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PICList Thread
'Temp Control'
1999\11\20@011651 by Sean Breheny

face picon face
Hi All,

Thanks for all the answers on the thermal conductivity of PCBs.

I am trying to make a temperature controlled cavity (this is part of the
autonomous helicopter project) to hold some accelerometers and rate gyros
that are temp. sensitive. I hope to keep the temperature constant to within
0.1 deg C (it doesn't matter at exactly WHAT temp, as long as it is stable
to within 0.1 deg C,and between 35 and 45 deg C).

Some preliminary experiments were promising, but I had overshoot problems
and it often never settles down to within 0.1 deg C. I tried both
thermostatic type control and proportional with various gains.

I suspect the problem is the slow response time of my temp sensor (in still
air, it is several minutes). SO, my idea is to put a micro fan (about 1"
square) inside my 2" cube volume to mix up the air. I figure that this will
speed up the temp. response time and tend to reduce gradients.

What do you think?

Thanks,

Sean


|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
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1999\11\20@050701 by Robert A. LaBudde

flavicon
face
<x-flowed>At 01:16 AM 11/20/99 -0500, Sean wrote:
>I am trying to make a temperature controlled cavity (this is part of the
>autonomous helicopter project) to hold some accelerometers and rate gyros
>that are temp. sensitive. I hope to keep the temperature constant to within
>0.1 deg C (it doesn't matter at exactly WHAT temp, as long as it is stable
>to within 0.1 deg C,and between 35 and 45 deg C).
>
>Some preliminary experiments were promising, but I had overshoot problems
>and it often never settles down to within 0.1 deg C. I tried both
>thermostatic type control and proportional with various gains.
>
>I suspect the problem is the slow response time of my temp sensor (in still
>air, it is several minutes). SO, my idea is to put a micro fan (about 1"
>square) inside my 2" cube volume to mix up the air. I figure that this will
>speed up the temp. response time and tend to reduce gradients.

1. I'm not a control system engineer, so I may use different terms for
effects in control loops.

2. Your problem is a time delay between switching the oven and the
detection at the sensor. This time delay allows buildup of heat which is
lagged with respect to the controller.

3. You need to put an integral on the controller so that the integrated
time of control is part of your feedforward.

4. Since I'm not an EE, it's easier for me to talk in terms of digital control.

5. The simplest solution to the problem is to apply a fraction of the
proportional correction with a time delay. For example, if the thermal lag
is   10 ms, turn on the controller for 1 ms and kill 9 ms until next
adjustment. This will allow the oven effect to be felt by the sensor. The
'damping factor' is the duty cycle of the on/sampling time.

6. You can make this adaptive by adjusting the damping factor by the
overshoot experienced. I.e., if you overshoot the temperature on the next
sample, reduce the damping factor by 10%. If you undershoot, increase it by
10%. You will eventually settle down to a limit cycle over/under-shoot. To
reduce the width, change the 10% to 5% etc.

7. A fan will reduce the thermal lag. If you could reduce it enough, the
problem will reduce back to proportional control. (I.e., the damping factor
will go to 1.0)

8. You will note that even a lag time of 'several minutes' still allows
pretty good proportional control (+/- 0.1 F).

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: .....ralKILLspamspam@spam@lcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================

</x-flowed>

1999\11\20@054553 by Russell McMahon

picon face
Sean,

- What sort of temperature sensor are you using. MUCH faster sensors should
be possible - time constants in the orders of seconds at most - eg v small
bare bead thermistors. Forced air should help. You have of course remembered
the fan motor energy dissipation :-).

- Adding thermal mass may help stability (large block of copper or whatever)
but may play havoc with loop time constants :-).

- Consider using, instead of a heater alone, a Peltier effect device which
you can heat and cool.
This allows both up and down control and also operation near or below
ambient if needs be.
Peltiers are cheap compared to the other equipment you are dealing with. You
only need a simple H bridge to drive them. This can be (depending on power
level) as simple as 2 resistors from one side of Peltier to ground and
supply and a rail to rail PWM signal applied to the other end. Probably a
full H bridge is desirable at more than low power levels.




     Russell McMahon
_____________________________

>From other worlds - http://www.easttimor.com
                               http://www.sudan.com

What can one man* do?
Help the hungry at no cost to yourself!
at  http://www.thehungersite.com/

(* - or woman, child or internet enabled intelligent entity :-))



From: Sean Breheny <shb7spamKILLspamCORNELL.EDU>
{Quote hidden}

1999\11\20@145502 by Robert A. LaBudde

flavicon
face
<x-flowed>I'm told I incorrectly described this problem as an 'integral controller',
when it's actually a 'derivative' controller.

The issue is the second-order system created by the thermal capacitance +
the thermal equilibration.

By using a fan, you eliminate one pole from the system, and proportional
control works fine.

Without the fan, you need to add phase shift for the lag time of the second
pole, and the PIC adaptive damping method I outlined should work fine. You
could also use an op-amp circuit with a capacitor on the input (sum a
derivative with the normal proportional control).

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: .....ralKILLspamspam.....lcfltd.com
Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
824 Timberlake Drive                            Tel: 757-467-0954
Virginia Beach, VA 23464-3239                   Fax: 757-467-2947

"Vere scire est per causae scire"
================================================================

</x-flowed>

1999\11\20@174924 by Reginald Neale

flavicon
face
 Sean asked:

>I suspect the problem is the slow response time of my temp sensor (in still
>air, it is several minutes). SO, my idea is to put a micro fan (about 1"
>square) inside my 2" cube volume to mix up the air. I figure that this will
>speed up the temp. response time and tend to reduce gradients.
>
>What do you think?

