NASA Technical Reports Server (NTRS) 19930002776: An 8-DOF dual-arm system for advanced teleoperation performance experiments

12 Pages·0.77 MB·English

by  NASA Technical Reports Server (NTRS)

N93-11964
AN 8-D.O.F. DUAL-ARM SYSTEM FOR ADVANCED
TELEOPERATION PERFORMANCE EXPERIMENTS
Antal K. Bejczy and Zoltan F. Szakaly
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
ABSTRACT and load handling capability together with
high positioning accuracy and repeatability,
This paper describes the electro-mechanical most existing manipulators have
and control features of an 8-D.O.F. manipula- shortcomings and have to be modified or
tor manufactured by AAI Corporation and rebuilt.
installed at the Jet Propulsion Laboratory
(JPL) in a dual-arm setting. The 8-D.O.F. arm The purpose of the research and development
incorporates a variety of features not found work described in this paper is to build an
in other laboratory or industrial manipula- end-to-end manipulator system to enable the
tors. Some of the unique features are: 8- performance of a broad range of realistic
D.O.F. revolute configuration with no lateral tasks not achievable by other manipulators.
offsets at joint axes; 1 to 5 payload to Of particular interest are tasks like the
weight ratio with 20 kg (44 Ib) payload at a real-life simulation of the Solar Max
1.75 m (68.5 inches) reach; joint position Satellite Repair (SMSR) in teleoperation
measurement with dual relative encoders mode. As known, this satellite was not built
and potentiometer; infinite roll of joint 8 for maintenance, and was still repaired in
with electrical and fiber optic slip rings; Earth orbit in the Space Shuttle bay by two
internal fiber optic link for "smart" end EVA astronauts in 1984. The question now
effectors; four-axis wrist; graphite epoxy is: can the SMSR task be performed remotely
links; high link and joint stiffness; use of an by the use of an advanced telemanipulator
upgraded JPL Universal Motor Controller system? If so, then what kind of
(UMC) capable of driving up to 16 joints. The performance can be expected for an SMSR-
8-D.O.F. arm is equipped with a "smart" end type work in teleoperation mode?
effector which incorporates a 6-D.O.F. force-
moment sensor at the end effector base and The SMSR type teleoperation work also
grasp force sensors at the base of the raises a number of interesting and important
parallel jaws. The 8-D.O.F. arm is interfaced issues regarding redundancy in the
to a 6-D.O.F. force-reflecting hand kinematics, sensing and control of
controller. The same system is duplicated manipulators and regarding the human
for and installed at the Langley Research operator interface to a redundant
Center. manipulator system.
INTRODUCTION The two 8-D.O.F. manipulators built by AA1
Corporation for JPL in 1990 (and two more
Most commercially available manipulators for LaRC in 1991) serve the purpose of
have been designed and built with a specific enabling the experimental evaluation of
application and performance domain in mind. application oriented performance issues
When it comes to application research and briefly indicated above.
development of a more general nature which
In the first part of the paper we summarize
typically requires some extra motion
the mechanical features of the AAI 8-D.O.F.
dexterity combined with some extra reach
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arm. Thecontrolelectronics,includingsome Highjointstiffnessis achivedby the useof
computationaal spects,are brieflyoutlined highstiffnessball bearingsandspecially
inthesecondpartofthepaper. stiffenedharmonicdrives. Togetherthe two
bearingsw, hichhavebeenusedinprevious
AAIARMMECHANISM spaceapplications,protectthe joint against
thrustandradialloading.Links3 and6
TheAdvancedResearchManipulatoIrI (ARM incorporatetubulargraphiteepoxy elements
II),Model1520-8Ai,s an 8-D.O.F.r,edundant which provide high stiffness, strength, low
manipulatodresignedandbuiltby AAI weight and structural damping. This last
Corporation(HuntValley,MD)to support point was experimentally verified on a
laboratoryteleroboticsR & Dwork. It hasa General Electric P50 robot arm and described
20kg (44Ib)payloadcapacityat a full in Ref. 2.
