Automated Assembly of Expander Plugs

Diogo Catarino M14, Hugo Hjerten E12, Ignacio De Rodrigo,Joel Sirefelt M13, Ludvig Malmros E13, Marcus Peterson F12

Swedish Powertrain

Applied Mechatronics - EIEF01

January 17, 2018

Contents

1 Introduction 2

2 Background - Problem Identification 2

3 Method 2

4 System Level Research 34.1 Transport To and From Assembly Station . . . . . . . . . . . . . 34.2 Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

4.2.1 Global Positioning . . . . . . . . . . . . . . . . . . . . . . 44.2.2 Fine Tuning Positioning and Insertion . . . . . . . . . . . 5

4.3 Feeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.4 Gripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74.5 Plugging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.5.1 Pull-Style Expander Plug . . . . . . . . . . . . . . . . . . 84.5.2 Push-Style Expander Plug . . . . . . . . . . . . . . . . . . 94.5.3 Push-Style Ball . . . . . . . . . . . . . . . . . . . . . . . . 9

4.6 Robot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94.7 Concept Selection Matrices . . . . . . . . . . . . . . . . . . . . . 10

5 Solution Overview 10

6 System Level Design 116.1 Robot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.2 Positioning and insertion . . . . . . . . . . . . . . . . . . . . . . . 126.3 Plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146.4 Robot Tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146.5 Feeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

7 Preliminary Cost Analysis 167.1 Compared to Manual Labor . . . . . . . . . . . . . . . . . . . . . 17

8 Discussion 17

1

1 Introduction

The company Swedish Powertrain has an assembly line where one station in-volves human interaction to mount expander plugs into four holes in an engineblock (so as to not leak when in operation). This is a costly part of the assem-bly since it is operated 24 hours a day, seven days a week. Hence, they haveasked of us to find a solution in which this process will be automated. Thereare some different factors to weigh in to the solution as per their specifications(60 seconds/plugged hole, 4 holes/casting part, 10-12 parts/hour), such as cost,time, etc. In this report the different pro’s and con’s of different solutions arepresented, and a case made for a solution with push-style expander plugs.

Mission Statement: To automate the mounting of the plugs in sucha way that the company saves time and money, with good accuracyin the mounting.

2 Background - Problem Identification

Today there is a bottle neck in the production of an engine block (see figure 1)for Swedish Powertrain, during the assembly of four expander plugs. Today thisassembly is done manually, which is very inefficient for the production in termsof time. The engine block is first be taken out of the production line followed bya laborer who uses a pneumatic hand tool1 to put pull-style expander plugs intothe holes (see section 4.5 for a more in depth explanation of expander plugs).Then the engine block is manually taken back into the production line.

Swedish Powertrain wants to automate this process and incorporate it withthe otherwise already automated assembly line. The problem can be dividedinto subsystems:

1. Transportation of engine blocks to and from the station

2. Automatic feeding of expander plugs

3. Gripping of a expander plug

4. Positioning to find the holes to be plugged, both global positioning andfine tuning

5. Plugging of the holes

These subsystems will be discussed and examined in section 4, along with apresentation of some different robot alternatives.

3 Method

There are six members in the team that have worked with this project. Toefficiently divide the workload, the team split up in two groups. One groupconsisting of Diogo Catarino, Ignacio de Rodrigo and Marcus Peterson focusedon the robot arm tool (both the gripping of, and the actual plugging of, the

1http://www.emhart.eu/eu-se/produkter-tjaenster/produkter-efter-kategori/pop-nitsystem/nitverktyg/proset1600.php

2

Figure 1: Engine block with arrows pointing to the location of the holes to beplugged

plug) that was to be used, as well as the plug feeding mechanism. The othergroup consisting of Hugo Hjerten, Joel Sirefelt and Ludvig Malmros focused onthe positioning as well as the choice of robot.

The sub groups have met on a regular basis during the project span, and theentire team met roughly once a week to brief each other on the groups progress.This weekly meeting was of vital importance to make sure that the individualgroups progress were compatible with one another. A good example of this waswhen it was discovered that the push-sphere (push-style expander plug) neededroughly 3.75 kN to be mounted in the hole, but the MotoFit sensor only couldhandle 1 kN. This realization meant that the sub groups had to adapt andchange their ideas to be compatible with one another.

