Procedia Chemistry Procedia Chemistry 1 (2009) 1363–1366 www.elsevier.com/locate/procedia
Proceedings of the Eurosensors XXIII conference
Self-Powered Sun Sensor Microsystems H. Wua, *, A. Emadia, G. de Graafa, J. Leijtensb and R F Wolffenbuttela a
Faculty of EEMCS, Department for ME/EI, Delft University of Technology, Delft, The Netherlands b TNO science and Industry, Delft, The Netherlands
Abstract An analog sun sensor has been designed based on shade profile proportional to the angle of incidence of incoming light projected onto a 2×2 array of photodiodes. This concept enables an autonomous self-powered optical system with two the main functions (electrical power generation for the amplifier and the optical position measurement) implemented in the photodiodes, by having these operated simultaneously in the photovoltaic and photocurrent mode respectively. The low-power current-to-voltage converter is used to readout the differential photocurrent, while powered from the photodiodes at minimum supply voltage level. Test structures have been designed, fabricated and used for validation of the concept. Keywords: Self-powering ; optical system ; sun sensor
1. Introduction Powering of electronic microsystems by batteries has become a limitation in the introduction of highly miniaturized autonomous sensor systems in many applications. Limitations are due constraints imposed by volume, weight, maintenance requirements and reliability of power sources in harsh environment1. Energy scavenging in combination with low-power circuit and system design has been identified as a promising solution to this problem. Energy scavenging concepts are usually based on extracting energy from vibrated movement2, temperature gradients using thermo-electric devices3 or rely on optical energy available and exploited using solar cells. An application that is served well using optical energy is the sun sensor, which is widely used for attitude control in satellites. With the increased interest in micro-satellites comes the interest in a micro sun-sensor. The sun sensor provides information on the attitude of the satellite with respect to the sun. Since the main interest in micro-satellites originates from the possibility of having multiple satellites operate in a cluster (a swarm), position information is crucial. The sun sensor is composed of a position sensitive detector (PSD) or an array of photodiodes. Power is typically supplied from the main solar panel or a small dedicated solar cell mounted on the sun sensor4. A semiconductor pn-junctions is basically operated either in the photovoltaic mode (solar cell) for electrical power generation or in the photocurrent mode (photodiode) for opto-electrical signal conversion. The concept
* Corresponding author. Tel.: +31-15-27-86285; fax: +31-15-27-85755. E-mail address:
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1876-6196/09 © 2009 Published by Elsevier B.V. Open access under CC BY-NC-ND license. doi:10.1016/j.proche.2009.07.340
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presented in this paper aims at combining these two modes of operation in order to enable the fabrication of selfpowered optical sensor systems. The ability of CMOS processes with a feature size of 0.35 μm and smaller to be operated using a supply voltage at 1V is an essential enabling factor for this concept. The reduced power supply voltage with reduced feature size is usually considered a disadvantage in analog circuit design, as it reduces the dynamic range at a given noise level. However, it does enable direct electrical powering of the circuit from a set of two photodiodes. The operating principle as compared with the conventional sun sensor is described in detail in the next part. The chip with photodiodes and current-to-voltage converter is designed and implemented. The experimental result shows the differential current between photodiodes can reflect the shadow imposed on photodiode where a needle is on top of chip. Conclusions are drawn in the final section along with an outline of future work. 2. Operating principle
2.1. Conventional sun sensor A conventional analog sun sensor measures the position of a satellite relative to the sun by projection of the sunlight through a hole (aperture) in a shield onto a PSD or a 2×2 array of detectors, as shown in Fig. 1(a). Digital sun sensors have also been investigated using a full CMOS imager and signal processing to calculate the sun position from the image5. The position of the projected light spot depends on the angle of incidence and, hence, on the satellite position relative to the sun. This information is used for directing an antenna or the solar panel or by the internal navigation system for attitude control. Since only a light spot is projected, only one photodiode is actually illuminated. The other diodes remain essentially idle. Therefore separate solar cells are required for electrical powering this optical system. Although die area in a small feature size CMOS process should be considered expensive real estate for implementing a solar cell, the benefits of small size and small weight outweigh such concerns when also considering the expenses of launching additional mass into space. Moreover, the loss of expensive die area is minimized when using the same diodes for powering and sun position detection, as is pursued in this project.
Fig. 1. (a) conventional sun sensor scheme; (b) sundial principle applied to sun sensor.
2.2. Sundial principle based sun sensor The approach requires a shadow mask positioned directly on top of 4 pn-junctions for projecting a shade profile proportional to the angle of incidence of the sun on the photodiodes. As shown in Fig. 1(b), all detectors are
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illuminated, thus can be used for electrical power generation and the position information is simultaneously available in the differential photocurrent6. 3. Design and fabrication The AMIS 0.35 μm CMOS process has been used to realize the chip. Two photodiodes in series together with current-to-voltage converter are designed into one unit cell. Two of these are required in the 2×2 detector array configuration. The differential photodiode is detected when the shadow is projecting on either surface of photodiodes while the series equivalent photodiode voltage and current is powering the readout circuit. The photodiode area is 550×350 μm2. With this dimension of photodiode, the photocurrent is large enough for electrical powering of the amplifier. IC compatible micromachining for fabrication of the pole has not yet been included in the initial devices. The package has been modified to include a pattern to cast a shade on the diode array in the initial experiments. 4. Measurement and characterization
4.1. Photodiode Characterization Figure 2(a) shows the different spectral responses of the diode formed by the p+top layer and the n-well as compared to that of the n-well-p-substrate diode and confirms the shift of maximum responsivity to longer wavelength of the latter type of photodiode7. The measurement is done by monochromator with a wavelength resolution of 0.3 nm. The 5 μm passivation layer causes the ripple in the spectral response. Figure 2(b) shows the photovoltaic performance of diodes for three different illumination intensities. The maximum power point is at 600 mV in case of a 7.6 μA load (115 W/m2 illumination) and drops to 400 mV in case of an 8.3 μA load.
Fig. 2. (a) spectral response of the two stacked diodes; (b) the photovoltaic performance of diode at three different illumination intensities. 4.2. Measurement of system performance The preliminary measurements confirm proper operation of the self-powered position sensor for one-dimensional position measurement using two diodes, which are connected to the readout circuit for electrical powering. The two diodes are separated by 36 mm. The needle with 0.75 mm diameter was put in between the light source and diodes to form a shadow. The distance between the needle and the diodes is 10 mm. The measurement result is shown in Fig. 3. The change in output voltage with the position of the needle is in reasonable agreement with the expected linear response.
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Fig. 3. measurement result of two diodes with moving shadow.
5. Conclusions A new concept of the self-powered sun sensor system has been presented. The concept suggests illumination of the whole photodiodes arrays instead of using a pin hole to illuminate particular photodiode in the array as in the conventional method. The detector array and basic analog read-out circuit have been integrated in the AMIS 0.35 μm CMOS process. The operation principle has been experimentally validated with one-dimensional position measurements using two diodes. The two diodes are separated by 36 mm with 0.75 mm needle in between to form a shadow in the preliminary experiment instead of applying the shadow mask on top of arrays. The differential current of two photodiodes has been changed during the movement of the shadow (from the needle) while the powering for the front-end readout is still supplied.
Acknowledgements This work has been supported in part by the Dutch technology foundation STW under grant DET.6667 and in part by MicroNed project MISAT 1C2.
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