Steppers & Linear encoders

What driver are you using for your steppers? I'm in the process of hooking up my 5i20 to some Gecko 203v drives...

As for the steppers with encoders I'm not exactly sure but have seen discussion on this before.

These are some links to some info on feedback with steppers.

wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Steppers_With_Encoders

wiki.linuxcnc.org/cgi-bin/emcinfo.pl?Com..._Devices_On_One_Axis

John

gtom wrote:

I know more or less how closed loop Pid works but i do not know how to connect em to the StepGen provided by the mesa haldriver,

is there a example how to integrate the stepgen in the pid?

regards Thomas

I don't know of any...

Just looking at my 5i20 configuration in the HAL configuration window I see that the stepgen only has two input pins enable and position-cmd. So I'm guessing you can't use any feedback directly on the stepgen. Maybe PCW or Seb might know a way.

John

gtom wrote:

is there a example how to integrate the stepgen in the pid?

Well, doing it in HAL is not that complicated.
The problem is with the results.
The idea is that steppers have a different torque/velocity curve than a regular servo.
If a servo gets behind the commanded position you simply increase the power so it catches up.
When a step motor gets behind commanded position it's usually because it doesn't have enough power. Increasing the command to it will only get it to lose more steps, and stall eventually (actually quite fast).
The only thing you could do is connect the position error (difference between commanded and feedback) to the adaptive-feed input to the motion controller. That way emc2 will slow down all axes if one motor starts to fall behind. In this case one can only hope the motor which started to fail will magically heal itself and catch up eventually.

It is (in theory) possible to drive a stepper like a servo, but you need a really high count encoder for that. At least 100 pulses / motor step. And you need to have a control (pc), drive, motor-encoder combination that can do things together. This only works if the PC control talks to the step driver and together they can decide on slowing down, increasing, etc.
And if you consider more than one axis, which all move synchronized, you see why it easily gets problematic.

I think the best way to make a stepper system work is to design it properly, so that it's never close to areas where it can stall or lose steps.
If you have encoders/linear encoders and you can watch if an motor stalls it's even better. IMO a motor will only stall if you had a collision, or something else which isn't part of a regular machine operation cycle.

Regards,
Alex

BigJohnT wrote:

Just looking at my 5i20 configuration in the HAL configuration window I see that the stepgen only has two input pins enable and position-cmd. So I'm guessing you can't use any feedback directly on the stepgen. Maybe PCW or Seb might know a way.

It works like this: motion -> pid -> stepgen, the feedback goes back to pid, not to stepgen.

Regards,
Alex

You can have a screw comp table if you have variations in your screw pitch.

www.linuxcnc.org/docview/html//config_ini_config.html#sub:[AXIS]-section

Not sure if this is what your trying to accomplish, but I thought I'd give it a stab.

Regards,
John

Ok I understand now what your after. Sorry if I'm slow sometimes.

EMC has the comp file ability so I'm wondering out loud what it would take to have adaptive compensation. Since your not looking to make up for lost steps (usually from a stalled stepper) but for inaccuracy in the drive mechanics it kinda makes sense to me.

On another note, I'm working on a THC that basicly hijacks the axis.n.motor-pos-cmd and adds to or subtracts from the stepgen.n.position-cmd as needed to maintain the proper tip voltage. It's not done yet as there is a lot to consider. You might think about writing a comp to add your scale.

Regards,
John