There was a continuous dialogue with the contact person Anders Goranssonat Swedish Powertrain, that provided the team with tips, information as well asmaterial for testing. His help was invaluable!

4 System Level Research

4.1 Transport To and From Assembly Station

The engine blocks (see figure 1) need to be transported from the previous stationto the station of the plug assembly. This can be done by using a conveyor beltor by utilizing either the robot from the current station or from the previousstation. The same way of transport can be used to transport the engine blockto the next station.

3

Figure 2: Two of four holes to be plugged

4.2 Positioning

4.2.1 Global Positioning

The global positioning is the problem of bringing the tool that is holding theplug, mounted on the robot, close to the holes. If you know that each engineblock is positioned in the exact same spot with marginal errors, the globalpositioning of the holes can be hard-coded. The problem would then be tofixate each engine block at the exact same place which can be done using aframe/fixture with pins that some specific holes in the casting are put through(see figure 3 for example).If the exact location of the engine blocks cannot be guaranteed, the holes can

Figure 3: Station Fixture

instead be located using computer vision by utilizing some specific details ofthe casting. The holes can be found by relating the robots position in threedimension to the details chosen on the casting, then relate the position of theholes to the position of the details. A camera would be needed for this approach

4

as well as software that can handle computer vision.

4.2.2 Fine Tuning Positioning and Insertion

When the hole has been globally located and the plug has been moved as closeto the hole as possible, the final positioning needs to be done when the plugis to put into the hole. Being a fairly trivial task for a human to do, it is nottrivial when automating, due to small clearance and tolerance constraints. Theproblem can be divided into two sub-problems (see figure 4).

Firstly, there is a positional/lateral error between the hole and the plugdue to tolerances of the hole placement on the part and the global positioningprecision. Secondly, there is an angular error between the hole and the plug.These two errors can result in large reactional forces causing jamming or wedgingand may also damage the robot. To handle these problematic errors severalsolutions have been explored.

Figure 4: Lateral and Angular Misalignment

Force ControlOne commonly tried solution to solve the problem is using force control together

Figure 5: Hybrid controller

with a hybrid controller for force and positioning control (see figure 5). In thisapproach a force sensor is added at the end of the robot arm right before a

5

gripper. This is used to feed back what forces are being applied at the end ofthe gripper allowing control of the forces being applied by the robot at the tooltip. Yaskawa’s MotoFit Force Control Assembly Tool 2 is an example of sucha force control tool, with the added benefit of already being compatible withSwedish Powertrain’s choice of robot supplier (Yaskawa). This tool exists in twoversions, 200 N and 1000 N, and it can not only measure force vertically, butalso at an angle.

Passive Remote Center Compliance (RCC)Another solution is to use Passive Remote Center Compliance or RCC. This

Figure 6: RCC concept structure

tool is mounted between the robot arm and the gripper, basically in the sameway as with the force sensor. The big difference compared to force control isthat it is a passive device and thus doesn’t provide any signal feedback to thecontrol system. The device itself is constructed of two or more plate frames heldtogether by flexing members(rotational and translational springs) which absorbthe initial lateral and angular error to be absorbed independently preventingjamming and wedging (see figure 6).

VibrationAnother passive solution is to create a controlled vibration either in the workingtable or in the robotic arm, causing the plug to vibrate with respect to the holewhich is supposed to prevent jamming and wedging.

Air Stream Assisted MethodsIn this solution an air stream is used to align and feed the plugs into their holes.The stream of air going through the hole creates reactional forces which guidesthe plug into the hole without jamming.

Computer VisionIn this solution a feed of images from a camera is used to locate the hole andguide the end effector with the peg towards and into the hole.

2https://www.motoman.com/hubfs/PDFs/motofit.pdf

6

4.3 Feeding

For different kinds of plugs different kinds of feeding systems are needed. Pull-style expander plugs (see 4.5.1) have a similar design to a T, in that it had astraight body with a bigger head. These need to be gripped in a way such thatthe body is turned up from the hole and the head inserted first.

