Vedran Bilas

Benefits of Wake-Up Radio in Energy-Efficient Multimodal Surveillance Wireless Sensor Network

Scarce energy budget of battery-powered wireless sensor nodes calls for cautious power management not to compromise performance of the system. To reduce both energy consumption and delay in energy-hungry wireless sensor networks for latency-restricted surveillance scenarios, this paper proposes a multimodal two-tier architecture with wake-up radio receivers. In video surveillance applications, using information from distributed low-power pyroelectric infrared (PIR) sensors, which detect human presence limits the activity of cameras and reduces their energy consumption. PIR sensors transmit the information about the event to camera nodes using wake-up radio receivers. We show the benefits of wake-up receivers over duty cycling in terms of overcoming energy consumption versus latency tradeoff (proved with two orders of magnitude lower latency-only 9 ms). At the same time, the power consumption of the camera node, including a wake-up receiver is comparable with the one having only duty-cycled main transceiver with 1% duty cycle (about 32 mW for 25 activations per hour).

Pulsed eddy current nondestructive testing of ferromagnetic tubes

In sinusoidal eddy current nondestructive testing (NDT) of thick ferromagnetic tubes, wall thickness is measured with the coils displayed for 2-3 tube diameters (remote field eddy current technique). The tube inner diameter is measured at higher frequency with another pair of coils displaced for around 1 tube diameter (electrical caliper). In this paper, we give a thorough analysis of the excitation frequency and the distance between the coils for measurement of the tube inner diameter and wall thickness as a background for the application of pulsed eddy current (PEC). We propose application of one pair of coils displaced for 1-2 tube diameters for measurement of those tube parameters employing the features of response to the pulsed excitation. Results of our experimental work confirm that PEC technique provides a significant improvement of present eddy current systems for NDT of the ferromagnetic tubes.

Design, Implementation, and Performance Evaluation of a Flexible Low-Latency Nanowatt Wake-Up Radio Receiver

Wireless sensor networks (WSNs) have received significant attention in recent years and have found a wide range of applications, including structural and environmental monitoring, mobile health, home automation, Internet of Things, and others. As these systems are generally battery operated, major research efforts focus on reducing power consumption, especially for communication, as the radio transceiver is one of the most power-hungry components of a WSN. Moreover, with the advent of energy-neutral systems, the emphasis has shifted toward research in microwatt (or even nanowatt) communication protocols or systems. A significant number of wake-up radio receiver (WUR) architectures have been proposed to reduce the communication power of WSN nodes. In this work, we present an optimized ultra-low power (nanowatt) wake-up receiver for use in WSNs, designed with low-cost off-the-shelf components. The wake-up receiver achieves power consumption of 152 nW (with -32 dBm sensitivity), sensitivity up to -55 dBm (with maximum power of 1,2 μW), latency from 8 μs, tunable frequency, and short commands communication. In addition, a low power solution, which includes addressing capability directly in the wake-up receiver, is proposed. Experimental results and simulations demonstrate low power consumption, functionality, and benefits of the design optimization compared with other solutions, as well as the benefits of addressing false positive (FP) outcomes reduction.

Low-Power Wearable Respiratory Sound Sensing

Building upon the findings from the field of automated recognition of respiratory sound patterns, we propose a wearable wireless sensor implementing on-board respiratory sound acquisition and classification, to enable continuous monitoring of symptoms, such as asthmatic wheezing. Low-power consumption of such a sensor is required in order to achieve long autonomy. Considering that the power consumption of its radio is kept minimal if transmitting only upon (rare) occurrences of wheezing, we focus on optimizing the power consumption of the digital signal processor (DSP). Based on a comprehensive review of asthmatic wheeze detection algorithms, we analyze the computational complexity of common features drawn from short-time Fourier transform (STFT) and decision tree classification. Four algorithms were implemented on a low-power TMS320C5505 DSP. Their classification accuracies were evaluated on a dataset of prerecorded respiratory sounds in two operating scenarios of different detection fidelities. The execution times of all algorithms were measured. The best classification accuracy of over 92%, while occupying only 2.6% of the DSPs processing time, is obtained for the algorithm featuring the time-frequency tracking of shapes of crests originating from wheezing, with spectral features modeled using energy.

Measurement of gait parameters from free moving subjects

In this paper a telemetry system for real-time force measurement in the legs and crutches or cane is presented. The system provides a quantitative gait analysis of orthopaedic patients with details, which are often hidden to a visual observation. Three vertical force components in each leg are measured. For this purpose flat and pliable capacitive sensors, which are placed on the sole of the shoes, were developed. The infrared telemetry is applied to enable free and unconstrained movements of the patient inside a wide indoor environment. Measured forces, after transmission and processing on the receiver side, are presented on the computer monitor as eight channel time diagrams. An original software package (GAS) for on-line monitoring and processing of vertical force data was developed. The processing enables calculation of many diagnostically important parameters (e.g. temporal parameters, average force, force-time integral, sum of forces, etc.), spectral and time-frequency analysis and many other. As an example of clinical application, temporal parameters during ten strides for 20 normal subjects and 20 patients with lower extremities degenerative joint diseases were calculated and compared.

