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Electromagnetic field interference.


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Hi,
The attached diagram is the base of my robot mower. The tank circuit is used to detect the electromagnetic field produced by the 555 timer with multiple loops of wires. When the field is detected, the robot will do a "U turn". Otherwise, it just go straight.
But my problem is, when i turn on the blades, it starts going crazy. It keeps doing U turn even at the center of the loops of wires. The electromagnetic field produced by the DC motor interfere my system.
I've used the shielding method by using the alluminium foil, but i was told that, that method won't solve my problem. Yes, it doesn't, i've tried it.
So, shall i change the direction of the DC motor so that they cut off each other?
Thanks!!!

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The magnetic fields from the motors might cancel if they run in opposite directions, if the sensor coil is equally between them and if they have exactly the same current.

Yes.. they have the same current. But they are in different direction (the left one turns anticlockwise; the right one turns clockwise), still interfere.
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You could try separating your digital and analog grounds and if needed, build shields around the motors. Perhaps you should post information about the electronics. It is possible that the electronic noise is transmitted through your circuit. If this is the case, all the shielding in the world will not help.

MP

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You could try separating your digital and analog grounds and if needed, build shields around the motors. Perhaps you should post information about the electronics. It is possible that the electronic noise is transmitted through your circuit. If this is the case, all the shielding in the world will not help.

MP

No, the blades (DC motors) are not common grounded with the rest. It is connected withe a battery source only.
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Even when you separate the digital and analog grounds, all common connections are put together at one common point. In this case, you basically have two electrical circuits interfering with each other. This is why I suggested that you post the circuit. I certainly understand if you do not want to make this design public.

MP

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Hi, they are not connected together. From the attachment, the red circle is the electromagnetic field detector. While the blue color is the DC motors for the blades. Try to figure it out with the previous attachment. Is it called electromagnetic field interference? When i use fully charged batteries, it interferes; when i use used batteries (around two third of full), it doesn't interfere. Is there any solution for my case?
Thanks

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Your observation, that with half charged batteries, you don't have a problem, might be a clue. The motor winding impedance is fixed, so as your voltage falls, so does the current. Current is directly proportional to flux field strength.

Where is the 555 located within the system? Is it some signal the the mower is suppose to detect when it gets to a "perimeter"?

MP is correct that the circuits should have "some common" reference point. Since the wheel motor and the blade motor will not always generate the same flux field, so cancelation probably won't work.

I can only suggest to minimize exposure of high impedance loops to the field (i.e. keep all wiring as short as possible, make sure the "input" components are as close to the LM393 as possible, make long runs with triaxial, coaxial, twisted shielded pairs or at bare minimum twisted pairs. If you can get you hands on some Mu metal, you could shield your circuit with that!!

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EMI and EMC are both a little bit of "black magic".  Why would you say that their ground wires have to be connected together?? What they need is a common reference and that reference doesn't have to be ground. In fact, sometimes it's to your advantage to not have ground at the reference.... it can help with common mode noise in some instances.

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No black magic there. You must have some reference that you are calling zero. Since all of these commons can be at a different potential, there is no "common" reference. Thus, your shield is just another voltage reference and is not protecting anything. If you want the shield to be at the same potential as the grounds, (you do want the shield to be grounded) you must connect all of the commons (grounds) at one point to insure they are all at the same potential. Then you have a zero reference and can work the rest from there.
There may also be other problems with the circuit, but this should be step number one.

MP

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The "common reference" can be anything I want it to be, it DOES NOT have to be something called ground or zero... just a "common point". In this circuit for example, you could take the +5 from back wheel battery, connect the +9 and the 5V return from the blade motor and call this my reference. It is a myth that shields have to be connected to ground or a single common to be effective in "protecting" signal integrity...

If what your saying is 100% correct, then this guy....

http://www.ieee-houston.org/Ads/DonWhite/DonWhite.htm


... is 100% wrong. I attended a couple of his seminars years back down in Florida (it was February... lucky me) for "Grounding and Shielding" and "EMI control in backplanes". Actually... now that I think about it, it was quite a while ago...1984

Anyway... these are 3 floating circuits... why would I "have to" tie the "commons" together?? I can define the supply for the DC motor as being -5V, so I would be tying the -5 return to the common. It might be found that it works "best" when the microcontroler signals are shielded with +9V..... when it comes to EMI and EMC, you OFTEN have to "think-outside-the-box"!!!

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No myth...just "Common" sense  ;D
That is what I said...a "Common" point. You do not have ANY point that is considered to be a "Common" point in this diagram. All systems are acting independently of each other and thus, they are all doing their own thing....not working in unison, if you will. How can you bleed the EMI to ground when the ground from the part of the circuit that is causing the EMI is at a different potential than the ground on the part of the circuit you want to bleed it from?
Ground is the best place to make these commons since other potentials can get complicated and can cause one voltage potential to interfere with another if you do not connect everything correctly.

MP

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This is not "my diagram"....

The point I was/am trying to make is that shielding doesn't necessarily have to tie to "ground" or a "common point" of a circuit all the time, and sometimes, the shield is more effective if tied elsewhere, this is a irrefutable fact. The notion that all shields have to be tied to a "zero" or a "ground" or a "common reference" or whatever you wan to call it, is indeed a myth!!! Driven guards are the perfect example of this. The number one choice of where to connect a driven guard to is too a buffered version of the signal you are trying to guard. The number 2 & 3 places to connect the guard are the power rails or ground.... sorry, the supply rail's return.  EMI energy can be diverted to anywhere (preferabaly to it's point of origin) as long as it stops interfering with a circuits operation. Whether or not is easy to do, as far as having to keep track of which shield is connected to what, is irrelevant..

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