LVDT question

[Marco Trapanese]

Take an LVDT (series-opposite), apply a sine wave on the primary coil
and measure the output signal on the secondary coil with an oscilloscope.

Do you agree you *must* see a change in the amplitude when you move the rod?
At least you should expect a null signal when the rod is place at null
point.

What if this doesn't happen? I mean the output signal changes very
slightly around it's maximum.

Double checked the connections, changed the LVDT with new ones - no chance.

What else?
Marco

 

[Jasen Betts]

Take an LVDT (series-opposite), apply a sine wave on the primary coil
and measure the output signal on the secondary coil with an oscilloscope.

Do you agree you *must* see a change in the amplitude when you move the rod?
At least you should expect a null signal when the rod is place at null
point.

What if this doesn't happen? I mean the output signal changes very
slightly around it's maximum.

Double checked the connections, changed the LVDT with new ones - no chance.

have you got the terminals mixed up?

swapping one of the primary terminals with one of the secondary
terminals would give the effect you describe.

 

[Marco Trapanese]

Il 09/04/2013 13:03, Jasen Betts ha scritto:

have you got the terminals mixed up?

swapping one of the primary terminals with one of the secondary
terminals would give the effect you describe.

I checked the coils measuring their resistance.
I'm pretty sure the connection is correct, anyway it's easy to swap two
pins and retry. I'll do it in a while.

Marco

 

[Marco Trapanese]

Il 09/04/2013 14:29, Marco Trapanese ha scritto:

I checked the coils measuring their resistance.
I'm pretty sure the connection is correct, anyway it's easy to swap two
pins and retry. I'll do it in a while.

Swapping two pins leads to a constant voltage with the same amplitude of
the excitation voltage.
With the correct connection the output voltage is quiet small.

I don't think the sensors are damaged because we have several of them
and all have the same behavior.

Marco

 

[John Devereux]

Take an LVDT (series-opposite), apply a sine wave on the primary coil
and measure the output signal on the secondary coil with an
oscilloscope.

Do you agree you *must* see a change in the amplitude when you move the rod?
At least you should expect a null signal when the rod is place at null
point.

Is it an AC LVDT?

Is the secondary coil in fact two coils? This is usually the case. But
possibly they have done this for you and there are only two wires
emerging.

In order to see a null you are supposed to connect the two secondary
coils in series and in antiphase. So with the core in mid position, or
absent, there is no net signal.

What if this doesn't happen? I mean the output signal changes very
slightly around it's maximum.

Double checked the connections, changed the LVDT with new ones - no chance.

What else?

I can see this happening if you are just using one of the two
secondaries, and it is unloaded (like with a scope).

 

[Syd Rumpo]

Il 09/04/2013 14:29, Marco Trapanese ha scritto:



Swapping two pins leads to a constant voltage with the same amplitude of
the excitation voltage.

Ok, thats right.

With the correct connection the output voltage is quiet small.

I don't think the sensors are damaged because we have several of them
and all have the same behavior.

Marco

I'm suspicious about the core (rod). Are you sure it's the original rod
and someone's not made a replacement from, say, Aluminium? Got a magnet
handy?

Cheers

 

[Marco Trapanese]

Il 09/04/2013 14:40, John Devereux ha scritto:

Is it an AC LVDT?

Yes


Is the secondary coil in fact two coils? This is usually the case. But
possibly they have done this for you and there are only two wires
emerging.

Yes, is a series-opposite configuration and the connection is made
internally. There are only four wires.

In order to see a null you are supposed to connect the two secondary
coils in series and in antiphase. So with the core in mid position, or
absent, there is no net signal.

This is what I knew and what I expect.
What about if the two coils are connected in series but not in antiphase?
I think I should read a double voltage span, with no null of course.

I can see this happening if you are just using one of the two
secondaries, and it is unloaded (like with a scope).

The secondaries are connected internally, and the two ends are connected
to my amplifier board (which works fine with several other LVDTs -
different models of course).

Marco

 

[John Devereux]

Ok, thats right.


I'm suspicious about the core (rod). Are you sure it's the original
rod and someone's not made a replacement from, say, Aluminium? Got a
magnet handy?

Haha, yes, BTDT.

 

[Marco Trapanese]

Il 09/04/2013 14:53, Syd Rumpo ha scritto:

I'm suspicious about the core (rod). Are you sure it's the original rod
and someone's not made a replacement from, say, Aluminium? Got a magnet
handy?

I'm agree. I will search for a magnet...
The rods were securely tied from the manufacturer along the sensor side.
I opened the shipping box myself.

Marco

 

[Marco Trapanese]

Il 09/04/2013 15:09, Dennis ha scritto:

Just a thought, is the applied frequency appropriate?

Both frequency and excitation voltage are within the specification
limits. They did the calibration at 3 kHz, I use 4 kHz... but it can't
matter at all.

Marco

 

[John Devereux]

Il 09/04/2013 14:40, John Devereux ha scritto:



Yes

What is the part number?

Yes, is a series-opposite configuration and the connection is made
internally. There are only four wires.




This is what I knew and what I expect.
What about if the two coils are connected in series but not in antiphase?
I think I should read a double voltage span, with no null of course.

Don't think it will work properly like that. Possibly if you put a load
on, few hundred ohms perhaps.

 

[John Devereux]

Il 09/04/2013 14:53, Syd Rumpo ha scritto:



I'm agree. I will search for a magnet...
The rods were securely tied from the manufacturer along the sensor
side. I opened the shipping box myself.

Should be OK then. Are you really really sure it is not a DC type?

 

[Bill Sloman]

Il 09/04/2013 15:09, Dennis ha scritto:


Both frequency and excitation voltage are within the specification
limits. They did the calibration at 3 kHz, I use 4 kHz... but it can't
matter at all.

It could, if somebody got cute, and tuned the primary. Self-resonance
can be a bitch. Seems unlikely, but some peopled do really weird
things.

 

[Marco Trapanese]

Il 09/04/2013 15:19, John Devereux ha scritto:

Should be OK then. Are you really really sure it is not a DC type?

Do you mean they could have an integrated amplifier?
If so I cannot measure about 180 ohm on the primary and secondary coils
in *both* directions...

Marco

 

[Marco Trapanese]

Il 09/04/2013 15:24, Bill Sloman ha scritto:

It could, if somebody got cute, and tuned the primary. Self-resonance
can be a bitch. Seems unlikely, but some peopled do really weird
things.

The ds says from 2 to 10 kHz.
Anyway I could try to change the frequency.

Marco

 

[Marco Trapanese]

Il 09/04/2013 15:18, John Devereux ha scritto:

What is the part number?

This is the datasheet:

http://www.waycon.de/fileadmin/pdf/Inductive_Transducer_SL.pdf


of course the version without the amplifier.
Marco

 

[Marco Trapanese]

Il 09/04/2013 15:57, John Larkin ha scritto:

Primary and secondary swapped? There is no standard for color coding these
things.

Was the first thing I tried to swap
The behavior is identical.

Marco

 

[Marco Trapanese]

Il 09/04/2013 16:07, John Larkin ha scritto:

OK, excite the assumed primary and slide a little probe coil, like an axial
inductor, down inside the barrel. Note induced voltage and phase.

Thanks John, I'll try to do this when I'll go back to the lab (next week).

Marco