Received March 23, 2020, accepted April 3, 2020, date of publication April 10, 2020, date of current version April 28, 2020. Digital Object Identifier 10.1109/ACCESS.2020.2987066 Pulsed-Temperature Metal Oxide Gas Sensors for Microwatt Power Consumption FRANCISCO PALACIO1,*, JORDI FONOLLOSA 2,3,4,*, JAVIER BURGUÉS 1,5, JOSE M. GOMEZ 1, (Senior Member, IEEE), AND SANTIAGO MARCO1,5, (Senior Member, IEEE) 1Department of Electronics and Biomedical Engineering, Universitat de Barcelona, 08028 Barcelona, Spain 2B2SLab, Departament d'Enginyeria de Sistemes, Automàtica i Informàtica Industrial, Universitat Politècnica de Catalunya, 08034 Barcelona, Spain 3Institut de Recerca Pediàtrica Hospital Sant Joan de Déu, 08950 Esplugues de Llobregat, Spain 4Networking Biomedical Research Centre in Bioengineering, Biomaterials, and Nanomedicine (CIBER-BBN), 28029 Madrid, Spain 5Signal and Information Processing for Sensing Systems, Institute for Bioengineering of Catalonia (IBEC), The Barcelona Institute of Science and Technology, 08036 Barcelona, Spain Corresponding author: Jordi Fonollosa ([email protected]) *Francisco Palacio and Jordi Fonollosa are co-first authors. This work was supported in part by the Spanish MINECO Program under Grant DPI2017-89827-R (ALPE) and Grant BES-2015-071698 (Severo-Ochoa), in part by the CERCA Programme/Generalitat de Catalunya, in part by the European Union's Horizon 2020 Program under Grant H2020-780262-SHARE4RARE, in part by the Departament d'Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya under Grant 2017 SGR 1721 and Grant 2017 SGR 952, in part by the Comissionat per a Universitats i Recerca del DIUE de la Generalitat de Catalunya, the European Social Fund (ESF), in part by the Networking Biomedical Research Center in the area of Bioengineering, Biomaterials, and Nanomedicine (CIBER-BBN), initiative of Instituto de Investigación Carlos III (ISCIII), in part by the Agència per la Competitivitat de l'Empresa (ACCIÓ) under Grant INNOTECRD18-1-0054, in part by the AGAUR, Agència de Gestió d'Ajuts Universitaris i de Recerca under Grant 2018-LLAVOR-00021, in part by the European Regional Development Fund, in part by the Institut de Bioenginyeria de Catalunya (IBEC), and in part by the FP6-IST Goodfood under Grant 508774. The work of Jordi Fonollosa was supported by the Serra Húnter Program (Generalitat de Catalunya) and the work of Fran Palacio by TECNIOSpring PLUS program (TECSPR18-1-0042). ABSTRACT Metal Oxide (MOX) gas sensors rely on chemical reactions that occur efficiently at high temperatures, resulting in too-demanding power requirements for certain applications. Operating the sensor under a Pulsed-Temperature Operation (PTO), by which the sensor heater is switched ON and OFF periodically, is a common practice to reduce the power consumption. However, the sensor performance is degraded as the OFF periods become larger. Other research works studied, generally, PTO schemes applying waveforms to the heater with time periods of seconds and duty cycles above 20%. Here, instead, we explore the behaviour of PTO sensors working under aggressive schemes, reaching power savings of 99% and beyond with respect to continuous heater stimulation. Using sensor sensitivity and the limit of detection, we evaluated four Ultra Low Power (ULP) sensors under different PTO schemes exposed to ammonia, ethylene, and acetaldehyde. Results show that it is possible to operate the sensors with total power consumption in the range of microwatts. Despite the aggressive power reduction, sensor sensitivity suffers only a moderate decline and the limit of detection may degrade up to a factor five. This is, however, gas-dependent and should be explored on a case-by-case basis since, for example, the same degradation has not been observed for ammonia. Finally, the run-in time, i.e., the time required to get a stable response immediately after switching on the sensor, increases when reducing the power consumption, from 10 minutes to values in the range of 10-20 hours for power consumptions smaller than 200 microwatts. INDEX TERMS Electronic nose, gas sensors, low-power operation, machine olfaction, pulsed-temperature operation, temperature modulation. I. INTRODUCTION other sensing technologies [1]. Furthermore, in the upcoming Metal Oxide (MOX) gas sensors have been successfully pro- era of connected devices and IoT, one can expect that cost- posed for a large diversity of applications due to its low-cost, efficient sensing devices will still encounter a new pool of easy operation, fast response, and sensitivity compared to applications. In fact, recent developments integrate miniatur- ized MOX sensors with digital and analog electronics on a The associate editor coordinating the review of this manuscript and single chip [2], [3] However, MOX sensors rely on chemical approving it for publication was Valerio Freschi . reactions that occur at high temperatures. Hence, built-in This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ 70938 VOLUME 8, 2020 F. Palacio et al.: Pulsed-Temperature MOX Gas Sensors