Europe's first confrontation with docking in space will require extensive testing to verify design and performance and to qualify hardware. For this purpose, a Docking Dynamics Test Facility (DDTF) was developed. It allows reproduction on the ground of the same impact loads and relative motion dynamics which would occur in space during docking. It uses a 9 degree of freedom, servo-motion system, controlled by a real time computer, which simulates the docking spacecraft in a zero-g environment. The test technique involves and active loop based on six axis force and torque detection, a mathematical simulation of individual spacecraft dynamics, and a 9 degree of freedom servomotion of which 3 DOFs allow extension of the kinematic range to 5 m. The configuration was checked out by closed loop tests involving spacecraft control models and real sensor hardware. The test facility at present has an extensive configuration that allows evaluation of both proximity control and docking systems. It provides a versatile tool to verify system design, hardware items and performance capabilities in the ongoing HERMES and COLUMBUS programs. The test system is described and its capabilities are summarized

Brondino, G.

Grimbert, D.

Marchal, PH.

Noirault, P.

English

NASA Technical Reports Server

A DYNAMIC MOTION SIMULATOR
FOR FUTURE EUROPEAN DOCKING SYSTEMS
G. Brondino and Ph. Marchal*
D. Grimbert and P. Noirault**
ABSTRACT
Europe's first confrontation with docking in space will require
extensive testing to verify design and performance and to qualify
hardware. For this purpose a Docking Dynamics Test Facility (DDTF)
has been developed by MATRA under a CNES contract. It allows re-
production on the ground of the same impact loads and relative mo-
tion dynamics which would occur in space during docking. It uses a
nine-degree-of-freedom, servo-motion system, controlled by a real-
time computer, which simulates the docking spacecraft in a zero-g
environment.
The test technique involves an active loop based on six-axis
force and torque detection, a mathematical simulation of individual
spacecraft dynamics, and a nine-degree-of-freedom servo-motion of
which three DOF's allow extension of the kinematic range to five
meters.
The configuration has been checked out by closed-loop tests
involving spacecraft control models and real sensor hardware. The
test facility at present has an extensive configuration that allows
evaluation of both proximity control and docking systems. It pro-
vides a versatile tool to verify system design, hardware items and
performance capabilities in the ongoing HERMES and COLUMBUS pro-
grams.
The paper describes the test system and summarizes its capa-
bilities.
INTRODUCTION
The HERMES spaceplane will develop on-orbit servicing capa-
bilities in Europe, which will open new horizons for European space
system designs and operations. In this context, rendezvous and
docking represents a major step in expansion of European orbital op-
erations, and is typical of HERMES mission requirements.
..........................
* Centre National d'Etudes Spatiales, Toulouse, France
** MATRA, Velizy, France
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https://ntrs.nasa.gov/search.jsp?R=19900012769 2020-03-19T23:07:10+00:00Z
Because mission success for HERMES will be dependent on the
docking function, which must be performed reliably and safely,
docking must first be verified by extensive ground tests tailored to
qualify the system hardware for flight. A research and technology
program has been sponsored by CNES at MATRA, to develop a mo-
tion-simulation facility to evaluate the relative control of closing
spacecraft, and the contact dynamics of docking systems. This test
facility is proposed for design, integration/verification and opera-
tional support in the development and operation of the HERMES
proximity and docking system. Currently, it is planned to utilize this
facility to define the requirements, to explore the technology and to
assess the dynamic performance of HERMES-baselined soft docking.
TEST FACILITY OVERVIEW
Different concepts may be considered for ground simulation of
two spacecraft docking in a zero-g environment. The concept se-
lected consists of a real-time, active loop based on a six-degree-of
freedom servo-motion with six-axis force and torque detection and
mathematical modeling of the docking spacecraft dynamics. The
docking hardware under test is physically installed in the motion-
generating device (Fig. 1).
At a high sampling frequency (125 Hz), the forces and torques
exchanged during contact of the docking interfaces are sensed and
transmitted to the computer as input data to the spacecraft dynamics
model. Compensation for gravity effects is concurrently made in the
computer. The derived relative kinematics are then transformed
into real motion.
This simulation technique is adaptive enough to accommodate
