which type of metal dedector circuit?

hello dear forum,

I was asked to build a metal dedector for
catching every type of metal particles
in the output of plastic shredding machine for recycling of used plastics

I need to dedect every kind of metal not just iron

I searched google and come upon 3 types of dedectors

Beat Frequency Operation, Pulse Induction, and Induction Balance

which type do you suggest me for my purpose ?

thank you

First - do you want to detect the types of metal, or do you want to remove the metal?

Recycling systems use several stages to remove metals, first magnets to remove ferrous material, then air blowers to segregate lighter plastic from heavy metals. Only then do they use metal detectors to find residual pieces.

The methods you mention will all work but are more suitable for finding buried treasure than large scale material separation.

Brian.

I dont want to detect the type of metal
I want remove the residual metal particles just like you mentioned

which type of detector is best for this job ?

thank you

It will take all of the methods betwixt described to you. Different methods for different materials. Magnets, eddy currents, blowers, tumblers, hand picking etc.

Consider that many plastics are actually composite materials. For example there may be metallic security tags or embedded nut inserts which are molded or pressed into the plastic. To some degree the sorting process depends on the size of the shredded particles, smaller bits will be easier to detect and less wasteful but more difficult to handle.

There are other methods you could consider, for example metals never pass light but some plastics are translucent, transparent or may fluoresce in UV light.

Brian.

I am trying to produce this dedector - what is your advice please? what type of dedector shall I use ?

https://www.sollau.com/katalog/en-i...for-the-plastic-and-recycling-industries.html

thank you

The first stage of that machine is not to detect the metal but to deflect it using powerful magnets, that is how all commercial sorters work. the process is:

1. the waste is first broken down (shredded or crushed) to make the particles smaller and lighter.
2. it is passed along a conveyor beneath a magnet, gravity pulls everything down but the magnet can lift ferrous metals away. (ferrous removed)
3. weight is used next, the residue is blown by a fan to separate it by weight, the idea being that heavier metals drop closer and lighter plastics travel further. (pure plastics removed)
4. residue then passed through an induction field to detect non-ferrous metals and these are mechanically separated. (non-ferrous removed)
5. finally, the remainder is hand sorted.

None of these are 100% effective, hence the manual stage at the end.

Brian.

I am trying to produce this dedector - what is your advice please? what type of dedector shall I use...

Click to expand...

They use electromagnetic detector: you use a pair of Helmholtz coils outside the plastic flow tube: the coil is excited with 100kHz (I am not sure about the exact frequency) and a metal particle (typically larger than a small coin) will produce a signal that will cause the particle to deflect and go into a different bin.

"Uses CR coil for reliable detection and is cost effective for the examination of bulk material in gravity free-fall applications."

Low-cost triple-coil separator offers superior sensitivity, simplicity, and reliability. Specifically designed for use in plastics and recycling.

hello dear friends,

I took these sentences from a metal dedector producers webpage

my questions ; what does CR coil mean ?

which of the tree types ( Beat Frequency Operation, Pulse Induction, and Induction Balance) of dedector is the CR coil and triple coil dedector ?

thanks

Never heard of a "CR coil" before, at least in the context of electronics.

It's probably some invented name to make it sound important. The principle used are:

BFO = the method used by hobby metal detectors, the coil is part of a tuned circuit in an oscillator. The output of that oscillator is mixed with a very stable one to produce a beat frequency, usually in the audible range. The presence of some metals near the oscillator coil causes a frequency shift which is heard as a change in audio pitch.

Pulse induction: works in one of two ways, either you pulse power through one coil and detect it in the other so the metal affects the amount of power transferred or, you pulse one coil and look for changes in it's impedance as energy is absorbed as eddy currents in nearby conductive (metal) items.

Induction balance = two identical energizing coils are used but with signals fed in opposite directions. The resulting magnetic fields cancel out. A third coil picks up the residual field which if balanced properly should be zero. If a metal object comes into the field, it disturbs it and the cancellation is no longer complete, the third coil then has a voltage induced into it.

Perhaps "triple-coil separator" is a reference to the three coils in an induction balance system.

Brian.

hello dear forum,

I assembled the 1st prototype of my puls induction metal dedector

as you can see in the video below ( only 1 min )it can dedect big parts but in the end of the video it cannot dedect
a 0.8 mm thick washer

https://www.youtube.com/watch?v=Rd9Mb1OVgvg&feature=youtu.be

I used a 2 coil PI dedector with transmitter and receiver sides isolated

I simply amplified 1000X the receiver coils voltage after clipping it with 2x 1N4148

and after an optocoupler I simply put this signal to the timer input of STM32
and counted the time between falling and rising edges of the signal

at this moment I take the difference of the previous count from the last count
if this difference is greater than some constant value I energize the LED
I adjust the constant value just a little greater than noise so that to eliminate false dedects

the transmitting coils current reaches 5 amperes in the end of the transmitting puls ( linearly increasing )

the transmit coil is 1mH receiving is 300 uH

what do you suggest me to increase the sensivity of my dedector ?

the professional companies claim that they can dedect 0,5 mm radius steel ball

shall I increase the transmitting coils current
or increase the receiving coil inductance

or better what else ? please advise

thank you

hello dear forum

here is the result of my PI metal dedector

as you can see in the video

it can dedect big metal parts but in the case of a 1 mm steel part
it has no response
(the osciloscope shows the output of difference amplifier)

my circuit is this - I tried to integrate (not shown in the circuit )the difference amp output however there is still no significant change in the resulting ( integrated ) signal

what do you suggest me to do further ?

thank you

Your Video above does not run.

Smaller Coils Detect Smaller Object Better.
It is Difficult to have just one coil that will detect both large and Tiny Objects.

From what I understand with other types of detectors frequency effects whether it will detect smaller objects. In general lower frequency will detect deeper. Higher frequency will not penetrate as deep but will detect smaller objects. I'm not sure if this also applies to pi detectors. Maybe something to consider.

Kajunbee said:

From what I understand with other types of detectors frequency effects whether it will detect smaller objects. In general lower frequency will detect deeper. Higher frequency will not penetrate as deep but will detect smaller objects. I'm not sure if this also applies to pi detectors. Maybe something to consider.

Click to expand...

No this does Not Apply to PI Detectors.
Increasing the Frequency on a PI Detector Just Increase the Coil Current, But than that Require a Longer Wait Time between Transmit and Receive.
So it gives Very little Improvement in Detection of either Large or Small items.
A Pin Pointing Coil is Best for Tiny Objects, But Distance of Detection is Quite Limited.

From what I understand with other types of detectors frequency effects whether it will detect smaller objects. In general lower frequency will detect deeper. Higher frequency will not penetrate as deep but will detect smaller objects. I'm not sure if this also applies to pi detectors. Maybe something to consider.

- - - Updated - - -

Sorry about the double post. If someone could delete it would be appreciated. With pi detectors does the spot where the object passes through the coil matter. Would object passing close to one side be detected easier than one passing through the center of coil. What I'm wondering is if the coil shape would make it more sensitive. Say instead of round coil, have a rectangular shaped coil. The area inside would be the same but the long sides of the rectangular shaped coil would be in closer proximity to one another. Do you think this would improve its sensitivity in any way.

A Rectangular Coil can be Better for Small and Larger objects.

Weither the Object Passes Through the Coil is Not so Important.
The Sensitive Area is at the Center, Either In it or Just Above or Below Top or Bottom of the Coil.