Re: [CAD_CAM_EDM_DRO] Re: servo vs stepper

Right - OK.

Do you have an oscilloscope? Access to one?

You should take a look at the pulse train coming from your controller
board. The Geckos have some specific requirements regarding pulse shape
(4 uS high time, 0.5uS low time, I believe). Look at the step and
direction inputs on the Geckodrive, and get your board to send a
moderately high speed pulse train to the Gecko (do back and forth rapids
over the full travel, on one axis)

There are several things to check with a scope:
1) Step pulse fall time. The falltime should be fast - like 0.1 uS or
so. The Gecko clocks a step on the falling edge of the pulse, so this
should be as clean as possible.

2) Low level of step pulse: This should go pretty much to ground. It
may not, depending on the output driver on your board. Anything below
about 1.5 volts should mean that the optoisolator LED is being turned
on. The driver on your board should be able to sink 16mA (I think).
Any less may take longer to get the signal through the optoisolator (or
not work at all).

3) High level of step pulse: This should go all the way to 5V (or
whatever the step/direction supply is). Incidentally, the control
supply on the Geckos should be 5V. You can run it at a higher voltage
if you put a resistor inline, but it's not a good idea. They aren't
compatible with 3.3V logic (there is a current thread on the Geckodrive
list regarding this).

4) Step pulse width: Make sure it's 4uS on and 0.5uS off. Even longer
is better (you probably don't want to get steps going at 200kHz, so
there's no reason to push the minimum step timings).

5) Make sure the direction line isn't changing

One of the boards available (Pico systems or Campbell - I don't remember
which) has a mode where it outputs direction changes synchronously with
the step pulse (on the rising edge of the pulse, I think - so there's no
uncertainty about which direction to step when the step line goes low)

It's possible that the output from your board isn't perfectly compatible
with the Geckos.

Out of curiosity, if you do 45 degree diagonal moves, can you go faster
than 20 IPM? I'm not sure what the answer would tell us, but it would
be interesting to know.

Keith Clark wrote:

> <>Yes, I have the Geckos set for 7 amps which is the rating for my
> current motors' current. I have a 75 volt 16 amp powersupply that
> is unregulated. I think the steps per inch is correct since I get
> accurate distance travel. I have the rapid setting set at just
> under 20 ipm since it stalls with higer settings and the acel and
> decel parameters do not seem to help but when not correct causes
> occasional stalling even at lower speeds.
>
> Steve, thanks for the consideration
> Keith Clark

Always up for an interesting question :)

- Steve