All Sky Pointing Attitude Control System

The present invention relates to space vehicle attitude control systems for precisely pointing the sensitive axis of scientific instruments at any arbitrary target in space in general and, in particular, to an attitude control system employing strapped-down gyroscopes, a sun and star tracker, and a method and error correction apparatus for compensating for gyro drift, integrator drift, scale factor error, gyro input axis triad non-orthogonality and gyro input and axis-optical axis misalignment.

A number of systems have been employed in the past to perform this function. They are of three basic types using either star-trackers to identify and lock-on to known stars at or near a target, gyroscopes with (or without) accelerometers on a stabilized platform to provide an inertial reference from which the target may be located, or a combination of gyroscopes and star-trackers wherein the gyros stabilize the vehicle and provide coarse pointing guidance while the star-tracker holds the vehicle on target. A programmable (general purpose) digital computer is usually required for the first two types of these systems.

The prior known systems, however, suffer from either being overly complex (hence, expensive and unreliable) or lacking the accuracy and precision demanded by experimenters, or both.

In view of the foregoing, a principal object of the present invention is an attitude control system employing two-axis rate gyroscopes, a sun-tracker, a star-tracker, control electronics and a small, special purpose digital computer which is characterized by simplicity of mechanization, higher reliability, lower cost, improved performance and versatility.

The gyroscopes are fixed ("strapped-down") to the vehicle with their input axes nominally aligned with the sun and star-tracker optical axes. The vehicle, using the sun and a star, such as Canopus, as a reference, is caused to execute a series of calibration maneuvers (rolling, then yawing through 360.degree.) which involves combining optical and gyro outputs to minimize the effects on pointing accuracy of the previously described error sources.

Upon solar acquisition after launch vehicle ejection, the instrument vehicle is rolled around the sun line to acquire Canopus. While the vehicle maintains the sun-star reference, gyro and integrator drift calibration figures are obtained and stored. Thereafter, the vehicle is caused to execute a series of roll and yaw maneuvers during which roll and yaw gyro output values are compared with the true roll and yaw angles and values corresponding to the degree of nonorthogonality between the roll axis and the pitch and yaw gyro input axes and the nonorthogonality of the roll and pitch axes relative to the yaw axis is determined. These values are stored and, together with the roll and yaw gyro output values, used to modify the outputs of the gyroscopes for subsequent acquisition of any arbitrary target independent of external references.

The above and other objects, features and advantages of the invention will be apparent in the following detailed description of the accompanying drawings in which:

FIG. 1 is a partially broken away perspective view of an instrument vehicle incorporating the attitude control system of the present invention.

FIGS. 2, 2A and 2B comprise a block diagram of the control electronics and computer of the system of FIG. 1.

Referring to FIG. 1, there is illustrated, in accordance with the present invention, an instrument-bearing space vehicle 100 having three axes represented by arrows designated roll, pitch and yaw. Vehicle 100 is provided at one end with a pair of experiment-viewing ports 102, 103 for use by optical instruments (not shown) mounted in the interior of the vehicle. It is to be understood, however, that any number of viewing ports may be used and that the location of the ports in the walls of the vehicle depends entirely on the vehicle instrumentation and the nature of the experiments to be performed and that other types of instruments requiring vehicle attitude control may be included in the vehicle instrumentation.

Coaxial with the roll axis and intermediate ports 103 and 102, there is provided a conventional sun-sensor 104. A conventional star-tracker 105 is positioned adjacent sensor 104. Star-tracker 105 is preferably, though not necessarily, orientated to have its optical null axis substantially normal to the optical null axis of the sun-sensor for providing maximum angular resolution. Considering the sun-sensor 104 and star-tracker 105 to be mounted on the front end of the vehicle and aligned with respect to a first set of orthogonal coordinates, there is provided aft of the front end and projecting from the wall of vehicle 100, a plurality of cold gas control jets 106 and 107. Jets 106 and 107 and others (not shown) are used to maneuver the vehicle 100 through selected roll, pitch and yaw attitude changes as hereinafter described.

Interior of vehicle 100, there is provided at its aft end a control jet gas tank 108 for use by jets 106 and 107. Forward of tank 108 and mounted to a bulkhead 109, there is provided three two-axis strapped-down gyroscopes 1, 2 and 3. Gyroscopes 1, 2 and 3 are, respectively, the roll, pitch and yaw gyros of the attitude control system of the present invention. The gyroscopes are fixed (strapped-down) to the vehicle with their input axes aligned with respect to a second set of orthogonal coordinates though nominally in alignment with the sun and star-tracker optical axes. Also mounted to bulkhead 109 are a control electronics module 110 and a small, special purpose digital computer 111.

