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Peltier elements

Hi

I was playing around with a peltier element I got from a friend, and I did not get the result I was looking for. I wonder how they make those mobile coolers.

By applying 12VDC it consumes 2-3A, stabilizing at app 2.4A = ~30W.

I used to metal plates from a CD player there was around, but that was not enough to set of the heat - it got quite hot, 90 deg. C, and with a fan some 45C. That means, that the "cold" side was still warmer the otherwise inside temperature (26C at work, at home 21-22).
Running it at 3V/0.5A it works better.

As this was a simple test I was not expecting it to work the best, but I amsurprised then instead if getting a cooled part I actually got a heater intotal.

I have a few options - running it at lower power, but the main thing would be to have a larger and more effective part for getting rid of the heat.

My idea was to make some kind of small fridge, for the fun of it.

What are your experiences with peltier elements?
Electronics for controlling it?

WBR
Sonnich
 
M

mike

Jan 1, 1970
0
Hi

I was playing around with a peltier element I got from a friend, and I did not get the result I was looking for. I wonder how they make those mobile coolers.

By applying 12VDC it consumes 2-3A, stabilizing at app 2.4A = ~30W.

I used to metal plates from a CD player there was around, but that was not enough to set of the heat - it got quite hot, 90 deg. C, and with a fan some 45C. That means, that the "cold" side was still warmer the otherwise inside temperature (26C at work, at home 21-22).
Running it at 3V/0.5A it works better.

As this was a simple test I was not expecting it to work the best, but I am surprised then instead if getting a cooled part I actually got a heater in total.

I have a few options - running it at lower power, but the main thing would be to have a larger and more effective part for getting rid of the heat.

My idea was to make some kind of small fridge, for the fun of it.

What are your experiences with peltier elements?
Electronics for controlling it?

WBR
Sonnich
Go lookup the specs/curves on a peltier of about the same size.
You get most heat transfer at zero delta
and not any heat transfer when the power in equals
the power/heat moved.
Bottom line, don't expect much deltaT if you want much heat transfer.

Peltier refrigeration has its place when you can't have moving parts.

Save yourself the hassle. Read up on it, then go buy a cheapo peltier
fridge on sale at the local big box store.

Or come by, I've got three in the attic I'll never use.
 
Hi

I was playing around with a peltier element I got from a friend, and I did not get the result I was looking for. I wonder how they make those mobilecoolers.

By applying 12VDC it consumes 2-3A, stabilizing at app 2.4A = ~30W.

I used to metal plates from a CD player there was around, but that was not enough to set of the heat - it got quite hot, 90 deg. C, and with a fan some 45C. That means, that the "cold" side was still warmer the otherwise inside temperature (26C at work, at home 21-22).
Running it at 3V/0.5A it works better.

As this was a simple test I was not expecting it to work the best, but I am surprised then instead if getting a cooled part I actually got a heater in total.

I have a few options - running it at lower power, but the main thing would be to have a larger and more effective part for getting rid of the heat.

My idea was to make some kind of small fridge, for the fun of it.

What are your experiences with peltier elements?
Electronics for controlling it?

WBR
Sonnich

the hot side need to get rid of the power you put in plus the power
you move from the cold side
unless you have a large heat sink on the hot side it get hot and and
the cold side will follow

-Lasse
 
D

Don Lancaster

Jan 1, 1970
0
the hot side need to get rid of the power you put in plus the power
you move from the cold side
unless you have a large heat sink on the hot side it get hot and and
the cold side will follow

-Lasse




With very few exceptions, Peltier coolers simply do not work and are an
outright scam.

The usual problem is that the temperature rise across the heatsink
ridiculously exceeds the temperature drop across the cooler.

See http://www.tinaja.com/glib/hack68.pdf for details.

They certainly should not be used above a 500 milliwatt power level.



--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com
 
J

Jamie

Jan 1, 1970
0
Phil said:
You keep going on about that, and it's frankly very misleading.
Peltiers are no use as replacements for heat sinks, but for temperature
stabilization near ambient, they're the bee's knees. (Besides, there's
no such thing as a real genuine heat sink, just extended surfaces for
heat transfer.)

We went round on this a month or so ago.

Cheers

Phil Hobbs
I bought a peltier chest cooler and warmer from our local club store and
it never worked. At least from what I could see. I did notice however,
the palette at the store had looked like it been pawed through and maybe
some returns. I picked one that was an original package.

