What Are Encoders?

Glossary
Encoders

Encoders are position feedback devices that convert mechanical motion into electrical signals, providing precise position, speed, and direction information for closed-loop motor control, metrology systems, and automated equipment. These sensors enable servo systems to achieve sub-micron positioning accuracy, verify commanded motions, and detect mechanical failures by continuously monitoring actual shaft or linear position.

The two fundamental encoder types serve different needs:

Incremental encoders generate pulse streams indicating relative position changes from a reference point, requiring homing sequences on power-up but offering simplicity and high resolution at low cost.
Absolute encoders provide unique position codes for every shaft angle or linear position, maintaining position knowledge through power cycles without homing but at higher cost and complexity.

Modern encoders communicate via digital protocols including BiSS (Bidirectional Synchronous Serial) and EnDat (Encoder Data), enabling high-resolution feedback, diagnostic capabilities, and multi-turn absolute position tracking. Selection depends on application requirements for position retention, homing tolerance, resolution, environmental conditions, and cost constraints.

How Do Absolute Encoders Store Position?

Absolute encoders store position through coded patterns on a rotating disk or linear scale where each position has a unique code, with single-turn encoders providing absolute position within one revolution and multi-turn encoders tracking total rotations through gear reduction or battery-backed counters.

Single-Turn Absolute Encoding

Optical absolute encoders use disks with multiple concentric tracks, each representing one bit of the position code. A 12-bit absolute encoder has 12 tracks with photodetectors reading each track simultaneously, producing a 12-bit binary or Gray code representing shaft angle. With 4,096 unique positions per revolution (2¹²), the encoder provides 0.088° resolution.

Magnetic absolute encoders use magnetic pole patterns read by Hall-effect or magneto-resistive sensors, achieving similar position coding in more compact, robust packages tolerating contamination better than optical designs. Resolution typically ranges from 12 to 17 bits (4,096 to 131,072 positions per revolution).

The position code is inherent in the mechanical pattern, not dependent on power or counting. Upon power-up, the encoder immediately reports the current shaft position with no movement required. This eliminates homing sequences critical for absolute positioning applications.

Multi-Turn Position Tracking

Gear-coupled multi-turn counting uses a mechanical gear reduction (typically 4,096:1) driving additional encoder disks. Each full rotation of the input shaft advances the multi-turn counter by one count. A 12-bit single-turn encoder with 12-bit multi-turn counting provides 24-bit total resolution (16.7 million unique positions across 4,096 revolutions), tracking shaft position across 340+ complete rotations.

Battery-backed electronic counting maintains revolution counts in non-volatile memory powered by an internal battery or supercapacitor. The encoder increments/decrements internal counters as the shaft rotates, preserving total position through power cycles. Battery life typically exceeds 10 years with minimal rotation during power-off periods.

Wiegand effect multi-turn uses wire-based energy harvesting to power internal counting circuits during shaft rotation, requiring no battery. The rotating shaft generates enough energy to maintain revolution counts in non-volatile memory, providing maintenance-free multi-turn capability.

Position Code Types

Binary coding assigns sequential binary numbers to positions, providing straightforward digital representation but risking multiple-bit errors during position transitions where several bits change simultaneously.

Gray code changes only one bit between adjacent positions, eliminating multi-bit error risks during transitions. Most absolute encoders use Gray code, converting to binary internally or in the drive controller.

What Are the Advantages of Incremental Encoders?

Incremental encoders provide higher resolution per dollar, simpler electronics, smaller package sizes, and adequate performance for applications tolerating homing sequences, making them cost-effective choices for servo systems where position retention through power cycles is unnecessary.

Absolute vs Incremental Encoders: Feature Comparison

Feature	Absolute Encoders	Incremental Encoders
Position at Power-Up	Known immediately, no movement required	Unknown, requires homing sequence
Position Retention	Maintains position through power cycles	Loses position without power or on error
Resolution (typical)	12-17 bits single-turn (4,096-131,072 counts/rev)	100-10,000+ lines (400-40,000+ counts/rev with quadrature)
Cost	Higher ($100-$1,000+)	Lower ($50-$300)
Size	Larger due to multi-track disk or counting mechanism	Smaller, simpler optical or magnetic disk
Communication	Digital serial protocols (BiSS, EnDat, SSI)	Differential line driver or digital protocols
Failure Mode	Reports position or errors clearly	May lose count without detection
Wiring Complexity	Serial interface, fewer wires	Multiple signal lines for A, B, Z plus differential
Diagnostic Capability	Extensive diagnostics via protocol	Limited, basic signal integrity monitoring
Best Applications	No homing allowed, safety-critical, vertical loads	Cost-sensitive, high resolution needs, homing acceptable

Cost and Complexity Advantages

Incremental encoders cost 50-70% less than comparable absolute encoders, making them economical for multi-axis systems where cost per axis significantly impacts total system investment. A 10-axis CNC machine using incremental encoders saves $3,000-$5,000 compared to absolute encoders while achieving equivalent or better resolution.

The simpler mechanical design (single track for quadrature, optional index) reduces failure modes and increases reliability in harsh environments. Fewer optical or magnetic sensing elements mean less can malfunction or degrade over time.