any change in the docking spacecraft dynamics to be evaluated with
a given set of docking hardware.
The docking is provided by a relative motion in all six degrees
of freedom between two structural rings representing the docking
interfaces. The motion is produced by a six-axis table, driven by six
electrical screw jacks. One of the rings is fixed in the laboratory co-
ordinate reference axes.
To represent the final approach (the last five meters), three
degrees of freedom have been added, consisting of large amplitude
translation along the docking line, and two rotations serving to simu-
late the angular misalignments about the transverse axes.
At the end of the final approach simulation, the test facility
must achieve a reference docking test configuration. Thus the three
additional degrees of freedom must be nullified, and the test system
76
must have no residual structural flexibility or backlash. During the
final approach there is no contact, so the rotations and lateral trans-
lations can be represented by moving the rendezvous sensor (two
rotations, using a two-axis, dedicated rotation device). For arresting
the translational motion, a clamping system utilizing air brakes is
employed for hardlocking the mobile mount, with emphasis on
eliminating flexibilities and backlash.
The algorithms which are used for handling the nine degrees of
freedom are based on optimization within the constraints on the
motion axes.
The facility design offers a high structural stiffness and a high
motion resolution. The geometrical configuration is optimized to
reach the best compromise between kinematic stroke and dynamic
loads in the docking hardware.
The screw jacks used on the six-axis table use precision ball
screws (3 mm per revolution) driven by stepper motors (3200 steps
per revolution). They provide a motion resolution of 1 _m and a load
capability of 1500 N at low speed (<2 cm/sec). The measured back-
lash is 50 _tm, and the maximum speed is 10 cm/sec. The load sen-
sors are mono-axis piezoelectric force transducers, selected for their
stiffness (108 N/m), their high linearity and their low noise (0.5 N
over a 1000 N range).
The test facility involves a real-time computer architecture
(Fig. 2), based on a 68020 work station for test operation and
monitoring, a 12 Mflop array processor for simulated dynamics
computation, and a GOULD 32/67 for data recording and output.
The software of the DDFT includes, in addition to the orbital
dynamics, the simulation of structural flexibilities (first combined
mode of the deployed solar panels and antenna) and liquid sloshing
(one tank in each vehicle). Plume effects are computed off line, and
their influence memorized to be used in real-time software.
In order to calibrate the facility and to verify validity of its
performance, an extensive series of open- and closed-loop runs were
made under a full range of operating conditions. The checkout
culminated in testing functional docking hardware.
During these checkout operations, some limitations on the
testing capabilities were identified:
1. The bandwidth is limited to 6 Hz, which is a limitation in the
range of spacecraft mass and docking hardware stiffness char-
acteristics that can be evaluated (Fig. 3).
2. The load and speed capabilities of the six-axis table make no
provision for accommodating "impact docking" hardware.
77
3. Vibrations associated with screw-jack activation cause degra-
dation of the contact force measurements.
4. Backlash and absolute accuracy need improvement to be more
representative of real motion.
PERFORMANCE VALIDATION
A one-axis testbed evaluation has been performed to derive
characteristics of the screw jacks and to validate the hardware and
software configurations. The measured backlash of the screw jacks is
now less than 5 t.tm.
The vibration level has been lowered, but it is still important
because of stepper motor resonance at some drive frequencies.
Closed-loop tests have been conducted to validate the computer
architecture and the management of software. The computing time
has been optimized with the sampling frequency at 125 Hz, so that
the simulator bandwidth attains 6 Hz.
A screw jack prototype based on a satellite roller screw has
been developed. It is used for the six-axis table and is characterized
by:
• Low noise level (no balls in the screw)
• Manufacturing accuracy of the screw at 10 I.tm
• No backlash
• Additional guidance of the screw jack (no collapse)
• Good motor-screw coupling
• Rotation lock for the linear part
• Stop pin for initial positioning
Limitations still exist. The most important one is the excessive
vibration level during operation, which can be effectively reduced by
substituting a DC motor/encoder assembly for the screw-jack actua-
tion.
This solution is being studied at present and will provide fur-
ther advantages (increased load and speed capabilities), so that the
