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Induction sensor

J

Joseph

I'd like to build a metal detecting induction proximity sensor. Something
that can pick up a stainless disk (size of a dime) at .2 inch. Flipping
through the Allied catalog I see page after page of cylindrical (M18)
induction sensors.

Is there a common technique "everyone" is using that I should look into?
Can someone give me some circuit details to point me in the right direction?
thx
 
P

Pete D

NJoseph said:
I'd like to build a metal detecting induction proximity sensor. Something
that can pick up a stainless disk (size of a dime) at .2 inch. Flipping
through the Allied catalog I see page after page of cylindrical (M18)
induction sensors.

Is there a common technique "everyone" is using that I should look into?
Can someone give me some circuit details to point me in the right direction?
thx
ot used one, but i think they detect metal moving past the sensor end at
speed
 
J

Joseph

Not used one, but i think they detect metal moving past the sensor end at
speed

What I really meant to ask...
Is there a typical circuit design they use and what is it? There are so
many of nearly identical size and specs that I'm taking a guess there is a
common circuit they all use.
 
J

John Popelish

Joseph said:
I'd like to build a metal detecting induction proximity sensor. Something
that can pick up a stainless disk (size of a dime) at .2 inch. Flipping
through the Allied catalog I see page after page of cylindrical (M18)
induction sensors.

Is there a common technique "everyone" is using that I should look into?
Can someone give me some circuit details to point me in the right direction?

All the units I have seen are based on an internal
oscillator (a few hundred kilohertz for he small ones) and a
means to measure the eddy current load on that oscillator,
with a decision point to switch the output logic signal,
indicating metal in the field at some specific magnitude of
eddy current load on the oscillator coil. The two main
types are ones that spray field out all around the end of
the coil, as well as in front of it, and those that have
field shaping cores that keep almost all of the field in
front of the coil (concentric poles), so that the unit can
be flush mounted in a hole in metal without seeing this
metal, or close bedside other units, without interacting
with each other. The down side of the shielded form is that
it cuts the sensing range for a given diameter unit, about
in half.
 
J

Jamie

Joseph said:
I'd like to build a metal detecting induction proximity sensor. Something
that can pick up a stainless disk (size of a dime) at .2 inch. Flipping
through the Allied catalog I see page after page of cylindrical (M18)
induction sensors.

Is there a common technique "everyone" is using that I should look into?
Can someone give me some circuit details to point me in the right direction?
thx

You may want to get a large surface diameter type. Stainless is not as
sensitive as other types of metals.
Depending on your application, 2 wire types require a 120/240 AC in
the loop of lets say a relay coil.., you can get normally
closed/normally open types and the average handling current is around
200ma, that is normally enough for a 120v 10amp type relay coil with a
medium mechanical mass.


The 3 wire types require DC voltage 12..24 volts DC on most of them
going to the Brown Wire (+), Blue wire(-) and the black wire will be a
transistor output.

You can get those in PNP, NPN, N-on/N-off etc..
the handling current on those are around 200 Ma DC>
for the NPN types, it is used as a sink (pulls to common when on). We
this type, you can switch a variety of devices that use a varying range
of DC voltages, you simply pull it to ground/common..
With the PNP type, it becomes a source device. What this does is
generates output from the same (+) as the sensor is connected too.
The PNP types are not so popular..

So you have your pick now.
 
J

John Popelish

Joseph said:
I'd like to build a metal detecting induction proximity sensor. Something
that can pick up a stainless disk (size of a dime) at .2 inch. Flipping
through the Allied catalog I see page after page of cylindrical (M18)
induction sensors.

Is there a common technique "everyone" is using that I should look into?
Can someone give me some circuit details to point me in the right direction?
thx

I don't think you can possibly build one of these sensors
for what you can but several.