Thus a sorting line is needed so that the plugs are sorted so the orientationbecomes true when it is time to grip. An early suggestion was a kind of machinein which the plugs are simply poured and the machine does the rest in termsof sorting and separating3. It looks like a big spiral in which the path becamesmaller and smaller, with a slit in the middle in the last part so that the bodyof the plug can fall down. Such machines are quite prevalent in the industry. 4

5

Another solution that is the integrated solution from a company specializingin plugs, POP. They have their own feeding system in which the plug packs arepoured into the system and it then handles the rest6. This has the advantageof being integrated with the tool solution for plugging with pull-style expanderplugs, a pneumatic gun. The plugs would literally be shot into the gun andthen held in place by the air suction.

For the the push-style ball (see section 4.5.3), which essentially is a steel ball,not much is needed in terms of feeding apart from some sort of line separatingthem for easier pick up. This does not need to be a complicated system sinceballs are readily mobile. Taking advantage of the shape of this kind of plug,another possibility arises. It consists on a bowl in which the steel balls arepoured. Gravity force guarantee that there will always be one steel ball at leaston the central point of the bowl. Thus, the robotic arm will be able to pick thiskind of plugs by just moving toward that point.

4.4 Gripping

To grip the plugs some kind of system is needed, especially to hold them inplace. For the pull-style plug (see section 7a) the pneumatic gun from POPwas suggested since it has pneumatic holding of the plug, which is desirable. 7

There was also speculations regarding a mechanical tool to grip and plug thepull-style plugs, but it was never fully explored. For the push-style ball (seesection 4.5.3) a version of electromagnetic gripping was explored since the ballis made of steel, and a round configuration is hard to grip.

4.5 Plugging

The requirement for plugging a hole is in this project to prevent leaking, up toa pressure of approximately 80 bar (with a safety margin). A common methodto do this is using an expander plug; a plug that, once in the hole, pressesagainst the edges of the hole to keeping it in place. Currently this is donemanually, using a pull-style expander plug. Another possibility is using push-style expander plug, or simply plugging the hole with a ball.

3https://www.youtube.com/watch?v=wX7lcQAa7s04https://www.assemblyauto.com/screw-feeders/5https://www.stoeger.com/en/bowl-feeder.html6http://poprivettooling.com/catalog/pop-riveting-systems/pop-rivet-presenters/7http://poprivettooling.com/catalog/air-pneudraulic/pop-proset-1600-series-rivet-gun/

7

(a) Pull-Style (b) Push-Style

Figure 7: Examples of Expander Plugs

4.5.1 Pull-Style Expander Plug

The main idea with a pull-style expander plug (see figure 7a) is to insert theplug in the hole, pulling the mandrel while keeping the sleeve in place, forcingthe head (that is wider than the sleeve) to be pulled through the sleeve whichin turn expands the sleeve towards the wall of the hole. It can only be pulledso far, until the top excess mandrel breaks off. The body breaks off from thehead due to a pre-induced fault, and the body must later be collected.

Swedish Powertrain uses the LK950 0408 plug model today (which is a pull-style expander plug), using a pneumatic gun to pull/plug. It uses vacuum topick up and suck the plug into the tool and pneumatics to create the force thatgrips and pulls the body back which makes the head expand. A ultimate tensilestrength (UTS) test was done on six pull-style expander plugs of model LK950040. The result can be found in table 1.

Table 1: UTS for Pull-Style Expander Plug

Test # 1 2 3 4 5 6 AverageForce [kN] 3.72 3.70 3.69 3.75 3.76 3.85 3.75

Since this kind of plugs are the ones used nowadys by Swedish Powertrain,the focus was set on finding the proper tool to work with them. As it hasbeen exposed on the Gripping section above, the POP tool satisfied both re-quirements. However, this kind of tool is also designed to be manipulated bya human while the main goal of this project is to dispense with the humaninteraction.

The first idea being explored consisted on a mechanical trigger replacing thehuman hand action. For this option, the robotic arm should hold the tool, anda mechanism composed by servos would push the built-in button which appearson production tools.

A more compact solution was considered. The goal of the nowadays button isjust to enable the air flow to run through the tool duct. So that, in order to have

8https://www.boltproducts.com/950-040-p-2536.html

8

a safer, faster and more accurate response to the plugging process, this button(and servos triggering) should be replaced by an air flow electro-mechanicalvalve, which only needs an electrical signal to be activated.

At last, due to all the constraints imposed by the Force Control Sensor andaccuracy problems, an own tool design was thought to be the best solution forthe Pull-Style Expander Plug.

