An overview is given of the work being done at NASA LaRC on developing the Control of Flexible Structures (COFS) 1 Flight Experiment Baseline Control Law. This control law currently evolving to a generic control system software package designed to supply many, but not all, guest investigators. A system simulator is also described. It is currently being developed for COFS-1 and will be used to develop the Baseline Control Law and to evaluate guest investigator control schemes. It will be available for use whether or not control schemes fall into the category of the Baseline Control Law. First, the hardware configuration for control experiments is described. This is followed by a description of the simulation software. Open-loop sinusoid excitation time histories are next presented both with and without a local controller for the Linear DC Motor (LDCM) actuators currently planned for the flight. The generic control law follows and algorithm processing requirements are cited for a nominal case of interest. Finally, a closed-loop simulation study is presented, and the state of the work is summarized in the concluding remarks

Lindner, D. K.

Montgomery, R. C.

Sulla, Jeff

English

NASA Technical Reports Server

N87-16043
DISTRIBUTED CONTROL FOR COFS I
R. C. Montgomery
NASA Langley Research Center
Jeff Sulla
PRC Kentron
D. K. Lindner
VPI & SU
First NASA/DOD CSl Technology Conference
Norfolk, Virginia
November 18-21, 1986
457
https://ntrs.nasa.gov/search.jsp?R=19870006610 2020-03-20T12:04:30+00:00Z
" d
PRESENTATION OUTLINE
This paper presents an overview of the work being done at NASA LaRC on
developing the COFS-I Flight Experiment Baseline Control Law. This control law is
currently evolving to a generic control system software package designed to support
many, but not all, guest investigators. A system simulator is also described. It
is currently being developed for COFS-I and will be used to develop the Baseline
Control Law and to evaluate guest investigator control schemes. It will be availa-
ble for use whether or not control schemes fall into the category of the Baseline
Control Law.
First, the hardware configuration for control experiments will be described.
This is followed by a description of the simulation software. Open-loop sinusoidal
excitation time histories are next presented both with and without a local control-
ler for the Linear DC Motor (LDCM) actuators currently planned for the flight. The
generic control law follows and algorithm processing requirements are cited for a
nominal case of interest. Finally, a closed-loop simulation study is presented, and
the state of the work is summarized in the concluding remarks.
INTRODUCTION
HARDWARE CONFIGURATION FOR CONTROL
COFS I REAL-TIME SIMULATOR
OPEN-LOOP EXCITATION OF MAST
USING THE TIP ACTUATORS
WITH/WITHOUT LOCAL CONTROLLER
GENERIC LINEAR CONTROL ALGORITHM
ALGORITHM PROCESSING REQUIREMENTS
CLOSED-LOOP SIMULATION STUDIES
WITH COLLOCATED RATE FEEDBACK
CONCLUDING REMARKS
Figure I
458
COFS I eF-FE_NCE LOCATIONS
For control experiment purposes, the COFS-I hardware consists of the space
shuttle, a 60-meter statically determinant truss beam, and a tip assembly. The
configuration of the experiment is shown in figure 2. Only the hardware of interest
in a control system experiment is listed. The truss beam, called the MAST, has 54
sections called bays. The bays are numbered outward from the shuttle, bay I being
nearest.
TIP LOCATION
BAY 54-- L_
44 _
38 ._
30 -
24 _.
12 --r
J
BAY1__ |
STEP PALLET_
SPACE SHUTTLE
Figure 2
459
CONTROL SYSTEM COMPONENTS
Control law processing will be done with a digital computer located on the STEP
pallet. Sensors are provided at the tip, at the STEP pallet and at five interior
MAST stations - bays 12, 24, 30, 38, and 44. The types of sensor equipment pro-
vided are rate gyros and linear and angular accelerometers. The only type of actua-
tor provided is the LDCM actuator. LDCM's are located at the tip and at three
interior MAST stations. The LDCM's at the interior stations are smaller than the
ones at the tip but are of the same type.
TIP LOCATION
BAYS 44, 30,
AND 12
BAYS 38
AND 24
STEP PALLET
3 RATE GYROS
3 LINEAR ACCELEROMETERS
4 LARGE LDCM ACTUATORS
2 LINEAR ACCELEROMETERS
1 ANGULAR ACCELERMETER
2 SMALL LDCM ACTUATORS
3 LINEAR ACCELEROMETERS
1 ANGULAR ACCELEROMETER
3 LINEAR ACCELEROMETERS
3 RATE GYROS
CONTROL SYSTEM COMPUTER
150 HZ UPDATE RATE
Figure 3
460
FLIGHT _OMPI_ER SYSTEM
The flight computer is located on the STEP pallet and communicates with it
