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

Home

NEXTPREVIOUS ê ê

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