Attitude control device for a satellite and method for attitude control of a satellite

Abstract

Attitude control device (1) for a satellite, comprising: at least three electric motors (2), wherein the at least three electric motors (2) are arranged such that a torque (8-1, 8-2, 8-3) with any orientation of an associated torque vector can be generated, and a controller (3), wherein the controller (3) is configured to control the at least three electric motors (2) on the basis of a torque control, and wherein the torque control is configured to operate the at least three electric motors (2) outside of a rest state only when a climbing torque (11) and a braking torque (12) are required to perform an agile attitude change maneuver (7).

Description

The invention relates to a position control device for a satellite and to a method for positioning a satellite.Due to the further development in the field of remote sensing systems, in particular with regard to higher-resolution and smaller capture ranges, current mission scenarios provide for a capture of multiple recordings within a single flight of a satellite over an area. In this case, in particular, widening of the swath and / or capturing of a plurality of targets are planned. This results in requirements that satellites must be rotated by up to 30 degrees within 10 seconds, for example.For such agile attitude change maneuvers (rotation rates >1 degrees per second), torques are needed that are greater than the torques necessary, for example, for the compensation of an orbit rotation rate (e.g., a permanent nadir orientation of the satellite), for the compensation of the trajectory movement of the earth around the sun or for rotations between different main orientations (e.g., in order to align solar cells of the satellite back in the direction of the sun). For these normal attitude change maneuvers at <1 degree per second rotation rates, reaction wheels, engines, or magnetic gate cross are typically used.Only two solutions are currently available for performing the agile attitude change maneuvers: Control Moment Gyroscope (CMG) or engines.A captive gyro is a gyro which rapidly rotates in a gimbal, the frame axes of which are not freely suspended, but in which one axis can be forced in a particular direction. Such a CMG therefore stores kinetic energy and angular momentum in a movable flywheel mass. When the non-freely movable axis is forced in one direction, a torque is thereby generated and the satellite is rotated due to the angular momentum conservation. In order to compensate for internal disturbances of the CMG, an internal regulator must continuously run along and power losses (e.g. due to friction) must be continuously compensated. The angular momentum always stored can also be a source of vibrations and can trigger control oscillations by a nonlinear coupling with desired satellite movements. Both effects can lead to a "movement blurring" of the acquired sensor data in the case of a high-resolution optical detection.Engines produce torque by aligning the thrust vector with a distance from the center of gravity of the satellite. However, when using engines, the obvious disadvantage is that the propellant used to generate the thrust can only be stored on board the satellite in a limited amount. Due to the mass loss accompanying the generation of the thrust, a constant change of the center-of-gravity position and the moments of inertia is added. This can also affect an orientation of the satellite.The use of high-torque wheels in satellites is known from P. Acquatella B., Fast Slew Manufacturers for High-Torque-Wheels BIROS Satellite, Transactions of the Japan Society for Aeronautical and Space Sciences, 61 (2), pp. 79-86, 2018.US 2012 / 0 199 697 A1 discloses a position control system for use with small satellites. According to various variations, the system enables fast retargeting (e.g., high slew rates) and full three-axis position control of small satellites using a compact actuation system. In certain variations, the compact actuation system includes a plurality of singly gimbaled steering torque gyroscopes (SGCMG) arranged in a pyramidal configuration and in a small satellite.From US 2016 / 0 355 279 A1 a method for fast panning and dropping of small satellites is known, based on four gyroscopes with a single degree of freedom control moment with flywheels (or reaction wheels) with variable speed in a pyramid configuration combined with a time-optimal control of the path and end point restriction. Path and endpoint constrained time-optimized control may be augmented by impulse management without the use of additional actuators.US 2011 / 0 006 162 A1 discloses an actuator device for changing the attitude of a spacecraft. This comprises a reversible conversion chain from electrical energy into mechanical rotational energy of a flywheel. The electrical energy is stored in a capacitive element, which may be a supercapacitor. The actuator device also comprises an electric power converter which is connected on the one hand to the capacitive element and on the other hand to be connected to the spacecraft power bus. The converter makes it possible to compensate for losses in order to keep a total energy of the actuator device constant.The object of the invention is to provide a position control device for a satellite and a method for positioning a