rts accuracy enhanced

8/14/2019 Rts Accuracy Enhanced

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Accuracy Enhanced Robotic Placement ofComputer Hard Disks

byBruce Fiala, Senior Software Engineer, Robotics and Vision Group

RTS Assembly Systems International

Introduction

Anytime a robot must move to a locationthat has not been taught, accuracy must

be considered. The reliance on visionguided positioning and offlineprogramming techniques using CADgenerated point data methods have putan increased emphasis on therequirement. Historically, robotcompanies have never specifiedaccuracy, but a few are beginning totake the requirement more seriously.Integrators who have accuracyrequirements can apply innovative

techniques to supplement the limitationsof the robot.

The Mechanics of AchievingAccuracy

An accurate robot attempts to minimizethe difference between the robotsencoder positions, related to atheoretical relationship model of the arm

and the position and orientation of itstool flange in space. The spatialposition of an axis true position is neveradequately defined by just its encoderposition.

Inaccuracies can be caused by linkexpansion due to heating, a result of

duty cycle and changing ambienttemperatures. For serially linked arms,more degrees of freedom imply moretolerance stackup and nonlinearity.

Different arms of the same robot modelwill have different characteristics.Critical differences include castingdissimilarities such as length and twist,bearing fits, drive train linearity, andadaptability to variable loading.Characterizing these differences on a

joint-by-joint basis and recalculating thekinematic parameters for the armprovides some basis for improvement,

but because of nonlinearities anddynamics, further improvement can becomplex or impossible.

The Challenge

Engineers at RTS Wright, a custommachine builder in Nashville Tennessee,were challenged to provide an accuratesolution to a historically difficult problem

for a large disk drive manufacturer.Engineers from both companiescollaborated on an automated diskhandling solution for the loading andunloading of vertically hung pallets usedfor the sputtering process. Therequirement had been around for years,but past attempts were inaccurate,


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difficult to maintain, inflexible, and didnot meet the companies overallexpectations. In the disk drive business,volume and yield requirements are high.Speed, throughput, and flexibility areparamount.

RTS provided an efficient and dynamicsystem that would adapt to varyingpallet and process conditions. Thepivotal aspect of success was the abilityto enhance the AdeptOne robots truepoint accuracy to less than 0.127 mm(0.005) in the vertically oriented palletarea. The application detailed here wasduplicated 24 times and deployed tofacilities in the United States and the FarEast.

Sputtering Process Specifics

Computer hard disks are polishedaluminum substrates, which resemble aflat donut. Outside diameters rangefrom 55 to 95 mm. Thickness is either0.76 or 1.27 mm. The hole in the centeris slightly less than 25 mm in diameter.In various locations, at the periphery of

the pallet, there are thru-hole fiducialfeatures.

Disks are brought in from a micro-cleaning process into the Class 10 loadroom. They are loaded, one at a time,into vertically hung pallets and thencycled through a 20-step sputteringprocess. During sputtering, themagnetic media adheres to the disk.The pallet and disks go through rapid

heating and cooling cycles.Temperatures can reach 250 degreesC. Improperly seated disks can be

jostled from the pallet due to turbid airwithin the environment.

Pallets are reused for 200 to 300 tripsthrough the process. After many trips

through the process, the pallets warpsignificantly, sometimes greater than +/-2 mm (0.08). Each pallet has uniqueand changing warpage characteristics.

Each disk placement hole location isslightly larger than the disk. Left-to-right

loading tolerance (world Y) is +/- 0.25mm (0.01). The bottom outsidediameter of the disk is seated in ashallow groove with less than 0.3 mm(0.012) total clearance in X (in and outof pallet). If a disk falls from the pallet inthe sputtering system, the wholeprocess may have to be shutdown,causing hours of downtime and processrecertification. It is also unacceptable totouch the pallet with any disk edge other

than the landing area. A dent or scratchon the edge is detrimental to disk drivedynamics.

Figure 1Pictured is an interface screen thatallows operator interaction to which

holes will be loaded with 95 mm disks

Each of the forty in-process pallets issuspended by two pins on a flight bar.Bars are recirculated through the


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process by a monorail-like conveyancesystem. Each pallet measures 1 metersquare and is machined from 6.3 mm(0.25) thick aluminum. The pallet is

designed to be disk-type specific (Figure1). There are five different pallet types,and depending on the disk size, a palletcan hold between 55 and 105 disks.

