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| Author |
Message |
shayglenn@gmail.com
science forum beginner
Joined: 09 Nov 2005
Posts: 12
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 Posted: Thu May 04, 2006 2:57 pm Post subject:
LVDT Transfer Function
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Dear all,
How do i go about creating a Transfer Function for an LVDT. What do I
need to know? This is the LVDT in question:
http://www.rdpe.com/displacement/lvdt/general/act-spring.htm
The LVDT I'm using is the ACT3000A model.
Ant help would be much appreciated.
Thanks,
Shay |
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Tim Wescott
science forum Guru Wannabe
Joined: 03 May 2005
Posts: 292
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| Posted: Thu May 04, 2006 3:29 pm Post subject:
Re: LVDT Transfer Function
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Shay wrote:
| Quote: | Dear all,
How do i go about creating a Transfer Function for an LVDT. What do I
need to know? This is the LVDT in question:
http://www.rdpe.com/displacement/lvdt/general/act-spring.htm
The LVDT I'm using is the ACT3000A model.
Ant help would be much appreciated.
Thanks,
Shay
The LVDT itself is immune to transfer function analysis, because with |
excitation it's a time-varying system. For all practical purposes the
LVDT itself is a very low-delay device, but it's 'transfer function' at
any instant in time is scaled by the instantaneous value of the excitation.
The transfer function of the LVDT along with its interface circuitry is
highly dependent on the interface circuitry. An analog LVDT interface
will take the return signal and demodulate it against the reference,
then low-pass filter the result. There's a distinct trade off between
the amount of ripple and the characteristics of the low-pass, so that's
where you'll see the differences based on the interface circuitry.
Look at the Analog Devices AD598 data sheet for information on how this
is done in practice -- I suspect that most LVDT signal conditioning
that's not done in DSPs is done with that chip.
If you're clever, and have a highly integrated system, you can
synchronize the LVDT sampling to the excitation and reduce the ripple to
zero. This allows you to really open up the filters and get some snappy
answers with the device.
To actually answer your question: Find out the transfer function of the
signal conditioning box. This will tell you what the delay behavior of
the system will be. To get the scaling constant of the system (in
volts/mm or whatever) you'll have to know the scaling constants of the
signal conditioning box and the LVDT.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/ |
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David Corliss
science forum beginner
Joined: 29 Nov 2005
Posts: 21
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| Posted: Fri May 05, 2006 5:15 am Post subject:
Re: LVDT Transfer Function
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The Analog Devices AD598 looks like the way to go here. Isn't this just a
straight line algebraic function?
Position is the independent variable, DC Volts is the output. I notice that
there is a spring force on the LVDT
armature. Would this be relevant in your system?
Dave
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:scKdndK1s-lRgMfZnZ2dnUVZ_s-dnZ2d@web-ster.com...
| Quote: | Shay wrote:
Dear all,
How do i go about creating a Transfer Function for an LVDT. What do I
need to know? This is the LVDT in question:
http://www.rdpe.com/displacement/lvdt/general/act-spring.htm
The LVDT I'm using is the ACT3000A model.
Ant help would be much appreciated. Thanks,
Shay
The LVDT itself is immune to transfer function analysis, because with
excitation it's a time-varying system. For all practical purposes the
LVDT itself is a very low-delay device, but it's 'transfer function' at
any instant in time is scaled by the instantaneous value of the
excitation.
The transfer function of the LVDT along with its interface circuitry is
highly dependent on the interface circuitry. An analog LVDT interface
will take the return signal and demodulate it against the reference, then
low-pass filter the result. There's a distinct trade off between the
amount of ripple and the characteristics of the low-pass, so that's where
you'll see the differences based on the interface circuitry.
Look at the Analog Devices AD598 data sheet for information on how this is
done in practice -- I suspect that most LVDT signal conditioning that's
not done in DSPs is done with that chip.
If you're clever, and have a highly integrated system, you can synchronize
the LVDT sampling to the excitation and reduce the ripple to zero. This
allows you to really open up the filters and get some snappy answers with
the device.
