WO2004071869A1

WO2004071869A1 - Device for controlling the attitude of a satellite by means of gyroscopic actuators

Info

Publication number: WO2004071869A1
Application number: PCT/FR2004/000264
Authority: WO
Inventors: Céline BEUGNON; Ange Defendini; Mehdi Ghezal; Julien Morand
Original assignee: Eads Astrium Sas
Priority date: 2003-02-07
Filing date: 2004-02-05
Publication date: 2004-08-26
Also published as: FR2850948A1; FR2850948B1; EP1590243A1; WO2004071869B1

Abstract

The device for controlling the attitude of a satellite by means of kinetic moment exchange comprises a cluster consisting of at least one gyroscopic actuator (10) having a gyro wheel (12) which turns about an axis (16), borne by a cardan drive (14), which can be oriented on the platform of the satellite by means of a stepper motor(22), and a system for controlling the attitude of the platform and the orbit inside an absolute reference mark, provided with sensors for sensing the orientation of the platform. The control system comprises a calculator (26) which is used to create a digital setpoint value for the orientation of the cardan drive (14) based on the differential between an attitude setpoint value of the platform and the current attitude. The actuator or the calculator (26) is provided with means for converting the digital oriental setpoint value to analog values of currents applied to the different phases of the stepper motor (22), whereby the resolution is such that the motor can be brought to and maintained in intermediate orientations between those corresponding to the mutual alignment of the poles of the stator (23) and the rotor (24).

Description

DEVICE FOR CONTROLLING THE ATTITUDE OF A SATELLITE BY GYROSCOPIC ACTUATORS

The present invention relates to devices for controlling the attitude of satellites by exchanging kinetic moments provided by a cluster of kinetic moment generating members, including at least one actuator with a rotary element, mounted on the platform of the satellite.

It relates more particularly to control devices of which at least one of the bodies for creating a kinetic moment is constituted by a gyrodyne or gyroscopic actuator, often designated by the acronym CMG (Control Moment Gyro).

The gyroscopic actuators comprise a wheel or spinning top driven in rotation, generally at constant speed, which rotates around an axis on a support, called cardan, orientable around at least one axis orthogonal to the axis of rotation of the top. Examples of gyrodynes are given in document EP-A-1002716 and FR 02 03569 to which reference may be made.

To allow the reorientation of a reference trihedron linked to the satellite platform in all attitudes, it is necessary to use at least three elements for creating kinetic moments. One can for example use a cluster of at least three gyrodynes, as indicated in the document EP 1002716 already mentioned, the application PCT / FR 02/02181 or the article by A. Defendini et al "lo cost CMG-based AOCS design "(ESA SP-425, Feb. 2000) to which we can refer, or two gyrodynes having cardan shafts whose axes of rotation are parallel and a reaction wheel whose axis has a fixed orientation relative to the body of the satellite and whose speed is adjustable. The invention is applicable to all devices which include at least one gyroscopic actuator capable of generating a torque of the platform of the satellite by angular displacements of the gimbal around its axis, so as to cause the router to precess.

Devices for piloting satellites are already known using gyroscopic actuators. Unlike the motor of a reaction wheel, that of a gyroscopic actuator is not controlled directly by the torque control provided by the computer for the satellite orbit attitude control system. In fact, the torque supplied by a gyroscopic actuator is the vector product of the angular momentum of the router by the speed of rotation of the gimbal around its axis. The use of a cluster of gyroscopic actuators on a satellite consequently requires a conversion of the torque commands to be applied to the satellites into speed controls of the universal joint motor.

The overall characteristics of the control device very much depend on the choice of the type of electric drive motor for the gimbal, allowing the axis of rotation of the router to be oriented. There are basically two types of motors, stepper and torque motors.

Torque motors use a speed or position control, in a closed local loop, to control the speed of rotation of the gimbal. The closed loop comprises an angular sensor, generally optical or inductive, making it possible to achieve a high resolution and a very good pointing position. The space-qualified torque motors have a maximum torque of a few Newton meters, sufficient for small actuators, providing a torque not exceeding approximately 50 Nm.

In return, the torque motors complicate the constitution of the actuator by the need for a sensor and control electronics, which must ensure the servo-control of the gimbal, the autopilot of the motor phases and the feedback of the measurement system. In particular on small satellites, often used in number, these drawbacks are penalizing in power, mass and size. The stepping motors currently used by gyroscopic actuators are designed to take only configurations in which the magnetic poles of the rotor align with the coils of the stator. The passage from one position to another is controlled by modification of the electrical states of the different phases of the motor, generally two-phase but which can be three-phase. Stepper motors have the advantage of simple control electronics, since a device for measuring the position or speed of the gimbal is not necessary; the piloting is done in open loop. On the other hand, the pointing accuracy of the router axis is limited to the angular pitch between poles of the motor, often 18 mrad on the actuators used in satellites. If one seeks to improve the definition by using a reducer, one comes up in particular with the problem of games during the reversal of the direction of rotation, of the weight and of holding during launching. Consequently, the use of stepping motors is not satisfactory on satellites which require a very stable attitude, typically less than 100 micro radians per second, which usually corresponds to 1 mrad of precision on the pointing of the axis of the router, for satellites weighing a few hundred kilograms. This is particularly the case for high-resolution earth observation missions using relatively small satellites in low or medium orbit.