System Features

?? True robot accuracy of less than 0.127 mm (0.005) in the pallet area.?? A self-calibrating system. Precise mechanical alignments of robot-to-robot or robot-

to-pallet are not necessary. Misalignments are compensated for in software. As aresult the two robots share the same coordinate system to within 0.05 mm (0.002).

?? Multitier calibration and verification routines.?? Mandrel tool offsets for each part type are self-discovered utilizing the modules-

driven laser system.?? Verification routines allow for easy sanity checking of each aspect of calibration.

?? Dynamically changing disk locations are measured to 0.0127 mm (0.0005)repeatability.

?? Each pallet hole location is determined in 5-degree space (X/Y/Z/Ry/Rz).

?? Dual, simultaneous hole discovery gauges for increased throughput.?? All disk placement locations are discovered there are no taught points.

?? Adaptive load/unload strategies.?? Loading and measurement mechanisms share access to the pallet.

?? Pallet instability detection.?? On-the-fly sensing of disk load integrity.

?? Location skipping if a disk presence is sensed prior to loading.?? Ability to bypass disks at load and compress at unload.

?? Tracking of groups of 25 disks from pallet load to pallet unload.?? AGV machine loading and unloading.

?? One version of software that is flexibly configured, based on file parameters, for ajob (load/unload), working level, disk type, etc.

?? Full error handling capability with retry options. If system is E-stopped during amove, the robot is immediately halted. Upon operator acknowledgment, the robotwill slowly complete the move in progress and continue with full speedloading/unloading.

?? The cell is completely controlled by the robot controller; there are no PLCs.

?? Application software is completely customized V+.?? System throughput of 3100 disks per hour for the smallest disk size, with an

efficiency rating of 99.9%. A load time of approximately 2.2 seconds per disk.

Cell Layout

An AdeptOne robot and two Adeptmodules provide the flexibility for thelocating and loading/unloading of palletlocations (Figure 2). Because of thepallets height and the robots 290 mm Z

stroke, three different machine platformheights are needed, for a total of sixworkcells, three load and three unload.

An overhead conveyance systemindexes pallets into the system. Thepallet is clamped at one point on the


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bottom. On the backside of the pallet,the orthogonal pair of Adept modules, ina Y/Z configuration, works in a planeparallel to the pallet. A lasertriangulation gauge held by the modulesrobot locates the pallet holes using edgeand distance sensing techniques. One

by one, each hole location is preciselydiscovered and transformed to a loadlocation for the AdeptOne robot in frontof the pallet. Hole locations are nottaught; they are mathematicallydetermined points in space. Thisrequires true robot accuracy.

Figure 2Plan View of an Unload Cell

The end-effector is simply two opposingmandrels passively carrying 13 disks.Disks are held vertically by tighttolerance grooves in the mandrel(Figure 3).

Carriers are shuttled in and out of thesystem by a U-shaped conveyancesystem. Once a carrier is indexed intothe load/unload area, a comb extendsup from below the conveyor to lift all 25disks. Each disk is held by a portion ofits circumference by a groove in thecomb. Disk carriers are staged in twos

so that carrier changing is not neededduring a single pallet cycle.

Techniques

?? Four of the five degrees of the diskposition are used in load/unloadposition compensation. The angleassociated with the fifth is comparedto a limit value to designate anextreme pallet warpage condition.

?? By relating CAD data of the disklocations relative to the pallets


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discovered fiducials, each disks Y/Zlocation is known

?? In order to read the backside of thepallet and relate the disk loadcenterline to the loading robot, thepallet is assumed to have a constantthickness

?? Clean room practices require thatdisks be loaded top-to-bottom andunloaded bottom-to-top

?? All disks in a comb must beexhausted before moving to the nextcomb.

Figure 3The robot end-effector is shown loading a disk after the location has been determined

by the modules based laser distance sensor

Calibration

The modules robot acts like acoordinate measuring machine; themodules reference plane is used as themachines master reference. All loadrobot inaccuracy characteristics andmechanical misalignments are mappedout relative to the modules. Themodules robot measurement system

must know how to precisely relate itslocation and distance measuring device

to the load robots quill centerline at anypoint in a 1 meter square work area

System calibration is only necessaryafter the machine is initially set in placeor when a module axis, robot, or laser ismoved. Daily calibration or calibrationafter power-up and homing is not


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necessary because the mechanicalsystems and their relationships remainconstant.

