On Wed, 09 Dec 2009 07:03:31 -0600, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Don Foreman fired this volley in
:

You'd need curtain
position feedback too.


I said that too, earlier, then realized that you don't need position
feedback.

A PID loop outputs a signal proportional to the amount of correction
required. One could position the curtain according to the correction
signal, and not require that the position be fed back to the controller.

That would still require more complex drive controls than simply "on-
off/up-down", but the position controlling mechanism could be part of the
curtain drive itself, rather than feeding back to the controller. Think
of how a PWM model aircraft servo works.

Then the feedback is in a local loop. Position sensing is still
required, and is provided by the servo mfr. PID provides a signal
proportional to error (and the integral of error, with some
differential modification). In a temperature control situation this
works: if cold, supply more heat. In this app, curtain position is
the time integral of motor excitation. Curtain position is the signal
required to be compared against setpoint to produce an error signal
that PID can use.

The receiver never "knows" the position of the servos. It merely outputs
a pulse width proportional to the position desired, and the servo goes
there - then nulls - on its own.

Yes, because the servo has internal position sensing feedback.

A string pot and a comparator could comprise the basics of such an auto-
positioning mechanism. In use, the controller would not have to output
on-off, or up-down outputs, just an analog signal or pulse width
modulated signal that represented the desired absolute curtain position.

LLoyd