A Miniaturized Autonomous Microsystem for Hydrogen Gas Sensing Applications

Naser Khosro Pour, François Krummenacher, Maher Kayal Ecole Polytechnique Fédérale de Lausanne, Lausanne, CH-1015, Switzerland Email: {naser.khosropour, françois.krummenacher, maher.kayal}@epfl.ch

Abstract— This paper presents a fully integrated, ultra-low power microsystem that is used for hydrogen gas sensing in an autonomous wireless sensor node. The proposed circuit harvests solar energy from a micro-power photovoltaic module to measure temperature and hydrogen concentration and transmits the measured value using wireless data transmission. A rechargeable NiMH microbattery is used to store harvested energy. Photovoltaic module charges this microbattery, using a highly area- and power-efficient power management circuit. In order to measure hydrogen concentration, conductance change of a miniaturized palladium nanowire sensor is measured and converted to a digital signal with 12-bit resolution, using an area-efficient readout circuit. The proposed microsystem has been implemented in a 0.18µm CMOS process and occupies a core area of only 0.47mm2. This circuit features a low current consumption of 200nA for power management circuit and an additional 1.1µA for sensor interface circuit. It operates with low power supply voltage in the 0.8V to 1.6V range.

I. INTRODUCTION

There is an increased demand for miniaturized autonomous sensor networks in different sensing applications. These emerging sensors incorporate energy source, energy storage, sensing and communication into a miniaturized system. As the use of hydrogen fuels becomes more common, an increased demand for low power autonomous hydrogen sensors is expected. Palladium (Pd) nanowire hydrogen sensors [1] that can be used in the room temperature have good sensitivity due to their large surface-to-volume ratio while maintaining low power operation and small form factor. Therefore, these miniaturized sensors are good candidates for ultra-low power (ULP) hydrogen sensing. The readout circuit should measure the change in conductivity of these nanowires upon hydrogen exposure. As these nanowires have an undesired thermal cross-sensitivity, temperature effect should be compensated for accurate measurement of hydrogen concentration [1].

Solar energy harvesting can be a good energy source for miniaturized autonomous wireless sensor nodes in the order of a few cubic centimeters size. However, as energy harvested from solar cell is intermittent, an energy storage device such as a rechargeable microbattery should be used for reliable operation of electronic circuits. An area- and power-efficient

power management circuit has been designed to harvest energy from nanowire solar cells [2] and store it in a Varta V6HR NiMH microbattery that has only 6.8mm diameter [3]. This microbattery not only provides power for the proposed sensor interface and power management circuits, but also provides required supply current during wireless data transmission in target Telran TZ1053 chipset [4].

II. SYSTEM ARCHITECTURE

Block diagram of the proposed microsystem and the external components that are required to realize a miniaturized autonomous gas sensor is depicted in Fig. 1. Required energy for autonomous operation of this sensor is provided by the series connection of four miniaturized nanowire solar cells. The power management unit (PMU) harvests energy from this miniaturized photovoltaic (PV) module to charge the target NiMH microbattery. As DC voltage that the PV module provides may differ from nominal voltage of the target battery, different DC-DC converters have been proposed to harvest energy from a PV module and charge a battery, including switched-capacitor (SC) and inductive DC-DC converters. In both SC and inductive DC-DC converters, large inductor or capacitors are required to maintain high efficiency. In order to reduce die area, some of these DC-DC converters use higher operation frequency and sophisticated control circuits that increases the power consumption of their control circuits.

Fig. 1. Block diagram of the proposed microsystem

This work was supported by European project SiNAPS under contract number 257856.

978-1-4673-0859-5/12/$31.00 ©2012 IEEE

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The proposed PMU achieves more than 90% efficiency during charging target NiMH battery by connecting the battery to the PV module through a switch, similar to low dropout regulators. However instead of using error amplifiers [5], low power SC circuits have been used to control the switch. Target V6HR microbattery, not only has a large nominal capacity of 6mAh, but also can provide a high discharge current of up to 18mA to be used during data transmission. This NiMH microbattery has much higher capacity and discharge current comparing to miniaturized thin film Li-ION microbatteries. In addition, as thin film Li-ION batteries have nominal voltages of more than 3V, additional DC-DC converters are required to use these batteries for ULP applications.

