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Demonstration of a Vibration Suppression System for the Space Station
Copyright ~ IFAC Automatic Control in Aerospace. Palo Alto. California. USA. 1994
DEMONSTRATION OF A VIBRATION SUPPRESSION SYSTEM FOR THE SPACE STATION TERRY R. ALT and JOHN T. HARDUVEL McDonnelI Douglas Aerospace, 5301 Bolsa Avenue, MIS 15-1, Huntington Beach, California 92647
Abstract. This paper presents test results of a prototype 3-axis vibration suppression system. This prototype system is analogous to a vibration suppression system that was designed to provide a micro gravity vibration environment on the space station . The vibration suppression system uses a set of proof mass actuators to control the vibration of a fluid filled container on a demonstration truss structure. Key Words. Acceleration control; actuators; decentralized control; robust control; vibration control
1. INTRODUCTION
modules to the module trunions (the trunions are the attach point to the Space Shuttle for launch), and therefore consume no volume within the modules. The VSSM are retrofittable, relocatable and operate under decentralized control, eliminating the need for high communication data rates between VSSM and providing robustness to a failure .
One of the key objectives of the space station is to provide a platform for microgravity research. Some of the planned experiments require acceleration levels to be below one micro-g at low frequencies, with the requirement gradually easing at higher frequencies . In order to achieve a one micro-g environment, a rigid space station would require that forces applied to it not exceed one half pound. Disturbances arising from crew motion, articulated appendages, including the Solar AJpha Rotary Joints (SARJ), Thermal Radiator Rotary Joints (TRRJ) and gimbaled antennas, and various other rotating equipment excite the low frequency, lightly damped flexible body modes of the structure. Current estimates place the environment over an order of magnitude above the requirement (Del Basso et al., 1991).
This paper presents results on a prototype 3-axis vibration suppression system. The experiment was set up to be analogous to the problem of suppressing the disturbance arising from the SARJ motor torque ripple on the space station. Without the planned modifications, the current estimates place the vibration environment due to this disturbance a factor of 15 above the requirement. The prototype system surpassed this goal by achieving a factor of 33 reduction in vibration amplitude.
Two proposed solutions to the microgravity problem are: 1) an active vibration isolation system for each of the microgravity pay loads (Edberg and von Flowtow, 1992), and 2) an active Vibration Suppression System (VSS) that maintains the entire laboratory volume of the U.S. Laboratory, ElD'Opean Space Agency module and the Japanese Experiment Module at the required environment (Alt and Harduvel, 1993). The VSS consists of a set of Vibration Suppression System Modules (VSSM). Each VSSM contains a proof mass acbJator (a linear momentum exchange device), accelerometer, electronics and a processor. The VSSM are mounted outside the laboratory and habitation
2. EXPERIMENTAL SETIJP The test article is a 9.5 meter Control/Structure Interaction (CSI) demonstration truss shown in Figure 1. The structure consists of 19 half meter bays typical of erectable space structures. A low frequency suspension system provides adequate separation between the rigid body pendulum mode frequencies (up to 0.5 Hz) and the first structural bending mode (7.25 Hz). A 100in. aluminum box was mounted to the center of the truss, simulating a lab module on the Space Station (Figure 2). A fluid ftlled container in the box represents the volume of a microgravity experiment. Three prototype Vibration 237
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Fig. 1. Vibration suppression system test setup
of the truss, one vertical and one horizontal, simulate SARJ disturbances exciting the fIrst two bending modes of the truss. 3. VSSM CONTROL SYSTEM DESCRIPTION
Fig. 2.
Figure 3 is a block diagram of the VSSM control system. Accelerometer feedback is used to null the motion of the box at the attach point of the VSSM . The accelerometer feedback splits into two paths through a complementary fIlter: a digital low-pass path and an analog high-pass path. The digital path provides flexibility in the compensator design for the primary frequency region of operation, while the analog path takes over at higher frequencies preserving the positive real stability guarantees for collocated, consistent sensing and actuating (McLaren and Slater, 1987). The digital path contains a high-pass fIlter. necessary to keep low frequency accelerometer noise and the pendulum modes associated with the suspension of the truss (analogous to Space Station rigid body modes) from causing large displacements of the proof mass. The high-pass characteristic of the complementary fIlter serves the same purpose for the analog path.