The gyros 1, 2 and 3 sense angular motion of vehicle 100 and output signal information to the control electronics module 110 and computer 111 which processes the signal information for positioning the vehicle.

Referring to FIG. 2, gyros 1, 2 and 3, control electronics module 110 and computer 111 comprise a roll, a pitch and a yaw channel.

In the roll channel there is provided roll gyro 1. The output of roll gyro 1 is coupled to an input of a three input summing amplifier 4. The output of amplifier 4 is coupled to a roll voltage-to-frequency converter 8 and, by means of a switch S18 and a line 130, to an input of a three input amplifier 31. The output of converter 8 is coupled to a roll up/down counter 14 and through a switch S14 to a roll drift up/down counter 13. The output of converter 13 is coupled to a roll drift digital-to-analog converter 7. The output of converter 7 is coupled to a second one of the inputs of amplifier 4 through a switch S1. Coupled to a third of the inputs of amplifier 4 via a switch S16 is a rate command module 120. Switches S16 and S18 are mechanically coupled. When switches S16 and S18 are closed, module 120 will output a signal which commands a roll rate of 1.5 degrees/second. The output of counter 14 is coupled to a contact 121 of a single pole, three position switch S7 having a selector 122 which is coupled to a central processing unit (CPU) 22 in computer 111.

Processed control signals from CPU 22 of computer 111 are fed into the roll channel via selector 123 of a single pole, three position switch S8. The input of a roll storage register 25 is coupled to a contact 124 of switch S8. The output of register 25 is coupled to a second input of amplifier 31 through a roll digital-to-analog converter 28 and a switch S9. The output of amplifier 31 is coupled to a roll gas jet 39 through a pulse width pulse frequency valve modulator 34. Jet 39 corresponds to one of jets 106 and 107 in FIG. 1.

The pitch channel comprises the pitch gyro 2. The output of gyro 2 is coupled to a two-input summing amplifier 5. The output of amplifier 5 is coupled to a pitch voltage-to-frequency converter 10. The output of converter 10 is coupled to a pitch up/down counter 16 and through a switch S5 to a pitch drift up/down counter 15. Counter 15 is coupled to the second input of amplifier 5 through a switch S2 and a pitch drift digital-to-analog converter 9. For providing signal information to computer 111, the output of counter 16 is coupled to CPU 22 through a contact 125 of switch S7.

Pitch control signals processed by computer 111 are fed into the pitch channel to a pitch storage register 26 coupled to CPU 22 by way of a contact 126 in switch S8. The output of register 26 is coupled to a pitch digital-to-analog converter 29. The output of converter 29 is coupled to a second input of the three-input amplifier 32 by means of a switch S10. The output of amplifier 32 is coupled to a pitch gas jet 40 through a pitch pulse width pulse frequency valve modulator 35. Jet 40 corresponds to one of the jets 106 and 107 shown in FIG. 1.

The yaw channel comprises the yaw gyro 3. Gyro 3 is coupled to an input of a three-input summing amplifier 6. The output of amplifier 6 is coupled to a yaw voltage-to-frequency converter 12 and, by means of a switch S19 and a line 131, to an input of a three-input amplifier 33. The output of converter 12 is coupled to a yaw up/down counter 18 and through a switch S6 to a yaw drift up/down counter 17. The output of counter 17 is coupled to a second input of amplifier 6 via a yaw drift digital-to-analog converter 11 and a switch S3. Converter 18 furnishes signal information to CPU 22 in computer 111 via a contact 127 in switch S7.

Control signals processed by CPU 22 are fed to the yaw channel via a contact 128 in switch S8 coupled to the input of a yaw storage register 27. The output of register 27 is coupled to a yaw digital-to-analog converter 30. The output of converter 30 is coupled to a third input of three-input amplifier 33 through a switch S11. Signals from amplifier 33 are coupled to a yaw gas jet 41 through a yaw pulse width pulse frequency modulator 36. As in the roll channel, there is further provided a yaw rate command module 128 coupled to amplifier 6 through a switch S17. Switches S17 and S19 are mechanically coupled. When switches S17 and S19 are closed, the vehicle is yawed at a rate of 1.5 degrees/second.

