LVDT Displacement Sensor For Sea Measurements

The submarine  is one of the harshest environments, and it is difficult for sensors to perform well. Especially when it needs to be used for 20 years in underwater. In a submarine environment, when the seawater depth reaches 15,000 ft, an external pressure of approximately 7,500 psi will be generated on the sensor. In such a high-pressure environment, many sensors will be incapable after working for a period, and the replacing  sensor will cost a lot. Normally, the maintenance service will also be a big expense.  So, extending sensor's life is a specified requirement in many applications. On the other hand, seawater will also corrode the sensor metal. Different seawater depth has different oxygen concentration, temperature, pH value, chlorine content, biological activity, conductivity and velocity, which accelerates the corrosion. Not only pitting, cracks but also inter-grain corrosion, they all can cause a malfunction.

Normally, LVDT is the only sensor that can provide an accurate and reliable performance under seabed conditions.

Turbid or contaminated water usually breeds many bacteria, which may also affect the performance of sensors. The high conductivity of sea water will promote the corrosion of the macro cell and will also rise up the temperature. Corrosion caused by microbialis is also a very serious problem. The installation conditions and sensor materials will affect the effects. especially when using a low-grade austenitic stainless steels. The most obvious manifestation is the degradtion of material performance, especially at the welded joints. If it is not examined and handled in time, it may cause a crack. Due to the pressure and the influence of seawater, making a reliable performance sensor become a big challenge. LVDT (linear variable differential transformer) with special alloy construction and airtight seal become the only sensor that can provide accurate and reliable performance under seabed conditions.

What is LVDT

The linear variable differential transformer (LVDT) is a position transducer that is noncontact, absolute reading, and has essentially infinite resolution. It comprises three or more coils within which a magnetically permeable core moves to provide variable coupling between the primary coil and the secondary coils (usually two). Although the detection technique is noncontact, there is often a mechanical arrangement added to keep the core positioned in the coil throughout the stroke. One example of this is the configuration called an LVDT gauge head. Practical linear sensors can be designed with a nonlinearity of less than 0.2% and full-scale ranges (FSRs) from less than 1 to over 100mm. Resolution is nearly infinite. Curved and rotary sensors are possible. Popular applications include industrial machinery, such as metal forming machines and in-process dimensional verification, as well as automotive and commercial products.

  LVDTs require a set of driving and conditioning electronic circuits. A typical LVDT is sold as a sensing element with a core, the electronic circuit being supplied as a separate device. A LVDT that includes all the required electronics within its housing is often called a dc LVDT because it operates on a dc power supply and has a dc output, although the internal operation includes the normal ac driving, demodulation, and signal-conditioning circuitry.