RE: [CAD_CAM_EDM_DRO] "lost steps", what is it and how do you know you lost 'em?

There are really two problems that sommetimes occur while setting up a
new machine or new control box. Lost steps and lost motion. Sometimes a
stepper/servo system will lose steps because of variety of reasons i.e
software settings, speed, resonance, encoder errors etc. But you can
lose mechanical position which could lead you to believe that you have
lost steps by having, a loose timing belt pulley, worn timing belt,
lost motion in the axis lead/ballscrew, racking of the axis, deflection
of the axis drive nut bracket etc. So the best way to test for lost
steps in a stepper system or a servo system like the gecko G320/G340 is
first disconnect the motor from the axis and to put a scible line on the
motor shaft. Tape a thin wire pointer onto a stationary adjacent parts
and zero the wire to the scribe line. Then you must set the software
configuration file for zero backlash and the stepper/encoder count set
for the number of steps for 10 full revolutions. Run the program to see
that when ordered it rotates to the exact point to line up with the
scribed line. A stepper would be similar. Then run the program again
reversing the direction and bringing it back to the starting point. For
example of a servo motor with a 250 count encoder time 4 for quadrature
you would have 1000 counts per rev, Assuming 10 turns we would set the
software to 1000X10=10000 counts. I use inches in my software so that I
can command the motor to turn 10 turns by using the command G01X1 F10.
Then G01X0. For stepper motor set in the half step mode it would be
200X2X10=4000 steps per inch.
If the motor starts and stops then reverses and lines up with the
scribed line then the lost steps are probably caused by lost motion in
the mechanism. There are always exceptions to the above for instance,
moving a stepper motor in a linear move may not replicate lost motion
observed if it was caused by stepper motor resonance. If stepper motor
resonance is suspected I would try the code G03X1y1 i1 f5 and see if the
X axis starts and stops on the scribed line.

Dan

-----Original Message-----
From: Tim Goldstein [mailto:timg@...]
Sent: Friday, January 31, 2003 9:25 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] "lost steps", what is it and how do you
know you lost 'em?


Step and direction servos are open loop between the computer and the
drive and closed loop from the drive to the motor. Yes they can loose
steps. The way you know it is you measure the table travel with an
indicator and see if it compares to the commanded motion. You can also
see it when cutting pockets if the cut levels shift. Another way it
turns up is when after the program finishes and you go to a know
reference point and it is not where it should be.

Tim
[Denver, CO]
Sherline products at Deep Discount
Mach1 & DeskCNC with credit card ordering www.KTMarketing.com/Sherline

> -----Original Message-----
>
>
> I think I know what the term "lost steps" is describing, but how do
> you know that you lost them? Is it a stepper motor thing only? I'm
> putting together a Taig mill with servo's, Gecko 320's (Camtronics
> kit, if it matters, but I don't think it does) and demoing Mach1 (I
> don't think that matters either, could be any step and dir controller
> program). I've read about servos and the encoders sending feedback
> "back to the computer", but in this sort of setup the encoder feedback

> isn't going "back to the computer", is it?
>
> The control program is feeding step and dir to the drivers. Are the
> drivers making sure that no steps are lost? Is this a true "closed
> loop" system, or just a way to use a step and dir controller program
> with servo motors, but still with the possible problem of lost steps?
>
> Monte (electronics almost done, waiting for mill, reading too much)
>
>
> Addresses:
> FAQ: http://www.ktmarketing.com/faq.html

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