ext:_nsion.A highpayloadtoweightratiois
achievedthroughthe useof lightweight The ARM II joint reference frames, following
graphiteepoxycompositematerialsfor the the Denavit-Hastenberg (D-H) convention,
arm links, lightweightmodularjoints, and together with the D-H parameters are listed
hightorqueservomotors.TheARMIIis in Figure 3. As seen, there are no lateral
basedon a modularjointdesign,which offsets of joint axes (the a; D-H parameters
permitsconstructionof a wide varietyof are zero for all links). This greatly
revolute kinematic configurations. The 'facilitates the handhng of torward and
modular joints are sized according to torque inverse kinematics and dynamic
requirements and can be mated to links, computations. The total reach capability of
regardless of twist angles. ARM II is 68.5 inches with the JPL Model C
Smart Hand. (Without end effector the reach
The special kinematic feature of the 8-D.O.F. capability is 60 inches).
ARM II is the four-axis, gimballed wrist
which allows singularity avoidance in a very Another important feature of ARM II is that
wide configuration range and permits small stiffness and conservative design
angular changes of the end effector with constraints allow it to be oriented as
little or no motion of the lower arm joints. desired with respect to gravity.
Another special feature of the design is the Nonetheless, ARM II is a lightweight
infinite roll capability of the last, 8th joint manipulator when considering its 1:5 payload
to which the end effector is mounted. to weight ratio. Note that the PUMA 560
This permits contiuous rotation about the payload to weight ratio is 1:13. More on the
roll axis of a tool heed by the end effector ARM II design characteristics can be found in
without requiring motion of the other joints. Ref. 1.
The mass and inertial characteristics of ARM
The ARM 11is driven by DC brash motors with
II links are listed in Table 2. Table 3 lists
integral brakes and encoders. Harmonic
drives are used as gear reducers. Each joint the ARM II joint natural frequencies. These
is equipped with two encoders for input and frequencies are based on motor and harmonic
drive inertias without link load inertias
output position sensing in the harmonic
drives. The encoders are relative position since the gear ratios for each joint are
encoders. In addition, each joint is equipped sufficiently large to essentially eliminate
with a potentiometer for sensing absolute the load inertia at the motor. Table 3
position at start up. Each joint also has a includes two natural frequencies for two
thermal sensor, electronic limit switch joint stiffnesses: one valid for the very low
(except the 8th joint) and a mechanical stop torque range, Ko. , and the other, K& , valid
(again, except the 8th joint) outside the over the remainder. The lowest natural
limit switch. Some of the motor, brake and frequency of the full system is calculated
gear characteristics are listed in Table 1. 5.3 Hz and is based on the vibration of joints
Figure 1 shows ARM II, Model 1520-8A as 2 and 4 under conditions of full load at full
installed at JPL for system integration. extension and assuming that K spring
Figure 2 shows the cabling system of the constant applies. The system natural
arm. frequency with no payload and full extension
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ORIGINAL PAGT_
BLACK AND WHITE PHOTOGRAPh
®
Figure 1. Eight D.O.F. ARM II(byAM Corporation)
is calculated for 8.4 Hz. This compares well at 30 VDC using only joint 1 motor at full
with the lowest natural frequency of the CM extension was somewhat more than 18
T3-776 robot arm measured by Tesar and inches per second.
