An Autonomous Wireless Sensor Node using a Solar Cell Antenna for Solar Energy Harvesting

Mina Danesh and John R. Long

Delft University of Technology, ERL/DIMES, 2628 CD, Delft, Netherlands

Abstract — An autonomous 3-5 GHz UWB sensor transmitter

node uses a single solar cell of 2 x 2 cm2, sufficient to generate energy up to 20 mW of peak power outdoors, and acting as a broadband monopole antenna in the 3-10 GHz range, thus miniaturizing the overall system package. The sensor node consumes 10 µA average current when transmitting data bursts every 8.5 s at 1 kbits/s using OOK modulation. The harvested solar energy is stored in a supercapacitor to ensure continuity of transmission.

Index Terms — Wireless sensor networks, energy harvesting, solar energy, ultra wideband antennas, ultra wideband communications.

I. INTRODUCTION

Integration of sensors and wireless transceivers for the networking of microsystems has been aimed at emerging applications that are highly integrated, ultra-low power, and low cost. Potential applications range from body-area sensor networks (WBAN) for health monitoring, environment monitoring, security and automotive sensors, to wireless personal area networks (WPAN) [1]. These wireless sensor systems rely on short-range communication (≤100 m), communicate at low data rates (<100 kbits/s), and are small in physical size (<1 cm3). Smart sensor systems can have an operational lifetime measured in years, however, as the wireless transceiver is often idle, average power consumption is just a few µWs for the transceiver electronics. This ultra-low power requirement makes energy scavenging feasible in the system design [2]. Harvesting of solar energy through photovoltaic conversion in an outdoor environment provides a typical power density of greater than 10 mW/cm2.

An autonomous wireless transmitter that harvests energy from its surroundings via a solar cell antenna which functions as both an RF and DC source [3] is described in this paper. Sharing the area consumed by the primary DC power source with the antenna reduces the size and overall cost of the transceiver.

Ultra-wideband FM (FM-UWB) is adopted for this work. Uniform spectral density at low transmit power and steep spectral roll-off for the transmit signal suitable for unlicensed UWB transmissions are realized through the use of a triangular sub-carrier wave shape [4]. This paper focuses on the 3.1 to 5.1 GHz frequency range with transmitter frequency bandwidths of 500 MHz.

II. SOLAR ENERGY HARVESTING WIRELESS SENSOR SYSTEM

A block diagram of the wireless transmitter sensor node that is powered by solar energy harvesting is shown in Fig. 1. The solar cell is fabricated using thin-film hydrogenated amorphous silicon (a-Si:H) technology in the DIMES facility, with a total area of 2 x 2 cm2. It provides the DC+ and DC- potentials which are isolated from the RF signal path by low-pass filters (i.e., Lf1, Cf1 and Lf2, Cf2, Lf3). The solar cell is operated close to the maximum power point tracking (MPPT) voltage of the solar cell (i.e., Vmp in Fig. 2). Therefore, the boost DC-DC converter is activated by the MPPT circuit when the solar cell voltage (Vcell) exceeds a reference voltage (Vr) of 0.5 V. A current of 7 mA is required by the boost converter from the solar cell in order to ensure proper start-up. After start-up, a minimum of 3 mA is drawn from the solar cell.

Fig. 1. Autonomous wireless smart sensor system block diagram. The boost converter is set to a maximum output voltage of

3.9 V. A Schottky diode (D1) isolates the storage device or supercapacitor (SC) from the solar cell and boost converter when the converter is turned off. The converter output voltage increases as it charges the supercapacitor when the solar cell is properly illuminated. Charge stored on the supercapacitor powers the sensor node when there is insufficient light to operate the boost converter via the solar cell. The voltage drop across D1 when charging is 0.3 V, so the supercapacitor is charged via the boost converter until the voltage across its terminals reaches 3.6 V (i.e., 0.3 V below the max. boost converter output voltage). An operating

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voltage between 2.1 V and 3.6 V is supplied to the microcontroller unit (MCU).

The MCU interfaces the sensor and the radio transmitter (RF-VCO and solar cell antenna). The MCU is a low-power microcontroller which has deep sleep functionality and consumes only 500 nA supply current in sleep mode. For this design, a low-power (<100 µW) temperature sensor with a 12 bit output word is employed. The sensor is controlled by the MCU via an I2C bus. The MCU also controls linear drop-out supply regulators (LDO) to set the bias voltages for the RF-VCO and sub-carrier modulator.