Here is a low priced source:
http://web4.automationdirect.com/ad...sors_-z-_Encoders/Inductive_Proximity_Sensors
 
J

Joseph

John Popelish said:
All the units I have seen are based on an internal oscillator (a few
hundred kilohertz for the small ones) and a means to measure the eddy
current load on that oscillator, with a decision point to switch the
output logic signal, indicating metal in the field at some specific
magnitude of eddy current load on the oscillator coil. The two main types
are ones that spray field out all around the end of the coil, as well as
in front of it, and those that have field shaping cores that keep almost
all of the field in front of the coil (concentric poles), so that the unit
can be flush mounted in a hole in metal without seeing this metal, or
close bedside other units, without interacting with each other. The down
side of the shielded form is that it cuts the sensing range for a given
diameter unit, about in half.

Despite the small costs of the available units I'd still like to build one
myself.

I have some pot cores and magnet wire for the coil, and uPs and relays etc
for the logic and output. But...the oscillator and detection circuit is new
to me and where I need some direction. I've googled and found some good
theory but very little on circuit specifics (except a 'hobby' metal
detector). Any help is appreciated.
 
J

John Popelish

Joseph said:
Despite the small costs of the available units I'd still like to build one
myself.

I have some pot cores and magnet wire for the coil, and uPs and relays etc
for the logic and output. But...the oscillator and detection circuit is new
to me and where I need some direction. I've googled and found some good
theory but very little on circuit specifics (except a 'hobby' metal
detector). Any help is appreciated.

I don't have a construction article for you, or even a
schematic, but I think you might get started by building a
Hartley Oscillator and sense the change in its DC supply
consumption as the sensed metal quenches its oscillation.
http://www.electronics-tutorials.com/oscillators/oscillator-basics.htm
 
R

Roger Dewhurst

John Popelish said:
I don't have a construction article for you, or even a
schematic, but I think you might get started by building a
Hartley Oscillator and sense the change in its DC supply
consumption as the sensed metal quenches its oscillation.
http://www.electronics-tutorials.com/oscillators/oscillator-basics.htm

How about an oscillator of which an induction coil is a component. The
frequency changes when metal approaches the coil. An appropriate filter
lets through the changed frequency. The level of the signal is detected and
if high enough a buzzer sounds. Would that work?

R
 
J

John Popelish

Roger said:
How about an oscillator of which an induction coil is a component.

Sorry, I meant to say that, but must have assumed it was
obvious. The oscillator inductor is the coil that produces
a field in front of the sensor.
The
frequency changes when metal approaches the coil. An appropriate filter
lets through the changed frequency. The level of the signal is detected and
if high enough a buzzer sounds. Would that work?

The ones I have seen have the oscillator actually lose
enough gain to stop oscillating, and the supply current
takes a step (down, I assume). The logic output just
detects the changes in the supply current to the oscillator.
 
J

John Popelish

Anthony said:
Is this attribute unique to Hartley oscillators? Does it not apply to other
types as well (Pierce, Colpitts, etc)?

I think it applies to any LC oscillator. I picked the
Hartley, because if you are going to wind your own inductor,
it puts more of the complexity into the coil, and less intro
the rest of the circuit. It is not hard to bring the tap
out on a home made coil.
 
J

John Popelish

John said:
I think it applies to any LC oscillator. I picked the Hartley, because
if you are going to wind your own inductor, it puts more of the
complexity into the coil, and less intro the rest of the circuit. It is
not hard to bring the tap out on a home made coil.

Man, it has been a long time since I last built a 1
transistor oscillator. I plinked around a little and
quickly discovered that I have the supply current situation
backwards.

The oscillator is biased to draw a significant and
continuous supply current, but once it gets going, the
rectification effect of the base junction kills the forward
bias for most of the cycle, greatly lowering the average
collector current, concentrating it into narrow pulses at
the positive peak of the cycle. Putting a piece of metal in
the inductor field not only lowers the inductance, but also
the Q, so the oscillation tends to start and die out ,
rapidly, so that the transistor spends most of the time in
an intermediate state between no oscillation (high current)
and active positive feedback oscillation (class C low
current operation). The hardest part for me to model is the
eddy current losses in the target as the distance is varied.
 
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