Pd nanowire grids fabricated on a silicon wafer [6] were used for gas sensing. Each grid consists of 14 Pd nanowires that half of them are covered with a passivation layer to prevent hydrogen from reaching the wire. However, the conductivity of these coated nanowires changes with temperature. The proposed sensor interface unit (SIU) measures conductance change of nanowires in a differential approach. Coated nanowires that are only sensitive to temperature are used as reference nanowires, while remaining nanowires which are sensitive to both temperature and hydrogen exposure, are used as sensing nanowires. This circuit measures conductance change of sensing nanowires in comparison with reference nanowires to calculate H2 concentration. By using matched reference and sensing nanowires in a differential approach, in addition to eliminating the effects of temperature by first order, the measurement is only sensitive to ratio of conductance of nanowires, instead of their absolute values. As a result, a higher accuracy is achievable. Although in the proposed approach temperature effect is eliminated by first order, however as stated in [1], the temperature coefficient of nanowire resistance may change by up to 35% according to H2 concentration that results in second order effects. In order to compensate this second order effect, temperature is measured using an integrated temperature sensor. Hydrogen sensing accuracy can be increased by incorporating measured temperature during sensor calibration.

Substrate PNP transistors have been used to realize an integrated temperature sensor [7]. Although saturation current and VBE of these transistors depend on process parameters, but the difference between VBE of two PNP transistors is a proportional to absolute temperature (PTAT) voltage that only depends on the ratio of bias currents or the emitter-base area of the two transistors. These ratios can be made precise and nearly insensitive to process variations. Summing this PTAT voltage and VBE, that is a complementary to absolute temperature (CTAT) voltage, with proper weighting factors, leads to a temperature-independent bandgap reference voltage [8]. The ratio between the PTAT and the bandgap reference voltage is measured to calculate temperature. By using low power SC circuits, sufficient accuracy is achievable by applying simple calibration techniques. An incremental analog to digital converter (ADC) [9] converts measured temperature and gas concentration to a digital value.

Common blocks are used in both power management and sensor interface units. Table 1 presents die area and simulated maximum power consumption of these common blocks.

TABLE I. DIE AREA AND ACTIVE POWER OF COMMON BLOCKS

Die Area (µm2) Active Power (nW)

Clock Generator 27500 165 Digital Control Unit 14200 90 Bias Circuit (10nA) 17300 35

Bias Circuit (100nA) 6700 190

A current-starved ring oscillator [10] was used to generate 1MHz clock frequency. A fixed 10nA beta-multiplier (BM) current reference was used to bias the power management circuit. As these blocks are used for both PMU and SIU, they are always active. In addition, a frequency-proportional 100nA BM current reference [10] is used for sensor interface circuit that is turned off when no sensing is in progress.

III. SYSTEM IMPLEMENTATION

A. Power Management Circuit

The main blocks of the proposed power management circuit can be seen in Fig. 1. Battery voltage (Vbat) is compared to open circuit voltage of the PV module (Vpv) using a dynamic comparator and if it is lower than Vpv, switch is turned on to charge the battery by the PV module. As neither Vpv nor Vbat changes rapidly, comparison is done every few seconds to minimize average power consumption of comparator. If Vpv drops below Vbat, switch between Vpv and Vbat is turned off to avoid battery discharge through the PV module. The voltage level detector (LD) circuit, shown in Fig. 2, is used for both initial power-on-reset (POR) of the system and detection of the end of charge (EOC) state [8] in the battery. If Vbat is not sufficient to power up the circuit, the PV module continuously charges the battery up to VPOR threshold voltage by keeping the switch closed. As soon as the LD block detects that Vbat has passed VPOR, the system starts its normal operation. The LD circuit also detects EOC and turns off the switch to avoid overcharge of the battery by comparing Vbat with VEOC threshold voltage. As the battery voltage decreases by increasing temperature, a CTAT voltage reference should be used to detect VPOR and VEOC threshold voltages. In total, 25 substrate PNP transistors were used as Q1 and Q2 to generate VBE1 and VBE2 voltages. This SC circuit operates using non-overlapping clock signals Φ1 and Φ2 and detects when Vbat passes VPOR specified in (1) by setting POR output in Fig. 2. Command signals used for VPOR detection and status of the switches during Φ1 stage, can be seen in Fig. 2. During VEOC detection, only βC capacitor is used to detect when Vbat passes VEOC specified in (2) by setting EOC output.