4.5.2 Push-Style Expander Plug

A variant of expander plugs is the push-style expander plug where the expansionis achieved by pushing a ball into a sleeve, instead of pulling a body through thesleeve. However, the idea of using push-style expander plugs were at an earlystage replaced by using the simpler solution of push-style balls (see section4.5.3).

4.5.3 Push-Style Ball

Instead of using a ball in sleeve solution, it is possible to simply use a steel ballthat is pushed into a hole that gradually gets a smaller diameter. This solutionbasically only needs a robot arm pushing down on the ball. Two UltimateCompression Strength (UCS) tests were done pushing a 5 mm diameter steelball into o hole with the same dimensions as the holes in the engine block. Theresults gave a value of roughly 5 kN needed to push the steel ball into the hole(see table 2).

Table 2: UCS for Push-Style Ball

Test # 1 2 AverageForce [kN] 4.76 4.9 4.83

4.6 Robot

Table 3: MH-Series Specification

HorizontalReach[mm]

VerticalReach[mm]

Payload[Kg]

Repeata-bility[mm]

Weight[kg]

MH3F9 532 804 3 0.03 27MH6-1010 2468 1422 10 0.08 130MH2411 1.730 3.089 24 0.06 268

MH180 12 2.702 3.399 180 0.2 970

When choosing a suitable industrial robot to use in this workstation, a briefresearch was made on what different companies could provide. The companieswhich were looked into was ABB, Yaskawa and KUKA. Since the contact personat Swedish Powertrain recommended Yaskawa robots and noted that they hadgood relations with that company, a choice was made to scale down the amount

9

of possible robots and only look further into Yaskawa robots. To be able toperform precision positioning in a 3D space, a six axis robot was needed whichfurther scaled down the amount of industrial robots suitable13. See table (3)for more details of the different robots.

Two armed robot were not considered for this application, and hence the”SDA” series was not examined and main focus was on the Yaskawa ”MH”series.

4.7 Concept Selection Matrices

In this section, an overview of the different pros and cons are presented for thedifferent solutions. In the table matrices, the following notation will be used forevaluation:

• + = ’better than’

• 0 = ’same as’

• − = ’worse than’

Table (4) presents an overview of the fine tuning positioning alternatives, table(5) of the different available plugs, table (6) of the different robot tool alterna-tives and table (7) presents the different feeding system alternatives.

Table 4: Fine Tuning Positioning Matrix Overview

ForceControl

RCCVibra-tion

CVAir

streamComplexity 0 + + −− −

Cost 0 0 − − −Precision + 0 −− − 0

Initial error handling ++ 0 0 0 +Flexibility + 0 − + 0

Insertion errorhandling

+ − − − −

Available solutions + − − − −Speed − + + − 0

Total + + + 0 − − 0

5 Solution Overview

After a thorough research and weighing of pros & cons, the team came up withthe solution below. For a more detailed explanation, see section 6.

The robot from the previous station will be used to place the engine blockin a custom made fixture frame. A Yaskawa MH24 robot will be used, witha mounted MotoFit Force Control Assembly Tool, as well as a custom builtgripping and plugging robot tool. The robot will receive one pull-style expander

13https://blog.robotiq.com/how-many-axis-my-robot-should-have

10

Table 5: Plug Matrix Overview

Pull-StyleExpander

Push-StyleExpander

Push-StyleBall

Complexity − − +”Grip”-ability 0 0 ++

Cost 0 0 ++Pressure Limit 0 0 0Force Needed + − −−

Previous Experience ++ − +Company Interest/Preference 0 0 +

Total + 0 ++

Table 6: Tool Matrix Overview

Electro-magnet

Pop-Gun

Me-chanical

tool

Self-designed

tool

Complexity − − + 0Ability to integrate + −− 0 ++

Cost − −− 0 ?Comp. w. force sensor 0 − 0 +

Comp. w. push-style exp. + −− + 0Comp. w. push-style ball ++ −− − 0Comp. w. pull-style exp. − + + ++

Total − − 0 ++

plug at a time from a bowl sorting machine. Thereafter the robot will move theplug to the holes using hard-coded coordinates, using the MotoFit for the finetuning positioning. Once the plug is in the hole, the custom designed robot toolwill apply the plug using pneumatics. After the robot will drop the body piecethat has broken off into a container. Finally the engine block is removed usingthe robot from the previous station.

