Absolute Encoder
Absolute encoders are critical displacement and angular position sensors used in modern industrial automation, robotics, and high-precision measurement systems. Unlike incremental encoders, absolute encoders provide unique, non-volatile position data at all times, even during power loss. This article provides a detailed analysis of absolute encoders, covering their definition, working principles, structural types, signal outputs, performance specifications, technical advantages, application areas, relevant standards, technical comparisons, common issues, and selection guidelines.
What is an Absolute Encoder
An absolute encoder is a sensor capable of providing a unique digital code representing the position at any given time. Its output signal corresponds to the absolute position of a rotating shaft or linear displacement, rather than relative movement pulses. Therefore, even after a power outage or system restart, the absolute encoder can accurately report the current position without requiring re-homing or reference point search.
Absolute encoders are commonly used in applications requiring high reliability and position retention, such as robotics, CNC machines, automated production lines, and elevator systems.
Working Principle of Absolute Encoders
1. Single-Turn Absolute Encoder
- Each position within one revolution is represented by a unique code combination.
- Suitable for applications with rotational ranges within 360°.
2. Multi-Turn Absolute Encoder
- In addition to single-turn coding, it tracks the number of rotations using mechanical gears or electronic counting techniques.
- Suitable for unrestricted rotation or applications requiring multi-turn position tracking.
Encoding Methods:
- Binary encoding
- Gray code
- BCD (Binary-Coded Decimal)
Sensing Technologies:
- Optical sensing (code disk and optoelectronic sensors)
- Magnetic sensing (magnetic scales and magnetoresistive/Hall sensors)
- Inductive sensing
Signal Output Types
| Signal Type | Description |
|---|---|
| Serial Interface | SSI, BiSS-C, EnDat 2.2, Profibus DP, Profinet, EtherCAT |
| Parallel Interface | Multi-bit digital output |
| Analog Output | Voltage (0-10V) / Current (4-20mA), for special applications |
Key Specifications
- Single-turn resolution: Up to 24 bits (16,777,216 positions).
- Multi-turn count: Up to 32 bits (4,294,967,296 turns).
- Maximum rotational speed: Typically 6000 RPM, higher options available.
- Protection rating: IP50 to IP68 (industrial heavy-duty models).
- Operating temperature: -40°C to +105°C.
- Vibration/Shock resistance: Compliant with IEC 60068-2 standards.
- Output latency: < 1 ms (for high-speed control requirements).
Technical Advantages
- Non-volatile position retention: Position data is preserved even during power loss.
- Simplified system design: No need for re-homing or initial calibration.
- Strong noise immunity: Differential signals and noise-resistant interface design.
- Flexible communication protocols: Supports various fieldbus and industrial Ethernet standards.
- High accuracy and repeatability: Suitable for precision positioning tasks.
Typical Applications
- Industrial robotics: Joint position control.
- CNC machining centers: Spindle and slide position feedback.
- Smart logistics: AGV and conveyor system path detection.
- Automated storage: Stacker crane and lift position control.
- Medical equipment: MRI and CT scanner rotary platforms.
- Wind and solar energy: Wind direction, blade angle, and solar tracking systems.
Industry Standards and Norms
- IEC 61800-5-2: Functional safety of electrical drive systems (including SIL classification).
- ISO 13849-1: Safety of machinery control systems.
- IEC 61131-2: Input/output requirements for industrial control equipment.
- IEC 60529: Degrees of protection (IP code).
- ISO 9001: Manufacturing quality management system.
Absolute vs Incremental Encoders
| Performance Indicator | Absolute Encoder | Incremental Encoder |
|---|---|---|
| Position Information | Provides unique absolute position | Provides relative position |
| Non-volatile Position Retention | Yes | No |
| System Complexity | Lower (no need for external re-homing) | Requires external counters and re-homing mechanisms |
| Cost | Higher | Lower |
| Accuracy | High (up to 24 bits or more) | Depends on PPR and counters |
| Application Scenario | High-precision, multi-turn tracking systems | General speed or position detection systems |
Maintenance and Troubleshooting
Routine Maintenance
- Regularly inspect mounting fasteners to prevent loosening or wear.
- Keep optical windows clean for optical encoders.
- Check the integrity of cables and connectors to prevent breakage or corrosion.
Common Issues and Solutions
| Issue | Possible Cause | Solution |
|---|---|---|
| No signal output | Power failure or disconnected wiring | Check power supply and wiring connections |
| Position data jumps or is lost | Interference, aging components, or loose parts | Check grounding, replace components, and tighten parts |
| Communication errors or delays | Incorrect protocol configuration or wiring issues | Verify protocol settings and inspect or replace cables |
Selection Guide
- Choose the type: Single-turn or multi-turn based on the motion range.
- Resolution requirements: Select the number of bits according to control precision needs.
- Output interface: Choose a serial or parallel interface compatible with the controller or PLC.
- Mounting specifications: Confirm shaft diameter, flange standard, and load capacity.
- Environmental requirements: Determine the protection rating and temperature range based on actual working conditions.
- System compatibility: Ensure communication protocols are compatible with existing systems.
- Safety certifications: For critical applications, prioritize products certified to SIL, ISO 13849, and other relevant standards.
By thoroughly understanding the construction, functions, standards, and technical advantages of absolute encoders, engineering professionals can efficiently select the appropriate model according to specific application requirements, enhancing system performance, reliability, and intelligence.
References / Cited Standards
- IEC 61800-5-2:2016
- ISO 13849-1:2015
- IEC 61131-2:2017
- IEC 60529:2020
- ISO 9001:2015