If your project requires an encoder, you’ll need to choose one that meets your needs. There is an incredible selection of encoders available for you to choose from, but if you don’t know what you’re looking for, there’s a good chance you’ll get something sub-optimal. Let’s run through what’s out there and maybe expose you to a few new options.
Types of encoder classes
As you shop for an encoder, your first question should be: What do I need my rotary encoder to do? Every encoder on the market measures rotation, but they do so in various ways. Here’s a quick guide to encoder classes:
- Tachometer: These are the most basic type of encoder. Tachometers indicate how far the device has turned but doesn’t track direction. These encoders use on/off signals to indicate that the shaft has turned a certain angle. In the real world, you can best recognize a tachometer in its application in a bike speedometer.
- Incremental (quadrature): Like a tachometer, incremental encoders pulse out signals as a wheel turns a certain angular distance. Unlike a tachometer, it outputs two separate signals in a quadrature arrangement so that it can imply both the distance and direction a shaft turns.
- Incremental with Extras: In addition to telling the direction and distance of rotation, some incremental encoders also feature an indexing feature at one point in the rotation. The indexing feature allows the shaft to come back to a known point. Other encoders have a button input, adding a new user interface.
- Absolute Encoder: For the most part, incremental encoders don’t inherently “know” a shaft position (except an index point). Absolute encoders use internal sensing to measure the angular position directly and maintain this measurement ability even if the power to it is turned off.
- Absolute Encoder Multi-Turn: In addition to knowing the shaft position, multi-turn absolute encoders can also determine how many turns an encoder has made in one direction or the other.
Learn when to use an absolute vs incremental encoder for your project.
Encoder sensing methods
Depending on the application you are planning for an encoder type, you may be interested in the principle behind angular sensing. This principle may affect the price or how well the encoder can handle external interference or contamination.
- Conductive: These encoders use a series of conductive pads. Incremental encoders’ pads transmit pulses in the case of incremental encoders, while absolute encoders’ pads position data.
- Optical: For optical sensing, encoders feature a light that is incrementally interrupted by a disk or other means attached to the shaft. This light transmits pulses for incremental encoders and transmits position data for absolute encoders.
- Magnetic: A magnet or series of magnets transmit rotational information.
- Capacitive: This newer technology senses the repeatable pattern of capacitance in a rotor setup.
Learn more about capacitive, magnetic, and optical encoders.
Encoder capabilities: Stats and extra features
Among rotation encoders that are of the same class and sensing method, there can be a wide disparity between capabilities. Encoder prices often reflect these disparities. Here’s a guide to these extra features:
- Resolution: The number of pulses or positions of an encoder per revolution. For absolute encoders, resolution measures the number of turns the device can record before “rolling” over to a zero position.
- Speed Capability: Encoders have a maximum mechanical speed in revolutions per minute, as well as a maximum frequency response of the electronics. You must account for both in your design, along with the frequency response capabilities of the reading electronics.
- Ruggedness: While many encoders require relatively clean environments, others come “hardened” for industrial use. This ruggedness can relate to the sensing method or the device’s seal, and you’ll need to consider your environment before choosing an encoder.
- Output Type: Encoders nominally output binary signals, but onboard electronics or attached boards may allow them to communicate more conveniently. Additional output types are protocols like I2C, as well as Ethernet/IP or Profinet in industrial applications.
- Manufacturer: While it’s possible to find a high-quality component from unknown suppliers, choosing an established manufacturer can be reassuring in critical applications. The cost for these devices may be higher, but the investment will bring you peace of mind.
The wealth of encoders on the market may be intimidating, but don’t be overwhelmed. Selecting the right part for your application can give you years of operation with minimal intervention.
Special thanks to John Davis of 3ML LLC—makers of the Rhubarb Raspberry Pi industrial interface board—for his input on this article.
