Re: Safety -- servo vs. stepper

I have been contemplating about this for some time.
No solution I am fully satisfied with yet.
I am thinking about possibly running the Servos
dc powerline through the limit switches.
Anybody with any input about this?

When my encoder gets unpluged or malfunctions the
servos take off at full speed and there is nothing
the software would be able to do about it.

I plan on using seperate Home switches so during
normal operation no limit switch should ever be
triggered.

Is this a reasonable aproach on a feedbackless system?


--- 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