I1

Fig. 2. Circuit diagram of voltage level detector

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In (1) and (2), α, β and δ coefficients are fractional ratios of αC, βC and δC capacitors that have been specified in Fig. 2.

. ⁄ (1) . ⁄ (2)

As VBE1 is CTAT and is PTAT, required CTAT reference voltage can be built by proper selection of α coefficient. By biasing Q1 and Q2 using 100 and using 25 4⁄ , 3 4⁄ and 5 8⁄ , VPOR of 900mV and VEOC of 1.5V can be detected in room temperature. As a CTAT reference voltage has been used, slightly lower voltage levels will be detected in higher temperatures.

B. Sensor Interface Circuit

The proposed sensor interface circuit is depicted in Fig. 3. Hydrogen sensing and temperature sensing are done in different times. Upon hydrogen exposure, individual Pd nanowires that have between 7K and 9K resistance, should be biased with a bias voltage between 50mV and 800mV for 4 to 12 seconds. The conductance of nanowires may change by 20% as H2 concentration varies from zero to 30% [1]. In Fig. 3, Rref and Rsense represent equivalent resistance of reference and sensing nanowires that are connected in series. The voltage around the reference nanowire, , is used as reference voltage, while the voltage of the sensing nanowire (V2) is used as input voltage for the following ADC. After initial reset of integrator and ADC counter, conversion is done in 3 periods. In the first period, 2 integration steps are performed, adding . ⁄ to Vo in each integration step. After each integration step, Vo is compared with VCM using a latched comparator and if the output of the comparator becomes 1, . ⁄ is subtracted from Vo and the ADC counter is increased by 1. If the output of the comparator becomes 0, . ⁄ is added to Vo and the ADC counter is decreased by 1. After these 2 steps, Vo changes by ΔVo1:

∆ ⁄ 2 . . 2 . (3)

In (3), NS1 indicates the number of subtractions of VR and 2 indicates the number of additions of

VR. Error introduced by the offset of opamp and charge injection of switches is shown by Ve. In the second period, Vo1

is converted to ∆ in 3 steps. In the first step, only SI and S9 switches are closed, to store Vo1 on C4. Next SI is opened and SR is closed to discharge C3. Finally SR is opened and S11 is closed to return . ∆ charge to C3. Assuming , the output voltage is thus ∆ .

Fig. 3. Circuit diagram of sensor interface

Third period is similar to period 1, but instead of , is applied in each integration step. In addition, instead of VCM, integration starts from ∆ .After 2 steps, the output of integrator is calculated as below:

∆ ∆ (4)

∆ 2 . . 2 . (5)

⁄ .

.. (6)

The value stored in the ADC counter at the end of third period is . As is not larger than . ⁄ , the difference between

⁄ and 2⁄ in (6) is less than 1 2⁄ and Nout is the n-bit digital representation of ⁄ . Conversion accuracy of this ADC is affected by noise, finite opamp gain and nonlinearity of the capacitors [9]. Target 12bits accuracy can be achieved by using 200fF metal-insulator-metal (MIM) capacitors as C3, C4 and C5 and a conventional 2-stage amplifier with at least 75dB gain for the integrator. A resistive divider was used to generate VCM for ADC. As 1MHz clock has been used for ADC, a 12-bit conversion takes nearly 8ms.

