The development of a docking system for aerospace vehicles has identified the need for reusable and variably controlled attenuators/actuators for energy absorption and compliance. One approach to providing both the attenuator and the actuator functions is by way of an electromechanical attenuator/actuator (EMAA) as opposed to a hydraulic system. The use of the electromechanical devices is considered to be more suitable for a space environment because of the absence of contamination from hydraulic fluid leaks and because of the cost effectiveness of maintenance. A smart EMAA that uses range/rate/attitude sensor information to preadjust a docking interface to eliminate misalignments and to minimize contact and stroking forces is described. A prototype EMAA was fabricated and is being tested and evaluated. Results of preliminary testing and analysis already performed have established confidence that this concept is feasible and will provide the desired reliability and low maintenance for repetitive long term operation typical of Space Station requirements

Carroll, Monty B.

Glenn, Dean

Stokes, Lebarian

English

NASA Technical Reports Server

N87-29878
AN ELECTROMECHANICAL ATTENUATOR/ACTUATOR FOR SPACE STATION DOCKING
LeBarian Stokes,* Dean Glenn,* Monty B. Carroll**
ABSTRACT
The development of a docking system for the Space Station and beyond
has identified the need for reusable and variably controlled attenuators/
actuators for energy absorption and compliance. One approach to providing
both the attenuator and the actuator functions is by way of an electro-
mechanical attenuator/actuator (EMAA) as opposed to a hydraulic system.
The use of the electromechanicai devices is considered to be more suitable
for a space environment because of the absence of contamination from
hydraulic fluid leaks and because of the cost-effectiveness of maintenance.
A smart EMAA that uses range/rate/attitude sensor information to preadjust
a docking interface to eliminate misalignments and to minimize contact and
stroking forces is described. A prototype EMAA has been fabricated and is
being tested and evaluated at the NASA Lyndon B. Johnson Space Center
Robotics and Mechanical Systems Laboratory. Results of preliminary testing
and analysis already performed have established confidence that this
concept is feasible and will provide the desired reliability and low
maintenance for repetitive long-term operation typical of Space Station
requirements.
INTRODUCTION
A technology development task titled "Construction Equipment/Soft
Docking Technology" was sponsored by the NASA Office of Aeronautics and
Space Technology to study Space-Shuttle-Orbiter-based construction
equipment required to support space construction, assembly, and satellite
servicing. Later, this task was expanded with emphasis on docking and
berthing. Requirements were proposed for minimum-disturbance (low force)
docking with large flexible structures and with satellites having sensitive
operating systems. This study was concentrated on the docking/berthing
function, with emphasis on isolating the requirements for, and exploring
the technology of, soft docking/berthing.
The methodology of this study comprised a simultaneous evaluation of
mission requirements, hardware design concepts, and systems performance.
The central study element was a combined conceptual-design/dynamic-
performance analysis that produced the identification of docking/berthing
component technology needs.
* NASA Lyndon B. Johnson Space Center, Structures and Mechanics Division,
Houston, Texas.
**Lockheed Engineering and Management Company, Houston, Texas.
275
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System design specialists reviewed the design evolution of past
docking hardware to maximize the benefit of that experience. In reviewing
component technology efforts, two key areas were identified for immediate
component development activity needs. Achieving soft docking for a
range of spacecraft masses and contact velocities requires the use of smart
attenuator/actuators which can provide variable force/stroke character-
istics and highly accurate proximity sensors to provide the necessary
intelligence. These design specialists identified sensors and smart
attenuator/actuators as the key components of the docking system that
warrant proof-of-concept development testing.
A prototype smart electromechanical attenuator/actuator (EMAA) was
fabricated for proof-of-concept development testing to support the
Construction Equipment/Soft Docking Technology study. The development of a
