Inductive proximity sensor / tachometer

Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
The sensor is just an inductor that is already made, I can't change its value or parameters, it measures around 900-1000 uH micro henries with LCR meter.
I am exciting this coil with a square wave 0 to 5 volts at 60khz to detect the turbine blades.
I have to be able to detect the blades and ignore the enclosure which is made out of another type of metal. (60khz seems to work)
Currently I am using a signal generator 50 ohms but later I have to make my own oscillator.
My circuit starts with a LR high pass filter to ground, and it is followed by a demodulator ( diode 1N4148, capacitor 1 nano ibelieve, and resistor 1Meg) which is fed into a LM358 as a comparator.
The demodulator has an idle voltage of 3.3v, and I have also set my refernce voltage to 3.3v.
When the blades are come close it knocks the voltage down to 3.2v ( 100mv change).
The circuit is currently working but that is with me tuning the refernce voltage down to the third digit or so... i don't think I will have this kind of flexibilty when I create my own oscillator.
I want to know if there is a better way of detecting such a small voltage change (100mV).
I can use an arduino to sense this little change but I am not sure ( haven't tried it yet).
Later I want to use the arduino to count the pulses using FrequencyCount library or something like that.
Online I have seen peopel use a differential amplifier before going into the comparator.
Do you have any suggestions for me? :/
I was thinking about adding a capacitor to the inductor to create an LC tank but I don't want to search for resonance since it is already working.
I have seen another user post this same circuit, I am pretty sure he/she found it on this website:
A DIY Crude Inductive proximity switch | Hum60Hz (wordpress.com)
Here is my circuit:
I would love to hear any feedback :D
Thank you for your time
 

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Sensacell

Joined Jun 19, 2012
3,505
What is the range of actual "blade pulse" frequencies you are expecting?

This would be vital for designing the demodulator for this idea.
 

michael8

Joined Jan 11, 2015
428
Is that the flow meter from this page (I searched for turbine-flow-teco-1-jpg from the picture URL)?

https://blog.teco-inc.com/2017/04/turbine-flowmeters.html

If so, it's designed to output pulses without outside excitation, so you don't need your 60KHz source,
just amplify the output and detect each blade going by. If you have it handy put an oscilloscope
on the output and spin the turbine. Once you know the output voltage you can figure out a circuit
for it. The output will be the frequency of the blades going by.

more on turbine flow meters:

https://www.omega.com/en-us/resources/turbine-flow-meter
 

Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
Is that the flow meter from this page (I searched for turbine-flow-teco-1-jpg from the picture URL)?

https://blog.teco-inc.com/2017/04/turbine-flowmeters.html

If so, it's designed to output pulses without outside excitation, so you don't need your 60KHz source,
just amplify the output and detect each blade going by. If you have it handy put an oscilloscope
on the output and spin the turbine. Once you know the output voltage you can figure out a circuit
for it. The output will be the frequency of the blades going by.

more on turbine flow meters:

https://www.omega.com/en-us/resources/turbine-flow-meter
Thank you for the feedback, Michael!
No, I found that image on google. I just wanted to include that image for a basic visual understanding.
There are 2 types of flowmeters: inductive and hall. We are using inductive.
Inductive and Hall Effect RPM Sensors Explained (linkedin.com)

I only have the coil; I need to create a circuit for it.
Yes, I have a scope.
When I blow into the flowmeter, I can see the pulses on the output.
My circuit is working but barely...I am trying to extend the range; trying a higher voltage didn't seem to help.
I want to know if there is a better way to detect the 100mv change.

This guy on YouTube is using a differential amplifier to do this.
Using a Hall Effect Sensor to Make a Tachometer - YouTube
He is using a hall effect sensor, but the concept is the same.
When an object comes close to the sensor it knocks the volage down.
For him the voltage goes from 2.5V to 1.5V
For me the voltage goes from 3.3v to 3.2v

I hope this makes sense.
 

LesJones

Joined Jan 8, 2017
4,227
Could you not make the coil part of a tuned circuit in an oscillator. As the feromagnetic part came close to it the coil's inductance would change causing a change in the oscillator frequency. If the signal from the oscillator was then pased though a bandpass filter (Just an L C tuned circuit.) the ampplitude of the signal after the filter would change as the frequency moved from the bandpass filter center frequency.
This is the same idea as shown in screenshot 2024 in post #1. (Except the value of capacitor and inductor does not resonate with the 220 Khz signal which it should.)
I picture of your coil and rotor assembly would help us see the relationship between the coil and the tips on the rotor.

Les.
 
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Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
Could yoy not make the coil part of a tuned circuit in an oscillator. As the feromagnetic part came close to it the coil's inductance would change causing a change in the oscillator frequency. If the signal from the oscillator was then pased though a bandpass filter (Just an L C tuned circuit.) the ampplitude of the signal after the filter would change as the frequency moved from the bandpass filter center frequency.

