Terry Pinnell wrote:
I made my Curtain Controller 6 years ago based on a motor I stripped from an
inexpensive battery-operated screwdriver drill (3V). In general it has worked
fine. But for the last year or two it has developed a strange intermittent
fault. Suddenly, instead of opening or closing the bedroom curtains in under
2 seconds, it will become glacially slow, taking maybe 10 seconds or so.
Although it always gets there in the end (closing the appropriate
microswitch), it's irritating.

I don't mean 'intermittent' in the usual sense. It continues in this slow
mode for several *months*. Then, just as suddenly, it will revert to its
normal fast mode. And stay that way for months.

A few days ago it once again reverted to slow mode, perhaps the 3rd or 4th
time in 2 or 3 years. I made an even more concerted but fruitless effort
than before to isolate the cause. It's definitely *not* the batteries, my
primary suspect. I've tried several sets of 3 x NiCd/NiMh C-types,
well-charged, all giving identical results. Voltage during operation remains
more than adequate for the 3V motor. I'm also sure it's not the mechanics,
friction, obstruction, etc.

Logically that seems to leave only the motor itself. Is it possible for such
a motor to exhibit this sort of behaviour and if so what's the likely cause
please? Is it fixable or am I going to have to find an identical motor?

When you were measuring voltages did you measure the voltage at the
motor, or upstream? You could have a bad connection between your
battery and your motor which would leave your battery as happy as a
clam, yet leave your motor under fed.

Have you checked for mechanical resistance? Perhaps the mechanism is at
fault, and the electronics are fine.

If the motor isn't seeing excess back-torque, and it's getting proper
voltage, then the problem is the motor. If not -- not.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com