Inductive Sensor with Contactless Interrogation for Conductive Target Detection ^†

Abstract

The contactless interrogation of an inductive sensor (IS) for conductive target detection is presented. The IS comprises a solenoidal coil of copper wire wrapped around a plastic pipe which is connected to a series capacitor to form an LC circuit resonating at the frequency f_r. A conductive target placed at different positions inside the pipe modifies the inductance of the coil, and in turn, f_r. An external interrogation coil (IC) electromagnetically coupled to the IS allows the f_r to be read through a contactless interrogation technique. The approach has been tested by varying both the position of a lead sphere adopted as the target and the interrogation distance d between the IS and IC. Without the sphere, the LC circuit has f_r0 = 2.51 MHz. The target sphere has been detected at up to |x| = 7.5 mm from the center of the IS coil with a frequency variation ∆ f_r = 180 kHz at x = 0.

1. Introduction

Inductive sensors (ISs) are widely used to detect conductive targets, such as, for example, wear debris in lubrication oil [1]. The detection relies on the variation in the IS inductance due to the interaction between the magnetic field produced by the sensor and the conductive target [2]. Conventional techniques exploit cabled solutions with the drawback of being unfeasible in enclosed or hermetic environments where cabling could be an obtrusive option [3]. This paper investigates the possibility of applying a contactless interrogation technique to an IS for conductive target detection by exploiting an external interrogation coil (IC). The proposed approach has been validated by adopting a millimeter-size lead sphere as the conductive target.

2. Materials and Methods

Figure 1a shows the block diagram of the proposed readout system, where R₁ and L₁ model the IC, R₂ and L₂ are the IS coil, and M is the mutual inductance dependent on the interrogation distance d between the coils.

The IS coil is connected in series with the capacitor C to form the LC circuit with resonant frequency f_r = ½π(L₂C)^1/2. As shown in Figure 1b, the IS coil and the IC are made by 100 and 20 turns of copper wire wrapped around a plastic pipe (8 mm diameter) and a FR4 support (32 mm diameter), respectively. The pipe is used to confine a lead sphere (3 mm diameter) which is moved along the x direction by a controlled positioning stage. The sphere induces variations in L₂ and R₂ according to its position x, thus modifying f_r. By measuring the IC impedance Z(f) = R(f) + jX(f), f_r can be obtained as the frequency f, where R(f) reaches its maximum value [3].

3. Discussion

Without the sphere, f_r0 = 2.51 MHz was measured. Figure 2a shows the R(f) for two selected sphere positions (x₁ = −0.5 mm and x₂ = −7.5 mm) while varying d, showing that d acts only as a scaling factor. Figure 2b shows f_r as a function of x for different values of d. The sphere was detected up to |x| < 7.5 mm, while at x = 0, a maximum frequency variation of ∆f_r = f_r − f_r0 = 180 kHz was measured in the explored range. The reported results show minimal residual dependences of f_r on d, which can be eventually reduced by adopting established compensation techniques [3]. The results validate the possibility of the contactless interrogation of an IS for conductive target detection. Ongoing tests are exploring the ability to detect downscaled target sizes such as conductive particles in nonconductive fluids.