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MIT MIDDECK ACTIVE CONTROL EXPERIMENT (MACE):
USING SPACE FOR TECHNOLOGY RESEARCH AND DEVELOPMENT

Recent Papers:

Mark E. Campbell, Jonathan P. How, Simon C. O. Grocott, David W. Miller
"On-orbit Closed-loop Control Results for MACE"
as it appears in the AIAA JGCD Journal.

David W. Miller, Jonathan P. How, Mark E. Campbell, Simon Grocott, Ketao Liu,
Roger M. Glaese, and Timothy Tuttle,
"Flight Results from the Middeck Active Control Experiment (MACE)",
as it appears in the AIAA Journal

IEEE-CST

Summary

The Middeck Active Control Experiment (MACE) is a United States Space Shuttle flight experiment launched on STS-67, (press photos), on March 2nd, 1995.

MACE (Figure 1a) was designed by the Space Engineering Research Center at the Massachusetts Institute of Technology, in collaboration with Payload Systems Incorporated, the NASA Langley Research Center, and Lockheed Missiles and Space Company. The goal is to explore approaches to achieving high precision pointing and vibration control of future spacecraft and satellites. In particular, MACE extends the bandwidth of conventional rigid body instrument pointing and attitude controllers to include the flexible modes of the satellite. Since the success of such flexible control is intimately dependent upon the accuracy of the spacecraft model used for control design, MACE is essentially a spacecraft modeling validation effort where success is determined by the control performance and predictability that is achieved in earth orbit. MACE builds upon the concept of the Middeck 0-Gravity Dynamics Experiment (MODE) , which flew on STS-40, STS-48, and STS-62 as a dynamics test facility to characterize fluid, Space Station structure, and crew motion dynamics in zero-gravity. MACE augments the MODE facility with real-time, digital control capabilities.

 

MACE Hardware

Figure 1a is a photo of the flight test article suspended for 1-g testing. Two instruments containing rate gyros are mounted to either end of the flexible structural bus using two-axis, direct drive gimbals. A three axis reaction wheel assembly (RWA) and a two axis piezoelectric bending strut are located near the center. Each strut is instrumented with strain gauges while each gimbal axis contains an angle encoder. A power and data umbilical, extending from the RWA to the bottom right corner of the photo, connects the actuators and sensors to the Experiment Support Module (ESM) shown in Figure 1b. The ESM contains all experiment sequencing, real-time control, signal conditioning, power amplification, data storage, and crew interface functions in a standard middeck locker. The real-time control supports 20 sensors, 9 actuators, and 80 state compensators at a control rate of 500 Hz. The control objective is to maintain the inertial pointing of one instrument while the other is undergoing either broad or narrowband excitation and slew maneuvers. Since, in the past, control system performance has been limited by the flexibility in the system (due to a phenomenon known as control - structures interaction), MACE represents a space flight validation of new technology which has the potential for revolutionizing the performance of space-based systems which cannot afford the massive, rigidizing support structure that would otherwise be required. This translates into earth-observing instruments with dramatically improved imaging capabilities for monitoring earth resources, pollution, ozone depletion and meteorological and oceanographic patterns. This technology also benefits astronomical instruments envisioned to detect objects at the edge of the universe and locate planets around other stars. More down to earth, this technology is already finding application in aircraft gust alleviation, computer disk drive head vibration suppression, noise control, sensitive instrument isolation, and precision machining. However, acceptance in the spacecraft community requires a comprehensive flight validation of this technology. Hence, the MACE program is designed to provide this flight validation.

HISTORY OF CONTROL - STRUCTURES INTERACTION

Control - Structures Interaction (CSI) occurs when control detrimentally interacts with flexibility in the system. Such interaction is caused by mis-modelling or lack of consideration for flexibility. The U.S. Space Program has a history of problems related to CSI, which have ranged from degrading spacecraft performance to causing catastrophic loss of the system: The contemporary solution to the CSI problem has been to analyze the system and limit the performance or bandwidth of the control to not include this flexibility. Therefore, flexibility has placed a limit on the performance of systems, particularly in space where rigidizing structure is obtained at high launch cost. Therefore, any excitation of the flexibility, due to thermal snapping of solar arrays, bearing noise or imbalances in reaction wheels, or scanning and slewing of instruments and manipulators, directly degrades performance. This was the case for many recent spacecraft such as Hubble, UARS, and the Shuttle RMS. The MACE program explores Controlled Structures Technology (CST) as a means for controlling rather than avoiding flexibility in space systems, thereby penetrating this artificial performance barrier. An extensive survey of historical occurrences of CSI (The Batelle Report, March 1989) culminated in the following recommendations:

MACE has responded to these recommendations.

