Precision Under Pressure: Subsea Linear Sensors and their Engineering

 

Putting a precision linear sensor into a subsea environment is one of the most difficult challenges in the sensing world. Sensors produce signals and they are inherently electronics-based devices. Liquid immersion is the enemy of all things electronic, including conductors, chips, circuits, and wiring connections. Anyone who has ever dropped their phone into salt water will know this well. Salt water immersion adds the factor of aggressive corrosion to the challenges of liquid immersion. Pretty much anything you put into salt water is going to degrade, or some cases even dissolve. This includes most metals, and it includes all electronics and electrical components.

What Doesn’t Work Under Water

Liquid immersion (including salt water) generally rules out the use of high tech non-contacting distance sensors like acoustic (ultrasonic, sonar), microwave (radars), and light-based (lasers, TOF). For example, microwave sensors simply don’t work in a liquid medium. That is why submarines don’t use radar when underwater. Subs DO use sonar however, but this is a poor choice for a precise linear position sensor. Sonar has to be specifically calibrated for the properties of the liquid that it operates in; especially the properties of pressure, temperature, and density. Even if calibrated for, these factors can fluctuate, inducing errors. Under perfect conditions sonar sensors don’t generally have the precision required for industrial systems, and they won’t get the job done when a precision linear subsea sensor is needed. Lasers, LIDARs, and other light-based distance measuring technologies also don’t work underwater. This is mainly due to the diffraction and attenuation of light under water, among other factors.

What Does Work Under Water

A mechanically-connected linear sensing system will work underwater. By this we mean a device that is physically connected between two objects whose distance from one another is to be measured. Since those objects typically move, the mechanically connected sensor system must be able to adapt its length (if the objects didn’t move, there would be no need for a linear sensor). Liquid immersion demands that only the most robust and practical mechanical sensor platform be used. For very short distances of up to about a meter, a rod-type magnetostrictive, or inductive linear sensor can be used (assuming the fixed-length rod can be accommodated to the system design). For anything over a meter, the best solution is usually a draw wire linear sensor. The CPI Subsea Draw Wire is a special type of draw wire sensor that will function reliably in a subsea environment. The presence of water (including salt water) doesn’t affect the way the sensor operates. This makes it one of the very few practical linear sensing platforms for subsea.

 

 

A Linear Sensor Made For Subsea

Most draw wire sensors utilize a rotary encoder or potentiometer to convert the rotation of the draw wire drum into position signals. This won’t work in subsea conditions. Most rotary encoders and potentiometers are not designed for submerged operation. Those that try to be must use dynamic rotary seals to keep the water out. This is a poor solution. Dynamic seals are wear items, and as depth increases, the outside water pressure builds up to force water past the seal. The use of heavier seals can significantly affect the rewind torque of the draw wire mechanism. It’s just not a good solution. The CPI Subsea Draw Wire sensor doesn’t use a rotary encoder or potentiometer. Instead it uses a micrometer thread coupled to a short-range magnetostrictive transducer. There are no dynamic seals needed. Magnetostrictive transducers are extremely accurate and are used routinely in industry as hydraulic system components. They are sealed to 5000 psi and above, and they are designed to operate in a liquid medium. CPI adds a specialized pressure-resistant subsea electronics connector to the magnetostrictive transducer. This provides an ideal short-range linear transducer to build into our subsea draw wire sensor.

A Highly-Engineered Draw Wire

The draw wire system that CPI uses in subsea applications is a derivative of the company’s LRL (Linear to Rotary to Linear) draw wire platform. It works this way: Pulling the draw wire out (long linear motion) causes the drum to rotate (rotary motion). The drum is threaded in the center and it moves along a threaded central shaft as it turns, like a micrometer (short linear motion). The micrometer action is precise and it is coupled magnetically with the precision transducer as noted above. This design avoids the need for complex and failure-prone sealed rotary transducers, while at the same time adapting the liquid-sealed high-pressure design of a common industrial sensor to the challenge of subsea environments.

 

Materials for Longest Life in Salt Water

Control Products engineers its subsea linear sensors to last as long as possible in the world’s harshest environment. We do this by carefully specifying materials for all of our mechanical components, including corrosion-resistant stainless steel, specialized marine brass alloys, mil standard anodized aluminum, UHMW polymers and high-density polyurethane. 

 

The CPI Subsea Draw Wire sensor is engineered to operate in submerged seawater environments. It’s the best choice for linear sensing applications in subsea systems. For  more information visit our website today: https://www.cpi-nj.com/

This blog was originally published at https://www.cpi-nj.com/precision-under-pressure-subsea-linear-sensors-and-their-engineering