thermal robotic arm controlled spraying...

THERMAL ROBOTIC ARMCONTROLLED SPRAYING (TRACS)

D. Breen, E. Coyle and D. M. Kennedy

Faculty of EngineeringDublin Institute of Technology

DublinIreland

[email protected]

Keywords: Thermal spraying, Robot Feedback Control.

Abstract

Design of a Thermal Robotic Arm Controlled Spraying(TRACS) system, is described. The research encompassesnovel design features for a robotic arm manipulator, includingcontinuous 3600 link rotation, together with automaticanalysis of the thermal spraying process for feedback controlof the robotic arm manipulator. The technical and simulationdesign will provide for the automatic application of advancedsurface coatings to enhance wear, low friction and corrosionresistance properties to substrates via a thermal sprayingprocess.Three key research areas in thermal spraying technology aredescribed. These include the test rig and equipmentconfiguration, a novel design of the robotic arm, and thesystem control strategy. Technical design, simulation, andcontrol of the robot arm form the more research-intensiveelements of the project. Software simulation, document andgraphics production has been carried out using Matlab andMicrosoft products and these are described in the paper.

1 Introduction Thermal Spraying Technology

Thermal spraying involves the application of wear andcorrosion resistant coatings to various substrates and has beentraditionally carried out in the aerospace, power generationand petrochemical industries [1]Improvements in the technology have resulted in opening upof additional markets, in particular in the biomedical andelectronic coating industries. It is further possible today toapply coatings to polymer-based materials [2]

1.1 Benefits of Thermal Spraying

There are many benefits to industry from coating substratematerials. Benefits accrue from a wide choice of coatings [1]now available to improve the characteristics of particular

materials when compared to uncoated materials. Benefitssuch as improved wear characteristics will result in reducedmaintenance and replacement costs [4].

1.2 Thermal Spraying Systems

Thermal spraying is a generic term for a range of thermalspraying technologies. There are four systems High VelocityOxyfuel Spraying (HVOF), Plasma spraying, Arc sprayingand Flame spraying. Flame spraying for example is used inthe application of corrosion resistance aluminum to off-shoreoilrigs [1,3]. Another example of surface coating isbiocompatible hydroxylapatite coating of prostheses, whichare made of materials such as titanium, this is achieved withthe HVOF system.To understand the robot arm design issues concerned, it isnecessary to expand on the powder flame spraying systemwhich is used for this project

1.3 Powder Flame Spraying

Powder flame spraying is the oldest thermal spraying process[1,3]. However it is a process that is still used extensivelytoday because of the range of coating material and alloysavailable. The lower energies required reduce the health andsafety risks although there are still serious health and safetyrisks associated with this flame spraying technology such ashigh temperatures, combustion emissions and general dustparticles resulting from un deposited metal, polymer orceramic powders.The vast majority of components are sprayed manually andthe development of an automatic robot arm to carry out thethermal spraying process will reduce costs and health andsafety risks. A schematic of the powder thermal-sprayingprocess is shown in Figure 1. Fuel is fed to the nozzle, whichis ignited to produce a flame. Powder coating material is fedinto the nozzle where an aspirating gas expels the powder intothe flame. The coating material melts in the flame, which inturn is directed at the substrate material to be coated.Automatic feeding of coating material into the flame willrequire consideration for the final robot arm design.

Control 2004, University of Bath, UK, September 2004 ID-212

The molten material bombards the substrate and uponcooling, the coating mechanically bonds to the substrate. Tominimize porosity the coating material should be directedperpendicular to the substrate. This is an important design andcontrol feature of the robot arm. Table 1 lists the maincategories of coating materials [9].

1. pure metals2. alloys3. nitrides4. carbides5. Graphite-iCTM

6. Polymers

Table 1: Coating Materials

Table 2 lists the characteristics of the thermal flame sprayingprocess [3].

1 Particle velocity 40 m/s2 Adhesion MPa < 803 Oxide Content 10 –15 %4 Porosity 10 – 15 %5 Deposition Rate 1-10 kg/hr6 Typical deposit thickness 0.2 – 10 mm

Table 2: Characteristics of thermal spraying

Some of these characteristics will be used to determine thequality of the robot design such as porosity control. This willbe a measurable quantity between manual spraying andautomatic spraying.

