[SOLVED] magnetic sensor to detect metal

[mohdsalim]

hi
i have a magnetic sensor circuit diagram which am trying to understand its concept of working but still not getting the idea. the following is an attachment of the circuit diagram. it uses the concept of an inductor and a capacitor in series with each other. it detects metal cans and differentiate them from plastic cans. please if anyone can help me on this. thanks

 

[erikl]

The circuit represents an electronic (low frequency) oscillator. If an electrically conducting material (all metals) gets close to the inductor, its electro-magnetic field induces eddy-currents in the metal, which deprive the e.m. field of power. This power loss can be detected and indicated.

 

[KTHOM]

it looks to me like the opamp circuit on the left is a regulator to bring the supply from 9V down to 4.5V. Im guessing the cans will be on some sort of conveyor belt that pass by the sensor? when the inductor is placed in an AC magnetic field, it will generate an AC current. C4/R6 and C6/R9 i believe form a bandpass filter for this current, so only metal moving by at a particular speed is detected.

 

[mohdsalim]

thank you all for the feedback.
@erikl you said that the power loss is detected and indicated, which power loss? and which em field of power? where is it detected? the project am doing is a robotic arm, which sorts these two cans(plastic and metal).

 

[erikl]

@erikl you said that the power loss is detected and indicated, which power loss? and which em field of power?

Your circuit represents an oscillator circuit, which converts power from the 9V supply into an e.m. field. Some of this power is present in the neighborhood of inductor L1. In metal coming close to L1, the e.m. field of L1 induces eddy currents, which means e.m. power from the coil is transferred to the metal, there being dissipated into heat. Thus the oscillator circuit looses power.

... where is it detected?

The ac voltage at the opAmp's output is transferred via C5 & R9 to a rectifier consisting of D1 & D2, flattened by C7, and transferred via R10 to the sensor output.

As more metal - or closer metal - is present, as more e.m. power will be transferred to it, as more the rectified voltage will decrease. This voltage dip will be detected and indicated by a following apparatus.

 

[miskol]

Dear erikl,

1) so this circuit acts as a transceiver. the circuit transmits e.m. power, and if a metal is nearby, the voltage received will decrease. is it like that?
2) other than metal, what other material will it responds to?
3) how far will the sensing range be?
4) stationary objects will not be detected?

Thank you.

 

[erikl]

1) so this circuit acts as a transceiver. the circuit transmits e.m. power, and if a metal is nearby, the voltage received will decrease. is it like that?

Not the "voltage received", there's nothing received from the absorbing metal; the power transfer from the oscillator's inductor coil (L1) to the metal is unidirectional. It's the oscillator voltage at the opAmp's output which decreases due to the power loss.

2) other than metal, what other material will it responds to?

Only material which can conduct electric current (at the oscillator frequency). In the existing application, this concerns only metals (in solid & fluid form, i.e also valid for mercury).

3) how far will the sensing range be?

About the order of the coil's size, perhaps 3times its length, but not 10 times.

4) stationary objects will not be detected?

Sure they will! Metal objects absorb power, if stationary or moving. But this depends also on the detecting method: If the detector senses an absolute voltage value, a stationary object can be detected. If it just senses a dynamic voltage dip change, only moving objects will be detected.

 

[mohdsalim]

thank you erikl. now i get the entire idea very clear. thanks again

 

[mohdsalim]

mr erik, can you highlight to me the importance of the op amp used in this circuit? i am now connecting the circuit.
thank you

 

[erikl]

... importance of the op amp used in this circuit

The op amp provides the necessary gain. The oscillation arises because of positive feed back from the resonator circuit (L1 || C8 ) via C2 to the non-inverting input (+) of the op amp.

 

[mohdsalim]

if i use the 9v supply, how much will it amplify it to?

 

[erikl]

if i use the 9v supply, how much will it amplify it to?

The op amp doesn't amplify its supply voltage!

 

[mohdsalim]

yeah, i meant is the voltage from the inductor L1 that is amplified in the amplifier and then taken to the diodes for rectification?

---------- Post added at 00:49 ---------- Previous post was at 00:48 ----------

and how much is it amplified to? using the IC TL071P amplifier

 

[erikl]

The output voltage will be in the order of 0.1 .. a few volts, depending on distance and volume of the metallic object. Output impedance is 10kΩ (R10).

 

[mohdsalim]

thank you sir for the help. it is really important to me. thanks again