Three VSSM attached to simulated lab module suppress motion in all directions
Suppression System Modules (VSSM) are mounted orthogonally to three faces of the box. The prototype VSSMs consist of a voice-coil type proof mass actuator having a 2-lb proof mass and a O.5-in. stroke, a Sunstrand QA-2000 accelerometer, electronics, and a PS/2 computer operating at 500 Hz controlling all three VSSM. Shakers at each end
A very important part of the VSSM control system is the servo loop shown in Figure 3. This high bandwidth analog feedback loop eliminates the nonideal characteristics of the actuator. The most signifIcant DOD-ideal characteristic is stiction-the friction effect that causes the proof mass to tend to remain fIxed to the case until a large enough force is
238
Analog. collocated. consistent sensWlg and actuating ensures high frequency stability
Postion S8r1SOI' 5 poles 0 1000Hz
Deoentraized oonIroI ensures stabiity after faiun!
Fig. 3. VSSM control sY8tem de8ign eliminate8 actuator friction and delivers robu8t performance
b. VSSOn
Fig. 4. Vibration Suppre88ion System eliminates an perceptible fluid motion
applied to break it away. The servo loop eliminates this effect by quickly applying whatever force is necessary to track the commanded relative motion of the proof mass using position feedback as shown. Although the high frequency content (harmonics) of the stiction-caused transient remains. its effects are acceptable. At low frequencies (the disturbance frequencies) where the requirement is stricter, the servo loop is very effective at eliminating the stiction effects. The benefit of this is that the requirements on the actuator are eased, eliminating the need for magnetic bearings or other elaborate, expensive friction reducing measures.
positive real. Second, it is possess performance robustness with respect to modal frequency uncertainty, since the gain is not being boosted at the modal frequencies of the nominal plant. Third, it has been determined that for this problem the near optimal decentralized (static) output feedback controller can achieve nearly the same disturbance attenuation as the optimal state feedback compensator (Doyle et al., 1988). Fourth, it lends itself to direct optimization of the exact performance function desired since the compensator is defined by only a few parameters for each VSSM. By optimizing the placement and gain of each VSSM, one can achieve the necessary level of attenuation. In this experiment, the motion of a single payload in all 3 axes is controlled by mounting a VSSM in each orthogonal direction. The gain of each VSSM was selected to achieve the same amount of attenuation required for the station to meet the micro-g requirements for a SARJ disturbance.
The compensator design was motivated by work done in Alt and Harduvel (1993). This work showed that when using collocated, consistent acruating and sensing for vibration control, simple decentralized (static) output feedback can achieve nearly optimal disturbance attenuation. This type of compensation for this problem has many advantages. First, it is positive real compensation, which guarantees robust stability in the frequency range where the plant is
239
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Fig. 5. Vibration Suppression System effectively cancels disturbances
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Fig. 6. VSS test results scaled to space station problem demonstrate requirement would be met
Tab. 1. Measured acceleration level's demonstrate at least a factor of 33 attenuation
Frequency (Hz) 7.25 14.3
Uncom :>ensated x-accel z-accel (g's) (g's) 0.2587 0.0438 0.4455 -
With VSS x-accel z-accel (g's) (g's) 0.0020 0.0013 0.0116 -
4. VSS TEST RESULTS
Factor attenuation x-axis z-axis 33
-
129 38
achieve at least a factor of 15 attenuation, the amount necessary for the Space Station to meet micro-g requirements due to an (unmodified) SARJ disturbance. We exceeded this goal by achieving a factor of 33 attenuation.
The SARJ disturbance was simulated by driving the nrst two bending modes of the structure, the primary modes affecting the environment at the payload. The frrst mode is an x-axis mode at 7.25 Hz, and the second mode is primarily a z-axis mode (vertical) at 14.3 Hz. During the test. the structure was excited at one end in the x-axis and the other end in the z-axis at these frequencies, simulating the frrst two excitation frequencies of the SARJ. The goal was to
Figure 4 compares pictures of our microgravity experiment with and without our Vibration Suppression System active . The fluid sloshes violently due to the structural excitation with no VSS . With the VSS active, no fluid motion is 240
perceptible. but significant motion of the proof masses is present and required to cancel the disturbances. A large reduction in motion occurs througbout the structure since the VSSM applied forces effectively cancel the disturbance forces. leaving only a small residual excitation of the two flex modes.
