Causes of magnetostrictive sensor problems

Magnetostrictive sensors, whether SSI, Start/Stop, or PWM, are rugged sensors, but like everything else, can fail. Troubleshooting can be difficult, and the symptoms can vary widely. At Delta, we say that the normal symptoms of a failed magnetostrictive sensor is “weird behavior”.

Here is a list of behaviors and causes we have encountered:

  1. Intermittent signal.

    • Internal wear due to vibration.
    • Bad cable.
  2. No signal.

    • Bad magnet
  3. Regular noise

    • Noisy power supply.
  4. Good signal, but doesn’t follow cylinder always.

    • Magnet fell off piston.
  5. Signal follows, but has lots of jumps of and down.

    • SSI Gray/Binary code setting is wrong in RMC.

Key Words: MDT, transducer, linear sensors

Other things I have seen:

  • Running a Balluff Transducer with an MTS/Temposonics magnet or vice versa does not always work. (Balluff can make a transducer for use with an MTS magnet as an option, MTS probably can do similar, I just have no experience)
  • Switching Data+/Data- on an SSI signal is not OK like it is for switching A+/A- on an incremental encoder.
  • Switching Data+/Data- on a Gray coded SSI transducer can make a 30000lb casting jump in the air (don’t ask)
  • Putting the transducer inside of the cylinder tells you exactly where the piston magnet is, not necessarily where the end of the rod or your end tooling is. The rod can grow with temperature and outside things like bearing play and machine flex are not measured with an internal transducer. Some applications require the transducer to be mounted outside of the cylinder.
  • The maintenance guy may think that to remove the transducer he has to take out the screws in the head of the transducer (the ones that hold the transducer together). We had a scenario where this happened and one screw was accidentally left loose. This caused random errors on the transducer. Our best theory was that this messed with the integrity of the ground in the transducer. Once we tightened the screw and everything worked, we didn’t worry about investigating further.
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