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.

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.

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.

Rotary encoders, such as these from HEIDENHAIN, come in a wide variety of configurations with different options such as with shafts, shaftless, or with a hollow through-shaft, and with or without an integral bearing.

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.

Incremental encoders generally supply square-wave signals in two channels, A and B, which are offset (or out-of-phase) by 90 degrees. This helps in determining the direction of rotation. The output signals of an incremental encoder only have information on relative position not absolute position like an absolute encoder. To provide useful position information, encoder position 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.

A traditional limitation of an incremental encoder comes with a loss of power. For instance, if a machine with an encoder is turned off, the encoder will not know its position when switched on again. The encoder has to perform a homing routine in order to know its exact position. Then, a counter or buffer will be zeroed and the system will determine where it is relative to fixed positional points. One way around this is to use a battery backup. Such a system ensures that the memory is backed up and can store the count information and provide an absolute count once power is restored.

These StepSERVO integrated motors from Applied Motion Products are equipped with HEIDENHAIN’s RENCO encoders. The RCML 15 rotary encoders are compact and reliable and provide easy-to-integrate motion control performance. The companies have collaborated to expedite critical components for the manufacture of ventilators, especially critical during the COVID-19 pandemic.

COMMON PERFORMANCE METRICS

Key to a good match between encoder and application is the resolution and accuracy. In other words, the ideal encoder has both the necessary resolution and is highly accurate.

Resolution is the distance over which a single encoder count takes place – it’s the smallest distance the encoder can measure.

For rotary encoders, resolution is typically specified in terms of measuring units, or pulses, per revolution (PPR). Linear encoder resolution is most commonly specified as the distance over which the count takes place and is given in terms of microns (μm) or nanometers (nm). The resolution of an absolute encoder is specified in bits, since absolute encoders output binary “words” based on the encoder’s position.

Accuracy is the difference between the true position (or speed) of the device being measured and the position (or speed) reported by the encoder. For rotary encoders, it is specified in arcseconds or arcminutes, and for linear encoders accuracy is typically given in microns.

Note that higher resolution does not mean higher accuracy. Consider two encoders – one with 100 PPR resolution and one with 10,000 PPR resolution, but both with the same accuracy specification. The lower resolution (100 PPR) encoder can report a movement of 90 degrees just as accurately as the higher resolution (10,000 PPR) model. The higher resolution encoder just has the ability to break up that 90 degree movement into much smaller increments.