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Author Topic: A axis speed limiting in Tangential Mode  (Read 2672 times)

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A axis speed limiting in Tangential Mode
« on: December 01, 2008, 06:00:08 AM »
Hi All,

Ok I have been tuning my fabric cutting table to get the most from it in terms of speed.

As the A axis has to rotate first to align to the x-y trajectory it is the speed of this A move that dictates the overall speed of the machine. I have wound up my A axis Velocity and accell values to the point where it is loosing steps at best I can acheive a feedrate of 6000mm/min cutting air

So what to do is the question. The A axis only has to move a few degree's at a time however it has to do it fast.

My A axis steps/deg is 5.55555 this is worked out as 2000/360 deg and my vel/acc is currently 15000 and 15000. which is where it starts to loose a few degress over the period of a large file run.

Also the machine runs it best/smoothest n Plasma Mode with G64 on but no CV settings ticked

Should I start to look for a servo??
Jason

« Last Edit: December 01, 2008, 06:05:19 AM by XLR84x4 »

Offline jimpinder

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Re: A axis speed limiting in Tangential Mode
« Reply #1 on: December 03, 2008, 05:42:02 AM »
The question is, why is it loosing steps.

Is it becasue you are indeed, trying to push the speed too fast, or is it the fact that at that speed, the pressure/drag is too much for the motor. Here is the dilema - because the answer to one is the opposite to the other.

If you want speed, but there is no appreciable pressure or drag, then you need to cut down your steps per unit. I don't know what drive is on the cutter, but can you change the gearing. I don't know what accuracy you are looking for, but the standard stepper motor is 1.8degree per step so a 2 to 1 gear would give you 0.9 degree - is this enough. A 3 to 1 would give you 0.6 degree and therefore the small changes would only require a few pulses.

If however, it is pressure and drag that is causing the problem, then the opposite is the case, and you need lower gearing to give greater torque to drag the whole cutter head round quickly.

Reading your post again - you say it looses steps over a large file run. I take it this is large number of the same pattern. Is it possible to incorporate a "return to zero" move either at the end or the start of a run, so that any missed steps would only be cumulative for the one cut.

Jim Pinder
Not me driving the engine - I'm better looking.
Re: A axis speed limiting in Tangential Mode
« Reply #2 on: December 03, 2008, 07:08:49 AM »
Hi Jim,

Thanks for the reply.

The knife is mounted directly on the end of the stepper and my tests have been dry runs. I have an optical homing switch on the A axis.
Yes the pattern/cuts are the same with as few as  20 up to 50 items. As far as accuracy I would like my machine accuracy to be 0.5mm but would settle for 1mm in tangential mode. I have also reduce feed rate to 5000mm/min with same lost steps

I have one file with +15 parts that runs great with no missed steps. I rescaled these parts and did a run of 20 with up to 8 deg lost at the end of the cycle. I rehome the A and run the original 15 parts for no lost steps. I have even set the linerisation tolerance on my iges file to 2mm up from 0.005mm to reduce the amount of lines and angle changes however this file still looses step. I have even stopped the cycle after each part and ran a A0 to check the a postion and it does lose postion after each cut. This has been tested with no cutter in place so no load on the stepper. the Gcode file Top large is the good file with the top small having issues. This is prior to rescale

I have a smooth stepper and have set the max step frequencies as such
A axis steps/deg 5.55555 with a feed rate of 10000mm/min = 166.66mm/sec x 5.55555 =925 steps/second set to 32khz

With Controller frequency at 2mhz.

I'm using Gecko 203V's and in motor tuning set step/dir pulses to 5us. The A is a vextra stepper 1.8deg wired as  unipolar with current set to 2 amps. It does get warm to touch

There is limited to no threads on tangental control so it would be great to hear from other users

Thanks
Jason

Offline jimpinder

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Re: A axis speed limiting in Tangential Mode
« Reply #3 on: December 03, 2008, 10:16:59 PM »
The motor on your A axis would seem to be the trouble.. When I was setting up my system, I wired my eight wire motors in series (I didn't know any different) and I could only get 4 inches per minute out of them before they missed steps (they didn't just miss steps, they stopped althogether).

I rewired them in parrallel and the difference was astounding - straight up to 40 inches per minute. I wound that right back for reliability but I can still get 20 inches per min out of them - reliably. Not up to your speed, perhaps, but this is a steel lathe I am talking about.

If your A motor is eight wire, in series, try it in parrallel, your Gecko will take it. My drivers were 2.5amp until I went onto Gecko, but even on that, performance was must better in parrallel than series.

My motors were rated at 2.5amps per coil (at 7.5v). With the Geckos  I have increased the current set to 4 amps,(feeding two coils in parrallel) and just lately put my voltage up from 24 to 36 volts. The motors will get hot (remember they are on all the time, even when not moving) - but if you can touch them, that's fine, if you can put a hand on and keep it there they are running cool. Reliability has improved tremendously.
« Last Edit: December 03, 2008, 10:24:18 PM by jimpinder »
Not me driving the engine - I'm better looking.