3D-Printed Robot Provides Head Scratches

David McDaid wanted to automate the use a cheap wire-armed scalp scratcher/massager. His initial plans were to use some sort of linear actuator to move the head scratcher up and down on the user’s noggin. But he found himself thinking that if he was going to bother with this project, he might as well go all-out and build something more capable. The result is this Stewart platform head massager than can move with six degrees of freedom.

In mechanical engineering, a “degree of freedom” is an independent motion possible in a system. The rear wheel on a bicycle, for example, has only a single degree of freedom: forward/backward rotation. But the front wheel has two degrees of freedom: forward/backward rotation and rotation along the axis of steering stem. This robot has a whopping six degrees of freedom (6 DoF). It can move linearly in each of the three axes of Cartesian space, but can also rotate in each of those three axes (pitch, roll, and yaw). This gives the robot the ability to move the head scratcher in many different ways and let McDaid setup six distinct patterns that can chain together to give a very thorough massage.

That 6 DoF movement is possible thanks to a 3D-printed Stewart platform and six servo motors. A Stewart platform is a mechanical system that relies on six linear actuators arranged between two plates. With the standard arrangement, programmers can use trigonometry to calculate the exact position of the output plate according to the length of each actuator. In this case, the actuation isn’t truly linear because each actuator is actually a rigid linkage connected to a servo arm. The translation of rotational motion to linear motion, as well as the offset angles, throws off the standard calculations. But it is still plenty accurate for this purpose.

An Arduino Mega 2560 development board controls the servo motors. Those are powerful AX-12A models that have more than enough torque to give scalps a good rubbing. Power comes from a beefy hobby LiPo battery pack. McDaid created a mechanical simulator that let him setup a variety of pre-programmed movements for the motors, so the Arduino doesn’t need to do any heavy processing during runtime.

This project ended up being quite expensive at £340.47 (about $411), but we can’t argue with the results.