Machine Design

Position feedback eliminates homing routines

An absolute position-feedback (APF) linear-displacement transducer built into linear actuators gets rid of the need for homing routines at power up or power loss.

Here's how APF works. The system generates an electromagnetic pulse that travels the length of the linear transducer, creating a circular magnetic field around the transducer. A magnet embedded in the carrier is mounted with its magnetic field at right angles to the transducer's electromagnetic field. Where the two fields intersect, a magnetorestrictive effect elastically deforms the transducer. This deformation moves away from the magnet in both directions at 9,285 fps as two mechanical waves. At the far end of the actuator a damper absorbs one wave. At the other end, a converter turns the mechanical wave into an electrical signal. The time it takes the mechanical wave to get there is proportional to the distance between the magnet and converter. By measuring this time, carrier position can be accurately determined.

Instead, the transducer sends analog feedback to the controller or PLC telling it the exact position of the load. The device also detects work-point variances throughout the cycle, so there’s no need to stop for manual setups. APF measures position directly, eliminating the effects of screw or nut backlash from positional feedback data. The APF option is currently available on BC3 Pneumatic and B3S Electric Series actuators from Tol-O-Matic, Hamel, Minn. ( Unlike fixed-length sensors, APF can be configured in incremental stroke lengths from 2 to 156 in.

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