Encoder Key Parameters Explained
In order to correctly evaluate and select encoders, it is crucial to understand the key parameters that influence various aspects such as measurement accuracy, speed range, environmental adaptability, and lifespan. This page provides a more professional and detailed explanation of the common electrical, mechanical, and environmental parameters of various encoders.
1. Resolution
1.1 Definition and Representation
- Incremental Encoders: Typically represented by PPR (Pulses Per Revolution) or CPR (Counts Per Revolution), with common values like 1000, 2500, 5000 PPR, etc. If quadruple frequency is used, the actual count can reach 4×PPR.
- Absolute Encoders: Usually represented by bit count (e.g., 13 bits = 8192 discrete positions) or as multi-turn bits + single-turn bits (e.g., 25 bits multi-turn).
- Linear Encoders: Often represented by LPI (Lines Per Inch), CPI (Counts Per Inch), or "µm/pulse" etc.
1.2 Resolution and System Performance
- Higher resolution allows the system to capture finer movements, achieving more precise positioning and speed control.
- Too high resolution requires high-performance controllers and faster processors; otherwise, pulses may be missed or delayed.
- In high-speed scenarios, higher resolution leads to higher pulse frequency, requiring cables and drivers with sufficient bandwidth.
1.3 Additional Details
- Some encoders support interpolation technology, which significantly increases resolution through internal subdivision.
- It is important to distinguish between mechanical resolution (actual marking on the disk) and electronic resolution (output after internal interpolation).
2. Accuracy
2.1 General and Narrow Accuracy
- General Accuracy: The maximum deviation between the encoder's output value and the actual position, influenced by multiple factors (disk errors, mechanical assembly, bearing play, etc.).
- Narrow Accuracy: Some manufacturers only provide the accuracy of the code wheel or sensor readings, excluding assembly and bearing errors.
2.2 Typical Measurement Units
- Angle Encoders: arcsec (arcseconds), arcmin (arcminutes), degrees (°), or resolution percentage.
- Linear Encoders: µm/m, ppm (parts per million), or other accuracy indicators.
2.3 Accuracy and Error Sources
- Code Wheel/Grating Manufacturing Errors: Uneven or eccentric line spacing.
- Sensor Non-linearity: Distortion at the edges of optical or magnetic sensors.
- Mechanical Assembly Errors: Eccentricity, bearing play, or flange misalignment.
- Environmental Influences: Temperature drift, vibration, contamination.
2.4 Accuracy Enhancement and Compensation
- Use high-quality disks or precision optical/magnetic components.
- Perform error calibration using tools like laser interferometers to create compensation tables.
- Improve bearing stiffness to reduce radial/axial wobble.
3. Repeatability
3.1 Concept
- Repeatability refers to the consistency of measurements when returning to the same position multiple times, regardless of the absolute deviation.
- Even if the absolute accuracy is average, good repeatability can still achieve high positioning precision through software corrections.
3.2 Influencing Factors
- Mechanical Backlash: Gears, couplings, or lead screws may have gaps or elastic deformations.
- Signal Jitter: Optical or magnetic sensors may fail to detect at critical points.
- Environmental Interference: Temperature changes, electromagnetic interference, vibrations, etc.
3.3 Improvement Measures
- Optimize the drive train, use high-quality couplings, and ensure shaft alignment.
- Perform multiple sampling and averaging or filtering.
- Reduce vibration and interference sources.
4. Maximum Speed and Frequency Response
4.1 Maximum Speed
- Rotary Encoders: Represented in RPM (revolutions per minute), such as 6000 RPM.
- Linear Encoders: Represented in m/s or mm/s.
- Exceeding maximum speed can result in pulse loss or distortion, affecting measurement accuracy.
4.2 Frequency Response
- Represents the encoder's ability to maintain full waveforms and correct counting as speed increases.
- Incremental encoder pulse frequency can be calculated by RPM × PPR / 60.
- Absolute encoders depend on serial communication rates or refresh rates.
4.3 Considerations
- The controller must have high-speed counting or serial parsing capability.
- Signal transmission cables and receiving circuits should match the required bandwidth to avoid signal attenuation and excessive noise.
5. Supply Voltage and Power Consumption
5.1 Supply Voltage Range
- Typical values: 5V DC (for TTL encoders), 10~30V DC (for HTL), and some high-end products may have a wide range of 4.5~30V.
- Industrial Ethernet encoders are commonly compatible with 24V industrial systems.
5.2 Power Consumption Evaluation
- Optical encoders, which include light sources, generally have higher power consumption.
- Magnetic/capacitive encoders consume less power and may require additional pre-heating in low-temperature environments.
- High-speed and high-resolution operation increases power consumption in the internal processors and drivers.
5.3 Compatibility and Stability
- Power ripple and short-term voltage drops can cause encoder reset or pulse loss.
- Large multi-axis systems should consider voltage drop and sufficient cable gauge for power lines.
6. Output Types and Signal Levels
6.1 Incremental Output
- A/B/Z Square Waves: The most common incremental encoder output, with TTL, HTL, or RS422 levels.
- Sine Wave Output (1 Vpp): Common in optical/magnetic encoders for high-precision measurement with interpolation to increase resolution.
6.2 Absolute Output
- Parallel Output: Uses multiple parallel bits to output absolute position information (e.g., Gray Code, Binary).
