This way the current is proportional to the reference signal.

Interesting things to watch for are that if the "speed squared" signal limits then the mill goes to constant current ( run away and avoid it). If the current signal limits then it aims for constant speed and that is nice to hold the things under control and induce a measure of stall at the furling point.

For those trying to follow this but not completely understanding, it is no way a mppt controller. It is a controller to force an alternator power out to follow speed cubed at the input. It has to be set up to match a given machine. The set up is not difficult, you have to set the pwm at cut in ( error amplifier offset) and set the current gain of the current amplifier to get the correct pwm at full load.