Les.
Hi Les:
Yes, when the blades come close it will change the frequency across the coil, which will also cause a change in voltage amplitude. That is what I am monitoring with the demodulator circuit, a change is voltage amplitude.
This is kind of like AM modulation...where the 60khz is my carrier frequency and the output is the "intelligence" demodulator part.
I understand that ferromagnetic metals will increase the voltage and non-ferromagnetic metals will decrease voltage, but once you up in frequency to certain level, you will see a drop in voltage for both types of metal.
So, you are suggesting using a band pass filter before going into the main circuit?
 

michael8

Joined Jan 11, 2015
428
My problem is that I am working with such a small voltage change (100mV)

No, your problem is that you are looking for a small change in a large value so it's hard to adjust to
the right point to decide if it's a pulse or not.

That's NOT the way the inductive sensor are designed to operate. They hae a magnet in them and directly
produce a, possibly small, voltage by themself without outside power. This is much easier to deal with
as it's against a near zero background. Here's the description from YOUR link:

https://www.linkedin.com/pulse/inductive-hall-effect-rpm-sensors-explained-kiril-mucevski

Inductive Sensor Operating Principles and Specification

The inductive sensor, also known as magnetic pickup sensor, during the operational work, as result of inductive effect, in the sensor’s coil is producing the oscillating voltage, i.e. one kind of sinusoidal waveform signal (∼ AC voltage).


When the trigger wheel with the teeth passes in enough close distance (G) to the pole pin of the sensor, the magnetic field surrounding the coil is changed. As the result of the magnetic field changes, in the coil a voltage is induced, which is proportional to the strength and rate of change of the magnetic field. One complete oscillation is produced for each tooth that passes beside to the sensor pole pin. Figure 1 shows the basic integral components and the shape of the generated signal of an inductive sensor.
 

Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
My problem is that I am working with such a small voltage change (100mV)

No, your problem is that you are looking for a small change in a large value so it's hard to adjust to
the right point to decide if it's a pulse or not.

That's NOT the way the inductive sensor are designed to operate. They hae a magnet in them and directly
produce a, possibly small, voltage by themself without outside power. This is much easier to deal with
as it's against a near zero background. Here's the description from YOUR link:

https://www.linkedin.com/pulse/inductive-hall-effect-rpm-sensors-explained-kiril-mucevski

Inductive Sensor Operating Principles and Specification

The inductive sensor, also known as magnetic pickup sensor, during the operational work, as result of inductive effect, in the sensor’s coil is producing the oscillating voltage, i.e. one kind of sinusoidal waveform signal (∼ AC voltage).


When the trigger wheel with the teeth passes in enough close distance (G) to the pole pin of the sensor, the magnetic field surrounding the coil is changed. As the result of the magnetic field changes, in the coil a voltage is induced, which is proportional to the strength and rate of change of the magnetic field. One complete oscillation is produced for each tooth that passes beside to the sensor pole pin. Figure 1 shows the basic integral components and the shape of the generated signal of an inductive sensor.
Thank you, Michael.
Not sure why you would refer me back to the same link I posted...
I am well aware of how these sensors work.
Right, that is how the inductive sensors work with a built-in circuit.
Again, I don't have a sensor, I just have a coil...
I don't have 3 wires coming out...I only have 2 wires...
Forget the pulses for now.
Imagine that it is just a metal detector for now and you want to extend the range.
 

Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
My problem is that I am working with such a small voltage change (100mV)

No, your problem is that you are looking for a small change in a large value so it's hard to adjust to
the right point to decide if it's a pulse or not.

That's NOT the way the inductive sensor are designed to operate. They hae a magnet in them and directly
produce a, possibly small, voltage by themself without outside power. This is much easier to deal with
as it's against a near zero background. Here's the description from YOUR link:

https://www.linkedin.com/pulse/inductive-hall-effect-rpm-sensors-explained-kiril-mucevski

Inductive Sensor Operating Principles and Specification

The inductive sensor, also known as magnetic pickup sensor, during the operational work, as result of inductive effect, in the sensor’s coil is producing the oscillating voltage, i.e. one kind of sinusoidal waveform signal (∼ AC voltage).


When the trigger wheel with the teeth passes in enough close distance (G) to the pole pin of the sensor, the magnetic field surrounding the coil is changed. As the result of the magnetic field changes, in the coil a voltage is induced, which is proportional to the strength and rate of change of the magnetic field. One complete oscillation is produced for each tooth that passes beside to the sensor pole pin. Figure 1 shows the basic integral components and the shape of the generated signal of an inductive sensor.
Oh, pardon me.
I think I know what you're talking about now.
I am trying it now without outside excitation.
I have the 2 wires from the inductor going into the scope, but when I blow into it, I don't see any change in voltage.
 