6 System Level Design

This section will provide a more detailed explanation for the choices made andthe design of the more critical parts of the system.

6.1 Robot

The robot that will be used for the plugging in this concept will be a YaskawaMH24. It will have the controller DX200. The Robot is chosen because itmatches our requirement relating to size and maximum payload. The key factorof choosing this robot was that it is compatible with the force control sensorMotofit. We could have chosen a bigger Yaskawa robot to have more layway in

11

Table 7: Feeding System Matrix Overview

RotationalVibration

FeederPop Feed Bowl

Ability to integrate 0 0 0Cost −− − 0

Comp. w. push-style exp. + − −Comp. w. push-style ball 0 −− ++Comp. w. pull-style exp. + + −−

Total + 0 −

maximum payload that it can handle, but the limiting factor then is it no beingcompatible with MotoFit.

6.2 Positioning and insertion

The engine block will be lifted from the previous station in the production lineby the robot in the previous station on to a fixture frame with pins that gothrough specific holes in the engine block. This fixture frame plate can be seenin figure (8).

(a) 3D model

(b) Design

Figure 8: Fixture Frame Plate

The holes in which the pins will hold the engine block in place are veryprecise tolerances according to our contact at Swedish Powertrain. This fixtureis always fixed in the same place which makes the global positioning easy toimplement. After mapping how the engine blocks are positioned on the fixturerelated to the robot, the rough position of the holes can be hard-coded suchthat the robot is able to position the plug on the article with a lateral andangular error small enough for the force control to take care of the fin-tuningpositioning. This calibration of this global positioning needs only to be doneonce per hole, such that the robot can do this preplanned action repeatedly foreach new engine block that is put on the fixture.

Out of the explored solutions for the plug insertion process, a force controlsolution was chosen as the best alternative. The reasons for this are as follows.

12

Table 8: Tolerances

Part in tolerance chain ToleranceGlobal placement previous robot ±0.2 mmPrevious robot gripper tolerance ±1 mm

Drill hole placement relative precice hole ±0.5 mmPlugging robot(MH24) position precision ±0.06 mmPlugging robot(MH24) angular precision ±0.5◦

Plugging tool tolerance ±0.2 mmHole angular ±0.5◦

Peg hole clearance 0.05 mmPlug hole chamfer allowed 0.1 mm

Hit area around hole 1 mm worst case

Total ToleranceTotal lateral error/position error less than 2 mm

Total angular error ±1◦

Total clearance 0.05 mm

Table 9: Motofit force control precision

Motofit information Value CommentFit clearance for insertion 10 µm At least

chamfer 0.1 mm At mostinitial position error ±1 mm Insertion without search strategyinitial angular error ±1.0◦ Insertion without search strategy

Firstly, it being an active solution unlike the other passive explored solutionsit has sensory feedback. With the help of this there is the possibility to detectinsertion failures and thereby also minimizes the risk of damage on the robot orarticle in case of an insertion error. Moreover, due to the sensory informationprovided this solution is more flexible using different algorithms and approachesfrom the research on the peg in hole problem.

Secondly, based on the numbers found from research papers it should be afeasible solution to the problem. It should be able to get a speed of the insertionprocess of around 1-5 seconds with no chamfer required, an allowable positionalerror of 2 − 3.5 mm, be able to handle high angular error at least for thoseinsertion strategies which purposely approach the hole at an extra tilted angleand achieve a clearance of 0.003 − 0.02 mm14.

Another reason for choosing force control is it being a well researched so-lution to the peg in hole problem allowing for a higher likelihood of successimplementing the solution. Also increasing the likelihood for success furtheris the availability of force sensors on the market from different robot manu-fatcturers. Currently there exist a lot for Yaskawa robots being used in therobotic park in Sibbhult. For this reason the force control sensor chosen wasthe Yaskawa MotoFit force control assembly tool as it is compatible with the

14https://core.ac.uk/download/pdf/2749955.pdf , p.20

13

chosen Yaskawa MH24H robot and the DX200 controller. Even though thereare other force control solutions on the market, this one was chosen as SwedishPowertrain has a previous relation with Yaskawa and knowledge about theirproducts which will increase the possibility of a successful implementation.

