Closed-Loop Performance of a Vibration Isolation and Suppression System Jeanne Sullivan*, Zahidul Rahman**, Richard Cobb*, John Spanos** *USAF Phillips Laboratory Kirtland AFB, NM 87117
[email protected],
[email protected]
Abstract This paper describes the initial closed-loop isolation testing of the Vibration Isolation and Suppression System (VISS) which can be used to isolate a precision payload from spacecraft borne disturbances. VISS utilizes six hybrid isolation struts in a hexapod configuration. The VISS experiment has recently been tested at the Phillips Laboratory and significant closed-loop vibration isolation was achieved. The control approach, the performance testbed, and the initial test results are described.
1 Introduction High precision optical sensors require high precision from the host spacecraft in order to perform their mission. Major sources of jitter for most satellites include momentum transfer devices such as momentum/reaction wheels, solar array drive mechanisms, and specialized devices with moving or rotating mass such as cryocoolers. The desired precision is often obtained by developing a low noise bus at a significant increase in system cost. The next generation of optical payloads will require ever increasing precision from the spacecraft bus. This trend towards higher quality sensors for the next generation space systems is incompatible with the nation’s move towards lighter and cheaper buses. One possible solution to this dilemma is to isolate the precision sensor from the rest of the satellite. Active and passive vibration isolation mounts which can be used to locally isolate vibration generating mechanisms from the rest of the satellite have been investigated by several researchers [1-9]. However, with few exceptions, only passive isolation technology has been accepted by satellite designers as mature for space applications. This paper describes the Vibration Isolation and Suppression System (VISS) which can be used to isolate a precision payload from spacecraft borne disturbances. VISS utilizes six hybrid isolation struts in a hexapod configuration. Central to the concept is a novel hybrid actuation concept which provides both passive isolation and active damping [7,9]. The passive isolation is provided by Honeywell’s flight proven D-Strut design. The passive design is supplemented by a voice coil based active system. For isolation, the active system is used to enhance the performance of the passive isolation system at lower frequencies. An additional benefit of the active system
**Jet Propulsion Laboratory Pasadena, CA 91109
[email protected],
[email protected]
is that it can be used for two additional functions: suppression and steering. Vibration suppression is needed to counteract disturbances from noisy devices, such as cryocoolers, that are directly attached to the optical payload. The steering function enables the VISS device to be used as a tracking gimbal for the optical payload. The design and fabrication of the VISS hardware was performed by Honeywell Satellite Systems Operations. The Phillips Laboratory was responsible for the overall program management, integration, and testing. The Jet Propulsion Laboratory was responsible for control system design. The VISS experiment has recently been tested at the Phillips Laboratory and significant closed-loop vibration isolation was achieved. This paper will describe the control approach, the performance testbed, and the initial test results.
2 Mission and Goals VISS is scheduled to fly as part of the Ballistic Missile Defense Organization’s (BMDO) Space Technology Research Vehicle (STRV) 2 in 1998. STRV-2 is an on-going collaborative effort between the BMDO and the United Kingdom Ministry of Defence to provide key space data to enhance design and risk reduction efforts for space based optical platforms. VISS will be used to isolate, suppress payload vibrations, and steer, in six degrees of freedom, an experimental mid-wavelength infrared (MWIR) sensor that is also part of STRV-2. The VISS hexapod configuration with the MWIR sensor is shown in Figure 1.
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Figure 1: VISS supporting MWIR sensor.
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x(t) m fa(t)
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Base Figure 2a: Vibration isolator with an active element.
