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Tough linear encoder with cylindrical scale offers 5µm accuracy

06 June, 2012

Renishaw’s Slovenian motion sensing associate, RLS, has developed a novel absolute linear encoder technology with an accuracy of ±5µm and capable of delivering full closed-loop control. The heavy-duty LinAce encoder is designed to be integrated into hydraulic, pneumatic or electromechanical actuators to provide position or velocity feedback.

The encoder (above) comprises a solid steel shaft with an integrated scale, and a sliding read-head module with no internal moving parts, making it highly resistant to shock and vibration. The shafts are available in diameters from 4–30mm and in measuring lengths up to 750mm.

Under the shaft surface there is an absolute scale formed from small circumferential grooves filled with a non-magnetic material such as chrome or copper, depending on the application. The surface is plated with hard chrome and polished to a fine finish. The images below show a cutaway view of the shaft (top) and a closeup cross-section of its surface (bottom).

Placing the scale on the actuator shaft delivers several benefits. Thanks to internal compensation inside the read-head, the system has an accuracy of ±5µm. And, because the scale is marked directly onto the actuator shaft, it gives a direct measurement of the actual position of the shaft, eliminating hysteresis and backlash, while improving repeatability and stability.

Because the scale is passive, external magnetic fields have little effect and the encoder is immune from contamination by dirt. The encoder has an operating temperature range from -40 to +85°C.

Because the scale extends around the shaft, it can be rotated as it moves in and out, without losing position. Furthermore, the shaft’s hard, smooth outside surface does not affect the operation of the actuator.

The scale is read by a module incorporating a custom-made ASIC with an array of Hall-effect sensors. Signals from the ASIC are processed to provide various industry-standard outputs, allowing the encoder to replace traditional analogue voltage or current transducers, potentiometers and LVDTs. Absolute positions can be communicated using a range of protocols, including CAN, SSI, SPI, RS-422, RS-485 asynchronous serial and PWM, with resolutions from 0.5–100µm.

Two bronze bearings are integrated into the read module’s stainless steel housing. They provide smooth movement while maintaining precise alignment of the sensor over the shaft, even under hostile conditions.

The LinAce can be fitted onto the front of an actuator, minimising system size, reducing complexity and eliminating potential failure modes by removing the need for parallel measuring systems. This will allow OEMs to consider new ways of using actuators in space-limited and demanding closed-loop applications.

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