manufacturing experiences of a high precision six axis ... · manufacturing experiences of a hi gh...

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

IIT Guwahati, Assam, India

08-1

Manufacturing Experiences of a High precision Six Axis Parallel Manipulator (Hexapod).

R. K Sachan*, J.J. Roy, H.B Panse, S.B. Jawale

Centre for Design and Manufacture

Bhabha Atomic Research Centre Trombay, Mumbai - 400 085. eeee----mail: mail: mail: mail: [email protected]@[email protected]@barc.gov.in, , , , [email protected]@[email protected]@barc.gov.in

Abstract

Six Axis Parallel Manipulator (Hexapod) is a very high precision robotic platform, having six degree-of-

freedom of movement. It consists of six motorized actuators, connected between the top moving platform and

the bottom fixed platform. The moving top platform obtains its six degrees of freedom movement from the

combined computed movements of six independent actuators. Travel range along X,Y,Z axis is ± 60mm with

positional accuracy of 20 µm. Rotational range about X,Y,Z axis is ±2degree with angular positional accuracy

of 10 Arc seconds. CDM has successfully designed & developed the technology for manufacturing these high

precision robotic platforms. Load capacity of this Hexapod is 1.5 T. The manufacturing of this sophisticated

robotic platform involved machining of precision components like end support Ball units for Actuators,

Actuator Housing & Motor Housing for achieving a very high degree of accuracy in positioning such a big and

heavy structure. The Ball unit & its locating elements were provided with Diamond like coating (DLC) to

reduce friction & improve wear resistance. Destination position of the top plate is fed to a controller pc based

programme in X, Y, Z, θx, θy&θz form. Control system works in closed loop with continuous feedback from

linear sensors and limit switches. This paper disseminates the varied experiences gathered in manufacturing

these very high precision robotic platforms.

Words:Parallel Manipulator, robotic platform, Hexapod, Precision-machining.

1. Introduction:

Hexapod is a Parallel manipulator and a

very high precision robotic platform, having six

degree-of-freedom of movement for positioning

and orienting, used for myriads of modern

applications.

Hexapods are still having the image of

being highly complex and are presently imported

in India, at exorbitantly priced tags. The design

& manufacturing development effort at CDM has

indigenised this exoticrobotic platformand has

made it, not only affordable, but extremely easy

to use.

Hexapodconsists of sixindependent

motorizedactuators, connected between the top

and the bottom platformplates,with the help of

specially designed end joints, at both ends of

theActuators thatprovide six degrees of freedom

to the Top Platform, from the combined

computed movements of these six independent

Actuators, while the Bottom platformsupportplate

remain stationary. It can move along X, Y & Z

axes and can also rotate about X, Y & Z axes.

The Bottom stationaryPlatformplateis supported

on three Manual course X - Y- Z Positioning

Mechanism

for initial coarse alignment of the manipulator.

The computer programme which

enables the controls of the Actuators from a

computer interface, have been also developed

in-house by CDM. Destination position of the

top plate is fed to the programme in X, Y, Z,

θx, θy & θz form. Control system works in

closed loop with continuous feedback from

linear sensors and limit switches.

This paper disseminates the varied experiences,gathered in the

successfulmanufacturing development of this

highly sophisticated robotic platform.

1.1 Hexapod Requirements& Challenges:

It is obvious that the performance of a

Hexapod depends on the quality and

symmetricity of the Actuators and has a strong

impact on the overall motion. But almost equal

importance isfor the joints, connecting the

actuators to the base and the moving top plate.

The entire design, manufacturing

assembling, testing & calibration of this

sophisticated system have been successfully

developed, first time in Indiaat CDM, with

high load capacity and positioning accuracy.

The challenging part of manufacturing

this sophisticated robotic platform was mainly


for machining precision

components,in batches mostly six

or its multiple,for achieving a very high degree of accuracy in positioning, such a big and heavy

structure.

