LVDT magnetic shield
Publication Date: 2014-Jul-31
The IP.com Prior Art Database
Linear variable differential transformers (LVDTs) measure displacement and may be used in a downhole tool to measure displacement of actuators based on magnetic principles. Any change in the magnetic field around an LVDT may alter the output and ultimately lead to a faulty displacement measurement. For example, a change in the earth’s magnetic field and static magnetization of the housing of the tool can significant impact the LVDT. Therefore, magnetic shielding of an LVDT used in conjunction with a downhole tool may improve the performance.
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A Linear variable differential transformer (LVDT) is used in a downhole tool to measure displacement of various actuators. The basic operation of an LVDT feeds a current to the primary of the transformer, and produces an induction current at its secondary. The magnitude of the output at the secondary is determined by the position of a magnetic rod inside the LVDT. Any change in the magnetic field can alter the output at secondary and ultimately indicate the wrong position when the displacement is measured.
When an LVDT is used for downhole tool position measurement, the change in the earth's magnetic field and/or the static magnetization of the housing of the tool may cause significant error in the output of the LVDT. Therefore, a magnetic shield may improve the performance of an LVDT used in connection with a downhole tool.
The proposed magnetic shield is fabricated of mu metal, and wraps around the LVDT. In one proposed form: the thickness of the hollow cylinder is 2 mils, the length is 9.510 inches ± 5mils, and the diameter
0.88 inches ± 5mils. The cylinder should cover the entire area of the LVDT except the two ends, where the rod is free to move. Such a cylinder is available in a pre-made assembly by a mu metal manufacturer.