Figure 1: Powder Thermal-Spraying Process.

1.4 Coating process

The coating process must occur in a very precise manor toensure quality control. Following surface preparation, therecan be up to three main stages in applying a metallic coatingto a substrate and the location of the torch for each stage is animportant design and control parameter. These include pre-heating the substrate, spraying the substrate with the coatingmaterial and finally fusing the coating to the substrate.Clearly the length of time taken for each stage is anotherdesign and control parameter, but this paper will concentrateprimarily on torch location. Locations in the torch flame ofthe three stages described above are shown in Figure 2.This can be programmed either by open loop or by automatedclosed loop control. Closed loop control will featureprominently in the finalised prototype system design.Methods to measure these locations and control of the torchare discussed in section 3.

Figure 2: Torch Control Locations

2 Test Rig and Equipment

2.1 Thermal Spraying Torches

A full range of powder thermal spraying equipment isavailable for use by the researchers of this project. Examplesof some of the types of thermal spraying torches available areshown in Figures 3a and 3b. (Type 3a. is a powder fed torchtogether with coating material feed bottle; 3b. is a polymerfed torch).

Figure 3a. Metal spray torch. Figure 3b. Polymer torch.


2.2 Two-axis Robot Test Rig

At this early stage in the research, the design anddevelopment of a two-axis robot is underway to enablepowder flame spraying in a semi-automatic environment, becarried out. A portion of the test rig, which will provide aprismatic / linear axis drive, is shown in Figure 4.

Figure 4 Prismatic Drive Unit

A new phase-angle power converter and microcontrollercontrol unit has been designed and built for this prismaticdrive. This drive with the rotary drive unit which has its owndedicated power controller provides the basis for a two-axisthermal spraying robot which will be used for thermalspraying feed back control testing and analysis.This setup will provide a test rig for thermal spraying of theexternal surface of cylindrical objects. A plan view of the testrig layout design is shown in Figure 5.It is intended that the prismatic drive will have the thermalspray gun attached so that is can be moved up and downalong a horizontal axis. The cylindrical object to be sprayedwill be fixed to the rotary axis.With the new control unit this set-up will provide areasonable degree of flexibility and thermal spraying optionsfor cylindrical objects, in addition to enable experience begained with automatic spraying.

Figure 5: Thermal Spraying Test Rig

One of the design problems was the attachment of theoxyacetylene torch hoses to the moving prismatic / linearaxis. This can be solved using a spring loaded oxyacetylenehose reel mechanism, which is commercially available.

3 Design and Control of the Novel Robot Arm

3.1 3600 Continuous Joint Rotation and Kinematics

Workspace analysis of robot manipulators shows thatmaximum workspace occurs when rotary joints have 3600

rotation. The first novel approach in the current research is inthe design of a robot joint that provides not only 3600 rotation,but also continuous 3600 rotation. One serious problem withthis type of joint is the cabling of equipment beyond the joint.Prior to addressing design issues it has been necessary tostudy a number of fundamental concepts, in particular robotkinematics and dynamics. This paper will address inparticular the forward kinematic equations of a five-axisarticulated robot arm with three-3600 continuous rotationjoints. A corner stone in design of any robotic system is thatof determining the kinematic equations required to model therobot arm. The kinematic equations describe themathematical relationships between robot joint space and tool(in this instance, torch) space. Joint space may be angles inthe case of rotary joints (drives) or distances in the case ofprismatic joints (drives). Torch space specifies the torchesposition and orientation in the working environment.Denavit-Hartenberg (D-H) [5,6] presented a process fordeveloping a mathematical model of robot manipulators,which relates joint space variables with torch space positionand orientation. This model is unique to each and every robot,yet requiring only four kinematic parameters. Theseparameters are listed in Table 3. There is only one variabledependent on joint type the remaining parameters are fixedvalues dependent upon robot construction. The variables arejoint angle for revolute joints or joint length for prismaticjoints.

