Position Sensing: Encoders & Linearization
High performance BLDC motors require position sensing for accurate control and efficient commutation. In other words, if the controller knows exactly where the motor is in its rotation (or relative to its initial position), the controller can apply the correct currents to achieve the desired speed, position, and/or trajectories. Position sensors, or encoders, come in several varieties and each have their own strengths and weaknesses. When selecting an encoder, it is important to consider the design implications, inherent encoder error, durability and cost. In this post, we will discuss a couple of the most popular encoders, including optical encoders and magnetic encoders.
Optical Encoders:
Optical encoders use the interruption of light to translate motor position into electrical signals. These types of encoders are made up of a reflective rotating disc with non-reflective, evenly spaced lines, a light source, and a light detector. The light source and light detector are typically located next to each other, so when the light shines on the reflective surface of the disc, the light detector “sees” it after the light bounces off the surface and returns. However, when the disc rotates and the light shines on a non-reflective line, the detector will see no light. These on/off readings are converted into digital square wave quadrature output signals. By knowing the number of lines on disc (which correspond to the “pulses per rotation”), the controller can translate the encoder output signal into position and speed.
Depending on the resolution, optical encoders can be highly accurate and precise, and they typically have very low error, making them ideal for high precision applications. Unfortunately, dirt, dust, condensation, and other contaminants can compromise the line of sight between the light source and detector, which will result in incorrect signals. To ensure robust functionality in non-ideal environments, many optical encoder manufacturers have designed solutions to seal the mechanism from environmental hazards. While this prevents line of sight failures, it adds mass and increases price. Many industrial servo motors use integrated optical encoders for position sensing, and these optical encoders are one of the main reasons why industrial servos are usually bulky and expensive.
Magnetic Encoders:
Magnetic encoders measure changes in magnetic field to read motor position. These types of encoders are made up of a magnetized component and a chip that has magnetic sensors and a conditioning circuit. The magnetized component can be a disc or simple diametric magnet, and it must be positioned near (usually directly above) the chip. As the magnetic component moves (either rotationally or linearly) the chip’s sensors, which can be Hall effect devices or magnetoresistive devices, detect the change in magnetic field. The conditioning circuit then takes that information to produce sine waves, which the controller can translate into position and speed.
The two primary advantages of magnetic encoders are their cost effectiveness and robustness. Magnetic encoders tend to be significantly (typically more than 50%) less expensive than optical encoders of similar resolution. They are also not affected by the contaminants that hinder the performance of optical encoders, making them more robust in harsh environments. Unfortunately, there are a few downsides to magnetic encoders that must be considered. First are the design complications. The distance between the chip and magnetic component must be just right, and any ferrous material near the sensor can manipulate the magnetic field, thus impacting position readings. Second, magnetic encoders experience more error than optical encoders of similar resolution. In other words, they aren’t as accurate or precise, which makes them worse for high precision applications.
Vertiq’s Experience and Calibration:
The magnetic rotary encoder has been an integral part of Vertiq’s products from the beginning. Their compact design and price point was conducive to building our earliest prototypes, and we still believe this component will enable us to build high performance, compact, and affordable motors and controllers that are the future of industrial automation and robotics.
One problem we had early on, however, was positional accuracy and precision. When comparing our solution to competitors using optical encoders, we couldn’t quite match their positioning abilities. So, we developed a calibration technique that would reduce magnetic encoder error. In doing so, we can make a magnetic encoder have less error than an optical encoder of similar resolution. We call this calibration process "Linearization." Below is a graph that shows Vertiq-calibrated magnetic encoders outperform optical encoders by 63%.
With the design lessons learned from early prototypes and our calibration technology, it is clear that magnetic encoders will be a vital component in Vertiq’s products going forward!