laboratory prototype microprocessor-controlled smart EMAA from which the
development technology can be applied to Space Shuttle Orbiter and Space
Station docking/berthing systems is described.
SOFT DOCKING SYSTEM OVERVIEW
A smart EMAA is one of four subsystems of a soft docking system
concept consisting of (I) a laser docking sensor (LDS) subsystem, {2) an
androgynous, four-fingered, ring-and-guide docking interface subsystem,
{3) the EMAA subsystem, and (4) a docking microcomputer system (DF_)
{Figure I). The LDS provides position, velocity, attitude, and attitude-
rate data of the approaching vehicle. The androgynous, four-fingered,
ring-and-guide docking interface provides the rigid structural coupling of
the two docking vehicles and permits 90 ° interval indexing of the two
mating ports, while providing two axes of inverse symmetry that coincide
with the two major axes of the vehicles. The DMS queries the LDS subsystem
for data necessary to process the kinematic equations that are required to
position the EMAA's. The EMAA's preadJust the docking interface. As a
result, intolerable misalignments are eliminated, and contact forces are
minimized. The DMS performs real-time processing to provide the EMAA's
with data including energy absorption, fault detection, and error
management during the attenuation process. The DMS also provides interface
to embedded Space Station management subsystems and crew systems. The EMAA
performs the soft docking energy absorption {attenuation) and actuation for
the system. The EMAA receives high-level control instruction and
attenuator/actuator performance characteristics from the DF_ and transmits
status information back to the DMS.
EMAA DESIGN REQUIREMENTS
The requirements for the smart EMAA were baselined upon the likely
event of soft docking an Orbiter weighing 108 775 kg (240 kips) to the
Space Station weighing 181 292 kg (400 kips) using a conceptual-design soft
docking interface system from the Construction Equipment/Soft Docking
Technology study. This interface system accommodates four pairs of smart
EMAA's to control the docking interface relative to the base ring. The
276
following are the general smart EMAA requirements for soft docking:
I. Absorb the relative kinetic energy necessary for the docking
interface between the two vehicles with typical approach velocity
of 0.003 m/s (0.01 ft/s) to 0.031 m/s (0.10 ft/s)
2. Draw the docking interface with mating vehicle together for
structured connection
3. Assure durability for repeated docking and undocking
4. Provide both attenuation and actuation functions
5. Change attenuation performance characteristics in real time
<
,,,
I
-I!PROCESSOR DIGITAL I ! DIGITAL DIGITALSERVO- SlmVO-- SERVO-CONTROLLER CONTROLLER CONTROLLER
SYSTEM BUs
f
I-I1t I CItOCOItPUTERSYSTEM(DtIS)
>
Figure 1 Soft Docking Interface System
277
6. Require little or no maintenance for long-term operations
7. Provide precise position control for prealignment of docking
interface
8. Allow 0.457 H (18 in.) of attenuation/actuation stroke
9. Provide constant force attenuation
EMAA COMPONENT DESCRIPTION
The eight components of an EMAA are (I) a digital servocontroller, (2)
a digital-to-analog converter (DAC), (3) an amplifier, (4) a direct-current
(dc) motor, (5) an optical encoder, (6) a gear pair, (7) a roller screw,
and {8) a mechanical housing {Figure 2).
C POS I T I ON CONTROL
OPT ICAL ENCODER
I
qo,o,...H .o.c.,o.kSERVO- PROCESSORCONTROLLER
+
DAC
DC MOTOR
HOUS I NG-_
Col
ROLLER SCREW
GEAR PAI R
ATTENUATOR FORCE EQUATIONS >
Figure 2 EMAA Components
27_
The digital servocontroller is a 16-bit microcontroller based on a
register-to-register architecture which allows digital filter control
algorithms to execute faster than in accumulator-based architecture in
which data transfer is bottlenecked. The controller receives data from
both the DMS and the optical encoder, calculates a digital voltage command,
and reports the current status and error conditions back to the DMS. This
digital voltage command is converted to an analog voltage by the DAC, a 12-
bit bipolar voltage-output device. The voltage is then applied to the
amplifier and held constant until the controller completes another sampling
period.
The pulse-width-modulated (PWM) amplifier receives the analog voltage
from the DAC and provides the electrical power to drive the motor. Based
on the average polarity of the amplified voltage, the dc motor will respond
by rotating clockwise or counterclockwise_. __Ar_ne"m_n_nh-m_n_-e_1_-o-v- ._ do
servomotor, with a peak rated torque of 0.490 N-m (70 oz-in.), converts the