satellite, in which the execution of agile position change maneuvers can be carried out in an improved manner.The technical object is achieved according to the invention by a position control device having the features of claim 1 and a method having the features of claim 10. Advantageous embodiments of the invention are evident from the dependent claims.In particular, a position control device for a satellite is provided, comprising at least three electric motors, wherein the at least three electric motors are arranged such that a torque can be generated with any orientation of an associated torque vector. Furthermore, the position control device comprises a controller, wherein the controller is designed to actuate the at least three electric motors on the basis of a torque control, and wherein the torque control is designed to operate the at least three electric motors outside a rest state only if an increase torque and a brake torque are required for carrying out an agile position change maneuver.Furthermore, a method for positioning a satellite is provided, wherein the satellite comprises a positioning control device having at least three electric motors and a controller, wherein the at least three electric motors are arranged such that a torque can be generated with any orientation of an associated torque vector, and wherein the controller is designed to actuate the at least three electric motors on the basis of a torque control, and wherein the torque control is designed to operate the at least three electric motors outside a rest state only if an increase torque and a brake torque are required for carrying out an agile positioning change maneuver, comprising the following steps: receiving a setpoint orientation of the satellite, calculating an increase torque and a brake torque on the basis of a comparison between the received setpoint orientation and an actual orientation of the satellite by means of the controller, generating the calculated increase torque and the calculated brake torque by driving the at least three electric motors by the controller.A basic idea of the invention is to actuate at least three electric motors by means of a control by way of torque control for carrying out an agile position change maneuver. In this case, the electric motors are only excited from a rest state, i.e. a rest position of the rotor, when a torque in the form of an increase and brake torque is required for carrying out the agile position change maneuver. As a result, the electric motors can remain in the idle state most of the time. If an agile position change maneuver is to be carried out, a setpoint orientation is transmitted to the controller, for example from a central controller of the satellite. The controller calculates a required increase and brake torque from the setpoint orientation and an actual orientation, which is likewise transmitted from the central controller of the satellite to the controller, for example, on the basis of a comparison between the received setpoint orientation and the received actual orientation of the satellite. An increase and a brake torque are intended to be torques generated in particular over a specific time interval. The increase torque can be, in particular, a torque increasing over the time interval. The braking torque can be, in particular, a torque dropping over the time interval. The controller then controls the at least three electric motors in such a way that the calculated increase torque and the calculated brake torque are generated by them and the satellite is thereby brought from its actual orientation into the desired orientation. It can be provided here that a current actual orientation of the satellite is continuously checked during the execution of the change-of-position maneuver.The advantage of the invention is that vibrations as occur in the CMGs due to the continuous operation of the gyroscope can be prevented, since the at least three electric motors remain at rest for the most time. Moreover, energy can be saved since the at least three electric motors are not permanently in operation and have to be kept in motion.The at least three electric motors are designed in such a way that they are able to provide sufficiently large torques for carrying out agile attitude change maneuvers. This is done by the electric motors being tuned accordingly with regard to their moment of inertia, their torque or their power to a mass or moment of inertia of the satellite in which the position control device is to be arranged. In particular, the controller is designed in such a way that it can be flexibly adapted to different electric motors or properties of the satellite, for example, by setting corresponding control parameters and storing them in a memory. In this way, the controller can be used as part of a modular concept in which controllers which are manufactured in the same way can be adapted to a respective application scenario in a flexible manner and without great effort, as required.In one embodiment, it is provided that the at least three electric motors each have a flywheel, wherein the flywheels are each mechanically coupled to a rotor of the electric motors. As a result, a moment of inertia of the at least three electric motors can be changed, in particular increased, in each case. This can be used to design the at least three electric motors to be flexible