Sun-sensor 104 and star-tracker 105 are also coupled to amplifiers 31, 32 and 33 for providing vehicle roll, pitch and yaw stabilization signals to jets 39, 40 and 41 in response to internally conventionally generated pitch and yaw optical error signals. The yaw axis output of star-tracker 105 is coupled to an input of amplifier 31 in the roll channel via a switch S12 and its pitch axis output is coupled to an input of amplifier 32 in the pitch channel via a switch S13. A pitch axis output of sun-sensor 104 is coupled to an input of amplifier 32 through a switch S14, and a yaw axis output of sun-sensor 104 is coupled to an input of amplifier 33 in the pitch channel via a switch S11.

Computer 111, which receives input control signals from the roll, pitch and yaw channels through switch S7 and provides processed control signals to the roll, pitch and yaw channels via switch S8, comprises, in addition to the conventional CPU 22, which includes means for addition, subtraction, multiplication and division, a calibration sequencer 19, a master sequencer 20, a computer sequencer 21, a computed word register 23 and a programmed word register 24. Master sequencer 20 is coupled to intermediate calibration and computer sequencers 19 and 21 and to sun-sensor 104 and star-tracker 105 and serves to control the operation of the calibration and computer sequencers 19 and 21 and the switching of switches S1-S6 and S9-S15 in response to yaw and pitch axis error signals from sun-sensor 104 and star-tracker 105. Computer sequencer 21 is mechanically coupled, as illustrated by a dashed line 132, to switches S7 and S8 and serves to sequence switches S7 and S8 in response to the master sequencer 20 coupled to its input. An output of computer sequencer 21 is also coupled to CPU 22, computed word register 23 and programmed word register 24. An output of registers 23 and 24 is coupled to the CPU 22 as is the output of calibration sequencer 19. An ouput of the CPU 22 is also coupled to an input of the computed word register 23.

The details of the construction and operation of the star-tracker 105 and sun-sensor 104 are omitted, as both are conventional apparatus. Similarly the details of calibration sequencer 19, master sequencer 20, computer sequencer 21 and CPU 22 are omitted as each employs only typical computer logic and switching networks in a conventional manner to control and execute the functional operations of the system as hereinafter described.

The operation of the all sky pointing attitude control system of the present invention comprises six modes of operation which commence upon ejection of the instrument vehicle 100 from a space-shuttle or other launch vehicle.

In Mode 1, Sun Orientation, the control system utilizes a standard solar pointing Aerobee rocket control system, the details of which are therefore omitted, to locate the sun and align the vehicle roll axis (as defined by the optical null axis of the sun-sensor 104) within 50 arc-seconds of the radiometric center of the sun. When the sun-sensor 104 indicates to the master sequencer 20 in a conventional manner that the pitch and yaw signals are less than 50 arc-seconds for a period of 10 seconds, switches S14 and S15 are closed to provide yaw and pitch stabilization, and Mode 2 is commenced.

In Mode 2, Stellar Acquisition, a predetermined star, preferably as close to ninety degrees from the sun as possible, such as Canopus, is acquired. The master sequencer 20 closes switches S16 and S18 at the output of the rate command module 120 in the roll channel to command the vehicle 100 to roll about the vehicle-sun line established during Mode 1 at 1.5 degrees/second. During this roll maneuver, with switches S14 and S15 closed, the sun-sensor 104 provides pitch and yaw stabilization while the yaw axis output of the star-tracker is monitored by the master sequencer 20. The roll maneuver is continued until the star-tracker, set to detect a preselected star, such as Canopus, indicates a yaw error of 7200 arc-seconds (2 degrees). The master sequencer 20 then opens switches S16 and S18 and closes switch S12 at the output of the star-tracker so that an error signal proportional to the angle between the star-tracker optical null axis and the vehicle-star line is generated. The vehicle thus becomes locked-on to both the sun and the star and now has an inertial reference. When the yaw error signal from the star-tracker and the yaw and pitch error signals from the sun-sensor have been less than 50 arc-seconds for a period of 10 seconds, Mode 3 is commenced.

