Encoders are common in motion control products, and rotary encoders are key components of motion control feedback loops, including industrial automation equipment and process control, robotics, medical equipment, energy, aerospace, etc.
As devices that convert mechanical motion into electrical signals, encoders provide engineers with basic data such as position, speed, distance and direction that can be used to optimize the performance of the overall system.
Optical, magnetic, and capacitive are the three main encoder technologies available to engineers. There are a number of factors to consider, though, to determine which technology is best for eventual application.
This article will give an overview of optical, magnetic and capacitive encoder technologies, and briefly discuss the advantages and disadvantages of each technology.
1. 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 infrared light source) and a photodetector, which are located on both sides of the encoder code plate.
The code plate is made of plastic or glass, with a series of transparent and opaque lines or slots arranged at intervals. When the code disk rotates, the LED optical path is blocked by the lines or slots arranged at intervals on the code disk, thus generating two typical square wave A and B orthogonal pulses, which can be used to determine the rotation and speed of the axis.
Technical analysis of optical, magnetic and capacitive encoders
Figure 1: Typical A and B orthogonal pulses for optical encoders, including index pulses (Photo credit: CUI Devices)
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 plate, thus blocking the transmission of LED light to the optical sensor.
The reliability and accuracy of optical encoder are greatly affected because the contaminated code disk may lead to the discontinuity or complete loss of square wave.
Leds have a limited service life and will eventually burn out, leading to encoder failure. In addition, glass or plastic code disks are prone to damage due to vibration or extreme temperatures, thus limiting the applicability of optical encoders in harsh environment applications; Assembling it into a motor is not only time-consuming, but also carries a greater risk of contamination.
Finally, if the resolution of the optical encoder is high, it will consume more than 100 mA of current, further affecting its application in mobile or battery-powered devices.
2. Magnetic encoder
Magnetic encoders are similar in structure to optical encoders, but use a magnetic field rather than a light beam. Magnetic encoders replace slotted optical code disks with magnetic code disks with spaced magnetic poles that rotate on a row of Hall effect sensors or reluctance sensors.
Any rotation of the code plate will cause these sensors to respond, and the resulting signal will be transmitted to the signal conditioning front-end circuit to determine the position of the shaft.
Compared to optical encoders, magnetic encoders have the advantage of being more durable, resistant to vibration and impact. Moreover, the performance of optical encoders is greatly compromised in the case of contaminants such as dust, dirt and oil stains, while magnetic encoders are not affected, making them ideal for harsh environment applications.
However, electromagnetic interference generated by motors (especially stepper motors) will have a great impact on the magnetic encoder, and temperature changes will also cause its position drift.
In addition, the resolution and accuracy of magnetic encoders are relatively low and are far less than optical and capacitive encoders in this respect.
3. Capacitive encoder
The capacitive encoder consists of three main parts: rotor, fixed transmitter and fixed receiver. Capacitive sensing uses a strip or linear pattern with one pole on a fixed element and the OTHER pole on a MOVING element to form a variable capacitor configured as a pair of receivers/transmitters.
The rotor is etched with a sine-wave pattern that produces a specific but predictable signal as the motor shaft rotates. This signal is then converted by the encoder's onboard ASIC to calculate the position and rotation direction of the axis.
Technical analysis of optical, magnetic and capacitive encoders
Figure 2: Comparison of encoder disks (Photo credit: CUI Devices)
4. Capacitive encoder
The Capacitive encoder works on the same principle as the digital vernier caliper, so it provides a solution that overcomes many of the disadvantages of optical and magnetic encoders.
The capacitance-based technology used in CUI Devices' line of AMT encoders has proven to be highly reliable and highly accurate.
Since no LED or line-of sight is required, capacitive encoders can achieve the desired results even when encountering environmental contaminants that can adversely affect optical encoders, such as dust, dirt, and oil stains.
In addition, it is less susceptible to vibration and extremely high/low temperatures than the glass code disks used in optical encoders.
As mentioned earlier, capacitive encoders tend to have a longer service life than optical encoders because leds do not burn out.
As a result, the capacitive encoder has a smaller package size and consumes less current over the entire resolution range of only 6 to 18 mA, making it more suitable for battery-powered applications.
Since the robustness, accuracy and resolution of the capacitive technology are higher than those of the magnetic encoder, the electromagnetic interference and electrical noise faced by the latter do not have a great impact on it.
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 plate, a new encoder is used each time other resolutions are required, resulting in an increase in the time and cost of the design and manufacturing process.
However, capacitative 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 PID control loop fine-tuning and system optimization.
Capacitive encoders allow digital alignment and indexing of pulse Settings when BLDC motor commences are involved, a task that can be repetitive and time-consuming for optical encoders.
Built-in diagnostic capabilities give designers further access to system data to optimize the system or troubleshoot in the field.
Technical analysis of optical, magnetic and capacitive encoders
Figure 3: Comparison of key performance indicators for capacitive, optical, and magnetic technologies (Photo credit: CUI Devices)
5. Weigh your options
In many motion control applications, temperature, vibration, and environmental contaminants are important challenge factors that encoders must deal with. It turns out that capacitive encoders can overcome these challenges.
Compared to optical or magnetic technologies, it provides designers with reliable, precise and flexible solutions.
Moreover, capacitive encoders add programmability and diagnostics, a digital feature that makes them more suitable for modern Internet of Things (IoT) and Industrial Internet of Things (IIoT) applications.