CHALLENGE OF CONTROLLED STRUCTURES TECHNOLOGY

Many CSI problems are not identified until after the system has been placed in operation. At this point, there is little that can be done to alleviate the problem. The concept of Controlled Structures Technology (CST) is to explicitly consider and control the structural flexibility in the system and to do so at an early stage in the design process when many more solutions are available. CST represents a marriage between high fidelity dynamic modeling and robust multivariable control system design. The more accurate the control design model, the more performance that can be achieved. The more robust the control, the larger the inaccuracies in the model that can be tolerated by the control in achieving improved performance. The challenge is to strike the appropriate balance between model refinement and robust control, particularly for a system which can only be tested in an environment (ground) other than that in which it will operate (space).

MACE OBJECTIVE AND APPROACH

The objective of MACE is to act as a pathfinder for a qualification procedure for flexible, precision-controlled spacecraft. This procedure will increase confidence in the eventual orbital performance of future spacecraft that cannot be dynamically tested on the ground in a sufficiently realistic zero-gravity simulation.

In the overall approach, illustrated in Figure 2 (click on a box to go to that subject), both finite element and measurement modeling techniques have been investigated to determine the advantages and limitations of each. A 1-g finite element model was developed which includes gravity and suspension effects. The accuracy of the FEM is improved through modal identification and model updating (Step A). Closed-loop updating (Step B) is performed because this improves the model from the perspective of control. Often, small errors in the open-loop dynamics can lead to large discrepancies between the experimental and predicted closed-loop behavior.

A similar process is performed using generally more accurate 1-g measurement models which are obtained by fitting a state space system to the transfer functions measured through the control hardware. These measurement models tend to provide accurate estimates of the modal parameters of the test article, which can be used to further update the physical parameters in the 1-g FEM (Step C). Controllers are also designed based on the measurement models and, by comparing the performance obtained with that achieved using the finite element based controllers, the designer can understand the cost-benefit of further FEM refinement (Step D).

One key advantage of a finite element model is that it is developed using analytic techniques, and thus can be used to predict the on-orbit system behavior (Step E). Note that the finite element updates are performed on the physical parameters of the model, which enables an explicit removal of gravity and suspension effects. This approach is in contrast to updating a particular 1-g state space model which implicitly contains the gravity and suspension effects. Of course, one would expect a variety of errors to still remain in the finite element model predictions for 0-g, and thus the need for robust control.

The activities listed in the bottom half of the approach figure occur during the mission. The 16 day Shuttle Endeavor STS-67 mission will contain six MACE operation days split into three main phases:

A comparison with the results of the preprogrammed controllers will enable an assessment of the accuracy of 0-g FEM predictions as they pertain to precision control. The control redesign based on the measurement models (Step F) will help identify the limitations of predicting 0-g closed-loop behavior from analysis and ground testing and the performance benefits that can be realized through on-orbit identification and control redesign. This discussion demonstrates the level of interaction between model development and control design. In the process, it also illustrates the need for an efficient control design methodology.

CONTRIBUTIONS OF THE MACE PROGRAM

Space Systems

The performance achieved by controlling the flexibility in the system is compared to standard industry practice where instrument pointing servo control is closed with a bandwidth roughly equal to one tenth of the frequency of the first flexible mode. The MACE program extends this bandwidth, and therefore performance, in two steps. First, the bandwidth of the instrument pointing servos are increased while maintaining 6 db of gain margin and 30 degrees of phase margin. Increasing the bandwidth to just above the frequency of the first flexible mode results in an order of magnitude improvement in inertial payload pointing. Second, dynamic CST compensation is closed around the extended bandwidth servo control to achieve an additional order of magnitude improvement in inertial payload pointing. The measured 1-g performance of this second layer of control is shown in Figure 3 by comparing the open and closed-loop performance auto-spectra. In all, an approximate 40 db improvement in inertial payload pointing has been achieved in 1-g tests over that obtained through standard industry practice.