Displacement transmissibility (dB)
Although VISS moves in six degrees of freedom, a single degree of freedom vibration isolator can be used to describe the performance objectives. Figure 2a shows a passive vibration isolator with an active element. The spring, with stiffness k, and damper, with damping constant c, act in parallel as a passive isolator with motion transmissibility between the base and the payload, m, shown by the dashed line in Figure 2b. The frequency where the transmissibility begins to roll off is known as the “bounce frequency” and is 2 1/2 equal to the damped natural frequency, ωn(1-ζ ) of the passive elements. The passive vibration isolator acts as a rigid connection between the base and the payload at frequencies below the bounce frequency. At frequencies greater than the bounce frequency, the isolator attenuates the motion of the payload relative to the base. As the bounce frequency of the passive isolator is lowered, isolation will be achieved over a wider frequency range. The frequency to which the bounce frequency can be lowered is limited by practical constraints on the compliance, 1/k, of the system. A very compliant system will be delicate and difficult to manufacture. However, the passive isolator can be made more compliant by the use of active control. Active control can effectively lower the bounce frequency of the passive isolator so that significant isolation is achieved over a broad frequency range. This effect is shown by the solid line in Figure 2b. At a frequency higher than the bounce frequency, the active control system can be allowed to roll off so that the transmissibility at higher frequencies approaches that of passive isolation alone. The passive stage significantly reduces the bandwidth requirement of the active control system and conserves power. 20 10
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Figure 2b: Displacement Transmissibility vs. Frequency.
The principal performance goal for VISS is to isolate the MWIR telescope by a minimum of 20 dB at frequencies of 5 Hz and above. The bulk of the isolation task is performed by the passive D-Struts with the active voice coil stage enhancing the system performance at lower frequencies. The voice coils, which are in parallel to the D-Struts, are designed to fail softly. Thus, in the event of failure of the active stage, the system reverts to its passive performance. Besides isolation, VISS will be used to demonstrate steering of the telescope by +/-0.3 deg with an accuracy of 0.02 deg. The steering profile required by the MWIR for this mission is composed of 2 Hz and 4 Hz Fourier components. The hybrid actuators have a linear stroke of +/-40 mils which is sufficient for the steering requirement. Finally, VISS is also used to mitigate the effects of the cryocooler vibrations on the MWIR performance. Specifically, VISS will demonstrate cryocooler vibration suppression by 20 dB for the first two cryocooler harmonics (55 Hz and 110 Hz) at the VISS/MWIR interface plate.
3 System Description Figure 3 shows a bipod configuration with two struts and a launch lock tower. VISS contains three such bipods. The voice coil and passive damper combination is located at the base of each strut. Each strut connects to a double blade flexure which simulates a universal joint. Accelerometers are located at the base and payload (MWIR) side of each strut and are colinear with the strut. In this way, the accelerometers are colocated with the voice coil actuators. The payload accelerometers are used as the feedback sensors and the base accelerometers were added to the system so that vibration isolation measurements could be made on orbit. The payload accelerometers, Allied Signal QA-3000, are an order of magnitude more sensitive (0.1 mg) than the base accelerometers, Allied Signal QA-2000 (1.0 mg). This is because, since VISS acts as an isolator, the payload will see 20 dB less motion than the base. Each strut also contains a proximity sensor so that measurements of the isolator extension and contraction could be made. The tower in the middle of the bipod contains a launch lock device so the system can be secured during launch. The overall control system architecture is shown in Figure 4. The plant consists of the six hybrid struts supporting the payload, including the voice coil actuators and payload accelerometers. The payload accelerometer for each strut is used as the feedback sensor for the voice coil on the same strut. Thus, each strut has colocated single input / single output (SISO) control. Flexible dynamics in the base, the struts, and the payload are captured in the measured plant transfer functions. The six payload acceleration signals are measured and then passed to two different signal paths. The filtering along the two different signal paths was chosen to separate the small acceleration signal due to base motion from the large acceleration signals due to the disturbance of the cryocooler and steering motion of the MWIR. If the low
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isolation, suppression and steering are combined in analog form. The output from the isolation DAC is filtered through another 5 Hz lowpass filter before the analog summation. This is necessary to smooth out the digital to analog converted isolation output signal from the low bit count. The reconstruction filter of 250 Hz is used to smooth out the suppression and steering control outputs. During ground testing, all accelerometer signals were sent through a high pass filter with a very low cutoff frequency before being sent to respective filters. This was done to remove the DC acceleration signal due to gravity. The scaling for the accelerometers was designed for micro-g operation on-orbit and so the effect of gravity without the high pass filtering would be to set the accelerometer scaled output to negative full-scale. The system sampling rate for all three control functions is 1,000 Hz. All of the acceleration signal paths go into a 16 bit analog to digital converter (ADC) with a multiplexer. The output signals are generated by a 16 bit DAC. A Texas Instruments C31 Digital Signal Processor (DSP) calculates the control inputs. The base acceleration signals are sent through the respective filters and are then sampled by the 16 bit ADC.
