A recently developed control system for capturing free-flying payloads with flexible manipulators is discussed. Three essential points in this control system are, calculating optimal path, using a vision sensor for an external sensor, and controlling active vibration. Experimental results are shown using a planar flexible manipulator

Iikura, S.

Komatsu, T.

Miura, H.

Shimoyama, I.

Uenohara, M.

English

NASA Technical Reports Server

N90-29784
CAPTURE OF FREE-FLYING PAYLOADS WITH FLEXIBLE SPACE HANIPI_TORS
T.Komatsu, H.Uenohara, S.Iikura
R&D Center, Toshiba Corporation
Kawasaki, Japan
H.Hiura, I.Shimoyama
The University of Tokyo
Tokyo, Japan
Abstract
The purpose of this paper is to discuss _t recently developed control
system for capturing free-flying payloads with flexible manipulators. Three
essential points in this control system are, calc_llating optimal path, using a
vision sensor for an external sensor, and active vibration control.
Experimental results are shown using a planar flexible manipulator.
1. Introduction
In the near future, capture of free-flying payloads, for example,
recovery of satellites, will become one of the most important tasks for
manipulators on the space station. This task of space manipulators are
different from those on the earth. Hanipulators have to capture payloads
without impact. Most space manipulators should be structurally flexible,
reflecting the necessity for their light-weight based upon minimum energy
comsumption and shipping cost, as well as handlin_ of large mass payloads in a
no gravity environment. Therefore, it is also ne_:essary to control structural
vibration in these space manipulators to accomplish this task, especially
after capturing satellites. The purpose of this paper is to discuss a recently
developed control system for capturing free flying payloads with flexible
manipulators. Experimental results are shown using a planar flexible
manipulator.
Three essential points in this control system are, (1)calculating optimal
path of manipulator tip, (2)direct sensing of position and attitude of a
payload using a hand eye camera, and (3)active vibration control of a
manipulator. Determining the optimal path is necessary to accomplish non
impact capture of a payload. This path is obtained by minimizing the
performance index which consists of relative position error, velocity error
and acceleration of manipulator tip. Direct sensing of the relative position
between a manipulator and a payload is necessary to move the manipulator tip
precisely along the optimal path. The authors used a hand eye CCD camera and
LED target marker for this sensor. Active vibration control is necessary for
precise and quick control of a manipulator. A local PD feedback of joint angle
and torque was used for robust positioning and vibration control.
An experimental equipment was built to investigate the validity of this
control system. This equipment consists of an ail suspended, flexible two-link
manipulator, a payload, and a controller. Experimental results of capturing a
moving payload demonstrates the effectiveness of this system.
2. Control System
The task of a manipulator considered in th:_s paper is automatic capture
35
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of free-flying payloads, for example, capture of satellites, with manipulators
on the space station or on the free flyer. This task was accomplished in the
Space Shuttle program by cooperation between RMS(remote manipulator system)
and EVA (extra vehicular activities). This task was done for repair of a
disabled satellite. Ill tile future, after constructing the space station, many
satellites will be assembled on the station and thrown into an orbit.
Therefore, maintenance and repair of satellites will be done frequently on an
orbit. In such a ¢-ase, doing this task manually will be expensive and
uneffective. Therefore accomplishing this task automatically is necessary.
To construct control system for the first stage,
assumptions were made for payload motion.
the following
(1)A payload rests or moves with constant velocity to the manipulator's
base.
(2)A payload does not revolve on its own axis.
(3)Movement of a payload is constrained on one plane.
For using this control system on an orbit, the following capabilities are
targeted:
(a)Real time manipulator path planning
(b)Direct sensing of the payload position
(c)Quick and precise motion control of a manipulator
(d)Easy integration
The system can be represented by Figure 1. The major subsystems are:
(1)Path planning
(2)Vision system
(3)Manipulator control
Explanation of the subsystem is as follows:
2.1 Path planning
In this subsystem, an approach path of a manipulator is calculated in
real time. The following assumptions were made:
(1)A free-flying payload moves straight with constant velocity.
(2)The position and velocity of calculated path become equal to those of
free-flying payload at the target time tf.
