Re: Safety -- servo vs. stepper

What I know about EMC fits in a thimble, but maybe the "charge pump"
circuit described (I think) on the Master5 Yahoo site might be useful.


Jeff

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Leslie M. Watts"
<leswatts@a...> wrote:

> Servo run away issues are always of concern, but I wouldn't
> let them prevent me from using a servo's performance advantages.
>
> I do think about this though... I am pushing around 800+ pounds
> at high speed!
>
> My primary machine protection is a hardwired e-stop at ends of

travel

> followed by a shock absorber tuned to dissipate the kinetic energy
> in a specific distance. The e-stop also shorts the servos with a ten
> ohm resistor to aid braking.
>
> As I think about it runaways would be caused by two basic scenarios:
>
> 1) the computer/interface fails or locks up
>
> 2) A servo amp output stage fails such that most of the supply
> voltage is applied to the motor.
>
> A watchdog of some kind might be the best way to deal with (1).
> I do not know if my emc control is set up for this (I think not).

To work

> the computer would have to send a signal periodically to the

hardwired

> estop system to keep it enabled. There is a estop_write bit on emc,

but it

> is a continuous level and could easily fail in an enable mode.
>
> So what is needed here is a real time function that priodcally spits
> out a pulse THRU the interface card (so it's function is also

checked).

> This might be pretty easy to do perhaps using an extra axis DAC.
>
>
> Now for (2) (servo amp short) something might work that continually

compares

> the output current to the input voltage , at least for DC types. If

it fails

> to be a reasonable facimile of a voltage controlled current source

hard

> wired
> e-stop could be actuated. This also might be pretty easy to do.
>
> A second at least partial safety that I use now is a software

velocity limit

> a bit lower than the system can do at full tilt. If the velocity

(checked by

> monitoring a separate tach) exceeds the soft limit e-stop is

actuated.

>
>
> These are just some ideas to improve the safety and robustness of a

servo

> system.
> I would like to hear more ideas from others.
>
> Les
>
> Leslie M.Watts
> L M Watts Furniture
> Tiger Georgia USA
> (706) 212-0242
> http://www.lmwatts.com
> Engineering page:
> http://www.lmwatts.com/shop.html
> CNC surplus for sale:
> http://www.lmwatts.com/forsale.html
> CNC carved signs:
> http://www.lmwatts.com/signwp.html
>
>
>
>
> -----Original Message-----
> From: Carl Mikkelsen [mailto:c.mikkelsen@c...]
> Sent: Thursday, May 20, 2004 12:32 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Safety -- servo vs. stepper
>
>
> 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