The extended bandwidth servos, along with the CST control, are achieved at a cost: increased sensitivity to modeling errors and time varying dynamics. However, the MACE program strives to minimize the resulting impact on performance through two operational means. First, extensive ground testing is combined with analysis to derive accurate models of 0-g behavior and develop models of the residual (deterministic) errors along with the remaining uncertainty (stochastic) bounds. Second, the MACE program realizes that there is no substitute for test data in the actual operational environment to aid in maximizing closed-loop performance. Therefore, a comprehensive identification and control redesign will be performed during the STS-67 mission. Such on-orbit redesign could be used for future spacecraft to aid in maximizing performance, working around unexpected problems, and adapting to changing environmental conditions.

CONCLUSIONS

MIT SERC has been very careful, through the MACE program, to conduct a comprehensive and unbiased evaluation of the control community's leading model development and robust control formulations as applied to the control of precision spacecraft. In doing so, SERC has developed model development and control design procedures which incorporate the best attributes of several of the leading techniques and has made the computational algorithms more mature and efficient due to the needs of the MACE hardware. In addition, MACE has developed not only an experiment but also a 0-g flight test facility for Shuttle and Space Station capable of conducting dynamics and control experiments on a diverse variety of test articles in the micro-gravity environment of earth orbit.

REFERENCES

Miller, D.W., Deluis, J., Stover, G., How, J.P., Liu, K., Grocott, S.C.O, Campbell, M., Glaese, R., and Crawley, E.F., "The Middeck Active Control Experiment (MACE) Using Space for Technology Research and Development," to be presented at the 1995 Ammerican Control Conference.
Liu, K. and Miller, D. W., "Identification of Structure Systems Using the ORSE Identification Technique," accepted to the ASME Journal of Dynamic Systems, Measurement, and Control, April, 1994.
How, J., Glaese, R., Grocott, S., and Miller, D., "Finite Element Model Based Robust Controllers for the Middeck Active Control Experiment (MACE)," presented at the 1994 American Control Conference, Baltimore, MD, June, 1994, pp. 272-277.
How, J. P. and Miller, D. W., "Assessment of Modelling and Robust Control Techniques for Future Spacecraft: Middeck Active Control Experiment," AAS Guidance Navigation and Control Conference, (editors) R.D. Culp and R.D. Rausch, Vol 86, Feb, 1994, pp. 395-414. Abstract
Grocott, S., How, J., Miller, D., MacMartin, D. and Liu, K., "Robust Control Implementation on the Middeck Active Control Experiment (MACE)," AIAA Journal of Guidance, Control, and Dynamics, Nov.-Dec., Vol. 17, No. 6, pp. 1163-1170.
Grocott, S. C. O., How, J. P., and Miller, D. W., "A Comparison of Robust Control Techniques for Uncertain Structural Systems," presented at the AIAA Guidance, Navigation and Control Conference, Scottsdale, AZ, August 1-3, 1994.
How, J.P., Hall, S.R., and Haddad, W.M. ``Robust Controllers for the Middeck Active Control Experiment using Popov Controller Synthesis,'' IEEE Trans. on Control System Technology Vol. 2, No. 2, June, 1994, pp. 73-87.

To request more information, please contact:

Dr. Jonathan P. How, Durand Building, Room 023a, Stanford University,
Stanford, CA 94305-4035 EMAIL. howjo@sun-valley.stanford.edu

Dr. David W. Miller, co-Principal Investigator. M.I.T. Room 37-371,
Cambridge, MA 02139, EMAIL. millerd@mit.edu

Dr. Javier de Luis, Payload Systems Incorporated, 270 Third St.,
Cambridge, MA 02142, EMAIL. deluis@payload.comSubcontractor.

Mr. Gregory Stover, Technical Monitor. MS 433 NASA Langley Research Center,
Hampton, VA 23681, EMAIL. g.stover@larc.nasa.gov