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Calculating strength of EM Field generated by motor


#1

Hey guys,

I’ve got a slight concern when it comes to my design - It incorporates a GPS module, RAZOR IMU, and two 6amp/7v2 Motors, together with a pololu dual motor controller.

The GPS module needs under 50microvolt fluctuations on its power line to function reliably, and I can confirm that this isn’t just a theoretical thing - Without at least 3 capacitors on a computer usb power line it is impossible to get a lock.

The IMU relays on magnetometres - Not something I’d put next to a Motor if my life depended on it.

How would I work out a reasonable distance to place those components at? With the GPS, too close to the motors and I will not get a lock altogether, however I would assume precision is directly proportional to the distance squared, so I need to work out the optimal point. This is taking into consideration that I have tight space constraints, and really, the closer I can put it and still be confident with the data the better.

Any advice on how to deal with this?


#2

Just out of curiosity, what gps module is this?

It should have decoupling caps on it, but if the motors are on the same power domain without proper filtering, you may have tons of issues.


#3

Using a GS407, I have loads of decoupling caps installed on the connector to it - 15pf, 33pf, 45pf, and 100nf.

It is using a seperate power supply - What I am worried about is the EM field generated by it and the cable which has to run in close proximity to the GPS module.


#4

I had massive trouble getting my GS407 to lock, and I never was able to finally get a fix, I sent it back. No excuse for that, it was just connected to a USB->UART bridge with clean power.

I like my Skytraq GPS far, far better. Gets a very quick fix.


#5

I purchased this module quite a few months back, so I can’t imagine I’d have much luck sending it back.

I’m thinking, twisting the motor power cables, wrapping them in a thin copper sheet and putting heat shrink over that. Then shielding the gps in a similar fashion. Hopefully that will kill some of the interference.

Have you got any other advice when it comes to stuff like that? Its the first time I’ve had to consider interference.

USB doesn’t provide enough power for the GS407 to get the initial fix, thats probably what your problem was.

Oh, and regarding the GS407 module, its connector is such a pain. Broke about 5 of them now. Ended up pernamently connecting it using some glue, double sided tape and a sheet of plastic.


#6

I had lab power going into it. No excuse for several amps to be too little :smiley:


#7

Ahhh fair enough. Well either way I’m stuck with it :slight_smile:

Btw any luck on that IMU you were planning to manufacture? I was had an operation and was high on painkillers for about 2 months, so I wasn’t following these forums too closely.


#8

Yup, accepting orders, want one?


#9

Not this very moment, but most likely in a months time. Just wrapping up this autonomous project, plan for summer holidays is a smartphone, hoping to enter it in the national science competition. How big is that IMU of yours? And what does it contain?

Also quick unrelated question, does the USBizi run standard .netmf, or ghis additions? And when porting .netmf is it possible to add GHIs additions?

I’m going to attempt to make it all as small as possible, first real experience with proper smd stuff :slight_smile:


#10

USBizi runs GHI’s port…

We are running engineering samples soon, but here is the thread… http://forums.netduino.com/index.php?/topic/1277-anybody-interested-in-a-cheap-imu/page__p__8855__hl__cheap+imu__fromsearch__1#entry8855

Also, see: http://www.chrisseto.com/wordpress/?page_id=309


#11

Thanks