Draw Wire vs. Variable Inductance Position Sensors

CPI Linear draw wire position sensor

A recent article in an EET Asia technology magazine discussed the advantages of linear position sensors based on variable inductance, as compared to those based on either magneto-strictive or potentiometer based sensors. The article was really a thinly veiled plug for variable inductance sensors (written by the company that makes them), and it chose to compare them to potentiometer based sensors, and traditional magnetostrictive sensors. Notably absent were the family of modern draw-wire sensors made by CPI that are having so much success in harsh duty applications.

LVDT Based, Draw-Wire Linear Position Sensors.

Readers of this blog will find some of the advantages of inductive sensors stated in the article sound vaguely similar to the advantages of CPI SL-Series Draw wire sensors that have been discussed previously.

  1. Can be used in subsea applications to 3000m and 3000 psig of hydraulic pressure.
  2. Completely non-contacting solution.
  3. Capable of absolute position measurement does not require dynamic scaling on startup.
  4. One button zero-calibration.
  5. “Easy” field replacement of magnetostrictive sensors.

Sound Familiar? While this sounds like a credible alternative for harsh duty usage, specific comparison to our LVDT based draw-wire sensors like the SL 1200 series is warranted. For starters and perhaps most glaringly, consider the hydraulic stroke length serviceable by this technology and you will immediately understand that for stroke lengths greater than 24” the technology is not viable. Furthermore the “Rod” inherent in Magnetostrictive sensor design is still present in Inductive sensor design, it is just used for a different purpose. As such it is still prone to the same mechanical and electrical failure modes related to vibration and electromagnetic interference, that a purely magnetostrictive sensor would be.

Finally for subsea applications, we note that a rating to 3000m and 3000psig may not provide the operating design margin required for many large scale hydraulics on the deep sea bed. It is in these applications that CPI sensors were born to live. The CPI SL-Series sensors have no dynamic seals, and no pressure vessels in a subsea deployment. Our sensors have been deployed at operational depths of 5000m and almost any hydraulic pressure.

CPI Linear Position Sensors for Subsea Applications

If you require robust operation on the sea floor at stroke lengths up to 10 meters, CPI sensors were designed for your application. Call one of our engineers today to discuss your project today.