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Author Topic: Mach III motor tuning with servos and G320X  (Read 1071 times)

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Re: Mach III motor tuning with servos and G320X
« Reply #10 on: June 29, 2018, 05:24:12 PM »
Hi,
the bottom line is to get the fastest and most powerful response from your servo you will need maximum current. Current is proportional
to torque which is proportional to acceleration. Thus you need to set the drive to deliver maximum current it can without fault.

The second thing you need to do to screw up the maximum Proportional gain you can within the limits of stability. You can't avoid this, it is absolutely
basic to any feedback control system, the higher the feedback gain the better. In order for you to do this you will need to turn the following error window
as wide as possible or even 'off' if you could.

In Machs motor tuning page specify an effectively infinite speed and acceleration, this will mean that the speed and acceleration that you see at the servo are the
most the servo can produce with this drive. You do not want the command to the servo limited in such a manner that you can't find the servos natural limits.
So in Machs tuning page put some ridiculously high numbers for max speed and acceleration. Then try tuning.

Next mistake you are making is that the current going into the drive from the power supply IS NOT THE SAME  as the servo current. The output of the drive is PWM,
that is to say the supply voltage switched on and off in proportion to the deemed required output voltage. Thus the output is in effect the output of a buck regulator,
it output voltage will always be somewhat less than the supply voltage but its output current can be GREATER than the supply current. How is this so?

Lets put some numbers  in to illustrate the idea.
Vsupply=100V
Vout(commanded)=10V
Thus the duty cycle for a 10V output is 10%.
The charge balance equation is:
100 X I(input)=10 X I(output)
I(output)/I(input) = 100/10
                          =10
So the driver is in effect behaving like a transformer, its input voltage is being transformed downwards and so its output current is being transformed upwards.
It makes no sense then to measure the supply input current because the output current could  differ markedly.

Craig
My wife left with my best friend...
     and I miss him!
Re: Mach III motor tuning with servos and G320X
« Reply #11 on: June 29, 2018, 05:57:12 PM »
Hi,
hopefully the attached pic may help.

In operation we want the error between the commanded position and the actual (encoder) position to be as small as we can, ie
small following error. The error (small) is amplified by the three gain blocks, P, I and D. In order that the current be maximized to the servo for maximum
acceleration and speed then the combined gains must be as big as we can make them because the input to the blocks is so small.

The G320 manual tells us to tune you turn the I down to zero, and then turn up a little D, then some P until it starts getting a bit unstable. Then dial in some
more D and then a little more P. You keep repeating it until you get the GREATEST amount of P gain you can AND still be stable.

This is the trick to tuning...you are trying to maximize the Proportional gain and use the minimum amount of Differential gain to keep it stable
AND then add a little I gain to get acceptable steady state error following.

Craig
My wife left with my best friend...
     and I miss him!