6. REFERENCES
All, T.R. and I.T. Harduvel (1993). A Vibration Suppression System for the Space Station. Proc. of the Fourth International Conference on Adaptive Structures. Del Basso. S. et al. (1991) . Space Station Microgravity Environment Definition . NASA Report SSC 18270. Doyle. 1.C.. K. Glover. P. Kbaragonekar and B.A. Francis (1988). State-space Solutions to Standard H2 and Hoo Control Problems. Proc. of the American Control Conference. Atlanta, GA. Edberg. D.L. and A. von Aowtow (1992). Progress Toward a Fligbt Demonstration of Microgravity Isolation of Transient Events. Proc. of the World Space Congress. Wasbington. DC. McLaren. M.D. and G.L. Slater (1987). Robust Multivariable Control of Large Space Structures Using Positivity. AIAA Journal of Guidance. Control. and Dynamics, Vol. 10, No. 4.
Figure 5 compares the acceleration time traces with and without the VSS. on the same scales. The attenuation as a function of frequency is illustrated in Figure 6 and Table 1. sbowing that a factor of at least 33 was acbieved in both the x and z axes. We expected small but measurable y-axis motion of the box without the VSS. but y-axis acceleration levels proved to be so small that no reliable measurement could be made with or without the VSS . However. we demonstrated that the y-axis VSSM did not adversely affect performance or stability of the other axes. wbicb was a second objective of the test. Figure 6 sbows the data scaled so that tbe uncompensated response matcbes the response at the space station laboratories due to the SARJ. relative to the micro-g requirement. Note that the data with the VSS contains the expected barmonics of the drive frequencies due to stiction in the proof mass actuator as described earlier. and that tbeir amplitudes are well within the requirement. This demonstrates the effectiveness our servo loop bas on eliminating friction effects in the actuator. An important feature of our system is that it is robust with respect to modal parameter and disturbance frequency variations. Its design is similar to that in AIt and Harduvel (1993). wbicb was sbown to demonstrate performance robustness. We expect our system to work just as well if the modal parameters were varied and disturbance frequencies were to sbift
5. CONCLUSIONS The benefits of a Vibration Suppression System to the Space Station are clear: it represents a viable global solution to meeting the microgravity requirements in the presence of both macbinery and crew disturbances that does not consume volume within the labs and eliminates the need for currently planned modifications to macbinery. In addition. it eases the disturbance requirements on other systems. like the Centrifuge. improves the environment througbout the structure. simplifying the C&T antenna design for example. and can also act as an ideal stimulus for modal identification experiments. The viability of this system was extended beyond the theoretical stage with these experimental results validating analytical performance predictions.
241
DEMONSTRATION OF A VIBRATION SUPPRESSION SYSTEM FOR THE SPACE STATION TERRY R. ALT and JOHN T. HARDUVEL McDonnelI Douglas Aerospace, 5301 Bolsa Avenue, MIS 15-1, Huntington Beach, California 92647
Abstract. This paper presents test results of a prototype 3-axis vibration suppression system. This prototype system is analogous to a vibration suppression system that was designed to provide a micro gravity vibration environment on the space station . The vibration suppression system uses a set of proof mass actuators to control the vibration of a fluid filled container on a demonstration truss structure. Key Words. Acceleration control; actuators; decentralized control; robust control; vibration control
1. INTRODUCTION
modules to the module trunions (the trunions are the attach point to the Space Shuttle for launch), and therefore consume no volume within the modules. The VSSM are retrofittable, relocatable and operate under decentralized control, eliminating the need for high communication data rates between VSSM and providing robustness to a failure .
One of the key objectives of the space station is to provide a platform for microgravity research. Some of the planned experiments require acceleration levels to be below one micro-g at low frequencies, with the requirement gradually easing at higher frequencies . In order to achieve a one micro-g environment, a rigid space station would require that forces applied to it not exceed one half pound. Disturbances arising from crew motion, articulated appendages, including the Solar AJpha Rotary Joints (SARJ), Thermal Radiator Rotary Joints (TRRJ) and gimbaled antennas, and various other rotating equipment excite the low frequency, lightly damped flexible body modes of the structure. Current estimates place the environment over an order of magnitude above the requirement (Del Basso et al., 1991).
This paper presents results on a prototype 3-axis vibration suppression system. The experiment was set up to be analogous to the problem of suppressing the disturbance arising from the SARJ motor torque ripple on the space station. Without the planned modifications, the current estimates place the vibration environment due to this disturbance a factor of 15 above the requirement. The prototype system surpassed this goal by achieving a factor of 33 reduction in vibration amplitude.
