Spindle overspeed?

I'm slowly re-configuring my mill and have run in to an issue I don't understand how to troubleshoot.  Pretty bog-standard setup for spindle control:

Lcnc 2.9 RIP
Qtpyvcp/Probe Basic
0-10v VFD
4096ppr encoder on motor (1:1 drive ratio)
Mesa 7i76e

Problem:
MDI "M3 S1000" results in 1000rpm
MDI "S2000" speeds it up to 2000rpm
MDI "S1000" speeds it up to ~4000rpm (undesired/unexpected behavior)
GUI displays "1000"
spin.out = ~3800
encoder.00.velocity-rpm =  ~4000
pid.s.command = 1000
pid.s.output = -3800 (trying to slow it down?)

Except:
MDI "M3 S1000" results in 1000rpm
MDI "S1500" speeds it up to 1500rpm
MDI "S1000" slows it down to 1000rpm (desired/expected behavior)
GUI displays "1000"
spin.out = 1000
encoder.00.velocity-rpm =  1015
pid.s.command = 1000
pid.s.output = 1000

If I MDI "M5" or press the GUI spindle STOP button it slows down properly and stops.  I can repeat the above behavior as many times as I want - this doesn't appear to be intermittent or some transient EMI thing.

Basically, if it's at speed and I command a slower speed while it's running it acts up... but not from every speed.  I haven't tried slowing from higher starting speeds as I don't want the spindle to go nuclear until I get a better handle on this.

Anyone have an idea where I should start?  Hardware/software?

Thanks,
Ralph

tommylight wrote:

Encoder or encoder counter.
 

 

Not sure what you mean Tommy?

EDIT - encoder, unless I messed up something in .hal

The simplest fix is to set the spindle P term to 0 (so you are running open loop)

EDIT: a perhaps better fix is to set:

OUTPUT_MIN_LIMIT = 0

In the spindle section of the ini file

The root of the problems is that the 7I76E analog output is unsigned
so does not work with negative numbers. You will get a negative PID
output when the (Commanded_RPM-Current_RPM)*P is
negative. In your example, you have a Current_RPM of 2000 and then
set a Commanded_RPM of 1000. This sets the PID output to -1100
which is interpreted  as a large positive number by the 7I76E's
analog output logic.

To do true PID control (that works well at low speeds) you would need to:

1. Drive the 7I76E analog out with the absolute value of the PID output
2. Drive the 7I76E analog direction with the sign of the PID output

PCW wrote:

To do true PID control (that works well at low speeds) you would need to:

1. Drive the 7I76E analog out with the absolute value of the PID output
2. Drive the 7I76E analog direction with the sign of the PID output
 

I will run open-loop for now, but do you have or can point me to an example of this I can start to work on?

You should be able to run closed loop if you set

OUTPUT_MIN_LIMIT = 0

Doing this alone will have poor performance because of discontinuities
around 0 speed.

It is debatable if closing the spindle speed loop
is of much use unless you have quite poor speed control
in your VFD

If you need PID speed control, (say for accurate control a low speeds
and varying loads) you would need to add one more
absolute function for the PID output and for extracting the sign of
the PID output for controlling the VFD direction signal.

PCW wrote:

You should be able to run closed loop if you set

OUTPUT_MIN_LIMIT = 0

Doing this alone will have poor performance because of discontinuities
around 0 speed.

It is debatable if closing the spindle speed loop
is of much use unless you have quite poor speed control
in your VFD

If you need PID speed control, (say for accurate control a low speeds
and varying loads) you would need to add one more
absolute function for the PID output and for extracting the sign of
the PID output for controlling the VFD direction signal.
 

I set OUTPUT_MIN_LIMIT=0 as you suggested and it seems to be working fine.  A little fluttering at low speeds (<750rpm).

I think I'll live with it as is unless it causes actual problems during machining.

Thanks again,
Ralph