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Test bed for verification of attitude control system

By Christian RASCHKE, Stephan ROEMER and Karsten GROSSEKATTHOEFER

1) Astro- und Feinwerktechnik Adlershof GmbH, Albert-Einstein-Str.12, 12489 Berlin, Germany

Phone: +49 30 6392 1000, Fax: +49 30 6392 1002 c.raschke@astrofein.com, s.roemer@astrofein.com, k.grossekatthoefer@astrofein.com

The test bed supports the development and verification process of attitude control systems by controlling components, generating sensor inputs and testing control algorithms and the subsystem performance, like attitude knowledge and orientation. Additional to the classical software-in-the-loop and hardware-in-the-loop tests (in the early phases of projects) it allows the hardware end-to-end test of the original system, with all its original behavior of software and hardware of the satellite.

Key Words: test bed, attitude control, test, simulation

1. Introduction Since 1993, Astro- und Feinwerktechnik Adlershof GmbH (AFW) offers solutions and services for aviation and aerospace industry. Besides engineering, precision manufacturing and environment simulation tests, small satellites technology is a key business segment of our company. In this area AFW offers the TET (Technologie-Erprobungs-Träger) satellite bus, an ACS (attitude control system) test bed and furthermore attitude control components, such as reaction wheels in different versions. This paper describes the ACS test bed of AFW, which supports the development and verification process of attitude control systems by controlling components, generating sensor inputs and testing control algorithms and the subsystem performance like attitude knowledge and orientation. The test bed consists of a magnetic field simulator, a sun simulator and an air bearing platform. The platform represents the satellite bus and is adaptable to the respective project. Therefor the moments of inertia can be adjusted to specific satellite characteristics and the same ACS components and onboard data handling system (OBDH) can be integrated and utilized on the platform. The platform itself supplies the bus (ACS components and OBDH) with a wireless communication system and power via a battery stack. Furthermore, the paper shows some results of the successful verification of TET-1 ACS (TET-1 is the prototype of the TET platform and is expected to be launched in summer 2011). Particularly during the testing of the different attitude control modes like sun pointing or nadir pointing, the test bed proved to be a valuable support for the verification process. Operating the complete attitude control system including sensors, actuators and software algorithms in the loop, the reaction of the system could be observed. The test bed demonstrated the

physical reaction of the satellite bus and stimulated the ACS sensors with realistic environment conditions referring to the Earth’s magnetic field and sunlight incidence. Hence the estimation of the attitude and the determination of the control torques could be performed using real sensor data, whereas the actuators applied the control torques to the test bed and thus close the test loop. The verification process of the TET-1 attitude control system was supported by a dynamic simulation model of the satellite. The model describes a closed loop simulation of the TET-1 satellite bus. On the top-level view, as shown in Figure 1, the model consists of four major elements: environment, satellite, dynamics, and mission. The environment sub-model considers the satellite’s orbit and all relevant space objects and their influences according to ECSS standards [5], whereas the mission block introduces the start time of the simulation and defines so the satellite’s mission time.

Figure 1: Overview simulation model

Another sub-model describes the internal configuration and subsystems, in our case power supply, thermal behavior, and attitude control, and thus represents the satellite itself. The dynamics block formulates the interaction between satellite and space environment, i.e. the equations of motion and three-dimensional disturbance torques induced by, for example, the magnetic field of the Earth [3]. The ACS model is mainly divided into the domains software, sensors and actuators. The software consists of state estimation, state control algorithm and actuator control. Both

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hardware and software parts are based on the control approach and hardware components included in TET 1 and represent their performance in a level of detail corresponding phase B of the space craft development process [4]. Use cases for the simulation model are amongst others the generation of reference data for complex test scenarios as well as the visualization of the satellite’s state and telemetry output. 2. The ACS Test Bed The ACS test bed is designed and made by Astro- und Feinwerktechnik Adlershof GmbH. The main task is the simulation of the satellite dynamics. This means that the attitude control algorithms can be tested and verified in the loop with the hardware. The test bed consists of a magnetic field simulator, a sun simulator and an air bearing platform. The platform represents the satellite bus and is adaptable to the respective project.

Figure 2: Overview test bed

2.1. Air Bearing Platform The test platform floats on a precision air bearing. This allows a vertical rotation of 360° and a horizontal rotation of 20°. The maximal load of the air bearing is 180 kg. This includes the platform and the test objects. The Air bearing platform consists of the platform for the test object, failsafe mechanism, power supply system, centre of gravity calibration and the air supply.

