Presentation title

Drift rejection front-end for long range capacitive sensors for human indoor localization

Authors

Giorgia Subbicini, Mihai Lazarescu, Luciano Lavagno

Institution(s)

Polytechnic of Turin

Presentation type

Technical presentation

Abstract

This research focuses on a drift rejection front-end for long range capacitive sensors for human indoor localization.

Capacitive sensors have a wide range of applications, including continuous person monitoring to recognize the early onset of diseases from long-term behavioral changes, assisted living systems, especially for elderly people who can be prone to falls or other accidents, household safety to detect unauthorized intrusions, and automatic management of home or building resources to reduce energy consumption.

Localization techniques may or may not require the person to wear a specific device to become visible to the system. Requiring wearables effectively reduces the reliability of the system because the persons may forget to wear or to regularly service them (recharge). On the other hand, relying on the installation of multiple mains-powered sensors can increase the costs and reduce the acceptance. Moreover, monitoring systems based on high-resolution cameras can raise privacy concerns, require a direct line of sight and adequate lighting, which increase the installation complexity and the system cost.

Load-mode single-plate capacitive sensors can localize the human body directly, without relying on additional devices. Among their advantages are simplified installation, low overall cost, unobtrusiveness, and privacy.

Capacitive sensors, operating in load mode, use a one-plate transducer and the human body as a constant potential plate. However, their sensing range is typically lower or comparable to their plate dimension, which is generally too short for our applications of interest. Moreover, the electromagnetic environmental noise can limit their sensitivity, accuracy, and stability.

To improve the capacitive sensor stability, we compare three analog front-ends and measurement techniques in terms of noise rejection, accuracy, and sensitivity: an RC period modulator, a constant current period modulator, and a slope modulator. The first two repeatedly charge and discharge the sensor plate capacitance between two fixed voltage levels. Variations of the plate capacitance change the free running frequency of the oscillation. The slope modulator is based on galvanostatic charge-discharge of the sensor plate, but a fixed period. Plate capacitance variations change the charge-discharge slopes and low frequency noise can be rejected using simple sensor signal post-processing.

We will detail the implementation of this differential measurement method, the characterization of the analog-to-digital converter for the signal acquisition, and will demonstrate analytically and using simulations how this measurement method can effectively reject low-frequency drift noise.

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Additional material

• Presentation slides: [pdf]