Magnetostrictive Technology Made Easy

In a Gemco Series position sensor, a current pulse is sent down a magnetostrictive wire in a specially designed waveguide (Figure A). The interaction of this current pulse with the magnetic field created by the movable magnet assembly produces a torsional strain pulse on the wire, which travels at sonic speed along the wire (Figure B). The strain pulse travelling up the wire is sensed by a small induction pickup coil in the head assembly of the LDT. The position of the movable magnet is determined with high precision by measuring the time between the launching of the current pulse and the arrival of the torsional strain pulse. The result is highly accurate non-contact position sensing with absolutely no wear on the sensing element. 

Hysteresis

The difference in indicated position when the same point is reached from two different directions. The deviation in indicated position when a point along a stroke length is approached repeatedly from the same direction.

Magnetostriction

A magnetic field produces a small change in the physical dimensions of ferromagnetic materials on the order of several parts per million in carbon steel and, conversely, a physical deformation or strain (or stress which causes strain) produces a change of magnetization in the material.

Linearity

The degree that the indicated position of the magnet at points along the stroke varies from the actual physical position. Linearity of an LDT is expressed in absolute error or as a percentage of the active stroke length.

Resolution

Resolution is the smallest incremental change in position along the stroke length of the sensor that can be detected and indicated in an output. When using LDTs with analogue output (i.e., voltage or current outputs), resolution is limited by the amount of power supply-induced output ripple and the sensitivity and/or design capabilities of the receiver electronics. Digital system resolution is defined by a specific value.

Recirculation

A method used to improve the resolution of a system using digital LDTs. The on time of the pulse width output is multiplied by a specific factor. This multiplication provides more counting time for the counter in the customer’s electronics, thus improving the resolution.