Spacecraft attitude or orientation system and method

Abstract: An attitude or orientation control system for spacecraft that provides for yaw axis error reduction and momentum dumping using a simple calculation involving only the momentum wheel speeds as input and which outputs two axis (roll and yaw) torque commands. The system utilizes the spacecraft 1) momentum wheels that store spacecraft momentum, 2) processing means for the calculation of control torques, and 3) torque actuators, e.g., magnetic torquers, that can provide a torque axis anywhere in the spacecraft roll/yaw plane, and involves three fundamental steps, 1) the input of the momentum wheel speed measurements, 2) the calculation of the required torques, using the wheel speed measurements, and 3) the outputting of the required torques in a form appropriate for controlling the available actuators.

Description: The present invention relates to spacecraft attitude control systems and more particularly to a system for controlling the attitude and orientation of spacecraft such as earth satellites. Many systems exist for automatically controlling the orientation of spacecraft to insure that a predefined travel path and desired attitude are precisely followed during flight. With the increasing use of earth satellites for research and communications the art in this regard has become quite sophisticated and complex and such practices as momentum biasing are used to achieve fine control and to reduce the magnitude of the required reorienting corrective forces. The operation of the components of these control systems generally involves complicated mathematical calculations, an example of which can be found in an article by K.L. LEBSOCK in the AIAA Journal of Guidance Controls, Vol.6, No.6, Nov.-Dec., 1983, pps 477 - 483, entitled "Magnetic Desaturation Of A Momentum Bias System." An example of a control system of the well-known Luenberger observer variety, which is the type used in such orbiting satellites as the INSAT, SUPERBIRD, INTELSAT VII, and GOES8, is disclosed in U.S. Pat. No. 4,521,855 issued in July 1985 to J.A. LEHNER and K.L. LEBSOCK. The systems used in the noted satellites have attitude error correction problems as they do not feature daily tracking of the yaw profiles so that there is no daily error information available, and the magnetic torquers used to make corrections are adversely effected by magnetic storms. Provisions to deal with these problems may involve additional complexity in the system. As the art develops it is desirable to simplify the component operations and the calculations required to achieve the desired attitude control and orientation manoeuvres, as well as minimizing the monitoring functions, the errors, and the amount of processor time used for corrections. Particularly regarding momentum bias spacecraft and satellites it is desirable to provide for efficient momentum unloading, to minimize yaw pointing errors, to maximize daily repeatability of yaw pointing profiles, and to minimize the computational and memory burden on the spacecraft processor in carrying out these functions. European Patent Application 0453096 discloses a method and apparatus for inclined orbit attitude control for momentum bias spacecraft. A pointing trajectory for an aim point is controlled with respect to one or more targets. A full state estimator, a feedback circuit, an integral controller, model-following logic, model-generation means and momentum thruster unload logic for long-term momentum management are used. An onboard computer capable of using orbit information available to the spacecraft and spacecraft hardware moves the aim point according to a predetermined tracking model. The tracking model allows the controller to remove short-term nutational and long-term orbit rate dynamical disturbances, beyond simple-order sinusoidal functions having a single frequency. Accordingly, the present invention seeks to provide an orientation system for spacecraft that offers better performance with a simpler design. The invention also seeks to provide an attitude control system for spacecraft that requires less computation and processor memory than the previously mentioned systems, while achieving improved yaw pointing and daily yaw repeatability. The invention may provide greater immunity to magnetic interference from storms and uses only momentum wheel speed measurements as input. According to one aspect of the invention there is provided an attitude control system for spacecraft having momentum wheels for spacecraft momentum storage, processing means for calculating control torques, and torque actuators that can provide a torque axis anywhere in the spacecraft roll/yaw plane, comprising means for inputting signals indicative of the momentum wheel speeds to said processor means, means in the processor means for calculating appropriate attitude torques, using the momentum wheel speeds indicative signals, comprising means, in accordance with a predetermined processor timing, for obtaining, from the momentum wheel speeds indicative signals, the spacecraft momentum wheel speed measurements and projecting said aircraft momentum wheel measurements onto an axis in the spacecraft roll/yaw plane, means for obtaining the spacecraft momentum on said roll/yaw plane axis, actuator command to control the torque actuators and means, responsive, for producing output signals indicative of the calculated appropriate attitude torques in a form for controlling the torque actuators, characterised by a plurality of simple filters with different time constants which are selected according to the particular spacecraft and spacecraft application, means for inputting the momentum on said roll/yaw plane