Optical Encoder

My most recent project was spurred from a lecture in my electronics design lab class. The goal of the class is to create a autonomous robot that preforms a unique function.  The robot consists of two driving wheels and a castor stabilizer wheel in the rear. Autonomy would be a relatively simple task for a micro controller. But there's a catch, in this class we must construct our own circuitry from the ground up. A type of component we are given are optical encoders, one for each wheel, which are used for speed and distance calculations.

Simply put, an optical encoder is a device that converts the angular position or motion of a shaft or axle to digital code. As shown in the figure below, a light source is shone through a rotating disk with a code track. If the disk is in a position such that the light source can shine through a code track hole, the photodetector puts out a high or low voltage, depending on the circuit.

http://zone.ni.com/cms/images/devzone/tut/b/82f98d1a421.gif

As the disk revolves, we start to see a square wave form. Note that a 50% duty cycle waveform, or a waveform that is on for 50% of the time, is produced only when the the code track has evenly spaced holes of equal parts hole and blocking material.

This sort of device seemed simple enough to replicate, so a week after that lecture, I sat down with some cardboard, a DC hobby motor, a bright red LED, and a photoresistor, to see what I could come up with.

As a prototype, I created a very basic cardboard box and disc. After connecting the disk to a DC hobby motor and running at high RPM, I foresaw an issue. Due to the inaccuracy of cutting with scissors, the center of mass of the disk was not at the axle. This resulted in a very unstable spin. Complications also occurred when testing the bright red LED and photoresistor.  When I flashed the LED at the photoresistor and probed the output, the flash was indistinguishable from the ambient light in my dorm room. At least, it wasn't giving me the nice square wave I was looking for.

The solution was to use an infrared emitter and receiver pair. To mediate the center of mass problem of the disk, I choose to construct a box and disk using a laser cutter, which is far more accurate than scissors.

Once constructed, the laser cut disk and IR emitter/reciever system worked beautifully, producing the pulse wave I was looking for. In my finished encoder, I only used four holes approximately 5mm wide at 0, 90,180, and 270 degrees. This produced a 90% duty cycle.

While this project was simple enough, it took a bit of time to work out some of the kinks. The end product was worth the effort, and I can now make calculations to determine the angular speed, frequency, and inertia of the disk. At least to me, this is quite exciting.

If you have any suggestions on how I might improve this design, please fell free to comment below!

Also, there are reasons we have web-controlled power supplies. One of those reasons is attached below.

-John "I was scared for my life filming this" Dunn