Un commanded movement

[PaulIngram]

Hi

I am converting a BP clone milling machine.

So far:

Controller powers up
Connection to Mach 3 seems good
Encoder feedback to Mach looks good

I have only so far connected the Z axis, I figure I would rather learn any issues on one axis than maybe fry 3 together.

At the moment, when I take the machine off e stop in Mach 3, the axis immediately jogs and then as soon as it moves, the estop trips again and it coasts to a halt. this usually gives an inch or so of travel so it's a pretty enthusiastic jog.

Sometimes it doesnt move so to check whether there was any holding torque at all, I tried to turn the servo pulley, instant movement (fast), I suspect in response to the encoder output.

I did wonder whether this was a servo problem and it maybe wasnt moving on command (stalled or something) but voltage only appears on the DC3IO output at the point that it jogs.

I suspect a mach 3 config issue but cant see what it is.

Any ideas ?

Thanks

Paul

[diycncscott]

Hi Paul,

It sounds like the encoder feedback for that axis is coming is indicating that the motor is moving in the opposite direction of what is expected. What I believe is happening is that the control sees a small error on motor powerup and tries to correct for it. The encoder moves in the opposite direction of what is expected which creates a larger error... the control applies even more correction.. and so on until the control generates a position error...

Try switching the "+" and "-" motor power leads at the DC3IOB and see if that fixes it.

The real problem is that I believe that you may have your A & B channels switched - you can check that by reviewing section 7.4 in the installation manual available here:

http://www.ajaxcnc.com/ajaxbb/viewtopic.php?f=20&t=76

[PaulIngram]

Hi Scott

Thanks, I swapped the + and - connections on the Z axis as suggested and that resolved the immediate trip problem.

I also checked the encoder connections as suggested and they are reading in the correct sense.

What now happens is that the axis can be jogged in the correct direcion but the axis during and after the jog motion oscillates quite strongly. It doesnt seem to settle and the only way out is to hit the estop before something gets damaged.

Is this likely to be a tuning issue ? or something else basic ?

Thanks

Paul

[diycncscott]

It sounds like your Kd is not set right for the Z axis

Go into Config->config plugins->ajax plug-in

When Kd is too low, you will get oscillations that are slow, back and forth like your are seeing. When Kd is too high the oscillations are much faster and more violent -more like bang-bang-bang

Try increasing your Kd by 25% -if it improves it ... keep going up by 25% as it continues to improve it. As you get closer to a value that is too high you'll notice that the motors will start to "sing" while holding -a little noise is normal but as you go higher it will get higher pitch and you may notice that the .0001 digit on the DRO will dither a tenth or two. If get there, just back it down a bit (10-25%)

If it's already too high when you start, just do the opposite of the above, reducing Kd by 25% each step.

[PaulIngram]

Thanks Scott

That worked a treat.

The only issue I am now seeing is that the motor only seems to have one speed, direction is good but about 30 rpm by the looks of things.

This feels like it's nearly there.

Any suggestions ?

Thanks again.

Paul

[diycncscott]

Paul,

Make sure you have room to move the axis a few inches in either direction and set zero for the axis. Use the "set zero" buttom next to the dro.
Use the MDI screen to command a 1" or 2" move (or 25mm-50mm) such as:

G1 Z50 F1000 (if in metric) or G1 Z2 F40 (imperial)

Does it increase the speed? What is the feedrate override set to?
Change the feedrates and issue a move to return to 0. Does it respond as expected?

What are your max velocities set to? Config->motor tuning

I'm sure it's just a setting in Mach but I can't think of what it could be.