Proximity sensors enable, in a simple manner, the position, size and material of a metal target to be established. Such sensors have the ability to obtain absolute analog signals proportional to proximity and are compact, robust and reliable constructions. Inductive sensors consist of a coil embedded in an open magnetic circuit.
The coil’s properties such as complex impedance depend on the magnetic resistance of the circuit. Metallic targets in the vicinity of the sensor affect the magnetic circuit and change the impedance and quality factor of the coil. These relatively small variations are detected by electronic circuitry and converted into desired length units.
Figure 1: Cut-Away snapshot section through the sensor
The geometry of the sensor was imported via the CST STUDIO SUITE® STEP Interface (Figure 2). The coil, also imported, was simplified by a faceted representation. A target was added which was parameterized to allow a set of S-Parameters, and subsequently the input impedance, to be obtained as a function of distance to the sensor.
Figure 2: 3D Model of the inductive sensor imported through STEP
A second order tetra-based mesh was applied to the geometry (Figure 3) and the full wave frequency domain solver of CST MICROWAVE STUDIO® was used for solving the field problem.
Figure 3: Tetrahedral mesh representation of the 3D model
The field plots in Figures 4-6 show different configurations of sensor and target with respect to the magnetic flux and current densities.
Figure 4: Magnetic flux density including a 2.5 mm spaced metal target
Figure 5: Current density at a spaced target of 2.5 mm
Figure 6: Current density at the windings at 6 Mhz resonance
The complex impedance can be directly derived from the S-parameters shown in Figure 7 for R +jωL as a function of frequency, a resonance can be identified at 6 MHz.
Figure 7: Complex impedance as a function of frequency
The Q-Factor gives an indication of the effectiveness of the sensor in terms of the frequency (Figure 8). A measure of the sensitivity can be obtained by the following equations:
Figure 8: Q-Factor formulae
This Q- Sensitivity / Q-measure is shown in Figure 9. A good working point is in the area of 100kHz at which the Q-Sensitivity is maximum. A blind spot is also obtained at 300 kHz where the Q-factor remains constant and the target distance can not be identified by the sensor.
Figure 9: Q-factor and Q-measure as a function of frequency
Finally, an equivalent of the sensor was derived for which the complex impedance was fitted over a wide range of frequencies.
Figure 10: Equivalent circuit representation and curve fitting of the impedance
This article has demonstrated how 3D EM field simulation in combination with S-Parameters and circuit optimization can be applied to the design and analysis of inductive proximity sensors.