for a corresponding application scenario, for example to take account of a higher mass or a higher moment of inertia of a satellite.In one embodiment, it is provided that the at least three electric motors can generate at least a torque of 0.09 Newton-meters per kilogram of self-mass, preferably of at least 0.45 Newton-meters per kilogram of self-mass. These conditions are advantageous in particular since mass and thereby starting costs can be saved in the satellites despite the high available torques.In a further embodiment, it is provided that the position control device has at least one further electric motor. The resulting at least four electric motors are arranged in a tetrahedron arrangement, so that even in the event of a failure of one of the electric motors with the respectively three remaining ones of the at least four electric motors, a torque can be generated with any orientation of an associated torque vector. The advantage is that by means of the tetrahedron arrangement, redundancy can be created by only one further electric motor, which makes it possible to continue rotating the satellite in any desired orientation even in the event of failure of one of the electric motors. Of course, the position control device can also comprise more than four electric motors in order to further increase the reliability of the satellite.In a further embodiment, it is provided that the position control device additionally comprises at least one constrained gyro (control torque gyroscope), wherein the controller is furthermore designed in such a way that it additionally actuates the at least one constrained gyro. As a result, the control can be optimized for a joint operation of the constrained gyro and the at least three electric motors for the agile attitude change maneuvers.In another embodiment, it is provided that the controller is furthermore designed in such a way that the at least three electric motors are also controlled for carrying out a normal position change maneuver of the satellite. As a result, the CMGs can be completely dispensed with, for example. This makes it possible to further increase energy efficiency and completely eliminate vibrations caused by the CMGs. In order to carry out the normal change-of-position maneuvers, the at least three electric motors are always brought from a rest state to a calculated torque by the controller. However, it can also be provided that the device is used in addition to the CMGs. This can provide redundancy, since normal position change maneuvers of the satellite can be carried out both by means of the CMGs and by means of the device. Reliability can thereby be increased.In a further embodiment, it is provided that the position control device is designed to at least partially recuperate an energy converted for applying the torque. This has two advantages: firstly, an energy efficiency can be further increased as a result, since not all of the energy required for applying the torque is converted into heat after completion of the position change maneuver, but is recovered, and secondly, heating of the satellite resulting from dissipation of the energy can be reduced, so that complicated methods for cooling can be avoided.In a further embodiment, it is provided that the at least three electric motors have a locking mechanism, wherein the locking mechanism is designed such that the at least three electric motors are capable of irreversibly releasing this locking mechanism independently by applying a corresponding torque. Such a locking can consist, for example, in a rotor of the electric motor being fixed to the stator by means of a connecting element. Such a connecting element can be designed, for example, in the form of a material bridge with a material bond. Since the rotor of the electric motor is fixed via the connecting member, the bearings of the rotor of the electric motor can be saved at the start of the satellite. After the launch of the rocket and the suspension of the satellite in space, the locking is irreversibly released by applying a corresponding torque by means of the electric motors, for example by irreversibly destroying the connecting element. The rotor of the electric motor can then move freely and the locking is released. The advantage is that the bearings of the rotor of the electric motor can be made less robust and therefore smaller and more material-saving, since they are no longer subjected to mechanical stress or at least are no longer subjected to such stress at the start of the satellite and an acceleration occurring in this case.In one specific embodiment, it is provided that the torque control is carried out on the basis of a torque characteristic curve. The torque characteristic curve indicates how a rotational speed is linked to a torque. The torque characteristic curve belonging to the at least three electric motors can be stored, for example, in a memory of the controller. By means of such a torque characteristic curve, a torque generated by this electric motor can be deduced via a detected rotational speed of the electric motor. The detection of the rotational speed can be effected, for example, by means of an encoder disk arranged on the rotor of the electric motor. The controller can then control or regulate the torques of the electric motors on the basis of the detected rotational speeds and the torque characteristic