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Strange motor speed on reversing....
« on: April 28, 2009, 10:00:27 PM »
Hi all.
Using Mach3 Turn for the Lathe...
I have a strange motor speed happening when I reverse direction on my Z axis.
I do not have this issue when using mach3 Mill software tho.
I was going to try to explain it in words,,, but I could not,,so I took a video to show it.

(Build history)
Lathe is the 7X14 Sieg  (micro mark Mill.) http://www.micromark.com/MICROLUX-7X14-MINI-LATHE,8176.html

I am using the hobbycnc pro driver board.
When I first started to convert the lathe to CNC,, I set up the X axis to drive the carrage only from the threading lead screw and used the Mach3MILL software to make some cuts.....  This worked well with backlash compensation, no problems.

I am using backlash compensation, .0135" on that leadscrew at backlash speed of 100%.  I do close and restart the program with any adjustment here.

Ive since mounted a motor (now the X axis) on the cross feed, and I have mounted the Z axis motor to the Z feed.
Motors are in 1/2 step and tuned properly per dial indicator verifications on each axis.

Now I am using the Mach3Turn software as I should be.
Here is the problem when using backlash compensation in Mach3Turn vs. Mach3Mill
http://tdracingrc.com/machining/Strange.wmv

In the video I jog, in one direction a few times,,, the velocity and accel work fine,, BUT when I reverse direction I get the strange movement.
I have made a few part with it this way,,, and accuracy is good on most features,, but,,,, this delay is causing some issues as the X takes off while the Z is winding down,, or up....

Can anyone offer some advice on this?
Any help is appreciated

I have changed accel rates and velocity rates to troubleshoot this issue.  They do change, per new inputs, but still the reversing motor speeds are still weird.
« Last Edit: May 04, 2009, 06:10:38 AM by thedude »
TheDude,

Offline jimpinder

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Re: Strange motor speed on reversing....
« Reply #1 on: April 29, 2009, 12:14:04 PM »
I have looked at the video and - yes - it does seem odd.

Reading your text, I wonder why you only have the system on 1.2 step ( which I assume means 2 microsteps) . It will work more smoothly if you work on 8 microsteps, or - for example - my Ghecko drivers work on 10)

I would increase the number of microsteps, then go to motor tuning, and make the appropriate adjustment to your steps per unit.  Your motors will be 200 step, if you use 8 microstep that will be 1600, you appear to have about a 2 to 1 reduction, so that will be 3200, then what ever your leadscrew is. Mine on the lathe was 10 turns per inch so this would be in the order of 32,000 per inch - do not worry, Mach can take it.

My lathe was 60,000 turns per inch - and Mach coped very well. I have now gone metric, with a 5mm ball screw, so I am down to 1200 per mm - but it is, in effect the same motor speed.

As regards you motors, if they are eight wire, they will be much faster if wired in parrallel. I wired mine in series at first, with disappointing results - only 4 ins per min.I wired mine in parrallel, and I shot up to 10 times that figure. I have eased it back to 24 ins per min to keep accuracy. My next move is to increase the voltage.My Ghecko drivers will take it, but my Z drive is limited to 24 volts so I am stuck at the moment.


Not me driving the engine - I'm better looking.
Re: Strange motor speed on reversing....
« Reply #2 on: April 29, 2009, 02:25:02 PM »
Thanks for the reply,,
I'll change the steps, maybe to 1/4, try it, then to 1/8, play around with it a bit.  I chose half step from the advice from the hobbycncpro instructions.
Mainly it was a recommendation as a good place to start.  Also, they mentioned not to use microstepping to increase accuracy.  I suppose if I don't run the motor faster than needed, I shouldn't have a problem with missing steps.

There isnt a problem when I do not use backlash comp, (or set it at zero.)  ie.  Backlash comp is on, X set to zero and Z set to .0113.  The X doesnt have the issue (as seen in vid)  but the Z does.  but if I add say .002 to the x,, it has the same issue.

This problem never occurs when I use the machMill program, only the Machturn....  I'm wondering if it might be a software issue???