Behi. This indicates the ARM II is similar to
In summary, ARM II incorporates a variety of
rigid, industrial robot arms in this regard.
features not found in other laboratory or
The ARM II positional accuracy and industrial manipulators. Some of the unique
features are:
repeatability under full load and at full
extension is less than t 0.1 inches and
_+0.01 inches, respectively. This is verified • 8 D.O.F. revolute configuration
experimentally. • Four-axis wrist
• No lateral offsets of joint axes
The measured maximum tip speed of ARM II • High, 1:5 payload to weight ratio
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\
\
Figure 2. Eight D.O.F. ARM IICabling Schematics
• 20 kg (44 Ib) payload at maximum was designed in our laboratory and it has
extension of arm (60 inches, without been commercialized. The commercially
end effector) available version is sold in four joint
• Modular joint design increments up to a 16 joint maximum per
• Graphite epoxy links UMC. We use the commercial UMC ourselves
• High joint and link stiffness for our various motor controller needs. The
• Infinite roll of joint 8 with electrical AAI arm has eight motors. Each joint, except
and fiber optic slip rings the last one, is equipped with two optical
• Internal fiber optic link for smart end encoders. There are a total of 15 encoders.
effectors These encoders count 4096 for every
• Two techniques for measuring joint revolution of the motor shaft. The two
position: dual relative encoders and encoders are connected to opposing ends of
potentiometer the harmonic drive. The gear reduction is
• Indirect measurement of joint torque 200 in every joint, so one encoder counts
from dual relative encoders 0.5% slower than the other. Since both
encoders are equipped with an index pulse,
the two index pulses shift about 2 degrees
CONTROL ELECTRONICS relative to each other for every motor
revolution. (Their relative position can
The arms are controlled by a 16 axis UMC eventually be used to determine absolute
(Universal Motion Controller) each. The UMC joint angle.) Since every joint also has a
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-8,
.........I....................... LE 8e(xT-)xs):180°
ZT
_7(X6--X)7):--90°
8s(Xs--_Xe): 90°
6s(X4--X*S):90°
dl
Xe
ds Z4,
Et
THE POSITION VECTOR OF THE ORIGIN OF THE LINK5 FRAME, Ps, IS
UNIQUELY DETERMINED BY THE GIVEN POSITION VECTOR, Pe,AND THE
Z3 84 APPROACHING VECTOR, ae, OF THE END-EFFECT=R: P5" P8- ds"as. NOTE
84(x3_ x4): 180° THATPsISNOTAFUNCTIONOF0s.
83(x2--*x3): 180°
DENAVIT-HARTENBER(DG-H)PARAMETERS
Z JOINT LINK TWIST
d3 t VARIABLE LINK DISTANCE d= LENGTH ANGLE
LINKi Oi (mm) al ai JOINT LIMIT
1 01 dl 0 90<' -'165 ° <01< 165=
Z, 02 2 e2 0 0 90° - 105° <02< 105=
.................... X2.,ql-.- )--*x, 82(x1-) x2): 180° 3 696.1 0 90° - 165° <%< 165=
z 81(Xo_ Xl): 180° 4 Q 0 90° - 105° <04< 1050
0t
dl 5 557 0 - 90° - 165° <85< 1650
6 0 0 90° - 102_<8G<131°
7 o 0 90° - 130° <87< 22°
8 e8 474.1 0 0° NO LIMIT
Figure3. ZeroConfigurationoftheEightD.O.F.ARMII. TheJoint Offset,81,i= 1..... 8,are
Defined astheD-HAnglesattheZeroConfiguration.
potentiometer, there are two ways to computations are currently done in this
determine the absolute position of the arm. environment. We are developing a new high
The two encoders provide two sources to performance multiprocessor system that
determine the relative position. This is the will be based on a custom designed high
primary quantity used for control. The load speed bus and processors in the 10 to 15
on the joint causes windup in the harmonic MIPS range each. We will describe that in
drive mechanism, this windup is precisely more detail subsequently. The other major
detected by the shift of one encoder position subsystem of the UMC is the motor
relative to the other. This is a way to controller. The motor controller consists of
determine joint torque. The alternative way the following:
is to read the motor current from the UMC.
