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Entry  Fri Jul 18 19:41:09 2008, Yoichi, Update, PSL, PMC PZT investigation Hysteresis.pngStepResponse.png
    Reply  Fri Jul 18 23:24:24 2008, rob, Update, PSL, PMC PZT investigation 
       Reply  Sat Jul 19 19:39:44 2008, rob, Update, PSL, PMC PZT investigation 
Message ID: 699     Entry time: Fri Jul 18 19:41:09 2008     Reply to this: 701
Author: Yoichi 
Type: Update 
Category: PSL 
Subject: PMC PZT investigation 
I measured the HV coming to the PMC PZT by plugging it off from the PZT and hooking it up to a DVM.
The reading of DVM is pretty much consistent with the reading on EPICS. I got 287V on the DVM when the EPICS says 290V.

Then I used a T to monitor the same voltage while it is connected to the PZT. I attached a plot of the actual voltage measured by the DVM vs the EPICS reading.
It shows a hysteresis.
Also the actual voltage drops by more than a half when the PZT is connected. The output impedance of the HV amp is 64k (according to the schematic). If I believe this number, the impedance of the PZT should also be 64k. The current flowing the PZT is 1.6mA at 200V EPICS reading.
The impedance of the PZT directly measured by the DVM is 1.5M ohm, which is significantly different from the value expected above. I will check the actual output impedance of the HV amp later.
The capacitance of the PZT measured by the DVM is 300nF. I don't know if I can believe the DVM's ability to measure C.

I noticed that when a high voltage is applied, the actual voltage across the PZT shows a decay.
The second plot shows the step response of the actual voltage.
The voltage coming to the PZT was T-ed and reduced by a factor of 30 using a high impedance voltage divider to be recorded by an ADC.
The PMCTRANSPD channel is temporarily used to monitor this signal.
After the voltage applied to the PZT was increased abruptly (to ~230V), the actual voltage starts to exponentially decrease.
When the HV was reduced to ~30V, the actual voltage goes up. This behavior explains the weird exponential motion of the PZT feedback signal when the PMC is locked.
The cause of the actual voltage drop is not understood yet.
From the above measurements, we can almost certainly conclude that the problem of the PMC is in the PZT, not in the HV amp nor the read back.
Attachment 1: Hysteresis.png  16 kB  | Hide | Hide all
Attachment 2: StepResponse.png  22 kB  | Hide | Hide all
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