In article ,
cavelamb wrote:

Joseph Gwinn wrote:
In article ,
cavelamb wrote:

[snip]
I would like to see limit switches at each end just for safety sake.
That would take up two bits of input per vent.
But if the device has comfortable "over-run" areas, it might not be needed.

I agree that limit switches are needed, but they should physically
interrupt power to the motor, and in no way depend on software.

The problem is that once these limits are hit, the computer can no
longer recover control, and a manual reset is needed.

The usual solution is to have two sets of limit switches, inner and
outer.

The inner limit switches are sensed by the computer, telling it to stop
moving in that direction.

The outer set physically interrupts power to the motor, to prevent
damage should the computer fail to do the right thing.

Joe Gwinn


There is an old saying that,
"If engineers built bridges the way programmers write software,
the first wood pecker than comes along could destroy civilization".

Yes, plus the sayings about "If Microsoft built xxx".


And there is some truth in that.

But no, I'd not design an autonomous system that need manual oversight.

IIRC, Karl said his vent was about 6 feet long.

My first fantasy was a fabric tape/shade (wide enough to cover the vent slot)
that rolls up on rollers at each end. Maybe it has some shaped supports
to keep it snug against the enclosure(?). But there is nothing to jam into
at the end of the run.

So all I'd be doing is rolling the shade up or down.

Well, the shade may tear. One may decide to simply clean up afterwards
if this happens, but I would think it would prove to be a big nuisance,
enough to justify some limit switches.

Given that shade replacement is manual, manual-reset limit switches (a
pair of normally-closed microswitches in series with the motor power)
may make sense, as reset is far easier and perhaps cheaper than shade
replacement.


The computer can monitor the drum and tell, within reason, where the opening
is at any time. If we are rolling up on a fairly small shaft, just count
turns. If it's a big drum, maybe glue some magnets around the drum and count
them with a Hall effect sensor.

You don't see that kind of proactive defense in other mechanical devices.
Printers, Plotters, Flight simulator motion platforms, CARS.

The usual solution is to make sure that the motor will stall without
harm to anything. Also using a pipsqueak motor saves money on the motor
and its power supply. But it takes a lot of engineering to achieve this
kind of balance, and so is worthwhile only for production items, not for
one-off projects.

Industrial gearmotors have *lots* of torque, so unless it doesn't matter
if the motor keeps on running forever, explicit limit switches or some
kind of mechanical torque limiter or mechanical fuse (like a shear bolt)
are necessary.


But you do have to be careful about writing any wood peckers into your code.

Even with care, one hears the pecking from time to time.

For an extreme example, fly-by-wire bet-your-life avionics code is
developed to the DO-178B standard, which increases the cost per line of
the resulting code by a factor of about ten over the process used to
develop for instance radar signal and data processing code.

The resulting avionics code is very reliable, and yet bugs are still
found from time to time, even in systems with many flight hours to their
names.


Joe Gwinn