Magnetic biasing and pre stressing

Since the magnetostriction is produced by the rotation of magnetic moments, a magnetostrictive transducer driven by an alternating magnetic field vibrate at twice the drive frequency and the motion takes place in only one direction. For dynamic applications, like vibration suppression, reversal of actuation strain is very much necessary. For such applications, these actuators are operated with a biased magnetic field to obtain reversal of strain with respect to the biasing centre [Figure 29.3].

Figure 29.3 Effect of magnetic bias on the strain produced by a magnetostrictive transducer

This leads to reduction in the available actuation strain by approximately 50 per cent. This has been the prime reason for magnetostriction loosing its cutting edge over the piezoelectric materials. By applying magnetic bias, the frequency of the input is preserved and the output is made bidirectional with substantial increase in the ratio of output per input. To operate the transducers in the vicinity of the desired bias point, a permanent magnet is employed in combination with a static field. Permanent magnet biasing has the advantage of substantial power saving but it adds to bulk and weight. Conversely, DC generates considerable power losses through ohmic heating but facilitate savings in bulk and weight.