Also for the radio controlled self made tank hull model, I designed a double differential steering unit. The principle is based on using two differentials to achieve a rotational speed difference between the two tracks of the tank. When the differential housings stand still, the incoming drive shafts exit with the same speed. When the housings start to rotate as well the speed of the outgoing shaft will change. The faster the housing spins the greater the difference in track speeds will be. Up to a curtain point where it actually reverses the direction. This principle works as well when driving or when standing still. In the latter case it will pivot on the spot, better know as a neutral turn. Although the last situation requires massive torque because all of the track friction with the ground surface has to be overcome.
The two differential housing are combined on a single motor driven axis, en the gears are placed in such a way that they rotate in opposite direction of course. It does require a separate motor to drive this shaft. In my model I used a motor with planetary gears out of an old drill. Both motors in the model are powered by a Sabertooth DC motor driver and are connected to the transmitter with an Arduino module.