Rather then jump directly to a sync-driven loop to control the motor speed of my new Nipkow televisor, I wanted to play with simulating the use of a synchronous motor. As shown below, I opted to modify the Peter Smith/Club motor control circuit. The picture that is displayed shows the schematic at low resolution, but you can download the full-resolution version if needed.
The strategy seemed simple - the 4046 PLL would be driven from the optical detector on pin 3 and, instead of the standard detected sync pulses, a crustal-controlled 400 Hz square wave on pin 14. The optical sensor is reflective, and looks at an evenly spaced set of black and white pairs - 32 pairs in all. It works fine. The 400 Hz tone source is a 4060 with a 3.2768 MHz crystal-controlled internal oscillator and I am taking the signal at pin two of the divider network (/8192) and the result is a 400 Hz square wave.
The resistors on either side of the 10K speed control are actually 10K pc trimmers so that I can play with the bias network as opposed to a pair of 4700-ohm resistors. I am using a 12V motor rated at 1800+ RPM at 2A under full load - the running load is notably less.
With the 400 Hz signal at pin 14 of the PLL and the optical chopper input at pin 3, I expected to be able to lock the system at 750 RPM with proper adjustment of the 10K speed pot and the associated bias resistors. That is not what I get!
With the 4046 in the circuit, I can get the motor up to slightly less than 750 RPM if I set the high bias resistor to 0 ohms and the low one to 10K. If the 10K ipeed pot is set to maximum, the motor speed settles into something a bit below 750 rpm.
If the optical sensor input to pin 3 of the PLL is disconnected, the motor speeds up and will peak out at its normal maximum speed at 12V - a bit over 1800 RPM. When I reconnect the sensor to pin 3 of the PLL, the motor immediately starts to slow down. As it passes through 750 rpm (determined by watching the image display), the picture jerks few times, as if it were trying to lock, but then it slows to something just under 750 RPM and stays there. A few times, by letting the disc speed up by breaking the connection at pin three and then restoring the connection, the system WILL lock at 750 RPM (confirmed by the image display and the strobe), but that happens rarely and eventually it snaps of of lock and settles to a constant speed just a bit slower.
With pin 3 disconnected, I can juggle the speed pot and bias resistors to get it to free-run at just about 750 RPM, but when the optical line is reconnected to pin 3, the disc immediately slows way down to something around 375 RPM.
If someone can point me to what I have over-looked, it would certainly be appreciated....
Ralph