curve.The invention is explained in more detail below with reference to preferred exemplary embodiments with reference to the figures. The following are shown here: FIG. 1 is a schematic illustration of an embodiment of the position control device for a satellite; FIG. 2 shows a schematic representation of the torques over time during a change-of-position maneuver; FIG. 3 shows a schematic flow diagram of an embodiment of the method for positioning a satellite.In FIG. 1, a schematic representation of an embodiment of the position control device 1 for a satellite is shown. The position control device 1 comprises three electric motors 2 and a controller 3. The arrangement of the three electric motors 2 is to be understood only schematically in the illustration, in the application scenario in the satellite, the three electric motors 2 are arranged with respect to their axis of rotation along the three spatial directions, such that a torque can be generated in each case with an associated torque vector along these spatial directions. The controller 3 controls the three electric motors 2 based on torque control. The torque control is configured to operate the three electric motors 2 outside a rest state only if an increase and brake torque is required for carrying out an agile position change maneuver.If an agile position change maneuver is to be carried out, a setpoint orientation 4 of the satellite is transmitted to the controller 3, for example from a central controller 5 of the satellite. Furthermore, an actual orientation 6 of the satellite is transmitted to the controller 3, for example likewise by the central controller 5 of the satellite. The controller 3 calculates a torque in the form of an increase and brake torque on the basis of a comparison between the received desired orientation 4 and the received actual orientation 6. Subsequently, the calculated increase and brake torque is generated by driving the three electric motors 2.It can be provided that the three electric motors 2 each have a flywheel 13, wherein the flywheels 13 are each mechanically coupled to a rotor of the electric motors 2. This achieves the effect that a moment of inertia of the movable part of the motors can be changed or set depending on an application scenario.It can furthermore be provided that the position control device 1 comprises a further electric motor 2, wherein these four electric motors 2 are arranged in a tetrahedron arrangement, so that even in the event of failure of one of the electric motors 2 with the three remaining electric motors 2, a torque can be generated with any desired orientation of an associated torque vector.It can furthermore be provided that the at least three electric motors 2 have a locking mechanism 14. The locking device 14 is designed in such a way that the three electric motors 2 are able to irreversibly release this locking device 14 independently by applying a corresponding torque.FIG. 2 shows a schematic time profile of a position change maneuver 7. On the Y axis, a torque 8-1, 8-2, 8-3 applied by one of the electric motors is shown in each case. In this simple example, it is assumed that, in order to carry out the position change maneuver 7, only a torque 8- 1, 8- 2, 8- 3 has to be applied, the torque vector of which coincides with a rotational axis of a rotor of one of the electric motors, in this case the electric motor, the torque 8- 3 of which is illustrated in the lower graph of FIG. 2. The position change maneuver 7 begins at an initial time point 9 and ends at an end time point 10. In order to carry out the position change maneuver 7, the electric motor illustrated in the lower part is actuated in such a way that an increase torque 11 and subsequently a braking torque 12 are generated with this electric motor. By the torque 8-3 generated in the form of the increase torque 11 and the brake torque 12, an orientation of the satellite is changed.If, during a position change maneuver 7, the torque vector of the necessary torque 8- 1, 8- 2, 8- 3 does not coincide with one of the rotational axes of the rotors of the electric motors, torques must also be applied in each case by the other electric motors in order to achieve a corresponding orientation of the torque vector of the torque to be applied.Referring now to FIG. 3, a schematic flow diagram of one embodiment of the method for positioning a satellite is shown. The satellite has a position control device with at least three electric motors and a controller. The at least three electric motors are arranged in such a way that a torque can be generated with any desired orientation of an associated torque vector. The control of the position control device is designed such that the at least three electric motors are controlled on the basis of torque control. The torque control is configured to operate the at least three electric motors outside a rest state only if an increase and brake torque is required for carrying out an agile position change maneuver.In a first method step 100, the controller receives a desired orientation of the satellite. The desired orientation can be provided, for example, by a central controller of the satellite. In a method step 101, the controller receives an actual orientation of the satellite. This actual orientation of the satellite can likewise be provided by the central controller of the satellite and