I was going to toss it one day and then I decided to take it apart to
see what made it tick. it has two fans in it that change direction via
the switch, depending on what you're after heat or cooling. I did a
little research and found that the fans were wired backwards with
peltier unit.
the peltier is 2 sided so you can select what you want. I switch the
wires to the switch to change fan direction and the damn thing actually
works now :)

It cools the inside when you want it and heats up too. its not a
supper cooler or heater but its fine for what it was designed for.

I am kind of thinking that maybe there was a lot of them at the store
that got returned.

Jamie
 
B

Bill Sloman

Jan 1, 1970
0
Peltiers are comparatively slow,

They don't shift all that much heat, so if you are using them to
regulate a large thermal mass you can't change the temperature all
that quickly.

For the work I wrote up in my 1996 paper, we compared three different
sized Peltier junctions, and there wasn't any evidence of any thermal
lag in the Peltier junctions themselves.

I don't think that Peltier junctions are slow at all, but they are
used in situations where you get to see the thermal time constants of
the stuff whose temperature you are regulating
so I often put a small heater right on
the cold plate.  That lets me run the Peltier flat out, or with an
appropriately slow auxiliary loop, while gaining the fast response of a
small heater with good thermal coupling.

Sounds crazy. The last thing you want to do with a Peltier is to shift
any more heat through it than you absolutely have to.

<snip>
 
G

gregz

Jan 1, 1970
0
Hi

I was playing around with a peltier element I got from a friend, and I
did not get the result I was looking for. I wonder how they make those mobile coolers.

By applying 12VDC it consumes 2-3A, stabilizing at app 2.4A = ~30W.

I used to metal plates from a CD player there was around, but that was
not enough to set of the heat - it got quite hot, 90 deg. C, and with a
fan some 45C. That means, that the "cold" side was still warmer the
otherwise inside temperature (26C at work, at home 21-22).
Running it at 3V/0.5A it works better.

As this was a simple test I was not expecting it to work the best, but I
am surprised then instead if getting a cooled part I actually got a heater in total.

I have a few options - running it at lower power, but the main thing
would be to have a larger and more effective part for getting rid of the heat.

My idea was to make some kind of small fridge, for the fun of it.

What are your experiences with peltier elements?
Electronics for controlling it?

WBR
Sonnich

I've played with them. Made led cooler for 45 watts, using good parts, CPU
coolers, copper, etc.

I worked on a mid sized lab cooler. As long as the fans keep spinning, your
good.
I own a small ice maker. It makes ice. What can I say?
I was considering buying a dehumidifier for my trailer. Most don't have
external drain lines.
Somebody bought me a neck cooler from sharper image. It works, but...

Keep the two sides and sinks separated and insulated, and use as much fan
power as you can.

Greg
 
G

George Herold

Jan 1, 1970
0
With very few exceptions, Peltier coolers simply do not work and are anoutright scam.

The usual problemis that the temperature rise across the heatsinkridiculously exceeds the temperature drop across the cooler.

Seehttp://www.tinaja.com/glib/hack68.pdffor details.

They certainly should not be used above a 500 milliwatt power level.

--
Many thanks,

Don Lancaster                          voice phone: (928)428-4073Synergetics   3860 West First Street   Box 809 Thatcher, AZ 85552
rss:http://www.tinaja.com/whtnu.xml  email: [email protected]

Please visit my GURU's LAIR web site athttp://www.tinaja.com- Hide quotedtext -

- Show quoted text -

Hi Don, Please stop saying this. TEC's work great, But you have got
to run the numbers and get rid of the heat on hot side if you want to
use them for cooling! Designed with no thought they have thermal run-
away. As the OP experienced.

George H.
 
G

George Herold

Jan 1, 1970
0
They don't shift all that much heat, soif you are using them to
regulate a large thermal mass you can't change the temperature allthat quickly.

For the workIwrote upin my 1996 paper, we compared three differentsized Peltier junctions, and there wasn't any evidence of any thermallagin the Peltier junctions themselves.

Idon't think that Peltier junctions are slow at all, but they areusedin situations where you get to see the thermal time constants ofthe stuff whosetemperature you are regulating


Sounds crazy. The last thing you want to do with a Peltieris to shiftany more heat throughit than you absolutely have to.

Grin, my last TEC project was stabilizing the B field of a permenant
magnet for NMR.
The 'trick' is to lock it to whatever the local temp is and then
change the frequency to match.

George H.
 
N

N_Cook

Jan 1, 1970
0
Hi

I was playing around with a peltier element I got from a friend, and I did
not get the result I was looking for. I wonder how they make those mobile
coolers.

