Spacecraft attitude control system

0001This invention relates to the field of controlling the attitude of a spacecraft with respect to a set of three orthogonal axes (roll, pitch and yaw).

0002EP-A-046 151 relates to a device for the positional stabilisation of vehicles with slightly dampened structural vibrations and intermittent positioning manoeuvres. Each axis of the vehicle, e.g. a spacecraft, is equipped with a tracking system of at most the third order, a conditional control, and a modulator circuit with relay characteristics. The commonly used modulator network with internal modulator return is eliminated and a feedback loop from the output of the modulator takes over the function of these two modulator elements.

0003According to the present invention, there is provided an apparatus for controlling the attitude of a spacecraft with respect to each of three orthogonal reference axes, comprising means for imparting angular velocity to the spacecraft about each of three orthogonal spacecraft axes that are fixed with respect to the spacecraft, means for measuring the angular deviation between each spacecraft axis and a corresponding one of the reference axes and for measuring the angular velocity of the spacecraft about each of the spacecraft axes, and means for periodically observing output signals produced by the measuring means and creating in response thereto a digital pulsetrain signature for each spacecraft axis for aligning said spacecraft axis with its corresponding reference axis; wherein each pulsetrain signature is generated by PWPF means situated within said observing and creating means and comprising a filter for smoothing out high frequency transients on the signals produced by the measuring means; and a hysteresis circuit having an input coupled to an output of the filter, wherein the filter's output is subjected to a hysteresis loop having a value of a logical one, a logical zero, or a negative one; characterised in that each digital pulsetrain signature has a varying number of pulses of varying duration, and is loaded into a shift register which serially outputs a signal for alternately switching on and off the imparting means for the corresponding spacecraft axis in response thereto, and in that the apparatus further includes a digital processor having an input coupled to an output of the hysteresis circuit and an output coupled to an input of the filter, the processor initiating operation of the PWPF means.

0004In its preferred embodiment, the invention provides a digital system for controlling the attitude of a spacecraft with respect to three orthogonal axes. Although it can be implemented by analog means, the digital implementation offers the following advantages. First, the digital processor can be shared with other spacecraft functions, saving weight and power. Second, there is no drift associated with temperature changes of analog components. Third, long time constant compensation can be represented as preprogrammed elements in a digital computer, saving the weight associated with heavy discrete inductors and capacitors.

0005The present invention can be used advantageously during all phases of the launch of spacecraft, e.g., in the case of a satellite, the invention can be used during the parking orbit as an automatic nutation controller; during initial transfer orbit manoeuvres, including sun acquisition, earth acquisition, and apogee motor firing; and during all phases of attitude control in final operational orbit, including station-keeping maneuvers and momentum wheel unloading.

0006The invention can be used whether or not spacecraft utilises momentum wheels for stiffening momentum about one or more of the orthogonal axes. When momentum wheels are used, part of the present circuit can be used for momentum dumping in the momentum wheel control mode.

0007Information concerning the orientation and angular velocity of the spacecraft about each of the three axes is gathered by attitude sensors, weighted by compensation electronics and fed to processors each processed error cycle period T1.

