Novel Encoder Feedback for Position and Velocity Accuracy

For applications that require position or velocity feedback integrated into electromechanical actuators, the LinACE absolute linear encoder is a novel approach. The encoder has an accuracy of ±5 µm and delivers full closed-loop control.

The encoder comprises a solid steel shaft with fully integrated scale and a sliding read-head module with no internal moving parts, making the entire system resistant to shock and vibration. It is reliable over a wide temperature range, with an operating temperature range from -40°C to +85°C.

Diagram of the absolute code under the shaft surface of the LinACE encoder

As with most actuators, the shaft is made from steel, to exploit its ‘soft’ magnetic characteristics. The absolute code under the shaft surface is composed of small circumferential grooves, which are filled with non-magnetic material such as hard chrome or copper, depending on the application. Finally, the surface is plated with hard chrome and polished to a fine finish.

Since the scale is passive, external magnetic fields have negligible effect and LinACE is immune to dirt contamination. Because the scale extends all round the shaft, it can even be rotated as it moves in and out without losing position. Furthermore, the scale manufacturing technique retains a hard, smooth outside surface on the shaft, so operation of the actuator is not affected by its presence. Shaft diameters range from 4 mm to 30 mm and measuring lengths are up to 750 mm.

LinACE absolute encoder being used on an example actuator

The scale is read by a module that incorporates a custom-made ASIC with an array of Hall-effect sensors, for a reliable, compact, solid-state detection method. Signals from the ASIC are processed to provide a range of industry-standard output configurations, allowing replacement of traditional analog voltage or current transducers, classical potentiometers and LVDTs. Furthermore, absolute positions can be communicated in a range of protocols including CAN, SSI, SPI, I2C, RS422, RS485 asynchronous serial and PWM, with a range of resolutions from 100 µm to 0.5 µm.

The module includes two bronze bearings that are integrated in the stainless steel housing for smooth movement while maintaining precise alignment of the sensor over the shaft, even under hostile conditions.

LinACE absolute shaft encoder.

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

LinACE can be fitted directly onto the front of an actuator, minimizing overall system size, reducing complexity and eliminating potential failure modes by removing the need for parallel measuring systems. Available in kit form, it can also be integrated into hydraulic and pneumatic actuators. The device lets designers consider new possibilities for the use of actuators in space-limited and demanding closed-loop applications.

Renishaw
www.renishaw.com