Draynor,
Based on your last post, you may want to use the analog-digital route as this will offer the most flexibility. However, the comparators can also offer this flexibility but without all the possible features of a microcontroller or PLC based system (interfacing to displays, having user adjustments, etc).
For the comparators, if you wanted it to stop in the middle, I assume the voltage would be 10V. This would require the reference voltage to be changed to 10V, which can be done. There are adjustable voltage regulators that would simply require a variable resistor to be moved to change the output voltage. Now how you going to move the variable resistor? Could use a digital pot (variable resistor) and control this through some other software/hardware. Just some more thoughts to get your wheels a spinning, good luck.