Microcontroller-Based Seat Occupancy Detection and Classification

This paper presents a microcontroller-based measurement system to detect and confirm the presence of a subject in a chair. The system relies on a single Force Sensing Resistor (FSR), which may be arranged in the seat or backrest of the chair, that undergoes a sudden resistance change when a subject/object is seated/placed over the chair. In order to distinguish between a subject and an inanimate object, the system also monitors small-signal variations of the FSR resistance caused by respiration. These resistance variations are then directly measured by a low-cost general-purpose microcontroller without using either an analogue processing stage or an analogue-to-digital converter, thus resulting in a low-cost, low-power, compact design solution.Peer ReviewedPostprint (published version

Measuring dynamic signals with direct sensor-to-microcontroller interfaces applied to a magnetoresistive sensor

This paper evaluates the performance of direct interface circuits (DIC), where the sensor is directly connected to a microcontroller, when a resistive sensor subjected to dynamic changes is measured. The theoretical analysis provides guidelines for the selection of the components taking into account both the desired resolution and the bandwidth of the input signal. Such an analysis reveals that there is a trade-off between the sampling frequency and the resolution of the measurement, and this depends on the selected value of the capacitor that forms the RC circuit together with the sensor resistance. This performance is then experimentally proved with a DIC measuring a magnetoresistive sensor exposed to a magnetic field of different frequencies, amplitudes, and waveforms. A sinusoidal magnetic field up to 1 kHz can be monitored with a resolution of eight bits and a sampling frequency of around 10 kSa/s. If a higher resolution is desired, the sampling frequency has to be lower, thus limiting the bandwidth of the dynamic signal under measurement. The DIC is also applied to measure an electrocardiogram-type signal and its QRS complex is well identified, which enables the estimation, for instance, of the heart rate.Postprint (published version

Bioimpedance plethysmography with capacitive electrodes and sole force sensors : Comparative trial

4th Latin American Conference on Bioimpedance 2021 (CLABIO 2021) 10-13 November 2021, San Luis Potosí, Mexico

DISEÑO DE UN DEMODULADOR DE FM MEDIANTE PLL PARA LA INTERROGACIÓN DE SENSORES INTERFEROMÉTRICOS DE FIBRA ÓPTICA

ResumenEn este trabajo se diseñó y se construyó un sistema de interrogación de sensores interferométricos. El sistema está constituido por una etapa que emula la señal interferométrica típica de un sensor de este tipo: Primeramente, una etapa de acondicionamiento que convierte esta señal en una señal de FM convencional y finalmente una etapa de demodulación de frecuencia; mediante el uso de la técnica de amarre de fase PLL, (del inglés: Phase Lock Loop). El proceso de demodulación, denominado en la literatura como “heterodino sintético”, utiliza un par de osciladores locales sintonizados a la frecuencia de la señal portadora y al doble de ésta. Así mismo, se requirieron una serie de filtros pasabanda tipo Butterworth de segundo orden para acotar el espectro de las señales de interés centrados en la frecuencia de la armónica necesaria para realizar el proceso de mezclado. Finalmente, la señal acondicionada se usó como entrada a un demodulador de FM mediante un PLL. Se consiguió recuperar señales del orden de miliradianes en el rango de 90 a 260 Hz. Se observó que este rango dependió del ancho de banda de los filtros pasabanda utilizados en el circuito. Se optó por esta técnica de demodulación basada en un PLL, pues logra la sintonización de una amplia gama de frecuencias, al ser también sintonizable el PLL a través de su VCO.Palabras Claves: Demoduladores de FM, fase óptica, sensores interferométricos, PLL. DESIGN OF AN FM DEMODULATOR THROUGH PLL FOR THE INTERROGATION OF OPTICAL FIBER INTERFEROMETRIC SENSORSAbstractIn this work, an interrogation system of interferometric sensors was designed and constructed. The system consists of a stage emulating the interferometric signal typical of such sensor: First a conditioning stage that converts the above signal into a conventional FM signal and finally a frequency demodulation stage, based in the Phase Lock Loop technique o demodulate FM signals (PLL). The demodulation process used here, referred in the literature as "synthetic heterodyne", uses a pair of local oscillators, one tuned to the frequency of the carrier signal and the other one tuned at twice of the carrier frequency. It also requires a series of second-order Butterworth bandpass filters to limit the signals of interest and maintain a constant amplitude in the passband. As well as a trimmer to minimize the amplitude changes, in the final part of the conditioning stage. Finally, the conditioned signal was used as input to an FM demodulator via a PLL and signals of the order of miliradianes were achieved; with frequencies of modulating signals in the range of 90 to 260 Hz. It was observed that this range depended on the bandwidth of the bandpass filters used in the circuit.Keywords: FM demodulators, Interferometric sensors, optical phase, PLL