payload accel location blade flexure
launch lock tower
base accel
Figure 3: Bipod configuration. amplitude base signal is not separated from the higher amplitude cooler and steering signals, then the isolation controller will be ineffective. For the target problem, the expected peak acceleration of 200 mg due to the base disturbance force is 200 times (46 dB) smaller than the peak acceleration due to the cooler and steering vibration of 40 milli-g. To circumvent this problem, the accelerometer signal is first subtracted by the acceleration steering profile thereby removing the steering component and then filtered by a 5 Hz, first order low-pass filter which reduces the cooler signal component by a factor of ten. The resulting small amplitude acceleration signal is subsequently amplified and digitized before it is used to drive the isolation compensator. The acceleration signals for the suppression and steering loops are passed through the 150 Hz anti-aliasing filters. Using this strategy, the large signal and small signal ratio can be reduced by at least 20 to 26 dB. Also, to alleviate the mixed large-small signal problem at the digital to analog converter (DAC) output, the three command signals for
4 Testbed Description The performance test objective is to obtain an accurate measure of the VISS system performance in a one-g environment. The VISS hardware in flight configuration, however, is designed to operate under zero-g conditions and will not operate in a one-g field without a gravity off-load system. The gravity off-load system used for VISS is composed of two pneumatic-magnetic suspension devices made by CSA Engineering, Inc. [10] and is connected to VISS from above with thin composite rods. The devices allow a vertical suspension mode in the x direction of 0.2 Hz and pendulum modes in the y and z directions of 0.2 Hz. An engineering model of the MWIR, with mass, inertias
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Figure 4: Overall control system architecture.
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and dynamic properties similar to the flight version, was used in the performance testing. This is important in the testing of VISS because the use of accelerometers on the payload side for closed-loop feedback makes the VISS compensators sensitive to flexible modes of the payload. The flight MWIR is not expected to have any modes under 130 Hz. The base of VISS is attached to a combination of components designed to approximate the mass and inertia of the satellite. These components include a plate, a support ring, and a Newport Corporation breadboard (a version of a Newport optics bench which is 2.25 inches thick). The plate, including its attachment to the support ring, is designed to approximate the dynamics of the composite structure to which VISS is attached for STRV-2. The Newport breadboard is attached to four pneumatic supports. Disturbances are then input to the breadboard using shakers.
the six VISS payload accelerometers. Three different transfer functions are shown for each strut. The largest, marked LK, shows the response with VISS locked. VISS was locked by using threaded fasteners in the launch locks. The passive performance is shown by the transfer functions marked, OL, for open-loop. In this case, the active control was not applied and the reduction in motion is caused by the passive isolator alone. The lowest magnitude transfer functions, marked CL, were measured when all six active isolation loops were closed. As can be seen from the plots, the active control attenuated the response at lower frequencies relative to the passive response, but the active control rolled off at higher frequencies so that the closedloop performance eventually matches that of the open-loop performance. Significant attenuation at the lower frequencies was obtained, 20 dB or greater, when the system was active.
