Written by: Radonix R&D Team
Short answer: An absolute encoder outputs a unique position value for every shaft angle, so the machine knows its exact position immediately at power-on, even after a power cut. Unlike an incremental encoder, which only counts movement from a reference, an absolute encoder needs no re-homing, which reduces start-up time and the risk of a wrong zero.
Absolute encoders are a core feedback device in CNC machines, robotics and automated production lines. They matter most where a restart without homing is valuable, such as vertical axes, rotary tables and machines with long cycles. This page covers how they work, the types, the interfaces and how to choose between absolute and incremental feedback. For the wider picture of encoder families, see encoders in CNC machines.
What Is an Absolute Encoder?
An absolute encoder converts shaft position into a unique digital (or analogue) code. Each position has its own value, so the position is available the moment power returns, with no calibration move or manual reset. An incremental encoder, by contrast, tracks only changes in position and loses its reference when power is removed.
How Absolute Encoders Work
- Optical absolute encoders pass light through a coded disc with several tracks. The pattern read by the photodetectors gives a binary or Gray code value that is unique for each position. They provide high resolution and repeatability and need clean conditions.
- Magnetic absolute encoders use a magnetised ring or magnet and magnetic sensors to produce the same unique position value. They are more tolerant of dust, humidity and vibration, which suits heavy-duty industrial equipment.
Types of Absolute Encoders
| Type | What it measures | Typical use |
| Single-turn | Position within one revolution | Rotary tables, valves, tool heads |
| Multi-turn | Position and the number of full revolutions | Long-travel or vertical axes where the shaft turns many times |
| Programmable | Adjustable resolution, zero point and output format | Flexible integration across different control systems |
Multi-turn encoders keep the revolution count either mechanically (gear stages) or electronically; check the datasheet for how the count is retained and whether a back-up battery is needed.
Connectivity and Interfaces
Common interfaces for absolute encoders include:
- SSI and BiSS: high-speed serial protocols with deterministic, real-time feedback.
- Industrial networks: EtherCAT, CANopen, PROFIBUS and Modbus, used for multi-axis coordination.
- Parallel and TTL/HTL outputs: voltage-level signals for short to medium distances.
- Analogue outputs (0–10 V or 4–20 mA): for legacy or PID-based systems.
- Fibre-optic links: immunity to interference over long distances and in noisy environments.
Always confirm that your controller or drive supports the specific interface, word length, code type and update rate of the encoder before ordering. For linear scales with serial absolute interfaces such as EnDat and BiSS-C, see the linear encoder guide.
Why Engineers Choose Absolute Encoders
- Position is available immediately after power loss, with no re-homing.
- High repeatability for demanding applications.
- Robust options for vibration, dust and heat.
- Simpler commissioning and fewer start-up errors.
- Reduced downtime after power interruptions or emergency stops.
Factors to Keep in Mind
- Higher initial cost than incremental encoders.
- A compatible controller or drive interface is required.
- Some models have maximum speed limits; verify the specification for your axis.
Real-World Applications
- CNC machinery: tool positioning, spindle synchronisation and axis alignment, especially vertical axes that must not lose position.
- Robotics: joint tracking and safe recovery after a pause.
- Automotive and aerospace: high-precision actuator and test-rig control.
- Factory automation: conveyors, servo systems and production lines.
Absolute vs Incremental Encoders
| Feature | Absolute | Incremental |
| Position at power-on | Known immediately | Unknown until a reference move |
| Homing required | No | Yes |
| Cost | Higher | Lower |
| Signal | Unique code per position | Pulses (A/B/Z) |
| Best for | Vertical axes, rotary tables, minimal downtime | Cost-sensitive axes where homing is acceptable |
If the system needs immediate, accurate position feedback without a start-up reference move, choose absolute. For less critical axes where cost matters more than restart time, incremental remains a sound choice; see incremental encoders in CNC.
Installation Best Practices
- Align carefully. Misalignment causes drift and wear; use precise couplings and fixtures.
- Minimise vibration. Rigid mounting improves accuracy and life.
- Protect from the environment. Match the IP rating to the conditions; see what IP rating means in industry.
- Wire correctly. Use shielded cable, ground properly and keep signal lines away from power cables.
- Configure the protocol. Match word length, code type and communication rate to the controller.
- Set the zero. Define the zero position and rotation direction before production.
Maintenance
- Inspect couplings, cables and connectors regularly.
- Keep sensing elements clean, especially on optical models.
- Route signal lines away from high-frequency power cables.
- Re-verify the zero position after major mechanical service.
FAQ
What is the difference between an absolute and an incremental encoder?
An absolute encoder gives a unique position value at all times, so it needs no homing. An incremental encoder only counts movement from a reference and must be homed after power-up.
Do absolute encoders need homing?
No. They report position at power-on. Machines may still define a machine zero and offsets, but no reference move is needed to know the encoder position.
What is a multi-turn absolute encoder?
It measures both the angle within a revolution and the number of full revolutions, which is needed on axes where the shaft turns many times over the travel.
Which interfaces do absolute encoders use?
Common ones are SSI, BiSS, EtherCAT and other fieldbus networks, parallel or TTL/HTL outputs and analogue outputs. Check that your controller supports the specific one.
Choose the Right Feedback for Your Axes
Absolute encoders give continuous accuracy and faster restarts, provided the controller interface and budget fit. Contact Radonix or use the chatbot in the bottom right corner to discuss encoder integration for your CNC system. For machine integration, compare Radonix CNC controller hardware for independent multi-axis motion, encoder inputs and industrial I/O.
For machine integration, compare Radonix CNC controller hardware for independent multi-axis motion, pulse/direction signals and industrial I/O. Radonix CNC controller hardware.