Seat occupancy detection based on a low-power microcontroller and a single FSR

This paper proposes a microcontroller-based measurement system to detect and confirm the presence of a subject in a chair. The system relies on a single Force Sensing Resistor (FSR), which is arranged in the seat of the chair, that undergoes a sudden resistance change when a subject/object is seated/placed over the chair. In order to distinguish between a subject and an inanimate object, the system also monitors small-signal variations of the FSR resistance caused by respiration. These resistance variations are then directly measured by a low-cost general-purpose microcontroller unit (MCU) without using either an analogue processing stage or an analogue-to-digital converter. Two versions of such a MCU-based circuit are presented: one to prove the concept of the measurement, and another with a smart wake-up (generated by the sudden resistance change) intended to reduce the energy consumption. The feasibility of the proposed measurement system is experimentally demonstrated with subjects of different weight sitting at different postures, and also with objects of different weight. The MCU-based circuit with a smart wake-up shows a standby current consumption of 800 nA, and requires an energy of 125 µJ to carry out the measurement after the wake-up.Peer Reviewe

Microcontroller-Based Seat Occupancy Detection and Classification

This paper presents a microcontroller-based measurement system to detect and confirm the presence of a subject in a chair. The system relies on a single Force Sensing Resistor (FSR), which may be arranged in the seat or backrest of the chair, that undergoes a sudden resistance change when a subject/object is seated/placed over the chair. In order to distinguish between a subject and an inanimate object, the system also monitors small-signal variations of the FSR resistance caused by respiration. These resistance variations are then directly measured by a low-cost general-purpose microcontroller without using either an analogue processing stage or an analogue-to-digital converter, thus resulting in a low-cost, low-power, compact design solution.Peer Reviewe

Measuring dynamic signals with direct sensor-to-microcontroller interfaces applied to a magnetoresistive sensor

This paper evaluates the performance of direct interface circuits (DIC), where the sensor is directly connected to a microcontroller, when a resistive sensor subjected to dynamic changes is measured. The theoretical analysis provides guidelines for the selection of the components taking into account both the desired resolution and the bandwidth of the input signal. Such an analysis reveals that there is a trade-off between the sampling frequency and the resolution of the measurement, and this depends on the selected value of the capacitor that forms the RC circuit together with the sensor resistance. This performance is then experimentally proved with a DIC measuring a magnetoresistive sensor exposed to a magnetic field of different frequencies, amplitudes, and waveforms. A sinusoidal magnetic field up to 1 kHz can be monitored with a resolution of eight bits and a sampling frequency of around 10 kSa/s. If a higher resolution is desired, the sampling frequency has to be lower, thus limiting the bandwidth of the dynamic signal under measurement. The DIC is also applied to measure an electrocardiogram-type signal and its QRS complex is well identified, which enables the estimation, for instance, of the heart rate

A Fully Differential Synchronous Demodulator for AC Signals

A novel fully differential (FD) demodulator is presented. Using different design strategies, the circuit can be used for processing amplitude-modulated (AM) signals obtained from impedance measurements or coming from modulating sensors with differential outputs where a high common-mode rejection ration (CMRR) and low noise are demanded. The circuit multiplies the AM input signal by a square wave with the same frequency and phase of the carrier of the input signal. This kind of wave is simpler to generate than a sine wave (homodyne detection) and narrow unit-amplitude pulses (synchronous sampling). The proposed circuit is not a perfect floating system, but yields a high CMRR if matched op-amps are used and does not depend on matched resistors. The system has been tested with off-the-shelf amplifiers; at 100 kHz, the CMRR is about 65 dB when fast and wide-bandwidth amplifiers are used. The spectral density of noise voltage obtained is lower than 55 nV/ Hz at 1 kHz; for a bandwidth of 15 Hz, this results in a noise voltage (rms) of 213 nV. Provided the circuit is implemented with low value resistors, the main contribution of noise comes from the noise voltage of the op-amps used to implement the demodulator.Peer ReviewedPostprint (published version