axis momentum in the filters, means for using the output of the filters to obtain two weighted linear sums, the weights of which are selected according to the particular spacecraft and application, means for transforming each weighted linear sum into a form compatible with the spacecraft actuator command to control the torque actuators and means, responsive to the transformed weighted linear sums, for producing output signals indicative of the calculated appropriate attitude torques in a form for controlling the torque actuators. According to another aspect of the invention there is provided a method for controlling the attitude of a spacecraft having momentum wheels for spacecraft momentum storage, processing means for calculating control torques, and torque actuators that can provide a torque axis anywhere in the spacecraft roll/yaw plane, comprising the steps of inputting signals indicative of the momentum wheel speeds to the processing means, calculating appropriate torques for controlling the spacecraft attitude in the processing means using the momentum wheel speed indicative signals, and outputting signals from the processing means indicative of the calculated appropriate attitude torques in a form for controlling the torque actuators, and wherein the step of calculating in the processing means comprises the steps, in accordance with a predetermined processing timing, of obtaining from the momentum wheel speeds indicative signals the spacecraft momentum wheel speed measurements and projecting said momentum wheel speed measurements onto an axis in the spacecraft roll/yaw plane, and obtaining the spacecraft momentum on said roll/yaw plane axis, characterised by the steps of inputting the momentum on said spacecraft roll/yaw plane axis momentum in several simple filters with different time constants which are selected according to the particular spacecraft and application, using the output of the filters to obtain two weighted linear sums, the weights of which are selected according to the particular spacecraft and application, transforming each weighted linear sum into a form compatible with the spacecraft actuator command to control the torque actuators, and sending the actuator commands to the torque actuators for carrying out the controlling operation. The present invention embodies an attitude or orientation control system for spacecraft or satellites that provides for yaw axis error reduction and momentum dumping using a very simple calculation involving only the momentum wheel speed measurements as input and which outputs two axis (roll and yaw) torque commands. This control system essentially requires three elements, 1) momentum wheels for spacecraft momentum storage, 2) processing means for the calculation of control torques, and 3) torque actuators, which may be low authority actuators, e.g., magnetic torquers, that can provide a torque axis anywhere in the spacecraft roll/yaw plane. These elements and steps are known from the prior art. System operation involves three fundamental steps, 1) the input of measurements of the momentum wheel speeds, 2) the calculation of the required torques, using the wheel speed measurements, and 3) the outputting of the required torques in a form appropriate for controlling the available actuators. In order that the invention and its various other preferred features may be understood more easily, an embodiment thereof will now be described, by way of example only, with reference to the drawings, in which:- Figure 1 is a schematic illustration of the basic components of an attitude control system constructed in accordance with the invention, and Figures 2A and 2B are the upper and lower portions of a block diagram illustrating the incorporation of an attitude control system in accordance with the invention in a prior art system including a Luenberger observer. The present invention is directed to the attitude control systems in spacecraft, such as earth-orbiting satellites, and involves a roll/yaw plane momentum controller employing the satellite processor in computing the necessary actuator commands to operate the satellite attitude torquers. The invention requires only the satellite momentum wheel speed measurements as input and outputs two axis (roll and yaw) torque commands to affect attitude control. A preferred embodiment, which will be referred as the HZ controller (HZC), may include a software component in its implementation, and essentially utilizes three of the components already found on such satellites, namely, momentum wheels, a control processing means, and torque actuators. As seen in Figure 1, a spacecraft S has momentum wheels, 1 and 2, used for spacecraft momentum storage, with momentum vectors H M\W #1 and H M/W/ #2 that are used in combination with suitable torque actuators, or torquers 3 and 4 to control the spacecraft attitude. Torque actuator 3 is a z-axis torquer for producing roll torques and torquer 4 is an x-axis torquer for producing yaw torques. These torquer actuators may be low authority actuators, e.g., magnetic torquers, that can provide a torque axis anywhere in the spacecraft roll/yaw plane. The orthogonal axes +X, +Y, and +Z shown are respective Velocity, South, and Earth oriented axes. A system processor 5, which, among other functions, assists in controlling spacecraft attitude, operates to perform three fundamental activities, namely, to receive the measurements of the speeds of the momentum wheels 1 and 2 as input, to calculate the required torques using the wheel speed measurements, and to output the required torques in a form appropriate for controlling the magnetic torque actuators 3 and 4. In operation, the detailed steps, in accordance with a