through a serial input/output data channel. The STEP pallet provides power for the
MAST flight systems and also data recording. High-speed data transmission is
available from the MAST flight system to the STEP pallet. All communications from
the STEP pallet to the MAST flight system are through the Modular Distributed Infor-
mation Subsystem, MDIS, which functions to send commands to and receive data from
the Actuator/Sensor network via the MIL-STD 1553B data bus. The MDIS also inter-
faces with the Excitation and Damping Subsystem, EDS, which processes the control
algorithms.
ACTUATOR_NSOR POWER
COMPUTERPOWER
EOS
COMPUTER
MIL-STD 1SUB
MOIS
POWER
MI)IS
(1551 CONTROLLER)
SENSOR
IIIETWORK
(7 STATIONS)
(MA T)
J CARRIER
DOS
POWER
SlO
COMMAND/CONTROLAND DATA),. | ]
STEP iHIGH RATEDATA CARRIER
Figure 4
461
ALGORITHM TIMING REQUIREMENTS
The sample rate of the system is 150 samples/sec. This allows 6667 micro-
seconds between samples. Because of projected system overheads and other necessary
functions, only 5033 microseconds can be counted on for algorithm processing.
_//_ t(k)834excilalJon_ustransfe_ to actuators
8C: t(k) sample of sensors; apply I(k) excitaiJon
I(k) sample .easier te EOS and MOiS
776 _s
t(k) sample t,ensfer te MOlS690 _s
,(k)
•",ocm_k)u,_,,
5033 _ rain at I SO Hz
i
t_
_1
1
-I
Process I/2 el t(k) coefficients; k even
Overhand processing
• 800 _s rain
l(k + 1) ezitalie.Uen_ef to KhmtOm
l o(k+ i1__i_
Figure 5
462
LIX:M ACTUATOR
The LDCM actuators are currently being designed, thus, only a shetch is
available. The Stator contains electrical wire windings and is hard mounted to the
MAST tip. The Moving Mass contains permanent magnets and can traverse finite limits
relative to the Stator. Relative force is generated between the Stator and a Moving
Mass by passing electrical current through windings of the Stator, thus, moving both
the Stator and the Moving Mass.
Because of travel limits, the Moving Mass is not able to store energy as would
more conventional devices such as torque wheels or CMG's. Its position must at all
times be a foremost consideration of the control algorithm since bi-dlrectlonal
control is lost at extremes. This complication is in addition to saturation which
is normally considered by control analysts. At high frequencies, saturation limits
the force output, but at low frequencies, force commands must be limited because of
travel limits. A local controller can be designed whose frequency response is
shaped according to these limits at frequencies of interest. This will somewhat
alleviate the problem, but does not allow the control algorithm to ignore position
management of the Moving Mass.
STATIONARY
'MOVING MASS
Figure 6
463
LOCAL ODNTROLLER
Because of the problems cited concerning the use of the LDCM's, a local
controller is being considered. The control computer sends commands to this
controller which, based on sensor inputs from a Stator-mounted accelerometer and a
sensor that measures relative position of the Moving Mass to the Stator, generates a
current signal for the Stator windings.
The schematic below describes one scheme currently being considered for the
local controller. Current command from the local controller is converted into a
relative force, F, through a linear gain. Any nonlinearities in the conversion are
modelled by f. The relative force is applied to the moving mass and in the opposite
direction to the structure through transfer function Q(s). 0(s) includes the
dynamics of the space shuttle, MAST, and Stator. The accelerometers mounted to the
Stator respond to the MAST motion with transfer function A(s). The accelerometer
output with error is twice integrated in the local controller to generate an
estimate of the position of the Stator. Each integration is accomplished by
transfer function l(s) which is stabilized. The sum of the estimated Stator and
relative position signals is an estimate of the inertial position of the moving
mass. Thus, the primary feedback of the local controller is the estimated inertial
position of the moving mass.
LOCAL CONTROLLER STRUCTURE/ACTUATOR
Command _ mO 2(s+k) 2
Kl(S+O) 2
I f
,
l ea E
• I °_° T
Figure 7
464
NODEL AND SINULATOR
Ground tests for COFS will involve the MAST flight system, a 20-meter truss
beam of similar construction called MINI-MAST, and a pendulum suspended shuttle,
truss beam, and antenna apparatus called SCOLE (in support of COFS II). The ground
test activities are expected to play an important role in evaluation of the guest
investigator control laws for flight. A simulator has been developed for the MAST
