somebody probably already thought of this...

[[email protected]]

i have a pyrometer (thermocouple and an analog gauge)...the setup take
a fairly long time to settle at its final temperature, and i'd like a
much faster response rate..

i was thinking that if whatever was reading the thermocouple analyzed
the RATE of change of the temperature, it could figure out what the
FINAL temperature would be..the time constant is a predictable thing,
right?

i.e. it sees where the current reading is on the exponential
"settling" curve and can therefore predict where it will end up

is this possible? is this done?

 

[Fred Bloggs]

i have a pyrometer (thermocouple and an analog gauge)...the setup take
a fairly long time to settle at its final temperature, and i'd like a
much faster response rate..

i was thinking that if whatever was reading the thermocouple analyzed
the RATE of change of the temperature, it could figure out what the
FINAL temperature would be..the time constant is a predictable thing,
right?

i.e. it sees where the current reading is on the exponential
"settling" curve and can therefore predict where it will end up

is this possible? is this done?

Yeah, they're called *anticipators* as in potators...

 

[vincent.thiernesse]

i have a pyrometer (thermocouple and an analog gauge)...the setup take
a fairly long time to settle at its final temperature, and i'd like a
much faster response rate..

i was thinking that if whatever was reading the thermocouple analyzed
the RATE of change of the temperature, it could figure out what the
FINAL temperature would be..the time constant is a predictable thing,
right?

i.e. it sees where the current reading is on the exponential
"settling" curve and can therefore predict where it will end up

is this possible?

In my opinion the answer is yes.

It is corrently a first order with time constant tau.

final temperature = tau * dT/dt + T

the only problem is tau won't be the same in water or free air.

another problem is that dT / dt will be hard to take out of noise.

 

[Spehro Pefhany]

i have a pyrometer (thermocouple and an analog gauge)...the setup take
a fairly long time to settle at its final temperature, and i'd like a
much faster response rate..

A smaller thermocouple is the obvious choice, but it might not work
(accurately) with your analog gauge.

i was thinking that if whatever was reading the thermocouple analyzed
the RATE of change of the temperature, it could figure out what the
FINAL temperature would be..the time constant is a predictable thing,
right?

No, it depends on the contact with the probe. Also, you are assuming
that there is only one time constant and the response is a simple
exponential increase- IOW you are assuming a certain model for the
system, and that might not be very accurate.

i.e. it sees where the current reading is on the exponential
"settling" curve and can therefore predict where it will end up

is this possible? is this done?

Yes, possible, but not preferable.

Best regards,
Spehro Pefhany

 

[Terry Given]

i have a pyrometer (thermocouple and an analog gauge)...the setup take
a fairly long time to settle at its final temperature, and i'd like a
much faster response rate..

i was thinking that if whatever was reading the thermocouple analyzed
the RATE of change of the temperature, it could figure out what the
FINAL temperature would be..the time constant is a predictable thing,
right?

i.e. it sees where the current reading is on the exponential
"settling" curve and can therefore predict where it will end up

is this possible? is this done?

not with pyrometers, but a few years back I wrote a least-squares
predictor for thermal test results. It calculates Tau, Tfinal and R^2.
At about 0.2Tau the predicted result was within 5% of the actual result.

This was when I did a whole bunch of thermal tests on a large box with a
~4hr thermal time constant, so I could do multiple experiments per day,
c.f. one. it worked well.

Cheers
Terry

 

[Phil Hobbs]

i have a pyrometer (thermocouple and an analog gauge)...the setup take
a fairly long time to settle at its final temperature, and i'd like a
much faster response rate..

i was thinking that if whatever was reading the thermocouple analyzed
the RATE of change of the temperature, it could figure out what the
FINAL temperature would be..the time constant is a predictable thing,
right?

i.e. it sees where the current reading is on the exponential
"settling" curve and can therefore predict where it will end up

is this possible? is this done?

Sure it's possible. In fact you can do a deconvolution and speed up the
response as much as you like (provided you know the impulse response
*very* accurately). The problem you run into is noise, due to having to
apply high gain at frequencies where there isn't much signal.

Estimating the settling transient isn't as noisy as trying to speed up
the response overall--you can do it using a least-squares fit of several
samples to the (assumed known) impulse response, and fitting the height
of the temperature step. That's a one-parameter fit, and quite well
conditioned. The key is that you need to know the shape of the curve
very accurately.

Cheers,

Phil Hobbs

 

[Vladimir Vassilevsky]

i have a pyrometer (thermocouple and an analog gauge)...the setup take
a fairly long time to settle at its final temperature, and i'd like a
much faster response rate..

i was thinking that if whatever was reading the thermocouple analyzed
the RATE of change of the temperature, it could figure out what the
FINAL temperature would be..the time constant is a predictable thing,
right?
i.e. it sees where the current reading is on the exponential
"settling" curve and can therefore predict where it will end up

is this possible? is this done?