for Microwatt Power Consumption heaters are integrated to bring the sensing layer to tempera- (Rs;max); and the steady-state resistance (Rs;end). Presenting tures where such chemical reactions happen more efficiently. CO at different concentration levels, in the range of 0-10 ppm, As a result, the power consumption of MOX sensors can and using PTO schemes with different duty cycle, DD(30, 50, be a limiting factor for its integration with other low-power, 70, 90) %, and period, TD(0.5, 1, 2, 5) s, authors explored long-lasting autonomous systems. Improvements in sensor the sensor (MiCS-5525, SGX Sensortech) performance for thermal isolation due to micromachining techniques have different operation schemes. Interestingly, they found that the largely decreased the power consumption up to a few tens sensitivity and the optimum D and T values change for the of mW [4]. different extracted features. Rossi et al. explored other fea- However, in many applications even lower power con- tures for MOX sensors under PTO modes. In particular, they sumption is desired, particularly for battery-operated sys- showed that the discrete cosine transform (DCT) applied to tems. For example, recent mobile platforms integrated MOX the gas sensor response can produce useful gas concentration gas sensors to patrol hazardous environments for gas leak information in the first 500 ms of the heating period, instead detection [5], [6]. These platforms operated the gas sensors at of the 5 s required to reach a steady-state value [16]. a constant temperature, and the power consumption of each Only a few authors proposed other features for MOX sen- sensor was in the order of magnitude of 100 mW. Given sors under PTO schemes with shorter duty cycles, showing that several MOX sensors were operated simultaneously, that MOX sensors are still sensitive to the volatile of inter- the power drained in the sensors may limit the total power est under more restrictive power conditions. For example, dedicated to the motion system and reduce significantly the Bicelli et al. proposed two features to characterize the sensor autonomy of the robot. Very recently, a mobile platform with response after a burst of heating pulses with DD1.4% and three MOX sensors reduced power consumption to 35 mW TD1s in a TGS 2442 (Figaro Inc.) sensor [17]. Both features per MOX sensor [7]. The requirement of bulky and expensive are computed as the differential resistance in the quasi-stable batteries may be a limiting factor for MOX sensors to be period that lasts from tD3s to tD6s after the heating pulse, integrated into smaller robot platforms, such as gas-sensitive divided by the differential resistance in the sensor transient nano-drones [8]. Moreover, the use of low-power devices is (tD2-3 s) or in the decay transient (tD6-9s), respectively. a strong requirement for certain applications. For example, They found a high correlation between these two features in food-logistics applications, where the system needs to and the CO concentration (only for CO<30 ppm) and low monitor food quality during several days and batteries need to sensitivity to variations in ambient temperature. Macías et al. be small and flexible to fit with the shape of the package [9]. compared the output of a PTO sensor with a sensor that was To extend the time range of the system, MOX gas sen- powered continuously for stability reference, when both sen- sors are typically operated under Pulsed Temperature Opera- sors were exposed to the same analyte in a gas chamber [18]. tion (PTO) schemes [10]–[15]. In the simplest PTO operation, Results showed that after a linear regression, the output of the sensor heater is switched ON and OFF periodically, and the continuously powered sensor can be predicted from the the ratio between the length of the ON period and the total output signal of the PTO sensor, enabling thereby low power cycle duration (i.e., the duty-cycle) is inversely proportional operation and fast measurement time. to the amount of power saved. When this strategy was applied Nevertheless, most of the research studies explore PTO to a wearable personal exposure monitor, the device auton- schemes applying waveforms to the heater with a time period omy improved by a factor of 2.7 [15]. Nevertheless, such PTO in the range of seconds and, generally, duty cycles above strategies, that successfully save power to the sensing system, 20%. Studies that use more restrictive PTO schemes do not come at the cost of sensor performance [12]. For example, show the sensor performance degradation as power-saving is faster response times are found at higher operating temper- increased. Here, instead, we propose a methodology to char- atures [5]. This is because immediately after turning ON the acterize the PTO sensors working under aggressive schemes. heater, the MOX sensor enters an unstable state characterized We employ our methodology to MOX sensors working under by a steady increase of the sensor resistance, also referred ultra-low power consumption (in the range of µW), reaching to as ``initial action''.