2. Manufacturing Methodology:

Before starting productionof a prototype of actual size, small size prototype is made to

understand the functionality of H

Functioning of actuators and end

checked by varying the length of the all

Actuators. Change in length for Actuators

made as per the programming. After giving the

actuation, positioning co-ordinates of upper plate

are measured by co-ordinate measuring

machine(UMM)for ensuring the

accordance with generated by programming

2.1 Selection of Machining Method:

The sequence of operations, selection of

machine tools and cutting tools and CNC

machine programming for manufacturing of

parts require thorough knowledge of

process capabilities and its limitations.

Extensive trials were conducted to

establish machining processes& its

and testing procedure to achieve the desired

accuracies of the various parts.

2.2 Selection of Machines:

Since the components were required to be

manufactured in small batches & required high

degree of accuracy & precision, high precision

CNC machines were selected.

2.3Typical Components &its Vari

The major sub assemblies

manufacturing the Hexapod areActuator Sub

Assemblies, End Joint sub assemblies

Actuators, X-Y- Z Motion sub assemblies and

Top & Bottom Plates/platforms.

2.3.1 Actuator Sub Assemblies:

The Fig.1 depicts the typical cross

sectional view of an Actuator sub Assembly. The

major components for manufacturing for this sub

assembly are Actuator Housing, Motor Housing, Gear Box Housing & End Joint attachment

components.

2.3.1.1 Actuator Housing

Itis the most critical part of the Actuator

assembly and is a large

gh precision Six Axis Parallel Manipulator (Hexapod).

andidentical

six numbers each

for achieving a very high degree of such a big and heavy

2. Manufacturing Methodology:

Before starting productionof a prototype small size prototype is made to

he functionality of Hexapod.

end joints is

ng the length of the all

for Actuators is

made as per the programming. After giving the

ordinates of upper plate

ordinate measuring

the output in

enerated by programming.

Selection of Machining Method:

The sequence of operations, selection of

machine tools and cutting tools and CNC

manufacturing of these

parts require thorough knowledge of available

limitations.

Extensive trials were conducted to

& its parameters

to achieve the desired

Since the components were required to be

& required high

degree of accuracy & precision, high precision

iations:

major sub assemblies involved for

the Hexapod areActuator Sub

Assemblies, End Joint sub assemblies for

otion sub assemblies and

depicts the typical cross

sectional view of an Actuator sub Assembly. The

for manufacturing for this sub

g, Motor Housing, Gear Box Housing & End Joint attachment

the most critical part of the Actuator

a large cylindrical

component,which houses the shaft, Ball screw and its end bearing

supports.It is made of Stainless steel (Grade

SS 304 -a material notorious for

in maintaining highmachining accuracy

needed process development for its successful

machining meeting the stringent dimensional

and geometrical tolerances.

The cylindrical component i

size and partially in split open condition

needs to maintain very high accuracy

Fig.2Sectional view of an Actuator

Fig.1Cross Section of

ActuatorAssembly

08- 2

the sliding Spline its end bearing

inless steel (Grade

for its difficulty

in maintaining highmachining accuracy) -

for its successful

machining meeting the stringent dimensional

component is big in

and partially in split open condition, but

o maintain very high accuracy

Sectional view of an ActuatorHousing

Cross Section of an

Assembly

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12


forconcentricity of ID dimensions.

tolerances of IT6/IT7 grades

geometrical features are to be achieved on diameter and 395 mm long cylindrical

Achieving close tolerances and to maintain its

straightness andconcentricity, within IT6

tolerance range is extremely difficult

of rigidity and material involved.

Multi stage machining process

followed to achieve thedimensional geometrical features. Inter stage annealing

process was done to get dimensional stability at finish machining stage by reliving heavy

machining stresses involved in pre

stage.CNC machining of IDs, keyway machining

and slot milling was adopted to achieve the

desired accuracy and tolerances. Finally

grinding of IDs, holding the partially split

Actuator housing with a dummy couplingthe Spline Shaft, got the desired accuracy

concentricity of ID dimensions and

performance results for the Actuator assembl

2.3.1.2Actuator Motor Coupling:

Fig.3Finish machined Actuator Housing

Fig.4Actuator Assemblies

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

ID dimensions. Dimensional

grades and close

are to be achieved on 95 mm cylindrical parts.

and to maintain its

, within IT6

tolerance range is extremely difficult, due to lack

Multi stage machining process was

edimensional tolerances& Inter stage annealing

dimensional stability at finish machining stage by reliving heavy

involved in pre-machining

keyway machining

was adopted to achieve the

Finally, finish

, holding the partially split

housing with a dummy coupling to suit got the desired accuracy for

concentricity of ID dimensions andgood

the Actuator assembly.