predetermined processor timing, each cycle, multiple cycles or multiplexed or regular intervals, for controlling a spacecraft momentum axis, i.e., the z-axis, may be as follows: 1) obtain the momentum wheel speed measurements of the spacecraft and project them onto the spacecraft z-axis; 2) obtain the spacecraft z-axis momentum; 3) input the z-axis momentum into several simple filters, e.g., first or second order filters, with specified time constants, the selection of which depends on the spacecraft and the spacecraft application; 4) take the output of the filters and obtain two weighted linear sums, the weights of which will again depend on the spacecraft and the spacecraft application; 5) transform each weighted linear sum into a compatible form for spacecraft actuator command, e.g., to control magnetic torquers; and 6) send the actuator commands to the actuators for carrying out the desired attitude adjustment operation. The time constants and weights, as noted, must be selected in dependence on the particular spacecraft and spacecraft application, and are obtained through an optimization that uses an objective function constructed for the particular target spacecraft and application. For example, a preferred working embodiment comprises: HZ = spacecraft z-axis momentum due to the momentum wheels as determined solely from the momentum wheel tachometers; HZ a = HZ first order filtered with a long term, i.e., multiple hours, e.g., 48 hour, time constant; HZ b = HZ first order filtered with an intermediate term, i.e., tens of minutes, e.g., 24 minute, time constant; HZ c = HZ first order filtered with a short term, i.e, minutes or fractions thereof, e.g., 6 minute, time constant; X-axis magnetic torquer coil amps = g 1 x HZ a + g 2 x HZ b + g 3 x HZ c ; where g 1 + g 2 + g 3 = 1; and Z-axis magnetic torquer coil amps = g 4 x HZ a + g 5 x HZ b + g 6 x HZ c ; where g 4 + g 5 + g 6 = 1. A particular implementation, in processor hardware or software or both, of the roll/yaw plane momentum controller (HZC) in an algorithm given as a FORTRAN like code fragment is as follows. This code or its equivalent is to be executed at 64 second intervals: h(1) = .997925 * h(1) + .002075 * hz ! 513.520 minute time constant h(2) = .962838 * h(2) + .037162 * hz ! 28.166 minute time constant h(3) = .854345 * h(3) + .145655 * hz ! 6.776 minute time constant xamp = 8.52 * h(1) - 7.97 * h(2) - 0.55 * h(3) zamp = -2.82 * h(1) - 8.48 * h(2) - 11.30 * h(3) where all of the variables are single precision; and hz = the spacecraft body z-axis component of the momentum due to the momentum wheels in newton-meter-seconds, +Z points toward the Earth; xamp = the commanded current in amps, prior to being limited, to the East face magnetic torquer, which produces z-axis torques; zamp = the commanded current in amps, prior to being limited, to the anti-earth face magnetic torquer, which produces x-axis torques. The recommended limits for this controller are a maximum of 0.4 amp and a minimum of -0.4 amp for both axes. By way of setting a context for the use of the invention in an existing system, although other uses are anticipated, the exemplary HZC of the invention may be incorporated in satellites presently using Luenberger observer type systems in the manner as shown in Figure 2. In the prior art system a sensor bank 10, including a No. 1 pitch wheel tachometer 11, a No. 2 pitch wheel tachometer 12, and No. 3 yaw wheel tachometer 13, inputs wheel speed signals to a Momentum Measurement Distribution Matrix device 14 which is part of the satellite micro-processor 30. These signals are also input to the control system for the Trim Tab positioner shown in the upper portion of the Figure. The lower portion of Figure 2 shows the micro-processor portion which acts as the control system for processing inputs from the Roll Horizon and Pitch Horizon sensors and producing outputs to the Tachometers for the Pitch Wheel and Yaw Wheel motor drives. Returning to device 14, upon receiving the three tachometer wheel speed measurement input signals, it outputs an appropriate first signal to a Pitch Axis Momentum Desaturation Logic device 15, and a second signal to a Yaw Axis Momentum Desaturation Logic device 16, and both a Luenberger Observer 17, and a Yaw Dipole Control Gains device 18 connected to the Observer 17. Devices 15 and 16 provide control signals to the satellite's Pitch Thruster 19 and Yaw Thruster 20. The Luenberger Observer 17 provides signals indicative of estimates of Yaw angle, Roll torque, and Yaw torque to the Gains device 18 which in the prior art, in addition to feedback signals, sends control signals to the Yaw Magnetic Coil Drive 21 and Roll Magnetic Coil Drive 22. However, in accordance with the invention, the HZC 23 has its outputs connected to the Yaw and Roll Magnetic Coil Drives, 21 and 22, with its input connected to receive the second wheel speed indicative signal from the Momentum Measurement Distribution Matrix device 14, which is a measurement of the spacecraft z-axis momentum due to the momentum wheels. Consequently, the HZC 23 is positioned to output two axis (roll and yaw) torque commands using momentum wheel speed indicative signals as input and thus controls the yaw attitude of a spacecraft by providing for torques and momentum dumping using a very simple calculation involving only the momentum wheel speed measurements. No earth sensor measurements are required. It will accordingly be seen that the foregoing descriptions illustrate an improved system for satellite attitude control that may be incorporated in satellites presently using Luenberger observer type systems and other similar modern type systems to achieve better performance with a simpler design.

