High-resolution rotary encoder

01

Separate resolution from accuracy

US Digital defines resolution as the number of increments per rotation: the disc's line count sets a native resolution, which can be increased by interpolation and then decoded up to four times using the two quadrature channels A and B. It calls the result before quadrature CPR (cycles per revolution) and after 4x decoding PPR, and notes that some vendors use these acronyms with different definitions. Accuracy, by contrast, is the difference between the target position and the actual position, specified in degrees, arcminutes or arcseconds. [1] [2]

Beckhoff's application note states that the encoder resolution sets the smallest measurable position difference, and that a 1 Vpp sine/cosine encoder with 1,000 signal periods per revolution, interpolated by its EL5021 terminal at 8, 10 or 13 bits, yields 256,000, 1,024,000 or 8,192,000 steps per revolution; it states that "Common interpolation rates are often around 4 to 8 bits" and notes that the interface corrects offset, gain and phase errors. [5] US Digital lists eccentricity error, caused by eccentric mounting of the disc, as a source of rotary encoder error. [4]

Editorial calculation: a 23-bit absolute encoder divides a turn into 8,388,608 steps, about 0.15 arcsec each; this is resolution only. By the small-angle approximation, a 10 µm eccentricity on a 20 mm code radius alone gives an angular error of about 0.010 / 20 rad ≈ 103 arcsec (assumed example values). Ask the supplier for the system accuracy in arcsec over one revolution, measured as installed, separately from the bit count.

Give the supplier the axis (motor, rotary table, conveyor), required accuracy and resolution, incremental or absolute (single-turn or multi-turn), signal or interface the controller accepts, maximum shaft speed, shaft or hollow-shaft size and coupling, supply voltage, cable length, protection class, temperature, shock and vibration, and test data required.

02

Match the signal type and check the speed limit

Beckhoff describes incremental encoders that output two channels in quadrature (A and B, 90° apart) so direction can be detected, with an optional reference pulse once per revolution, as rectangular 5 V TTL or 30 V HTL signals referenced to ground, or as differential 1 Vpp sine/cosine signals that are interpolated for higher resolution. Absolute encoders output a unique position value that remains known after a power failure, whereas incremental systems must home after switch-on. [5]

US Digital explains that each encoder is rated at a frequency it can read and that higher resolutions may reduce the read rate, limiting the speed at which the encoder is effective. [3] Beckhoff notes that interpolation computing time limits resolution at high speed, so its EL5021 uses about 13 bits at low speed and about 8 bits at higher speed, with a 250 kHz maximum input frequency. [5] Hengxiang's E50S8 incremental encoder lists a 300 kHz maximum response frequency with resolutions from 50 to 23,040 per turn. [6]

Editorial calculation: maximum shaft speed ≈ response frequency × 60 / pulses per turn. At 300 kHz, a 1,000-pulse encoder can be followed to 18,000 rpm, but a 10,000-pulse version only to 1,800 rpm; the controller's input frequency and the encoder's mechanical speed rating may set a lower limit. [6] [3]

03

Compare what suppliers offer

Suppliers linked to this page list high-resolution rotary encoders; each card links to the page reviewed.

Listings include incremental, absolute and fieldbus absolute encoders with solid or hollow shafts; ask each for accuracy in arcsec, signal type, maximum frequency and the CPR/PPR definition used.

04

Trace the encoder parts

This is a functional reference built from the sources cited; it is not one maker's specification.

05

Fill the specification fields with values for your axis

A rotary encoder quotation needs values for your axis. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the axis, signal and accuracy data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Axis: [motor; rotary table; other]
Accuracy: [system accuracy; resolution; repeatability]
Type: [incremental; absolute single-turn or multi-turn]
Signal: [TTL; HTL; 1 Vpp; interface]
Speed: [maximum rpm; controller input frequency]
Mechanical: [shaft or hollow; coupling; protection]
Documents: [accuracy test; datasheet]

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