Optical or magnetic linear encoders are traditionally used to provide position feedback in applications with frequent product changes or and sophisticated machining. Linear encoders improve the positioning accuracy of electromechanical actuators in these situations by providing direct position feedback at the load to allow machines to compensate for backlash or other sources of error. Linear encoders are also used as the primary feedback device for linear motors (which directly convert electrical energy into linear motion.)

However, design engineers are increasingly replacing encoders with magnetostrictive linear position sensors to improve productivity and lower cost of ownership. The latest magnetostrictive sensors are sophisticated for installation on most electrical linear motors, electromechanical actuators, and pneumatic, hydraulic, spindle, or power-grip belt drives that perform high-accuracy, dynamic positioning.

Some designers continue to specify linear encoders for applications in which magnetostrictive sensors would work, the former is a familiar technology. However, as we'll explore, magnetostrictive sensors can replace encoders to reliably deliver dynamic closed-loop control.

Continuous feedback

One key benefit over incremental linear encoders is inherent absolute positioning. A linear encoder is basically a ruler or scale marked with increments that a reading head counts as it travels past. Each mark encodes a specific distance, so the encoder can determine how far it has moved from a reference point. There are limitations to that approach, because there are maximum and minimum speeds at which the reading head can travel before losing track of marks it scans.

Why is there a minimum speed limit? Dropping below minimum speed causes cogging because reading in between marks produces no feedback for the amplifier — so forces it to speed up until the next signal is received. The result is an uncontrolled jerky movement unacceptable in most low-speed applications. In addition, contamination or disturbances can interfere with the encoder's ability to read the marks. Magnetostrictive position sensors, on the other hand, have no maximum or minimum speed limitations because they are absolute position devices, and sample position at a fixed rate and resolution — independent of speed.

Absolute position sensors also eliminate returns to a reference mark or home position after a power reset. At any point in time, an absolute position sensor can report its location without requiring a movement. This boosts safety and productivity.

Magnetostrictive sensors report position (and any error) to a motion system's controls. Then the motor drives output motion commands to compensate.

Magnetostrictive sensorss with two different interefaces are suitable for use with linear motors.

Accuracy

Older magnetostrictive sensors did not have the resolution and accuracy of better optical linear encoders. However, advancements in magnetostrictive technology over the years have now increased the overlap in performance. Magnetostrictive sensors can reach sub-micron resolutions to permit displacements at speeds of only 0.5 mm/sec; measurement cycle times down to 100 µsec, to track fast motion; linearity to ±0.01% and typical repeatability of 2.5 µm. Real-time linearity correction is available for measurement accuracy down to 20 µm or better.

Switching to magnetostrictive sensors may be appropriate in harsh environments, where an optical encoder's reading head (glass and laser) can be contaminated by dust or oil, making the marks unreadable and causing optical signal loss. The switch may also be appropriate where mechanical vibrations can damage an encoder's glass.