Abstract
In this study, two types of direct interface capacitive sensors, self- and mutual-capacitance, were developed and compared experimentally. Electromagnetic Compatibility (EMC) tests—International Electrotechnical Commission (IEC) 61000-4-3, IEC 61000-4-4, IEC 61000-4-6—were applied in an accredited laboratory to measure the immunity of the sensors against radiated and conducted interference. The frequency hopping algorithm could be implemented for the mutual-capacitance sensor without using any particular circuit. The effects of EMC disturbance were detected by means of a new noise detection algorithm and when the signal-to-noise ratio (SNR) became lower, the operation frequency of the sensors switched to an undisturbed frequency to ensure safe operation. For this purpose, a new noise detection algorithm was developed and frequency hopping was performed with a standard controller. Both cards were tested under several conditions and their performances compared.
Highlights
Capacitive sensors currently find a wide range of applications in many areas including consumer electronics, industry, and automotive sectors
The charge transfer method was implemented with a switched-capacitor (SC) technique in order to measure relative capacitance variation at the sensors when touched
In the first phase charge is accumulated on the Cx and in the second phase the charge stored on Cx is discharged into the larger Cs
Summary
Capacitive sensors currently find a wide range of applications in many areas including consumer electronics, industry, and automotive sectors. The capacitance variations measured at sensors are quite small and the values obtained are extremely weak, in picofarad (pF) range, or sometimes even below They are rather sensitive to interference and affected and disrupted by electric noise originating from nearby devices, neighboring sensor wires (coupling) or from the power grids(conducted noise). This paper presents a novel method of decreasing the conducted noise It can be used in any low-end MCUs which do not need any particular analog- and digital modules. Frequency hopping is implemented without using a special module (PRS) and clock source (excitation); only a resistance-capacitance (RC) oscillator is used Both a simple analog-digital converter (ADC) and an RC are common modules for each controller.
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