Parameter SymbolJoint angle θJoint length dLink length aLink twist α

Table 3 Kinematic Parameters

Following consideration of various robot workspace envelopedesigns, including cartesian, cylindrical, spherical andarticulated, a decision was made to utilise the articulatedsystem, as this is a more suitable form from ananthropomorphic perspective. The D-H process [5] on asimple 6-axis articulated robot arm shown in Figure 6,produces a highly non-linear and coupled homogeneousmatrix, as shown in equation 1. Unit vectors n, o and a


z0 y0

x0Base

Torch

oa

n

p

provide the orientation of the torch tip in terms of basecoordinates and vector p provides torch tip position in termsof base coordinates.

)1(

1000

=zzzz

yyyy

xxxx

HR

paon

paon

paon

T

Figure 6. Six-Axis Articulated Robot Arm

Each component of RTH is a non-linear coupled equation, anexample of which is shown in equation (2)

)2()( 6516234652341 CSSSSCCCCnx −−=

where C1 represents cosθ1.

An analytical solution will provide joint angles based onposition and orientation data, for example, the equation forjoint angle θ1

is shown in equation (3).

)3(2tan1

=

x

y

p

paθ

The robot design for this project will require a five-axisarticulated robot arm, three for position each with continuous3600 rotation and two for tilt and pitch. There is no addedadvantage in having roll for thermal spraying, and this willreduce the cost of the robot arm. However to providecontinuous 3600 rotation the shoulder and elbow joints mustbe offset in the same direction, as shown in Figure 7.

Figure 7. Five-Axis Robot Arm with 3600 Rotations

These offsets complicate the Kinematic equations, but by wayof an example the transformation matrix, developed for thisrobot arm, from frame 1 to 2 is given by equation (4).

2 2 2 2

2 2 2 22

2

cos sin 0 cos

sin cos 0 sin(4)

0 0 1

0 0 0 1

a

aA

d

θ θ θθ θ θ

− =

The overall transformation from base to torch tip is given byequation (5).

RTH = A1A2A3A4A5 (5)

These equations are too large to reproduce here. The methodused to obtain this multiplication was to use the Matlabsymbolic toolbox. Work is ongoing to develop the inversesolution to this matrix. This will provide joint angles givingtorch tip position and orientation for a particular torchtrajectory. The forward kinematic equations may be used todetermine other robot control equations, such as the robot’sJacobian Matrix.In addition to kinematics the control of robot manipulatorsmust consider robot dynamics and the development of controllaws to manage those equations. However the dynamicequations of a robot manipulator are particularly complexwhen we are considering articulated robot manipulatorsbecause they are non-linear and highly coupled. The generaldynamic model of a robot manipulator is a second orderdifferential equation with highly non-linear and coupledparameter matrix coefficients. The matrix form of thedynamic model [7] of a robot manipulator is given inequation (6)

Z0 y0

x0

Base

Pitch

Yaw

ano


where D(i,j) is an inertia tensor matrix term, Ccent(i,j) is acentrifugal matrix term, Ccor (i,j,k) is a coriolis matrix term, hi

is a gravity term, b is a friction term and τi is a torque term. Anumber of methods of analysis are under investigation, inparticular gravity compensation techniques as gravity plays asignificant part in the dynamic model of articulated robotmanipulators. Due to the twisting nature of the offsetsproviding the 3600 robot arm capability, the friction term isalso likely to play a more than normal role in the dynamicmodel. Speed and acceleration will not pose a significantchallenge in the robot design, therefore the centripetal andcoriolis forces are likely to play a somewhat less significantroll in the dynamic model of this design.

3.2 Cabling 3600 Continuous Rotation Joints

One significant problem with 3600 continuous rotation jointsis the cabling of power and data along the robot arm. TheHarmonic Drive and Megatorque motor have hollowshafts that allow cabling through the device, but to date donot allow continuous rotation.A solution being investigated is to make the electricalconnections via slip-rings and brushes as shown in Figure 8,but omitting traditional materials such as copper for the sliprings and carbon brushes, as these materials would not besufficiently robust.