electrical power into rotary motion, which drives the optical encoder and
the gear pair.
Shaft position feedback is determined by a 500-count-per-revolution
optical incremental encoder. Two channels in quadrature are transmitted by
the optical encoder to the controller, which resolves rotational direction
by determining that channel "A" leads "B," or vice versa. By counting
encoder increments or decrements and knowing the lead of the roller screw,
the shaft's linear position can be determined.
Through the 1:1 gear pair, the mechanical motion from the dc motor is
applied to the roller screw. One of the gears is attached to the motor
shaft, and the other is attached to the nut of the roller screw.
The high-efficiency (0.845) roller screw with a lead of 0.005 m (0.2
in.) consists of a threaded shaft and an internally threaded nut with
threaded rollers. The nut assembly rotates at a fixed location in the
mechanical housing so that only the shaft is allowed to translate.
Rotation of the shaft is restrained by a keyed bushing located at the end
of the shaft sliding in the mechanical housing.
An aluminum structure mechanically supports the dc motor/encoder
assembly and the.roller screw. The housing is built to be easily mounted
to a test fixture so that performance evaluations can be made.
EMAA FUNCTION
The smart EMAA is unique in its capability to provide programmable
attenuator forcing functions. The digital servocontroller permits real-
time, external-sensor data inputs to its attenuator force equations and
allows for real-time performance parameter changes. Most attenuator
forcing functions can be implemented using only position control and an
algorithm to calculate a position profile of that function as it relates to
279
energy absorption. This discussion will be limited to constant force
attenuation using position control.
In position control, a digital servocontroller will often command a
motor to move and lock onto a final position. This action is accomplished
by the controller determining a desired position and then calculating the
position error, the difference between the desired position and the actual
position. The position error is then digitally filtered, and the filtered
output is applied to a motor through a DAC and an amplifier.
Constant velocity of a motor using position control is obtained by
changing the desired position by constant discrete amounts every sampling
period. Since the desired position is changing, a controller can take
position feedback and compare it to the new desired position to obtain
position error. A smooth constant velocity is sustained by minimizing this
error through a digital filter. The controller follows the constant
discrete changes allowing the filter to maintain stable motion.
Position control can also be used to accelerate or to decelerate a
motor. The method is similar to the previous constant velocity control,
except that the discrete changes in desired position are not constant.
Again, the digital filter minimizes position error and maintains a stable
acceleration or deceleration position profile.
Constant force attenuation is obtained from the EMAA using only
position control. The force Fcan be expressed using Newton's second law of
motion
F - m_
where m is the combined mass of the capture mechanism and the approaching
body, and the acceleration a is defined by the deceleration position
profile. The total work required to absorb the kinetic energy of the
combined mass with a velocity u is expressed as
112 mu2 : Ys
where s is the differential displacement (EMAA stroke) of the combined mass.
Figure 3 best illustrates this method. Figure 3a is a velocity versus time
(u(t)) profile of the roller screw shaft, and Figure 3b is the shaft's
parabolic position profile (s(t)), the double integral of the constant
acceleration. In phase A, the roller screw is accelerating to match the
velocity of the approaching mass. Phase B shows the constant deceleration
of the roller screw during the capture and the constant force attenuation
of the mass. This parabolic position profile is generated by the digital
servocontroller and applied to the dc motor, which drives the roller screw
to provide constant force attenuation.
280
v %)
I
PHASE A I PHASE B
I
!
a, VELOCITY PROFILE
s(-r)
I
I
/J
b. POSITION PROFILE
""_ "1"
"'- "1"r
Figure 3 Capture and Attenuation Profile
'3.'1
t.I)]
EMAA TESTBED EVALUATION
A smart EMAA testbed was developed to test a single-axis EMAA system.
The testbed is shown in Figure 4. It consists of (i) a test stand, (2) the
load cells, (3) a mass simulator, (4) the EMAA, (5) a sonar ranging system
