HDD laser steering
Brian Quist and Mark Locascio took apart two computer hard drives and used the read/write actuator arms to control two mirrors, to steer a laser beam in two dimensions.

 

Under computer control the mirrors can direct the laser beam through a solid angle. Here the beam traces out a triangle.

The read/write arm of the disk drive is blurred in this photo as it moves. The arm carries a mirror. The laser beam reflects off this mirror, and then another one which controls its second angle.

The arm also carries a magnet. A Hall effect sensor senses the arm's angle, and a small coil produces a voltage proprtional to its velocity.

PID control and actuator driver circuit diagram.

Sensor inputs are the angle of the mirror (from the analog hall sensor) and the angular velocity of the mirror (from the coil)

Response of the mirror (red) to a square wave input command (blue). The vertical axis is hall voltage, corresponding to mirror angle.

m files, circuit diagrams, and drawings