Inductive Position Sensors based on Coupling of Coils on Printed Circuit Boards for Demanding Automotive Applications

TL;DR

This work presents a PCB-based inductive rotor-position sensor that relies on MHz-range coupling between a transmitter coil and multiple receiver coils with a passive, non-ferromagnetic target to encode angle in the RX signals. The system achieves robust performance with intrinsic stray-field rejection through synchronous demodulation and differential RX geometry, and it leverages a compact transmitter LC tank and a multi-layer PCB to optimize coupling and air gap. Finite element simulations and experimental measurements demonstrate sub-degree mechanical angle errors and harmonic suppression, with the design being highly tunable via coil geometry, windings, and target configuration. The approach offers a scalable, low-cost alternative to resolvers and magnetic sensors, suitable for automotive applications and capable of integration with standard automotive interfaces such as SENT.

Abstract

Rotor position feedback is required in many industrial and automotive applications, e.g. for field-oriented control of brushless motors. Traditionally, magnetic sensors, resolvers or optical encoders are used to measure the rotor position. However, advances in inductive sensing concepts enable a low-cost, high-precision position measurement principle which is robust against magnetic stray fields exceeding 4000 A/m. The operating principle is based on the coupling of a transmitter coil with several receiver coils in the megahertz frequency range. The coils are part of a printed circuit board (PCB) which also comprises circuitry for demodulation and signal processing. The transmitter coil induces eddy currents in an electrically conductive passive coupling element, which provides position-dependent amplitude modulation. The voltage induced in the receiver coils encodes the rotor angle information, typically in quadrature signals. The coupling element requires no rare-earth materials and can be made of stainless steel, for instance. The PCB-based design of the sensor offers considerable flexibility in optimizing its performance. By tailoring the coil geometry and arrangement, accuracy, air gap and overall sensor dimensions can be adjusted to meet a broad range of application-specific requirements. A sensor design sample exhibits a mechanical angle error less than 0.02° (0.1° electrical) in both, finite-element simulation and test bench measurement, with good agreement.

Figures (8)

• Figure 1: Operating principle of a rotary inductive position sensor. An AC current in the megahertz frequency range flows through the transmitter coil (TX) and generates a magnetic excitation field $\mathbf{B}_e$ which induces eddy currents $\mathbf{j}_r$ in an electrically conductive target material, which in turn generates an opposing reaction field $\mathbf{B}_r$ that induces a voltage in the receiver coils (RX). Due to the shape of the target and the spatial phase shift between the two receiver coils, the target angle can be measured by calculating the angle of the demodulated receiver coil voltages. The differential nature of the RX coils suppresses direct coupling of TX coil into the RX coils due to the inverted surface normals $\mathbf{n}$ of the RX coil area segments. Figure published in Kuntz2024.
• Figure 2: Simplified schematic of a coupled-coil inductive position sensor as an equivalent transformer. The transmitter (TX) coil on the PCB forms a parallel LC-tank together with two discrete capacitors $C_\mathrm{TX}$. The coil resistance $R_\mathrm{TX}$ is parasitic. The LC-oscillator is excited at the resonance frequency $\omega_0$. The target (\ref{['fig:target']}) acting as a coupling element provides a position-dependent mutual inductance between TX and RX coils, which leads to a varying output voltage amplitude of the modulated receiver coil signals $V_\mathrm{RX1},\,V_\mathrm{RX2}$.
• Figure 3: Simplified block diagram of an inductive ASIC. The TX driver provides an AC excitation current for the LC circuit with a resonant frequency in the range of 3MHz to 5MHz. The induced voltage in the receiver coils is demodulated synchronously with the TX excitation as reference. The receiver coil signal amplitude in the order of $\sim10mV$ is amplified, typically with an automatic gain control loop. Typical outputs are analog differential sine/cosine signals or digital automotive interfaces such as SENT.
• Figure 4: Signal processing of inductive position sensor signals. The induced voltage (a) in the receiver coils is demodulated synchronously with the TX voltage (\ref{['fig:RLC']}) and amplified (b). In the Lissajous figure, the signals $a$ and $b$ approximately trace a circle (c). The encoded angle can be recovered with the arctangent function (d). The resulting electrical angle repeats $p$-times within a full mechanical rotation (e), according to the periodicity $p$ of the sensor.
• Figure 5: Inductive sensor printed circuit board (PCB) sample for research purposes. Top side (a) with ASIC, passive components, and connector; bottom side (b) with coil system, facing the target. The PCB consists of four layers, the bottom two layers are used for the coil system. The two receiver coils have periodicity $p=5$ and three windings each ($n_w=3$) with a cosine shape function. The hole in the center of the PCB allows for end-of-shaft as well as on-axis applications. This sensor provides single-ended non-differential analog sine/cosine output signals on a 4-pin interface together with 5V supply and ground pins.