By applying 12VDC it consumes 2-3A, stabilizing at app 2.4A = ~30W.

I used to metal plates from a CD player there was around, but that was not
enough to set of the heat - it got quite hot, 90 deg. C, and with a fan some
45C. That means, that the "cold" side was still warmer the otherwise inside
temperature (26C at work, at home 21-22).
Running it at 3V/0.5A it works better.

As this was a simple test I was not expecting it to work the best, but I am
surprised then instead if getting a cooled part I actually got a heater in
total.

I have a few options - running it at lower power, but the main thing would
be to have a larger and more effective part for getting rid of the heat.

My idea was to make some kind of small fridge, for the fun of it.

What are your experiences with peltier elements?
Electronics for controlling it?

WBR
Sonnich

+++++++


With all this global warming around these days, will I be able to make up a
Peltier electric blanket for summertime use?
 
B

Bill Sloman

Jan 1, 1970
0
BillSlomanwrote:



Not so.  You can get an order of magnitude faster response, i.e. 20 dB
better forcing rejection at all frequencies.

Were you trying to work with a constant gain around your thermal
control loop? The Peltier output - in terms of heat transferred per
amp through the junction - is heavily dependent on the temperature
difference across the junction. If you are working with a simple
analog control loop (no multiplier) this can force you to settle for
slow settling over most of the range to avoid having the loop unstable
at one end of the range. My 1996 paper spelt out how we avoided this
in our digital control loop and my 2004 comment on Flaxer's 2003 paper
spelled out the implications in some detail.

Sloman A.W. “Comment on ‘Implementing of a precision fast
thermoelectric cooler controller using a personal computer parallel
port connection and ADV8830 controller’[Rev.Sci. Instrum. 74, 3862
(2003)]” Review of Scientific Instruments, 75 788-9 (2004).”

Using an extra heat source is much clumsier than using a sigma-delta
converter and realising an adjustable control loop in a
microcontroller. You do need to monitor the temperature of the exhaust
side of the Peltier junction, but you don't have to do it particularly
precisely, and it's a good thing to do in any event - you can even use
the information to add a bit of feed-forward control as well, if
you've characterised your system properly.
 
G

George Herold

Jan 1, 1970
0
Not so.  You can get an order of magnitude faster response, i.e. 20 dB
better forcing rejection at all frequencies.

Interesting, I've never looked at the time response of a Peltier.
Do you have any 'rule of thumb' guesstimate of the time constant for a
(say) one inch square element?

Is it just the extra mass involved?

A heater can be pretty small while a Peltier has to drag all that
semiconductor mass around.

George H.
 
G

George Herold

Jan 1, 1970
0
Peltiers are not as fast as small heaters, and they have weird transient
responses due to the cooling occurring near the cold plate and the I**2
R heating occurring throughout the length of the bismuth telluride bars,
as well as the heat sink warming up.  Multistage Peltiers can melt if
you turn them on too abruptly, because the later stages dump a
significant amount of their heat through lateral conduction in the cold
plates of the earlier stages.  Before the lateral gradient gets
established, they'll run very hot, so you have to turn them on gently.

Using a Peltier near delta-T_max (or with a slowish and well-behaved
control loop), with a small heater on the cold plate, gets you the same
delta-T_max as the Peltier alone, obviously, but as an actuator it's
very much faster because it can be closer to the thing you care about.
Thermal transport speeds up quadratically as you make the distances
shorter, so the difference can be fairly startling, like a factor of 10.

It does waste some electricity, but a factor of 10 in speed for a
temperature control loop is worth a lot.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text -

- Show quoted text -

Thanks Phil, I'll file that away in the back of my brain. Lot's of
times 'my' thermal loops are just plodding along with a time constant
of seconds. So TEC response time is not so important.

One of the fun things about instrument design are the 'onion' layers.
It doesn't help to fix some problem that is two layers down. The
'art' is being able to identify the top layer problem. (Not that I'm
particularly good at it... I sometimes have to eat the whole onion to
find the top layer :^)

George H.
 
B

Bill Sloman

Jan 1, 1970
0
BillSlomanwrote:




Peltiers are not as fast as small heaters,

True - they are more mechanically complicated. Even single stage
Peltiers consist of lumps of bismuth telluride sandwiched between two
layers of alumina. This doesn't make them slow.

