Alessandro Danisi

Design of a Linear Variable Differential Transformer With High Rejection to External Interfering Magnetic Field

The sensitivity of linear variable differential transformer (LVDT) position sensors to external slowly varying magnetic fields represents a critical issue when these sensors are installed close to high-current cables or electrical motors with significant fringe fields. The resulting position error can reach several hundreds of micrometers against a specified uncertainty normally below a few micrometers. In this paper, the design of a LVDT position sensor with high rejection to external constant or slowly varying magnetic fields is addressed by exploiting the finite element method (FEM) simulator FLUX. A shield, isolated from the sensors magnetic circuit, has been considered to reduce the effect of magnetic fields on the secondary voltages of the LVDT. In addition, a dc current is used in order to polarize the magnetic circuit to reduce the sensitivity of the sensor to external interferences.

Characterization of Magnetic Immunity of an Ironless Inductive Position Sensor

The ironless inductive position sensor is a novel linear position-sensing device that should exhibit immunity to external magnetic fields while simultaneously guaranteeing high-precision measurements in harsh environments. This paper focuses on the characterization of the sensors working principle and magnetic field immunity. The sensor is tested, with both voltage and current supply, with different dc and slowly varying magnetic field patterns in order to prove its immunity and analyze the differences and similarities between the two supply cases. The measurements are performed with a test bench on a custom prototype, which is especially manufactured for this purpose.

Ironless position sensor with intrinsic immunity to external magnetic fields

A novel sensing structure for linear position measurement is presented. It is based on variable inductive coupling between different coils, as in Linear Variable Differential Transformers. Nevertheless, the novelty of the concept stays in the intrinsic immunity of the proposed sensor to external DC/slowly varying magnetic fields, achieved by avoiding the use of magnetic materials (i.e. magnetic cores). Features like absence of sliding contacts, infinite resolution and magnetic field immunity make this solution very attractive for harsh environment applications. Experimental measurements have been performed on a first prototype, in order to test the working principle and the immunity in a case study (longitudinal interfering field) with constant and ramped magnetic flux density profiles. The sensor showed to be practically insensitive to external fields.

Electromagnetic model of an ironless inductive position sensor

An ironless position sensor is a novel sensing structure which exhibits intrinsic immunity to external magnetic fields, since it is characterized by air-cored supply and sensing windings. This new solution may be of significant interest for applications where external magnetic fields are present. In this paper, an analytical model of the working principle of the sensor is proposed. The effect of the moving coil flux on the overall sensed magnetic flux is described. The modeling of such a sensor acts as a design tool of primary importance in the framework of the synthesis of the device. The model is verified by simulations of a finite element structure of the sensor, expressly prepared for this purpose.

Design optimization of an ironless inductive position sensor for the LHC collimators

The Ironless Inductive Position Sensor (I2PS) is an air-cored displacement sensor which has been conceived to be totally immune to external DC/slowly-varying magnetic fields. It can thus be used as a valid alternative to Linear Variable Differential Transformers (LVDTs), which can show a position error in magnetic environments. In addition, since it retains the excellent properties of LVDTs, the I2PS can be used in harsh environments, such as nuclear plants, plasma control and particle accelerators. This paper focuses on the design optimization of the sensor, considering the CERN LHC Collimators as application. In particular, the optimization comes after a complete review of the electromagnetic and thermal modeling of the sensor, as well as the proper choice of the reading technique. The design optimization stage is firmly based on these preliminary steps. Therefore, the paper summarises the sensors complete development, from its modeling to its actual implementation. A set of experimental measurements demonstrates the sensors performances to be those expected in the design phase.

Study of magnetic interference on a LVDT prototype

This paper presents a prototype of Linear Variable Differential Transformer, which has been designed, simulated and produced in order to observe, study and characterize the effects on its working of an external interfering magnetic field. The LVDT prototype has been first simulated, both in standard conditions and in case of external interference, by using the FEM software FLUX. The design guidelines coming from the simulations have been used to develop the prototype and a complete set of measurements has been performed on it. A comparison between simulations and measurements is then proposed.

Impact evaluation of environmental and geometrical parasitic effects on high-precision position measurement of the LHC collimator jaws

Measuring the apertures of the Large Hadron Collider (LHC) collimators, as well as the positions of their axes, is a challenging task. The LHC collimators are equipped with high-precision linear position sensors, the linear variable differential transformers (LVDTs). The accuracy of such sensors is limited by the peculiar parasitic effect of being rather sensitive to external magnetic fields. A new type of inductive sensor, the Ironless Inductive Position Sensor (I2PS), that keeps the advantages of the LVDTs but is insensitive to external magnetic fields has been designed, constructed, and tested at CERN. For this sensor, a detailed description of parasitic effects such as high-frequency capacitances and the presence of conductive shields and electric motor, in the surroundings is given, from analytical, numerical, and experimental viewpoints. In addition, proof is given of the I2PSs radiation hardness. The aim of this paper is to give a complete and exhaustive impact evaluation, from the metrological viewpoint, of these parasitic effects on these two fundamental sensor solutions.

Modeling and Compensation of Thermal Effects on an Ironless Inductive Position Sensor

The Ironless Inductive Position Sensor can be the ideal candidate for linear position sensing in harsh environment and in presence of external magnetic fields. Starting from the validated electromagnetic characteristics, this paper presents a model of thermal effects influencing the sensors position reading and an effective algorithm to compensate them. The compensation is performed without affecting the nominal sensors functioning and without using additional temperature probes, which would complicate the sensors assembly. The model constitutes the basis of this algorithm, which is then validated through experimental measurements on a custom Ironless Position Sensor prototype.

Modelling and compensation of thermal effects on an Ironless Inductive Position Sensor

The Ironless Inductive Position Sensor can be the ideal candidate for linear position sensing in harsh environment and in presence of external magnetic fields. Starting from the validated electromagnetic characteristics, this paper presents a model of thermal effects influencing the sensors position reading and an effective algorithm to compensate them. The compensation is performed without affecting the nominal sensors functioning and without using additional temperature probes, which would complicate the sensors assembly. The model constitutes the basis of this algorithm, which is then validated through experimental measurements on a custom Ironless Position Sensor prototype.

Influence of External Conductive Objects on the Performance of an Ironless Inductive Position Sensor

The ironless inductive position sensor (I2PS) is a novel device that measures high-precision linear position without being affected by radiation and external magnetic fields. Built on the basis of the linear variable differential transformer, the I2PS senses the variation of flux linkage between the supply and sense coils which is related to the linear position of the moving coil. This paper characterizes the magnetic performance of the I2PS through a detailed analysis of the impact of axisymmetrical external conductive objects on the sensor. This characterization is performed through a set of finite element simulations and through dedicated experiments. Axisymmetrical conductive objects result in offset voltages, but the differential measurement techniques combined with high-resolution calibration curves mitigate this effect.