Absolute Linear Position Sensors

RF25x sensors are contact-type displacement transducers used for high-precision measurement of:

• Linear displacement and position of moving components
• Part dimensions — width, height, step geometry
• Surface runout of rotating or reciprocating parts
• Flatness deviations across a surface
• Object deformation under load or thermal influence
• Profile of surfaces requiring contact-based measurement

With accuracy of ±1 µm (±2–3 µm depending on model and range) and resolution selectable down to 0.1 µm, RF25x sensors are suited for laboratory-grade dimensional inspection, precision machining quality control, and any application where sub-micron resolution is required from a contact measurement device.

The two series are designed for different deployment environments.

RF251 is built for industrial conditions. It carries an IP57 enclosure rating, operates across a wide temperature range of -40°C to +50°C, and is available in two measurement ranges: 3 mm and 25 mm. Output is digital only via RS422. There is no display, no analog output, and no synchronization input — the RF251 is designed for integration into automated systems where data is read by a controller, not an operator.

RF256 is oriented toward laboratory and quality control environments. It offers three measurement ranges — 15 mm, 35 mm, and 55 mm — and supports a broader set of outputs: RS485, SSI, RS232, analog (0–20 mA or 0–10 V), quadrature encoder emulation, synchronization input, and two programmable logic outputs. An optional built-in display and battery power option make it suitable for manual inspection and standalone gauging stations. Enclosure rating is IP50, operating temperature -10°C to +50°C.

In short: RF251 for industrial automation and harsh environments; RF256 for laboratory measurement, manual inspection, and applications requiring analog output or encoder emulation.

Output options differ by model series.

RF251 provides RS422 digital output only — suited for direct connection to motion controllers, PLCs, and measurement systems supporting RS422.

RF256 supports a wider range:

• RS485 with SSI — for high-noise industrial environments and long cable runs
• RS232 — for direct PC connection and laboratory instruments
• Quadrature encoder emulation (EncD5 / EncD10) — 5 µm or 10 µm period, allowing RF256 to replace incremental encoders in existing systems without controller changes
• Analog output: 0–20 mA (load ≤ 500 Ω) or 0–10 V — for PLC analog input cards and data acquisition systems
• Two programmable NPN logic outputs — for limit switching, tolerance signaling, or process control
• Opto-isolated synchronization input — for triggered measurement synchronized with external events

Note that analog output and encoder emulation (EncD) cannot be combined in the same device, and analog output cannot be used simultaneously with logic outputs. Interface configuration is specified at the time of ordering.

Resolution is selectable at the time of ordering: 0.1 µm, 0.5 µm, 1 µm, 5 µm, or 10 µm depending on the application requirement. For most precision dimensional measurement tasks, 0.1 µm resolution is the appropriate choice.

Accuracy at 20°C is ±1 µm for RF251-3 and ±2–3 µm for larger range models. This level of performance places RF25x sensors in the same accuracy class as high-grade contact gauges and bench micrometers, with the advantage of digital output and direct integration into measurement systems.

These specifications make RF25x sensors applicable to precision tasks such as bearing race inspection, gauge block verification, runout measurement of precision shafts, and flatness checking of reference surfaces — anywhere sub-micron resolution and micron-level accuracy are required from a contact displacement sensor.

Yes, in most cases. The RF256 series supports quadrature encoder signal emulation with periods of 5 µm (EncD5) or 10 µm (EncD10), which are standard incremental encoder signal formats. This means an RF256 sensor can be connected to a controller already configured for an incremental encoder without modifying the controller firmware or signal processing logic.

The advantage of making this substitution is that the system gains absolute position measurement — eliminating homing cycles and position loss on power-down — while maintaining full compatibility with existing hardware. This is particularly useful in retrofitting precision measurement stations, CMM accessories, or gauging fixtures where the controller is fixed but the measurement performance needs to be improved.

Three parameters drive the selection: measurement range, environment, and required output type.

Measurement range determines the model. RF251 covers 3 mm and 25 mm. RF256 covers 15 mm, 35 mm, and 55 mm. Choose the smallest range that comfortably accommodates your full stroke — smaller ranges generally deliver better accuracy and resolution performance.

Environment determines the series. If the sensor will be installed in a machine tool, production line, or any environment with coolant, dust, or temperature below -10°C, RF251 with IP57 and -40°C operating range is the correct choice. For laboratory, quality control room, or inspection station use, RF256 provides the additional output flexibility and display option that manual operation requires.

Output type finalizes the model configuration. If your system uses analog inputs, specify the 0–20 mA or 0–10 V output on RF256. If you are replacing an incremental encoder, specify EncD5 or EncD10 emulation. If you need direct digital integration with a PLC or measurement controller, RS485 with SSI is the most robust option for noise-sensitive environments.

For applications with specific accuracy requirements or unusual installation geometry, contact Riftek engineers — sensor parameters can be adjusted for task-specific configurations.