Need advise on PCB design. Hm55b compass placement.

diafysal

Hi
I'm thinking about placing my stamp and some of my sensors on a PCB so I can easily move them from one robot to another. But regarding the compass, is there large problems involved in placing it in a corner of the PCB. Should the compass always be separated from the rest of the electronics?

Advise/opinions anyone?

Chris Savage

Hello,

·· Not necessarily all the electronics, but power wires and motor leads can be two source of EM fields that can affect the operation of any compass module.· This is also true to magnetic devices such as motors and servos.· So if your compass module is too close to some of these things without shielding, you can expect degraded accuracy.


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Chris Savage
Parallax Tech Support
csavage@parallax.com

diafysal

I will keep the PSC, servos and related voltage regulator and power leads away from the compass. On the PCB there will be (in order of power consumption, order of placement?):

Voltage regulator 6-15V to 5V @ 30-240mA
Stamp
pcf8574 ( up to 200mA)
Accelerometer ( 4mA)
ADC ( 1mA)
Compass ( 13mA)

The power leads to the servos can have a top current of 3,4A. And of course that will produce a strong magnetic field. But how strong fields does an IC produce(not the power leads to the IC). I think that the pcf8574 can consume most power of the components, but I'm not sure how large fields 200mA in a power lead produce.

Yanroy

You can calculate the strength of the magnetic field... I think the formula is B = u_0 * I / 2*pi*r where B is the magnetic field strength in Tesla, I is current, r is the distance from the wire in which the current is flowing in meters, and u_0 is the permeability of free space... 4*pi*10^-7.· 1 tesla = 10000 gauss.· earth's magnetic field is ~0.5 gauss.· If you don't come close to overpowering that, I'd assume your compass would work fine.· This is all just theoretical and I'm spitting out stuff I learned in EE classes 2 years ago... but it makes sense in my head :P

diafysal

I think you know those formulas way better than I do !

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0.2A and 1cm away = 0.04 Gauss ( I think)

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And placing + and - wire parallel next to each other would further reduce the field (i think)

Post Edited (diafysal) : 8/2/2005 4:46:24 PM GMT

Ed Stennie

Twisting the power and return (ground) leads to motors an solenoids should help reduce of cancel EMI that could effect the compass module.

diafysal

I ended up with no room for the compass on the PCB.
So it (compass) will be placed separate from the PCB.

(nearly done with the design)

steve_b

Wouldn't a couple of ferrite beads play a role here?

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."