Figure 3: Air bearing with platform

The shown platform is design for a microsatellite that means moments of inertia of ca. 5 kgm² to 7 kgm². Another platform for small satellite moment of inertias between 15 kgm² and 30 kgm² is available. The COG calibration is done in two steps. The first step is a coarse manual calibration. In the second step the COG is adjustable by fine masses (100 g) in steps of 0.01 mm via WLAN. So a high performance is possible. Due to the use of a precision air bearing and the accurate COG calibration follows very low disturbance torques of <10-5 Nm. The power supply consists of rechargeable batteries and a power distribution unit. The performance of the batteries is greater than 400 Wh. They are mounted under the platform. Thus the rotation is not limited by any cable. The power distribution provides voltages between 3.3 V and 32 V. The test bed especially the air bearing is saved by special mechanism against blackout or pressure loss. The failsafe mechanism consists of two pressure tanks and three pneumatic cylinders. In a failure case the pneumatic cylinders are automatic deployed so that the platform is fixed and no destroying of air bearing occurs. 2.2. Magnetic Field Simulator The magnetic field simulator consists of three pairs of Helmholtz coils for homogeneous field generation and special controller software. The coil system generates a field up to 200,000 nT around each axis with 1% accuracy. The magnetic field controller bases of the SGP4 orbit model and the IGRF earth magnetic field model. So a dynamic simulation of the magnetic field depending on the orbit is possible. Furthermore the local magnetic field can be set, so that the local field is compensated by the magnetic coil system. 2.3. Sun Simulator The sun simulator consists of a HMI vapor discharge lamp and a special structure. The spectrum of the HMI vapor discharge lamp is similar to the sun with an intensity of light of 50,000 lux (measured at the platform). The lightning spot at

Sun simulator Magnetic coil system

Air bearing platform

Platform

Fine COG calibration

Power supply system

Failsafe mechanism

WLAN interface

Air tanks for failsafe

Air supply

Coarse COG calibration

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the test object has a diameter of 50 cm. The design of the structure allows a free rotation of the sun around the platform, so different incident angles can be realized. A reflecting mirror allows the cancellation of big parts of the infrared radiation, to avoid the heating of the test object, and to have a better homogeneity of the light. 3. Test Object The test bed is used for the verification of attitude control system of TET-1 (launch 2011). So the ACS of TET-1 is integrated at the test bed. The integrated system consists of four reaction wheels, the magnetic coil system, one magnetic field sensor, one gyroscope, the data processing unit of the star tracker and the sun sensor system, so all ACS components are presented, but without the redundant sensors. Additionally the board computer is integrated with the original software (ACS software and operating system).

Sun sensor system

Magntic coil system

Board computer

Magnetic field sensor

Gyroscope

Data processing unit

Reaction wheels

With the integrated system test and verification of the complete ACS is possible that includes test of algorithm and filter, test of communication between hardware and board computer as well as satellite and ground segment, test of physical reactions of the system like direction of rotation, pointing etc. The commanding and monitoring of the ACS is done by a identical ground segment like the flight system.

Extra room for test staff Command console

Test bed with EM ACS of TET-1

The extra room for the test personal should minimize the disturbances to the satellite model.

4. Simulation reference data During the verification of the TET-1 ACS the measurements are compared to simulated data representing the test scenario. Furthermore, a three-dimensional animation of the measured telemetry illustrates the satellite’s behavior and assists to identify possible errors. Especially when the test facility physical limitations, e.g. a 360° rotation is impossible about all axes is impossible, hinder the execution of test cases, simulation and visualization are a useful support.

An example for a simulation’s contribution to a successful test is shown in Figure 7. The executed test case concerned target pointing mode, in this case to the geographical North Pole. The plot includes the measured angular rate in comparison to simulated reference data. Particularly in the case of target pointing, the EM ACS motion on the air bearing table is difficult to validate, as the distance of satellite and Earth in contrast to their relative proportions make the results difficult to comprehend.

Figure 4: EM ACS of TET-1

Figure 7: Analyses of test data supported by simulation data

Figure 6: Visualized target pointing

Figure5: overview lab (clean room)

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References

1) Robuste und fehlertolerante Lageregelung des DLR-TET-1

Satelliten; Zizung Yoon, Thomas Terzibaschian, Christian Raschke DGLR 2008

2) Support of ACS development and test by dynamic simulation models, Karsten Großekatthöfer, Christian Raschke, 8th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, April 2011, to be published.

3) V. Schaus, K. Großekatthöfer, D. Lüdtke, and A. Gerndt, Collaborative Development of a Space System Simulation Model, COMETS 2011 - 2nd International Track on Collaborative Modeling and Simulation, Paris, France, 2011, submitted

4) ECSS-E-ST-10-04A Space Engineering - Space Environment, ECSS Secretariat, ESA-ESTEC, Requirements and Standards Division, Noordwijk, The Netherlands, November 2008

5) ECSS-M-ST-10C Space project management - Project planning and implementation, ESA ESTEC, Rev. 1, March 2009