The minimum supply voltage for the load circuits (i.e., the MCU, sensor, and LDO) is 2.0 V. Switch S1 closes to power the load when the voltage on the supercapacitor is at least 2.1 V. When the boost converter is not operating and the supercapacitor discharges, the voltage at Vc decreases slowly from 3.6 V to the minimum supply voltage of 2.1V.

Figure 2 shows the solar cell DC characteristics, measured separately under best-case illumination from a direct light source of 1000 W/m2 at 25oC. The open-circuit voltage (Voc) and short-circuit current density are 0.9 V and 13 mA/cm2, respectively, resulting in an overall efficiency of 6.3%. The maximum output power point (Pmp) occurs at an output voltage (Vmp) of 0.59 V and current (Imp) of 34 mA. Hence, the solar cell can deliver up to 20 mW of power (i.e., Pmp = 20mW). However, the intensity of illumination is not constant so the solar cell can only be expected to deliver a few mW to power the radio transceiver under typical outdoor conditions. A minimum of 2 mW is required to power the entire system when the boost converter is operating and the supercapacitor is fully charged.

Fig. 2. Solar cell DC characteristics.

Data transmission is initiated when switch S3 is closed by

the MCU. Low data rate on/off keying (1 kbits/s OOK) and duty cycling of the load under control of the MCU is used to minimize power consumption by the sensor system and prolong its operating time. The wireless sensor transmits 16 bits of data in each data burst. A 4 bit preamble ‘1010’ precedes the 12 temperature data bits, as seen in Fig. 3.

Fig. 3. Output RF signal with OOK modulation at 1 kbits/s.

Figure 4 shows the overall timing sequence of the system. A conversion time of 32 ms allows the temperature sensor to perform its A/D conversion after a start instruction is initiated by the MCU. After the temperature data is received across the I2C communication bus, the LDOs are turned on by the MCU to power up the RF transmitter. The peak output current of the LDOs during start-up is 3 mA, as seen in Fig. 4. The RF-VCO is then modulated by a 70 mVp-p triangular wave to produce a 500 MHz bandwidth FM-UWB signal at the transmitter output when transmitting a '1', as seen in Fig. 5. The output buffer amplifier driving the antenna is turned on/off (OOK) according to the transmit data sequence. Fig. 4. Timing sequence of voltages and current. Fig. 5. Output power spectrum of the FM-UWB signal and comparison with a single tone.

In a typical application, the sensor data is collected and transmitted periodically to a host. Thus, the transmitter duty cycle (i.e., transmitter node active time over a period) will determine the overall power consumption of the sensor node. The wireless system should remain operational for a

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minimum amount of time while light intensity is insufficient, which depends upon the size of the supercapacitor. Moreover, the size, weight, and time required to charge the supercapacitor fully must be taken into account in the overall system and package design.

For this work, a 10 mF supercapacitor (rated at 4.5V) and a sleep time of 8.5 s, equivalent to a 0.3% duty cycle, were chosen for experimental assessment. The supercapacitor charging and discharging curves are shown in Fig. 6. The light source is turned off after charging the supercapacitor fully. A 1 V change in the capacitor terminal voltage (i.e., Vc from 3.6 to 2.6 V) is completed in 770 s, resulting in a 10 µA average current and 29 µW average power. The sensor node continues transmitting data packets for 20 minutes, until Vc reaches 2.1 V. The average energy per bit is thus 29 nJ/bit. For an application where system autonomy is required to last more than 2 days while sending temperature data every minute, a 500 mF capacitor rated at 3.6 V would be sufficient based on the measured performance.

Fig. 6. Charging and discharging curves of the 10 mF supercapacitor.

III. SOLAR CELL ANTENNA

As seen in Fig. 7(a), the solar cell backside contact is shared by both the RF signal path well as the DC supply. The solar cell measures 2 cm x 2 cm2, suitable for a planar antenna structure to operate in the GHz range. The copper patterns on the FR-4 substrate and the solar cell form a monopole antenna suitable for ultra-wideband transmission, as seen in Fig. 7(b). The DC supply, antenna, and RF circuits are co-integrated as shown in Fig. 7(c).

The simulated 1-port input reflection coefficient (S11) from 1 to 12 GHz at the antenna input, is shown in Fig. 8(a). The antenna input impedance matches 50 Ω across the UWB band, as seen in Fig. 8(b). The resonant peak below 2 GHz is due to the solar cell DC+ contact interconnects. At 4 GHz, the antenna behaves as a monopole, as seen in Fig. 9. Below 5 GHz, the gain of the solar cell antenna is similar to a copper-only monopole [5], thus its maximum gain ranges from 0 to 3 dBi.