The component couples the Ball

Screw & Motor at ID side and Actuator

Housing & motor Housing on OD. Coupling

has a bottle bore, needs good dimensional and geometrical accuracies.

The coupling was initially

in split pieces and welded together

the bottle bore and then finish CNC

for achieving dimensional and geometrical

accuracies.

2.3.2 Actuator End Joint Assemblies

End joint connects the Actuators to the Top & Bottom plate, with a good amount of

multi directional flexibility required to achieve

the six axis movement of Top platesame time with high accuracy

strength to transmit the required

and load to the top platform.

It was tried with all types of possible

joints & came up with joint designs that are

not only simple, but also provide significantly

good performance joints. This resulted in developing a Hexapod that provides more

rigidity and higher load capacity.

variations of the joints were manufactured and

tried & tested.

Finish machined Actuator Housing

Fig.6Finish machined ActuatorMotor

Coupling

Actuator Assemblies

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,

08-3

couples the Ball

Screw & Motor at ID side and Actuator

Housing & motor Housing on OD. Coupling

has a bottle bore, needs good dimensional and

initially machined

welded together to achieve

CNC machined

for achieving dimensional and geometrical

Assemblies:

the Actuators to the a good amount of

required to achieve

the six axis movement of Top plate, but at the and sufficient

required movement

It was tried with all types of possible

joints & came up with joint designs that are

not only simple, but also provide significantly

good performance joints. This resulted in developing a Hexapod that provides more

load capacity. Following

variations of the joints were manufactured and

ActuatorMotor


2.3.2.1 Hinged Joints:

Hinge joints are the simplest types of

joints, but if they are not aligned correctly, the

hinged object will not operate, the way it is

intended and will not operate on its intended

plane.

The important component

assembly are connectingBlock & Rod and

support brackets. Connecting block & Rod

of 17-4PH Stainless steel and finish machined in

hardened condition,maintaining very high

of accuracy. The assembly requirement

unit was to achieve co-planarity of axes of

Connecting Block & Rod within 0.010mm

proper selection of machine tools & sequencing

of machining operations, the required accuracy

could be achieved.

The limitation of the joint is it gives only

two axes flexibility, but gives sufficient strength

and accuracy of the joint more or less depe

alignment & co-planarity of the hinge pins

2.3.2.2 Ball Joints

A ball joint consisting of a ball in a

socket is a more flexible joint and

range of motion. Though it giveswider movement in all directions,and allow

movement also, its strength capacity is limited

and gives lesser accuracy of mo

transmission.

Fig.8Balls Joint components & Assemblies

Fig.7Hinges Joint Assembl

gh precision Six Axis Parallel Manipulator (Hexapod).

joints are the simplest types of

if they are not aligned correctly, the

the way it is

operate on its intended

components of this sub

lock & Rod and

support brackets. Connecting block & Rod were

4PH Stainless steel and finish machined in

very high degree

requirement of the

planarity of axes of

within 0.010mmBy

proper selection of machine tools & sequencing

operations, the required accuracy

The limitation of the joint is it gives only

axes flexibility, but gives sufficient strength

joint more or less depends on

planarity of the hinge pins.

consisting of a ball in a

flexible joint and allows wide

giveswider free and allow rotational

, its strength capacity is limited

accuracy of movement

The important component

assembly are Ball & its Socket taper bush and

this need to machine with very high accuracyThese are made of is of 17-

Steel, pre-machined & hardened to 45 HRC

and finish machined to high accuracy.

Spherocity for ball was archived within 5

micron & angularity for taper socket within 10

micron.

The Ball unit & its locating bushes were provided with Diamond like coating

(DLC) to reduce friction& improve wear

resistance. This coating is of 5

and surface hardness more than 70 HRC &

coefficient of friction less than 0.

