The Microsatellite Laboratory of the University of Bologna in Forlì is involved since 2004 in the design and development of its first microsatellite mission, named ALMASat-1, now scheduled for launch at the end of 2010 as part of the secondary payloads of the VEGA Maiden Flight. As the spacecraft is three-axis stabilized, in Nadir-pointing mode, making use of a momentum wheel and three orthogonal magnetic coils, it was decided to embark two attitude sensors, a magnetometer and a sun sensor, to ensure a deterministic three-axis attitude determination, at least in sun-light. While for the magnetometer a commercial unit was selected, it was decided to develop in-house the sun sensor, mainly for R&D purposes but also due to the current high cost of commercial sensors, even of "low-cost" units. This paper describes the whole design process, the materials and part selection, the assembly phase, the calibration procedure and, finally, the functional and qualification tests performed on the Engineering Qualification Model (EQM) of the Sun Sensor units to be flown on ALMASat-1. The developed Sun Sensor is a low-power, small-size and lightweight unit. The working principle is based on a Position Sensitive Device (PSD) sensor, on which the Sun light is concentrated by a commercial wide angle lens. Inside the Al case the reading electronics is based on an Atmel microcontroller and data communication can be through selectable RS232, CAN, I2C data buses. The calibration procedure, a critical step of the sensor integration because of the low-cost assembly techniques employed, is necessary to accurately define both lens distortion and focal length, and the position of the optical axis on the PSD. It is conducted using an optical bench equipped with a light collimator and an image analyzer used to define the exact contours of the light spot. The procedure, described in detail below, consists in taking digital pictures of the light spot using a magnifier lens for the entire lens aperture, starting from the optical axis to the lens full field. Each image is analyzed in order to find the spot light edges, then an appropriate algorithm is applied to retrieve the coordinates of the spot centre. Centre coordinates are used to define the lens distortion function throughout a polynomial fitting. At each coelevation angle, a statistically significant number of readings were acquired to characterize also the electronic noise. The systematic errors are within 0.15° in the range 0-60° of the coelevation angle (mainly due to a low order polynomial curve), while the electronic noise, which gives rise to random errors, is smaller than the systematic error in the entire range of coelevation angles admitted by the optical front-end.
Design, Development and Qualification of a Low-Mass, Low-Power Digital Sun Sensor / C. Bianchi; N. Melega; P. Tortora. - ELETTRONICO. - (2010). (Intervento presentato al convegno Small Satellite Systems and Services Symposium tenutosi a Funchal, Madeira – Portugal nel 31 May – 4 June 2010).
Design, Development and Qualification of a Low-Mass, Low-Power Digital Sun Sensor
MELEGA, NICOLA;TORTORA, PAOLO
2010
Abstract
The Microsatellite Laboratory of the University of Bologna in Forlì is involved since 2004 in the design and development of its first microsatellite mission, named ALMASat-1, now scheduled for launch at the end of 2010 as part of the secondary payloads of the VEGA Maiden Flight. As the spacecraft is three-axis stabilized, in Nadir-pointing mode, making use of a momentum wheel and three orthogonal magnetic coils, it was decided to embark two attitude sensors, a magnetometer and a sun sensor, to ensure a deterministic three-axis attitude determination, at least in sun-light. While for the magnetometer a commercial unit was selected, it was decided to develop in-house the sun sensor, mainly for R&D purposes but also due to the current high cost of commercial sensors, even of "low-cost" units. This paper describes the whole design process, the materials and part selection, the assembly phase, the calibration procedure and, finally, the functional and qualification tests performed on the Engineering Qualification Model (EQM) of the Sun Sensor units to be flown on ALMASat-1. The developed Sun Sensor is a low-power, small-size and lightweight unit. The working principle is based on a Position Sensitive Device (PSD) sensor, on which the Sun light is concentrated by a commercial wide angle lens. Inside the Al case the reading electronics is based on an Atmel microcontroller and data communication can be through selectable RS232, CAN, I2C data buses. The calibration procedure, a critical step of the sensor integration because of the low-cost assembly techniques employed, is necessary to accurately define both lens distortion and focal length, and the position of the optical axis on the PSD. It is conducted using an optical bench equipped with a light collimator and an image analyzer used to define the exact contours of the light spot. The procedure, described in detail below, consists in taking digital pictures of the light spot using a magnifier lens for the entire lens aperture, starting from the optical axis to the lens full field. Each image is analyzed in order to find the spot light edges, then an appropriate algorithm is applied to retrieve the coordinates of the spot centre. Centre coordinates are used to define the lens distortion function throughout a polynomial fitting. At each coelevation angle, a statistically significant number of readings were acquired to characterize also the electronic noise. The systematic errors are within 0.15° in the range 0-60° of the coelevation angle (mainly due to a low order polynomial curve), while the electronic noise, which gives rise to random errors, is smaller than the systematic error in the entire range of coelevation angles admitted by the optical front-end.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
