Types of Shaft-Angle Encoders

When operating within a motion control system, a servosystem feedback sensor can detect and convert a physical variable into a standard electrical signal that can be read by a motion controller device. These types of monitoring and control units are useful in a wide range of mechanical systems because of their capabilities for regulating and improving performance. Resolvers, linear variable differential transformers, and encoders are some of the common types of feedback sensors used in mechanical applications. Specialized devices, such as linear velocity transducers, potentiometers, and laser interferometers, are less frequently employed in servosystem control networks but still serve the important function of providing operational data.

Shaft-angle devices are types of encoders that rely on the position of a rotating shaft, sometimes in combination with light reflected at a specific optical angle, to supply electrical readings of physical motions. In most cases, the closer a feedback sensor is to the variable it is measuring the more effectively it can support a mechanical system in error correction or repositioning. A direct measurement of linear position along a single axis tends to be more accurate than an indirect measurement based on angular positions and an understanding of system geometry, but these angle-based measurements are also capable of performing under certain conditions and producing complex readings that would not be feasible for other devices.

Rotary Shaft Encoders

Rotary encoders are essentially electromechanical transducers designed to transform mechanical shaft rotations into a series of electrical output pulses, which provide positioning and motion rate data in feedback loops. A standard rotary shaft-angle encoder detects points per revolution, which is the number of discrete positions in a complete shaft rotation and corresponds to the steps per revolution in a stepper motor. Encoder speed is designated by the number of counts taken per second. In addition to taking indirect measurements of the motor shaft or lead screw angle to determine positions, these encoders can also directly measure the responses of rotating machines.

Besides standard configurations, commercial and industrial rotary encoders can also be custom designed to meet specific application requirements, such as functioning in high-stress environments. Rotary encoders are typically packaged in cylindrical cases and can feature detection rates ranging from a few dozen cycles to several million cycles per shaft revolution. The hollow shaft encoder is an alternative to the standard version and helps reduce difficulties with shaft run-out or installation.

Incremental Optical Shaft-Angle Encoders

An incremental optical shaft-angle encoder typically relies on a plastic or glass code disk that is attached to a shaft and rotates between a photodetector unit and an internal light source, such as a light-emitting diode (LED). The disk features a series of uniformly spaced transparent or opaque segments extending from its center. An electronics assembly transmits signals to a motion controller that determines the position and velocity data used for feedback. Higher resolution encoders employ code disks with finer gradations, while sturdier plastic disks are used in applications involving vibration strain.

A quadrature configuration is one of the most common forms of incremental encoders, dividing the light from the LED that passes through the code disk before it reaches the photodetector unit. Output signals are converted into separate pulse channels, with the number of pulses in each channel corresponding to the number of code disk segments that pass the photodetectors during rotation. In a quadrature encoder, the position and direction of rotation are determined by identifying the number of pulses and relative signal phases from each channel.

For more information on quadrature encoding principles, see ProtoTalk.net.

Absolute Optical Shaft-Angle Encoders

Absolute optical shaft-angle encoders rely on multiple light sources and photodetector assemblies, as well as code disks with segment patterns arranged as a series of annular rings. This code disk variant provides an absolute measurement by using a binary output to represent each individual shaft angle as the disk rotates between a radial arrangement of photodetectors and a linear sequence of light sources. The length of the rotational arcs for both opaque and transparent segments on the disk decrease based on their radial distance from the shaft.

The accuracy of the shaft position generally depends on the quantity of the disk’s annular rings, and as the disk rotates the light that passes through each ring produces a continuous signal stream. The electronics array then converts this stream of output into a binary code. Unlike incremental encoders, absolute encoders have code disks that retain the last angular position of the shaft from the point at which it stopped moving, even if the system shuts down. This provides a fail-safe by preserving data in case of power failure or malfunctions.