2.3.2.2.1 Machining parameters

hardened Spherical Ball part:

The finish machining parameters of

Spherical ball for Ball Joint was established

with extensive trials and cutting tool details

get the required Surface finish, Spherocity &

dimensional accuracies. The

parameters,thus established for the

hardened ball for ball joint are given

Table-1.

Table – 1 Finish machining parameters

Material SS 17-4-

Hardness 45 HRC.

Machine tool Spinner CNC Lathe.

Cutting Tool Carbide

ISO VNMG 12 T3 04

Spindle Speed 1500 rpm.

Feed 0.005 mm

Depth of cut 0.02 mm.

Dimensional

control

Within 0.002 mm.

2.3.2.3 Modified Universal Joints

Modified Hinge joints are the similar

types of hinge joints, butthe hinge

better achieved by making all hinge pins as

single integral unit and bearing housing is

replaced by to Retainer. Theseare

is of 17-4PH Stainless Steel, pre

hardened to 45 HRC, and finish machined to

high accuracy. Advantage for this design of

end joint is that manufacturing easierat the

same time retains almost all the advantages of

hinge joints Balls Joint components & Assemblies

Hinges Joint Assembly

08- 4

components of this sub

are Ball & its Socket taper bush and

very high accuracy. -4PH Stainless

machined & hardened to 45 HRC,

and finish machined to high accuracy.

Spherocity for ball was archived within 5

micron & angularity for taper socket within 10

The Ball unit & its locating bushes were provided with Diamond like coating

DLC) to reduce friction& improve wear

of 5 to 10 micron

surface hardness more than 70 HRC &

coefficient of friction less than 0.10.

parameters used for

finish machining parameters of

was established

cutting tool details to

Surface finish, Spherocity &

ies. The cutting

the heat treated

are given below in

Finish machining parameters

-PH.

45 HRC.

Spinner CNC Lathe.

inserts

NMG 12 T3 04.

1500 rpm.

mm/revolution

mm.

Within 0.002 mm.

Universal Joints:

joints are the similar

types of hinge joints, butthe hinge alignment is

better achieved by making all hinge pins as

single integral unit and bearing housing is

Theseare also made of

4PH Stainless Steel, pre-machined &

hardened to 45 HRC, and finish machined to

for this design of

end joint is that manufacturing easierat the

same time retains almost all the advantages of

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014,


08-5

The assembly of the Hexapod thus

manufactured gave exceptionally good results

and the test result of the Hexapod is tabulated

below in Table-2

Table – 2

The Specifications &Accuracies achieved

For the Hexapod

Specification Range Accuracy

Range of Individual

Actuators

± 125

mm

±0.5

mm

Magnitude Range of

Motion in

X,Y,Z Direction

±60

mm ±0.02 mm

Magnitude Range of

Rotation about

X, Y, Z axes.

±2

degree

±5

Arc

second

Magnitude of

Maximum

Perpendicular load

1500 Kg

3.0 Conclusions:

i) The design & manufacturing development

effort at CDM has indigenised this exotic

robotic platform at affordable cost.

ii) Hexapod is a very promising positioning

platform for manipulationapplication for

nuclear beam systems.

iii) It has wider range of applications such as

six axis CNC machine platforms, robotic

surgical machines etc.

iv)Further development on curvilinear motion

actuators will enhance the usefulness of such

system.

4.0 Acknowledgements:

The authors thank the staff of CDM,

especially of Tool Room & High Tech. shop

and for the cooperation extended for the

accomplishment of this job.

5.0 References:

i)Development of Hexapod Based Machine

Tool - Hans-Jurgen Warneckel1, Reimund

Neugebauerz2, Frank Wielandz

2 (1)

Fraunhofer-Gesell schaft, Munchen, Germany (2)Fraunhofer-

lnstitutWerkzeugmaschinen und

Umformtechnik, Chemnitz, Germany

ii)Modeling, Simulation & Development Of

Parallel Manipulator (Hexapod)Singh J.V,

Mishra Vinay, Sinha A.K,

SutharR.L.Centre for Design & Manufacture, Bhabha Atomic Research

Centre, Mumbai-94

Fig.9ModifiedHinges Joint components

& Assemblies

Fig.10Hexapod Platform Assembly

manufacturing experiences of a high precision six axis ... · manufacturing experiences of a hi gh...

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