The present invention aims to provide a device for controlling the attitude of a satellite by exchanging kinetic moments, using organs for creating a kinetic moment, including at least one gyroscopic actuator, making it possible to maintain the simplicity of control inherent in stepper motor, while achieving the high precision required for certain satellites, which until now required a torque motor.

To this end, the invention proposes in particular a device for controlling satellite attitude by exchanging kinetic moments, comprising a cluster of several kinetic moment generation members including at least one gyroscopic actuator having a wheel or spinning top rotating on a axis carried by a universal joint orientable on the satellite platform by a stepping motor (on the platform ...) around at least one axis orthogonal to the axis of rotation of the router and having a system for controlling the attitude of the platform and orbit in an absolute reference (known as SCAO or, in English, AOCS), provided with sensors for orienting the platform, characterized in that the control system d attitude and orbit comprises a computer provided for developing a digital orientation instruction for the gimbal of said gyroscopic actuator from the difference between an attitude instruction for the platform and the current attitude, and in that l actuator or calculated it tor is provided with means for converting the digital orientation setpoint into analog values of currents applied to the different phases of the stepping motor, with a resolution such that the motor can be brought and maintained in intermediate orientations between those which correspond to the mutual alignment of the poles of the stator and of the rotor.

In the case of a two-phase motor, the holding currents and I supplied to the two phases are:.

- I ₂ = I ₀ .sin (p.θ) where I ₀ is a motor setpoint current, p the number of motor poles (number of steps divided by A, for a two-phase motor) and θ the position setpoint issued of the SCAO calculator. For example, the local control of the currents I] and I ₂ can be achieved by using read only memory tables (PROM) giving the cosines and sines. The currents and I ₂ can be generated by linear gain amplifiers controlled by analog values supplied by conversion of values supplied by at least one table in cosine and sine memory. We can thus arrive at very improved fine pointing performances, limited only by the resolution of the calculations with local actuator control electronics which remains very simple compared to conventional piloting using a torque motor looped back on an angular encoder (the electronic can be analog and the local loop being eliminated) and a simplified mechanism (no angular encoder, stepper motor simpler than a torque motor, requirement of stiffness in relaxed torsion). These results are achieved with a SCAO interface which remains classic.

The above characteristics as well as others will appear better on reading the following description of particular embodiments of the invention, given by way of nonlimiting example. The description refers to the accompanying drawings, in which:

Figure 1 is a block diagram showing the implementation of the invention with a cluster of four gyroscopic actuators;

Figure 2 is a block diagram of the functions of an attitude control device usable with the configuration of Figure 1; and

Figure 3 is a diagram showing another possible configuration. The device shown diagrammatically in FIG. 1 comprises four gyroscopic actuators in a configuration which is that given in the document

EP-A- 1 002716, to which reference may be made (associated with a control system which is not that according to the invention) or in the article by A. Deferdini et al. “Low cost CMG-based AOCS design” (ESA SP-425, February 2000) already mentioned.

FIG. 1 shows a cluster of four identical gyroscopic actuators 10, each having a router 12 mounted on a gimbal 14 so as to be able to rotate on the gimbal around an axis 16. A motor not shown keeps the router in rotation, generally at constant speed. Each gimbal is mounted on the platform of the satellite, represented schematically at 25 in FIG. 3, so as to be able to rotate around an axis 18 orthogonal to the axis 16. The axes 18 have different orientations. In the case shown, they occupy the edges of a regular pyramid with vertex 20.

Each of the universal joints 14 is provided with a motor 22, only one of which is shown, making it possible to rotate it around the respective axis 18. The gimbal 14 is generally devoid of an angular sensor. Each motor 22 is of the stepping type and comprises a stator 23 and a rotor 24. In certain cases, the gimbal 14 will be provided with a low resolution angular encoder. But it will not be used for slaving and will only aim to give an approximate indication to initialize the device.

Maintaining the satellite in a set attitude in an inertial frame is ensured by an attitude control system. It comprises a member 26 for calculating and controlling the motors 22 which receives orientation (and possibly angular speed) instructions from the satellite, supplied by a transceiver 28 for connection with the ground and signals 30 coming from non-sensor represented, such as gyrometers, star sensors, terrestrial horizon sensors, etc. ... The member 26 controls the power circuits 32 supplying the motors 22.