- Serial Output: SSI, BiSS, EnDat, and other synchronous serial protocols, reducing wiring and enhancing anti-interference.
- Bus/Ethernet: CANopen, Profibus, EtherCAT, Profinet, etc., enabling networked multi-node control.
6.3 Analog Output
- Voltage (0~5V, 0~10V) or current (4~20mA) signals, suitable for traditional analog control systems.
- Precision can be affected by power supply ripple and cable impedance.
7. Protection Rating (IP Rating)
7.1 IP Standard
- IP65: Dust-tight, resistant to low-pressure water jets.
- IP67: Immersion-resistant for short periods, strong sealing.
- IP68: Can work normally even submerged or in deep water environments.
7.2 Selection Strategy
- High IP rating encoders are necessary for outdoor, food processing, washing, and other high moisture or dust environments.
- High IP ratings usually come with higher costs, requiring a comprehensive evaluation of cooling, bearing resistance, etc.
8. Bearing Load and Mechanical Life
8.1 Bearing Types and Load
- Common precision ball bearings or angular contact bearings.
- Radial Load: Force applied perpendicular to the shaft; Axial Load: Force along the shaft direction.
8.2 Lifespan and Maintenance
- High-speed, high-load, or extreme temperature environments can shorten bearing lifespan.
- Some high-end encoders use ceramic bearings or special lubricants to enhance durability.
8.3 Installation Considerations
- Flexible couplings or external supports can reduce radial force on the encoder bearings.
- Ensure shaft alignment during installation to avoid additional torque or wobbling.
9. Environmental Parameters
9.1 Temperature Range
- Operating Temperature: For example, -20°C to +85°C, ensuring normal measurement accuracy.
- Storage Temperature: Can be a wider range, but excessive heat or cold can degrade optical components or lubricants.
9.2 Humidity, Condensation, and Corrosion
- High humidity or condensation conditions require enhanced sealing and rust prevention.
- Corrosive environments (acid, alkali, salt mist) need encoders with stainless steel or corrosion-resistant coatings.
9.3 Vibration and Impact Resistance
- Measured in g or m/s², encoders for machines with high vibration, such as spindle machines or automotive test rigs, need higher specifications.
- External shockproof pads or mounts can also enhance impact resistance.
9.4 EMC/ESD Resistance
- In environments with high electromagnetic interference, shielded cables and proper grounding solutions are necessary.
- ESD measures (e.g., grounding rings, TVS) protect internal circuits.
10. Functional Safety and Redundancy
10.1 Safety Integrity Level (SIL / PL)
- SIL: Defined by IEC 61508, IEC 62061; PL: ISO 13849 provides safety performance levels.
- For safety-critical areas like AGVs, elevators, and collaborative robots, it is recommended to select encoders with SIL2 or SIL3 certification.
10.2 Redundant Design
- Dual Heads or Dual Channels: Cross-checking data from two sensors or switching to a backup in case of failure.
- This greatly reduces the risk of downtime due to sensor failure.
10.3 Built-in Self-Test
- Alerts when light sources degrade, magnetic anomalies occur, or temperature is too high.
- Implements “safe shutdown” or “speed reduction” logic, improving overall system safety.
11. Linearity and Calibration
11.1 Linearity Errors
- Non-uniform distribution of disk or magnetic track periods.
- Gaps or angular misalignments between the read head and scale.
11.2 Calibration and Compensation
- For high-precision applications (e.g., semiconductor, precision inspection), laser interferometers are often used for encoder calibration and error curve generation.
- Some encoders support internal compensation tables, automatically correcting errors before output.
11.3 Local Errors and System Integration
- If mechanical movement is confined to a small range, focus on calibrating and compensating for that specific working range.
- Coupled with good repeatability, extremely high positional accuracy can be achieved.
12. Comprehensive Selection and Integration Recommendations
12.1 Target Application Requirements
- Type of Motion: Rotary/Linear, speed range, acceleration/deceleration characteristics.
- System Accuracy: Resolution, accuracy, repeatability requirements.
- Environmental Constraints: Protection level, temperature range, dust/moisture, vibration interference, etc.
12.2 Mechanical and Electrical Matching
- Check shaft diameter, flange type, bearing load, and compatibility with mechanical drive systems.
- Ensure compatibility of power supply voltage, signal levels, and communication protocols with controllers.
- Estimate cable length, voltage drop, and signal attenuation, considering shielded or twisted-pair cables.
12.3 Installation, Debugging, and Maintenance
- Ensure concentricity and avoid large radial or axial forces on encoder bearings.
- Using flexible couplings can compensate for slight eccentricities.
- Periodic inspections of sealing, dust rings, cables, and connector integrity are recommended.
13. References and Conclusion
Understanding the key parameters of encoders is crucial for proper system selection. These parameters include:
- Resolution, Accuracy, Repeatability: Determining the level of fine control and precise positioning in motion control systems.
- Maximum Speed and Frequency Response: Preventing pulse loss and signal distortion in high-speed applications.
- Protection Rating, Bearing Load: Ensuring stable lifespan in complex environments and under heavy loads.
- Electrical Compatibility and Communication Interfaces: Ensuring efficient connection with system drivers, PLCs, and industrial networks.
- Functional Safety and Redundancy: Meeting the reliability needs of safety-critical applications.
By mastering these parameters and their testing standards, more informed decisions can be made in encoder selection, integration, and maintenance, providing higher efficiency and safety in industrial automation systems.