Encoder scale pitch: how to choose the right pitch for speed and resolution.

What does encoder scale pitch mean?

Encoder scale pitch is the repeat distance of the optical pattern on the scale. The readhead detects phase changes as it moves across those periodic features. Electronics then interpolate the signal to produce a smaller digital resolution than the physical pitch itself.

Pitch therefore should not be confused with accuracy or resolution. A 20 µm pitch may be interpolated to a sub-nanometer output increment in a specified configuration, but scale accuracy, interpolation error, mounting and machine mechanics still determine the real position result.

Fine pitch and coarse pitch: the engineering trade-off

A finer pitch gives the sensing system more cycles over the same travel and can support fine interpolation. It may also impose tighter optical, alignment, contamination and signal-quality requirements. A coarser pitch produces fewer cycles per unit distance, which can simplify high-speed signal handling and preserve margin at demanding velocities.

Neither choice is automatically better. The selection should start with the required resolution and error budget, then check the maximum speed, output frequency, controller input and installation envelope. The scale is part of a matched readhead-and-interface system.

• Finer pitch can support finer interpolation
• Coarser pitch can reduce signal frequency at a given speed
• Interpolation quality depends on signal amplitude and alignment
• Scale pitch does not establish scale accuracy

How pitch, speed and output frequency are connected

As linear speed increases, the optical signal frequency increases. A simplified first check relates speed to pitch: the faster the axis moves and the smaller the pitch, the more cycles the interface must process. Interpolation and output coding then determine the required electrical bandwidth.

Use the supplier’s resolution-versus-output-frequency data for the exact configuration. Do not publish or design around a single maximum speed without stating the resolution, interface frequency and controller conditions that produce it. This is especially important when comparing a fine-pitch miniature encoder with a high-speed coarser-pitch encoder.

Examples from the SENFU encoder range

The SMG20 uses a 20 µm scale pitch and offers resolution options down to 0.5 nm in the documented configuration. Its compact readhead makes it relevant to miniature precision stages, but the final speed must be selected together with the output frequency and interpolation setting.

The SMG26 uses a 256 µm pitch and is positioned for high-speed linear motion, with documented configurations down to 4 nm resolution. These two products illustrate why pitch should be evaluated with motion profile and interface data rather than used as a standalone ranking.

A practical encoder pitch selection workflow

First write down the travel, maximum speed, acceleration, resolution, accuracy, repeatability and controller interface. Then define the scale material, mounting direction, readhead gap, cable route and temperature range. Finally, ask for the configuration-specific signal, frequency and installation data.

For rotary axes, the same principle applies to disc diameter, line count, angular pitch and interpolation. The correct decision is the one that leaves measurable margin in the complete axis, not the one with the smallest number in a catalogue column.

• Define the machine error budget
• Calculate signal frequency at maximum speed
• Check interpolation and controller bandwidth
• Verify scale accuracy and temperature
• Confirm the mechanical installation drawing