POWER TRANSMISSION REFERENCE GUIDE
Encoders — the
I
n motion systems, encoders provide information on a number of parameters including position, distance, and speed. They can be classified a number of ways including as rotary or linear, incremental or absolute, or according to their operating principle as optical, magnetic, or capacitive. The most important performance parameter for encoders is resolution. For incremental encoders, resolution is typically specified in pulses per revolution (PPR), or, in the case of linear encoders, pulses per inch (PPI) or pulses per millimeter (PPM). These square-wave pulses are precisely spaced, and the encoder determines its position by counting the number of pulses generated during a movement. Incremental encoders generally supply square-wave signals in two channels, A and B, which are offset (or out-of-phase) by 90
basics
degrees and help determine direction of rotation. The output signals of an incremental encoder only have information on relative position not absolute position. In order for the encoder to provide any useful position information, the position of the encoder has to be referenced in some way, traditionally using an index pulse. So the incremental encoder sends incremental position changes to electronic circuits that perform the counting function. In contrast, absolute encoders have a unique code for each shaft position. The encoder interprets a system of coded tracks to create position information where no two positions are identical. Another feature is that absolute encoders do not lose position when power is switched off. Because each position is distinctive, the verification of true position is available as soon as power is switched on without the need for a homing routine. Encoders for industrial uses typically are either optical or magnetic. While optical encoders were, in the past, the primary choice for highresolution applications, improvements in magnetic encoder technology now
allow them to achieve resolutions down to one micron, competing with optical technology in many applications. Magnetic technology is also, in many ways, more robust than optical technology, making magnetic encoders a common choice in industrial environments. Then there are capacitive encoders, a relatively new introduction. They offer resolution comparable to optical devices, with the ruggedness of magnetic encoders. Currently, there are only a handful of vendors for capacitive encoders, but their suitability for applications requiring high precision and durability make them a good choice for the semiconductor, electronics, medical, and defense industries. Magnetic rotary encoders rely on three main components: a disk, sensors, and a conditioning circuit. The disk is magnetized, with a number of poles around its circumference. Sensors detect the change in magnetic field as the disk rotates and convert this information to a sine wave. The sensors can be Hall effect devices, which sense a change in voltage, or magnetoresistive devices, which sense a change in magnetic field. The conditioning circuit multiplies, divides, or interpolates the signal to produce the desired output.
HEIDENHAIN’s RCN 6000 series absolute sealed angle encoders use the company’s METALLUR process in which the graduation is applied directly to the bearing ring and uses a reflected light scanning method which gives it compact dimensions. The graduations consist of lines and gaps at defined intervals with minimal deviation, forming structures with high edge definition and making them resistant to mechanical and chemical influences as well as to vibration and shock.
26
DESIGN WORLD — MOTION
Encoders— Power Transmission HB 05.19_v2.indd 26
5 • 2019
F w
EP • • • • • •
Ca
W
motioncontroltips.com | designworldonline.com
5/14/19 8:02 AM
1