0008A prior art search was conducted in the public records of the U.S. Patent and Trademark Office and disclosed the following references: <ul id="ul0001" list-style="none"><li>U.S. patent No. 3,866,025 is a digital three axis attitude controller, but is not a true PWPF (Pulse Width Pulse Frequency) system as in the present invention, each pulse train has a variable number of pulses each having a fixed pulse width, whereas the present invention varies the width as well as the frequency of pulses in response to spacecraft dynamics and to control same. The above patent does not employ shift registers as in the present invention. It discloses a system mainly for precessing the angular momentum vector of the spacecraft, without the capability to perform station-keeping and control spacecraft attitude simultaneously as in the present invention. It requires expensive rate integrating gyros in addition to sun sensors and/or earth sensors, whereas the present invention does not require rate integrating gyros. The above reference combines information from all axes at once, while the present invention operates upon the axes independently with simple uncoupled control laws.</li><li>U.S. patent No. 3,937,423 discloses a technique for nutation damping while maintaining attitude control with respect to just the roll axis; the present invention, on the other hand, controls the attitude along each of three orthogonal axes. The latter reference is an analog signal processing technique, whereas the present invention, in its preferred embodiment, is a wholly digital system, offering concomitant advantages. The cited patent issues thruster pulses timed according to the spacecraft nutation period, whereas the present invention does not phase the thruster firings with nutation periods, and has a quasi-real-time response to fast spacecraft dynamics that is much shorter than the nutation period. The latter patent pertains to on-orbit control whereas the present invention has a broader range of applicability encompassing acquisitions, station-keeping, or any other maneuver that requires thruster controls. The cited patent does not produce pulses of variable widths and is therefore not a PWPF system as in the present invention.</li><li>U.S. patent No. 3,994,172 is an attitude control system which could possibly be extended to three axis control, although just one axis is described. It is not a PWPF system because just the pulse width, and not pulse frequency is varied within a sampling period. The cited patent does not employ shift registers, which the present invention advantageously uses to mete out the thruster command signals while freeing up its processor (2) to perform other spacecraft functions.</li><li>U.S. patents Nos. 3,643,897 and 4,161,780 disclose techniques associated with spin stabilization, and not three axis spacecraft control.</li></ul>

0009The present invention achieves better resolution of attitude control than prior art systems. In part, this is attributable to the PWPF apparatus comprising the processor, filters, limiter circuits, hysteresis circuits, and data registers. A PWPF loop for each axis is actuated once for each thruster command period T2 (there are preferably several T2's per processed error cycle period T1, to improve resolution). There is thus generated each T1, a thruster pulse train signature for instructing each thruster whether and when to fire during the next T1. It is assumed that there are six spacecraft thrusters positioned to impart both directions of angular momentum to the spacecraft about the three orthogonal axes (a positive rotation thruster and a negative rotation thruster for each axis). Each thruster pulse train signature is a sophisticated sequence of a variable number of logical plus one pulses, minus one pulses, and zero level pulses, each having variable width.

0010A set of pulse train signatures is loaded into shift registers each T1 and outputted to the spacecraft thruster coils each T3, where T3 is preferably less than T2 to minimize the impact of synchronization errors. Data extenders are serially connected to shift registers and ensure that there is continuity between each T1's series of pulses so that the thruster valves do not unnecessarily open and close.

0011The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0002" list-style="none"><li>Figure 1 is a functional block diagram of a preferred embodiment of the present invention;</li><li>Figure 2 is a timing diagram showing a typical thruster pulse train signature as a function of processed error cycle period T1 and thruster command period T2;</li><li>Figure 3 is a table showing the status of formatter (30, 32) corresponding to the thruster pulse train signature depicted in Figure 2;</li><li>Figure 4 is a schematic representation of each limiter circuit 20, and</li><li>Figure 5 is a schematic representation of each hysteresis circuit 22.</li></ul>

Detailed Description of the Preferred Embodiments

0012In Figure 1, spacecraft 14, which can be a missile but for the purposes of this discussion is assumed to be a satellite, must be stabilized about three orthogonal axes commonly designated roll, pitch, and yaw. Attitude sensors 12 measure the angular deviation of spacecraft 14 with respect to the desired orientation of each of the three axes, e.g., by means of earth sensors and sun sensors. Sensors 12 further measure the angular velocity of spacecraft 14 about each of the three axes, e.g., by means of rate gyros. These six signals are fed by attitude sensors 12 into control loop compensation electronics 10, which weights the angular deviation and the angular velocity signals according to their relative importance in attitude control according to control laws. Compensation electronics 10 also contains circuitry to insure that the frequency of thruster firings on board spacecraft 14 does not interact with the structural frequencies of spacecraft 14 to cause instability.

