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On-Orbit Measurement and Analysis of the Micro-vibration in a Remote-Sensing Satellite

  • Dengyun Yu
  • Guangyuan WangEmail author
  • Yu Zhao
Original Paper
  • 29 Downloads

Abstract

With the improvement of the image resolution produced by remote-sensing satellites, the cameras onboard are increasingly sensitive to the micro-vibration caused by the moving parts of the satellite buses. The disturbance measurement in ground tests is often with low confidence, since the dynamic characteristics of the moving parts as well as the satellite structure are strongly influenced by many environmental factors which are quite different in space, such as atmosphere pressure, air damping or acoustic transmission. Therefore, on-orbit micro-vibration measurement is an important way to learn the characteristics of the disturbing source and the micro-vibration transmission along with the satellite structure in vacuum and weightless environment. In the present study, the on-orbit micro-vibration measurement system onboard a remote-sensing satellite is introduced. The measured data are analysed in three aspects of background noise level, disturbance source characteristics, and satellite structure transmission characteristics. From the measured data, it is found that local vibration caused by control moment gyro (CMG) and momentum wheels have the highest level, while the vibration transmitted to the payload is mainly caused by CMG and two-axis antenna. The response at the payload interface is much lower than the disturbance source interface, which means that the vibration level is attenuated largely by the satellite structure.

Keywords

Remote-sensing satellite Micro-vibration On-orbit measurement Data analysis 

1 Introduction

Micro-vibration induced by the moving parts onboard spacecraft may affect the pointing stability and imaging quality of sensitive payload. With the improvement of the image resolution produced by remote-sensing satellites, the cameras onboard are increasingly sensitive to the micro-vibration caused by the moving parts of the satellite bus. Attenuation of micro-vibration became a necessary progress to ensure the image resolution [1, 2]. Nevertheless, evaluation of the satellite bus micro-vibration is the foundation for micro-vibration attenuation and improvement of image quality.

Evaluation of micro-vibration through mechanical analysing or ground test has run into many difficulties. Application of FEM in micro-vibration analyse is limited due to large error in middle-high frequency band, with difficulty in describing the structure uncertainty induced by micro-deformation and lacking evidence for structure parameters with micro-deformation, etc. [3, 4]. The ground test needs to make up for the test error resulting from different environments between ground and onboard: different boundary conditions, different mounting status of flexible accessories, structure dynamic bias induced by gravity, air damping, etc.

Evaluating the micro-vibration level at the spacecraft structure using on-orbit micro-vibration measurement unit (OMMU) is the most direct and accurate way. NASA has measured the on-orbit micro-vibration for many times from 1970s, along with the development of OMMU such as SAMS-II [5] and AMAMS [6]. PAX developed by ESA were mounted on several satellites to measure the on orbit micro-vibration, and perform SPOT-4 to compare micro-vibration on earth with on orbit [7]. There are several satellites launched with OMMU and the micro-vibration level in orbit was measured in China as well since 2011 [8, 9].

On-orbit micro-vibration measurement scheme is presented in this paper; the test data were analysed from three aspects: background noise, moving part properties, transfer characteristics of satellite structure.

2 OMMU Hardware Configuration

OMMU consists of four parts, which are data acquisition module, control and storage module, power module, and sensors, which its configuration is shown in Fig. 1. Data acquisition module collects the signal from three-dimensional acceleration transducers (3DAT) and generates data codes. Control and storage module records the data and recalls the memory when necessary. Three sensors (3DAT) were installed at different positions on the satellite and connected with the control and storage module by cables.
Fig. 1

OMMU configuration

Parameters of the data acquisition system are shown in Table 1.
Table 1

Data acquisition parameters

Item

Specification

Scale

0.001–0.1 g

Sensitivity

1000 mV/g

Accuracy

8%

Frequency band

0.1–150 Hz

Channels

3 × 3

2.1 Transducer Allocation

The moving parts on remote-sensing satellite mainly include momentum wheel, control moment gyroscope (CMG), Solar Array Driving Assembly (SADA) and antenna driving assembly (ADA), etc. Sensitive payloads including two optical cameras, namely camera 1 and camera 2. 3DATs of the OMMU were allocated near those moving parts and sensitive payloads. The location of 3DATs is shown in Table 2.
Table 2

Location of the micro-vibration transducers

Measure point number

Measure point location

a

+ Y solar array SADA holder mounting point

b

Service cabin top plate − Y − Z CMG mounting point

c

Service cabin top plate − Y + Z momentum wheel mounting point

d

Camera 1 bottom plate + Z near mounting point

e

Camera 2 side plate − Y + Z near mounting point

f

Antenna assembly mounting point

2.2 Measurement Cases

OMMU started operation at 30 min before launching, recorded all the vibration data through the launching process, and shut down 10 min after the deployment of solar array wings. Afterward, OMMU performed micro-vibration measurement several times according to ground instructions. There were mainly two working conditions when the payloads work; the micro-vibration data of the two cases were analysed in this paper; the description of cases is listed in Table 3.
Table 3