My 1996 paper reports an initial 1.7sec exponential time constant on
the cooling curve that preceded the 249, 414 and 589 second decay
constants determined by the thermal mass of the block controlled and
the thermal resistance to ambient.

The thermistor we used had a thermal time constant of about one
second, so the lag inside the Peltier junctions - which were
respectively 16cm^2. 9cm^2 and 4 cm^2 - can't be more than one second.
That's not long.
and they have weird transient
responses due to the cooling occurring near the cold plate and the I**2
R heating occurring throughout the length of the bismuth telluride bars,
as well as the heat sink warming up.

We didn't see anything like that.
 Multistage Peltiers

<snip - I've never used a multistage Peltier - they are rather exotic
beasts and it doesn't make much sense to lump them with regular single
stage Peltier junctions>
Using a Peltier near delta-T_max (or with a slowish and well-behaved
control loop), with a small heater on the cold plate, gets you the same
delta-T_max as the Peltier alone, obviously, but as an actuator it's
very much faster because it can be closer to the thing you care about.

This doesn't make any sense to me. Thermal time constants are
determined by mass of material and the geometry of its structure, and
adding a heater isn't going to change that at all, nor move anything
material closer to "the thing you care about".

It may move the operating point into a region where the transfer
function of the Peltier junction - in terms of watts moved across the
junction per amp driven through the junction - is more nearly stable,
but that transfer function is known, calculable and predictable, and
it's a lot more sensible to calculate the amount of heat you need to
move and supply the current that you need to move that much heat
rather than adding extra heat to be moved to make the calculation
easier.
Thermal transport speeds up quadratically as you make the distances
shorter, so the difference can be fairly startling, like a factor of 10.

How does adding a heater make "the distances" shorter?
It does waste some electricity, but a factor of 10 in speed for a
temperature control loop is worth a lot.

Controlling the heat transferred - rather than the current moving the
heat - does let you tune the control loop to be dead-beat over the
whole range, rather than forcing you to use a proportional term that
is too low over most of the range to avoid instability at one end.

It's more computationally demanding, but thermostats don't need fast
control loops, and the extra computation involved didn't come anywhere
near overloading our micro-controller, in our application. Smaller,
faster set-up could require more frequent up-dates, but there is a
whole spectrum of cheap computational power available if you need
something more potent than the Siemens SAB80C517A 8-bit
microcontroller we used.
 
G

gregz

Jan 1, 1970
0
gregz said:
I've played with them. Made led cooler for 45 watts, using good parts, CPU
coolers, copper, etc.

I worked on a mid sized lab cooler. As long as the fans keep spinning, your
good.
I own a small ice maker. It makes ice. What can I say?
I was considering buying a dehumidifier for my trailer. Most don't have
external drain lines.
Somebody bought me a neck cooler from sharper image. It works, but...

Keep the two sides and sinks separated and insulated, and use as much fan
power as you can.

Greg

I was going to make a fridge once. You have to worry about condensation and
freezing possibility. That ice maker I have is neat, and makes ice pretty
fast. Makes hollow cylinder pieces, that stay wet. The ice builds up in the
storage, and keeps melting. That reduces the total refrigeration needed,
and keeps the ice from sticking together.

I always had this thing at work on display. 2 foot copper probe, 1/2 inch
thick, with heavy base. Must weigh 5 lbs. Experiment gone bad. They were
attaching peltier devices on the base to cool the rod. I forget what the
probe was for, but I don't want to get into that. Ridiculous.

I've worked with tons of microscope heater/cooler stages. Newer ones use
peltier for that. Some use water tubes for heat transfer. Switching
supplies no good for things that need low electrical noise. The control
voltage is a slow ramp DC. Going positive or negative. Usually the small
working area is small, so external sinking is minimal

Greg
 
B

Bill Sloman

Jan 1, 1970
0
Sure, you can get 2 second-ish time constants on barefoot Peltiers, but
that isn't necessarily fast enough.  Normally you don't try slewing the
temperatures of macroscopic objects that fast, because the gradients are
hard to control.  The butterfly-packaged laser I used in my downhole
gizmo had a time constant of almost 20s  barefoot, but a bit of local
feedback a` la Phelan got that down to about 2s, which is where the
uncontrolled phase shifts due to thermal diffusion took over.  So time
constants of that order with barefoot Peltiers are very doable.

Where I'm going with the Peltier-plus-heater scheme is rejection of
ambient thermal forcing.