0013The output information from compensation electronics 10 is fed into data register 8, a buffer between the data gathering equipment just described and processor 2. Register 8 is typically a digital serial-in parallel-out register; its composite error signal output is fed to processor 2 once every processed error cycle period T1, with the error information typically separately identified for each of the three axes. It is desirable to make T1 as small as possible to improve the stability of this sampled data system and the resolution of the processed signals. In an attitude controller which has been built following the teachings of the present invention, T1 is 64 milliseconds. With T1 = 64 ms, a wide bandwidth controller up to 49 radians per second can be symthesized without violating Shannon's Theorem.

0014Sequencer 4 is a controller such as a programmed digital computer which bidirectionally communicates with processor 2. Every T1, sequencer 4 sends a first signal informing gating circuit 36 that a processed error cycle has ended, and sends a second signal to processor 2 instructing it to accept new data from register 8 and commence actuation of a new round of PWPF loops. Said first signal could also be sent directly from clock generator 34 to gating circuit 36. Sequencer 4 also acts as a time sharing interface so that other functions (not illustrated here) associated with controlling spacecraft 14, such as TT&amp;C, magnetic control torquer loops, etc., can also use the services of processor 2 on a time shared basis. As mentioned above, this is a major advantage of employing this digital attitude control system because it enables an overall savings of weight and power on board spacecraft 14.

0015Processor 2 is essentially a microprocessor arithmetic logic unit. It communicates bidirectionally with temporary storage register 6, typically a RAM scratchpad memory used for making calculations and for storing, for each of the three axes, the output of the last PWPF loop of each processed error cycle for use in the first PWPF loop for the next processed error cycle.

0016The PWPF apparatus, which performs PWPF loops for all three axes, comprises processor 2, three filters 16 (one for each axis), three limiter circuits 20 (one for each axis), three hysteresis circuits 22 (one for each axis), and three data registers 24 (one for each axis). To avoid cluttering the drawing, Fig. 1 illustrates just one each of items 16, 20, 22, and 24 using the letter n in parenthesis to indicate that the item illustrated is but one of three siblings. Whenever used in this specification, n means one of 1, 2, and 3, representing roll, pitch, and yaw, respectively. Wires 18 connect processor 2 with filters 16; wires 26 connect registers 24 with processor 2. For each n, n = 1,2,3, the output of filter 16(n) is connected to the input of limiter circuit 20(n), the output of which is connected to the input of hysteresis circuit 22(n), the output of which is connected to the input of data register 24(n).

0017The amount of time assigned to the output of each PWPF loop for each axis is designated the thruster command period T2. It is desirable that T2 be smaller than T1 to improve thruster command signal resolution, although T2 could equal T1. In the above mentioned working embodiment, T1 = 4T2, i.e., T2 is 16 milliseconds. Thus, a PWPF loop for each axis is actuated four times during each T1. The output of each of these twelve negative feedback loops is either a logical +1, -1, or 0. +1 indicates that the positive thruster for that axis, but not the negative thruster, should be fired. -1 indicates that the negative thruster for that axis, but not the positive thruster, should be fired. 0 indicates that neither the positive nor the negative thruster for that axis should be fired. The actual firing of the thrusters occurs during the next processed error cycle. Together, the T1/T2 outputs of each PWPF loop constitute one T1's-worth of the thruster pulsetrain signature for the thruster associated with that axis. A typical pulsetrain signature is show in Fig. 2 over a time of 6T1.

0018It is assumed that spacecraft 14 has six thrusters (or other angular velocity imparting means) positioned to impart angular velocity to spacecraft 14 about each of the three axes in each of a positive rotational sense and an opposite negative rotational sense. A prudent spacecraft 14 designer will provide a set of six backup thrusters to be used if one or more of the primary thrusters should fail. The thrusters impart motion to spacecraft 14 pursuant to Newtonian laws of physics, typically by firing a gas. The firing may be actuated by electrically charging a coil associated with the selected thruster.

0019For each axis, sign detector 28(n) determines the sign of each output of the PWPF loop (typically stored as the most significant bit of the data word presented to the input of 28(n)) and gates the sign information to sign formatter 30(n). Each of the three formatters 30/32 stores the pulsetrain signature for one of the axes and breaks it up into smaller bits. In the illustrated embodiment, a minus sign is represented as a logical one and a plus sign is represented as a logical zero within sign formatter 30(n).

