Re: [CAD_CAM_EDM_DRO] Re: tach output

mariss92705 wrote:

> Hi,
>
> Three points on this subject.
>
> (1) You can always take the encoder signals back to the PC as well.
> The drive uses the encoder to run the motor but that does not mean
> you can't send them to the PC as well as a DRO input. You have the
> encoder anyway, why not use it if the PC is set up to take the
> inputs? It would give a reading even if the drive faults.
>
> (2) The "send a step and hope it gets there" is the same with any
> other servo as well. The only reason it wouldn't "get there" is if
> the motor was overloaded. At that point the drive is pushing the
> motor to 100% capacity. What would any other servo drive do? Find a
> way to push it to 101% of capacity? It would also have to fault out
> because the motor would have fallen to far behind.

Yes. Most servo drives have great capacity for momentary peak torque
requirements. A stepper stalls suddenly, and somewhat unpredictably,
so you have to leave it some substantial margin of extra torque.
A servo can deliver a momentary burst of enormous torque, often
4 - 10 X continuous rated torque. Of course, if this ocurrs in some
steady-state move, rather than just during acceleration, it likely
indicates a crash. Like when I drove the vise into the side of a
3/8" end mill in the stationary spindle. The end mill broke off cleanly
and sailed across the room, and my very meek servo system didn't
even fault.

> (3) A linear encoder would only work if there is zero backlash. If
> there is backlash, the motor will ping pong between the backlash
> limits. You can still send the linear encoder back to the PC, and
> based on its readings, output the required step / direction signals
> to the drive. This takes out leadscrew error as well.

Without separate velocity feedback from a tach on the motor, this is
almost unavoidable. If the backlash is substantial, the system would be
totally unusable. But, if you can reduce backlash to near zero, then
leadscrew error is eliminated. This is used on high-end machining
systems, and they have to watch out for backlash developing due to
wear on the ballscrews.

Jon