Which I'd describe as feed-forward control. If you know what's
happening on the exhaust - heat-sink - side of the Peltier junction,
your scheme can change the amount of heat being generated in the
controlled volume in such a way that the amount of heat that needs to
be transported through the Peltier junction remains unchanged. You
then need to know the temperature of the heat-sink almost as
accurately as you know the temperature of the controlled volume - the
thermal inertia of the controlled volume and the integral term of your
PID control loop mean that you don't need to know the temperature of
the heat-sink quite as accurately as you know the temperature of the
controlled volume - but that's just making the circuit marginally more
expensive.
If your heater, sensor, and controlled volume are small, you win speed
quadratically.  A factor of 3.2 in size is enough for a factor 10 in
speed, in situations where thermal diffusion dominates, and going from
alumina to copper theoretically gets you another factor of 8.

Even just one factor of 10 gets you at least 20 dB better rejection of
thermal forcing at all frequencies, and, more than that, it allows you
to move the zero of your second order control law to a frequency 10x
higher.  That, in turn, means that frequencies below the zero benefit to
the tune of 40 dB additional forcing rejection.  That's a very big deal,
since in many cases the diurnal or other low frequency forcing terms are
by far the strongest.



Try turning a Peltier on abruptly at constant current, and watching what
the cold plate temperature does.  It overshoots and then recovers, and
the phase shift of the recovery part is gigantic since it involves
thermal diffusion in some millimetres of ceramic, which is slow, slow,
slow.

That's pretty much what we did to measure the time constant of the
controlled block plus Peltier junction - though we did use turn-off
rather than turn-on - and what we saw was monotonic - a quick small
exponential decay with a time constant of about 1.7 seconds followed
by the much slower exponential of the block cooling as a whole.
Heaters on the cold plate are even more useful with stacked Peltiers,
for all the same reasons except more so.

Heater plus feed-forward control.
Only in the thermal-mass (read 'slow') limit.  In that limit, the phase
of the transfer function is a nearly constant -90 degrees on the rolloff
portion, so you can always crank up the gain and get more bandwidth.
(Slew rate is another issue, and depends on how hard you can drive the
actuator.)

Once you're dominated by thermal diffusion, though, things get a lot
nastier, because the phase shift increases without bound with
frequency.  (See e.g.http://www.electrooptical.net/www/book/thermal.pdf
.)  So small size is key, if you want a bandwidth greater than ~0.1 Hz.

And a temperature sensor that's got low thermal mass and is in really
good thermal contact with volume whose temperature you are trying to
control, which is to say a low thermal time constant.
It can be smaller, and closer to the volume of interest (e.g. a diode
laser), and embedded in the same small piece of copper or aluminum,
which diffuse heat about 6-8 times faster than alumina.  A simple
example is using the monitor PD in a diode laser as a temperature
sensor. It's only a couple of mm away, and embedded in the same copper
block, so it responds in a fraction of a second.

Okay. It's a separate local control loop, inside the the larger
control loop that uses the Peltier junction to shift the bulk of the
heat and generate the bulk of the temperature difference from ambient.

Basically another version of your two-zone heat temperature control
scheme
It isn't a control-loop problem that I'm talking about.

No, it sounds more like two essentially independent nested control
loops.
 You can't make
a feedback loop work with thermal diffusion, because the frequency
dependence isn't

H_LF(omega) = 1/( 1 + j*omega*tau )

which has a nice, well-behaved, asymptotically constant 90 degree phase
lag, it's

H_HF(omega) = exp((1-i)*omega*L**2/Kappa),

in which every 1/e rolloff in amplitude gets you another radian's worth
of phase shift.  Once that sets in, you can't recover by tuning a
PID--the only solution is to reduce the distance L over which heat
diffuses.  I go through the math of this in the chapter referenced
above.

It's a completely different regime, but if you can make stuff work
there, the forcing rejection improves amazingly.

But what you are now describing sounds more like two nested control
loops, the outer one doing the heavy lifting, and the inner one
providing the agility.

To change the subject a bit, one scheme I've come across in precision
calorimetry uses a Peltier junction as the sensor in a fast local AC-
only local control loop.

The Peltier - okay, Seebeck in this application - junction is
sandwiched between the calorimeter and a solid, well-insulated block
of copper - a thermal capacitor - which eventually thermally
equilibrates with the calorimeter, so the Seebeck junction only
monitors the transient temperature differences.

The advantage of this approach is that the Johnson noise on the output
of the Seebeck junction is much lower than that of a platinum
resistance sensor and the sensitivity - in volts per degree - is very
much higher.