To actually answer your question: Find out the transfer function of the
signal conditioning box. This will tell you what the delay behavior of
the system will be. To get the scaling constant of the system (in
volts/mm or whatever) you'll have to know the scaling constants of the
signal conditioning box and the LVDT.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/ |
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Tim Wescott
science forum Guru Wannabe
Joined: 03 May 2005
Posts: 292
|
| Posted: Fri May 05, 2006 5:20 am Post subject:
Re: LVDT Transfer Function
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It's not so simple if you need to know the dynamic behavior.
David Corliss wrote:
| Quote: | The Analog Devices AD598 looks like the way to go here. Isn't this just a
straight line algebraic function?
Position is the independent variable, DC Volts is the output. I notice that
there is a spring force on the LVDT
armature. Would this be relevant in your system?
Dave
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:scKdndK1s-lRgMfZnZ2dnUVZ_s-dnZ2d@web-ster.com...
Shay wrote:
Dear all,
How do i go about creating a Transfer Function for an LVDT. What do I
need to know? This is the LVDT in question:
http://www.rdpe.com/displacement/lvdt/general/act-spring.htm
The LVDT I'm using is the ACT3000A model.
Ant help would be much appreciated. Thanks,
Shay
The LVDT itself is immune to transfer function analysis, because with
excitation it's a time-varying system. For all practical purposes the
LVDT itself is a very low-delay device, but it's 'transfer function' at
any instant in time is scaled by the instantaneous value of the
excitation.
The transfer function of the LVDT along with its interface circuitry is
highly dependent on the interface circuitry. An analog LVDT interface
will take the return signal and demodulate it against the reference, then
low-pass filter the result. There's a distinct trade off between the
amount of ripple and the characteristics of the low-pass, so that's where
you'll see the differences based on the interface circuitry.
Look at the Analog Devices AD598 data sheet for information on how this is
done in practice -- I suspect that most LVDT signal conditioning that's
not done in DSPs is done with that chip.
If you're clever, and have a highly integrated system, you can synchronize
the LVDT sampling to the excitation and reduce the ripple to zero. This
allows you to really open up the filters and get some snappy answers with
the device.
To actually answer your question: Find out the transfer function of the
signal conditioning box. This will tell you what the delay behavior of
the system will be. To get the scaling constant of the system (in
volts/mm or whatever) you'll have to know the scaling constants of the
signal conditioning box and the LVDT.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
|
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html |
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| Back to top |
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David Corliss
science forum beginner
Joined: 29 Nov 2005
Posts: 21
|
| Posted: Fri May 05, 2006 5:49 am Post subject:
Re: LVDT Transfer Function
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Well, all things considered, if you are concerned about a realistic model,
why not improvise a sinusoidal generator to input a test waveform on the
armature, traverse the frequency through a likely interval, and get some
sort of Bode Plot characterization of the input/output? Get the dominant
time constant, and determine if it is significant. If the system in question
has time constants of significantly slower components, it won't even
matter.
If you are dealing with some really low mass parts, then you need to examine
the situation in more detail.
..... The point here is ... why worry about electronic 'fast time constants'
when you probably have some 'mass' time constants that will overwhelm them.
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:HOudndzQZ7UNfcfZnZ2dneKdnZydnZ2d@web-ster.com...
| Quote: | It's not so simple if you need to know the dynamic behavior.
David Corliss wrote:
The Analog Devices AD598 looks like the way to go here. Isn't this just a
straight line algebraic function?
Position is the independent variable, DC Volts is the output. I notice
that there is a spring force on the LVDT
armature. Would this be relevant in your system?
Dave
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:scKdndK1s-lRgMfZnZ2dnUVZ_s-dnZ2d@web-ster.com...
Shay wrote:
Dear all,
How do i go about creating a Transfer Function for an LVDT. What do I
need to know? This is the LVDT in question:
http://www.rdpe.com/displacement/lvdt/general/act-spring.htm
The LVDT I'm using is the ACT3000A model.
Ant help would be much appreciated. Thanks,
Shay
The LVDT itself is immune to transfer function analysis, because with
excitation it's a time-varying system. For all practical purposes the
LVDT itself is a very low-delay device, but it's 'transfer function' at
any instant in time is scaled by the instantaneous value of the
excitation.