Unfortunately every solution also comes with potential issues. We haveidentified a few potential issues with the choice of our positioning and insertionsolution.

Firstly, the global positioning might not bring the tool close enough to thehole. Table (9) and table (8) does however show that the angular error of thesystem and the clearance are within the Motofit force sensor tolerances. Theuncertainty consists of many different parts as seen in table (8), some givenby manufacturer and some more self made rough estimates. This makes itvery difficult to guarantee an initial positioning error of ±1 mm required for theMotofit fitting process (table (8). Fortunately, when using our solution it isenough to have precision enough to hit the surrounding surface and then use asearch strategy to get within ±1 mm of the hole. This should not be a problemfor a majority of the holes. However, one of the holes has an edge close by whichin worst case only give 1 mm extra room outside the hole. Luckily with forcecontrol feedback a hit on this edge should be able to be detected and as thereis more room on the other sides around the hole the reference position could inthis case be set slightly further away from the edge and a search strategy usedto approach the hole.

Secondly, as force control is sensitive to peg geometry it is hard to knowif there might be problems with the groves on the finally chosen pull-style pegpotentially increasing the risk of jamming during the insertion. On the otherhand this may also be seen as an argument for choosing force control due to itsfeedback which increase the possibility to be able to handle this issue.

Lastly, as seen in table (9) the Motofit force controller require an chamferededge around the hole which the current part does not have. This is easily solvedby drilling an extra chamfer in the drilling process although we recommend totry it without first when implementing the system.

6.3 Plug

Even if there was a potential for simpler mating geometry with the push-styleplugs when using force control and additionally also a simpler plug pickup forthe push-style balls, the plug chosen was determined to be the pull style plug.This was due to the very high forces needed to insert the push style plug; theMotofit sensor that was chosen can not handle those high forces.

To facilitate for Swedish Powertrain, the same model pull style plug waschosen that is used for manual assembly today; LK950 040.

6.4 Robot Tool

Having chosen the slightly less easy-to-handle pull-style plug, there are not anygiven tools on the market that fit the requirements perfectly.

A tool that is compatible with the robot is required, but the majority on themarket are for manual handling. Additionally, the tools on the market do notmanage to hold the plug in place in a fixed, static manner. This is problematic

14

for the positioning since the force sensor is chosen. Having to conjugate thesedifferent aspects, a custom designed tool was chosen.

The custom design uses the same technique as the POP-rivet gun in that ituse pneumatics to both initially grip the rivet, as well as for the plugging. It iscylinder shaped with a hole in the middle, where the mandrel fits but not theplug, and with an air-tube on the side leading to it. The design can be seen infigure (9).

Two small pneumatic cylinders that work as a gripper are to be found insidethe tool to secure the plug for the fine-tuning positioning. They act as soon asthe robot starts the positioning, keeping the plug fixed, and retreat after thedisposal of the mandrel, waiting for a new plug to be picked up. For the toolto work, air tubes must be attached to both valves and to the side tube. Thisgripper is essential for the fine control since with just the air flow of the toolthe plug will wobble, due to uneven suction around the plug.

(a) Cross section(b) Exterior

(c) Transparent view

Figure 9: Robot tool

When the plug has found the hole and been inserted, the effector will createa suction high enough to cause the plug to expand and seal the hole. Themandrel will stay in the effector due to the suction, until the robot has movedto the ”waste collection”; a simple holder/collector within reach of the robotarm. Then the suction turns off, the cylinders retreat and the mandrel falls intocollector.

15

6.5 Feeding

The feeding of the plugs will be done through a bowl sorting machine, whichwill be bought from a company like Stoger Automation15 or Weber16. Theabsence of a definite choice is due to lack of answers from the companies, whichis suspected to stem from the author’s position as a student.

Their solution supports good feeding with refills far between, as well as anupside down orientation of the rivets when they come out of the feeder, whichcauses the gripping to become much easier for the robot. Another aspect to betaken into account is the cost, which again, has not been readily available.

7 Preliminary Cost Analysis

This section aims to present a rough cost analysis of the solution. Table (10)gives an overview of the fixed costs associated with the solution, and table (11)of the variable costs.

Since Yaskawa adapt the price of the products depending on many factors(order size, custom design etc...), we have based the Yaskawa product priceson a package price that we received from a mail conversation with a Yaskawasalesperson. Similarly, we have not received a price suggestion for the feedingsystems.

