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
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.
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Il 09/04/2013 13:03, Jasen Betts 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.
Marco
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. 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
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.
I can see this happening if you are just using one of the two secondaries, and it is unloaded (like with a scope).
-- 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?
Cheers
-- Syd
Il 09/04/2013 14:40, John Devereux ha scritto:
Yes
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.
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
Haha, yes, BTDT.
-- John Devereux
I haven't used LVDTs for many years..
Just a thought, is the applied frequency appropriate?
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.
Marco
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.
Marco
What is the part number?
Don't think it will work properly like that. Possibly if you put a load on, few hundred ohms perhaps.
-- John Devereux
Should be OK then. Are you really really sure it is not a DC type?
-- John Devereux
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.
-- Bill Sloman, Sydney
Il 09/04/2013 15:19, John Devereux ha scritto:
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
Il 09/04/2013 15:24, Bill Sloman ha scritto:
The ds says from 2 to 10 kHz. Anyway I could try to change the frequency.
Marco
Il 09/04/2013 15:18, John Devereux ha scritto:
This is the datasheet:
of course the version without the amplifier. Marco
Primary and secondary swapped? There is no standard for color coding these things.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
Il 09/04/2013 15:57, John Larkin ha scritto:
Was the first thing I tried to swap :) The behavior is identical.
Marco
OK, excite the assumed primary and slide a little probe coil, like an axial inductor, down inside the barrel. Note induced voltage and phase.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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