In Mode 3, Gyro Drift Calibration, gyro drift is observed for a predetermined interval while sun and star pointing attitudes, established in Modes 1 and 2, are maintained utilizing them as optical references. The master sequencer closes switches S4, S5 and S6 so that the pulses coming from the voltage-to-frequency converters 8, 9 and 10 will be counted by the roll, pitch and yaw drift up/down counters 13, 15 and 17 for a predetermined interval, such as, for example, 40 seconds. In this way, the net drift of the roll, pitch and yaw gyros 1, 2 and 3 over said specified period of time will be converted from a slowly fluctuating voltage at the summing amplifiers 4, 5 and 6 to digital numbers stored in binary code in the drift up/down counters 13, 15 and 17. At the end of the predetermined interval, the master sequencer opens switches S4, S5 and S6 and closes switches S1, S2 and S3. Upon closure of switches S1, S2 and S3, which couple digital-to-analog converters 7, 9 and 11 to summing amplifiers 4, 5 and 6, respectively, the three drift values stored in counters 13, 15 and 17 are converted to appropriately scaled analog voltages and substracted from the gyro outputs at the summing amplifiers. If, at this time, the optical error signals in all three axes have remained less than 50 arc-seconds for at least 10 seconds, Mode 4 will also be initiated.

In Mode 4, Roll Axis Calibration, a roll axis scale factor is generated and an orthogonality calibration value determined. Switches S16 and S18 are closed by the master sequencer causing the vehicle to roll about the vehicle-sun line established during Mode 1 at 1.5 degrees/second until the star-tracker yaw error signal is less than 7200 arc-seconds with respect to the selected star Canopus as in Mode 2. When the desired yaw error signal is achieved, switch S12 is closed and switches S16 and S18 are opened to allow the vehicle to stabilize using the star image as a reference.

In contrast to the system operation in Mode 2, however, the star-tracker yaw error signal is not monitored by the master sequencer until 10 seconds after start of the roll maneuver. This precludes locking-on to the star without a full revolution of the vehicle.

During the roll calibration maneuver, the roll gyro output is compared with the true roll angle (360.degree.) measured by sensing Canopus. Simultaneously, the pitch and yaw gyro outputs are measured to determine the values corresponding to the degree of nonorthogonality between the roll axis and the pitch and yaw gyro input axes.

To perform the necessary computations, the computer sequencer 21 sequences switch S7 whereby the central processing unit (CPU) 22 sequentially samples the output of the roll, pitch and yaw up/down counters 14, 16 and 18. The calibration sequencer 19 in response to the master sequencer directs the CPU 22 to process the data to provide roll scale factor, roll-into-pitch and roll-into-yaw cross coupling factors and store these numbers in the computed word register 23 for later use. Provided that all optical error signals have remained below 50 arc-seconds for a ten-second interval, the master sequencer 20 commences Mode 5 operation at the completion of the Mode 4 computations.

In Mode 5, Yaw Axis Calibration, the vehicle is yawed 360.degree. and reacquires the sun and Canopus in a manner similar to Modes 1 and 2. Calculations identical to those of Mode 4 are made for yaw axis scale factor error and nonorthogonality of the roll and pitch axes relative to the yaw axis.

In order to revolve about the yaw axis and still retain optical pitch and roll attitude information, the vehicle is maneuvered about its pitch axis until the star-tracker pitch error signal is zeroed. This is accomplished by the master sequencer 20 when the master sequencer 20 closes switch S13 and opens switch S14 on the output of the sun and star-tracker 104 and 105, respectively. After 10 seconds, if the star-tracker 105 pitch error signal remains below 50 arc-seconds, the master sequencer 20 opens switch S15 and closes switches S17 and S19 to command the vehicle to yaw about the vehicle-star line at a rate of 1.5 degrees/second. In a manner identical to that in Mode 4, but using the sun-sensor 104, the vehicle 100 monitors the sun-sensor 105 yaw error signal (and binary sun presence indication, as in all sun acquisition maneuvers), and opens switches S17 and S19 and closes switch S15 to reacquire the sun when they yaw error signal from the sun-sensor 104 drops below 7200 arc-seconds. When 10 seconds of less than 50 arc-seconds of optical error on all three axes have elapsed, the original state of the pitch orientation at the beginning of Mode 5 is re-established by a closure of switch S14 and an opening of switch S13 on the outputs of the sun and star-tracker 104 and 105, respectively, under the control of master sequencer 20.

When the vehicle has returned to the orientation it had at the beginning of Mode 5 and the optical error signals in all three axes have remained less than 50 arc-seconds for at least 10 seconds, the master sequencer 20 activates the computer and calibration sequencers 20 and 19, respectively. As in Mode 4, but with respect to yaw, the computer sequencer 21 sequences switch S7 to sample the contents of the roll, pitch and yaw up/down counters 14, 16 and 18, while the calibration sequencer 19 controls the computation in the CPU 22 of the yaw scale factor, yaw-into-pitch and yaw-into-roll cross coupling factors and stores these factors in the computed word register 23. Upon completion of the Mode 5 computations, Mode 6 is initiated.

