Does this help?
Regarding implementing a governor, how about this:
1) Process that reads the voltage, and adjusts a "Power Factor" variable. For example, when voltage is > 9 volts, Power Factor = 1. When it drops below that, the value declines.
2) In your process that sets the speed of the motors (let's say the value is Speed), multiply that number by the Power Factor variable (Speed * Power Factor).