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Need Help, Magnetic Field Detector

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JK108

New Member
Hi. I am currently building a robot which is suppose to be able to detect and follow the magnetic field lines emitted from a beacon at a specific frequency.

Here is a simple run down:

source:

the beacon which is a coil of wires is connected to a wave generator which emits the magnetic field.


receiver:

there is a small 1mH inductor which is connected in parallel to a capacitor to make it resonate at the frequency of the beacon (not sure if needed)

the current induced by the magnetic field produces a voltage which is first buffered before being passed through an active bandpass filter.

the active bandpass filter consists of a simple second order highpass filter immediately followed by a second order lowpass filter. Both are unity gain with a bandwidth of 20000Hz.

Next the filtered voltage is passed through a series of amplifiers (4 stages)

Depending on the distance away from the beacon, a different stage is switched by a relay which connects it to a peak detector.

All opamps in use are TL071's and the problem that I have is that when it gets to the 3rd stage, there is some degree of noise.

I have been told to use smaller gains and when I connect a stage to the peak detector, I notice that the voltage drops considerably. I have tried buffering it before it enters the peak detector but no luck.

I have also put capacitors at the +/-V terminals of the opamp to prevent self oscillation

Any suggestions or comments about my circuit and this approach??

I was also wondering if its better to put the bandpass before or after the amplifiers and whether there should be gain, reduced bandwidth, etc..

Thanks!
 

Russlk

New Member
Noise is proportional to the square root of bandwidth, so reducing bandwidth will help. 20kHz bandwidth is very wide, what is the basic frequency? You should try for 5% bandwidth unless the frequency is crystal controlled, then 1% or less bandwidth could be used. If you give the frequency, I can post a circuit that is better than the hipass, lowpass.

It appears that you are trying to measure distance by the amplitude of the signal. Are you aware that the amplitude varies as the square of the distance?
 

JK108

New Member
I am trying to detect a magnetic field at 100kHz

And this project is an autonomous robot which is suppose to be able to locate and move towards the beacon by following the magnetic field line...

The thing is when I put an oscilloscope at the end of the third stage, and the inductor is far away from the beacon, it doesnt seem to pick up the magnetic field lines anymore.

At one point for a certain configuration, the frequency detected by the third stage was say around 103khz. I was wondering if this is caused by the opamps or is it possible that it is caused by the LC circuit.

Thanks for your prompt reply
 

Russlk

New Member
Magnetic lines are more or less circular, so how does the robot know which way to go?
If you have a signal strength X at a distance Y, at Y/10 the signal will be .01X and at Y/100 the signal is .0001X. That is why there is no signal.
The tuned circuit could easily be off 3% and would respond to noise at that frequency.
Physically large coils would increase the signal. Doubling the size should double the distance that you can detect a signal.
 

JK108

New Member
do you have any recommendations on any other opamps that may work better for this circuit?

I also have another problem, at every output of each opamp, there is an offset. I have tried putting a passive high pass filter at every at every output to get rid of the dc voltage but it only seems to decrease the amplitude of the signal and add in more noise.

Is there a way to calculate values for the R and C?? so that there is more voltage going into the next opamp?

or is there a better method? to get rid of the offset?

thanks
 
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