DDTF will allow evaluation of impact docking dynamics involving ini-
tial velocities to 10 cm/sec, forces to 5000 N and a lower noise level.
78
Validation
The introduction of simplified approach-control laws refer-
enced to real test rendezvous sensor information is currently under-
way. This will allow validation of the closed-loop operation of the
test facility through the full range of its testing capabilities.
The Imaging Rendezvous Sensor (IRS) uses a new process mode
called Flash During Transfer (FDT), developed by MATRA, which al-
lows work under marginal lighting conditions (even with the sun in
the sensor field of view). The sensor uses a special target pattern,
composed of five optical retro-reflectors (fig. 5), illuminated by a
pulsed laser source. This equipment can measure with high accuracy
the relative attitude and position of the two bodies(three rotations
to 20 meters and three translations from 250 m to contact).
The Telemetry Rendezvous Sensor (TRS), which is used for
distances from 1000 meters to 2 meters, measures the phase differ-
ence between the emitted and the retro-reflected wave, of an ampli-
tude modulated laser source. This sensor can use the same target
pattern as the IRS.
The characteristics of the sensors are as follows:
IRS
Field of view 10 °
Attitude range +2 °
Distance range 0-250 m
Attitude resolution 0.25 °
Line of sight precision 0.05 °
Distance resolution 1%
TRS
10 °
+2 °
2-1000 m
0.25 °
0.05 °
1-3%
Pulse 100W 100mW
Duration 200 nsec
Modulation rate 1 Hz 15 MHz
Measurement rate I Hz 1-10 Hz
Target 2-3 Kg 1 Kg
0.1-1.5 m 0.1 m
Functional Synthesis:
The DDTF can perform docking tests from the last five meters
of the controlled final approach (based on real sensor measurements)
down to linkup of the docking interfaces (by use of simplified dock-
ing mechanisms) (Fig. 4).
79
Within the computer, rigid-body mass properties from 4000 kg
to 60,000 kg and inertia properties from 10 4 kg.m2 to 10 7 kg.m2 can
be simulated, providing significant margins over today's known mis-
sion needs.
The testbed bandwidth is about 6 Hz, so that the test hardware
stiffness may vary between 103 N/m and 3x106 N/m in translation,
and 104 Nm/rad and 3x106 Nm/rad in rotation.
The attainable performance is subject to the testbed operating
capabilities as follows:
Final Approach (emphasis on motion)
Position Range
Velocity Range
Resolution
Accuracy
Acceleration range
Translation
5cm inside a 10 °
conical envelope
axial 10 cm/sec
radial 2 cm/sec
0.1 mm
1 mm
5.10 .3 m/sec 2
Rotation
+5 °
5°/sec
0.001 o
0.01 °
1 0 -s rad/sec 2
Docking (emphasis on contact loads)
Position Range
Velocity Range
Resolution
Accuracy
Acceleration range
Load range
Tran slati on Rotation
+7.5 cm +2.5
1.5 cm/sec axial
0.5 cm/sec radial 0.2°/sec
1 I.tm
1 mm
0.1 m/sec 2 10 -2 rad/sec 2
1.500 N axial
900N radial 500 Nm
The basic architecture of the testbed facility is shown in fig. 5,
with hardware items separated from computer software. A comput-
erized graphic display allows visualization in real time of the indi-
vidual spacecraft response motion during a test run, in either space-
craft or orbital coordinate reference axes. In addition, provision is
included in the facility to accommodate enhanced spacecraft dynamic
models and pilot interfaces so that manual proximity control and
docking can be evaluated.
80
CONCLUDING REMARKS
A dynamic testing capability for docking systems has been described.
The technology, based on current off-the-shelf components, has been
optimized to eliminate backlash and to minimize vibrations (which
produce the most significant limitations on the testing performance).
Computations and computers in the control loop incorporate complete
motion generation and spacecraft dynamics simulation, so that the
facility acquires a potential for implementing both spacecraft
proximity control and docking systems (sequentially or
independently) without limitation on the testing requirements.
This unique concept provides a versatile test tool to validate
system design, to demonstrate hardware items, and to verify dy-
namic performance capability and reliability.
Currently, application of this facility is being planned for test-
ing HERMES docking model hardware in a joint CNES-ESA program,
and evaluating the required manual control performance for docking.
Because of use in HERMES missions of the FREEDOM space station
docking system, this testing is primarily intended to evaluate the
design revisions of the system in order to accommodate the HERMES
soft-docking requirements.
_CE
G. Brondino and Ph. Marchal, Centre National d'Etudes Spatiales,
Toulouse, France; D. Grimbert and P. Noirault, MATRA, Velizy, France,
"DDTF Improvements for More Accurate Space Docking Simulation",
EIOOTS, September 1989.
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A dynamic motion simulator for future European docking systems

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