Optical encoders in drive technology: incremental vs. absolute, resolution and accuracy

1. Introduction to precise position detection

In modern industrial automation technology, especially in CNC machine tools, industrial robots and highly dynamic servo drives, the exact recording of position and speed is essential. The control quality of a drive system depends directly on the quality of the feedback system. In this area, optical encoders represent the industrial standard for applications that require the highest precision.

This technical article analyzes the physical principles of optical encoders, differentiates between incremental and absolute measuring methods and defines the critical parameters of resolution and accuracy. The aim is to provide maintenance engineers and system operators with a well-founded decision-making basis for the selection, installation and error analysis of these system-critical components.

2. Physical and electrical engineering basics

Optical rotary encoders primarily work based on the photoelectric scanning principle. A measuring standard (usually a glass pane with a vapor-deposited grid) is illuminated or reflected by a light source. The transmitted light method is the dominant technology for high-resolution encoders.

2.1 The sampling principle

The system consists of an LED light source, a condenser lens, a scanning plate (reticle), the rotating code disk and a photodetector array. The light from the LED is parallelized by the lens. As the code disk rotates, the bar grid modulates the light beam. The photodiodes behind it convert the light intensity into sinusoidal photocurrents. In high-resolution systems, the interferential scanning principle is used, which uses the diffraction of light on fine gratings (line periods < 8 μm) to generate signals of the highest quality.

2.2 Incremental signal acquisition

Incremental encoders record relative position changes. They provide two signals that are 90 electrical degrees out of phase (track A and track B). This phase offset enables direction detection. The so-called quadrature evaluation in the control quadruples the physical number of lines on the disc. There is also a reference mark (Z-track) that delivers an impulse once per revolution to define a mechanical zero point.

2.3 Absolute signal capture

Absolute encoders assign a unique digital code to each angular position. The system knows the exact position immediately after switching on the supply voltage, without the need for reference travel. In order to avoid read errors at the track boundaries, the Gray code is used as standard, in which only a single bit changes from one step to the next.

A distinction is made here:

• Singleturn encoder: Record the absolute position within exactly one revolution (360°).
• Multiturn encoder: Additionally record the number of revolutions. This is typically achieved by a downstream, optically or magnetically scanned reduction gear. Battery-free multiturn technologies use the Wiegand effect to generate energy for the revolution counter.

3. Technical specifications and standards

The design and operation of rotary encoders in industrial systems are subject to strict standards to ensure reliability and personal safety.

3.1 Resolution vs. Accuracy

These two terms are often confused in practice, but must be evaluated strictly separately:

• Resolution: Defines the smallest measurable step size. For incremental encoders it is specified in lines per revolution (e.g. 2048 ppr), for absolute encoders in bits (e.g. 25 bits = 33,554,432 measuring steps per 360°).
• Accuracy: Describes the maximum deviation of the measured position from the actual mechanical position. It is given in arc seconds (arcsec) or arc minutes. A high-resolution encoder is not necessarily highly accurate. The accuracy is limited by pitch errors of the glass pane, eccentricity of the bearing and wobble errors of the shaft. Typical accuracies of optical precision encoders are ±10 to ±20 arc seconds.

3.2 Relevant industry standards

• DIN EN 61800-5-2: Adjustable speed electric power drive systems - Part 5-2: Safety requirements. Defines safety features such as Safely-Limited Speed ​​(SLS) and Safe Operating Stop (SOS).
• IEC 60529: Degrees of protection through housing (IP code). Industrial encoders require at least IP64, in harsh environments IP67.
• DIN EN 60068-2-6 / DIN EN 60068-2-27: Environmental tests regarding vibration and shock resistance. Optical encoders with a glass pane typically achieve shock resistance of up to 2000 m/s² (6 ms) and vibration resistance of up to 300 m/s² (10...2000 Hz).
• Directive 2014/34/EU (ATEX): For use in potentially explosive areas (Zone 1/21 or 2/22), encoders must have flameproof enclosures (Ex d) or intrinsic safety (Ex i).

4. Selection and Sizing Guidelines

Selecting the correct encoder requires analyzing the mechanical path and the control technology requirements. The following formula is used to calculate the required encoder resolution for a linear axis (driven via a ball screw spindle):

Resolution (steps/revolution) = Spindle pitch (mm) / Desired linear resolution (mm)

Example: A ball screw with a pitch of 5 mm should be positioned with an accuracy of 0.001 mm (1 μm). The minimum resolution of the encoder is 5 mm / 0.001 mm = 5000 steps per revolution.

