Types of Rotary Encoders
Just as there are many methods of commutating current to a motor, there are many types of encoders that can perform that task.
Optical encoders with commutation output currently dominate the market, most often used in precision applications and built in to electronic devices to control motion.
- Generates output code using infrared light and phototransistor
- The most common type of encoder available
- Most often used in precision applications and built in to electronic devices to control motion
Magnetic encoders with commutation output are often used in applications where there are extreme temperatures, high humidity or exposure to particulates or liquids.
- Generates output code by detecting changes in magnetic flux fields
- Most often used in adverse environments
- Resistant to most airborne contaminants
Fiber optic commutation encoders are sometimes called 'explosion proof' and are used in applications where methane, propane, or other highly combustible gases are present.
- Generates output code by using a laser and phototransistor
- Most often used in explosion-proof applications where extremely flammable gasses are present
The AMT31 and AMT33 are not recommended for explosion-proof applications but can withstand similar environmental factors as magnetic encoders and generally outperform optical encoders thanks to their proprietary capacitive technology.
- Generates output code through detecting changes in capacitance using a high frequency reference signal
- Relatively new compared to the other types listed
- Technology has been used for years in digital calipers and has proven to be highly reliable and accurate
How a Capacitive Encoder Works
The AMT series consists of three basic parts as shown in the image. The ac field transmitter emits a signal that is modulated by the metal pattern on the rotor as it turns. The sinusoidal metal pattern on the rotor creates a signal modulation that is repetitive and predictable. This occurs as a result of varying capacitive reactance between the signal generated by the transmitter and the metal on the rotor. The field receiver uses a proprietary ASIC to convert the modulated signal into output pulses that can be read by the same circuits used to receive optical encoder output.
If you have ever used digital calipers, you are already familiar with capacitive encoding. The code generation used in digital calipers for decades is the same technology built into the AMT. This capacitive code-generation technology has been shown to be reliable, accurate, economical and rugged enough to outlast other types of optical encoders.