You are still reliant on some other temperature sensor to keep the
controlled temperature stable in the long term, but the short term
stability can be improved - if, of course, only up to the limit set by
thermal diffusion times.
 
G

George Herold

Jan 1, 1970
0
Sure, you can get 2 second-ish time constants on barefoot Peltiers, but
that isn't necessarily fast enough.  Normally you don't try slewing the
temperatures of macroscopic objects that fast, because the gradients are
hard to control.  The butterfly-packaged laser I used in my downhole
gizmo had a time constant of almost 20s  barefoot, but a bit of local
feedback a` la Phelan got that down to about 2s, which is where the
uncontrolled phase shifts due to thermal diffusion took over.  So time
constants of that order with barefoot Peltiers are very doable.

Where I'm going with the Peltier-plus-heater scheme is rejection of
ambient thermal forcing.

If your heater, sensor, and controlled volume are small, you win speed
quadratically.  A factor of 3.2 in size is enough for a factor 10 in
speed, in situations where thermal diffusion dominates, and going from
alumina to copper theoretically gets you another factor of 8.

Even just one factor of 10 gets you at least 20 dB better rejection of
thermal forcing at all frequencies, and, more than that, it allows you
to move the zero of your second order control law to a frequency 10x
higher.  That, in turn, means that frequencies below the zero benefit to
the tune of 40 dB additional forcing rejection.  That's a very big deal,
since in many cases the diurnal or other low frequency forcing terms are
by far the strongest.





Try turning a Peltier on abruptly at constant current, and watching what
the cold plate temperature does.  It overshoots and then recovers, and
the phase shift of the recovery part is gigantic since it involves
thermal diffusion in some millimetres of ceramic, which is slow, slow,
slow.





Heaters on the cold plate are even more useful with stacked Peltiers,
for all the same reasons except more so.





Only in the thermal-mass (read 'slow') limit.  In that limit, the phase
of the transfer function is a nearly constant -90 degrees on the rolloff
portion, so you can always crank up the gain and get more bandwidth.
(Slew rate is another issue, and depends on how hard you can drive the
actuator.)

Once you're dominated by thermal diffusion, though, things get a lot
nastier, because the phase shift increases without bound with
frequency.  (See e.g.http://www.electrooptical.net/www/book/thermal.pdf
.)  So small size is key, if you want a bandwidth greater than ~0.1 Hz.





It can be smaller, and closer to the volume of interest (e.g. a diode
laser), and embedded in the same small piece of copper or aluminum,
which diffuse heat about 6-8 times faster than alumina.  A simple
example is using the monitor PD in a diode laser as a temperature
sensor. It's only a couple of mm away, and embedded in the same copper
block, so it responds in a fraction of a second.










It isn't a control-loop problem that I'm talking about.  You can't make
a feedback loop work with thermal diffusion, because the frequency
dependence isn't

H_LF(omega) = 1/( 1 + j*omega*tau )

which has a nice, well-behaved, asymptotically constant 90 degree phase
lag, it's

H_HF(omega) = exp((1-i)*omega*L**2/Kappa),

in which every 1/e rolloff in amplitude gets you another radian's worth
of phase shift.  Once that sets in, you can't recover by tuning a
PID--the only solution is to reduce the distance L over which heat
diffuses.  I go through the math of this in the chapter referenced
above.

It's a completely different regime, but if you can make stuff work
there, the forcing rejection improves amazingly.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

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Wow! That's great! The next project looks to be on the 'slow road to
china' for exactly the above reason. But it's hard to get people to
understand. It's 'just' a bit longer time constant isn't it..
(I may copy and paste that, perhaps it will mean more coming from your
mouth.)

I sometimes think I'd like to build a thermal test-bed 'thing'.
Different sensors, 'plants', hunks of metal of various lengths,
loads ... Hook it up and control the T, and then beat on it in
different ways. I'd learn a lot!

I think I 'get' the TEC + heater idea. It's a bit specialized, but
you keep the TEC cooling (a heavy idle.) and then control things short
term with the heater. It's a 'class A' brute force solution... Just
my style :^)

Bill, you should give Phelans book a read. I got a copy for ~$15,
delivered.

Speaking of thermal diffusivity, I saw an ad for graphite thermal
sheets.

George H.
 
B

Bill Sloman

Jan 1, 1970
0
BillSlomanwrote:



No,Bill, it's just plain feedback.  You just get way wider bandwidth,
so the loop gain is higher at any frequency below the unity gain cross.
This really isn't a hard concept.

