Re: Lahman, how ya doing?

Responding to Hansen...

The output circuit looks like

r1 shunt heater
Vout ----/\/\/-+--/\/\/----/\/\/--
| |
/ gnd
r2 \
/ |
gnd

r1 and r2 form a resistaive divider, Vout'=Vout*r2/(r1+r2). Since it's digitized (13 bits) there's a minimum voltage step dV, therefore a minimum power step dP=(dV)^2/4R. A 1/20 attenuator increases the power resolution by a factor of 400, at the cost of losing some dynamic range.

The shunt resistor is a precision Vishay resistor, about 20K, the voltage across that gives the current since I=V/R. The controller isn't involved in that, it's just saved to disk for later analysis. And the heater resistor is a metal film resistor, also 20K, epoxied to the target, the voltage across that is measured, and again saved to disk for later analysis. And the power delivered is P=IV. 

More than I wanted to know.  B-)

I was trying to get a handle on how the temperature gets to the controller. The hangup I had with the list above -- that I extracted

from your description of what I /thought/ was the thermometer -- was the

order. The square root seemed to be done before the attenuator and the DAC output seemed to be fed to the shunt resistor.

However, the schematic above seems to be the hardware on the other side of the controller and Vout is what is produced by the controller after the feedback loop. So I think we are talking about two different things.


Oh, well, it's a traditional PID controller, except instead of outputting a PV + I\int V dt + D dV/dt, it outputs a sqrt(PV + I\int V dt + D dV/dt). The final calculation at the bottom of the routine is a square root, before it's sent to the DAC.

The temperature measurement is just a straight voltage input from the lock-in. If the lock-in measures 2 nV, then the proportional term is P*(2e-9), the integral is increased by I*(2e-9)*dt, etc.

Again, what the computations are is not what I was after. Originally I assumed the basic configuration was:

[Thermometer HW] <-----+
      |                |
      |                |
      V                |
[Controller SW]        |
      |                |
      |                |
      V                |
[Heater HW] -----------+

However, as I interpreted your descriptions of the processing sequence, things were much more complicated, as in:

[Thermometer HW] <-----------+
      |                      |
      | delta-V              |
      V                      |
[sampling SW]                |
      |                      |
      | 7/60th trigger       |
      V                      |
[DAC->???->ADC HW]           |
      |                      |
      | ???                  |
      V                      |
[Feedback Controller SW]     |
      |                      |
      | value                |
      V                      |
[ratio transformer HW]       |
      |                      |
      | power                |
      V                      |
[square root SW]             |
      |                      |
      | delta-V              |
      V                      |
[attenuator HW]              |
      |                      |
      |                      |
      V                      |
[Heater HW] -----------------+

For example, the 7/60th event seemed to be going to a DAC, presumably to trigger a HW read of a value placed there by the sampling SW. Since that was part of the preprocessing of the temperature samples, it had to be before the main feedback computations. So what I was trying to get a handle on was the alternation of HW vs. SW "blocks" with a view towards defining which events were HW vs. SW.

However, your clarifications seem to be moving back to my original assumption. (At least for the square root; I still don't understand why the 7/60th event goes to a DAC.)


*************
There is nothing wrong with me that could
not be cured by a capful of Drano.

H. S. Lahman
hsl@xxxxxxxxxxxxxxxxx
Pathfinder Solutions  -- Put MDA to Work
http://www.pathfindermda.com
blog: http://pathfinderpeople.blogs.com/hslahman
(888)OOA-PATH



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