Joint processor
The UMC is a cage housing two major Joint interface
subsystems, the multiprocessor and the Power amplifiers
motor control and sensing subsystems. (See Input filters
Figure 4). Currently the multiprocessor
subsystem consists of up to 8 processors. The joint processor is one 32016 board
These are NS 32016 boards interconnected dedicated to controlling the joints. It
by a MULTIBUS -I backplane. These interfaces to the joint interface cards via a
processors perform about 1 MIPS. All of our 16 bit i/o bus. This i/o bus is built to the
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Table i. Some Drlve System Features of ARM II
MOTOR
Brake
Rated Rated Rated Rated Rated Gear
Joint Speed Torque Current Volt Torque Ratio
(rpm) (oz-ln) (A) (V) (oz-ln)
1 1970 861 12.9 120 1200 200
2 1970 861 t2.9 120 1200 200
3 2164 285 10.5 60 560 200
4 2164 285 10.5 60 560 200
5 1600 162 7.25 40 240 200
6 3000 86 7.8 35 128 200
7 1600 t62 7,25 40 240 200
8 2050 51 4.9 30 240 200
Table 2. Mass/Inertia Parameters for Dynamic Model of AAI ARM II,
Given in the Individual Link Reference Frames
Link & Reflectec
Refer- Mass Mass Center Moments of Inertia Rotor
ence (lb's_/ (in) (in.lb.s_) Inertia
Frame in) MXi Myi MZi IXXi Iyyi Izzi Iai **
I Mi (in.lb s_)
X X X X X X X 356
1 .1612 0.0 3.50 -4.05 12.34 5.882 7.334 356
2 .0764 O.O .83 18.15 29.09 28.96 .4023 80.7
.0682 0o0 2.43 -2.06 2.499 1_78 1.528 80.7
._682 3.22 0.0 15.62 18.81 20.45 1.904 52.1
i0455 -.37 0.0 -.90 .2199 .3419 .2044 16.3
.0336 ..._.3.53 -.04 1.60 .269g .9204 .7414 52.1
7 * .0077 0.0 0.0 9.5 .7090 .7090 .0195 27.1
*) Without EndEffector Data
**) Includes ALL Input Shaft Inertias Multiplied by the Square of the Gear
Ration Between Input/Output
287
Figure 4. Eight D.O.F. ARM II Control Electronics, in Front and Rear Views
Intel iSBX standard. The ilo bus makes the
Table 3.Joint Natural Frequencies
joint motion parameters memory mapped to
Natural Freq. Sprleng Constant the joint processor's address space, The
Jolnt (Hz) (lO',Ibinlrad) joint interface card performs input data
W_ W A B conversion and output control functions to
the power amplifiers.
} 6.9 I0,0 9,0 I&8
Each joint interface card has the following
2 6,9 10,0 9,0 18.8
functions"
3 6,9 9.9 2.5 5.15
16 analog input channel A/D converter
4 6.9 9.9 2.5 5, t5 at a 12 bit accuracy.
4 optical encoder position counters.
5 8.7 123 I.I 2J4
4 digital tachometers.
4 digital control units for the PWM
6 11.l 27.1 0.48 1,20
amplifiers.
7 8.7 12.7 1.1 2,34 EEPROM non-volatile memory to store
joint parameters.
8 27 1 39.5 1.t 2,34 Watchdog timer.
288
The optical encoder interface is an up/down motor generate high energy noise spikes on
counter that can be used in 8 or 12 bit all of the incoming signals. Such spikes are
modes. The count has to be read periodically relatively easy to filter out because they are
by the software to avoid more than one narrow. The incoming digital signals are
wraparound. -The software computes the first filtered by an R-C low-pass filter,
incremental change from one reading to the after which a four stage digital sampling
next and adds this change to a counter in filter eliminates all pulses that are
memory. narrower than 2 microseconds. The analog
signals are R-C filtered once in the input
The digital tachometers are devices that filter section and a second time on the joint
measure the time lapse from one positive interface card.
