development of four-piece servo manipulator
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60 Special Issue | October 2014
BARC NEWSLETTERFounder’s DayDEVELOPMENT OF FOUR PIECE SERVO MANIPULATOR
R.V.Sakrikar, U. Sarkar, D. D.Ray, B. Sony, D. C. Biswas and K. Jayarajan Division of Remote Handling and Robotics
Abstract
Traditionally, the Master Slave Manipulator (MSM), installed through the standard wall-sleeve, is used as dexterous,
remote handling tool inside the hotcells for carrying out various operations. The next generations of manipulators
are the electrically powered servo manipulators, which are installed fully inside the hotcells, usually on a movable
platform or gantry, thus providing larger flexibility and reach. However large numbers of hotcells in the nuclear
industry are designed for the use of wall mounted manipulators. The need for extending the superior features
of a servo-manipulator to the wall mounted MSMs led to the conceptualization and development of the Four
Piece Servo Manipulator (FPSM). The FPSM, which can be installed in the hotcell through a standard wall sleeve,
is a bilateral master-slave servo-manipulator with force feedback. The FPSM control system is based on a tightly
coupled distributed digital micro-processing technique. The paper describes the overall design and implementation
of FPSM.
Shri R.V. Sakrikar is the recipient of the DAE Scientific &Technical Excellence Award for the year 2012
Introduction
Remote handling plays a vital role in all nuclear
installations. In such facilities, the operators can
handle the material only behind thick shields, using
remote handling tools. The remote handling tools
are expected to carry out varied tasks inside the Hot-
cells. Master Slave Manipulators (MSMs) are the most
dexterous and widely used general-purpose remote
handling tools in the nuclear industry. A typical MSM
has a Slave arm located in the active area of the hotcell
and a Master arm, which controls the Slave arm, in the
operating area of the hotcell. The motions imparted by
the operator to the master arm are replicated by the
slave arm to carry out the desired tasks. In MSM, the
two arms are connected mechanically, across a wall-
mounted through-tube [1].
The designs of the various MSM models, varying in
their reach and payloads, have been standardized and
deployed in the hotcells, in the department, in large
numbers.
The generation of MSM, was followed by Servo
Manipulators [2], where the Master and the Slave
arms are linked together electrically by an appropriate
control system. Mounting of the slave arm on a
movable transporter, like a gantry or mobile platform,
inside the hotcell, results in higher flexibility and reach.
In a mechanical manipulator, the efforts for performing
a task have to be provided by the human operator,
whereas in servo manipulators they are provided by an
external power source, making them more operator-
friendly.
The endeavor to provide the obvious advantages of
the servo manipulator technology to the conventional
hotcells users led to the conceptualization, design and
development of the unique design of the Four-Piece
Servo Manipulator (FPSM), which can be installed
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CONTENTS
Special Issue | October 2014 61
BARC NEWSLETTERFounder’s Daythrough the standard wall sleeves provided in the
hotcells.
The Design
Mechanical Design
The FPSM has four distinct parts or subassemblies:
the Master arm, the Slave arm, the Through-tube and
the Motor-drive-unit. Fig. 1 shows the schematics of
the FPSM along with its four distinct subassemblies.
The FPSM is an adaptation of the existing Three Piece
Manipulator (TPM). The Slave Arm and the Through
Tube of FPSM are identical to those of the TPM. The
Master Arm of TPM is replaced by the Motor Drive
Unit, which is interfaced to the Through Tube. This
design improves the availability of the system as the
Motor Drive Unit can be replaced by the standard
Master Arm in case of faults in the Motor Drive Unit or
the control system.
with the position sensors necessary for their operation.
The various slave arm motions are illustrated in Fig. 2.
Fig. 3 shows the actual slave arm of the FPSM and the
Table 1 shows the range of motion of the FPSM Slave
Arm.