Claims: An attitude control system for spacecraft having momentum wheels for spacecraft momentum storage, processing means (30) for calculating control torques, and torque actuators that can provide a torque axis anywhere in the spacecraft roll/yaw plane, comprising means for inputting signals indicative of the momentum wheel speeds to said processor means, means in the processor means for calculating appropriate attitude torques, using the momentum wheel speeds indicative signals, comprising means, in accordance with a predetermined processor timing, for obtaining, from the momentum wheel speeds indicative signals, the spacecraft momentum wheel speed measurements and projecting said spacecraft momentum wheel measurements onto an axis in the spacecraft roll/yaw plane, means for obtaining the spacecraft momentum on said roll/yaw plane axis, actuator command to control the torque actuators and means, responsive, for producing output signals indicative of the calculated appropriate attitude torques in a form for controlling the torque actuators, characterised by a plurality of simple filters with different time constants which are selected according to the particular spacecraft and spacecraft application, means for inputting the momentum on said roll/yaw plane axis momentum in the filters, means for using the output of the filters to obtain two weighted linear sums, the weights of which are selected according to the particular spacecraft and application, means for transforming each weighted linear sum into a form compatible with the spacecraft actuator command to control the torque actuators and means, responsive to the transformed weighted linear sums, for producing output signals indicative of the calculated appropriate attitude torques in a form for controlling the torque actuators. A system as claimed in claim 1, wherein said roll/yaw plane axis is the Z-axis, +Z oriented from the spacecraft to the center of the earth. A system as claimed in claim 1, wherein the filters are first order filters. A system as claimed in any one of the preceding claims, wherein the torque actuators comprise an X-axis magnetic torquer coil and a Z-axis magnetic torquer coil, +X oriented in the direction of the spacecraft velocity and +Z oriented from the spaceship to the center of the earth, the means for inputting signals indicative of the momentum wheel speeds comprises momentum wheel tachometers, and wherein said roll/yaw plane axis is the Z-axis, said filters are first order filters, and said time constants and weighted linear sums are obtained through an optimisation that uses an objective function constructed for the particular spacecraft and spacecraft application, said constructed objective function comprising the values: HZ = spacecraft Z-axis momentum due to the momentum wheels as determined solely from the momentum wheel tachometers; HZ a = HZ first order filtered with a long term time constant; HZ b = HZ first order filtered with an intermediate term time constant; HZ c = HZ first order filtered with a short term time constant; and wherein, X-axis magnetic torquer coil amps, which correspond to the value produced by one of said two weighted linear sums, = g 1 x HZ a + g 2 x HZ b + g 3 x HZ c ; where g 1 + g 2 + g 3 = 1; and Z-axis magnetic torquer coil amps, which correspond to the value produced by the other of said two weighted linear sums, = g 4 x HZ a + g 5 x HZ b + g 6 x HZ c ; where g 4 + g 5 + g 6 = 1. A system as claimed in any one of the preceding claims, wherein the torque actuators comprise an East face magnetic torquer and an anti-earth face magnetic torquer and wherein the means comprising the means for calculating appropriate attitude torques comprise means for executing the following code at 64 second intervals; HZ a = .997925 * HZ a + .002075 * HZ ! 513.520 minute time constant HZ b = .962838 * HZ b + .037162 * HZ ! 28.166 minute time constant HZ c = .854345 * HZ c + .145655 * HZ ! 6.776 minute time constant Xamp = 8.52 * HZ a - 7.97 * HZ b - 0.55 * HZ c Zamp =-2.82 * HZ a - 8.48 * HZ b - 11.30 *HZ c where all of the variables are single precision, and HZ = the spacecraft body Z-axis component of the momentum due to the momentum wheels in newton-meter-seconds, +Z points toward the Earth; Xamp = the commanded current in amps, prior to being limited, to the East face magnetic torquer, which produces Z-axis torques; Zamp = the commanded current in amps, prior to being limited, to the anti-earth face magnetic torquer, which produces X-axis torques; and the current is limited to a maximum of 0.4 amps for both axes. A system as claimed in claim 4, wherein the long term time constant comprises multiple hours. A system as claimed in claim 4 or 6, wherein the intermediate term time constant comprises tens of minutes. A system as claimed in claim 4, 6 or 7, wherein the short term time constant comprises minutes or