0020Sign detector 28(n) also extracts the amplitude information from the output of each PWPF loop (typically the least significant bit of the data word presented at the input of 28(n)) and gates this information, either a logical one or a logical zero, to amplitude formatter 32(n).

0021Formatters 30/32 are each typically serial-in parallel-out shift registers in combination with data splitters. The input shift register portion of each formatter has T1/T2 columns, one for each of the PWPF loop outputs produced each T1. In the example discussed herein, the number of columns is 4. The data splitter portion of each formatter 30, 32 can be, e.g., simply a set of wires to split each data bit into T2/T3 clone data bits, where T3 is the period of the Output Shift Clock used to output data from shift registers 44 and 46. The reason for the data splitting is to minimize the impact of synchronization discrepancies between the Output Shift Clock and the Data Update Clock used to load data into registers 44, 46; e.g., with the data splitting, an error of duration T3 would be made due to a missed clock pulse, rather than an error of duration T2.

0022In the embodiment described herein, T3 is four milliseconds, so each thruster pulsetrain signature residing within a formatter 30 or 32 has sixteen bits for each period of time T1. Within each period of time T2, all bits are the same. This is shown in Fig. 3 for the signature depicted in Fig. 2.

0023Clock generator 34 produces all the clocks necessary for timing the Fig. 1 circuit, starting with a first clock at the fastest rate needed by the circuit. In the embodiment illustrated, this fastest clock signal is 500 KHz. and is sent to gating circuit 36. This is the rate at which the Data Update Clock loads extenders 40 and 42 and registers 44 and 46 from formatters 30 and 32. Clock generator 34 divides the fastest clock into three slower clocks, e.g., by a set of flip-flops. These clocks have periods of T1, T2, and T3, respectively. The T3 clock signal is also sent to gating circuit 36 where it is subsequently used as the constantly-running Output Shift Clock. The T1 and T2 clock signals are sent to sequencer 4.

0024Gating circuit 36 receives a signal from sequencer 4 (or generator 34) at the end of each T1 period, and in response to this signal, actuates T1/T3+x cycles of the Data Update Clock, causing Tl/T3+x bits from each amplitude formatter 32(n) to be read into data extenders 40(n) and 42(n) and shift registers 44(n) and 46(n). A bit from amplitide formatter 32(n) will be read into the combined register consisting of positive data extender 40(n) and positive shift register 44(n) if the sign of that bit, as evidenced by the corresponding bit position within sign formatter 30(n), is zero. On the other hand, a bit from amplitude formatter 32(n) will be read into the combined register consisting of negative data extender 42(n) and negative shift register 46(n) if the sign of that bit is negative, as evidenced by a logical 1 appearing in the corresponding bit position for sign formattter 30(n). This gating into the positive versus the negative logic is accomplished by means of gating circuitry for each axis consisting of inverter 37(n) and AND gates 38(n) and 39(n). The output of sign formatter 30(n) is connected to the input of inverter 37(n) and to a first input of AND gate 39(n). The output of amplitude formatter 32(n) is connected to the second input of AND gate 39(n) and to a first input of AND gate 38(n). The output of inverter 37(n) is connected to the second input of AND gate 38(n). The output of AND gate 38(n) is connected to the input of positive data extender 40(n) and the output of AND gate 39(n) is connected to the input of negative data extender 42(n).

0025Since the Data Update Clock is much faster than the Output Shift Clock, the Output Shift Clock can be left running continuously. The value of x should be at least 1. In the illustrated embodiment, x was chosen to be 2. Thus, at the end of each T1, 18 pulses of the Data Update Clock are activated, loading 18 bits from amplitude formatter 32(n) into data extenders 40(n), 42(n) and shift registers 44(n), 46(n).

