Design of Sun Sensors for Cubic Stars

To obtain the current attitude of the satellite in attitude control of CubeSat，a digital sun sensor for CubeSat is designed． Through the linear CCD( Charge Coupled Device) and the corresponding slit-type optical path design，the double-angle measurement of the point light source relative to the normal line of the installation plane is achieved，and the design index of the field of view angle of 40° and the theoretical measurement accuracy of 0． 01° is achieved． The sun sensor uses commercial off-the-shelf products and technologies ( COTS: Commercial Off-The-Shelf) ，and has the characteristics of small size，low power consumption，simple design，high stability，and easy integration． As a COTS resource that satisfies the geometric dimensions of the CubeSat circuit board，it has potential application value in the future development of the CubeSat and plays a vital role in the further research of other CubeSats．

1. Sun sensor

Sun sensors are photoelectric sensors used by spacecraft to determine their own flight attitude during orbital flight. It can provide spacecraft with information on the angular position relationship between the current solar incident light and a certain plane on the spacecraft, and is one of the key sensors for achieving spacecraft attitude control and accurate orientation of mission loads. Currently, solar sensors can be mainly divided into two types: analog solar sensors and digital solar sensors. A typical analog solar sensor uses sensing units such as photocells and photodiodes to determine the direction of the sun by measuring electrical analog quantities; The digital sun sensor uses linear CCD (Charge Coupled Device), CCD array and other sensing units to sample the imaging position information of the solar incident light passing through the optical front-end on the sensing plane, and calculates the sun vector through a microcontroller.

2. Cubic Star

Cubic stars are micro and nano satellites with strict size and weight requirements, enabling simple tasks such as ground photography and atmospheric observation. The size, weight, and system power requirements of the equipment on the Cubic Star result in different design requirements from those used in traditional spacecraft. Due to the difficulty in installing complex attitude control systems on cubic stars, simple attitude control methods are often used or not used. The users of satellites are mostly non aerospace related departments such as universities and scientific research institutions, and the number of ground measurement and control stations is relatively small. The operators have a short measurement and control time for cubic stars. Therefore, in tasks with directional requirements, it is necessary to use solar sensors to obtain flight attitude, ensuring that the task payload points towards the observation target within a limited effective measurement and control time.

3. System architecture design

The sun sensor mainly consists of the following three parts: optical front-end, sensor, and signal processing part. The optical front end restricts the light from the sun or other light sources, causing them to produce light spots that fall on the detection plane of the sensor. This section can adopt structures such as slits, pinholes, or diffraction gratings. The sensor part converts the intensity of the beam into analog electrical signals with different voltage values by sensing the light spot projected on the detection plane by the optical front-end. Typical sensor structures include photodiodes and their arrays, photocells, CCD, etc. The signal processing section will process the electrical signal generated by the sensor, perform analog-to-digital conversion (ADC: Analog Digital Conversion), and finally obtain the distribution of the incident light. This enables the microcontroller to calculate the vector relationship of the incident light relative to the plane. In this design, a combination of slit, linear CCD, and microcontroller is used to form the overall architecture of the system.

(1) Optical path design

In this design, the optical front-end adopts a lensless design, which controls the shape and size of the spot formed by the incident light on the sensor detection plane through the structure of optical openings. In terms of selecting the shape of the opening, the commonly used designs for solar sensors mainly include pinholes, slits, and rectangular windows. Pinholes and rectangular windows are commonly used in photoelectric sensors such as photocells and area array CCD, with high machining accuracy and low difficulty. For linear CCD, due to the limited longitudinal height of the smallest photosensitive element, when using pinholes or rectangular windows, it is necessary to have holes very close to the sensor surface to avoid light spots falling outside the effective photosensitive area. This structure is difficult to process and install as a whole, so this design adopts a slit method for opening holes. The designed optical slit width depends on the minimum photosensitive element width of the sensor. The width of the slit is 34 times that of the photosensitive element, based on how one spot can be detected by multiple smallest photosensitive elements. This design can prevent blind spots in angle sensing caused by damage to individual photosensitive components. Its material is stainless steel, and the seam opening process is a chemical corrosion method that can obtain smoother seam edges. The inner and outer surfaces of the slit are treated with a blackening process, and in addition to the slit, a blackened light shield is installed around the linear sensor. Avoid adverse effects of reflected and scattered light inside the sensor on the sensor.

(2) System hardware design

The components used in the solar sensor are all COTS (Commercial Off The Shelf) components. The photosensitive element of the sensor is selected as TSL1401CL linear CCD. The geometric dimensions of the solar sensor meet the design specifications of a 1U cubic star, with a length of 95 mm and a width of 90 mm. Using PC-104 32 ×  The 2-pin inter board connector is directly connected to other circuit boards of CubeStar. The data is transmitted through the onboard bus, and the power supply is provided by the CubeStar power bus.

(3) System software structure

The microcontroller of the sensor runs the open-source real-time operating system FreeRTOS. After the sensor is powered on, the main process polls and operates two linear CCD exposures, performing tasks such as reading analog voltage information, binarizing, and outputting the final solar vector information; The communication process monitoring board selects signals and on-board buses, responds to computer instructions received on the satellite, and implements data transmission. Information exchange between processes is carried out through queues, and resources are reused through mutexes. The logical flowchart of the system is shown in Figure 2. The sun sensor, as a sub board in the cubic star system, is connected to the onboard bus of the cubic star. The onboard computer retrieves the sensing reading of the current sun sensor through polling.

The binarization operation process obtains the judgment threshold by sorting the voltage values of the input signal, taking the middle part and averaging it. Then, all voltage values are compared with the threshold to convert it into a processing result that only includes 0 and 1 states.

This article analyzes the structure of a solar sensor and designs and implements a slit optical structure, achieving a solar sensor with a 40 ° field of view range and a theoretical measurement accuracy of 0.01 °. The feasibility of the sensor theory was verified through testing, and the basic indicators of the sensor were measured. The solar sensor adopts COTS components, which comply with the mechanical and electrical standards of CubeStar and is suitable for use on CubeStar. The future research direction will focus on the error analysis of sensors, improving the detection field of view angle of sensors, resolving accuracy of sensors, and other technical indicators.