flight system and is under development for the MINI-MAST and ground test systems.
For the flight system, the physical apparatus -- the space shuttle, the MAST, sensor
and actuators were modelled using finite element analysis techniques. The output of
the finite element analysis is a set of mode shapes and frequencies that are used by
the system real-tlme simulator resident on the LaRC CYBER 175 simulation system.
The control computer is functionally simulated assuming an update rate of 150 Hz.
The detailed bit manipulation, computation, and communications required by the
flight system are not simulated. The number of modes used in a simulation is a
variable and is typically 15 (including the 6 rlgld-body modes of the system).
Physical Apparatus t
X
MAST, MINI-MAST, SCOLE
f
Topology, _ Finite Element
Element Characteristics i Model
I (EAL)
Output
Graphical/Numerical
Time Histories,
Control System Design
and Analysis Parameters
(STOLE)
Mode Shapes, Frequencies
Simulator
StructuralDynamics,
Actuator/Sensor Dynamics,
Control Laws
T
Initialization
Open-Loop Inputs
Figure 8
465
EFFECTS OF LOCAL _ONTROLLER
FIRST BENDING FREQUENCY
Figure 9 is time histories of the MAST flight system in response to sinusoldal
input commands to the LDCM's to excite the first bending mode of the MAST in the
direction normal to the plane of symmetry of the space shuttle. On the left are the
responses using direct current command to the LDCM's. This set illustrates one
fundamental problem -- that the relative position of the Moving Mass continually
drifts to one side. When the same simulation is conducted with the local controller
engaged, the result is shown on the right. Here, the local controller corrects the
drift and produces a zero steady-state response.
OPEN- LOOP CLOSED- LOOP
o/
8-5oL-
A
f 10
L , I I , I
0 5 10 0 5 10
TIME, SEC TIME, SEC
Figure 9
466
CONTROL EXPERIMENT SOFTWARE
The Baseline Control Law software package is envisioned to contain three sub-
systems. One provides excitation and disturbances, one is the control algorithm,
and the last is performance evaluation.
EXCITATION AND
DISTURBANCE INJECTION SUBSYSTEM
CONTROL ALGORITHM SUBSYSTEM
PERFORMANCE EVALUATION SUBSYSTEM
Figure I0
467
EXCITATIONANDDISTURBANCE INJECTION SUBSYSTEM
The excitation and disturbance injection subsystem will be able to generate
sinusoidal inputs at a single frequency or inputs that are a finite sum of several
weighted sinusoidal signals (multi-mode sinusoidal). Other input types include
amplitude-limited random signals. These inputs may be used for excitation in an
open-loop manner or may be used during a control law test to provide disturbances.
A necessary feature of the system is command limiting. This will be done to
avoid exceeding load and deflection limits on the MAST and relative position limits
on the LDCM's.
SINGLE & MULTI-MODE SINUSOIDAL
AMPLITUDE-LIMITED RANDOM
COMMAND LIMITING
DEFLECTION AND LOAD MONITORING
Figure II
468
CONTROL ALGORITHM SUBSYSTEM
The control algorithm will include variable order sensor prefilters that are of
the ARMA form wherein the user provides the filter constants. These are scalar
digital filters for each sensor signal used. A modest form of fault detection will
be used to insure that the data passed along in the control algorithm are valid.
This will include reasonability tests based on past sensor data. The primary
control algorithm is linear. It is based on a state estimator followed by a control
command calculation. Efficient software will be developed to take advantage of
sparse matrices. The matrices in this subsystem are user supplied. The output from
the control algorithm subsection is passed to scalar compensators. The compensators
are also of variable order ARMA form with the coefficients user supplied. The
output of the compensators is command limited prior to being sent to the MDIS.
SENSOR PREFILTERS
SCALE FACTOR & BIAS CORRECTIONS
ARMA FILTER FORM
FAULT DETECTION
LINEAR ESTIMATION & CONTROL ALGORITHM
ESTIMATOR:
CONTROLLER:
Xk+l -- Axk+Bk+CYk
Uk+l -- Dxk+Ek+FYk
ACTUATOR COMPENSATORS
SCALE FACTOR & BIAS CORRECTIONS
ARMA COMPENSATOR FORM
COMMAND LIMITING
Figure 12
469
PERFORMANCE EVALUATION SUBSYSTEM
Measures of control law performance will be time histories of the sensor
outputs (raw and filtered), state estimates, control commands (raw and compensated),
and commands transmitted to the MDIS. This will be augmented by RMS calculations
and on-line estimates of frequency and damping of selected scalar signals.
RECORDS & RMS OF
SENSOR OUTPUTS
