From: jeedom74000@gmail.com   
      
   Many thanks Phil,   
      
   OK I think we are more or less on the same page but I am coming more from the   
   mechanics angle rather than electronics, so the semantics could be a little   
   different. From what I recall from my Univeristy course in automation, a   
   pragmatic approach to    
   getting coarse tuning parameters for a PID feedback system is to measure the   
   time constant of the system, its resonance frequency (when possible), and the   
   "Ziegler-Nichols" method lets you estimate reasonable PID parameters. However   
   in this case we are    
   only dealing with a P loop so I was    
   thinking of just playing with the low pass filter and gain setting to tune it.   
   But I am likely oversimplifying the problem. And the more I think about it the   
   more I am thinking a pure Integrator type of loop could be more accurate (in   
   removing the last    
   DC offset error but does it really matter in my case).   
      
   Anyhow, believe it or not I don't have any instrument to measure capacitance   
   (please advise a good HP, Fluke or alike instrument and will look it up on   
   eBay) but the spec sheet of the Murata 7BB-20-6L0 piezo specifies a 10nF   
   capacitance, a 6.3KHz    
   resonance freuquency and 1KOhms Resonant Impendance. On the bench I measured a   
   resonant frequency of the whole system as closer to 10KHz which is surprizing   
   as I would have expected it to be less since we have a mass (the mirror) glued   
   on the piezo.   
      
   Then I tried to measure the time constant of the system by inputing a step   
   function into the piezo and measure the output of the PDs on a scope but alas   
   the signal generator is completely messing up the signal when switched on.   
   Analog electronics fun for    
   you :)   
   I will retry by plugging/unplugging by hand a battery output into the piezo   
   input and see if I can measure anything on the scope.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)