This is certainly possible; however if you try to speed up the things by
the factor of N, then all errors will be also multiplied by the factor of N.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[Martin Brown]

If the time constant or more likely time constant(s) of your setup are
stable and reproducible then there are feedforward filter designs
around to compensate for a step change. These are commonly used to
compensate for the non-ideal behaviour of very high resistances of
10^9 ohms in Faraday current amplifiers for mass spectrometry.

They have to be very carefully trimmed to match the actual behaviour
of each amplifier resistor combination and so unless you have very
rigorous control of the system it is probably a lost cause. You might
still be able to improve the time to settle a bit by applying an
undercorrection with about the right time constant though.

Swap the thermocouple for an optical pyrometer would give the fastest
response - no thermal inertia at all.

This is certainly possible; however if you try to speed up the things by
the factor of N, then all errors will be also multiplied by the factor of N.

It should not be quite that bad unless you do something silly.

Regards,
Martin Brown

 

[MooseFET]

This is certainly possible; however if you try to speed up the things by
the factor of N, then all errors will be also multiplied by the factor of N.

I think in this case the increase in errors will be more. There is
likely more than just one time constant at work.

You can get better settling without increasing the noise, by removing
as much of the RC type filtering as you can and then applying a FIR
filter. This is because a FIR can make the same cut off without
having to include the data from a very long time ago.

 

[Vladimir Vassilevsky]

I think in this case the increase in errors will be more. There is
likely more than just one time constant at work.

Yes, the error is increased as the order of the function, and this is
the fundamental property. I assummed the 1-st order contribution would
be dominant.

You can get better settling without increasing the noise, by removing
as much of the RC type filtering as you can and then applying a FIR
filter. This is because a FIR can make the same cut off without
having to include the data from a very long time ago.

To me, it looks like a good application for the Kalman-like estimator.
May be, Tim Wescott would suggest some better ideas.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[John Larkin]

Sure it's possible. In fact you can do a deconvolution and speed up the
response as much as you like (provided you know the impulse response
*very* accurately). The problem you run into is noise, due to having to
apply high gain at frequencies where there isn't much signal.

Estimating the settling transient isn't as noisy as trying to speed up
the response overall--you can do it using a least-squares fit of several
samples to the (assumed known) impulse response, and fitting the height
of the temperature step. That's a one-parameter fit, and quite well
conditioned. The key is that you need to know the shape of the curve
very accurately.

Cheers,

Phil Hobbs

Forests have died to make papers discussing the deconvolution problem.
It's a member of the family of "ill-posed problems."

A faster thermocouple, or an ir thermometer, might be a better
approach.


John

 

[Phil Hobbs]

Forests have died to make papers discussing the deconvolution problem.
It's a member of the family of "ill-posed problems."

A faster thermocouple, or an ir thermometer, might be a better
approach.


John

Depends on the transfer function you're trying to deconvolve. Gaussians
and suchlike, where the high frequency information really goes away
fast, are evil.evil.evil to deconvolve. Polynomial things are
easier--you can get a factor of 2 or 3 speed increase for a one- or
two-pole rolloff pretty easily, at a cost of 6 to 20 dB of SNR in the
worst case (where all the noise is added after the low pass operation).
That's sometimes a pretty good trade.

Other times, the thing is slow because its low frequency response has
been artificially boosted, e.g. the carefully insulated pixels in my
Footprints sensor. Deconvolving that gave a big SNR _improvement_
compared with just increasing the thermal conduction to make it
faster--more signal and less thermal conduction noise (like Johnson noise).

Another time deconvolution helps a lot is if you're starting with a
transfer function with a cusp or a discontinuity of some sort, which
gives rise to ugly long settling tails. Back when I was a grad student,
I got a factor of 2 improvement in the lateral resolution of a laser
microscope by measuring the complex amplitude (mag and phase) vs
position and applying various deconvolutions. Getting rid of the
settling tail was a big win all by itself, and the noise gain of the
filter was 1 dB.

I agree entirely with your basic point that there's no substitute for
good data. It's only when sensor improvements are exhausted that
deconvolution ought to come in.

Cheers,

Phil Hobbs

 

[Joerg]

Forests have died to make papers discussing the deconvolution problem.
It's a member of the family of "ill-posed problems."

Maybe that's why we have such a wood pellet shortage right now ;-)

 

[MooseFET]

Yes, the error is increased as the order of the function, and this is
the fundamental property. I assummed the 1-st order contribution would
be dominant.