On-orbit measurement cases

Case number

Case description

I

CMG, momentum wheel, SADA, antenna working stable, camera I imaging

II

Camera 2 imaging, only antenna, momentum wheel and SADA work at same time

3 Data Analysis

3.1 Background Noise

In the time period between the satellite separated with rocket and the satellite control system started, the flexible accessories were not deployed and the driving assemblies were shut off and all the moving parts were not moving, the data acquired was considered as background noise. Time domain data of background noise are shown in Fig. 2. Quantified precision of measure system was 6.04 × 10−5 g; the peak–peak value of background noise was 3.02 × 10−4 g which is 5 times of quantified precision. This measurement indicates that there is hardly any measurable disturbance in orbit, except the moving part excitations.
Fig. 2

Time domain data of background noise

3.2 Moving Part Excitation Properties

3.2.1 CMG Excitation Property

Antenna, SADA, and momentum wheel also worked during the stable working period of CMG. Comparing the mounting point acceleration data before and after the CMG started, it was found that the peak–peak value increased from 36.8 × 10−3 to 114.3 × 10−3 g and RMS increased from 4.3 × 10−3 to 16 × 10−3 g. From the power spectrum density (PSD) of CMG disturbance (Fig. 3), it can be seen that the disturbing energy was concentrated at the frequency band of 60 Hz, 100 Hz, 180 Hz, 200 Hz, as four peaks arising after CMG started.
Fig. 3

Acceleration PSD at CMG mounting point

According to CMG disturbance analysis and ground tests, the rotation speed is 6000 r/min, which means the relative frequency is 100 Hz. There would be harmonic disturbance due to bearing defects at 0.6 times of the relative frequency. So the disturbance of CMG mainly composed of 60 Hz, 100 Hz, and their frequency multiplication [10]. The frequency components tested on-orbit were similar to ground test and analysis result.

From the spectrum of CMG disturbance tested on-orbit, it can be found that the amplitude at the rotation frequency is higher than other peaks. The natural frequency of the CMG structure is 110 Hz, which indicates that the disturbance near this frequency band would be amplified. Disturbance in the high-frequency band can be attenuated by isolation, caused by structural flexibility or be amplified by coupling effect with the high-order modes. Because of the limitation of the measurement system, current data cannot cover the disturbance above 250 Hz, which is more sensitive to optical imaging payload. Therefore, it is necessary to carry on wide-band measurement.

3.2.2 Double Axis Antenna Assembly Excitation Property

The acceleration measured in time domain at the mounting point of the double axis antenna is shown in Fig. 4. The peak–peak value is 6.6 milli-gravity (mg) and the root mean square value is 0.98 mg. The PSD of the acceleration data is shown in Fig. 5; main peaks in middle-high frequency range are 53.6 Hz, 107.2 Hz, 160.8 Hz, respectively, that are the step frequency and the multiplication frequency of the stepping motor in X direction.
Fig. 4

Acceleration in time domain at two-axis antenna mounting point

Fig. 5

Acceleration PSD at two-axis antenna mounting point (0–250 Hz)

In lower frequency range, the main disturbance peaks are 1.621 Hz and the odd number multiplication frequency, as shown in Fig. 6. 1.621 Hz is the step frequency of the stepping motor in Y direction. The peak value oscillates while attenuating, until the signal enlarges at 85 Hz. This phenomenon is cause by the structure dynamic property in the transmission path.
Fig. 6

Acceleration PSD at two-axis antenna mounting point (0–60 Hz)

3.2.3 Momentum Wheel Excitation Property

The acceleration measured in time domain at the mounting point of the momentum wheel is shown in Fig. 7. The peak–peak value is 28 mg and the root mean square value is 3.7 mg. The PSD of the acceleration data is shown in Fig. 8. The rated rotation speed corresponds to working frequency 24.8 Hz, while the disturbance at working frequency is smaller, and disturbance at higher frequency range is larger, mainly at 131.6 Hz, 137.7 Hz, 145 Hz. Those disturbances are caused by harmonic part of bearing defects and holder follow-up.
Fig. 7

Acceleration in time domain at momentum wheel mounting point

Fig. 8

Acceleration PSD at momentum wheel mounting point

3.2.4 SADA Excitation Property

The acceleration measured in time domain at the mounting point of SADA is shown in Fig. 9. The peak–peak value is 3 mg and the root mean square value is 0.356 mg. The PSD of the acceleration data is shown in Fig. 10. Compared with the figures above, the disturbance of SADA is far lower than other moving parts.
Fig. 9

Acceleration in time domain at SADA mounting point

Fig. 10

Acceleration PSD at SADA mounting point

4 Conclusion

Micro-vibration were measured on-orbit with OMMU mounted on a remote sensing satellite. Valuable data was obtained for moving part excitation property analysis, giving basic data for micro-vibration suppression. These are helpful to analyse the effect of micro-vibration on the image quality. From the two measurement cases, it is found that:
  1. 1.

    Background noise was relatively small when all the moving parts were shut down; thus, the moving part excitations were the main disturbance.

     
  2. 2.

    Disturbance of CMG and momentum wheel was relatively large.

     

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Copyright information

© Chinese Society of Astronautics 2019

Authors and Affiliations

  1. 1.China Aerospace Science and Technology CorporationBeijingChina
  2. 2.Beijing Institute of Space System EngineeringBeijingChina

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