Mode 6, Target Acquisition, is in two parts: a roll maneuver, Modes 6-1, and a yaw maneuver, Mode 6-2.

In Mode 6-1, Target Acquisition Roll Maneuver, the master sequencer 20 closes switch S16 and opens switch S12 to command the vehicle 100 to roll at a rate of 1.5 degrees/second through a target angle calculated from a predetermined roll target angle prestored in the programmed word register 24. During the roll maneuver with switches S14 and S15 closed, the pitch and yaw axes are stabilized by signals from the sun-sensor 104. The computer sequencer 21 sequences switch S7 to sample the data in the roll up/down counter 14 and controls the computation of a corrected roll signal and the derivation of a roll position error signal. The corrected roll signal is derived by multiplying the roll scale factor stored in the computed word register 23 in Mode 4 by the number, at each sample, in the roll up/down counter 14. The derivation of the roll position error signal, a function of the roll gyro output, is obtained by subtracting the corrected roll signal (a digital number) from the predetermined roll target angle (another digital number) prestored in the programmed word register 24. When the roll position error signal is less than 7200 arc-seconds, the master sequencer 20 opens switch S16 and S18 and closes switch S9 at the output of the roll digital-to-analog converter 28. The roll position error signal is transferred from the CPU 22 to the roll storage register 25. The roll digital-to-analog converter 28 converts the contents of the roll storage register 25 to an analog voltage and applies the analog voltage to the input of the roll pulse width pulse frequency value modulator 34 through the amplifier 31 which drives the roll gas jets 39 to roll the vehicle through the computed roll target angle.

During Mode 6-1, the CPU 22 computes roll-into-yaw and roll-into-pitch correction factors which are arrived at by multiplying the corrected roll signal by the roll-into-yaw and roll-into-pitch cross coupling factors computed during Mode 4 for each sampling of the contents of the roll up/down counter 14. When the optical error signals in all three axes have remained less than 50 arc-seconds for at least a ten-second duration, the master sequencer 20 resets the yaw and pitch up/down counters 16 and 18 with the negative of the most recent value of the appropriate cross coupling correction factor. This insures that the pitch and yaw axes have the correct initial conditions at the start of the yaw target acquisition maneuver.

In Mode 6-2, Target Acquisition yaw Maneuver, at the completion of the loading of the pitch and yaw up/down counters 16 and 18, the master sequencer 20 closes switches S17 and S19, and switch S10, and opens switch S14 and switch S15. The vehicle 100 is then completely stabilized by reference to the three rate gyros, and is rotating about its yaw axis at 1.5 degrees/second.

During Mode 6-2, the computer sequencer 21 is cycling switch S7 and switch S8 synchronously so that the roll, pitch and yaw data is obtained from, computed for, and directed to each axis sequentially. Corrected error signals for the Mode 6-2 maneuver are computed in a series of steps as follows:

a. The four cross-coupling correction factors, roll-into-yaw, roll-into-pitch, yaw-into-roll and yaw-into-pitch, are obtained by multiplying the most recent value of the corrected yaw and roll angles (digital numbers) by the appropriate previously computed cross coupling factors stored during Modes 4 and 5.

b. An intermediate value of corrected pitch, yaw and roll angles is found by substracting the cross coupling correction factor determined in the previous step from the contents of the up/down counter corresponding to a particular axis -- i.e., an intermediate pitch number would be obtained by subtracting the yaw-into-pitch and roll-into-pitch cross coupling correction factors from the number found in the pitch up/down counter.

c. New corrected roll and yaw angles are determined by multiplying the intermediate value found in the previous step by the respective scale factor determined and stored during Modes 4 and 5.

d. Roll and yaw error signals are produced by subtracting the corrected roll and yaw angles from the numbers prestored in the programmed word register 24.

Since the pitch axis always remains at a null, there is no necessity to compute cross coupling correction factors such as pitch-into-roll, or multiply the intermediate value of the corrected pitch angle by a scale factor. The error signal transferred to the pitch storage register 26 is the intermediate pitch angle number computed above.

The yawing motion is braked when the yaw error signal drops below 7200 arc-seconds.

When this occurs, the master sequencer 20 monitoring the yaw error signal opens switches S17 and S19 and closes switch S11 to allow the yaw angle to be driven to the preselected value. Mode 6-2 is terminated when the error signals in all three axes remains below 50 arc-seconds for a ten-second period. The master sequencer may be programmed to hold at the target for any desired length of time, recycled through the six modes, or move to a new target by utilizing a new set of programmed roll and yaw members.