast from a central location or an out of band impulse (UWB) radio signal.

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