edge in the input encoder pulse stream to the
next. The software divides a constant with Control Computations
this time to get a quantity that is
proportional to the joint velocity. The multiprocessor system presently
contains a total of three processors. The
The PWM amplifiers in the UMC are computer hardware system, including
controlled by digital signals that are communication to the control station
generated on the joint interface cards. Each computer mode (and to the VME bus at the
joint has two controlling registers, the LaRC installation) are shown in Figure 5. The
motor is driven at a duty cycle that computational functions are distributed
corresponds to the smaller value of the two. among the three processors as follows:
This arrangement ensures software control
of the motor voltage even in case of Remote communication, trajectory
component failures in the system. The generation, Cartesian servo and
amplifiers used to drive the AAI arm can harmonic motion generator.
deliver 20A of current at 60V each. Inverse and forward kinematics,
gravity compensation, smart hand
The PWM amplifiers in the UMC are unique in interface and compliance.
the sense that they do not have an integral Joint servo
current feedback loop. Conventional PWM
circuits are strongly non linear. This The remote communication consists of a
necessitated the use of a feedback loop that packet exchange via the fiber optic link. The
linearizes the current transfer function of hand controller node transmits a packet to
the amplifier. Such PWM amplifiers take an the robot node. This packet contains a mode
analog signal as input and produce a motor byte, a six byte (one for each degree of
current on the output. The amplifiers in the freedom) relative motion command, a one
UMC are designed such that the relationship byte grasping force command and a
between duty cycle and motor voltage, checksum. When the robot node receives this
current and energy output is linear. This packet it replies with its own that contains
makes it unnecessary to have a feedback loop the following: Currently active control
inside the amplifier. The output can be mode, robot position in the task space and
controlled directly with a digital signal, joint space, the forces on the end effector,
eliminating an intermediate analog stage. It the finger position and a checksum.
is also because of this that the amplifiers
produce an inherent velocity damping due to The trajectory generator receives the
the tachometer effect of the motor. This incremental motion commands from the
reduces the magnitude of additional velocity communication and generates the desired
damping needed. In such a system the motor joint or task space positions for the robot.
itself is used as a tachogenerator, without In one of the joint modes this is simply an
any additional hardware. More on the UMC addition of the incoming command to the
can be found in Refs. 4 and 5. joint setpoint. In task mode the input matrix
for the inverse kinematics has to be
Input filters are also implemented since generated. This consists of the end effector
they are needed because the sharp rising and tip position which is generated by simple
falling edges of the PWM signals driving the accumulation and the end effector
289
FRHC:FORCE-REFLECTHINAGNDCONTROLLER
FO: FIBEROPTIC
UMC: UNIVERSAMLOTORCONTROLLER
VME
INTERFACE
CPU-1
MOD-C
CPU-2. HAND
VMEINTERFACE JOINT CPU
FOINTERFACE
USERCPU JOINT DRIVE
JOINTCPU JOINT DRIVE
JOINTDRIVE JOINT DRIVE
JOINTDRIVE JOINT DRIVE ARM-r[
FRHC
POWERSUPPLY POWER SUPPLY
UMCBYMTEK UMC BY MTEK
Figure 5. Eight D.O.F. ARM HOverall Control Schcmatics (Also Indicating VME Bus Interface atLaRC)
orientation matrix which is computed by The inverse and forward kinematic
consecutive rotations of this matrix while computations are solved as an integrated
maintaining its orthonormality. When package. The mathematics are based on the
switching from one mode to another work performed in our group at JPL. (See
continuity of the robot position is Ref. 3). The governing principle in the
maintained. This is accomplished by feeding computations is to use geometric reasoning
the output of the forward kinematics into to achieve optimum performance. One joint
the input of the inverse kinematics while not of the first four and one joint of the last
in task mode. four are parametrized, they maintain their
positions from the last time they were
The Cartesian servo improves trajectory moved in joint mode. The remaining six
tracking accuracy by establishing a servo joints are computed based on the tip position
loop in the task space. This servo loop and orientation requirement.
compares the output of the forward
kinematics to the desired Cartesian position The gravity compensation precomputes the
and applies a correction to the input of the torque load of each joint based on the static
inverse kinematics to reduce the error ...... weight of the links. This value is added to
the feed forward field of the UMC joint
The harmonic motion generator allows the servo. Such compensation improves the
robot to execute autonomous motions. These robot positioning accuracy substantially.
