Adder/subtracter drive

May 2008


I built a differential drive rover based on an adder/subtracter design I had seen floating on the web many years ago. One motor drives the wheels and the other provides addition or subtraction or rotation to cause the wheels to turn opposite amounts, resulting in a steering effect.

If you want a robot to truly go straight you have to drive both wheels from the same motor to ensure that they turn at the same rate. But how to steer such a robot? For this you can use an adder/subtractor built using LEGO differentials. The adder/subtractor uses a second motor to add or remove rotation from the wheels; causing the robot to turn. When the second motor stops rotating the wheels resume turning at the same rate and the robot drives straight again.

I based my design off some I found on the net over the last few years: one of the first I saw was Alex’s design at It was in turn based on a design by Leo Dorst at I also examined the design from Denis Cousineau at My design is loosely based on their approaches, modified to use the LEGO Power Functions motors.


I’ve posted step-by-step building instructions for my design and the MLcad file (LDRAW compatible).


Last updated 23 May, 2008

All content © 2008 Mark Crosbie

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