After finishing gas sensing, temperature is measured to further increase accuracy of gas sensing. The ratio between a PTAT and a bandgap reference voltage is used to measure temperature. In Fig. 3, 25 substrate PNP transistors were used as Q1 and Q2 in a common-centroid layout to generate VBE1 and VBE2. By biasing Q1 and Q2 with 100 , the voltage ∆ , can be used as a PTAT voltage to measure temperature after proper amplification. A bandgap reference voltage can be generated by summing VBE1 and ΔVBE with proper weightings. In order to apply a bandgap reference voltage to the integrator, VBE2 and VBE1 are applied to the bottom plate of C2 in the first and second phases of non-inverting integration. Meanwhile bottom plate of C1 is connected to ground and VBE1 to add Vref as (7) to Vo. In order to generate the required PTAT voltage for temperature measurement, only C2 is used to add Vptat as in (8) to Vo.

⁄ . ⁄ . (7)

⁄ . (8)

By using a 50fF MIM capacitor as C1 and a 360fF MIM capacitor as C2, a temperature-independent reference voltage of approximately 310mV has been generated. Similar to gas sensing, n-bit digital representation of ⁄ is stored in the ADC counter after temperature sensing. Accuracy of temperature sensing is mainly limited by mismatch between Q1 and Q2 and nonlinearity in temperature dependence of ΔVBE and VBE1. Although these errors can be minimized using dynamic methods presented in [7] to reach 0.1 accuracy, such power-consuming techniques are not needed here. The proposed low power temperature sensor can achieve 1 accuracy by only calibrating C2 and I1 at room temperature.

IV. SYSTEM INTEGRATION AND PERFORMANCE

The circuit has been implemented in 0.18um CMOS process and the total active area of whole microsystem is only 0.47mm2. The layout of the circuit can be seen in Fig. 4.

203

Sensor Interface

Unit (Digital)

PMU

Sensor Interface

Unit (Analog)

Common Blocks

790µm59

0µ

m

Test Blocks

Fig. 4. Circuit layout of whole microsystem

Simulation results in Fig.5a, using equivalent circuit model of the PV module [11], shows the power delivered to the battery under AM1.5 illumination. Starting from a fully discharged NiMH battery and charging it up to fully charged state, average efficiency is 90.9%. The PV module can provide a maximum power of 2.88mW at its maximum power point (MPP). In Fig. 5b, although illumination is reduced by a factor of 50, the average efficiency is still 87.8% and the PV module delivers average power of 48.6µW to the battery.

Table 2 presents die area and simulated maximum active power of main blocks in both PMU and SIU. As the comparator and LD blocks are activated every 10seconds, their average power consumption is less than 1nW. As a result, the average power consumption of whole microsystem during sensing is less than 2µW. An additional 7µA current is used for initial biasing of Pd nanowires for 10 seconds. After sensing, the average power consumption of the system is only 300nW. As the average current consumption of TZ1053 is 11.6µA to send one sample every 10seconds, the total average power consumption of whole sensor node is less than 30µW.

Fig. 5. Efficiency of power management system, under simulated AM 1.5 illumination (Fig. 5a), under simulated 2% light intensity (Fig. 5b)

TABLE II. DIE AREA AND ACTIVE POWER OF MAIN BLOCKS

Die Area (µm2) Active Power (nW)

SIU: Temperature Sensor 16900 180 SIU: ADC SC Integrator 8200 600

SIU: ADC Latched Comparator 780 200 SIU: ADC Vcm Generator 16400 150 SIU: ADC Digital Blocks 81400 230

PMU: Level Detector 27000 520 PMU: Dynamic Comparator 650 55

V. CONCLUSION

An ultra-low power microsystem is proposed for hydrogen sensing using miniaturized Pd nanowires. The circuit has been realized in a 0.18µm CMOS process with only 0.47mm2 die area to measure both hydrogen concentration and temperature. An area- and power-efficient power management system has been proposed to charge a NiMH microbattery using energy harvested from the nanowire solar cells. Simulation results show that even under reduced illumination, the harvested energy is enough for autonomous operation of whole system. Meanwhile, as the whole sensor node consumes less than 30µW for sensing and data transmission, target 6mAh battery can provide enough power for approximately 200 hours operation, even without energy harvesting.

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