I'll change the steps and try that first..

Thanks again!

dude..
TheDude,

Offline Hood

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Re: Strange motor speed on reversing....
« Reply #3 on: April 29, 2009, 02:30:12 PM »
Check that you have the same shuttle accel value in Turn as you have in Mill.

Hood
Re: Strange motor speed on reversing....
« Reply #4 on: April 29, 2009, 03:10:17 PM »
Check that you have the same shuttle accel value in Turn as you have in Mill.

Hood

Checked it,,,, they are not the same.
Shuttle Accel.
Mill  .004
Turn 1

This could be it huh?
TheDude,

Offline Hood

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Re: Strange motor speed on reversing....
« Reply #5 on: April 29, 2009, 03:11:31 PM »

Almost definitely :)

Hood

Offline jimpinder

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Re: Strange motor speed on reversing....
« Reply #6 on: April 29, 2009, 03:18:41 PM »
I will bow to Hood, who knows a bit more than me.

To go back to the microsteps issue. Hobbycnc are quite right, you do not use microsteps to increase accuracy - although you would think it would. It does, infact, but the reason that advice is given is that the distance between microsteps is not even and you cannot guarantee that say 5 microsteps of a 10 micro step driver is exactly in the centre of a motor step. It does, however, give the maths of Mach more to go at when making the divisions necessary to drive complex fractions of measurement. Mach works to a 0. (12 figures) of accuracy - althoug only 4 are shown on your DRO's.

The reason you use microsteps is for smoothness of running, and you will find that 8 is a good smooth run with most motors and drivers.
Not me driving the engine - I'm better looking.
Re: Strange motor speed on reversing....
« Reply #7 on: April 29, 2009, 03:24:06 PM »
I assumed the "shuttle Accel" was a setting for the MPG,, which I do not have.
MAN!  I hope this is the problem.  I will check tonight when I get home.

Thanks!
TheDude,
Re: Strange motor speed on reversing....
« Reply #8 on: April 29, 2009, 10:10:16 PM »
Worked like a charm!!!  So much that I spent the evening in the shop playing and making a few trial parts.
Thank you all and thanks Hood for your help.  Nice to know there is a dedicated and helpfull forum of the software for us new to CNC.

I did change to 1/8 step and reset the steps, etc.  I do like the smoother action and sound of it this way.
Some "motor banging/EEEGHRRRR!" sounds when changing direction, but I got it worked out through velocity and accel in motor tuning.
Although when rapid jogging in Z,, then adding X jog during the Z jog,, caused some bad motor noise and disruption in steps.

I wonder if 1/4 step would be a happy medium for the hobbycncpro board and my setup?

I also found this in a document other than the manual,,,,  just after hood suggested it, of coarse.. haha

"Shuttle Wheel Setting (Shuttle Accel.____ Seconds)
Determines how much time is allowed for the backlash takeup movement to take place.
For my servo systems I set this to a VERY small value (0.00001). This will prevent the
backlash motion from affecting the smoothness of the machine because the step pulses
are sent out as fast as possible (to the limit of the kernal speed). A large number may be
required in stepper systems to prevent lost steps. I also find it very helpful to set the
backlash distance to some HUGE visible number (10mm), then it is easy to see how the
different backlash settings affect machine motion."

TheDude,

Offline jimpinder

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Re: Strange motor speed on reversing....
« Reply #9 on: April 30, 2009, 01:31:58 PM »
You will probably find the "motor banging" (although that sounds a bit harsh) on reversal is the backlash compensation system, which you say you have turned on. There is very little you can do about it. The backlash seems to (as you reverse direction) - start in the reverse direction - suddenly realise this and stop - apply backlash (at whatever speed you have indicated) - stop - start again to complete the move

I have my backlash comp speed set at 50% (although I recently fitted ball screws and got rid of most of the backlash) and I found this reasonable. It's not too bad when you realise what it is - and it certainly works very well. The problem is, of course, on some moves you have to apply backlash on both axis, and these have to be independently applied to each axis. It is quite fun to listen to.
Not me driving the engine - I'm better looking.