The transfer function of the LVDT along with its interface circuitry is
highly dependent on the interface circuitry. An analog LVDT interface
will take the return signal and demodulate it against the reference, then
low-pass filter the result. There's a distinct trade off between the
amount of ripple and the characteristics of the low-pass, so that's where
you'll see the differences based on the interface circuitry.
Look at the Analog Devices AD598 data sheet for information on how this
is done in practice -- I suspect that most LVDT signal conditioning
that's not done in DSPs is done with that chip.
If you're clever, and have a highly integrated system, you can
synchronize the LVDT sampling to the excitation and reduce the ripple to
zero. This allows you to really open up the filters and get some snappy
answers with the device.
To actually answer your question: Find out the transfer function of the
signal conditioning box. This will tell you what the delay behavior of
the system will be. To get the scaling constant of the system (in
volts/mm or whatever) you'll have to know the scaling constants of the
signal conditioning box and the LVDT.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html |
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Tim Wescott
science forum Guru Wannabe
Joined: 03 May 2005
Posts: 292
|
| Posted: Fri May 05, 2006 6:07 am Post subject:
Re: LVDT Transfer Function
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David Corliss wrote:
| Quote: | Well, all things considered, if you are concerned about a realistic model,
why not improvise a sinusoidal generator to input a test waveform on the
armature, traverse the frequency through a likely interval, and get some
sort of Bode Plot characterization of the input/output?
|
Because you'd get a bunch of pointless information. It's not the
response to the excitation you're concerned about, it's the dynamics of
the response from position in to voltage out.
| Quote: | Get the dominant
time constant, and determine if it is significant. If the system in question
has time constants of significantly slower components, it won't even
matter.
If you are dealing with some really low mass parts, then you need to examine
the situation in more detail.
.... The point here is ... why worry about electronic 'fast time constants'
when you probably have some 'mass' time constants that will overwhelm them.
|
I have two clients who need to position > 50lb mechanisms in fractions
of a second, using torquer motors. System bandwidths need to be above
10Hz, which means that a sensor system with a second-order pole pair at
100Hz seriously crimps the loop's style -- and if you're running the
excitation at 2.5kHz and filtering out ripple you'll be pushing some
phase shift into your loop, even with a closing frequency of 10 or 20Hz.
In fact, read the section of the data sheet that says
"To use an LVDT in a closed loop mechanical servo application,
it is necessary to know the dynamic characteristics of the transducer
and interface elements. The transducer itself is very quick
to respond once the core is moved. The dynamics arise primarily
from the interface electronics."
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html |
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David Corliss
science forum beginner
Joined: 29 Nov 2005
Posts: 21
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| Posted: Fri May 05, 2006 7:21 am Post subject:
Re: LVDT Transfer Function
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For a situation such as you describe, you really need to configure both the
LVDT and the AD598 in a test bed and see what you get with the Bode Plot.
The graphs shown in the AD598 Application Note appear to be for the AD598
only, not in conjunction with an LVDT. These show 1k or so of usable
bandwidth for the AD598 by itself, but you have to wonder what will happen
when they are hooked up to the LVDT.
For your barrel positioner, I am guessing that you have other issues than
the sensor and associated electronics. In order to get the order of time
constants you are talking about, you really need to get some numbers
together in order to approximate instantaneous power requirements. If the
motor saturates, you won't get what you expect to get.
regards,
Dave
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:o7ednQJZi_4HdsfZnZ2dnUVZ_umdnZ2d@web-ster.com...
| Quote: | David Corliss wrote:
Well, all things considered, if you are concerned about a realistic
model, why not improvise a sinusoidal generator to input a test waveform
on the armature, traverse the frequency through a likely interval, and
get some sort of Bode Plot characterization of the input/output?
Because you'd get a bunch of pointless information. It's not the response
to the excitation you're concerned about, it's the dynamics of the
response from position in to voltage out.
Get the dominant
time constant, and determine if it is significant. If the system in
question has time constants of significantly slower components, it won't
even matter.
If you are dealing with some really low mass parts, then you need to
examine the situation in more detail.
