If Colin is going to create a map of the space he’s in, he needs to be able to find the distance between objects in the space. If he’s going to accurately control his own motion, he needs to be able to control the speed of his motors. For both of these things he needs information about how far and how fast his motors are turning. To give Colin that information I added encoders to his motors. Encoders are special sensors that track both how far his motor shafts have turned, and in what direction.
The following tutorial will discuss how to use shaft encoders with DC motors. First I’ll discuss the principles the encoders work on, then I’ll show you how to wire up an encoder and verify that it’s working. This tutorial owes a large dept to the Penn State Robotics Club Wiki, which has an excellent page on wheel encoders.
For this tutorial I’ll be using these magnetic encoders from Pololu. They are great for use with an Arduino because their output is a nice, easy to read, digital square wave. Pololu also offers an analog optical encoder, but to reliably read its output you need to use a ADC (analog to digital converter). Also, the optical encoder is susceptible to interference from bright light. So magnetic encoders are easier to use and more reliable for our purposes.
Encoder Principles
shaft encoders add a 6 pole magnetic disc to the shaft of the motor, as well as two Hall effect sensors. As the motor turns the disc rotates past the sensors. Each time a magnetic pole passes a sensor, the encoder outputs a digital pulse, also called a “tick.” The encoder setup is pictured below.
The encoder has two outputs, one for each Hall effect sensor. The sensors are separated by 90 degrees. This means the square wave outputs of the sensors are 90 degrees out of phase. This is called a quadrature output. The picture below (taken from Pololu’s website) shows the typical output of an encoder.
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