Why speed and current PID controller use different algorithm?

Hi Qian.Miao,

This is a very good question, which I can only speculate as to why this occurred.  It goes back to the legacy code from which InstaSPIN was derived.

As you know, both the series and the parallel form are identical from the perspective of a linear transfer function.  I suspect that in the original code, they were both probably in parallel form.  But then it was discovered that for the current PI controllers, the PI coefficient calculation is more straightforward if the structure is series rather than parallel.  You can read more about this in my blog series on designing PI controllers:  http://e2e.ti.com/blogs_/b/motordrivecontrol/archive/2013/02/28/teaching-your-pi-controller-to-behave-part-i

So I would guess that the current PI controllers were converted to series form to accommodate the self-tuning algorithm, but they didn't bother to also change the speed PI regulator since the same tuning advantage doesn't exist for the speed loop.

That's my best guess...

Regards,

Dave