Fig. 7. Front and back views of the solar cell in (a), front (b) and back (c) view of the integrated solar cell antenna.

Fig. 8. Simulated (a) input reflection (S11) and (b) antenna real and imaginary input impedances.

Fig. 9. Simulated radiation patterns in the x-z and y-z planes as a function of antenna gain (in dB) at 4 GHz.

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IV. RF TRANSMITTER

The FCC permits a maximum transmit power density of -41.3 dBm/MHz, or -14.3 dBm radiated signal power across a 500 MHz bandwidth. Since a wideband FM signal is transmitted, the stability requirements for the VCO are relaxed (-100 dBc/Hz at 10 MHz offset is sufficient).

The integrated transmitter consists of a ring oscillator and an antenna output buffer, as shown in Fig. 10. The single-ended topology allows for a direct integration with the monopole antenna. The oscillator NMOS and PMOS transistors M7-M14 which are biased in the deep triode region, regulate the bias current in each of the four CMOS inverter stages. The oscillator is directly modulated using a triangular wave on its core supply voltage (VDD_VCO), as seen in Fig. 4. The two-stage inverter buffer (M15-M18) is designed to minimize loading at the ring oscillator output. The last cascode buffer amplifier (M19-M20) provides a 50 Ω match using the matching network C3, L1, C4, and L2.

Fig. 10. Schematic of the ring oscillator and output buffers.

The FM-UWB transmitter VCO and buffer IC shown in

Fig. 11 is fabricated in a 90 nm bulk CMOS technology. The total chip area including bondpads is 725 x 900 µm2. Coupled inductances L1 have a high coupling factor (k) of 0.88 and total inductance of 3.5 nH. Capacitor C4 and inductor L2 (1.8 nH) contribute to the wideband (i.e., 3-7 GHz) output matching network. Inductors L1 and L2 have a Q of 7.5 and 10 at 4 GHz, respectively. Capacitors C1 and C2 filter the RF signal from the biasing lines.

The biasing circuitry is supplied by VT (1 V) and VBIAS (0.3 V) which draws 5 µA. With VDD_VCO and VBUF set at 1.1 V, the VCO core consumes from 200 to 360 µA, thus tuning from 3.1 to 5.1 GHz, and the output buffers consume 650 µA. Hence, the total DC power consumption of the transmit IC is about 1 mW. The frequency tuning characteristic is close to linear, as shown in Fig. 12, with a tuning sensitivity of 7 GHz/V. The unmodulated maximum RF output power (i.e., single tone) is -15 dBm. The measured maximum phase noise is -100 dBc/Hz at 10 MHz offset

across the VCO tuning range, which is sufficient to meet the performance requirements for an FM-UWB communication link.

Fig. 11. Microphotograph of the RFIC oscillator.

Fig. 12. Measured output oscillation frequency and single tone output power of the RFIC oscillator.

V. CONCLUSION

A solar energy harvesting wireless transmitter sensor node using a solar cell antenna for its DC power source and wideband monopole antenna is integrated into a smart system. The overall package consists of a temperature sensor, a supercapacitor, DC power management, digital, analog, and RF circuitry. The sensor node is capable of transmitting a 500 MHz bandwidth FM-UWB RF signal at a data transfer rate of 1 kbits/s. It consumes 29 µW average power when waking-up to transmit data packets every 8.5 s.

REFERENCES

[1] W. Weber, J. M. Rabaey, and A. Aarts, Ambient intelligence, New-York: Springer, 2005.

[2] S. Roundy, P. Wright, and J. Rabaey, Energy Scavenging for Wireless Sensor Networks, Kluwer Academic Publishers, 2003.

[3] M. Tanaka, Y. Suzuki, K. Araki, and R. Suzuki,” Microstrip antenna with solar cells for microsatellites”, Electronic Letters, vol. 31, no. 1, pp. 5-6, Jan. 5th 1995.

[4] J. F. M. Gerrits, M. H. L. Kouwenhouven, P. R. van der Meer, J. R. Farserotu, and J. R. Long, “Principles and limitations of ultra-wideband FM communications systems,” EURASIP J. of App. Signal Proc., vol. 2005:3, pp. 382-396, 2005.

[5] M. Danesh, J. R. Long, and M. Simeoni, “Small-area solar antenna for low-power UWB transceivers”, 4th Eur. Conf. on Ant. and Prop. EuCAP’2010 Digest, April 2010.

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