0026Shift registers 44, 46 are all serial-out shift registers whose output shifting is controlled by the Output Shift Clock and whose input loading is controlled by the Data Update Clock. Data extenders 40, 42 are, e,g., latches, and each imparts a delay of xT3. Extenders 40, 42 are also loaded by the Data Update Clock and shifted out by the Output Shift Clock. Each positive data extender 40(n) is connected in series with a positive shift register 44(n) and each negative data extender 42(n) is connected in series with a negative shift register 46(n), making composite shift registers of 18 bit positions each for the example illustrated. Because of the latching action of each data extender, the last bit to be loaded into its associated shift register is repeated twice by the data extender. For example, if the 16th bit shifted into positive shift register 44(1) is a 1, then the two bits within positive data extender 40(1) are also 1's. The reason for the redundancy provided by the data extenders is to provide continuity between processed error cycles, avoiding data gaps in case, e.g., the signal to gating circuit 36 announcing the end of a processed error cycle arrives late. By this means, unwanted turn-on's and turn-off's of thrusters is avoided.

0027A positive output bit from a register 44 or 46 is typically an electrical signal having a potential of 5 volts, indicating that a thruster is desired to be actuated for that time T3, which a logical zero output bit is an electrical signal having zero volts. The three positive high side drivers 48 and the three negative high side drivers 50 simply serve to increase the 5 volt signals to a higher level more suitable for spacecraft 14, e.g., 26 volts, to overcome noise and transients on spacecraft 14, while passing-through zero volt signals unchanged. Thus, the six high side drivers 48 and 50 can be voltage translators.

0028For each axis, a signal is fed from the positive high side driver 48(n) and the negative high side driver 50(n) to a positive thruster valve coil driver 52(n) and a negative thruster valve coil driver 54(n), respectively. Each driver 52, 54 typically increases the voltage for a logical one to 42 volts, applying this voltage to a thruster coil which energizes the proper thruster. A zero volt output from a high side driver 48, 50, results in a zero volt output from its associated coil driver 52, 54, which deactivates the associated thruster.

0029The PWPF apparatus will now be described in more detail. Each filter 16 is typically a single-pole low pass integrating filter having a transfer function of the form K/(TS+ 1 ), where S is the integrating operator, T is a time constant, and K is proportional to the gain. Filter 16 smooths out high frequency voltage excursions on the input error signal. Filter 16 can be implemented as part of a digital computer, e.g., each filter 16 can be part of processor 2. In an analog embodiment, filter 16 might have a capacitor and an inductor. The output from each filter 16 is designated AI and becomes the input to its corresponding limiter 20, the function of which is shown is Fig. 4, wherein the input is represented as being along the X axis and the output along the Y axis. If the absolute value of A1 is less than or equal to pre-established threshold A5, A1 will pass through limiter 20 unchanged, because the slope of the limiter's curve in the vicinity of the axes' intersection is one. Positive values of A1 in excess of A5 are limited to A5, and negative values of A1 less than -A5 are limited to -A5. The function of limiter 20 is to lower the recovery time of the loop. Such limiters 20 are conventionally available in digital hardware. An analog example of such a filter is a pair of back-to-back Zener diodes.

0030The output from each limiter 20 is passed to the hysteresis circuit 22 for that axis, shown schematically in Fig. 5. Assume that the initial input and output for circuit 22 are each zero, and the input starts to increase. Once the input exceeds A7, the output of circuit 22 will be a 1. The output of circuit 22 will remain high as the input decreases below A7 until the input reaches -A6, whereupon the output of circuit 22 will become zero. As the input declines even further, the output of circuit 22 will remain zero until the input is less than -A7, whereupon the output of circuit 22 will be -1. As the input passes -A7 from a negative direction, the output of circuit 22 will remain a -1 until such time as the input exceeds A6, whereupon the output of circuit 22 will become zero. It is desirable to vertically overlap the hysteresis loop (e.g., a zero input can result in a -1, 0, or 1 output depending upon the input's history) because overlapping 'the hysteresis loop modifies its phase relationship with respect to the error signal from register 8 and improves the phase margin of the compensation.

