Re: Safety -- servo vs. stepper

Use AC or brushless servo motors. An amp/drive failure is much less
likely to leave this type of motor in motion as it requires the
amp/drive to perform the commutation.

~petev

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Carl Mikkelsen"
<c.mikkelsen@c...> wrote:

> I have a choice of using stepper drive or servo drive for my next
> hexapod. My concern is how to cope with safety issues.
>
> My previous hexapod is stepper driven, with fairly slow maximum
> speed of just less than 1 inch per second. My next hexapod will be
> faster, with a designed intention of fast movements of about 3-5
> inches per second.
>
> With my hexapod design, it is very difficult to have limit switches
> on all axes. In software, it is easy to limit motions in various
> ways, such as to prevent any joint angles or actuator lengths from
> being exceeded. It is not easy to place a fixed limit switch on an
> actuator to serve as an E-stop of last resort.
>
> Under hardware fault conditions, it seems that steppers are safer.
> In particular, most failures in the high-power drive system will

not

> result in actuator motion. With a servo system, it seems that an H-
> bridge failure is likely to result in uncontrolled movement.
>
> Has anyone experience with drive failures, and the reprocussions?

I

> was about to order servomotors, and my stomach went cold as I
> imagined the danger of an sudden, RAPID, uncontrolled movement of a
> 50 pound mass moving in 3D.
>
> Any thoughts or suggestions?
>
> -- Carl