In the fields of industrial automation and motion control, the selection of servo systems has a decisive impact on equipment performance, precision, and stability. Among these, absolute and relative servos represent two common control modes, each suited for distinct application scenarios. Understanding their selection criteria is crucial for engineers and system designers.
I. Fundamental Concepts of Absolute and Relative Servos
Absolute servo systems continuously capture and retain the absolute position of the motor shaft or load in real time. Even after power loss and subsequent restart, the system instantly recognizes its current position without requiring a return-to-zero operation. This capability relies on absolute encoders, which record position data using unique coding methods (e.g., Gray code). Absolute servos are the preferred choice for applications demanding high-precision positioning where position loss is unacceptable, such as CNC machine tools and robotic joints.
Relative servo (also known as incremental servo) uses incremental encoders to track position changes incrementally. Position information is lost after power loss, requiring a homing operation upon power-up (e.g., using limit switches or Z-phase pulses to establish a reference point). This lower-cost approach suits applications with less stringent initial position requirements or where periodic homing is acceptable, such as standard conveyor belts and packaging machinery.
II. Core Considerations for Selection Criteria
1. Position Retention Requirements After System Power Loss
If equipment must resume operation immediately after power loss without positional loss (e.g., semiconductor lithography machines, medical devices), absolute servo is the only viable option. For instance, search results indicate that absolute encoders in high-end manufacturing prevent repeated recalibration due to unexpected power outages, significantly boosting efficiency.
Conversely, if equipment can tolerate returning to zero at each startup (e.g., standard printing presses), incremental servo systems offer greater cost advantages.
2. Precision and Repeatability Requirements
Absolute encoders typically offer higher resolution and multi-turn position recording capabilities (e.g., 17-bit single-turn accuracy, 16-bit multi-turn range), making them suitable for micron-level positioning scenarios. While incremental encoders can achieve comparable single-turn accuracy, multi-turn applications rely on external counting circuits. Long-term operation may compromise precision due to cumulative pulse errors.
3. Cost and System Complexity
Absolute servo systems entail higher hardware costs (encoder prices may be 2-3 times that of incremental types) and require drivers supporting absolute communication protocols (e.g., SSI, BISS, or EtherCAT). Incremental servos only need simple pulse interfaces (e.g., A/B/Z signals), resulting in lower overall costs. For projects with limited budgets or less stringent performance requirements, incremental solutions offer greater cost-effectiveness.
4. Environmental Adaptability and Reliability
Absolute encoders demonstrate superior interference resistance in environments with high vibration, dust, or electromagnetic interference. For instance, an automotive welding line experiencing incremental signal loss due to electromagnetic noise saw a 70% reduction in failure rates after switching to absolute servo systems. Additionally, absolute systems eliminate the need for battery backup (mechanical multi-turn encoders record revolutions via gear sets), reducing maintenance requirements.
5. Ease of Commissioning and Maintenance
Relative servos require repeated zero-return operations during commissioning, whereas absolute servos enable direct position reading, shortening equipment installation time. However, if the encoder fails, replacing an absolute system may necessitate recalibrating the mechanical zero point, making the process more complex.
III. Comparison of Typical Application Scenarios
1. Suitable Scenarios for Absolute Servo Systems
● High-precision machining equipment: e.g., five-axis CNC machines with complex tool paths requiring processing resumption after power loss.
● Collaborative robots: Frequent start/stop cycles and human interaction necessitate absolute positioning for safety.
● Aerospace test benches: Continuous operation without interruption; absolute encoders offer lifespans exceeding 100,000 hours.
2. Applications for Relative Servo Systems
● Logistics sorting lines: Conveyor belts require only relative motion control, prioritizing cost efficiency.
● Standard injection molding machines: Repetitive cyclic operations where zero-return startup does not impact production cycles.
● Educational lab equipment: Student comprehension of principles outweighs performance demands, making incremental systems more intuitive.
IV. Hybrid Solutions and Future Trends
Some high-end systems adopt a "incremental + absolute" dual-encoder design, balancing dynamic response and power-off safety. For example, a photovoltaic silicon wafer cutter uses an incremental encoder at the motor end (for real-time control) and an absolute encoder at the load end (for absolute precision). Additionally, with the proliferation of industrial Ethernet, the transmission rate of absolute protocols has increased from 1MHz to 100MHz (e.g., EtherCAT FSoE), further narrowing the real-time performance gap with incremental encoders.
V. Recommended Selection Decision Flowchart
1. Define Requirements: Is position retention during power loss mandatory? Does accuracy exceed ±0.1mm?
2. Assess Environment: Are strong vibrations, oil contamination, or electromagnetic interference present?
3. Calculate Costs: Does the budget permit an absolute system? What are long-term maintenance costs?
4. Verify Compatibility: Does the drive support the selected encoder protocol? Is mechanical installation space sufficient?
In summary, selecting between absolute and incremental servo systems fundamentally involves balancing performance, cost, and reliability. As Industry 4.0 demands greater intelligence, absolute servo market share is growing annually (projected to reach 45% by 2025). However, for most standardized equipment, incremental solutions remain cost-effective. Engineers must align with specific process requirements to avoid extremes of "over-engineering" or "under-performance," thereby achieving optimal system configuration.