Session: 11-07-01 Measurements of Thermophysical Properties

Paper Number: 72946

Start Time: Tuesday, 03:30 PM

72946 - High Temperature LVDT Design and Modeling 

The linear variable displacement transducer (LVDT) is a one-dimension displacement sensor. It is widely applied in the engineering and industrial field due to its wide measurement range and high resolution. In nuclear engineering, LVDT is being developed for the high-temperature and radioactive environment. The Halden Reactor Project (HRP) developed a series of LVDT sensors to measure the nuclear fuel pressure, fuel rod expansion, fuel swelling, and cladding elongation. The Idaho National Laboratory has conducted LVDT experiments in the Advanced Test Reactor (ATR) to explore its high-temperature performance. They used LVDT to do an in-pile creep test in reactors. However, no reliable theoretical model exists to predict the high-temperature LVDT results accurately and guide future nuclear applications. The objective of the current work is to build predictive capability.

The LVDT typically consists of one primary coil in the middle, two secondary coils on two sides, and a sliding magnetic core inside a cylindrical bobbin. When an alternating current goes through the primary coil, the voltage difference between the two secondary coils represents the magnetic core's axial position. The relationship between core displacement and voltage difference within two secondary coils is almost linear. And when the magnetic core is in the middle, the voltage difference is zero. Therefore, based on voltage difference, the core displacement from the middle position can be accurately obtained.

Neubert developed the traditional approximate LVDT theory. However, the distribution of magnetic flux is assumed to be linear, and the non-uniformity part is ignored, resulting in a significant error compared with experiments. Later, Tian built a three magnetic flux circuit model to achieve a better result. But the approximation in obtaining the magnetic conductance in the model leads to errors. And essential parameters, such as core permeability and coil thickness, were still ignored. Therefore, we still need a high-temperature LVDT performance model because the magnetic core changes permeability significantly near the material curie temperature.

This paper utilizes FEM to directly solve the corresponding Maxwell equations to analyze the LVDT performance and improve the existing magnetic circuit model. The research focus is the permeability of the magnetic core. It is found that, as long as the permeability is greater than 1000, increasing it will have little influence on the LVDT voltage output. When the temperature rises to near curie point, which means the permeability is decreased significantly, any change in permeability will influence the voltage output considerably. Other parameters, such as the outer and inner radius of coils, are also investigated to help future LVDT design and application in high-temperature and radioactive environments.  

Presenting Author: Yuan Gao University of Pittsburgh

Authors:

Yuan Gao University of Pittsburgh
Heng Ban University of Pittsburgh
Austin Fleming Idaho National Laboratory