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acceleration values question
« on: December 15, 2014, 12:13:33 PM »
So I'm currently running a machine that has factory set velocity/accel values at 750/45, 750/45, 750/20 for the x, y, and z axes, respectively.  I am trying to increase the quality of my v carving by adjusting down the acceleration values as the mechanical properties of the machine have changed (i.e. the z axis has multiple heads and weighs more than it originally did).  Say I keep the velocity values the same but go down to 10 for each axis.  Do I have to reboot the computer in order to avoid losing steps on these stepper motors?  I adjusted them and I like the smoothness of the movement, but I have lost position in the z axis twice today.  Any insight as to adjusting acceleration values resulting in lost steps?

Thanks,
Bill

Offline rdean

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Re: acceleration values question
« Reply #1 on: December 15, 2014, 05:12:50 PM »
You may want to go with a counter balance of some type on the Z axis to compensate for the extra weight.

There are several types available just do a search.

Ray   

Offline RICH

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Re: acceleration values question
« Reply #2 on: December 15, 2014, 07:16:38 PM »
Like the velocity value, best way is to by trial and error. Once a reliable velocity is found, say 30% below where it skipped, then just increase the accel
in steps. Over time one finds something that is reliable. Yes there are programs which will evaluate motor requirements  for  given
conditions, but frankly, it gets you approx numbers to be verified through use of the machine.  

Counter balancing, as suggested, assists in reducing the required motor torque. You can also consider a different stepper motor.

If you change accel values in motor tuning make sure you click Config> Save Settings in the Tab. There is no need to reboot the computer.


RICH
Re: acceleration values question
« Reply #3 on: December 15, 2014, 08:37:13 PM »
Yes, I'm using two gas shocks from Suspa on the z axis.  At any rate, that computer crashed a few times today and didn't do it until I started messing with these values.  I have removed some unnecessary programs and tried to pare down what XP is doing.  I guess we'll see what happens tomorrow.  I just didn't know if messing with the acceleration would cause a missed step problem/computer crash.
Re: acceleration values question
« Reply #4 on: December 15, 2014, 09:08:06 PM »
if you want mach to work correctly there should be no other programs on the pc.

I'm sure someone will pipe up with "i run this program with no problems" but you should consider windows thinks this is a printer port, so any program that has the ability to access the port can cause havoc.

Computers are so cheap it's just stupid to have one for all, these days you'll pay more for a cutter than a pc.
Re: acceleration values question
« Reply #5 on: December 16, 2014, 06:23:05 AM »
Thanks for the reply.  It is dedicated.  It is not hooked up to the internet.  It is hard wired into my LAN.  I'm just trying to isolate if it's a computer problem, or if it's a Mach settings problem.

I know if I set the accel too high, I will lose steps.  However, I didn't know if I set the accel too low, or if you have high velocity with low accel, if it could lose steps.

Could lost steps be a processor problem?

Of course, there was a power outage two weeks ago and I'm wondering about the BOB in the control box too.  It's just aggravating the thing will work fine for awhile, but then when I'm away and the operators are in there, the machine will lose position (I'm assuming because of lost steps and not mechanical) and ruin material ???
Re: acceleration values question
« Reply #6 on: December 16, 2014, 07:01:25 AM »
have you ever had the machine on and sitting for long periods and heard the odd thump ?
Re: acceleration values question
« Reply #7 on: December 16, 2014, 01:45:51 PM »
No, what makes the sound?

Offline RICH

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Re: acceleration values question
« Reply #8 on: December 16, 2014, 04:28:04 PM »
Quote
However, I didn't know if I set the accel too low, or if you have high velocity with low accel, if it could lose steps

Velocity is constant movement of the axis, and as long as the stepper can maintain the required torque to deliver
 the force necessary to keep the axis movement at that speed it will not skip. Now the stepper looses torque the faster it
rotates.

Acceleration is the change in the velocity from a higher to lower or lower to a higher value in some time.
The stepper must have adequate torque to provide that acceleration to the axis. One of the factors is the mass
which it is trying to speed up or slow down. You know F=MA

So if you are trying to increase the axis velocity and it is at max velocity / unreliable you may not have the torque available to do it
and the stepper will skip. Also you may want to move the axis a small amount  in a very short time such that you need  adequate
torque to provide that move. IE; say the Z must move 0.0003" in a fraction of a second if can't do it it will skip.

So try all axis at say velocity = 100 and accel =10, then just double the velocity values until it skips, then reduce the
velocity 30%. Then double the acceleration values until it skips. Now reduce the accel value by say 30%.
You do the above  in a linear move with the axis from a dead start to rapid stop for both a long and short move.
You will quickly find approx velocity and acceleration values for your machine and you should use the values of
the weaker axis.  The 30% provides some headroom for different materials when cutting and you may find from experience that
you want to additional reduction of values. IE; what happens when you hit a knot in the wood?

Rapiding forward.........   ;) you want power from the motors which is function of speed times torque.

Since you modified the machine as compared to what it was designed to do ....whatever that was  ???.......you need to
change some values to account for the modifications.

Do the above before getting into other possible system contributing factors and quit quessing what is wrong!

RICH
Re: acceleration values question
« Reply #9 on: December 18, 2014, 04:31:27 PM »
Thank you