The member 26 comprises the means and the peripherals necessary to constitute the control loop shown diagrammatically in FIG. 2, which can be viewed as comprising on the one hand a sub-assembly 40 involving the cluster of actuators 10 provided with electronics. analog control and secondly a sub-assembly 41 implementing the digital computer 26 of the SCAO. In fig. 2, the frames correspond to the different functions.

The SCAO receives a correction command as input. This command is produced, for example, in a subtractor 44 from an attitude setpoint θ and / or speed v of the platform, supplied for example by remote control from the ground, and from current values resulting from the dynamics. satellite, shown diagrammatically at 46. These current values of attitude and speed can be supplied by sensors 48 such as gyrometers or star sensors.

The functions performed by the sub-assembly 41 include attitude control 50 from the error signal supplied by the subtractor 44. The output signal from the attitude control module 50 is representative of the torque to be supplied. The local guide module 52 calculates, from the value of the torque, the speeds to be given to the universal joints 14 of the different wheels 12, possibly taking into account the selection instructions for the wheels 12 aimed at choosing the three most advantageous wheels 12. The speed signal supplied at 54 is integrated at 56 to provide, on the SCAO bus 58, new cardan reference positions.

The sub-assembly 40 comprises a module 60 which develops optimized controls of the currents to be applied to the different phases of the cardan motors. For this, this module 60 generally includes a PROM ROM 62 of digital-analog converters and linear amplifiers with controlled gain. The different currents supplied at 64 are applied to the motors 22 and, due to the dynamics of the gyroscopic actuators 10 shown diagrammatically by the frame 66, cause internal exchanges of angular momentum with the platform of the satellite. The dynamics of the satellite, shown diagrammatically by the frame 46, causes a reorientation of the satellite body, which is detected by the sensors 48.

The intensity control of the phase currents of the motors 22 is thus looped back to the speed measurement of the platform such as in FIG. 3 of the satellite (supplied by the sensors), which provides a directional pointing instruction. 14 by conservation of the angular momentum. The loopback is thus carried out through a simple update of the universal joint position setpoint 14, supplied by the SCAO on its bus 58, which does not add any complexity to the electronics of the cluster. actuators 10. Each gimbal 14 is controlled by simple processing of the instructions provided by the SCAO.

There is no loop intended to control the speed of the universal joints 14 to a set value supplied by the computer and consequently no need for speed or angular position sensors for the universal joints.

In the variant embodiment shown in FIG. 3, the cluster of internal angular momentum creation organs is of a type which is in particular usable on microsatellites. These members include a kinetic wheel or reaction wheel 64 with variable speed of rotation around the yaw axis z and two gyroscopic actuators 68 whose axes of rotation of the universal joints are parallel to each other and to the yaw axis z and of which the axes of rotation of the routers can thus be oriented in the plane defined by the roll x and pitch y axes.

The kinetic wheel 64 is placed so that its axis z is parallel to the direction of the axis of rotation of the gimbals of the gyroscopic actuators 68. In such a case, the control of the kinetic wheel 64 is carried out in a conventional manner and the invention is only implemented on actuators 68.

Claims

claims

1. Satellite attitude control device by exchange of angular momentum, comprising a cluster (40) of several angular moment generation members including at least one gyroscopic actuator (10) having a router (12) rotating on an axis (16) carried by a universal joint (14) orientable on the satellite platform by a stepping motor (22) around at least one axis (18) orthogonal to the axis (16) of rotation of the router (12) and having a system for controlling the attitude of the platform and orbit in an absolute reference frame, provided with sensors (48) for orienting the platform, characterized in that: the control system attitude and orbit comprises a computer (26) provided for developing a digital setpoint (58) for orienting the gimbal (14) of said gyroscopic actuator (10) from the difference (44) between a setpoint attitude (28) of the platform and of the current attitude (30) and the said actuator (10) or the computer (26) is provided with means (60) for converting the digital setpoint (58) for orientation into analog values of currents (64) applied to the different phases of the stepping motor (22), with a resolution such that the motor (22) can be brought and maintained in intermediate orientations between those which correspond to the mutual alignment of the poles of the stator (23) and the rotor (24).

2. Device according to claim 1, characterized in that, in the case of a two-phase motor (22), the holding currents Iχ and I ₂ supplied to the two phases are

= I ₀ .sin (p.θ) where I ₀ is a motor setpoint current, p the number of motor poles (22) and θ the position setpoint from the computer (26, 41) of the control system attitude and orbit.

3. Device according to claim 2, characterized in that the currents

Il and I ₂ are generated by linear amplifiers (60) with gain controlled by analog values provided by converting values provided by at least one ROM (62) cosine and sine table.

4. Device according to claim 1, 2 or 3, characterized in that the cluster comprises only gyroscopic actuators (10).