Re: PID question


<hondgm@xxxxxxxxx> wrote in message
news:1164400422.640183.260120@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Now I'm understanding what you were suggesting way back. It sounds
like it is essentially swapping the current and voltage controllers,
along with the addition of this low selector. I was thinking something
like this (paste to Notepad to view):


|| to current sense
.---||---. |
| || | | ||
| | o-------||--.
| |\| | | || |
voltage output feedback -|-\ | | |\| | |
| >---| '---|-\ | |/
.----|+/ | | >----o-----|
| |/| '---|<----o--|+/ |>
| | |/| |
| |
V setpoint .-|<----'
|
I setpoint
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

I thought about how to implement an easy low-selector and came up with
this. It does have a problem with the diode voltage drop, and needs a
pull-up resistor at the op-amp, but the basic idea still holds. I
don't know, maybe a comparator circuit is better after all is said and
done.

Almost but not quite. Tie the VEA output directly to the CEA. You know the
sensitivity of the CEA so all you have to do is implement a local clamp
around the VEA so it cannot demand more than the current setpoint.

Almost every bench PS design I find out there is one of these:
VCC
+
|-----.
| |
.-. |
| | |
| | |
|\| '-' |
from current sensor -|-\ | |
| >----|<-------| |
-|+/ | |/
current ref |/| o---|
| |>
| |
| .--o output
|\| | | |
.---------|-\ | | |
| | >----|<-------' | |+
| -|+/ | ===
| volt ref|/| ___ | /-\
| .-|___|------' |
| | ___ |
'----------------o------|___|----o
|
===
GND

This seems like a bad design because one of the op-amp outputs will
swing between the rails uncontrolled.


Yes, it's one of those things that gets used repeatedly because everbody
copies it. As you say it has problems with amplifiers going into and coming
out of saturation.

DNA


.