Author: Nima Rad
Short answer: A linear encoder, also called an electronic linear scale, measures the real straight-line position of a machine axis and turns it into an electrical signal. It feeds a digital readout (DRO) for manual machines or gives a CNC controller direct position feedback for closed-loop control, so errors from backlash, screw pitch and thermal growth can be corrected.
What Is a Linear Encoder (Electronic Linear Scale)?
A linear encoder is a high-accuracy sensor made of two parts: a scale, which carries a coded pattern, and a readhead, which detects that pattern as it moves along the scale and converts it into electrical signals. In machine tools these signals are used in two main ways:
- Displaying accurate axis position on a digital readout (DRO).
- Providing direct position feedback to a CNC controller or servo drive for true closed-loop control.
Unlike a rotary encoder mounted on a motor, a linear encoder measures the actual table or slide position. That is why the controller can compensate for backlash, ball screw pitch error, mechanical compliance and thermal expansion that a motor-side sensor cannot see. If you are comparing rotary and linear feedback, start with the overview of encoders in CNC machines.
How a Linear Encoder Works
As the readhead moves along the scale, it detects changes in the pattern and outputs analogue or digital signals:
- Analogue outputs (sine/cosine) allow fine interpolation, so resolution can be much finer than the scale period.
- Digital outputs (A/B quadrature with an index Z pulse) provide clean, discrete position counts.
Linear encoders may be open (exposed) for maximum accuracy in clean conditions, or sealed for protection against coolant, oil and chips in harsh shop environments.
Types of Linear Encoders by Sensing Technology
| Technology | How it works | Strengths | Limitations | Typical use |
| Optical | LED or laser light passes through or reflects from a fine grating on a glass or steel scale, read by photodetectors | Very high resolution and accuracy | Sensitive to dirt, oil mist and contamination unless protected | Precision machine tools, metrology, inspection |
| Magnetic | A magnetised tape with defined poles is read by Hall or magnetoresistive sensors | Tolerates oil, dust and chips; larger air gap | Lower resolution; sensitive to strong magnetic fields; accuracy depends on tape quality and installation | Heavy-duty, dusty or oily environments |
| Inductive | Measures inductive coupling changes in an AC circuit | Rugged against vibration, coolant splash and airborne contamination | Can be sensitive to temperature changes | Demanding industrial CNC machines |
| Capacitive | Measures capacitance changes between readhead and scale | Simple and inexpensive; some designs give battery-free absolute position | Sensitive to dust and humidity in basic designs | Digital calipers, low-precision measurement |
| Eddy current | Detects inductance changes with an encoded metallic scale | Resistant to harsh environments | Moderate resolution | Specialised industrial applications |
Optical vs Magnetic: Which One for a CNC Machine?
Optical scales offer the finest resolution and suit clean, controlled machining and inspection. Magnetic scales allow larger air gaps and withstand dust, coolant and oil, which makes them practical on rugged machines where ultimate resolution is less important. Inductive scales sit between the two for contaminated environments. Final accuracy and thermal stability depend on the scale quality, installation and environment in every case.
Output Types and Interfaces
Incremental Linear Encoders
Incremental encoders generate position changes through quadrature A/B signals plus a reference (Z) pulse. Common formats are TTL or RS-422 digital outputs and 1 Vpp sine/cosine signals. They remain widely used in DRO systems and many CNC installations, but the axis must find a reference point after power-up. For how the signals work, see incremental encoders in CNC.
Absolute Linear Encoders
Absolute encoders report a unique position value immediately at power-on, with no movement to a reference mark. Typical serial protocols are EnDat, BiSS-C and SSI; they differ in data structure, integrity checking, speed and flexibility. Absolute scales can remove repeated homing cycles, subject to controller compatibility. Read more in absolute encoders in CNC and automation.
Key Specifications When Selecting a Linear Encoder
- Resolution: the smallest measurable increment, such as 5 µm, 1 µm or 0.1 µm. Display resolution does not automatically equal positioning accuracy.
- Accuracy: the maximum permissible positioning error over a defined measuring length, usually stated in µm over a fixed distance or µm per metre.
- Repeatability: how consistently the encoder reports the same position over repeated forward and backward moves.
- Maximum speed and bandwidth: every encoder has a speed and signal-frequency limit; exceeding it causes signal loss or counting errors.
- Environmental resistance: coolant, oil, chips, dust and vibration tolerance.
