Hall effect detector

[wyndham]

I was reading up on the discovery of the Hall effect back in the late 1800's where a current was passed thru a rectangular gold foil length wise and a meter was connected on either side perpendicular to the currentand as a magnet was passed over the foil it created a current that was measueredfrom these contacts(I hope this is the correct way of expressing the experiment)
Could a PI TX coil induce an eddy current in both the target and the and a large scale hall sensor inside the coil housing(if the TX coil is 12 in dia then maybe a 6x8 Hall effect sensor) either using the TX induced current or a low constant current as in the Hall experiment did, so that it would be the RX sensor for the target signal and the charge gathered at the edge of the sensor be the signal brought back to the PI about the target.
If the Hall effect were to work the signal might be amplfied by the current or the TX induction and since the target signal would be derived from the charge build up on the side of the sensor, it might be isolated & not be too noisy
On the other hand, If the Tx induction were to overload the sensor, shielding it from the TX, then maybe the sensor could respond to the induced target mag field.
If this were to work, then tap points along the sensor could possibly give target info not presently available
I realize this maybe comparing apples and Aardvarks here but what does anyone think. Wyndham

[Carl-NC]

Wyndham,

I've never heard this one asked before. Very unique thinking.

The only way I can think to make this work is to put a Hall sensor in the middle of the coil, like you said, and pass a DC current through it. Then, the magnetic field from the TX coil will establish an AC Hall-effect voltage on the Hall sensor. When a metal target distorts the TX field, it should cause a change in the AC Hall-effect voltage, both amplitude and phase, similar to what you get in a normal IB system.

The problem I see is sensitivity. In an IB system, you start out with close to no RX signal, so it doesn't take much of a target signal to show up. With Hall-effect, you start out with a relatively large AC voltage, so that small perturbations from targets are really difficult to see.

But, in theory, it should work.

- Carl

[goldfinder]

Hall Effect

You are right Carl. In theory it would work and probably would for strong signals. The basic problem is that Hall Effect sensors are not sensitive enough to be of any use. You'd have to use a fluxgate magnetometer sensor.

Goldfinder

[daverave]

if i wanted to try and use hall effect sensors for iron discrimination would 2 sensors have to be used for common mode rejection of the earths magnetic field...if so would the hall effect sensors have to be postioned correctly in the coil to reject earth magnetic field.

[Waikiki_Sweep]

A1301 - Continuous-Time Ratiometric Linear Hall Effect Sensor ICs

I will get one this Monday from Digikey and I want to put it into my SurfPI coil as pinpointer or probably may be use it as substitute of RX coil for discrimination.

With proper orientation this small IC probably can be nulled for magnetic field from transmit coil just like two box metal detector balanced on 90 degrees between TX and RX.

It has PREAMPLIFIER inside. Also it sensing changes in magnetic fields up to 20 KHZ frequency. Looks perfect for the solution.

http://www.allegromicro.com/~/media/Files/Datasheets/A1301-2-Datasheet.ashx

The A1301 and A1302 are continuous-time, ratiometric, linear
Hall-effect sensor ICs. They are optimized to accurately provide
a voltage output that is proportional to an applied magnetic
field. These devices have a quiescent output voltage that is
50% of the supply voltage. Two output sensitivity options
are provided: 2.5 mV/G typical for the A1301, and 1.3 mV/G
typical for the A1302.
The Hall-effect integrated circuit included in each device
includes a Hall circuit, a linear amplifier, and a CMOS Class A
output structure. Integrating the Hall circuit and the amplifier
on a single chip minimizes many of the problems normally
associated with low voltage level analog signals.
High precision in output levels is obtained by internal gain
and offset trim adjustments made at end-of-line during the
manufacturing process.



Looks like hall effect is a future of metal detecting !!!

[daverave]

i think hall effect sensors may be an easy solution in pi detectors...i saw a product called a ferrous hound which sits on top of pi coil and gives iron id...but very expensive...i think it uses hall effect sensors...ive ordered some sensors so i may do some experimenting....i think 2 sensors will be needed as common mode rejection of earths magnetic field...anyone with any more knowledge on this pls advise.

[Waikiki_Sweep]

Allegro makes even more sensitive sensors:
A1324, 5 mV/G Internal Bandwidth 17 kHz
A1362, 4.5 to 16 mV/G but needs some fanny programming by 3 level voltages
A1359, 5 mV/G Matched analog and PWM outputs.

It is good idea to use two sensors to regect earth magnetic field. But you can also use one sensor and two sampling intervals, like most of PI detectors. First sending an impulse to create eddy currents in target then sample their magnetic fields then second sample for background (including Earth) magnetic field.
Sensors working on high frequency (20kHz). First to try I think is to put sensor as replacement of receiving coil into PI detector and put sensor on edge to make it 90 degrees to transmitting coil.

Such sensors, like magnetometers can measure very small changes in magnetic field with "symmetry broken" method:


Please keep us informed. It is very interesting new space for experimenting.

