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Development, construction and signal processing of a high-frequency inductive measurement system for precise, absolute position and angular measurement

Dipl.-Ing. Bernhard Aschenbrenner

In industrial applications, there is an increasing demand for measuring the absolute linear or angular position of objects which can be integrated in highly dynamical drives (see Figure 1). This kind of sensor system has to meet criteria such as being compact, showing a good reliability even in harsh industrial environments, a wide temperature range, insensitivity to moisture, dust, vibrations and mechanical offsets and should have a long service life.

Contactless inductive sensors meet these requirements even under harsh industrial environment. One type of inductive position sensor is based on inductive resonance and utilizes the physical principle of mutual inductance between the antenna and target (see Figure 1).

Figure 1: On the hand left side the developed and used contactless inductive position sensor (antenna) can be seen with two different periods for the sine and cosine structures for precise and absolute measurements. The transmit coil of the antenna is formed by tracks, which extends around the periphery of the PCB forming three loops. At the right hand side of the picture the high Q-resonant target PCB is illustrated with the rectangular coil winding to form the inductance for the LC resonant circuit.

The antenna consists of a planar arrangement of tracks which forms the sine shaped receive coils with two different pitches and the rectangular transmitter coil which extends around the periphery of the antenna PCB. In the region of the transmitter coil a uniform, elongated and alternating electromagnetic field is formed because of the excitation current. The target is a high Q-resonant circuit whose resonance frequency is tuned to the excitation frequency.

When the target enters the alternating electromagnetic field, currents are induced to flow in the resonant circuit. These currents generate their own flux field, which induce a voltage in the receiver coils which oscillates at the resonant frequency but whose amplitude varies as a function of the relative position of the antenna and the target. An important property is the ratiometric sensor design, which means that the target position is calculated via a ratio of two quantities from two receiver channels.

The sensor system has to determine linear target displacements smaller than 10 µm or has to determine angle differences smaller 0.1 degree for radial speeds up to 5000 revolutions per minute.

Figure 1 shows the developed and manufactured antenna and target PCB for a measurement setup.

Analytical analysis about signal crosstalk, disturbing signals and radial target offset has been undertaken. Further analysis with COMSOL Multiphysics will be done to optimize the sensor-structure for a better position sensitivity.

Figure 2: The basic block diagram shows the main components of a possible analog and digital signal processing electronic for the excitation coil and receiver coils. At the left side of the picture the quantitative signal behavior is shown of the induced voltages dependent of the target position along the antenna.

Additionally to the sensor design and sensor analysis of different evaluation and demodulation methods for the amplitude modulated receiver signals such as ring modulators, active demodulators and digital demodulation methods will be analyzed. Figure 2 shows a basic block diagram of a possible analog and digital signal processing electronic with analog mixer.

Keywords: contactless, inductive sensor, synchronous demodulation, absolute-position, angle measurement, under sampling.