19 Jul
2010
19 Jul
'10
4:59 p.m.
On 2010 Jul 19, at 4:03 PM, Dave McGuire wrote:
Every vacuum-column drive I've seen has used pressure sensors to
determine the position of the tape loops within the columns. Granted
I've not seen a HUGE variety of different drives, but...just sayin'.
On the desk beside me, I'm looking at a part from a Honeywell vac-col
tape drive. It's an aluminum extrusion 1/2-inch wide and about 6 inches
long, with a masked glass plate on one side. Inside, spread along the
length are 8 selenium photocell strips.
I think this was located somewhere around the mid-point on the vacuum
column, that is, the target area in the column to keep the tape loop
at. Perhaps optical was chosen for servo response time compared to
mechanical pressure switches.
I haven't examined a large variety of drives either, but at least on
some drives, the sensors are not distributed evenly along the length of
the column, but are concentrated around the target area, and become
sparser towards the extremities. Never seen it, but one could imagine a
hybrid design with optical sense around the target area and pressure
sense further out.