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

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" <da-mack@e...>
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.