If the robot already has a speed regulator, then this is the way to control its speed. What do you hope to achieve by feeding the controller with PWM? In general most, non low-inertia motors have sufficient mechanical inertia with their added L/R time constant that they will appear to run properly at any PWM frequency over a few hundred hertz. The switching frequency once over the lower limit, can be anything,limited only by the switching losses and radiation problems. The motor speed is governed by the mark/space ratio of the PWM not its frequency.
Frank