5 Isolation Control Design and Results Magnitude (dB)
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Figure 5a: Strut 1 open loop transfer function
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The isolation compensator can be characterized as a broadband linear time-invariant filter while the suppression and steering compensators are adaptive Least Mean Square (LMS) filters designed for narrowband inputs. The isolation layer is designed using classical control theory as six decoupled broadband acceleration feedback loops which provide robust stability even if some of the accelerometers fail in flight. The stability of the system is checked using the generalized Nyquist theorem [11]. At high frequencies, the performance of the active isolation system diminishes gradually to that of the passive isolator as the control gain rolls off. Two representative open-loop plots of the isolation controllers, for struts 1 and 3, are shown in Figure 5a and 5b. This data was measured by breaking the control loop at the payload accelerometer signal output and injecting random noise. The open-loop transfer functions for the struts following strut 1 were measured using the successive loop closure method. For strut 1, the low frequency and high frequency gain margins are 8 dB and 15 dB respectively. Both the low and high frequency phase margins are 45 degrees. For strut 3, the high frequency gain margin is 15 dB. The low frequency phase margin is 60 degrees and the high frequency phase margin is 42 degrees. The open-loop transfer functions for both struts were designed to have two gain crossover frequencies, therefore the phase margin for the highest gain crossover frequency should be used when determining stability [11]. All open loop transfer functions were stable with gain margins above 6 dB and phase margins greater than 30 degrees. The closed-loop response, for three representative struts, of VISS is shown in Figures 6 a, b, and c. A VTS 100 lb shaker was used to provide a random force input to the breadboard. The input location and direction was chosen so that all six degrees of freedom were excited. This data shows transfer functions taken between a 50 lb PCB load cell and
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Figure 5a: Strut 3 open loop transfer function (Struts 1, 2 closed)
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6 Conclusions
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The initial results from VISS indicate that it performs well as an isolation system in both the passive and active/passive modes. It acts as a broadband isolation system and therefore will be able to attenuate most satellite disturbances, including solar panel motion, momentum wheel vibrations, thrusters, and motion caused by other payloads on the satellite. Future work will focus on the suppression control loops for the payload cryocooler motion and the steering control loops. These will be designed using LMS control. The drive signal of the cryocooler will be sensed directly and the desired steering trajectory will be used as disturbance measurements for these control loops.
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References
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1. Cunningham, D. and Davis, P., “A Multiaxis Passive Isolation System for Magnetic Bearing Reaction Wheel,” Proceedings of the 16th Annual AAS Guidance and Control Conference, AAS 93-066, Feb 1993. 2. Havenhill, D. and Wolke, P. J., “Magnetic Suspension for Space Applications,” NASA CP-10066, March 1991. 3. Spanos, J., Rahman, Z., and Blackwood, G. “A Soft 6-axis Active Vibration Isolator,” AIAA American Control Conference, Seattle, June 1995. 4. Rahman, Z., Spanos, J., and Bayard, D., “Multi-tone Adaptive Vibration Isolation of Engineering Structures,” AIAA paper 95-1237, Proceedings of the 36th Structures, Structural Dynamics and Materials Conference, April 1995. 5. Rahman, Z. and Spanos, J., “Active Narrow-band Vibration Isolation of large Engineering Structures,” Proceedings of the 1st World Conference on Structural Control, Paper #176, Pasadena, Calif., August 1994. 6. Davis, P., Cunningham, D., and Harrell, J., “Advanced 1.5 Hz Passive Viscous Isolation System,” Proceedings of the 35th AIAA Structures, Structural Dynamics, and Materials Conference, April 1994. 7. Hyde, T. and Anderson, E. H., “Actuator with Builtin Viscous Damping for Isolation and Structural Control,” AIAA Journal, 33(1), January 1996. 8. Hyland, D. C. and Phillips, D. J., “Advances in Active Vibration Isolation Technology,” VPI&SU 10th Symposium on Structural Dynamics and Control, May, 1995. 9. Davis, P., Carter, D., and Hyde, T.T., “Second Generation Hybrid D-Strut,” Proceedings of the SPIE Smart Structures and Materials Conference, February 1995. 10. Kienholz, D.A., “Defying Gravity With Active Test Article Suspension Systems,” Journal of Sound and Vibration, April 1994. 11. Ogata, K., Modern Control Engineering, Prentice Hall Inc., Englewood Cliffs, NJ, 190.
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Figure 6c: Strut 6 disturbance response.
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