MOTION SYSTEMS HANDBOOK
Fundamentals of encoders for motion control Encoders can be classified in several different ways – by the sensing method, whether the output is absolute or incremental, as well as by some common performance parameters. They’re also commonly divided into rotary (the more common) or linear types.
SENSING METHODS The most common sensing methods used by encoders include optical, magnetic, and capacitive sensing. Optical encoders use a light source and a photo-detector to determine position, but their use of light makes them sensitive to dirt and debris, which can disrupt the signal. Optical encoder performance is influenced by the gap between the sensor and the scale, which must be properly set and maintained to ensure that signal integrity isn’t compromised. This means that mounting must be done carefully, and shocks and vibrations should be avoided. Optical encoders have historically been the only option for resolutions below 5 microns.
However, improvements in magnetic scale technology now allow them to achieve resolutions down to 1 micron. Magnetic encoders use a magnetic reader head and a magnetic scale to determine position. Unlike optical encoders, magnetic encoders are mostly unaffected by dirt, debris, or liquid contamination. Shock and vibration are also less likely to affect magnetic encoders. However, they are sensitive to metallic chips, such as steel or iron, as they may interfere with the magnetic field. Capacitive encoders offer resolution comparable to optical devices, with the ruggedness of magnetic encoders. The basic principle behind capacitive encoders is that they detect changes in capacitance using a high-frequency reference signal. This is accomplished with three main parts—a stationary transmitter, a rotor, and a stationary receiver. The rotor is etched with a sinusoidal pattern, and as it rotates, this pattern modulates the high-frequency signal of the transmitter in a predictable way. The receiver disk reads the modulations, and on-board electronics translate them into increments of rotary motion. The main concern when using capacitive encoders is their susceptibility to noise and electrical interference, though there are methods to minimize these effects.
OUTPUT TYPES There are two types of encoder outputs; absolute or incremental. Absolute encoders have a unique code for each shaft position, so that every position is distinct. The encoder interprets a system of coded tracks to create position information where no two positions are identical. Absolute encoders also don’t lose position if there is power loss. Because each position is distinct, true position is verified as soon as power is restored, eliminating the need for a homing routine. Absolute encoders can be further classified as either single-turn or multi-turn. Single-turn encoders are well suited to short-travel motion applications where position verification is needed within a single turn of the encoder shaft. On the other hand, multi-turn encoders are better for applications that involve complex or lengthy positioning requirements. Absolute encoders have a number of advantages, with safety being a key advantage. In some applications where a loss of position could lead to operator injury or machine damage, an absolute encoder automatically provides position verification when power is restored. Absolute encoders also have good immunity to electrical noise because they determine position by reading a coded signal. Stray pulses from electrical noise wont build up so an accurate position is presented again on the next reading.
Encoders from Posital are available with IP69K level environmental protection. They’re designed to withstand repeated exposure to the hot, high-temperature water jets used in pressure washing systems, and are available with a wider range of mechanical options including more shaft diameters and more flange types, hub and square. Interface options includes analog, CANopen, J1939, SSI, Modbus and IO-Link, with HTL/TTL serial interfaces for incremental encoders.
50
DESIGN WORLD — MOTION
8 • 2020
motioncontroltips.com | designworldonline.com