Figure 8. Slip-Rings for 3600 Rotation

A material with low friction, low wear and low resistivity, isrequired. Such material would not wear out significantly overthe life of the actuator, would not introduce any significantfriction and owing to its low resistivity would provide goodelectrical connections.Having researched available material types with the requiredproperties, a material referred to as TM 117C [8] may besourced and used in this application. This material is made of

a nickel-phosphorus matrix with particles ofpolytetrafluoroethylene (PTFE) uniformly distributedthroughout the matrix. A key advantage of the material is,that as wear occurs new PTFE particles are exposed,maintaining a low coefficient of friction. Further research andtesting of this is required, however some of its featuresinclude the following;

TM 117C heat-treated 590 0F

Coefficient of friction between 0.1 - 0.7 Electrical resistivity 130-200 µΩ-cm

3.3 Real-time Measurement of Surface Coating Thicknessas a Robotic Arm Control Parameter

Implementation of closed-loop trajectory planning andexecution will require feedback control signals. An importantcontrol parameter of thermal spraying is the automatic depthmeasurement of the coating being applied to the substratematerial. To achieve this, a low-cost light emitting diode(LED) and charged coupled device (CCD) cameracombination and triangulation, is being considered. A typicalconstruction is shown in Figure 9.

Figure 9. Light Emitting Diode (LED) and Charged-Coupled Device (CCD) Camera Combination

Investigation of the correlation between change in depth ofsubstrate material and movement across the CCD chip isongoing. A single wavelength LED and matched CCDcamera will be required, however interference will besignificant from the thermal spray. Implementation of theDigital Signal Processing algorithms will be achieved withprocessors from the Texas Instruments TMS320C6000 familyof DSP chips. A typical LED-CCD combination maycomprise a Gallium Phosphide LED with a wavelength of 550nm and a monochrome CCD camera with peak response inthis region.

)(:1

)6(),,(

),(),(

1.

2

1

actuatorsofnumberNifor

bhkjiC

jiCjiD

iii

N

jkkjcor

jcent

N

jj

=

=+++

+

+=

=

τθθθ

θθ


3.4 Real-time Control of Torch Perpendicularity toSubstrate

In order to minimize surface coating porosity, the torch mustmaintain a 900 angle with the substrate. To achieve this it isintended to set up an experimental system comprising twoCCD cameras positioned at right angles as shown in Figure10, enabling image processing of the edge of the torch bluecone area at the tip of the torch as shown in Figure 11. It maybe possible using edge detection techniques such as Roberts,Sobel or Canny edge detection algorithms [10] to use theedges of the blue cone as control signals for maintaining thecorrect torch position. This area will require further researchand testing.

Figure 10. CCTV Set-up to Check Perpendicularity

Figure 11 CCTV Set-up: Image Processing of the Edge ofthe Torch Blue Cone Area at the Tip of theTorch

4 Conclusions

The design and control of a robotic arm for depositing surfacecoatings onto substrates in hazardous environments has beendiscussed and presented. A number of novel features havebeen discussed which would make this robotic arm userfriendly and robust. The application of robots for conductingdemanding and precision type work in hazardousenvironments is not new but the ability of a robot system toapply surface coatings to a prescribed thickness, making use

of feedback vision systems and allowing a greater degree ofmovement offers new potential for this technology. Based onthe present research and investigation, a final prototype of arobot arm incorporating the design features discussed in thepaper will be developed and tested in specified workingenvironments.

References:

[1] Air Products “Thermal Spraying” (accessed 6/03)

http://www.airproducts.com/Products/CylinderGases

/MAXX/ThermalSpraying/thermalspraying_techpap

er

[2] G. England “Thermal Spray Coatings on Carbon and

Glass Fiber Reinforced Polymers” (accessed 6/03)

http://www.gordanengland.co.uk/frpapp

[3] Richard Halldern “Flame Spraying” TWI (2001)

http://www.twi.co.up/j32k/protected/band_3/ksrdh00

1.html

[4] David M. Kennedy “Surface Engineering

Addressing Maintenance Applications” DIT

[5] Saeed B. Niku, Introduction to Robotics Analysis,

Systems, Applicatiosn, pp 67-75, Prentice Hall 2001

[6] Robert J. Schilling, Fundamentals of Robotics

Analysis and control, pp 51-54, Prentice Hall 1990

[7] Wesley E Snyder, Industrial Robots: Computer

Interfacing and Control, pp208-209, Prentice-Hall

[8] Techmetals Inc. “TM 117C coating”

[9] Teer Coatings Ltd “Coatings”

[10] Paul F.Whelan, Derek Molloy, Machine Vision

Algorithms in Java Techniques and Implementation,

pp 82-90 Springer 2001.

Substrate

Torch head to becontrolled

CCD imager


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