Several of my topics in the past have dealt with gauging and measurement. Most of the systems I have worked on in the past used LVDTs, encoders, optical sensors or machine vision when it came to accurately measuring the physical dimensions of an object.
The decision on what kind of measurement technique may be used to measure an object involves several considerations. Can the object be touched? If not, optical methods such as machine vision, photoeyes or lasers may be used. If the object can be contacted physically methods such as LVDTs, encoders and magnetostrictive methods can be used. LVDTs are quite accurate but typically have a very short range. Since the output is analog, resolution can also be a problem if precision is needed over a large dimension.
An excellent option for measuring distance is time-based magnetostrictive position sensing. Magnetostriction uses a ferromagnetic measuring element known as a waveguide, along with a movable position magnet. The magnet generates a direct-axis magnetic field within the waveguide. When a current or “interrogation pulse” is passed through the waveguide, a second magnetic field is created radially around the guide. The interaction between the two fields generates a strain pulse that travels at a constant speed from its point of generation at the magnet (the measuring point) to the end of the waveguide. A sensor detects the pulse and generates a highly accurate positional reading through the electronics of a high-speed counter.
Magnetostrictive sensors provide an absolute position reading that never needs recalibration or homing after a power loss. This can be a significant advantage over using LVDTs and encoders. The only limitation of this technology is that it cannot be used for short distance dimensional measurements; the minimum range is about 25mm. A well-known manufacturer and the first to develop products using this technology is MTS Systems, developer of Temposonics sensors.
Magnetostrictive sensors are also often used in motion control applications where the stroke of an actuator is defined. The sensor can be built right into the axis or easily adapted. Though somewhat more expensive than an encoder or resolver, they are very useful in the right motion application.
I haven’t used magnetorestrictive sensors, and probably won’t, because their resolution and repeatability isn’t that good compared to a linear encoder. Although they’re probably a good alternative to LVDTs, when I’ve used a linear feedback device, I’ve always needed encoder-level performance.
For example, a SSI-interface MTS R-series MR sensor has a best resolution of 100 nm, a repeatability of at least 2500 nm, and a non-linearity of at least 40000 nm. The MicroE Mercury II linear encoder has a best resolution of 1.2nm, and a repeatability of 20 nm.
I do use them. I don’t think there’s a big issue regarding their resolution. There’re plenty of different sensors with different capacities here http://www.directindustry.com/cat/position-measurement/magnetostricitive-position-sensors-AG-1262.html for instance. Just have to pick a good one.