NOVA4/X -- running!

From: Tom Jennings <tomj_at_wps.com>
Date: Mon Feb 14 11:08:21 2005

On Mon, 14 Feb 2005, Tore S Bekkedal wrote:

>> It attempted to load, but acted strangely, sucked tape into the
>> buffers (vacuum columns) then THWACK the tape, then go slack.
>
> That's the same thing happening with the NORD-10's drive. Although it
> fortunately cuts out in time, before harming the tape :)

Well I learned a lot when the servo motors were turned off: That
let me verify that the limit stops worked precisely right.

(For those not familiar with vacuum column tape buffers, there's a
deep rectangular box, open at one end. At the bottom of the box is
vacuum generated by a pump (a noisy pump). There's two boxes, one
upside down, for each reel. It sucks a loop of tape down into the
box. Picture a loop of tape sucked into the box, but held so that
it goes half way; it wants to suck all the way to the bottom of
course. Then...)

For each column, there's two limit (vacuum) switches, one near the
open end. The switch near the bottom should have vacuum on it
(tape blocks off the bottom of the box), the one at the top should
not (tape is sucked past the switch).


That's the limit stuff. The servo loop is actually
straightforward, sort-of. In addition to the limit switches, the
column has a row of small holes into the column box, which on the
backside go into a manifold; as the tape moves up and down the box
more or less holes are passed by the tape. The holes towards the
bottom of the box apply vacuum, the ones above the tape loop are
at atmospheric pressure. Therefore, given a whole bunch of givens
(pump capacity >> flow of the little holes, etc mostly static
physics) the amount of vacuum (OK pressure you pedants) in the
manifold is proportional to how far down the column the tape is!

Then there's this hokey-looking (but reliable) piston/lever/spring
thing that bends a strain guage, sniffing the vacuum manifold.
This produces a resistance proportional to the amount of tape in
the box.

This is the feedback for the 10-ampere at 12-volts opamp that
drives the tape motor. Linear! Heat! Tape-snapping torque! Dumb as
a bag of hammers. It really needs some fast and sensitive
current-limit (rate ramp-up) to avoid tape-snapping.

Its the sort of thing you make a microprocessor do. Ahem.


Anyways, the point of this: I did not do this step, but you could:
manually arrange for the right amount of slack tape to fill the
buffers half way, and tape down one of the reels. UNPLUG THE
MOTORS from their drivers. Go through the "LOAD" sequence, with
one hand on the free reel, and position the tape into the columns
correctly. This is fairly easy.

Now you've isolated the free-reel's servo system for open-loop
testing: you can put a DVM on the feedback resistor strain guage
thingie and rock the free reel back and forth and see it produce
an error voltage.

Or better yet, put a "big" resistor in series with the motor to
limit it's torque to something absurdly low (so it won't tear all
the skin off you hand and snap the tape). The D.G. drive wouldn't
mind this, other designs might get upset. DG's tape servo is
simply a gargantuan discrete opamp, all linear and ceramic .1-ohm,
50-watt resistors. No PWM, no nothing! Many watts! Tres simple.

I wouldn't have known to do this at the time, but with the system
LOADed, and the reels taped down into place, motors off,
physically checking for vacuum leaks (pressing covers, wiggling
hoses, etc) would have produced a BIG feedback signal change.
This would have been Wrong and a big hint on what to fix.



Oh yeah, I forgot to say, the hoses that pressed onto the plastic
vacuum switches were stretched out and likely leaking; they were
loose, on and off. I cut off .25" and fixed that. It was an
accumulation of problems. The other end of the hose, that pressed
onto a brass nipple, was NOT stretched out, so it was a
plastic::plastic problem, probably one of them outgassing etc.
Received on Mon Feb 14 2005 - 11:08:21 GMT