0031Each data register 24 is the recipient of the output produced by its corresponding hysteresis circuit 22. This output is fed to processor 2, is inverted, and is added to the portion of the error signal received from data register 8 for that axis for inputting to filter 16 as the error input for the next iteration of the PWPF loop for that axis. This negative feedback via register 24 is used even when said next PWPF loop iteration occurs during the next T1 interval, i.e.. the data from register 8 has been updated.

0032Elements 16, 20, 22, 28, 30, 32, 37, 38, and 39 could be replaced by a programmable read-only memory (PROM) connected to sequencer 4 and processor 2, whereupon sequencer 4 feeds preselected constants to the PROM, which then instructs processor 2 to execute the twelve PWPF loops. Such a PROM has been constructed, programmed in the FORTRAN language as follows:

Line

0033<ul id="ul0003" list-style="none"><li>1. SUBROUTINE PWPF</li><li>2. IMPLICIT INTEGER^*2(E-S)</li><li>3. ^*SBR PWPF</li><li>4. COMMON/VARI/A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11</li><li>5. COMMONNARl2/11,12,13,14,15,16,17,18,19,110,111,112,</li><li>6. 11 = 0</li><li>7. 13 = 0</li><li>8. 14 = 15</li><li>9. DO 1 I = 1,4</li><li>10. ^*CALC XRP, XPP, YP</li><li>11. AI = A4*(A2-A3) + A8^*A1</li><li>12. * CALC YR, YYP, YY</li><li>13. IF(A1.GT.A5) A1 = A5</li><li>14. IF(A1.LT. -A5) A1 = -A5</li><li>15. IF(A1.GE.A6.AND.A3.EQ. -1.) A3 = ₀.</li><li>16. IF(A1.GE.A7) A3 = 1.</li><li>17. IF(A1.LE. -A6.AND.A3.EQ.1.) A3 = 0.</li><li>18. IF(A1.LE. -A7) A3 = -1.</li><li>19. ^* CALC THRUSTERS</li><li>20. IF(A3.EQ.1.) 11 = IOR2(11,14)</li><li>21. IF(A3.EQ. -1) 13 = IOR2(13,14)</li><li>22. 1 14 = ISHFT2(14,4)</li><li>23. 17 = 32768*A1</li><li>24. RETURN</li><li>25. END</li></ul>

0034In the above program, A4 = K/T and A8 = (1―1/T), where K and T are the aforementioned filter 16 constants.

0035Line 11 of the program embodies the operation of filter 16(n) with constants supplied by sequencer 4. Lines 13 and 14 embody the function of limiter circuit 20(n), lines 15 through 18 embody the functioning of hysteresis circuit 22, and lines 20-22 embody formatting and sign assignment.

0036This one PROM program embodies the PWPF loops for all three axes; the individual constants for each loop are fed into the PROM by sequencer 4.

0037Sequencer 4 could command processor 2 to perform the PWPF loops in any order. In the illustrated embodiment, the twelve PWPF loops are separated axis-wise so that the four roll loops are first performed, followed by the four pitch loops, then the four yaw loops. Each of the PWPF loop outputs corresponds to the 16 milliseconds of the thruster command period, while the execution of each loop takes just one millisecond of processor 2 time. The remaining processor 2 time is used to perform other spacecraft 14 functions, as described above.

0038In selecting suitable PWPF constants, it should be noted that the constants influence each other synergistically, and perhaps for this reason, their selection is somewhat heuristic. In general, increasing A4 (filter gain) increases the on/off frequency of the pulses while lowering the average pulse width. Increasing filter constant A8 enables the filter to smooth out lower frequency spikes and makes for a longer average thruster pulse width. If A5 were larger it would take longer for thruster pulses to change state. Naturally, A7 must be greater than A6. If minus A6 were a positive number, thrusters would turn off faster in response to downward decreasing signals on the input of hysteresis 22. If A7 were greater, it would be more difficult to turn on a thruster. The following example gives the values of constants used in the illustrated embodiment: <tables id="tabl0001" num="0001"><img file="EP0101302B1_D0001.tif" /></tables>