The authors chose the optimal path obtained by minimizing the following
performance index as the approach path satisfying upper assumptions.
1 +lft f a t
(_tsf X) tf 2It0 a dt (1)
j--
2 m m
where
K----IK --K
P m
_p'The position vector of
_m The position vector of
a free-flying payload
a manipulator path
36
x=[:]=,vxvy zxYz,t
Sf=d i ag (C 1 , C 1 , C 1 , C 2, C 2, C 2) (C 1 , C 2-_o)
m m
By using this path, non impact capture is realized because position error
and velocity error between a manipulator and a payload become zero at tf.
Horeover, smooth path is obtained because of minimizing total sum of (am)2.
Considering a as the input to the system, the following system equation
is obtained becaus_ the acceleration of a payload is zero.
where
X=F-X+G- a (2)
m
--
I
0 t 0
I
------+ ------
lOOl
0101 0
0011
G_
-1 0 0
0-1 0
0 0--1
0
Therefore, solving optimal path from equatiou (1) is equal to the design
of the linear inhomogeneous regulator. In general, for this problem, the
system equation, the cost function and its general solution become as follows
[1]:
y=F (t) y+G (t) m+C (t) (3)
1
i/tf t
t
+ (ytAy+u Bin) dtJ=- (y Sfy) tf
2 tO
m (t)=_B-1GtSy-B-1GtK
_=-SF-FtS+SGB-1GtS-A
(4)
(5)
(6)
Sf=S(tf)
K+ (Ft-SGB -1Gt)K+SC= 0 (7)
K (tf) =0
Compairing equation (1) and (2) with (3) and (4),F and G equal equation
(2), and y,u,B,A,C become as follows:
y:X m:a
m
(a)
37
I] 00]
B= £10
001
Substituting these equations
solution is obtained as follows:
am X
amy --
a
mz
A=C=O.
into equation (5)_(7), the analytical
4 6
VX+ X
(tf-t) (t f-t) 2
4 6
Vy+ y
(t f-t) (t f-t) 2
4 6
Vz+ z
(tf-t) (t f-t) 2
(9)
Tile optimal path is calculated by the integral of equation (9)
substituting Yx V Yz, X Y, Z which are obtained from vision system in real
time. ' Y' '
2.2 Vision system
In this system, position and attitude data of a payload are sensed in
real time. This system consists of a CCD camera, a marker and an image
processing unit. A CCD camera watches a target marker on a payload. This
marker consists of a rectangle formed by 4 LED points. Next, video signal from
a CCD camera is sent to image processing unit. In this unit, coordinates of 4
LED points in the frame are detected using a simple hardware.
In three dimensional space, when the geometric relation of 4 points on a
plane are known and corresponding points to 4 points are obtained in the
frame, three dimensional coordinates of these points are determined by using
inverse translation of projection. Therefore, three dimensional coordinates of
a payload can be detected only by a monocular camera. Figure 2 shows
coordinates of a ca_eraf( _ C) which consist of Xc, yC and Z , and a frame
which consist of X- Y-. For each 4 LED points, the following equations are
obtained from trans ation of projection.
X. c
1
y.C
1
Z. c
1 J
f
X.
1
= k. y1 a
f
Y.
1
where
(i=l, • - -, 4) (10)
L 1
f I f 2 f f 2 I
kl (X1 -k3 X3 ) Jr (Y1 -k3 Y3 ) ÷ (l-k3 )2y _
38
=- I
k_ l --
j
and
x2f -x3f
y -y
a a
f f
Y2 -Y3
f" --i
x 4
y
a
f
Y4
" fl
X 1 "
y k . =k k'.
a I l 1 x
f I
Y 1 I
.J
(i=2, 3,4)
X C-x4C Z C-z c
q_=tan-1 ( 1 ) O=cos-1 ( 1 4 ) (12)
C-z4CZ 1 L14cos4>
_ 1 c c) co.s_- (Z c ctb=cos 1 [_ { (X 2 -X 1 2 -Zl ) sin_t ]
L 12
_, 8, _ denote role, pitch, yaw of a marker. LIj denotes the
actual length between No. i and No. j. Therefore, three dimensional data
Xc, ¥c, Zc, @, _, _ of a marker are detected by sensing Xf and
Yf. In this system, data are obtained at 50 msec intervals.
2.3 Manipulator control
Most of space robots must be structurally flexible, reflecting the
necessity for their light-weight based upon minimum energy consumption and
shipping cost, as well as handling large mass payloads in a no gravity
environment. Therefore it is necessary to control the structural vibration in
the flexible arm for quick, precise tracking of the trajectories and
accomplishment of tasks.
Figure 3 shows a blockdiagram of manipulator control. The outer loop is
for motion control. The inner loop is for vibration control. In this method,
joint angle and torque sensors are basically needed, which are ordinarily used
in remote manipulator systems for nuclear power plants.
Ne derived this method from the standpoinl of energy control in the
system, where the energy was both kinetic and potential. We considered a
flexible manipulator system as a completely mechanical system. From this
standpoint, it is possible to consider that the control of a flexible
manipulator is the same as the control of two types of mechanical springs in a
system. The first one is springs in an elastic link. These springs cause
structural vibrations of all modes when the accumulated potential energy
converted to kinetic energy. So it is necessary to minimize both kinetic and
potential energy at the target point for vibration restraint.
The second one is a spring in an actuator. For example, if the
manipulator motion is controlled by simple proportional and derivative (PD)
feedback compensation, as the general industrial robots, the work accomplished
