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Magnetostrictive Linear Displacement Transducers

A magnetostrictive linear displacement transducer (hence forth to be called a MLDT) is a device used to precisely track the position of a linear actuator.

But what does that mean? How do they work?

Let's take a closer look at the mysterious MLDT!
We will examine the structure of these devices and the principles behind their operation.

The Structure

A MLDT looks suspiciously simple.

There's a metal rod with some fancy computer bits on the end and a magnet that moves along the rod.

But there are some really complex things going on internally.

First of all, the rod is, in fact, hollow.

Ferromagnetic Wire
Non-ferromagnetic Wire
Dampener
Brace

Inside the rod there is both a ferromagnetic wire and a non-ferromagnetic wire, several non-conductive braces to keep everything in place, and a dampener on the far end.

The ferromagnetic wire is known as the waveguide. This becomes important later.

The rod is connected to the electronics housing.
This contains the electronics required to emit and detect signals from the rod.

Housing with onboard processing
Housing with detached processing

The electronics housing may also include the processing electronics needed to interpret signals from the rod, or it may send the data to be processed elsewhere.

The other key component to a MLDT is the magnet, or in some cases multiple magnets.

These are permanent magnets that can come in various shapes, depending on the application.

The magnet is positioned in such a way that it is near the rod, but never actually makes contact.

Since there is no contact between the rod and the magnet there is no wear and tear on the sensor.

This allows MLDTs to have very long operational lifespans.

Mounting

There are many ways to mount a MLDT.

The most common method is internal mounting. Let's look at an example using a hydraulic cylinder.

This means having the MLDT rod inserted into what is known as a "gun drill hole" which passes through the center of both the cylinder rod and cylinder piston. The magnet is attached to the piston.

Gun Drill Hole
Piston
Magnet
Magnet Isolator

A magnet isolator (a non-magnetic spacer) is often used to ensure that the metal of the piston doesn't interfere with the magnetic field of the permanent magnet.

As the cylinder piston cycles it moves the magnet along the MLDT's rod.

An MLDT can also be mounted externally.

In this situation the MLDT will be mounted in parallel with the actuator being measured and then physically linked together.

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