Urban crowd sensing demonstrator: Sense the Zagreb Air

We demonstrate an urban crowd sensing application for monitoring air quality by use of specially-designed wearable sensors and mobile phones. The application is built upon the OpenIoT platform1 with the goal to support context-aware and energy-efficient acquisition and filtering of sensor data in mobile environments while ensuring adequate sensing coverage. We demonstrate how sensors and mobile devices jointly collect and share data of interest to measure air quality. In particular, we outline the main features of our wearable air quality sensors, present the data acquisition process as well as the user view of the system, which, in contrast to similar applications, provides a personalized real-time notification mechanism to mobile application users. The solution was used in an air quality measurement campaign “Sense the Zagreb Air” performed in the City of Zagreb, Croatia, in early July 2014 with 20 participants.

Validation of a coil impedance model for simultaneous measurement of electromagnetic properties and inner diameter of a conductive tube

In eddy-current nondestructive testing of conductive tubes such as oil-well casings, the measured tube wall thickness and inner diameter must be compensated for variations of electromagnetic properties of a tube material -magnetic permeability and electrical conductivity. Commercial systems for eddy-current nondestructive testing of oil-well casings incorporate multiple coils and multifrequency excitation for measurement of the inner diameter and the electromagnetic properties. This paper investigates the feasibility of the electromagnetic properties and inner diameter measurement with only one coil excited at one frequency. The authors have derived a coil impedance model and developed an inversion procedure for the determination of tube properties. Measurements have been performed on several tubes made of different materials. In order to validate the model and the optimization algorithm, measurement results were compared with the predictions of the model and finite-element analysis. Applicability of the method has been confirmed for the investigated frequency range (1-50 kHz).

A Novel Approach to Wheeze Detection

Wheezing often accompanies pulmonary pathologies and its detection is considered of great importance for the diagnosis and management of respiratory diseases. Our aim was to develop a simple and robust algorithm for wheeze detection in respiratory sound spectra to be used for long-term monitoring and early stage assessment of asthma episode in children. The robustness of the algorithm enables wheezing detection in presence of noise and moving artifacts. Children cannot perform respiratory function tests such as peak-flow measurement and therefore we find continuous recording and processing of respiratory sounds as an alternative. The algorithm we used for wheeze detection is based on the idea of frequency domain peak detection proposed by Shabtai-Musih et al. because of its simplicity and scoring used for specifying the likelihood that the peaks in power spectra represent wheezes. In our algorithm, we have modified the way of searching peaks in the spectrogram. Before searching for peaks, wavelet denoising was used in order to remove the noise in spectrum without affecting the peaks that we were searching for. Using the scoring algorithm we were able to create a binary image of the spectrogram of the sounds - wheezes and score the length (duration) of connected components considered as wheezing. The components that did not meet length criterion were rejected and were not considered as wheezing. The algorithm was tested on respiratory sound signals from public signal databases and on our own signals recorded in a group of 26 asthmatic children. The algorithm successfully detected wheezes in all signals containing wheezing.

Design of sensor node for air quality crowdsensing

Information on air-quality in urban environments is typically measured only at limited number of sites, due to cost of measurement of atmospheric concentrations of toxic gases (CO, NO2, SO2) within accuracy boundaries defined by regulative bodies. Low spatial resolution of the mentioned environmental parameters hinders their applications in localization of the air-pollution sources, traffic regulation or studies of chronic respiratory diseases related to personal pollution exposure. Thus, we propose complementing the existing air quality monitoring infrastructure by a network of mobile sensors enabling the citizens to participate in measurement (e.g. “crowdsensing”). In this paper, we present the design of such battery-powered, wearable sensor node, housing two electrochemical gas sensors, temperature, relative humidity and atmospheric pressure sensors, with Bluetooth connectivity. Electrical, mechanical and software design are shown. Next, sensor node was characterized by evaluating the sensing accuracy and the autonomy in laboratory conditions. Accuracy within ±1 °C, ±2% RH, ±2 hPa, and ±0.6 ppm CO is shown. Autonomy is estimated at 65 h. Preliminary results of the outdoor functional test are demonstrated.

Empowering smartphone users with sensor node for air quality measurement

We present an architecture of a sensor node developed for use with smartphones for participatory sensing of air quality in urban environments. Our solution features inexpensive metal-oxide semiconductor gas sensors (MOX) for measurement of CO, O3, NO2 and VOC, along with sensors for ambient temperature and humidity. We focus on our design of sensor interface consisting of power-regulated heater temperature control, and the design of resistance sensing circuit. Accuracy of the sensor interface is characterized. Power consumption of the sensor node is analysed. Preliminary data obtained from the CO gas sensors in laboratory conditions and during the outdoor field-test is shown.