Two proposed solutions to the microgravity problem are: 1) an active vibration isolation system for each of the microgravity pay loads (Edberg and von Flowtow, 1992), and 2) an active Vibration Suppression System (VSS) that maintains the entire laboratory volume of the U.S. Laboratory, ElD'Opean Space Agency module and the Japanese Experiment Module at the required environment (Alt and Harduvel, 1993). The VSS consists of a set of Vibration Suppression System Modules (VSSM). Each VSSM contains a proof mass acbJator (a linear momentum exchange device), accelerometer, electronics and a processor. The VSSM are mounted outside the laboratory and habitation
2. EXPERIMENTAL SETIJP The test article is a 9.5 meter Control/Structure Interaction (CSI) demonstration truss shown in Figure 1. The structure consists of 19 half meter bays typical of erectable space structures. A low frequency suspension system provides adequate separation between the rigid body pendulum mode frequencies (up to 0.5 Hz) and the first structural bending mode (7.25 Hz). A 100in. aluminum box was mounted to the center of the truss, simulating a lab module on the Space Station (Figure 2). A fluid ftlled container in the box represents the volume of a microgravity experiment. Three prototype Vibration 237
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Fig. 1. Vibration suppression system test setup
of the truss, one vertical and one horizontal, simulate SARJ disturbances exciting the fIrst two bending modes of the truss. 3. VSSM CONTROL SYSTEM DESCRIPTION
Fig. 2.
Figure 3 is a block diagram of the VSSM control system. Accelerometer feedback is used to null the motion of the box at the attach point of the VSSM . The accelerometer feedback splits into two paths through a complementary fIlter: a digital low-pass path and an analog high-pass path. The digital path provides flexibility in the compensator design for the primary frequency region of operation, while the analog path takes over at higher frequencies preserving the positive real stability guarantees for collocated, consistent sensing and actuating (McLaren and Slater, 1987). The digital path contains a high-pass fIlter. necessary to keep low frequency accelerometer noise and the pendulum modes associated with the suspension of the truss (analogous to Space Station rigid body modes) from causing large displacements of the proof mass. The high-pass characteristic of the complementary fIlter serves the same purpose for the analog path.
Three VSSM attached to simulated lab module suppress motion in all directions
Suppression System Modules (VSSM) are mounted orthogonally to three faces of the box. The prototype VSSMs consist of a voice-coil type proof mass actuator having a 2-lb proof mass and a O.5-in. stroke, a Sunstrand QA-2000 accelerometer, electronics, and a PS/2 computer operating at 500 Hz controlling all three VSSM. Shakers at each end
A very important part of the VSSM control system is the servo loop shown in Figure 3. This high bandwidth analog feedback loop eliminates the nonideal characteristics of the actuator. The most signifIcant DOD-ideal characteristic is stiction-the friction effect that causes the proof mass to tend to remain fIxed to the case until a large enough force is
238
Analog. collocated. consistent sensWlg and actuating ensures high frequency stability
Postion S8r1SOI' 5 poles 0 1000Hz
Deoentraized oonIroI ensures stabiity after faiun!
Fig. 3. VSSM control sY8tem de8ign eliminate8 actuator friction and delivers robu8t performance
b. VSSOn
Fig. 4. Vibration Suppre88ion System eliminates an perceptible fluid motion
applied to break it away. The servo loop eliminates this effect by quickly applying whatever force is necessary to track the commanded relative motion of the proof mass using position feedback as shown. Although the high frequency content (harmonics) of the stiction-caused transient remains. its effects are acceptable. At low frequencies (the disturbance frequencies) where the requirement is stricter, the servo loop is very effective at eliminating the stiction effects. The benefit of this is that the requirements on the actuator are eased, eliminating the need for magnetic bearings or other elaborate, expensive friction reducing measures.
positive real. Second, it is possess performance robustness with respect to modal frequency uncertainty, since the gain is not being boosted at the modal frequencies of the nominal plant. Third, it has been determined that for this problem the near optimal decentralized (static) output feedback controller can achieve nearly the same disturbance attenuation as the optimal state feedback compensator (Doyle et al., 1988). Fourth, it lends itself to direct optimization of the exact performance function desired since the compensator is defined by only a few parameters for each VSSM. By optimizing the placement and gain of each VSSM, one can achieve the necessary level of attenuation. In this experiment, the motion of a single payload in all 3 axes is controlled by mounting a VSSM in each orthogonal direction. The gain of each VSSM was selected to achieve the same amount of attenuation required for the station to meet the micro-g requirements for a SARJ disturbance.