Decision matrix for the feedback system

5. Installation and commissioning practices

More than 50% of all encoder failures are due to mechanical or electrical installation errors.

5.1 Mechanical coupling

Rotary encoders must never be rigidly coupled to the drive shaft if they have their own bearings. A radial or axial misalignment leads to constraining forces that destroy the encoder's ball bearings within a very short time (according to ISO 281 service life calculation).

• Solid shaft encoders: Must be connected via a backlash-free compensation coupling. Permissible radial offset typically < 0.1 mm, angular offset < 0.2°.
• Hollow shaft encoders: Are plugged directly onto the motor shaft. The torque arm (stator coupling) compensates for the radial play and prevents the housing from rotating.

5.2 Electromagnetic compatibility (EMC)

According to DIN EN 61000-6-2 (interference immunity in the industrial sector), a strict shielding concept is required. The signal cables must be twisted pairs (twisted pair) and shielded. The shield must be connected over a large area (not via pigtails) at both ends (encoder housing and switch cabinet ground). If there are potential differences between the machine and the control cabinet, a separate potential equalization conductor (at least 6 mm²) is absolutely necessary to prevent compensating currents across the encoder cable shield.

6. Failure Modes and Root Cause Analysis

The systematic analysis of failures is critical for increasing system availability.

• Bearing damage (mechanical overload): Shown by increased running noise and vibrations. The cause is usually an alignment error during assembly that exceeds the maximum permissible shaft loads (e.g. 40 N radial, 20 N axial).
• Signal loss due to contamination: If moisture or oil penetrates the housing (e.g. due to defective shaft seals or temperature change breathing), the optical disc fogs up. The photocurrents decrease, which leads to interpolation errors or total failure.
• Glass breakage: Occurs when the shock load (e.g. from hammer blows when assembling pulleys) exceeds the specification limit of typically 2000 m/s².
• LED degradation: The luminosity of the transmitting LED decreases over time. High-quality encoders compensate for this by automatically increasing the LED current. The MTBF (Mean Time Between Failures) of the electronics is typically > 100,000 hours at room temperature.

7. Predictive maintenance and condition monitoring

Modern absolute encoders with digital interfaces (such as EnDat 2.2, BiSS-C or IO-Link) offer extensive diagnostic functions that enable condition monitoring before the system comes to a standstill.

• Monitoring of the signal amplitude: The control continuously reads the internal signal reserve. If the light intensity falls below a threshold value (due to aging of the LED or incipient contamination), a warning bit is set.
• Temperature monitoring: Integrated temperature sensors measure the internal temperature of the housing. The permissible operating range (often -40 °C to +100 °C) can be monitored in order to detect overheating due to bearing friction or external heat at an early stage.
• Position monitoring: Continuous checking of the internal plausibility of the code. If an error occurs, an alarm bit is sent and the drive can be controlled and shut down in a safety-related manner.

8. Comparison matrix: encoder technologies

The following table provides a technical comparison of the common encoder classes to make component selection easier:

9. Summary

The choice of the optimal encoder determines the performance, accuracy and reliability of the entire drive system. Optical encoders offer unsurpassed resolution and accuracy for highly dynamic control loops. The decision between incremental and absolute systems depends on the specific application requirements for referencing, cabling effort and diagnostic capability. Mechanical tolerances and EMC guidelines must be strictly adhered to during installation to ensure the calculated service life of the components.

As a specialized partner to the industry, UNITEC-D GmbH offers a comprehensive portfolio of standard-compliant, certified encoders and spare parts for drive technology. Visit our electronic catalog for detailed specifications and availability: https://www.unitecd.com/e-catalog/

10. References

• DIN EN 61800-5-2:2017 - Electric power drive systems with adjustable speed.
• IEC 60529:1989+A1:1999+A2:2013 - Degrees of protection through housing (IP code).
• DIN EN 61000-6-2:2019 - Electromagnetic compatibility (EMC) - Interference immunity for industrial areas.
• ISO 281:2007 - Rolling bearings - dynamic load ratings and nominal service life.