Fig. 1: FPSM Schematic
The Slave arm
The FPSM slave arm is designed to handle a payload
of 20 kg in any position. The slave arm has six degrees
of freedom and gripper. It has five articulated joints (X
and Y canting, Azimuth rotation and Wrist Rotation
and Elevation), a double co-axial telescopic joint and
gripper. All the joints are powered by the motors
contained in the motor drive unit. Each joint is provided
Fig. 2: FPSM Slave Arm Movements
Fig. 3: FPSM Slave Arm
Description ValueX Motion (X) -60° to +60° Y Motion (Ym), (Ye) -20° to +25°, -20° to +90°Z Manual Motion (Zm) 970 mmZ Electrical Motion (Ze) 900mmAzimuth Rotation (Az) -170° to +170°Wrist Rotation(Wr) -170° to +170°Wrist Elevation(We) -24° to +116°Gripper opening 80 mm
Table 1: Range of motions of FPSM Slave Arm
62 Special Issue | October 2014
BARC NEWSLETTERFounder’s DayThe Master Arm
The Master arm (Fig. 4) is iso-kinematic, scaled down
version of the Slave arm, with handgrip and HMI
keypads. It can be mounted on a trolley to offer
flexibility in terms of its stationing in the operating
area of the hotcell cold. The smaller size of the master
arm facilitates its positioning close to the hotcell
window resulting in improved operator visibility inside
the hotcell. The master arm is provided with position
sensors on all joints and force feedback actuators on
the major axes (X, Y and Z) and handgrip. The sensors
on the master arm generate the appropriate position
command for the slave arm joints, resulting in Master
Slave Follower operation. The master arm is fitted
with two keypads providing facilities for operational
parameter setting and operation of indexing motions
and joint locks.
can be carried out remotely. Sealing and shielding
can be provided in the through-tube, as per the site
requirements.
The Motor Drive Unit
The motor drive unit contains eight sets of servomotors,
gearboxes and position sensors. Motor drive unit
is designed in such a way that it is accommodated
within the standard clearances available around the
manipulator sleeve on the Hot-cell wall. The balancing
counter weights for major axes motions are provided
on the motor unit. The motor unit is coupled with
the Through-tube with an adapter. Apart from its use
in traditional hotcell architecture, the slave arm with
the motor unit can also be mounted on a transporter,
installed inside a hotcell, thereby exploiting all the
flexibilities of a typical servo manipulator. Fig. 5 shows
the motor drive unit mounted on a Through-tube
assembly.
Fig. 4: FPSM Master Arm
The Through tube
The Through-Tube of FPSM contains a set of parallel
shafts for transmitting motion from the motor drive
unit to the slave arm. The through-tube shafts have
slotted couplings at its either ends for engaging them
with shafts in the slave arm and the motor drive unit.
The coupling arrangement is such that the assembly
and disassembly of Slave arm and Through-tube
Fig. 5: Motor Drive unit mounted on Through-tube
Control System Design
The FPSM has a multi axis, bilateral, tightly coupled,
digital distributed control system [3], [4]. It provides
a master follower configuration with the position
control loop from master to slave. Force feedback
loops provide force reflection to the major axes (X, Y,
Z) and gripper of the master arm.
Special Issue | October 2014 63
BARC NEWSLETTERFounder’s DayThe main components of the FPSM control system
are the actuators, motor drives, joint controllers,
HMIs and the master control computer. The control
schematic is as shown in Fig. 6.
hardware for all master and the slave drives is identical,
with different master and slave firmware. The joint
configuration of the drive is carried out using the
onboard DIP switch settings. The control architecture
is designed for high speed data transfer between the
master and the slave drives for satisfactory operation
of the manipulator. The manipulator can operate up
to a distance of 100 m between the control panel and
the slave arm.
The Servo-drive (Fig. 7) is fabricated in two parts. The
main base board contains the power electronics and
servo controller components with necessary isolations.
The piggyback board, which interfaces with the base
board, contains the master micro-controller executing
the control loop, Resolver to Digital (R/D) conversion
circuit, brake control and the position sensing circuitry.Fig. 6: FPSM Control Schematic
Actuators and Sensors
The FPSM uses wash-down duty Brushless AC
servomotors, with inbuilt resolver and failsafe brake, as
actuators on both the Master and the Slave arm. The
motors operational characteristics suitable for accurate
position control and also have linear torque-current
relationship, essential for generating accurate force
feedback to the operator. The resolver is used to sense
the rotor position for the electronic commutation and
providing positional feedback for the control loop.
A multi-turn potentiometer, mounted on each joint,
provides absolute joint position for initializing the
Resolver readings at power up. All the joints are
provided with failsafe brakes for locking of the joints
on power failure and system faults. The operator can
also lock the joints in desired positions, whenever
required. The load gripper can be locked individually
for operator convenience during load handling.
Control Hardware
The manipulator joint controls are based on
indigenously developed BLAC servomotor drives. The
Fig. 7: The BLAC Servo Motor Drive
Operator Interface
The master control program, deployed on an Industrial
Computer, provides facilities for initialization of the
manipulator parameters, display of joint status, alarm
logging, and fault conditions. The GUI (Fig. 8) of
the Master controller provides access control for the
manipulator operation and continuously displays its
operational status.
The two keypad based HMIs, on the master arm (Fig. 9),
provide facilities for the operation of Joint brakes, slave
gripper locking, force reflection ratio (FRR) selection,
slave arm torque limit selection and Indexing motion.
