Encoders - the basics

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.

In 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 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.

This magnetic absolute linear encoder, the MSA213C from SIKO, features absolute resolution of 1 μm and supplies absolute position without referencing and without requiring a backup battery.

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.

The resolution of a magnetic rotary encoder is determined by the number of magnetic poles around the disk and by the number of sensors. Incremental encoders (whether magnetic or optical) use quadrature output and can employ X1, X2, or X4 encoding to further increase resolution. The primary difference between incremental and absolute encoders, regardless of sensing technology, is that absolute versions assign a unique binary code, or word, to each measuring position. This allows them to track the encoder’s exact position, even if power is discontinued.

The operation of linear magnetic encoders is analogous to their rotary counterparts, expect that they use a linear scale (also referred to as a tape, since they typically have an adhesive backing) and a read head. The read head can employ either a Hall effect or a magnetoresistive sensor, and detects signals generated by the magnetic code on the scale to provide position information. For absolute linear magnetic encoders, each position on the scale represents a unique binary word, indicating the exact linear position of the read head. For incremental versions, one or more reference marks are included on the scale, to enable homing after a power-off situation. Linear magnetic scales can be provided in long lengths—up to 100 meters from some manufacturers.

The most significant advantage of magnetic encoders may be their robustness. Unlike optical encoders, magnetic versions are insensitive to contaminants such as dust, dirt, liquids, and grease, as well as to shocks and vibrations. Similar to optical encoders, magnetic encoders do require an air gap between the magnetic disk and the sensor. However, the air gap in a magnetic encoder does not need to be clean and transparent, as it does for an optical encoder. As long as no ferrous material is present between the disc and the sensor, the magnetic pulses will be detected. Two important specifications for proper operation of magnetic encoders are the radial placement of the sensor in respect to the disk (or tape), and the gap distance between the sensor and the magnet.