Controlling the motor torque can be done with a constant current. Since it is a robot, I expect efficiency will be an issue, so a a switching regulator will be needed in the end. After you experiment please post how things are going and I will suggest something.
For now I would breadboard the circuit shown and test to see if this is realy what is needed, namely current control.
The motor current is simply:
Im=(Vdac-Vbe)/R1
You should select R1 so as to obtain the maximu current required for the motor when Vdac is at maximum (I suppose 5V).
Vdac is the voltage you get from the DAC (I suppose you will have a DAC). For testing you can just use a pot and vary the voltage from 0 to 5V. The battery shown will be your bench supply for now. Adjust the voltage to about 10-12V. Do not worry about details now. You just need to test the concept. Later we can deal with the other issues such available voltage, power, etc.
The transistor shown may get hot, depending on the motor current and power suply voltage. I suppose a 2N2219 or similar should do the trick. A heatsing may be required.
Let me know how it goes.
Cheers,