fractions thereof. A system as claimed in any one of claims 6 to 8, wherein the long term time constant is 48 hours. A system as claimed in any one of claims 6 to 9, wherein the intermediate term time constant is 24 minutes. A system as claimed in any one of claims 6 to 10, wherein the short term time constant is 6 minutes. A method for controlling the attitude of a spacecraft having momentum wheels for spacecraft momentum storage, processing means for calculating control torques, and torque actuators that can provide a torque axis anywhere in the spacecraft roll/yaw plane, comprising the steps of inputting signals indicative of the momentum wheel speeds to the processing means, calculating appropriate torques for controlling the spacecraft attitude in the processing means using the momentum wheel speed indicative signals, and outputting signals from the processing means indicative of the calculated appropriate attitude torques in a form for controlling the torque actuators, and wherein the step of calculating in the processing means comprises the steps, in accordance with a predetermined processing timing, of obtaining from the momentum wheel speeds indicative signals the spacecraft momentum wheel speed measurements and projecting said momentum wheel speed measurements onto an axis in the spacecraft roll/yaw plane, and obtaining the spacecraft momentum on said roll/yaw plane axis, characterised by the steps of inputting the momentum on said spacecraft roll/yaw plane axis momentum in several simple filters with different time constants which are selected according to the particular spacecraft and application, using the output of the filters to obtain two weighted linear sums, the weights of which are selected according to the particular spacecraft and application, transforming each weighted linear sum into a form compatible with the spacecraft actuator command to control the torque actuators, and sending the actuator commands to the torque actuators for carrying out the controlling operation. A method as claimed in claim 12, wherein the roll/yaw plane axis is the Z-axis, +Z oriented from the spacecraft to the center of the earth. A method as claimed in claim 12 or 13, wherein the filters are first order filters. A method as claimed in any one of claims 12 to 14, wherein the torque actuators comprise an X-axis magnetic torquer coil and a Z-axis magnetic torquer coil, +X oriented in the direction of the spacecraft velocity and +Z oriented from the spacecraft to the center of the earth, said signals indicative of the momentum wheel speeds are input by momentum wheel tachometers, and the time constants and weighted linear sums are obtained through an optimisation that uses an objective function constructed for the particular spacecraft and spacecraft application, said constructed objective function comprising the values: HZ = spacecraft Z-axis momentum due to the momentum wheels as determined solely from the momentum wheel tachometers; HZ a = HZ first order filtered with a long term time constant; HZ b = HZ first order filtered with an intermediate term time constant: HZ c = HZ first order filtered with a short term time constant; X-axis magnetic torquer coil amps which correspond to the value produced by one of said two weighted linear sums, = g 1 x HZ a + g 2 x HZ b + g 3 x HZ c ; where g 1 + g 2 + g 3 = 1; and Z-axis magnetic torquer coil amps which correspond to the value produced by the other of said two weighted linear sums, = g 4 x HZ a + g 5 x HZ b + g 6 x HZ c ; where g 4 + g 5 + g 6 = 1. A method as claimed in any one of claims 12 to 15, wherein the torque actuators comprise an East face magnetic torquer and an anti-earth face magnetic torquer and wherein the steps comprising the calculating step comprise executing the following code at 64 second intervals: HZ a = .997925 * HZ a + .002075 * HZ ! 513.520 minute time constant, HZ b = .962838 * HZ b + .037162 * HZ ! 28.166 minute time constant, HZ c = .854345 * HZ c + .145655 * HZ ! 6.776 minute time constant, Xamp = 8.52 * HZ a - 7.97 * HZ b - 0.55 * HZ c Zamp = -2.82 * HZ a - 8.48 * HZ b - 11.30 * HZ c where all of the variables are single precision, and HZ = the spacecraft body Z-axis component of the momentum due to the momentum wheels in newton-meter-seconds, +Z points toward the Earth; Xamp = the commanded current in amps, prior to being limited, to the East face magnetic torquer, which produces Z-axis torques; Zamp = the commanded current in amps, prior to being limited, to the anti-earth face magnetic torquer, which produces X-axis torques; and the current is limited to a maximum of 0.4 amps for both axes.

Inventors: CHAN KAM KIN, GAMBLE DONALD W, PRICE XENOPHON H, CHAN, KAM KIN, GAMBLE, DONALD W., PRICE, XENOPHON H.
Applicants: LORAL SPACE SYSTEMS INC, SPACE SYSTEMS / LORAL INC.
Application year: 1996
Main classification: G05D1
IPC codes: B64G1, G05D1