[daverave]

i was thinking about using a bar graph as an indicator of iron rather than audio...i think anything that can help us id iron is worth trying...and hall effect sensors are small and inexpensive to buy....where would you position the sensor in the coil...maybe centre ???? can you explain more what you mean about putting sensor on edge at 90 degrees...i was just going to place it face down flat in coil...is this wrong idea ????

[Waikiki_Sweep]

Sensor shows S and N magnetic field strength and polarity. Results depend on orientation. If you bring magnet to the edge of sensor it will show nothing. Branded side and back side of sensor gives maximum value and opposite outcomes. If you set magnet near edge and put sensor precisely into symmetry (as on picture) you can "balance" it to symmetry to not let it to see the magnet at all. Then if iron chunk goes to the area it will change symmetry of magnetic field and sensor will show few millivolts or microvolts shift you need to amplify.

With sinusoidal or rectangular signal from the coil there are more ways to get target detected even for color metal.

First I want use it as pinpointer because large PI coil makes me digging to much. And small sensor have to show precise position of target. So I want to put it on top left corner of the coil, that place I am using usually to pinpointing by coil edge.

Then I want to try to build something that may be will discriminate by sampling phase shift in PI transmit signal. Idea is to use balanced sensor as balanced TX coil used in VLF detectors.

[daverave]

i guess i will have to do some experimenting with these sensors...i did read somewhere about sampling the tx on waveform...is that what you mean about phase shift in PI signal....i saw a german pi detector which had a sensor in the coil like a bulge in the middle of the coil housing ...maybe a hall effect in there or magnometer sensor.

[Waikiki_Sweep]

i guess i will have to do some experimenting with these sensors...i did read somewhere about sampling the tx on waveform...is that what you mean about phase shift in PI signal....i saw a german pi detector which had a sensor in the coil like a bulge in the middle of the coil housing ...maybe a hall effect in there or magnometer sensor.

If you build PI detector with balanced RX coil (like all VLF detectors using) you will see on the oscilloscope how ferric / color metal targets changing reaction in RX coil in the beginning of TX pulse. I have tried it on the bench and hearing difference between US 25 cents and beer bottle cap from 6-8 inches.



I got the idea from Sascho Bulgaria "PULSE IB DISCRIMINATOR".



And PI detector that will discriminate ferric targets with using RX balanced coil should looks probably this way:



But if Hall sensor will work it may be cheaper. We will see.

[daverave]

very interesting...i noticed the diagram shows concentric coil instead of 2D....does that make a difference ???? it would be so good to get some sort of ferrous discrimination as im getting too old for digging deep holes !!!! at present im nearly at the end of making the surf pi detector...im just doing the coil but worried about using the resin as i feel it may warp the plastic coil housing also im not sure about how much hardner to put into the resin...im afraid to ruin the coil that ive made so well.

[Waikiki_Sweep]

very interesting...i noticed the diagram shows concentric coil instead of 2D....does that make a difference ???? it would be so good to get some sort of ferrous discrimination as im getting too old for digging deep holes !!!! at present im nearly at the end of making the surf pi detector...im just doing the coil but worried about using the resin as i feel it may warp the plastic coil housing also im not sure about how much hardner to put into the resin...im afraid to ruin the coil that ive made so well.

"i noticed the diagram shows concentric coil instead of 2D....does that make a difference ????" - No. Any way to build balanced coil will work for practically any IB detector.
You can make it DD, OO, Concentric, "8" shape, e.t.c. Any construction require that RX coil does not receive signal from TX coil if there is no target.
But some IB detectors required not a complete nulling but proper phase shift so adjustment may take some hard work.

Probably you do not need resin at all. To make strong hard coil is required for induction balance coils because even small deformation will destroy balance. So IB coils use big amount of resin and they are heavy.

For PI detectors there is no balance requirement so you can bend and deform PI coil as you wish. So you can use any light stuff to cover the coil and give it any shape.
Important is to shield and to waterproof the coil. Anything else can be made from foam not from epoxy.

Once you put a pound epoxy on it think how you will sweep that "brick on stick" for several hours a day. I like to make my 140 gram PI coils from fiberglass.

[Waikiki_Sweep]

Oops... I have fried both my sensors accidentally thinking that pinout is similar to transistors (branded side flat - but it is wrong).
So they did not survived inverted power and now giving only 0.6V instead of 2.5V output and having low reaction on the magnet.
I have to wait for next supply of it to make my experiments.
To prevent loosing of your sensors please check the picture below for correct pinout.

[Waikiki_Sweep]

Shame on me. I have messed up with pinout again.

CORRECTED


Also sensors are working and I have make first quick experiment.
Two sensors placed opposite and on two inches distance.
One sensor inverted on 180 degrees to compensate Earth magnetic field.
Magnet is placed on the center between sensors.
Each sensor have to provide 2.5 volts if there is no magnetic field.
I have connected voltmeter between sensors then nulled signal by moving magnet to precise position
when it shows close to 0 volts.
Then if I move beer bottle cap up to two inches high over sensors I can see 1 mV or more readings on voltmeter.
So it looks promising. There should be some ways to increase range of detecting like increase distance between sensors,
use stronger magnet, use amplifier, e.t.c.
I will look into it more.