Calibration consists of relating the tworobots to one another and then creatinga lookup table of load robot

inaccuracies. Together each robotmoves to a grid of X/Y/Z data points.With the load robot stationary, themodules robot measures the position ofthe load robot and calculates the errorbetween the actual and desiredpositions. A grid pitch of 20 mmprovides an adequate number ofhorizontal points while keeping thecalibration time to a minimum.

At runtime, after the measurement robothas relayed a hole location to the loadrobot, inaccuracy factors in X/Y/Z areinterpolated from the lookup table andadded to the location.

The interpolation between 20 mm gridpoints can only be valid if the robot canaccurately achieve positions betweenthe grid points. The Adept SCARA,while not inherently accurate to the

desired level of 0.127 mm (0.005), waslinearly accurate between grid points.

This grid technique mitigates threedifferent types of error simultaneously:the mechanical misalignment betweenthe modules robot and the load robot inX/Y/Z, the load robot inaccuracies, andthe real-world conversion factor

differences between the laser and therobots.

Figure 4 demonstrates the systemrealignment and robot accuracyenhancement. Eight vertical rows ofload robot grid point data were read bythe measurement robot. The graphsshow a quasi-repeating pattern in eachrow. There are 47 points in each row.The Before Enhancement data is the

inaccuracy data taken at each of thegrid points. The After Enhancementdata was taken when the load robot wasmoved to a point midway between gridpoints after applying the interpolatedcompensation data from the lookuptable. This could be considered as theworst case positioning error. There aremore Before data points because theAfter data points are midpoints withinthe larger mapped area.


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Point Inaccuracy Before Enhancement Worst Case Point Inaccuracy After Enhancemen

Figure 4


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The linear trending of points in a row isdue to the mechanical misalignmentbetween the two robots. The nonlinearrelationship of points in a rowdemonstrates the load robotsinaccuracies.

Inaccuracies of theta about Z are notmapped out because of the nonlinearitybetween points. The Afterenhancement theta data presented isonly readings at the enhanced X/Y/Zlocations. The deviation shown is withintwo or three encoder counts, which is

within the positioning tolerance of theaxis.

Enhancement of robot world Y accuracyis minimal because the theta deviationdirectly affects Y point-to-point accuracyas defined by the SCARA robot module.

Since the application is most tolerantalong the Y axis, this is not an issue.The application is most critical of worldX (in and out of the pallet), whichdemonstrates the least amount of errorafter correction.

Application Challenges

??

Because of interference between the robot quill cover and the pallet conveyor, theouter-most mandrel slot is 7 inches out from robot quill centerline (Figure 3).Because of that, theta resolution and repeatability are crucial. One encoder tickrelates to a 0.127 mm (0.005) translation in Y and a 0.05 mm (0.002) translation inX, the most critical axis.

?? SCARA robots are designed to work in a horizontal plane, this application works in avertical plane. The benefits of Z axis rigidity in a horizontal workspace is of nobenefit here.

?? Each robot has unique mechanical differences.?? The coordinate system origin and, reference frame of the modules robot are adopted

by the load robot cell. The modules inherent accuracy must be an order of

magnitude greater than the minimum accuracy requirement of the cell.?? While inaccuracies at any fixed location in the pallet plane were mapped out, the

accurate control over the path for the loading motion was still critical for good loadintegrity.

?? Regardless of finish or color of the pallet, the laser must read linearly over the entiresurface.

?? The laser sensor edge triggering capabilities must have low hysteresis.

?? The robot controller must be able to latch encoder locations repeatably, at highspeed, as a result of that trigger.

?? The vertical reach of the SCARA arm was limiting. A six axis robot was tested in thisapplication and failed. The arm could not linearly move between grid points within

the required accuracy.

Conclusion

It is feasible to enhance the accuracy of some robots even though the precisionrequired is not available off-the-shelf. It is not possible to bridge the gap between


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inherent capability and enhanced capability without a powerful robot controller and anin-depth understanding of robots, controls, coordinate transformations and sensingtechniques. Once a level of accuracy can be guaranteed, a new class of robust andadaptable applications can be realized.

________________________________

Bruce Fiala is a Senior Software Engineer in the Robotics and Vision Group at RTSWright Industries LLC, 1520 Elm Hill Pike, Nashville, TN 37075; 615-361-4111 ext.3241, [email protected].

For this application, RTS Wright was presented the Advanced Technology AchievementAward by Adept Technologies.

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rts accuracy enhanced

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