 In my experience, it's crucial to have close thermal coupling
 between the source and the detector. Without that, you'll
 never get a stable temperature or the system response time
 will be abysmal.

 Your goal is to reach equilibrium without overshooting and stay
 there. If you can make the system do that, it's possible for
 the temp of your target device to be stable, even if there is
 a gradient between the sensor and the target.

 Also, since the heating/cooling profile of your system will
 presumably be constant, you should be able to use on/off response
 until you approach the setpoint, then switch to proportional
 control. This is easier to implement than full PID.

 Reg Neale

1999\11\20@182024 by Wagner Lipnharski

flavicon
face
Sean Breheny wrote:
>
> Hi All,
>
> Thanks for all the answers on the thermal conductivity of PCBs.
>
> I am trying to make a temperature controlled cavity (this is part of the
> autonomous helicopter project) to hold some accelerometers and rate gyros
> that are temp. sensitive. I hope to keep the temperature constant to within
> 0.1 deg C (it doesn't matter at exactly WHAT temp, as long as it is stable
> to within 0.1 deg C,and between 35 and 45 deg C).

Hi Sean, 40¡C is easy to keep steady if you build a thermal chamber. If
you build a metal box, with heaters inserted or attached mechanically to
the walls, insert it in a styrofoam box, and measure the temperature
directly to the metal walls.  Small holes are necessary to generate a
refrigeration curve.  The only problem is that your chamber should be
stable before you launch the aircraft, it can take few minutes.  If you
have enough tools, why not use the aircraft exaust temp to heat the box?
use somehow a air valve to control it.

1999\11\21@153608 by Tom Handley

picon face
  Sean, I've seen the comments from others and their suggestions but
why do you need +/- 0.1 C? How Large is this autonomous helicopter?
In a 2" `insulated' cube, you could just heat a resistor but what is the
weight and power penalty? Given the thermal environment, I don't see how
you can maintain that accuracy in flight. PID (or P or I or D, etc)
control would not be of any use. It all boils down to thermal mass.
If you are trying to maintain a sensor to an exact temperature, then I
would pre-heat the environment given a large thermal mass, before flight.
Kind of reminds me of old INS systems where we had to both heat up gyros
and align them (gyro-compass) before takeoff in an F4 Phantom... I assume
that, in your case, you are talking about a short flight in an R/C `hobby'
helicopter?

  - Tom

At 01:16 AM 11/20/99 -0500, Sean Breheny wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

1999\11\24@011011 by Sean Breheny
face picon face
Hi all,

Thanks to all who responded, both publicly and privately, to my questions
about temperature control. The response was OVERWHELMING, especially since
I have been very busy with classes. So, I have to respond to you all
together (at least for right now).

Essentially, we(the helicopter team) are tossing around the ideas that you
gave us. We have gotten a temperature controlled chamber to stabilize
reliably to 0.1 deg C within 11 minutes. We haven't fully tested its
resistance to external temp changes yet,nor have we determined what kind of
gradients we have in the box. We are just using proportional only (with a
high gain and feeding a resistor so that it is actually sorta
square-proportional as far as power is concerned) and a small fan inside
the box.

It is still not certain what we will ultimately implement on the heli, but
your ideas were helpful.

By the way, the heli is electric (so the suggestions about using exhaust
are out). For now, it will be tethered,but we eventually want it to be able
to do short flights from batteries. Right now, my box consumes about 6
watts, so it isn't TOO bad, especially when compared to the motor current
drain!

I was also amazed at the level of interest in our gyro and accels and the
methods we plan on using to get usable data from them. At this point, I am
still designing the test setup for a single axis accelerometer,so I can't
say anthing with experimental certainty yet.

We plan on using ADXL-105 accelerometers. We have not decided on the gyros
yet, but possible candidates range from Horizon Gyros from Systron-Donner,
to Murata gyros, and even to pre-production samples from a local company
making MEMS gyros.

The basic idea we have is to mount three accels and three gyros at 90 deg
angles to each other and mount them very firmly, so they can't change their
spatial relationship to eachother. Then, we maintain the environment at a
constant temperature, to within 0.1 deg C. We then use the Analog Devices
AD73360 6-channel 16-bit sigma-delta ADC, complete with programmable gain
amplifiers and programable digital low-pass filtering to grab the data from
all six sensors at once.

A program running on an Atmel AT90S8535 (sorry PICLIST <G>) will read the
output of the ADC and perform a complex linear cross-channel correction on
it. What I mean by this is that it will remove offset error by subtracting
constants from each channel, adjust for scale error by multiplying by
constants, adjust for cross axis and alignment error by subtracting from
each channel a small multiple of several other channel's outputs. It will
also look at the derivative of roll rate and use it to correct for the
accels not being at the CG, etc. The data will then be integrated to obtain
velocity and position/orientation,and it will be sent at about 50Hz over a
radio link (using Radiometrix RPC transceivers - this is where it DOES
contain a PIC!).

All of the constants needed for the correction algo will be obtained by
running a calibration routine which involves moving the heli around by hand
to various points, letting it stand still (to see zero offset error), and
tilting it to known angles. This calibration will (we hope) only need to be
run after every 10 flights or so.

By the end of all of this, we hope to have an INS which is capable of
determining the position of the craft to 1 mm and 0.1 deg along all three
axes within one second. A global vision system consisting of three
orthogonal USB cameras attached to a PC will be used to reset the velocity
and position/orientation error of the INS approximately once per second.

This is our plan and I'm sure much will change before the final
implementation.

Thanks again for all your help,

Sean



|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
Personal page: http://www.people.cornell.edu/pages/shb7
EraseMEshb7spam_OUTspamTakeThisOuTcornell.edu ICQ #: 3329174

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