[sdiy] lessons learned from temperature chamber design II

[Ian Fritz]

Hi Martin --

>some may laugh about it, but here are some practical
>experiences from my temperature chamber and servo
>experiments.

I think this is a great topic, especially for people wanting to do careful 
experiments on VCO drift.  Nothing to laugh about at all -- this is not as 
easy as it may sound.

>-1/f noise: I started the first servo circuit on a Sunday.
>  So I used what I had, jfet op amps. For a DC servo loop,
>  this causes serious 1/f-noise problems. So now the
>  high gain part uses OP07, lower 1/f, cheap, available
>-I made a mistake with the sensor, then in haste fixed
>  that, but building another mistake: circuit supply
>  inluences sensor output. Since the sensor output is only
>  3mV/K and needs very high amplification, this can cause
>  trouble. So I went back and built a bridge circuit with
>  instrumentation amplifier, virtually eliminating any
>  CM errors. Needs three OP07, but is cheaper then a
>  high precision voltage source (burried zener type).

The LM335 makes a great T sensor and is inexpensive.  Also, it puts out 3 V 
at 300K.  No bridge or amplification needed.

>-the thermo chamber behaviour is of course unknown,
>  until you start playing with it. So some components
>  of the circuit (gain, integrator, differentiator)
>  will need adjustment in servall orders of magnitude,
>  i.e. you have to replace caps and resistors.
>  This will damage the circuit board. Therefore matching
>  or unknown value resistors are now soldered on pins
>  a few mm above the board (solder nails (sp?)). I can
>  exchange them whenever I like.

Some suggestions for when you get to adjusting your controller:

1.)  Do not use derivative feedback (use P or PI, but not PID).  The 
derivative can speed up the response, but it is difficult to tune. It will 
need to be varied as the set point is and the effort to map this out just 
isn't worth what you gain.  At least try without it at first.

2.)  Start out with just pure, simple proportional control.  Adjust the 
loop gain to where there are no oscillations, or at least to where they 
damp out rapidly (within two cycles).  Proportional control involves an 
error between the set point and the actual temperature, but this may not 
matter to you -- just calibrate your set-point pot against the actual 
temperature.

3.)  If you really think you need the integral feedback, set it to a very 
low level.  This will take out the error mentioned above, if you are 
relying on sensor calibration.  It's actually best to do the integration 
mechanically with a motor driven pot, as the integration then does not have 
a decay.

>-the supply rails for all active components are now
>  parallel silver/copper wires, a few mm apart.
>  Inductance is thereby minimised. I soldered
>  some 100nF blocking capacitor for each op amp,
>  but did not connect them. Until now, I could see
>  no ringing problems (well, the OP07 is not
>  particulary fast).

It's worth taking these precautions, since it isn't really any more 
difficult to construct.

>-I have inserted jumpers, in order to be able
>  to seperate circuit parts from other parts.
>  This makes triming, CM rejection trimming
>  much easier.
>-the 12V regulator (for the fans) was thrown
>  out, since it will dissipate much heat and thus
>  influence the circuit
>
>so far, so good...
>m.c.

Sounds like a a good start!  Please continue to keep us posted.

   Ian