MIDDECK ACTIVE CONTROL EXPERIMENT (MACE):
SPACE FOR TECHNOLOGY RESEARCH AND DEVELOPMENT
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
as it appears in the AIAA Journal
The Middeck Active Control
Experiment (MACE) is a United States Space Shuttle flight experiment launched
STS-67, (press photos),
on March 2nd, 1995.
MACE (Figure 1a) was designed
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,
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.
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
HISTORY OF CONTROL - STRUCTURES
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
- Problems with spacecraft
flexibility started as early as the first U. S. satellite, Explorer I (1958).
Unexpected energy dissipation in the flexibility of the four whip antennas
on the spin stabilized satellite caused it to tumble.
- Attitude oscillation
caused by the control system interacting with boom and solar panel flexibility
occurred on OGO III (1966), OVI-10 (1966), DMSP (1972), and Mariner 10 (1973)
which was almost lost.
- Thermal warping and
snapping proved a major source of agitation in Alouette I (1962), Explorer
XX (1964), OGO IV (1966), Voyager (1977), and Landsat (1982, 1984).
- Unstable interaction
between the control system and liquid propellant slosh modes occurred on Leasat
(1984). Flexibility in the docking unit between the Apollo Command Module
and the Lunar Module was modeled in order to ensure that the gimbaled thruster
would not cause instability during trans-lunar injection.
- Approximately half the
operational time of the shuttle Remote Manipulator System (RMS) is spent waiting
for flexible motion to decay. Finally, pogo, which is an unstable interaction
between thrust and compressibility in the propellant system, has plagued many
- structural analysts
need more accurate/less computationally intensive models.
- low-gravity test capability
- Multibody testing is
- Testing needs to done
in a closed-loop fashion.
- the capability to experiment
in space and qualify hardware and control techniques is crucial.
MACE has responded to these recommendations.
CHALLENGE OF CONTROLLED
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.
- A system identification
will be performed during the first phase to obtain time response data which
will be downlinked over the Ku-Band system. The data will be used to assess
the accuracy of the FEM predictions and to develop 0-g measurement models.
- Numerous controllers
that have been designed prior to flight using the 0-g FEM will be implemented
during the second phase. Also, during this period of the mission (approximately
72 hours), new 0-g models and robust controllers will be developed on the
- These compensators will
then be uplinked and implemented during the last phase of the experiment.
CONTRIBUTIONS OF THE MACE
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
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
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
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,
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.
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,
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
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,
Dr. Jonathan P. How, Durand Building, Room 023a, Stanford University,
Stanford, CA 94305-4035 EMAIL. email@example.com
Dr. David W. Miller, co-Principal Investigator. M.I.T. Room 37-371,
Cambridge, MA 02139, EMAIL. firstname.lastname@example.org
Dr. Javier de
Luis, Payload Systems Incorporated, 270 Third St.,
Cambridge, MA 02142, EMAIL. email@example.comSubcontractor.
Mr. Gregory Stover,
Technical Monitor. MS 433 NASA Langley Research Center,
Hampton, VA 23681, EMAIL. firstname.lastname@example.org