Rotating encoders are key components of motion control feedback loops in a variety of applications, including industrial automation equipment and process control, robotics, medical equipment, energy, aerospace, and more. As a device that converts mechanical motion into electrical signals, encoders provide engineers with basic data such as position, speed, distance and direction to optimize the performance of the entire system.

Optical, magnetic, and capacitive encoder technologies are the three main encoder technologies available to engineers. However, there are a number of factors to consider in determining which technology is most suitable for eventual use. To assist engineers in their selection, this article Outlines three encoder technologies, optical, magnetic, and capacitive, and Outlines the trade-offs of each.

Overview of encoder technology

Optical encoder

Optical encoders have been a popular choice in the motion control application market for many years. It consists of an LED light source (usually an infrared light source) and a photodetector located on either side of the encoder's code disk. The code tray is made of plastic or glass and is spaced with a series of lines or grooves that are transparent and opaque. When the code is circled, the LED light path is blocked by the lines or slots spaced on the code tray, resulting in two typical square wave orthogonal pulses of A and B, which can be used to determine the rotation and speed of the shaft.

Although optical encoders are widely used, they still have several drawbacks. In dusty and dirty environments such as industrial applications, contaminants can accumulate on the code tray, preventing the transmission of LED light to optical sensors. The reliability and accuracy of optical encoders are greatly affected because the contaminated codeplate may cause discontinuity or complete loss of square wave. LEDs have a limited lifespan and will eventually burn out, leading to encoder failure. In addition, glass or plastic codeplates are prone to damage due to vibration or extreme temperatures, thus limiting the scope of application of optical encoders in harsh environments. Assembling it into a motor is time-consuming and carries a greater risk of contamination. Finally, if the optical encoder has a high resolution, it will consume more than 100 mA of current, further affecting its application to mobile devices or battery-powered devices.
Magnetic encoder
A magnetic encoder is similar in structure to an optical encoder, except that it uses a magnetic field instead of a beam of light. The magnetic encoder uses a magnetic code disk instead of a grooved photocode disk with spaced magnetic poles that rotate on an array of Hall effect sensors or reluctance sensors. Any rotation of the code plate causes these sensors to respond, and the resulting signal is transmitted to the signal conditioning front-end circuit to determine the position of the shaft. Compared with optical encoders, magnetic encoders have the advantages of greater durability, vibration and shock resistance. Moreover, the performance of optical encoders is compromised by contaminants such as dust, dirt and oil, while magnetic encoders are not affected, making them ideal for harsh applications.
However, the electromagnetic interference generated by the motor (especially the stepper motor) can have a great impact on the magnetic encoder, and the temperature change can also cause its position shift. In addition, the resolution and accuracy of magnetic encoders are relatively low, far inferior to optical and capacitive encoders in this respect.
Capacitive encoder
Capacitive encoder mainly consists of three parts: rotor, fixed transmitter and fixed receiver. Capacitive induction uses strips or linear patterns with one pole on the fixed element and the other pole on the moving element to form a variable capacitor and to be configured as a pair of receivers/transmitters. The rotor is etched with sinusoidal lines that produce a specific but predictable signal as the motor shaft rotates. The signal is then converted by the encoder's onboard ASIC to calculate the position and rotation direction of the shaft.

Advantages of capacitive encoders
Capacitive encoders work on the same principle as digital vernier calipers, so it provides a solution that overcomes many of the shortcomings of optical and magnetic encoders. The capacitor-based technology used in Cui Devices' AMT line of encoders has proven to be highly reliable and highly accurate. Because no LEDs or stadia are required, capacitive encoders achieve the desired results even in the presence of environmental contaminants such as dust, dirt and oil that can adversely affect optical encoders. In addition, it is less susceptible to vibration and very high/very low temperatures than the glass code plates used in optical encoders. As mentioned earlier, capacitive encoders tend to have longer service life than optical encoders because there is no LED burnout. As a result, capacitive encoders have smaller package sizes and lower current consumption over the entire resolution range of only 6 to 18 mA, making them more suitable for battery-powered applications. Because capacitive technology has higher robustness, accuracy and resolution than magnetic encoders, it is not affected by electromagnetic interference and electrical noise that the latter is faced with.
In addition, the digital nature of capacitive encoders offers key advantages in terms of flexibility and programmability. Because the resolution of an optical or magnetic encoder is determined by the encoder disk, a new encoder is used each time other resolutions are needed, increasing the time and cost of the design and manufacturing process. However, capacitive encoders have a range of programmable resolutions, saving designers the trouble of replacing the encoder every time a new resolution is needed, which not only reduces inventory, but also simplifies fine tuning of the PID control loop and system optimization. Capacitive encoders allow digital alignment and indexing pulse setting when BLDC motor commutations are involved, a task that can be repetitive and time-consuming for optical encoders. Built-in diagnostics provide designers with further access to system data for optimization or field troubleshooting.

Weighing the options
Temperature, vibration and environmental pollutants are important challenges that encoders must address in many motion control applications. It turns out that capacitive encoders can overcome these challenges. Compared to optical or magnetic technologies, it provides designers with reliable, accurate and flexible solutions. In addition, the capacitive encoder adds programmable and diagnostic capabilities, a digital feature that makes it more suitable for modern Internet of Things (IoT) and industrial IoT (IIoT) applications.