The compensator design was motivated by work done in Alt and Harduvel (1993). This work showed that when using collocated, consistent acruating and sensing for vibration control, simple decentralized (static) output feedback can achieve nearly optimal disturbance attenuation. This type of compensation for this problem has many advantages. First, it is positive real compensation, which guarantees robust stability in the frequency range where the plant is
239
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Fig. 5. Vibration Suppression System effectively cancels disturbances
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10' Fnqaoocy ( Ib)
Fig. 6. VSS test results scaled to space station problem demonstrate requirement would be met
Tab. 1. Measured acceleration level's demonstrate at least a factor of 33 attenuation
Frequency (Hz) 7.25 14.3
Uncom :>ensated x-accel z-accel (g's) (g's) 0.2587 0.0438 0.4455 -
With VSS x-accel z-accel (g's) (g's) 0.0020 0.0013 0.0116 -
4. VSS TEST RESULTS
Factor attenuation x-axis z-axis 33
-
129 38
achieve at least a factor of 15 attenuation, the amount necessary for the Space Station to meet micro-g requirements due to an (unmodified) SARJ disturbance. We exceeded this goal by achieving a factor of 33 attenuation.
The SARJ disturbance was simulated by driving the nrst two bending modes of the structure, the primary modes affecting the environment at the payload. The frrst mode is an x-axis mode at 7.25 Hz, and the second mode is primarily a z-axis mode (vertical) at 14.3 Hz. During the test. the structure was excited at one end in the x-axis and the other end in the z-axis at these frequencies, simulating the frrst two excitation frequencies of the SARJ. The goal was to
Figure 4 compares pictures of our microgravity experiment with and without our Vibration Suppression System active . The fluid sloshes violently due to the structural excitation with no VSS . With the VSS active, no fluid motion is 240
perceptible. but significant motion of the proof masses is present and required to cancel the disturbances. A large reduction in motion occurs througbout the structure since the VSSM applied forces effectively cancel the disturbance forces. leaving only a small residual excitation of the two flex modes.
6. REFERENCES
All, T.R. and I.T. Harduvel (1993). A Vibration Suppression System for the Space Station. Proc. of the Fourth International Conference on Adaptive Structures. Del Basso. S. et al. (1991) . Space Station Microgravity Environment Definition . NASA Report SSC 18270. Doyle. 1.C.. K. Glover. P. Kbaragonekar and B.A. Francis (1988). State-space Solutions to Standard H2 and Hoo Control Problems. Proc. of the American Control Conference. Atlanta, GA. Edberg. D.L. and A. von Aowtow (1992). Progress Toward a Fligbt Demonstration of Microgravity Isolation of Transient Events. Proc. of the World Space Congress. Wasbington. DC. McLaren. M.D. and G.L. Slater (1987). Robust Multivariable Control of Large Space Structures Using Positivity. AIAA Journal of Guidance. Control. and Dynamics, Vol. 10, No. 4.
Figure 5 compares the acceleration time traces with and without the VSS. on the same scales. The attenuation as a function of frequency is illustrated in Figure 6 and Table 1. sbowing that a factor of at least 33 was acbieved in both the x and z axes. We expected small but measurable y-axis motion of the box without the VSS. but y-axis acceleration levels proved to be so small that no reliable measurement could be made with or without the VSS . However. we demonstrated that the y-axis VSSM did not adversely affect performance or stability of the other axes. wbicb was a second objective of the test. Figure 6 sbows the data scaled so that tbe uncompensated response matcbes the response at the space station laboratories due to the SARJ. relative to the micro-g requirement. Note that the data with the VSS contains the expected barmonics of the drive frequencies due to stiction in the proof mass actuator as described earlier. and that tbeir amplitudes are well within the requirement. This demonstrates the effectiveness our servo loop bas on eliminating friction effects in the actuator. An important feature of our system is that it is robust with respect to modal parameter and disturbance frequency variations. Its design is similar to that in AIt and Harduvel (1993). wbicb was sbown to demonstrate performance robustness. We expect our system to work just as well if the modal parameters were varied and disturbance frequencies were to sbift
5. CONCLUSIONS The benefits of a Vibration Suppression System to the Space Station are clear: it represents a viable global solution to meeting the microgravity requirements in the presence of both macbinery and crew disturbances that does not consume volume within the labs and eliminates the need for currently planned modifications to macbinery. In addition. it eases the disturbance requirements on other systems. like the Centrifuge. improves the environment througbout the structure. simplifying the C&T antenna design for example. and can also act as an ideal stimulus for modal identification experiments. The viability of this system was extended beyond the theoretical stage with these experimental results validating analytical performance predictions.
241