Hi again,I am guessing there is a processor in the Upper unit that takes the RPM sensor output and calculates the speed when compared to the entered value.
Then provides the commutation angle for the Triac bridge device .In that case, you would need to come up with a duplicate method.
I was able to figure out the circuit well enough to write an arduino program that works to spin up the motor and adjust the speed. The triggering pulse needed to be inverted so that it could be re-inverted on the lower board. There are now two challenges. The motor experiences erratic triggering and speeds up randomly. I am unsure why this is. The zero cross circuit generates a square wave. Not sure if this would have any effect.
The other challenge is when the motor is spinning it lacks power. I can stop the motor with my hand even at high speed. I cannot do this if it is running using the original upper board. Is it possible that the triac is not fully opening to let full power through?