Encoders provide precise position feedback for both rotary and linear servo motor control, as well as accurate position information for applications. Rotary encoders are truly at the heart of high-performance automation. This article offers a detailed insight into the working principles of rotary encoders.
Rotary encoder technologies include optical, magnetic, capacitive, and inductive. The optical encoder is preferred for applications requiring the highest resolution and accuracy. Magnetic and inductive encoders excel in harsher environments. The primary focus in this article is the rotary optical encoder.
Celera Motion has developed a suite of precision rotary encoders for cutting-edge applications. Miniature components are employed to meet the needs of the smallest electromechanical systems. The latest generation is the Aura™ Series.
As mentioned, there are different types of rotary encoder technologies. The optical rotary encoder is preferred for applications requiring the highest resolution, accuracy, and repeatability. Magnetic and inductive rotary encoders excel in harsher environments, but magnetic encoders are sensitive to external magnetic fields and can suffer from accuracy drift over the specified temperature range. The remainder of this article narrows the focus to the optical rotary encoders.
Rotary encoders can be incremental or absolute devices. Incremental encoders generate position change information only. An additional index/marker signal defines zero position, which is detected during a homing routine. Absolute devices provide actual physical position – eliminating time-consuming homing, which requires movement to locate the index.
Incremental rotary encoders employ optical scanning of a rotary scale. The scale is made of reflective and non-reflective lines of precisely equal width. Incident light from an LED creates a projection of the scale (light/dark lines) which can be detected by a photoelectric sensor. As the scale moves relative to the sensor, the lines can be counted to provide incremental position information.
Incremental Rotary Scale
The number of reflective lines on the scale corresponds to the resolution of the rotary encoder. With careful design of sensor geometry, however, the variation in light intensity from line to line appears sinusoidal. This enables the generation of sinusoidal signals which can be interpolated to much higher resolution.
Absolute rotary encoders also typically incorporate an incremental scale. To determine absolute position, an additional pseudo-random pattern of reflective lines is illuminated and projected to a second sensor. This essentially creates a barcode which, at startup, is used to identify a specific line on the incremental track i.e. the rotary encoder is now reading line 128 which is at exactly 43.5°.
The interface to an incremental rotary encoder is known as ABZ. A and B are square waves, phase shifted by 90°. Whether A leads B or B leads A indicates the direction of motion. The controller counts transitions on the AB signals. The Z signal is the index signal or zero reference. Absolute rotary encoders typically have a high speed synchronous serial interface such as the open standard BiSS-C.
A modular rotary encoder solution requires that the scale disc be mounted to a hub. Ensuring the scale centre is concentric with the axis of rotation is critical to minimize angular error. Eccentricity (difference in centres of rotation) can have a significant impact on angular error. As can be seen from the following formula, the error is magnified for smaller scale discs.
Angular error = arctan (eccentricity-error/radius) degrees
For high performance applications, an eccentricity of less than 25 microns is preferred. This can be challenging and two readheads are sometimes employed to average out the error.
The sensor in the readhead must also be aligned correctly relative to the scale. Wide alignment tolerances can significantly reduce installation time, reducing production cost.
Celera Motion Aura™ Series rotary encoders provide 18 to 22 bit resolution, corresponding to as many as 4,194,304 discrete positions per rotation. Accuracy is ± 0.01 degree. The small format encoder is also easy to install with wide alignment tolerances. This Celera Motion rotary encoder delivers advanced features including scale eccentricity compensation, eliminating the potential need for two averaging readheads.
Looking for rotary encoders?
Aura™ Series rotary encoders consume minimal power. Comprehensive connectivity includes low-latency BiSS-C as well as SSI and SPI. An incremental ABZ output with configurable resolution adds additional interfacing flexibility.
Want to learn more? Contact a member of the Celera Motion team today to learn about our new flagship absolute rotary encoder.