Sun sensor design and test of a micro satellite

2.1. The PSD working mechanism

With the advantages of high position resolution, simple reaction current and quick response, PSD is widely used in the non-contact optical displacement detection system [10]. The PSD section structure is shown in Fig. 1. When the incident light beam irradiating on the surface of the photosensitive, the charge is generated at the incident position and the intensity, which is proportional to the light intensity. The charge flows through the P-layer to the electrode. Since the resistance of the P-layer is uniform, the current output from the two poles is inversely proportional to the distance from the point of light to the poles.

The distance of the two electrodes is 2$L$, and the output light current of the electrode (1) and (2) are respectively $I_1$ and $I_2$, and the total output current of electrode (3) is shown as the Eq. (1):

Fig. 1Cross-section structure schematic plan of PSD[11]

Regarding PSD as the establishment of the center of the coordinate system, the distance between the point of light spot and the center points $x_A$, we get:

From the Eq. (1)-(3), the distance between the center point and the point of light, we get:

The Eq. (2) and (3) show that the movement of the carrier in the substrate is separated by the Ohm’s Law, which means

The position information $x_A$ is determined by detecting the movement of the photo generated charge on the substrate, while it has no relationship with the intensity, distribution, symmetry and size of the light spot on the surface [12]. The structure and equivalent circuit of one dimensional PSD are shown in Fig. 2.

Measurement range and measurement accuracy are the main basis for the selection of photoelectric detector. The measuring range of the system is ±0.5 mm, and measuring precision is ±0.001 mm. In this paper, we select the one-dimensional PSD, that its light receiving area is 81 mm², nonlinear RMS is less than 0.15 %, and typical position error is ±5 μm, as the photoelectric displacement detection device, which produced by the Hamamatsu company.

Fig. 2One-dimension PSD [11]

a) Structure schematic plan

b) Equivalent circuit

2.2. The principles of optics

The sun imaged on the PSD through the hole on the diaphragm. By calculating the distance from the initial center position to the sun image point, we can determine the relationship between the azimuth angle of the sun light and the fixed coordinate system. The digital sun sensor optics principle is shown in Fig. 3 [13].

Fig. 3Sun sensor optics principle

Assuming that the distance between the diaphragm and the PSD plane is $h$, and the relative moving distance between the sun facula that the incident angle of the sun ray is $\theta$ and the vertical incident sun ray is $l$. $\gamma$ represents the two axis ($X$-axis and $Y$-axis) sun angle. $l_r$ represents the distance between the sun facula centroid to $o$. So, the sun angle is expressed as Eq. (5):

Resolution is:

where, $d{l}_{\gamma }$ is the minimum resolution for PSD.

The calculation formula for the single axis viewing angle of the sun sensor is:

where, $l_d$ is the length of the PSD sensitive element in the horizontal ($X$-axis or $Y$- axis).

4.1. Mechanical test

The component-level random vibration forced acceleration power spectral density RMS value of a micro-satellite is 14.53 g. The test conditions are shown in Table 2. The $X$-axis, $Y$ axis and $Z$- axis random vibration test were respectively carried out with 20 mechanical vibration test bench to assess the structural strength of the sun sensor. The mechanical test site is shown in Fig. 6. The random vibration response curves of $X$-axis, $Y$-axis and $Z$-axis of the sun sensor are shown in Fig. 7.

Fig. 6Mechanical test site

From Fig. 7, we can see that the RMS values of the random vibration response of the sun sensor are all in 20 g, which can meet the overall design index of the satellite.

4.2. High temperature test

For the assessment of sun sensor in thermal stability, according to a small satellite component level thermal cycling test conditions, using DHS-800 high low temperature test box to test sun sensor at atmospheric pressure with –5 °C-+35 °C in 8.5 times at low temperature and high temperature cycling test. The high and low temperature test site is shown in Fig. 8.

Fig. 7The random vibration response curves

a)$X$-axis

b)$Y$-axis

c)$Z$-axis

Fig. 8High and low temperature test site

4.3. The performance test

In order to test the performance of the sun sensor after mechanical test and thermal cycling test, the field experiment was carried out on the sun sensor. As shown in Fig. 9, the sun sensor test system including sun sensor, a tripod, a turntable fixing piece, a three axis turntable composition. Testing carried out under cloudless weather. Turntable console can accurately control the rotation angle and position of the turntable; the accuracy can reach 4″. Deflect two axis measurement accuracy tests on sun sensor.

According to the test data, the repeatability measurement error is less than 60″. In the whole field of view rotation measurement, the maximum error of the sun angle measurement and the actual value is within 40′. The sensor realized (120°)×(120°) to measure the sun angle, the field of view the default position accuracy is superior to 6″.

Fig. 9Performance test site