Then it's one feedback loop controlling two actuators.

How do you partition the feedback between the slower Peltier junction
doing the grunt work, and the little local heater providing the
agility? Does the Peltier junction just get the integral term from the
PID output?

I don't want to try and re-invent that particular wheel - if you were
prepared to be more explicit about what you do, it would be a waste of
effort and bandwidth.
Nonsense.  It's just ordinary feedback, it just has a much faster
actuator.  No mysteries.

Apart from the unspecified way you managed to develop to partition the
output of the control loop into a control signal for the Peltier
junction and a control signal for the local heater.
No, it isn't a polynomial rolloff at all at higher frequency.  Do the
math if you don't believe me.

One could do a more or less infinite amount of math on a more or less
realistic physical model of the thermal mass being controlled,
including the thermal lag from the mass to the sensor, the extra
thermal lag from the local heater back to the controlled thermal mass,
and the different - and somewhat larger - thermal lag from the Peltier
junction back to the controlled thermal mass. For extra credit you
could throw in the thermal mas of the heat sink on the exhaust side of
the Peltier.

Without real experimental feedback to test your choice of the
simplifications required to keep the model tractable, it wouldn't be a
particularly useful activity.

Since I never saw anything that vaguely looked like your weird
overshoot, I'd probably be doing the math on a rather different
system.
No, once again, no feedforward.  You don't have to measure the sink
temperature at all, and it wouldn't help you if you did.

Of course it could help - the effectiveness of the Peltier junction
depends on the temperature difference across it, and if you don't know
that temperature difference you can't fully optimise your control
loop.

I can accept that you don't bother calculating exactly what the
Peltier is doing for you, though I would see it as a missed
opportunity, but until you tell us how you partition the feedback
between the Peltier and the local heater you are essentially saying
that you have a magically better way of managing your system which you
aren't going to tell us about in enough detail to let us copy it.
The sensor and the controlled item have to be small, sure.  I said that
already, I believe.  But once again, unless you have a fast actuator,
*it isn't a time constant problem at all*, because the math is different
once diffusion starts to dominate.

The control theory books that I've looked at distinguish between
controlling an inertial mass and and controlling an inertial mass with
a time delay in the feedback loop. The math for these two conditions
is - obviously - different, but the texts did point out that time
delay was always present in any real system.
No, you only need one sensor per controlled zone.  It would be quite
possible to have multiple zones on the same cold plate if you needed to,
each with its own heater.  You'd have to manage their mutual
interactions, but that wouldn't be too hard if the bandwidth were large
enough.

That I can see. but I am still interested in how you partition the
feedback signal between the Peltier junction and the local heater.

True, but you are being a little mysterious about the details of the
control loop that you use to get around it.
No, once again, it's a fundamental speed limit due to the onset of
diffusion-limited propagation.

Or - in control theory language - the presence of a fixed delay around
the feedback loop. Because the delay through the heater is less than
the delay through the Peltier junction, you've got two different
feedback loops in the one system, and you haven't told us how you
split the error-correction signal to exploit these two separate
actuators with their two different sets of dynamic properties.

While your inner heater doesn't have as large a pure delay as the
Peltier junction, it is still in series with the temperature sensor,
whose pure delay forms part of the pure delay around both of the
control loops. Feedforward does have the advantage that it by-passes
the delay in the sensor (but not the actuator), but it is a purely
open-loop correction.
I think I've been adequately clear.

I'd beg to differ. You haven't been at all clear on how you contrive
to exploit the different properties of the two different actuators -
and my recent posts make my incomprehension perfectly obvious.
 The chapter I posted has a lot more
detail.

Spamming your book? Have sales been slow recently?
And does that chapter spell out how to partition the feedback signal
between the two actuators?

If it does I'll probably buy the book (but not until I'm safely in
Australia and can stick it on the shelf next to "The Art of
Electroncis" and Williams and Taylor on filter design, that are out
there already.
Sounds potentially reasonable, but if you can insulate the copper block
well enough that ambient forcing doesn't move it around, why not just do
the same thing to the controlled volume and save trouble?  Is it mainly
for dealing with variable dissipation in the controlled zone?

As I said, I came across it in a paper on precision calorimetry, which
is - of course - all about measuring the heat that gets dissipated on
the controlled volume aka calorimeter.
 
B

Bill Sloman

Jan 1, 1970
0
Wow!  That's great!