.... The point here is ... why worry about electronic 'fast time
constants' when you probably have some 'mass' time constants that will
overwhelm them.
I have two clients who need to position > 50lb mechanisms in fractions of
a second, using torquer motors. System bandwidths need to be above 10Hz,
which means that a sensor system with a second-order pole pair at 100Hz
seriously crimps the loop's style -- and if you're running the excitation
at 2.5kHz and filtering out ripple you'll be pushing some phase shift into
your loop, even with a closing frequency of 10 or 20Hz.
In fact, read the section of the data sheet that says
"To use an LVDT in a closed loop mechanical servo application,
it is necessary to know the dynamic characteristics of the transducer
and interface elements. The transducer itself is very quick
to respond once the core is moved. The dynamics arise primarily
from the interface electronics."
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html |
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MLD
science forum beginner
Joined: 04 May 2005
Posts: 15
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| Posted: Fri May 05, 2006 2:38 pm Post subject:
Re: LVDT Transfer Function
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I used to define my instrumentation time constants by just putting in a step
input. Recorded the input and output and used that data to define the
response time. May not be as neat as getting a Bode plot but it didn't take
too long and always seemed to flush out undesirables.
MLD
"David Corliss" <dcorliss@ieee.org> wrote in message
news:At-dnWrLEohkYcfZRVn-vQ@comcast.com...
| Quote: | For a situation such as you describe, you really need to configure both
the
LVDT and the AD598 in a test bed and see what you get with the Bode Plot.
The graphs shown in the AD598 Application Note appear to be for the AD598
only, not in conjunction with an LVDT. These show 1k or so of usable
bandwidth for the AD598 by itself, but you have to wonder what will happen
when they are hooked up to the LVDT.
For your barrel positioner, I am guessing that you have other issues than
the sensor and associated electronics. In order to get the order of time
constants you are talking about, you really need to get some numbers
together in order to approximate instantaneous power requirements. If the
motor saturates, you won't get what you expect to get.
regards,
Dave
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:o7ednQJZi_4HdsfZnZ2dnUVZ_umdnZ2d@web-ster.com...
David Corliss wrote:
Well, all things considered, if you are concerned about a realistic
model, why not improvise a sinusoidal generator to input a test
waveform
on the armature, traverse the frequency through a likely interval, and
get some sort of Bode Plot characterization of the input/output?
Because you'd get a bunch of pointless information. It's not the
response
to the excitation you're concerned about, it's the dynamics of the
response from position in to voltage out.
Get the dominant
time constant, and determine if it is significant. If the system in
question has time constants of significantly slower components, it
won't
even matter.
If you are dealing with some really low mass parts, then you need to
examine the situation in more detail.
.... The point here is ... why worry about electronic 'fast time
constants' when you probably have some 'mass' time constants that will
overwhelm them.
I have two clients who need to position > 50lb mechanisms in fractions
of
a second, using torquer motors. System bandwidths need to be above
10Hz,
which means that a sensor system with a second-order pole pair at 100Hz
seriously crimps the loop's style -- and if you're running the
excitation
at 2.5kHz and filtering out ripple you'll be pushing some phase shift
into
your loop, even with a closing frequency of 10 or 20Hz.
In fact, read the section of the data sheet that says
"To use an LVDT in a closed loop mechanical servo application,
it is necessary to know the dynamic characteristics of the transducer
and interface elements. The transducer itself is very quick
to respond once the core is moved. The dynamics arise primarily
from the interface electronics."
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html
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Jerry Avins
science forum Guru
Joined: 03 May 2005
Posts: 534
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| Posted: Fri May 05, 2006 7:00 pm Post subject:
Re: LVDT Transfer Function
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Tim Wescott wrote:
...
| Quote: | In fact, read the section of the data sheet that says
"To use an LVDT in a closed loop mechanical servo application,
it is necessary to know the dynamic characteristics of the transducer
and interface elements. The transducer itself is very quick
to respond once the core is moved. The dynamics arise primarily
from the interface electronics."
|
I ran into this years ago in someone else's design. The dynamic response
of the amplifier/detector is easy to overlook. I designed new
electronics (all analog) that used a second synchronous detector with
its clock at 90 degrees, a pair of analog multipliers to square the
output of each, and a third to extract the square root. With no
significant ripple to filter (and no consequent phase shift) the loop
settled down. Too late, I found a square-root-of-the-sum-of-the-squares
chip. I forget who made it.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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shayglenn@gmail.com
science forum beginner
Joined: 09 Nov 2005
Posts: 12
|
| Posted: Fri May 05, 2006 9:51 pm Post subject:
Re: LVDT Transfer Function
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Hi there all,
Many thanks for your help.