(similar to the LDS), (6) a docking computer (a scaled-down DMS), and (7) a
data acquisition system.
The mass simulator is an independently computer-controlled electro-
mechanical mechanism similar to the EMAA. It drives the mating surface to
simulate a free-moving mass in space. The mass-simulator computer receives
force information from the load cell and uses the selected mass value to
calculate a deceleration position profile to drive the mating surface.
Testing and verifying the soft docking concept required the fabrica-
tion of the single-axis EMAA system. The docking computer coordinates the
data between the sonar and the EMAA; then, using the sonar range-rate data,
the computer sends control parameters to the EMAA to capture and attenuate
the mass. The sonar is an inexpensive system which is used only in this
test setup for proof-of-concept purposes. In proximity, it provides range
rate information similar to that of a laser sensor.
_
ul
iV
I S1I{II_L I
GONDIT IONII_ I
Figure 4 EMAA Testbed
282
The digital servocontroller, employing a position control system, was
breadboarded and tested with laboratory equipment to verify the DACupdate
time, which corresponds to the sampling period of the position control
algorithm.
In order to establish torque versus speed curves, the dc servomotor
was first dynamometertested at voltages ranging from 5 to 40 volts de.
Peak torque established the maximumconstant force output of the
attenuator. In its current configuration, this output was found to be
515 N (115 ibf).
The roller screw selected provides a 0.457-m (18 in.) stroke and the
capability to withstand a 5440-N (1223 ibf) axial load (safety factor 3).
Load-handling capability was obtained from the manufacturer's published
specifications, and after numerousEMAAtests, no degradation has been
observed. Also, from analysis of the load data based on a six-degree-of-
freedom, three-body simulation program (SOFTDOCKSIM),it was concluded that
these actual force loads to the roller screw shaft in a docking ring
configuration would not be exceeded.
The data acquisition system obtained force-versus-time, position,
velocity, and motor-current data from the soft docking attenuation process.
These data verified the performance requirements of the smart EMAA.
CONCLUDINGREMARKS
A proof-of-concept smart EMAAsystem has been described. The
technology derived was based on current off-the-shelf componentsand can be
applied to present and future space docking systems. Special new designs
have been considered that incorporate a brushless dc motor around the
roller screw and include a permanent-magnetrotor attached directly to the
roller screw nut. This concept will eliminate the use of gears by
producing a more compact size with reduced friction and inertia of moving
parts. The attenuation technique, which uses a position controller to
provide a motion profile, gives a high degree of flexibility to any
attenuator/actuator system. This technology applies not only to a docking
system, but also to berthing and to positioning and holding aids by
controlling the motion of large, massive objects.
Someinherent, unresolved difficulties remain which include both the
complexity of coordinated control of an overconstrained multiaxis system
and the instability and mechanical binding of a coupled system. The
control system development for the soft docking system will be the prime
instrument in overcoming the difficulties of stability and control.
This unique concept has great potential for current and future
docking/berthing systems. The use of microcomputers enables the real-time
updating of attenuator/actuator parameters and allows for programmable
attenuator forcing functions. This capability greatly enhances the
283
performance of the docking/berthing system because of the high degree of
flexibility and programmability of the microcomputer.
ACKNOWLEDGMENTS
The authors wish to express their appreciation to the NASAOffice of
Aeronautics and Space Technology and to membersof the Robotics and
Mechanical SystemsLaboratory at the NASALyndon B. Johnson Space Center.
REFERENCES
I. Construction Equipment/Soft Docking Technology. NASALyndon B. Johnson
SpaceCenter 506-64-27, November1982.
2. LASERDocking Sensor (LDS) System. NASALyndon B. JohnsonSpace
Center, JSC-22031,January 1986.
3. Tal, J.: Motion Control by Microprocessors. Galil Motion Control,
1984.
4. Soft DockSimulator (SOFTDOCKSIM).NASALyndon B. JohnsonSpace
Center, LEMSCO-21763,JSC-20624, January 1986.
284


An electromechanical attenuator/actuator for Space Station docking

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