by this actuator is the same as the work in a gefleral linear spring. In this
case, the proportional control gain equals the spring constant. It is also
necessary to minimize both the kinetic and potential energies of this spring
at the target point for motion control [2].
39
In the case of flexible manipulators, both vibration and motion control
are needed at tile same time. Therefore, it is necessary to control both
springs. In this control, total energy control for the system is accomplished
by composing both joint angle and torque PD feedback loops. The position
feedback loop realizes grobal motion control, while the joint torque feedback
loop realizes vibration control of all modes [3].
3. Experimental setup
An experimental equipment was built to investigate the validity of this
system. The authors named this equipment TESRA-I (teleoperated elastic space
robot arm). This equipment consists of a two dimensional air suspended
flexible manipulator, a payload and a controller. Figure 4 shows configuration
of this equipment.
The flexible manipulator is about 1.5 meter long. It has two flexible
links and three degrees of freedom (shoulder, elbow, wrist). An actuator is
installed at each joint. It consists of a DC motor and a planetary gear
reducer (1:100 reducer ratio). The sensor system consists of a potentiometer
for sensing the joint angle, a tachogenerator for sensing the motor velocity,
and the strain gages at the joint axis for sensing the joint torque. Flexible
links for this manipulator are made form strainless steel. The link diameter
is 6mm. The total weight for each joint and hand are 4kg and lkg. This arm
floats on an acrylic plate base, using four air bearings so as to simulate a
no gravity environment in the horizontal plane. A small CCD camera, 35mm(W) x
43mm(H) x 70mm(L), is installed on the manipulator's hand (Fig.5).
The payload consists of lead sheets, and the weight is 40kg. A handle for
grasping is installed at one side of this payload and a target marker is
attached on it. This marker consisted of a rectangle formed by 4LED points,
40mm(W) x 30mm(H).
This manipulator is controlled by a MOTOROLA digital computer VME-IO
system as the main computer. Its MPU is the 16-bit 68010, and the VERSAdos
multitasking system is used as the operating system. Sensor outputs are
sampled at 15msec intervals through a 32ch h/D board. Commands are fed to the
servo amplifiers through the 4ch D/A board. In the vision system, 32-bit
MPU(68020) are used for calculation of the target position.
4.ExperiRental result
Figure 6 shows one of experimental results of capturing a free-flying
payload, and the paths of manipulator tip and a payload are indicated.
The task sequence from capture to stop is as follows. The time of capture
t_ is 60sec. The path are planned so that manipulator tip goes to the
location of about lcm from a handle for grasping a payload. And, after
arriving at the location, manipulator tip goes ahead and captures a payload.
(1)A payload is manually accelerated to get to about 5cm/sec speed, and
released.
(2)An optimal path is calculated, and a manipulator is controled so as
to track the planned path.
(3)A manipulator tip arrives at the location of about lcm from a handle
for grasping a payload at 6.0 seconds.
40
(4)A manipulator tip goes ahead for grasping a handle.
(5)Capturing of a payload finishes at 10 seconds.
(6)A manipulator stops a payload at 30 seconds.
It is obvious that a manipulator tip moved along a smooth path and
captured a payload without impact. A little impact shown in figure 6 was
caused when two fingers of a manipulator closed. No link vibration occured
after stopping the payload.
5. Conclusions
In this study, the authors proposed a recently developed control system
for use in automatic capturing free-flying payloads with a flexible
manipulator. The features of thls system are as follows:
(1)real time path planning
(2)direct sesing of payload position by a vision system
(3)quick and precise control of a flexible manipulator
(4)easy integration.
The effectiveness of this system has been verified by experimental
results. The next step for this sytem is to expand its ability so as to adapt
it to capture a payload with acceleration and revolution on its own axis, and
finally adapt it to real three dimensional system.
References
1. V.Carber: Optimal Intercept Laws for Acceler._ting Targets, AIAA Journal,
No.6, Vol.11, 6-11, (1968)
2. M.Takegaki and S. Arimoto' A New Feedback M_thod for Dynamic Control of
Manipulators, ASME 3ournal of Dynamic Systems, Measurement, and Control, Vol.
102, 119-125, (1981)
3. T.Komatsu, M.Uenohara, S.Iikura, H.Miura and [.Shimoyama: Active Vibration
Control for Flexible Space Environment Use Manipulator, IFAC, Zurich,
Switzerland (1988)
41
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Capture of free-flying payloads with flexible space manipulators

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