Re: Geckos, Servos, VFD and twitching on Z axis

We tried the wire separation and isolation, we even swapped motors
and encoders with the Y axis. We are at the mercy of the machine's
construction. The Z axis servo is mounted right up front of the
spindle head. The spindle motor is directly behind that about 10"
away from the servo and encoder. The VFD we wanted mounted to the
side of the spindle head for ease of use. We could have mounted the
VFD in a cabinet behind the machine and rigged up a remote keypad
for operation but we didn't realize that it would cause such noise.

The noise would result in a slight "twitch" of the Z axis usually
causing a .005-.010 jump or error in the Z axis motion. As for now,
everything seems to be working and repeating fine. I ran a program
over and over and there were no position errors or twitches. As for
other noises, the machine is located in a high school machine shop.
It is surrounded by other machines, 3 phase and single phase and
machines powered from the some bus. We also have quite a few
machines with VFDs on them as well. So far, we have no noise showing
up. We are going to try some 3D contouring tomorrow. Any noise
produced by any of the axii will definitely show up then as a divot
in the surface of the part. We have our fingers crossed.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, R Rogers <rogersmach@y...>
wrote:

> I had the same crosstalk issues when I retrofitted my Bridgeport.

I was able to just seperate the motor leads and encoder cables to
rectify. It seems there should be a standard to eliminate cross-talk
and interference issues with encoders, or at least a more absolute
fix and safeguard. I'm wondering what difference moving the 10 uf
cap on the encoder current a few inches away from the Gecko made. I
dont have any caps on my encoder wiring and wondering if I need any.
Once I see a servo act extremely erradical and resolve it by just
moving wires, It's not a very secure feeling for future operation.
What else could possibly make it happen? Arcing in the spindle
motor? Noise from another piece of equiptment coming through the
line? Cell phone? The kids RC car? I think about this when I have a
1.25"dia. X .875"dia. shank Reamer buried 10" deep in a large
workpiece. Having 5hp at the ballscrews from the servos, that would
be a disaster.

>
> Ron
>
> yethdear0 <da-mack@e...> wrote:
>
>
> Everything is shielded.
>
> By chance, we installed a 10 microfarad capacitor on the 5 volt

power

> wire that powers up our encoder and "Bigo!" all is great in the
> universe. We also found out that the capacitor had to be installed
> within a few inches of the encoder. If we installed it at the

Gecko,

> it wouldn't help. All 3 axis repeat now on a travle indicator
> within .0005in.
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "vavaroutsos"
>
> wrote:
> >
> > Sounds to me like you are getting noise on your Z encoder. Are

you

> > running single ended encoders, or differential with converters?

Are

> > the encoder cables shielded? Did you use shielded VFD cable from
> the
> > VFD to the spindle motor? How are your ground connections

oriented?

> >
> > ~petev
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "yethdear0"
> > wrote:
> > >
> > > Hiya,
> > >
> > > We have an old Hurco knee mill that we have stripped and retro-

ed

> > > with a pc, CNCPro, Geckos and a Baldor VFD for the spindle

motor.

> > We
> > > just got it all up and running but have a problem with the Z

axis

> > > servo or controller. Using a travel indicator on the Z axis,

the

> > > movement is smooth and exact. We have the machine running a
> program
> > > that move Z -.100" pauses for 1 second and then moves Z +.100"
> and
> > > then runs in a loop. When we turn on the VFD and the spindle,

the

> Z
> > > axis seems noisy and the travel on the indicator will start to
> > > drift .001" to .005" each time the program cycles. The drift

is

> > > random, sometimes positive and sometimes negative but positive
> the
> > > majority of the time. When we slowed the motion to 1 ipm, you

can

> > > see the motion twitch at random during the movement. This only
> > > happens when the spindle is running and only the on the Z

axis.

> We
> > > swapped out the servo motors with the Y axis and the problem
> stayed
> > > with the Z axis controller. On the machine, the Z axis servo

is

> > > mounted on the front of the spindle head of the machine, the
> motor
> > > (5hp, 230V, 3ph,) is on the back of the head. The VFD (a new
> > Baldor)
> > > is mounted on the side of the head about 6in from the Z servo.

We

> > > also dismounted the Z servo and located it away from the motor
> and
> > > VFD to test for magnetic flux or noise and it still twitches.

We

> > > have also played with the gain and damping adjustment but

nothing

> > > there would correct the problem. The twitching is only in Z,

all

> 3
> > > Geckos are getting their power from the same DC power source
> which
> > > baffles us why the twitching is only on the Z axis. We are

also

> > > using new US Digital encoders. When we swapped servos though,

we

> > > also swapped encoders and the twitching stays on the Z axis.

We

> > also
> > > rerouted cables as far from our VFD and motor as possible but
> still
> > > have the problem.
> > >
> > > Any suggestions or input will be greatly appreciated, thanks.
>
>
>
>
>
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