Capacitive sensor to differentiate metal and plastic

[kkdelabaca]

Hello!

I need detect when an object is made with plastic or with metal.
I can use a capacitive sensor? Somebody knows some capacitive sensor? (please, NON EXPENSIVE).

Somebody has another idea?

Regards!!!

 

[VVV]

Perapas an inductive sensor would make sense. The change in inductance in the proximity of a metal object is large, while plastic should produce no such change.
This is actually the principle of the good ol' metal detectors.

 

[kkdelabaca]

Thanks VVV, but it the metetal is ALIMINUM?

 

[VVV]

It does not matter if the metal actually causes the inductance to increase or decrease. You are looking for a CHANGE in inductance. Metals will cause the inductance to change significantly, while plastics will actually cause no change.

 

[watermelon]

You can use a capacitive sensor.

Use two plates positioned side by side (or opposite) as two plates of a capacitor. Use this capacitor in an oscillator ( digital oscillators will be more suitable, in some cases). The presence of metal (any metal) will increase the capacitance, lowering the frequency substantially while plastic will barely effect the frequency compared to no material present. If you know the change in capacitance (measure the frequency change), you can use a capacitor in parallel with this capacitor to keep the change with in manageable limits.

Requires placing the plates close to the object.

You need to shield against electrical and magnetic fields.

Detect oscillator frequency change using a PLL (or a monostable + D flip flop combination). You can have long lines (subjected to noise interference and magnetic / electrical fields and current drive capability of osc (+buffer)) between oscillator and PLL.

Plates could be mounted on a solid plastic sheet by bolting, assuming that you need this for some kind of objects moving on a conveyor belt.

Alternatively, you could sense change in phase of a waveform. There are more ways. Depends on your requirement and the scenario of usage. If you describe your requirement in detail and what do you want to achieve, probably a more suitable solution can be worked out.

 

[kender]

I need detect when an object is made with plastic or with metal.
I can use a capacitive sensor? Somebody knows some capacitive sensor? (please, NON EXPENSIVE).

I would soleve your problem with a combination of a capacitive sensopr and an inductive (eddy current + variable reluctance) sensor.

Also, I would like to invite your question to a speciallized forum dedicated to sensors: **broken link removed**.

 

[VSMVDD]

the standard way to do this
{fruit machine ticket machine coins or note's accepter tester etc...}

is to electrostaticaly charge the unit under test
then measure its ev over a given time
this way it will be able to sense any material under test
and its cheep to do this
using say a 1000v charge

not so good for es sensitive devices however

but is used in some type of manufacture

another method is to burst xray it if it travels thru
if its plastic no scatter of the focus

if there is scater beyond a point its metal
use xray ccd and led's
so.. two usefull methods

lets see if there is more

 

[watermelon]

Well, unless the scenario of usage is described, all the suggested solutions will remain equally useful (useless).

I also feel that, at the end of the day, the project needs to be economically viable, technically repeatable and reliable to provide continuous value.

Exotic methods should be used only when all else fail.

kkdelabaca needs to be specific on the requirements.

 

[kender]

BTW, here's a place to read-up on capacitive sensors https://www.analog.com/library/analogDialogue/archives/40-10/cap_sensors.html