motions currently consist of straight line
The smart hand interface controls the
segments but curved segments will later be
introduced. The tip velocity of the robot is trimming values of the input channels of the
controlled as a function of the position along smart hand force sensor, it controls the
the line of motion. This velocity has a grasping and it reads the wrist force torque
sinusoidal profile. The compliance information. The forces and torques are
parameters can be independently preset for converted to the laboratory frame and they
every motion segment. This allows the are also low pass filtered. From the basic
robot, for example, to autonomously track 500 Hz force readings a 100 and a 5 Hz force
the inside edge of a hole or the outside is generated. More on the Smart Hand can be
envelop of an object. More on Cartesian found in Ref. 8.
Servo and Harmonic Motion Generator can be
found in Refs. 6 and 7.
290
The compliancefunctionmodifiesthe The two joint modes allow the operator to
desiredCartesianpositionaccordingto the control the joints using the hand controller.
forcesdetected. Therearetwotypesof
In joint-1 the upper and lower arm rotations
compliances,pringandratetype. Thespring are controlled, in join-2 the remaining six
typeof compliancecausesthe robotposition joints move.
to beproportionatlo theforce
acting on it.
This is equivalent to a spring. The Task mode controls the end effector position
integrating compliance causes that the and orientation via the inverse kinematics.
velocity is proportional to the force on the The current implementation freezes the
tip. This is equivalent to a viscous damping. joint 3 and 5 positions.
The two types of compliances can be mixed
with each other as desired. More on Advanced Bus, Advanced Processor
compliance control can be found in Refs. 5
and 6. Due to the limited processing performance of
our current system (1 MIPS per processor)
The joint servo function is performed by a and the limited transfer rate of our bus
dedicated processor. This processor runs the (MULTIBUS-I) we coded all of our
code generator software. The code generator computations in 32000 assembly language.
has a set of menues on which the user The arithmetic is performed using binary
defines the robot being controlled. Based on fixed point instructions to gain execution
this information the code generator writes time. Currently the control systems for the
highly optimal machine code that controls two (right and left) robots are not tightly
the robot. This makes it possible to switch coupled to each other. It is desirable for us
polarities of various devices at runtime to control both robots from the same tightly
without changing the hardware, to move a coupled environment and to increase our
joint from one output port to the next to processing and bus communication
facilitate debugging and to safely setup a performance. The only viable commercial
new robot without risking damage to the alternative at hand would be a VME bus
hardware. This software also performs system. It is generally agreed upon by the
calibration at power on time to establish the research community today that the VME bus
robot absolute position accurately. is no longer fast enough to support a high
Currently we cannot utilize the multiple number of high performance processors
redundancies of the robot fully because working in a closely coupled environment. In
special software will have to be written fact, it is our view that the very concept of
that recognizes the failure of various the bus for our application has to be
devices and switches over to their rethought instead of just trying to build yet
alternative. another bus that is a little faster then the
previous ones.
The control modes are the following:
For these reasons we came up with an
Freeze mode advanced bus concept to support our new
Neutral mode high performance multiprocessor
Joint- 1 environment. This system will have up to 16
Joint - 2 processors on a bus with 10 or 15 MIPS of
- Task performance each. This new bus concept
delivers about 10 times the performance of a
Freeze mode means that the robot does not VME bus at the same clock rate, and will be
move and the brakes are all set. This is an clocked at 20 MHz so we expect a better than
alternative to the robot being completely 20 fold performance increase relative to
turned off. VME bus systems. The processors will use
fiber optic links as their primary means of
Neutral mode allows the robot to be moved communication outside the cardcage. Each of
by hand, it is gravity compensated but the our processors will have 4 Mbytes of
control gains are set to 0. dynamic memory as well as a floating point
processor and various i/o functions.
291