OP, thanks for Best Answer.
Grobe, sorry I typo'd your name wrong. I've never used Speedfan, so I am not aware it can be used to control to an actual speed target - I'll take your word for that. I was speaking of the systems used by mobo headers and their BIOS software tools. Neither of those actually tries to achieve a target speed, nor do they care. The older system basically did this: here's 9.37 Volts, so run with that. (And whatever speed that produces is OK, until the BIOS decides that the temperature measurement has changed, and sends out a new Voltage.) The newer PWM system does it this way: I want you to run with full power on for 47% of the time, whatever speed that produces. Etc. The PWM signal is an on-and-off signal cyclcling at about 25KHz with varying "% On". It does not specify any speed, but you could create a circuit to scale that from 30 to 100% On into a speed target of, say, 400 to 2,000 RPM, and then create a speed control circuit to drive the motor (using what method?) to meet that target, based on a speed feedback signal. That is more complex than what a mobo BIOS does. Oh, and you'd have to arrange for the scaling system to be adaptable to different fans with different speed range capabilities.