I have the same router and found that it already has some form of electronic feedback for the speed control. If you remove the cover of the router you will find a module that sits over the shaft end. Removing this you will see a circular magnet on the shaft end which fits into a pocket in the module where the magnetic encoder is enclosed. It's been my intention to obtain another router so I can have a spare and then open up the module (it's hermetically sealed in what looks like silicone). Although hacking whatever chip is in there may be difficult, I imagine that the encoder signal could be used with an arduino to create an external PID controller. Take a look and see what you think.The logic for such a controller is the easy part, I think. The more daunting task to me would be designing a safe and reliable power circuit.