If not explained in any kind of useful detail.
The next project looks to be on the 'slow road to
china'  for exactly the above reason.  But it's hard to get people to
understand.  It's 'just' a bit longer time constant isn't it..
(I may copy and paste that, perhaps it will mean more coming from your
mouth.)

I sometimes think I'd like to build a thermal test-bed 'thing'.
Different sensors, 'plants', hunks of metal of various lengths,
loads ... Hook it up and control the T, and then beat on it in
different ways.  I'd learn a lot!

I think I 'get' the TEC + heater idea.  It's a bit specialized, but
you keep the TEC cooling (a heavy idle.) and then control things short
term with the heater.  It's a 'class A' brute force solution... Just
my style :^)

Bill, you should give Phelans book a read.  I got a copy for ~$15,
delivered.

I've not yet got a text on control theory. I've dipped into quite a
few of them, and got what I've needed, but the ones that I saw
confirmed Sturgeon's Law, that 90% of everything is rubbish.
If Phelan's book is part of the 10%, it's quite likely that I'll buy
it.
Speaking of thermal diffusivity, I saw an ad for graphite thermal
sheets.

Used it back in 1993, so it's referred to the 1996 paper. It's
electrically conductive, which can be inconvenient with power
transistors, but it's great for power resistors aka heaters and
Peltier junctions. Unlike Thermopads, you did seem to get the thermal
conductivity that the manufacturer predicted.
 
B

Bill Sloman

Jan 1, 1970
0
Not exactly.  The phase does go linearly with frequency, but unlike a
normal delay, the amplitude also falls off exponentially.  Effectively
at any given frequency, the loop loses sight of anything more than a
few diffusion lengths into the material.


The problem with feedforward is that it has to be very accurate, or it
limits the forcing rejection rather than improving it.   Insulation and
feedback don't have that problem.  Which is not to say that feedforward
isn't useful in some situations, it's just not what I was talking about.


You can do it that way, or alternatively you can run the Peltier at a
fixed current and rely on the heater to do all of the work.  Arranging
the loop so that at zero error you're dissipating 1/4 of maximum heating
will allow both actuators space to work in without having two control
loops fighting.

What's magic about a quarter? I imagine you mean that the close-in
resistive heater is dissipating 25% of the maximum power you can
afford to have it dissipating. I'd have though that that running it at
half the maximum dissipation you think that you can get away with
would maximise your headroom, but obviously there may be subtleties in
there that I don't know about.
From other folks' comments, they seem to have understood.  We all start
from different places, of course, and there's no dishonour in that.

They may not have known enough about the gritty details of putting a
temperature controller together to have actually noticed that you were
being vague and elliptical and skating over important detail, or -
more likely - weren't interested enough to care. Obviously, there's no
dishonour in being vague and elliptical - it saves a lot of typing,
and if you been immersed in a particular technology for a while one
does tend to slide over the practical details which have been drummed
into your head by repeated exposure.
That chapter is a free download, and speaks exactly to the point at
issue.

That would be Chapter 19 which didn't actually make it into your book
- which could be just as well. The phrase "carbon thermistors" came up
twice, and thermistors aren't any kind of carbon resistors but a
sintered mix of metal oxides or sulphides. Faraday apparently made the
first one out of silver sulphide.

http://www.technologystudent.com/elec1/therm1.htm

I couldn't find an answer in it to the question of how you partition
your control signal between the fast local heater and the slower
Peltier junction - which is not to say that it isn't a brilliant and
totally admirable bit of work. The "carbon thermistor" pratfall
doesn't detract from that at all - you are perfectly entitled to the
occasional human error, and - if pressed - you can always claim that
your publisher insisted that you included a couple of deliberate - and
obvious - errors as anti-piracy devices.
It would be nice to be able to argue without quarrelling.

Who is quarrelling? I'm trying to get you to be more explicit about
what you actually do, and I'm getting a bit grumpy about the time that
it's taking to define exactly what it is that you are actually doing,
but I certainly don't hold you in lower esteem now now than I did when
this thread first got going.
Read it in good health.

I'm about to turn 70. My shiny new aortic valve is working fine, but
the other heart valves are a bit leaky - nowhere near enough to create
any kind of problem, but I may see more of my Australian cardiologist
than I like (and he's a very pleasant character, if totally brilliant
in his field - one of my friends from undergraduate days is a
professor of oncology with a dodgy heart, and he got his cardiologist
to take me on too).

I'll do my best to follow your instruction, but I can't promise to
succeed.

<snip>
 
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