Weighing the Advantages and Tradeoffs of Encoder Technologies
Rotary encoders are key components in the motion-control feedback loop of a variety of applications, including industrial automation equipment and process control, robotics, medical devices, energy, aerospace, and more. As devices that translate mechanical motion into electrical signals, encoders provide engineers with essential data, such as position, speed, distance, and direction that can be used to optimize the performance of their overall system.
Optical, magnetic, and capacitive represent the three main encoder technologies at an engineer’s disposal, but deciding on which technology will best serve the end application comes with several considerations. To aid in this selection process, this article will provide an overview of optical, magnetic, and capacitive encoder technology, while outlining the advantages and tradeoffs of each.
Encoder technology overview
Optical encoders have been the popular choice in the motion control market for many years. They are comprised of an LED light source (typically infrared) and photodetectors placed on opposite sides of an encoder disk. This disk is constructed of plastic or glass and contains a series of alternating transparent and opaque lines or slots. During the rotation of the disk, the LED light path is interrupted by the alternating lines or slots on the disk, which in turn produces the typical square wave A & B quadrature pulses used to determine shaft rotation and speed.
Figure 1: Typical A & B quadrature pulses of an optical encoder, including an index pulse (Image source: CUI Devices)
Although widely used, optical encoders do suffer from several drawbacks. In dusty and dirty environments, such as industrial applications, contaminants can build up on the disk and prevent the LED light from passing through to the optical sensor. This greatly impacts the reliability and accuracy of optical encoders as the contaminated disk can cause square pulses to be sporadic or missed completely. LEDs also have limited lifespans and will eventually burn out, leading to encoder failure. In addition, the glass or plastic disk is prone to damage from vibrations or extreme temperatures, limiting its useful range in rugged applications, while its assembly onto motors can be time consuming and open it up to more risk of contamination. Finally, at higher resolutions optical encoders can consume current upwards of 100 mA, further impacting their usefulness in mobile or battery-powered devices.
Similar in structure to optical encoders, magnetic encoders utilize a magnetic field rather than a beam of light. In place of the slotted optical wheel, magnetic encoders have a magnetized disk with alternating poles which spins over an array of hall-effect or magneto-resistive sensors. Any rotation of the wheel produces a response in these sensors, which goes to a signal-conditioning front-end circuit to determine shaft position. Magnetic encoders hold the advantage of being much more durable and resistant to shock and vibration over optical encoders. Where optical encoders also struggle with contaminants such as dust, dirt, and oil, magnetic encoders are unaffected, making them well suited for harsh environments.
However, magnetic encoders are greatly impacted by magnetic interference caused by electric motors, most notably stepper motors, and suffer position drift due to changes in temperature. They also fall short of optical and capacitive alternatives due to their relatively lower resolution and accuracy.
The three main components of a capacitive encoder are a rotor, stationary transmitter, and stationary receiver. Capacitive sensing uses patterns of bars or lines, with one set on the fixed element and the other set on the moving element, to form a variable capacitor configured as a transmitter/receiver pairing. The movement of the rotor and its sinusoidal pattern attached to the motor shaft produce a unique but predictable signal that is interpreted by the encoder’s on-board ASIC to calculate the position of the shaft and direction of rotation.
Figure 2: A comparison of encoder disks (Image source: CUI Devices)
Capacitive encoder benefits
Capacitive encoders are adapted from the same principles used to develop the digital Vernier caliper, resulting in a solution that overcomes many of the shortcomings of optical and magnetic encoders. CUI Devices’ AMT encoder series implements this capacitive-based technology which has proven to offer high reliability and accuracy. With no need for LEDs or line of sight, capacitive encoders perform as expected even when presented with environmental contaminants such as dust, dirt, and oil that negatively impact optical encoders. They are also less susceptible to vibration than an optical encoder’s glass disk as well as hot or cold temperature extremes. As mentioned earlier, with no LED to burn out, capacitive encoders have a longer lifespan than their optical counterparts. This further results in a smaller package size and lower current consumption of 6 to 18 mA across their entire resolution range, making them better suited for battery-powered applications. Magnetic interference and electrical noise that present problems for magnetic encoders are also less of an issue for capacitive technology, giving them added ruggedness compared to magnetic encoders as well as improved accuracy and high resolution.
Flexibility and programmability are additional key benefits afforded by the digital nature of capacitive encoders. Because an optical or magnetic encoder’s resolution is determined by the encoder disk, a new encoder must be used each time another resolution is needed. This can add both time and cost during the design and manufacturing process. With a range of programmable resolutions, capacitive encoders eliminate a designer’s need to replace the encoder each time a new resolution is needed, which reduces inventory holding and simplifies the fine tuning of a PID control loop and system optimization. When it comes to BLDC commutation, capacitive encoders allow for digital alignment and the setting of an index pulse, which can be an iterative and time-consuming task for optical encoders. Built-in diagnostic capabilities give designers further access to system data used for optimization or troubleshooting in the field.
Figure 3: Comparing key performance indicators of capacitive, optical, and magnetic technology (Image source: CUI Devices)
Weighing the options
Temperature, vibration, and environmental contaminants are important factors that an encoder must deal with in many motion control applications. Capacitive encoders have proven to overcome these challenges, providing designers with a reliable, accurate, and flexible solution compared to optical or magnetic technology. Their digital nature also brings capacitive encoders into the modern age of Internet of Things (IoT) and Industrial IoT (IIoT) applications with added programmability and diagnostic capabilities.
Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.