Servo - July 2010

THIS MONTH:

I Am Encoding (And So Can You!)

THE POSITION CONTROLLER FROM PARALLAX.

This month, we have the pleasure of presenting the Position Controller Kit from Parallax. In the tradition of Parallax products like the Scribbler robot, the position controller is easy to implement, a breeze to program, and is versatile enough to keep novice and expert roboticists engaged and entertained. We thought a good way to test the module’s accessibility was to implement it with the classically hackable Scribbler in a project that could appeal to even less experienced members of the SERVO Nation.

The Run Down

The position controller is designed to take over wheel operation functions that would otherwise require the attention of the robot’s microcontroller. The position controller can impressively be interfaced with any microcontroller, and it can also be used in conjunction with HB- 25 motor controllers to help control 12 VDC motors. But even without acting as a motor controller, the position controller can perform a host of useful functions. The most apparent use without the motor controller is to use the position controller as an encoder to measure speed and distance.

The position controller uses optical interrupter switches to sense the position of a handy encoder wheel that comes with the kit. The encoder wheel has nine fins, but because the optical interrupter switches make measurements at four positions, it can sense 36 positions for one rotation of the wheel. The documentation for the kit says that this corresponds to about 0.5 inches of travel per position reading using a six inch wheel. For young hackers wondering how the good folks at Parallax came up with that number, they can be introduced to the immortal equation of s = rθ.