Table 10: Fixed Costs

ComponentCost[SEK]

Info.Source/Observations

Industrial Robot: YaskawaMH24

- Package Price

Force control sensor: YaskawaMotofit 1000N

- Package Price

Controller Yaskawa Dx200 - Package Price

Yaskawa Package Price 400 000 Salesperson

Feeding System 350 000 Salesperson quote

Fixture Station 10 000 Estimate

Installing robot 30 000 Estimate

Total fixed cost 790 000

Table 11: Variable Costs

ComponentCost

[SEK/h]Info. Source/Observations

Expander plug, pull style 110 Rough calculation

Own Tool Design X Unknown

Total variable cost X

15https://www.stoeger.com/en/16http://www.weberusa.com/feeding-systems/bowl-feeder-zeb/

16

When calculating the variable cost for the plug, the following requirementswere taken into account done: the cost for the LK950 040 plugs is 2.19 SEKeach; 60 seconds/plugged hole; 4 holes/casting part; 10-12 parts/hour. Thistranslates to a rough maximum hourly cost of 110 [SEK/hour].

The custom designed tool has economical implications which are difficult todetermine at this stage. The development of the tool is linked to a period oftime in which it must be debugged. Furthermore, the different prototypes whichshould be developed add an extra cost which is directly related to the numberof iterations needed to get the final tool.

7.1 Compared to Manual Labor

Estimate of todays manual labor cost: If the worker makes 20000 [SEK/month],the total cost for the laborer would be around 31000 [SEK/month]17. In realitythis is a shy estimate since the workers make more during night shift and week-ends. We count with 4 people working at the station each month thinking it op-erates everyday 24/7. That would be a total monthly cost of 31000∗4 = 124000[SEK/month]. Using this estimate the return of investment would only takeabout 7 months (790000/124000). This estimate does not take into account thevariable costs into consideration, such as maintenance and energy cost.

8 Discussion

This section presents some of the potential flaws and areas of concern.

Bottleneck on the series productionUsing the robot in the previous station to place the engine block in the fixture,could create a bottleneck. There is no way to store any units processed by theearlier station. A solution for this could be to have some kind of conveyor beltin between the production stations so that the previous station can still processthe engine block while the automated assembly of plugs station is working.

Changing to a of the steel balls solutionAlready at an early stage of the project, Swedish Powertrain informed that usingsteel balls as a solution was desirable -as a future feasible option-. The solutionpresented in this report is however based on the pull-style expander plug, withtools adapted to that type of plug. Therefore, should Swedish Powertrain wishto change plugs to steel balls in the future, the transition is not entirely easysince the tools are adapted to pull-style expander plugs. For a successful sucha transition, there are primarily three things that need to be changed.

Firstly the robot tool needs to be able to hold a spherical shaped plug. Thiscould most easily be done using a magnetic gripper; the benefit of a sphere isthat the orientation is not of importance. Secondly, a force sensor controller isneeded that can handle a force that is larger than 5 kN (which is required toplug a ball), and potentially the force sensor control algorithm must be adaptedto handle a spherical shape. Thirdly, a different feeding system would be neededto feed spherical plugs. This is probably the easiest to fix however, since thethe plug is identical in all orientations.

17https://www.blinfo.se/kostnad-for-en-anstalld-20110502

17

Adaptations Needed for New Casting Block Designs The componentshave been selected in that way that geometrical changes on the design of thecasting block can be perfectly managed by the system. For example, by justusing software, a new hole position (new distance reference, new angle...) couldbe plugged.

Nevertheless, other kinds of changes could occur. Change could for instanceoccur on the dimensions of the hole (due to a greater required oil flow intothe duct). This could lead to bigger expander-style plugs dimensions and inconsequence, to some changes on the dimensions of the tool (gripper, feedingsystem, etc).

Debugging time and budget cost of the custom designed toolAs there is no available solution on the market for the robot tool that fitsall the requirements, a custom tool was designed. However, the design andmanufacture of this tool is linked to a period of time of development, debuggingand improvement.

When buying a tool from a manufacturer, such as the POP-rivet gun, awarranty is provided and the implementation of the system is faster than in thiscase. The cost which has to be payed in order to have a custom tool design isa transition testing period which can lead to delays and budget costs.

18