I'm sorry I didn't let you all know this earlier but I'll be using
this: http://www.rdpelectronics.com/electronics/e309.htm
This will provide excitation.
My output from this will be an -5 to +5 V from 2 wires. This is the
Data Sheet:
http://www.rdpelectronics.com/support/technical-manuals/cd1602m-e309.pdf
At present I'm trying to equate this system to a spring mass damper to
try and come up with a solution. I'm thinking this because of the
friction (even though its very low) and the spring (that causes the
probe to be extended all the time).
I'm I on the right lines?
Thanks for all your hlp guys,
Shay |
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Tim Wescott
science forum Guru Wannabe
Joined: 03 May 2005
Posts: 292
|
| Posted: Fri May 05, 2006 10:35 pm Post subject:
Re: LVDT Transfer Function
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Shay wrote:
| Quote: | Hi there all,
Many thanks for your help.
I'm sorry I didn't let you all know this earlier but I'll be using
this: http://www.rdpelectronics.com/electronics/e309.htm
This will provide excitation.
My output from this will be an -5 to +5 V from 2 wires. This is the
Data Sheet:
http://www.rdpelectronics.com/support/technical-manuals/cd1602m-e309.pdf
At present I'm trying to equate this system to a spring mass damper to
try and come up with a solution. I'm thinking this because of the
friction (even though its very low) and the spring (that causes the
probe to be extended all the time).
I'm I on the right lines?
Thanks for all your hlp guys,
Shay
If the LVDT is rigid to the thing it is measuring then the dynamics of |
the assembly aren't going to resemble the dynamics of the LVDT in
isolation. If you are using this in a control system what you really
care about is the transfer function from the drive signal out of your
controller to the feedback from your LVDT interface box. This transfer
function, in turn, will be a cascade of the driver electronics' transfer
function, the transfer function of the mechanical assembly including the
LVDT, and the transfer function of the LVDT electronics.
If the LVDT _isn't_ rigid to the thing it is measuring then you got
problems! If you connect the LVDT with something springy then that
mechanical compliance working with the friction in the LVDT is going to
cause hysteresis. If the mechanism could move so suddenly that it
outruns the LVDT then you're going to have a very difficult to manage
nonlinearity in your system. Either way mechanical redesign is indicated.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html |
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miles
science forum beginner
Joined: 27 Apr 2006
Posts: 4
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| Posted: Sat May 06, 2006 1:56 am Post subject:
Re: LVDT Transfer Function
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Tim Wescott wrote:
| Quote: | If you connect the LVDT with something springy then that
mechanical compliance working with the friction in the LVDT is going to
cause hysteresis.
|
Can you explain this? The spring would be used to force the LVDT
against the item who's displacement is of concern. I would tend to
think if the LVDT is not able to travel as fast as the item being
measured then you would have hysteresis. The spring would reduce it.
In my systems if nonlinearity is greater than acceptable then software
is used to linearize it. This does assume that whatever is causing the
nonlinearity is repeatable. |
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Tim Wescott
science forum Guru Wannabe
Joined: 03 May 2005
Posts: 292
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| Posted: Sat May 06, 2006 3:55 am Post subject:
Re: LVDT Transfer Function
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miles wrote:
| Quote: | Tim Wescott wrote:
If you connect the LVDT with something springy then that mechanical
compliance working with the friction in the LVDT is going to cause
hysteresis.
Can you explain this? The spring would be used to force the LVDT
against the item who's displacement is of concern. I would tend to
think if the LVDT is not able to travel as fast as the item being
measured then you would have hysteresis. The spring would reduce it.