ESTIMATED VARIABLES
CONTROL COMMANDS
ON-LINE ESTIMATES OF
FREQUENCY & DAMPING FOR
SELECTED SCALAR SIGNALS
Figure 13
470
ALGORITHM PROCESSING REQUIREMENTS
To get a measure of the demands the control algorithm makes on the EDS
computer, calculation of the number of floation-point multiply and add operations
required for a specific case has been made. The assumptions of the algorithm are
that all data are processed 150 times per second. This may not be required. For
example, in a modal controller, the lower frequency mode calculations may not
require complete updating at 150 tlmes/sec. The sensor outputs are assumed to be
filtered by a 6th order ARMA filter. Twelve sensors are assumed. The six rigid-
body and eight of the flexible modes are included in the control algorithm. This
produces fully populated matrices with a state dimension of 28. Each scalar output
of the linear control algorithm is passed to a 4th-order, ARMA form compensator.
The time frame used in the calculation taken from a previous chart is 5033 mlcro-
seconds and 20 percent overhead is used to represent operating system and logic
required to process the control law. To process this algorithm under the assump-
tions discussed, the EDS processor must be capable of performing a multiply and add
operation in 1.2 microseconds. If only the eight flexible modes are used, the
processing time requirement reduces to 2 microseconds.
ALGORITHM ASSUMPTIONS:
150 SAMPLES/SECOND
12 SENSORS 6th-ORDER FILTERS
6 RIGID-BODY & 8 FLEXIBLE MODES
9 ACTUATORS -- 4th-ORDER COMPENSATORS
SYSTEM ASSUMPTIONS:
TIME FRAME -- 5033 #SEC
20% OVERHEAD -- LOGIC & SYSTEM
SPEED REQUIREMENT -- ptSEC/MAD:
(MAD -- MULTIPLY & ADD OPERATION)
1.2 #SEC/MAD
2.0 #SEC/MAD FLEXIBLE MODES ONLY
Figure 14
471
COLLOCATED ILATE FEEDBACK
CLOSED-LOOP
One obvious control law to process with the algorithm is collocated rate
feedback using estimated velocity of the Stator. Figure 15 shows a simulation run
sending direct current commands to the LDCM's, i.e., no local controller. The left
set of charts is the excitation phase of the simulation and the right is the
controlled phase. The applied force was generated by open-loop slnusoidal force
commands at the frequency of the first flexible mode. The excitation phase lasted
I0 seconds. No attempt was made to limit or monitor motion of the Moving Mass.
This would be required for the flight software. At the end of the lO-second
excitation phase, a collocated rate feedback control law assuming perfect estimation
was engaged. The collocated rate feedback gains were precalculated to produce 5
percent damping in the mode excited. The 15 lowest frequency modes of the MAST
flight system were included in the simulation. The design damping was indeed
obtained; however, during the experiment approximately 1.2 meters of relative motion
of the LDCM's was observed. It is indeed unfortunate that their travel is projected
to be only 20 CM. A study using the local controller is in the process of being
made but is not available at this time. Hopefully, the local controller will
produce similar results and not exceed the travel limits of the LDCM's.
EXCITATION CONTROLLED
o o
o
1 2
_0
_ 0 _ 0
i I I i i
0 5 i0 i0 15 20
TIME, SEC TIME, SEC
Figure 15
472
CONCLUDING REMARKS
This paper has proposed a Baseline Control Law software package that will,
hopefully, support the research of many guest investigators. Elements of the
package are an Excitation and Disturbance Injection Subsystem, a Linear Control
Algorithm Sybsystem, and a Performance Evaluation Subsystem. Definition and
evaluation of the Baseline is in progress, but it must be preceeded by a precise
definition of the LDCM controllers. A real-time simulation of the MAST flight
system is currently available at LaRC and will be used with hardware in the loop to
evaluate the Baseline and software for potential guest investigators. The hardware
to be used in the simulation are actuators similar to LDCM's save that they use
conventional rotary DC motors to accelerate the Moving Mass. These are being
developed in-house at LaRC and are expected to be available for use between January
and March of 1987.
CONTROL EXPERIMENTS SOFTWARE
PROPOSED FOR COFS I
EXCITATION
GENERIC LINEAR CONTROL ALGORITHM
PERFORMANCE EVALUATION
DEFINITION AND EVALUATION IN-PROGRESS
DEPENDS ON DEFINITION OF
ACTUATOR WITH LOCAL CONTROLLER
REAL-TIME SIMULATION READY
TO EVALUATE VARIOUS LINEAR
CONTROL SCHEMES
HARDWARE IN LOOP SCHEDULED
1st QTR 1987
Figure 16
473


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