Resolution Advantages

High line count incremental encoders achieve 10,000 lines per revolution or higher, providing 40,000 counts per revolution with quadrature (x4 multiplication), yielding 0.009° resolution. Equivalent absolute encoder resolution requires 16-bit encoding (65,536 counts/rev), available only in premium absolute encoders at significantly higher cost.

Applications requiring sub-arc-second angular positioning or micron-level linear resolution benefit from incremental encoder high resolution without the cost penalty of ultra-high-resolution absolute encoders.

When Homing Is Acceptable

Many applications power up with motors disabled, allowing homing sequences to execute safely:

CNC machines - Home all axes on power-up before production starts
3D printers - Home sequence part of startup routine
Pick-and-place systems - Return to home position between production runs
Laboratory equipment - Operators expect initialization procedures

These applications realize incremental encoder cost savings without operational penalties since homing sequences fit naturally into startup procedures.

When Absolute Encoders Are Necessary

Vertical loads - Gravity pulls loads downward during power loss, creating safety hazards if position is unknown on power restoration. Elevators, hoists, and vertical automation require absolute position to prevent uncontrolled motion or collisions during startup.

Safety-critical applications - Medical devices, aerospace actuators, or equipment where unexpected motion causes injury or damage need guaranteed position knowledge.

No homing space - Applications where mechanical travel limits prevent homing sequences or where starting position varies widely benefit from absolute encoders eliminating homing requirements.

Frequent power cycling - Equipment powered down between cycles (battery-operated devices, energy-saving modes) benefits from instant position knowledge without repeated homing.

What Are BiSS and EnDat Communication Protocols?

BiSS (Bidirectional Synchronous Serial) and EnDat (Encoder Data) are digital serial communication protocols enabling high-resolution position feedback, encoder diagnostics, parameter configuration, and multi-turn absolute position transmission over simple cable interfaces with built-in error detection.

BiSS Protocol Characteristics

BiSS provides unidirectional or bidirectional serial communication using clock and data lines. The master (servo drive) generates clock signals, and the encoder synchronously transmits position data during clock cycles. Key features include:

High-speed operation - Clock frequencies up to 10 MHz enable sub-microsecond position updates
CRC error detection - Cyclic redundancy checking ensures data integrity, critical for safety applications
Flexible frame structure - Supports various position resolutions and diagnostic data transmission
Simple wiring - Requires only clock, data, and power connections (typically 5-6 wires total)

BiSS enables both position transmission and parameter access, allowing drives to read encoder serial numbers, configure settings, or retrieve diagnostic information like temperature, signal strength, or bearing wear indicators.

EnDat Protocol Characteristics

EnDat (developed by Heidenhain) provides bidirectional serial communication specifically designed for encoder feedback with extensive diagnostic and configuration capabilities:

Mode-based operation - Supports position-only mode for minimal latency or enhanced modes adding diagnostics
High resolution - Handles 25+ bit position values for multi-turn absolute encoders
Advanced diagnostics - Monitors signal quality, contamination, and potential failures before they cause positioning errors
Parameter storage - Encoders store configuration data, scaling factors, and calibration information accessible via protocol

EnDat 2.2 (current version) supports cycle times down to 1 microsecond for position updates, suitable for high-bandwidth servo applications requiring rapid feedback.

Protocol Advantages Over Analog Signals

Digital protocols provide significant advantages over traditional analog sine/cosine or incremental line driver outputs:

Noise immunity - Digital signaling resists electrical noise affecting analog signals, critical in EMI-heavy industrial environments
Extended cable lengths - RS-485 differential signaling enables 50-100m cable runs versus 5-10m for analog incremental
Integrated diagnostics - Real-time monitoring of encoder health enables predictive maintenance
Simplified installation - Single cable carries position, configuration, and diagnostics versus multiple cables for power, signals, and programming

BiSS vs EnDat Selection

BiSS suits applications prioritizing:

Open standard implementation (no licensing fees)
Simple protocol structure for custom controller development
Multi-vendor encoder support

EnDat provides advantages for:

Advanced diagnostic requirements
Heidenhain encoder integration
Applications requiring extensive encoder configuration capability

Both protocols achieve comparable performance for position feedback, with selection often determined by drive controller support or encoder manufacturer preference rather than fundamental technical limitations.

Conclusion

Encoders provide essential position feedback enabling precise motion control, with selection between absolute and incremental types depending on application requirements for position retention, homing capability, and cost constraints. Incremental encoders offer high resolution and lower cost for applications tolerating homing sequences, while absolute encoders provide immediate position knowledge critical for safety applications, vertical loads, or systems without homing capability.

Modern digital communication protocols like BiSS and EnDat enhance encoder functionality beyond simple position feedback, providing diagnostic capabilities, configuration flexibility, and improved noise immunity that legacy analog interfaces cannot match. These protocols enable predictive maintenance through continuous encoder health monitoring and simplify system integration through standardized digital interfaces.

Understanding encoder characteristics, resolution requirements, environmental considerations, and communication protocol capabilities enables appropriate selection balancing performance needs against cost and complexity constraints. The trend toward absolute encoders continues as costs decline and protocols mature, though incremental encoders remain optimal for many applications where their advantages prove decisive.

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Encoders