In my systems if nonlinearity is greater than acceptable then software
is used to linearize it. This does assume that whatever is causing the
nonlinearity is repeatable.
|
We're talking about different springs. If you let the plunger of the
LVDT ride against a rigid body then you're fine, unless the thing can
accelerate away from the LVDT faster than it can go. Should that happen
then you'd certainly see that the LVDT's reported position is different
from the thing you're trying to measure, but I'm not sure that
'hysteresis' would be the best term to use to describe it.
What I was talking about is if you let the LVDT ride against some light
springy thing that's attached to a rigid body then that springiness,
plus the friction, will cause hysteresis.
And hysteresis is a very hard nonlinearity to overcome, because when
your plant reverses direction the reading simply stops moving for a
while, and you just can't tell where you are within the backlash. It's
best to try to eliminate hysteresis from your measurements entirely, and
only cope with it on the drive side.
You can see more detail about ways to compensate for hysteresis (on the
drive side at least) and friction in my article:
http://www.wescottdesign.com/articles/Friction/friction.html.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/
"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html |
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shayglenn@gmail.com
science forum beginner
Joined: 09 Nov 2005
Posts: 12
|
| Posted: Sat May 06, 2006 8:34 am Post subject:
Re: LVDT Transfer Function
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At the moment, to try and model my LVDT I'm thinking of it allowing it
to probe a solid rigid object so that I can avoid any unwanted
oscillations etc.
Thanks for for pointing that out to me.
What I'm mostly concerned about at this point is the LVDT. E.g I If
this LVDT was sitting on a table, connected to its power supply etc and
I were to move it in and out at exactly the same rate e.g 2mm/sec then
how could I explain this mathematically?
What I'll hope to do is to expand on this mathematical description, say
when I probe the position of a sheet of metal, or allow it to detect
the presence of an actuator.
Is it the change in voltage proportional to the probe position if it
was running in a linear state?
Many thanks for all you help,
Shay |
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David Corliss
science forum beginner
Joined: 29 Nov 2005
Posts: 21
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| Posted: Sat May 06, 2006 10:08 am Post subject:
Re: LVDT Transfer Function
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Consider the following proposition or experiment:
Find a small DC motor with a shaft to which you can affix a piece of
wood, plastic, or other 'shapeable' material. In some way, shape the shaft
attachment into a cam, that is a form which has a 'lobe' that will allow
the LVDT armature to rise and fall some arbitrary distance when placed at an
appropriate position adjacent to the armature piece. Run the motor at
varying speed settings and see if there are any interesting features of the
output variable, that is the LVDT output voltage. ...It would be useful to
be able to measure the angular speed of the motor. This can be done with a
simple Hall Effect device, a magnet, and an oscilloscope. If you have an A
to D converter board, you should be able to record the LVDT voltage waveform
so that you can study its characteristics on a computer.
If you do observe a significant signal characteristic at some frequency of
the output variable, try to relate this to the characterists of a spring,
mass damper system ... resonant frequency or whatever. Maybe you can obtain
values for the spring and armature mass of the LVDT, and see if they are
relevant to your observations.
I guess the first question to ask is what characteristics of the output
variable, in relation to the input, will you observe? ....
.... The motivation for suggesting this scheme is that it approximates a
fundamental systems identification experiment that is commonly used to
characterize unknown plants.
.... glad to be of assistance if possible,
Dave
"Shay" <shayglenn@gmail.com> wrote in message
news:1146904473.475835.25370@e56g2000cwe.googlegroups.com...
| Quote: | At the moment, to try and model my LVDT I'm thinking of it allowing it
to probe a solid rigid object so that I can avoid any unwanted
oscillations etc.
Thanks for for pointing that out to me.
What I'm mostly concerned about at this point is the LVDT. E.g I If
this LVDT was sitting on a table, connected to its power supply etc and
I were to move it in and out at exactly the same rate e.g 2mm/sec then
how could I explain this mathematically?
What I'll hope to do is to expand on this mathematical description, say
when I probe the position of a sheet of metal, or allow it to detect
the presence of an actuator.
Is it the change in voltage proportional to the probe position if it
was running in a linear state?
Many thanks for all you help,
Shay
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