Re: Belt tensioners on toothed belts

Tom,

If I'm understanding your description properly:

Sounds like an old design; or based on one... (An oxy-fuel setup
upgraded to plasma, perhaps?)

The link-arm feedback tells us one thing immediately:

They're servo-ing velocity instead of position. Since one relates to
the other, it can work.

Nowadays we generally servo position digitally, and derive analog
velocity. Here it appears the mfr. was servo-ing velocity to derive
position.

Likely the arm has a pot that is closing the loop, and the motor
speed is set by this, as in old CNC and computer tape readers, and
the reel-to-reel tape players you mentioned. (Would've seemed
the "obvious" choice at the time, to a designer.)

Pots are less expensive than encoders AND they're analog, instead of
digital. So they found much more widespread use as feedback devices
in our once fully analog world. The analog feedback alternatives of
the time were/are LVDT's and resolvers. Either of these is FAR more
expensive to implement than a pot driven by a swing arm. So...

Considering that torch cutting/plasma is/WAS considered pretty low-
rez; it doesn't surprise me at all to hear you describe this sort of
feedback. The positional errors are/WERE just an "acceptable"
tradeoff/ byproduct of using a lower priced fully analog controller
methodology.

Hope this helps,

Ballendo

P.S. Anybody who's done mfg. maintenance work has likely seen many
swing-arm feedback devices on machines doing all sorts of work;
they're pretty common IME.
(It's interesting to note that this swing-arm technology pre-dates
electronics; at one time the swing arm mechanically amplified the
feedback error to effect changes in the speed of the system.)

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <thom@...> wrote:
>
> I just finished helping a customer with a servo driven plasma table
> and I was surprised to see spring loaded idler wheels on swing arms
> (like the old reel-to-reel tape drives) being used to tension the
> belts on the toothed pulleys. The tension mechanism was a coil
spring
> and was not very taunt so that when the motors changed direction the
> belt would bow out or in, depending on the direction. Seems to me
the
> flexing of the belt would introduce some errors. Do any of you have
> any experience with a design of this type? Seems like a way to move
> the motor to maintain belt tension would be a better approach. I
> guess if the belt tension was strong enough it might be a better
> solution (?)
>
> I do know that in this circumstance it ended in disaster. The pivot
> arm became loose and the arm got cocked at an angle. It was okay
with
> the axis running in one direction but it acted almost as a brake in
> the other direction and since the motors were large it just ran
until
> the fuse blew (about 30 msec after the servo drive blew!).
>
> Tom Caudle