transmitted to the controller, for exampleBy means of the control, in a method step 102, an increase and brake torque is calculated on the basis of a comparison between the received desired orientation and the received actual orientation of the satellite.In a method step 103, the calculated increase and brake torque is generated by actuating the at least three electric motors. For this purpose, the controller controls the at least three electric motors by torque regulation, so that the torques generated by the at least three electric motors in each case result in the calculated increase and brake torque by interaction and the actual orientation of the satellite is changed to the desired orientation in accordance with the position change maneuver.List of reference characters1 Position control device 2 Electric motor 3 Control 4 Setpoint orientation 5 Central control 6 Actual orientation 7 Agile position change maneuver 8-1 Torque 8-2 Torque 8-3 Torque 9 Starting time 10 Ending time 11 Increase torque 12 Braking torque 13 Flywheel 14 Locking mechanism 100-103 Method steps

Claims

Position control device (1) for a satellite, comprising: at least three electric motors (2), wherein the at least three electric motors (2) are arranged such that a torque (8-1, 8-2, 8-3) can be generated with any orientation of an associated torque vector, and a controller (3), wherein the controller (3) is designed to actuate the at least three electric motors (2) on the basis of a torque control, and wherein the torque control is designed to operate the at least three electric motors (2) outside a rest state only if an increase torque (11) and a brake torque (12) are required for carrying out an agile position change maneuver (7).Position regulating device (1) according to Claim 1, characterized in that the at least three electric motors (2) each have a flywheel (13), wherein the flywheels (13) are each mechanically coupled to a rotor of the electric motors (2).Position control device (1) according to Claim 1 or 2, characterized in that the at least three electric motors (2) can generate at least one torque (8-1, 8-2, 8-3) of 0.09 Newton-meters per kilogram of intrinsic mass, preferably of at least 0.45 Newton-meters per kilogram of intrinsic mass.Position regulating device (1) according to one of the preceding claims, characterized byat least one fourth electric motor (2), wherein the at least four electric motors (2) are arranged in a tetrahedron arrangement, such that even in the event of a failure of one of the electric motors (2) with the respectively three remaining ones of the at least four electric motors (2), a torque (8-1, 8-2, 8-3) can be generated with any orientation of an associated torque vector.Position control device (1) according to one of the preceding claims, characterized in that the position control device (1) additionally comprises at least one constrained gyro (control moment gyroscope), wherein the controller (3) is furthermore designed in such a way as to additionally actuate the at least one constrained gyro.Position regulating device (1) according to one of the preceding claims, characterized in that the controller (3) is furthermore designed in such a way that the at least three electric motors (2) are also controlled for carrying out a normal position change maneuver of the satellite.Position control device (1) according to one of the preceding claims, characterized in that the position control device (1) is designed in such a way as to at least partially recuperate an energy converted for applying the torque (8-1, 8-2, 8-3).Position regulating device (1) according to one of the preceding claims, characterized in that the at least three electric motors (2) have a locking mechanism (14), wherein the locking mechanism (14) is designed in such a way that the at least three electric motors (2) are capable of irreversibly releasing this locking mechanism (14) independently by applying a corresponding torque (8-1, 8-2, 8-3).Position control device (1) according to one of the preceding claims, characterized in that the torque control is carried out on the basis of a torque characteristic curve.Method for regulating the position of a satellite, wherein the satellite comprises a position regulating device (1) having at least three electric motors (2) and a controller (3), wherein the at least three electric motors (2) are arranged such that a torque (8-1, 8-2, 8-3) can be generated with any orientation of an associated torque vector, and wherein the controller (3) is designed such that the at least three electric motors (2) are controlled on the basis of a torque control, and wherein the torque control is designed to operate the at least three electric motors (2) outside a rest state only if an increase torque (11) and a brake torque (12) are required for carrying out an agile position change maneuver (7), comprising the following steps: - receiving a setpoint orientation (4) of the satellite, calculating a rise torque (11) and brake torque (12) on the basis of a comparison between the received desired orientation (4) and an actual orientation (6) of the satellite by means of the controller (3), generating the calculated rise torque (11) and the calculated brake torque (12) by actuating the at least three electric motors (2) by means of the controller (3).

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