Which Controller?

The discussion referred to was on a somewhat different topic - this
one was more specific - that of which controller might be
I too would love to know just exactly what is fed back in terms of a
signal in order to achieve what?
Are we discussing a power source which sets a desired voltage level
but compares the actual instantaneous output with the desired or is it
some measure of a pulsed variable frequency square wave which is
regulated by the reflected component voltage?
I thought that all my years in electronic control engineering left
little room for non knowledge but I am definitely lacking in this
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Feedback controllers use a pulse output (the power is switched on and off). While the power is off the motor is still turning and acts as a generator producing a voltage proportional to its speed (BEMF). A feed back controller measures this voltage and adjusts the pulse duty cycle (ratio of on to off time) to maintain a constant speed.
In analog systems the pulse frequency is quite low (and audible) as the signal has to travel some distance from the controller to the loco. In a digital system a BEMF controller is inside the loco, resulting in quite short wire runs, which allows a much higher frequency to be used. This frequency can be outside the audible range.
Mark Thornton
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Mark Thornton

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