When engineers evaluate a Video Measuring Machine (VMM) or a Vision Measuring System (VMS), most of the attention naturally gravitates toward the optics — the magnification range, the camera resolution, the illumination quality. These are visible, tangible, and intuitive. What often goes unnoticed, however, is the component that actually determines how accurately the machine knows where it is at any given moment: the optical linear encoder, also known as the linear scale or linear encoder.

At DONGGUAN CITY HANDING OPTICAL INSTRUMENT CO., LTD., we manufacture both the measuring machines and the encoder systems that power them. This dual expertise gives us a perspective on linear encoder performance that few pure equipment vendors can offer. In this article, we walk through the technical fundamentals of exposed linear encoders, their role in optical measuring systems, and what separates a high-performing encoder from one that quietly introduces errors into your measurement data.

What Is an Optical Linear Encoder?

An optical linear encoder is a displacement sensing device that uses a graduated scale — typically a glass or steel substrate engraved or etched with a precisely spaced grating — to measure linear position or motion. A read head containing a light source and photodetector moves along the scale, detecting the light-dark pattern of the grating. As the read head traverses the scale, it generates a series of electrical signals (typically sinusoidal or square wave outputs) that a controller uses to calculate position.

The term exposed linear encoder refers to an encoder configuration where the scale and read head are openly mounted on the machine structure, as opposed to a sealed (enclosed) encoder where the scale is protected inside a housing. Exposed encoders offer several advantages for precision measuring machines:

• Higher accuracy: Without the mechanical coupling of an enclosed housing, exposed encoders can be mounted directly to the machine's motion axis, minimizing Abbe error caused by offset between the measurement axis and the actual axis of motion.
• Lower friction: There is no contact between the scale and read head (non-contact optical sensing), which means no mechanical wear over time.
• Easier installation and maintenance: The open design allows for straightforward alignment, cleaning, and inspection.

Incremental vs. Absolute Encoders: Which Do You Need?

Incremental encoders generate position signals by counting grating lines as the read head moves. They are highly accurate and cost-effective, but they require a reference return (homing) procedure after each power cycle, since the controller loses position memory when power is removed.

Absolute encoders, by contrast, encode a unique position identifier at every point along the scale, allowing the system to know its exact location the moment power is restored — with no homing required. Absolute encoders are preferred in applications where frequent power cycling is expected, or where manual axis repositioning could occur between measurement sessions.

For Video Measuring Machines and Optical Measuring Systems used in industrial quality control, incremental linear scales are the dominant choice, given their cost efficiency and the fact that most automated VMMs perform a homing routine at startup. However, for certain non-contact measuring machine configurations — particularly those used in semiconductor or medical device inspection — absolute linear encoders may be specified.

Grating Pitch and Interpolation: Understanding the Accuracy Chain

The native accuracy of a linear encoder is fundamentally determined by the grating pitch — the spacing between adjacent lines on the scale. Common pitches in precision encoders range from 20 μm down to 4 μm or finer. However, the raw output period is typically much larger than the desired measurement resolution.

This is where electronic interpolation comes in. Interpolation circuitry within the read head or encoder interface electronics subdivides each grating period into finer increments. For example, a 20 μm pitch scale with 1000× interpolation yields an effective resolution of 0.02 μm (20 nm). The degree to which interpolation can be reliably performed depends heavily on the signal quality (amplitude, symmetry, phase accuracy) of the encoder's raw output.

Our optical linear encoders are designed with precision-lapped glass scales and low-noise optical read heads that produce clean sinusoidal signals, supporting interpolation factors up to 2000× while maintaining sub-micron accuracy across the full scale length.

How Linear Encoder Performance Affects Your VMM Measurements

Consider a typical 2D Video Measuring Machine or 3D Video Measuring Machine configured with X, Y, and Z linear encoders. Every dimensional reading the machine produces is ultimately a calculation based on the encoder positions recorded at the moment the measurement edge is detected. If the encoder introduces any non-linearity, periodic error, or thermal drift, these errors propagate directly into the measurement result — regardless of how good the optics are.

Common encoder-related errors in VMMs include:

• Periodic (cyclic) error: Caused by signal harmonics in the encoder output, this appears as a repeating positional error with a period equal to the grating pitch. It is often invisible in single-point measurements but becomes apparent in form measurements such as roundness or straightness.
• Thermal expansion mismatch: If the encoder scale's thermal expansion coefficient does not match that of the machine structure, temperature changes will introduce a scale error proportional to the axis length and the temperature delta.
• Contamination sensitivity: In production environments, oil mist, coolant spray, and metallic particles can settle on exposed scale surfaces, degrading signal quality and creating localized positional errors.

Our encoder design addresses these concerns through the use of low-thermal-expansion glass scales, hydrophobic scale coatings that resist contamination, and digital signal processing within the read head that filters harmonic noise before the position signal is output.

Integration with HANDING OPTICAL Measuring Systems

Our linear scales and optical linear encoders are standard components in our VMM and VMS product lines, but we also supply them as standalone components for customers who need to retrofit existing machines, build custom metrology fixtures, or upgrade the position feedback systems on older measuring equipment.

Encoder specifications available from us include:

• Scale lengths from 50 mm to 3000 mm (custom lengths available on request)
• Grating pitch options: 20 μm, 10 μm, 4 μm
• Resolution after interpolation: down to 0.1 μm
• Accuracy grade: ±1 μm/m (standard), ±0.5 μm/m (precision grade)
• Output signal: 1Vpp sine wave, TTL square wave, or RS-422 differential
• Operating temperature range: 0°C to 50°C

Beyond encoders, HANDING OPTICAL manufactures a full range of optical measuring instruments including Video Measuring Machines (VMM), Instant Vision Measuring Machines, Metallographic Tool Microscopes, and PPG Thickness Gauges for lithium battery production lines.

Conclusion

The linear encoder is the backbone of positional accuracy in any optical measuring machine. Selecting the right optical linear encoder — whether incremental or absolute, glass or steel scale, exposed or enclosed — is a decision that has lasting consequences for the measurement quality your VMM delivers over its service life.

If you are specifying a new Vision Measuring System, retrofitting an existing machine with higher-accuracy encoders, or simply want to understand more about how encoder technology influences your measurement results, we welcome your inquiry. Our engineers are available to provide detailed technical consultation tailored to your specific application.