Monobot, the single servo robot

thingiverse

Human: How many motors do you need to make a robot which can move in two dimensions? Most robots require at least 2 motors, either left-right differential like a tank or one for going forward and one for steering like cars. There are experiments to achieve steerable robots using just one actuator, notable ones being the 1STAR from UC Berkeley (which has six legs, the middle legs are elastic) and the Monospinner from ETH Zurich (which can even fly), but they require complex and delicate mechanisms and control systems. Introducing Monobot, a single servo robot. This robot consists of only two parts: the base and the foot, which are connected by a servo motor. But how could this robot move? The key is center of mass, friction, and acceleration. The mass of the robot is located on its upper part ('the body') and is offset to the front, near the batteries. When the motor accelerates in one direction, normal force (N) on one side of the foot becomes greater than the other, and since static friction limit (fsmax) is proportional to N, then one side will have more tendency to slip and 'steps' forward. By accelerating left and right repeatedly, the robot will move forward... ideally. If the foot sides have dissimilar friction constants due to imperfect surfaces, the robot will veer off from a straight line. However, you can make the robot turn around in place (which 1STAR cannot) by moving the motor to one direction slowly then suddenly reversing, so the body with larger inertia continues rotating that way, but the foot slips. You can see the demonstration in the video: https://www.youtube.com/watch?v=g27qqC4I8kU Print the base, battery holder, and choose a foot (I used 5 degrees; haven't tried the others, but Blender Rigid Body simulation shows that higher angles give faster movement). Screw the servo horn to the foot using servo mounting screws, insert your preferred controller (I used an Arduino Nano and Bluetooth receiver), 2x 18650 batteries and battery holder, a 5V step-down converter for the servo (I don't like powering servos through the nano's regulator), and there you have it: a robot as simple as can be. I recommend using high-quality servos, preferably metal gear, since this robot will put high stress on the gears, motor, and motor driver from repetitive direction reversing. I haven't broken any servo yet. Friction is crucial here. Slippery PLA produces little movement, so I covered the foot with masking tape. I measured the coefficient of friction on wood; the result is about 0.4. Too high friction causes the robot to jump around or topple. Arduino code included. How could you improve the performance of this robot?