- Mounting tolerances: gap, parallelism and alignment between readhead and scale directly affect signal stability and long-term accuracy.
Where Linear Encoders Are Used
| Application area | Examples | Why a linear encoder |
| Machine tools and motion control | CNC milling and turning, PCB assembly, printers, semiconductor equipment | Closed-loop position feedback without intermediate mechanical error |
| Metrology | CMMs, laser scanners, digital calipers, tensile testers | High accuracy and repeatability in length measurement |
| Heavy machinery | Cranes, grinders, large linear platforms | Stable measurement under vibration, temperature change and contamination |
| Research | Optical instruments, semiconductor R&D, precision stages | Repeatable motion for scientific work |
The Role of Linear Encoders in CNC Machines
In DRO-based systems, the linear encoder is mainly a position display aid for manual or semi-automatic machines.
In closed-loop CNC systems, scales are fixed along the X, Y and Z axes with the readhead moving with the slide. The controller compares commanded and measured position in real time and corrects the error, compensating for transmission errors, backlash, coupling compliance and thermal effects. This improves contouring performance, surface finish and dimensional accuracy, especially in high-speed or multi-axis work, and is a common step when retrofitting older machines. For the control principle behind this, see open loop vs closed loop control in CNC.
Benefits and Limitations
Benefits: true position measurement, high repeatability, less scrap and support for higher cutting and positioning speeds.
Limitations: optical types need protection from contamination, magnetic types can be disturbed by strong magnetic fields, high-resolution models cost more, and careful alignment is required to reach the rated performance.
Common Errors: Why Accurate Numbers Can Still Give Inaccurate Parts
- Cosine error: the scale is installed at an angle to the true axis motion, so measured displacement differs from actual movement.
- Abbe error: the measuring line is offset from the axis of motion and angular deviations exist, which amplifies positioning error.
- Thermal expansion: temperature changes affect the machine structure and the scale itself, particularly over long travels.
- Electrical noise and grounding: poor shielding, interference or wrong grounding cause unstable counts, reference loss or intermittent errors.
- Contamination: oil, chips and debris degrade signal quality, especially on optical systems without adequate protection.
Installation Checklist
Correct installation is as important as encoder quality.
- Mount the scale on a flat, mechanically stable surface.
- Use shims or spacers as the manufacturer specifies.
- Verify parallelism over the full travel with a dial indicator.
- Keep the specified readhead gap and offset.
- Route encoder cables away from power cables and inverters.
- Use proper shielding and single-point grounding.
- Protect the scale with covers or scrapers.
After installation:
- Run bidirectional travel tests.
- Verify reference or index repeatability.
- Check signal stability at low and high axis speeds.
Quick Selection Guide by Application
- Manual milling or turning machines: a DRO kit with a protected glass scale.
- Oily or heavily contaminated environments: magnetic or rugged industrial encoder systems.
- High-precision CNC with tight tolerances: industrial linear feedback encoders, often optical or inductive.
- Fast start-up without homing: absolute linear encoders, if the controller supports the interface.
Common Terminology
Electronic linear scale = linear scale = linear encoder. Readhead = scanning head. Reference mark = index = Z pulse. Accuracy, repeatability and resolution keep their standard metrology definitions.
FAQ
What is the difference between a linear encoder and a rotary encoder?
A rotary encoder measures motor or screw rotation, so mechanical errors after it go unseen. A linear encoder measures the actual slide position, so the controller also corrects backlash, pitch error and thermal growth.
What is a linear encoder used for in a CNC machine?
It gives direct position feedback for closed-loop control, or drives a DRO on manual machines. It improves accuracy, repeatability and contouring performance.
Is an optical or magnetic linear encoder better?
Optical gives the finest resolution in clean conditions. Magnetic tolerates dust, oil and coolant better. Choose by environment, required accuracy and budget.
Does a linear encoder make a CNC machine more accurate?
Only if installed correctly. Misalignment, cosine and Abbe errors, thermal effects and poor shielding can still produce inaccurate parts despite accurate readings.
Should I choose an absolute or incremental linear encoder?
Absolute scales give position at power-on without homing but need a compatible interface. Incremental scales are simpler and common but need a reference move after start-up.
Add Linear Feedback to Your Machine
Linear encoders turn mechanical estimates into measured position. Chosen and installed correctly, they raise accuracy, repeatability and process stability on CNC machines and DRO systems.
Contact Radonix or use the chatbot in the bottom right corner to discuss encoder feedback for your machine. For 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.