You know what happens when you assume.

Some years back I designed a system with a very simple thermistor and
heater servo. When I characterized the "plant", I found that it had a
huge transport delay style phase lag. It also had more poles and
zeros than you could shake a stick at. One that really surprised me
depended on how the thermistor was epoxied in place. Epoxy on top of
the thermsistor turns out to be bad because the heat has to flow
through the thermistor to get there.

To me, it looks like a good application for the Kalman-like estimator.
May be, Tim Wescott would suggest some better ideas.

Using a model of the system and then correcting the model for any
small errors it may have would likely make better results.

 

[Terry Given]

You know what happens when you assume.

Some years back I designed a system with a very simple thermistor and
heater servo. When I characterized the "plant", I found that it had a
huge transport delay style phase lag. It also had more poles and
zeros than you could shake a stick at. One that really surprised me
depended on how the thermistor was epoxied in place. Epoxy on top of
the thermsistor turns out to be bad because the heat has to flow
through the thermistor to get there.





Using a model of the system and then correcting the model for any
small errors it may have would likely make better results.

internal model control. great stuff.

Cheers
Terry

 

[Glen Walpert]

internal model control. great stuff.

Is "internal model control" the same as what I would call "adaptive
model reference control", with continuous monitoring and statistical
analysis of the fit of the model to measured system response, and
continuous adjustment of the model for best fit?

 

[Vladimir Vassilevsky]

of N.

Using a model of the system and then correcting the model for any
small errors it may have would likely make better results.

Still the fundamental problem remains with the error growth in proportion
with the order of the model. Different methods can only give more or less
accurate estimates.

VLV

 

[Terry Given]

Is "internal model control" the same as what I would call "adaptive
model reference control", with continuous monitoring and statistical
analysis of the fit of the model to measured system response, and
continuous adjustment of the model for best fit?

nope. there is a great description of it in the CRC Press Control
Systems Handbook. OTTOMH the basic idea is to develop an ideal model of
the plant, use the ideal model to provide feed-forward control, then
apply a feedback controller to correct any errors. which is similar to
MRAC, without the adaptation, except the implementation of IMC involves
subsuming the model into the controller itself.

I came across this at the turn of the century, when I was working on
250kW 3-phase regen rectifier control. we had a very low sample rate
(IGBT Fsw) and a very small line inductor - 2%, so a 2% voltage error
caused 100% current error. which made closing the loop and getting it
all right quite tricky. I spent a lot of time working on dead-time
compensation (DTC. an un-related paper was published in IEEE TIA just
recently which describes in detail how I did it), because I needed to
very accurately control the inverter output voltage.

the low sample rate meant getting high bandwidth was quite tricky (hence
doing ATAN calculations for pre-warping BLTs etc), so I turned to IMC.
my first IMC current controller performed as well without DTC as my sync
ref frame PI controller did with DTC, but with higher BW. by getting
smarter with the plant model, and tossing in DTC, I got very good
results. never again shall I use anything else

HTH

Cheers
Terry

 

[Glen Walpert]

Glen Walpert wrote:

nope. there is a great description of it in the CRC Press Control
Systems Handbook. OTTOMH the basic idea is to develop an ideal model of
the plant, use the ideal model to provide feed-forward control, then
apply a feedback controller to correct any errors. which is similar to
MRAC, without the adaptation, except the implementation of IMC involves
subsuming the model into the controller itself.

I came across this at the turn of the century, when I was working on
250kW 3-phase regen rectifier control. we had a very low sample rate
(IGBT Fsw) and a very small line inductor - 2%, so a 2% voltage error
caused 100% current error. which made closing the loop and getting it
all right quite tricky. I spent a lot of time working on dead-time
compensation (DTC. an un-related paper was published in IEEE TIA just
recently which describes in detail how I did it), because I needed to
very accurately control the inverter output voltage.

the low sample rate meant getting high bandwidth was quite tricky (hence
doing ATAN calculations for pre-warping BLTs etc), so I turned to IMC.
my first IMC current controller performed as well without DTC as my sync
ref frame PI controller did with DTC, but with higher BW. by getting
smarter with the plant model, and tossing in DTC, I got very good
results. never again shall I use anything else

Thanks, sounds like something worth learning. The CRC Control Systems
Handbook is about to come out in a second edition, so I recon I'll
wait for it.

Glen

 

[Terry Given]

Thanks, sounds like something worth learning. The CRC Control Systems
Handbook is about to come out in a second edition, so I recon I'll
wait for it.

Glen

the IMC section, IMNSHO, paid for the book.

Cheers
Terry