Many motion systems require the use of an encoder to determine position, speed or direction. But with all the different variations and types of encoders on the market, the designer or engineer may have a hard time choosing the correct fit for the application. The purpose of this guide is to help explain the different types of encoders and where each type is applied in typical motion applications. So let us start with the basics to get the most out of your applications: incremental vs. absolute encoders.
What is an Encoder?
An encoder is a sensor of mechanical motion that generates digital signals in response to motion. As an electro-mechanical device, an encoder is able to provide motion control system users with information concerning position, velocity and direction.
There are two different types of encoders: linear and rotary. A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion. The construction of these two types of encoders is quite similar; however, they differ in physical properties and the interpretation of movement. In addition, each type of encoder can be generally categorized by the means of its output, either incremental or absolute. An incremental encoder generates a train of pulses which can be used to determine position and speed. An absolute encoder generates unique bit configurations to track positions directly.
The type of output from an encoder can be produced using several different technologies. The most common technology used in encoders is optical index counting for measurement, but some encoders use mechanical or even magnetic technology.
How Does an Optical Rotary Incremental Encoder Work?
Incremental rotary encoders utilize a transparent disk which contains opaque sections that are equally spaced to determine movement. A light emitting diode or LED light is used to pass through the glass disk and is detected by a photo detector. This causes the encoder to generate a train of equally spaced pulses as it rotates. The output of incremental rotary encoders is measured in pulses per revolution which is used to keep track of position or determine speed and/or direction. See Figure 1:
Figure 1 – Pulse Train Produced from Optical Incremental encoder
A single-channel or uni-polar output is commonly implemented in applications where position or the direction of movement is not significant. Instances in when direction sensing is required, a 2-channel or bi-polar output should be used. The two channels, typically denoted as A and B, are commonly 90 electrical degrees out of phase and the electronic components determine the direction based off the phase relationship between the two channels. The position of an incremental encoder is done by adding up all the pulses by a counter.
One of the disadvantages of the incremental encoder is count loss, which occurs during a power loss or machine shutdown. When restarting, the equipment must be referenced to a home position to reinitialize the counter. However, some incremental encoders come equipped with a third channel called the index channel. The index channel, typically denoted as Z, produces a single signal pulse per revolution of the encoder shaft and is often used as a reference marker. The reference marker is then denoted as a starting position which can resume counting or position tracking.
How Does an Optical Rotary Absolute Encoder Work?
An absolute encoder contains similar components also found in incremental encoders. They implement a photodetector and LED light source but instead of a disk with evenly spaced lines on a disc, an absolute encoder uses a disk with concentric circle patterns with multiple tracks.
Absolute encoders utilize stationary mask in between the photodetector and the encoder disk as shown in Figure 2. The output signal generated from an absolute encoder is in digital bits which correspond to a unique position. The bit configuration is produced by the light which is received by the photodetector when the disk rotates. The light configuration received is translated into gray code. As a result, each position has its own unique bit configuration.
Figure 2 – Components of an Optical Absolute Rotary Encoder
Unlike incremental encoders absolute encoders can maintain count information during a power loss or machine shutdown. This eliminates the need to “rehome” the equipment on initial startup.
Now that we've covered the basics of an encoder and examined incremental vs. absolute encoders, we're ready to look at some of the more rare encoders and discuss the best applications for each.
Stay tuned for Part 2 of the Easy Encoder Guide!
In the meantime, if you have any questions about using encoders or you're seeking consultation for your next project, contact our experienced staff for a free consultation.
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