LesJones

Joined Jan 8, 2017
4,227
Re post #8. From your original post I thought you were feeding a 60 khz signal to the coil and monitoring the change of voltage across it. My suggection was to make the coil part of an oscillator so that the change of inductance caused a change in frequency. (Frequency modulation)
A variation on this idea is to add a capacitor in parallel with the coil to make it resonate at 60 Khz . Then feed your 60 Khz signal to the tuned circuit via a low value capacitor or a reasonably high value resistor. When the metal tip on the rotor was close to the coil it would no longer resonate at 60 Khz so the voltage across the coil and capacitor will drop. Adding a ferite core to the coil woul probable increase the magnetic coupling between the coil and the tips on the rotor. This would also increase the inductance which would change the required capacitor value to resonate at 60 Khz. For resonance with your 1 mH coil you would require a a capacitor of about 7 nF
(The reactance of the coil ot capacitor at 60 Khz is 377 ohms.)

Les.
 

LesJones

Joined Jan 8, 2017
4,227
Re your post #11 You have not done exactly what Michael described. You need a magnet close to the top end of the coil core. See the first picture in the link in post #9.

Les.
 

Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
Re post #8. From your original post I thought you were feeding a 60 khz signal to the coil and monitoring the change of voltage across it. My suggection was to make the coil part of an oscillator so that the change of inductance caused a change in frequency. (Frequency modulation)
A variation on this idea is to add a capacitor in parallel with the coil to make it resonate at 60 Khz . Then feed your 60 Khz signal to the tuned circuit via a low value capacitor or a reasonably high value resistor. When the metal tip on the rotor was close to the coil it would no longer resonate at 60 Khz so the voltage across the coil and capacitor will drop. Adding a ferite core to the coil woul probable increase the magnetic coupling between the coil and the tips on the rotor. This would also increase the inductance which would change the required capacitor value to resonate at 60 Khz. For resonance with your 1 mH coil you would require a a capacitor of about 7 nF
(The reactance of the coil ot capacitor at 60 Khz is 377 ohms.)

Les.
Right,
In the beginning I was using a Colpitts/clap oscillator, and my inductor was part of the oscillator.
Now I am just using a square wave to excite the coil and monitoring the voltage with the demodulator.
In my opinion it will be easier to create a square wave in the future for the excitation as opposed to a sine.
I will try the resonant idea with the 7nano cap like you suggested and will get back to you.
Thank you very much Les!
 

Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
Re your post #11 You have not done exactly what Michael described. You need a magnet close to the top end of the coil core. See the first picture in the link in post #9.

Les.
Yes, I agree with both of you guys.
I don't know how I would do that?
This is my setup:
The coil gets screwed into the flowmeter.
When I blow into it nothing happens, but Michael did suggest amplifying it, which I have not done yet.
I was just thinking of using a non-inverting op-amp for the amplification...
 

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michael8

Joined Jan 11, 2015
428
Re your post #11 You have not done exactly what Michael described. You need a magnet close to the top end of the coil core. See the first picture in the link in post #9.

The magnet is (supposed) to be built into the sensor.
I had a auto distributor pickup coil (instead of ignition points) and that is how it worked.
Don't be surprised if the signal level output is anywhere between 10 mV (0.010) to 10 volts.
And it's likely a pulse as the blade goes by.
 

Thread Starter

RoyerDominator

Joined Apr 22, 2024
20
Re your post #11 You have not done exactly what Michael described. You need a magnet close to the top end of the coil core. See the first picture in the link in post #9.

The magnet is (supposed) to be built into the sensor.
I had a auto distributor pickup coil (instead of ignition points) and that is how it worked.
Don't be surprised if the signal level output is anywhere between 10 mV (0.010) to 10 volts.
And it's likely a pulse as the blade goes by.
I see I see.
Thank you for all of the insight Michael, you're the man :)
Tried amplifying the signal and it didn't do anything.
I believe those are the "older" type of flowmeters that kind of work like a guitar pickup amp.
Also, I believe those type of sensors cause drag because of the magnet.

These "newer" type use modulated signal.
I think I am going to try using an Arduino with analogRead() instead of the comparator stage to detect the 100mV change.

I am still curious about how this individual is using a differential amplifier as a comparator.
At least that's what it looks like to me.
Using a Hall Effect Sensor to Make a Tachometer - YouTube
 

michael8

Joined Jan 11, 2015
428
I am still curious about how this individual is using a differential amplifier as a comparator.
At least that's what it looks like to me.

Using a Hall Effect Sensor to Make a Tachometer - YouTube

You said that you measured the inductance of your "inductive prximity sensor". A hall effect sensor is totally
different, it measures the magnetic field strength (or if a digital hall effect sensor says high or lower than
some built in threshold). And it would not measure as inductive...

It's possible that the signal output from your inductive sensor is in the mV range so make sure your scope can
see signals that low. Or put some A/C gain in front of the scope to increase the signal. This is still quite simple as
there will be NO signal between blades passing the sensor.

Or you have something else, you've refered to things all over the web but never said what you actually have.
Make & Model number... (and URL?)
 
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