64 Special Issue | October 2014
BARC NEWSLETTERFounder’s DayThe status of the parameters selected is indicated by
the associated LEDs on the HMI.
b) Force Feedback mode
In this mode, the slave motor current is used for
the calculation of joint load. This is applied on the
corresponding master joint, which is operated in
the torque control mode after necessary scaling and
filtering. The direction of the master motor torque
is always opposite to the torque generated by the
corresponding slave motor. The resulting joint-
torques contributes to the reproduction of the Slave
environment force at the Master handgrip.
The Master–slave follower mode and the Force
Feedback Mode can work together to operate the
manipulator in bilateral mode.
The control system has additional provision for setting
up of soft limits, beyond which the slave arm motions
are not allowed and a stall torque is reflected on the
master arm.
c) Indexed Motion mode
In this mode, the operator can independently move
the selected major slave joint, without moving
the corresponding master joint, using the keypad
interface. This can be used to move the manipulator,
along with the load, to the desired location, thereby
reducing the operator efforts. This mode can also used
to pre-position the Slave arm at an optimal position, as
per the operational requirements, before commencing
with the Master Slave mode thereby extending the
reach of the operator.
Performance Evalution
Master to Slave Positional tracking is a basic criterion
for performance evaluation of a master slave
manipulator. The main criteria for the evaluation are
the faithfulness of the system and operational effort
required for load handling. With the servo manipulator
the operator effort can be easily controlled by the FRR
settings. The evaluation of the Master to Slave tracking
Fig. 9: HMI Interfaces on Master Arm
Fig. 8: FPSM Supervisory controller GUI
Operation Modes
The following operator selectable modes are provided
through the control software:
a) Master Slave Follower mode
In this mode the slave arm, which is in the remote
area, follows the movement of the master arm and
the handgrip to execute the necessary tasks. The
primary role of the control system, in this mode, is the
continuous Master to Slave position tracking for all the
joints.
Special Issue | October 2014 65
BARC NEWSLETTERFounder’s Dayability hence becomes the most vital parameter. Since
the configurations of both the arms are same, only
the rotating angle of corresponding axes needs to
be compared for testing the tracking performance.
Fig. 10 shows a single axis tracking performance as a
representative result.
is carried out on the Slave torque feedback to filter out
the noise, to avoid spurious reflection of slave torques
on the master arm.
Conclusion
FPSM is a unique design, which can be installed through
the standard wall-sleeves of existing hotcell, thereby
providing a viable alternative to the conventional
MSMs. The FPSM provides advantages such as
reduced operator efforts and increased availability
along with facilities for sealing and shielding as per
site requirements. The manipulator is modular and
hence requires less space for installation and is easier
to maintain.
References
1. K. Jayarajan, and Manjit Singh, “Master-
Slave Manipulators: Technology and Recent
Developments”, BARC News Letter, Issue No. 269,
June 2006, pp. 2-12.
2. D.P. Kuban and H.L. Martin, “An Advanced
Remotely Maintainable Force Reflecting Servo
Manipulator Concept,” Proc. 1984 National
Topical Meeting on Robotics and Remote Handling
in Hostile Environments, pp. 407-415.
3. Jouve & D. Bui, “High Performance Servo Drive
Design for Distributed Motion Control,” Proc.
June19-21, 2001 PCIM’ conference, Nurnberg,
pp.1-6.
4. H.L. Martin, W.R. Hamel, S.M. Killough and R.F.
Spille, “Control and Electronic Subsystems for
the Advanced Servo Manipulator,” Proc. 1984
National Topical Meeting on Robotics and Remote
Handling in Hostile Environments, pp. 417-424
5. R. Kress, J. Jansen , M. Noakes and J. Herndon,”
The Evolution of teleoperated manipulators at
ORNL ,” Proc. of ANS 79th Topical Meeting on
Robotics and Remote Systems , pp 623-631
Fig. 10: Master to Slave Position Tracking
Fig. 11: Slave Torque and Reflected Master Torque
It is observed that the Positional tracking is in general
satisfactory. Although, there are errors in tracking,
in the range of 2-3 degrees at the motor shaft end,
the actual tracking error at the end effector will be
significantly less due to the mechanical reductions
between the motor and the joint output. Also, due to
presence of man in the loop, these errors get corrected
easily and hence are not significant [5].
Due to similar kinematic configuration of Master
and Slave, testing of a single axis torque tracking is
sufficient to test force reflection. Fig. 11 shows the
result of torque tracking with a FRR of 0.25. Smoothing
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