LOCATING LCD GLASS FOR ROBOTIC HANDLING

J. A. Smith

Advanced Engineering, Corning Incorporated, Coming, NY 14831

ABSTRACT. The display industry is trying to improve yields by refining the automation processes inthe manufacturing of Liquid Crystal Display (LCD) Glass. To help achieve better yields, it isnecessary to locate with better precision the LCD panels with respect to robotic tools. Twoapproaches will be discussed to make ranging measurements on LCD Glass: a laser confocaldisplacement sensor and ultrasonic rangers. The knowledge gained from these measurements willimprove the positioning of the robotic tools with respect to the LCD glass during the manufacturingprocess.

INTRODUCTION

Corning and Coming's customers are trying to improve yields by refining theautomation processes in the manufacturing of Liquid Crystal Display (LCD) Glass. Tohelp achieve better yields, it is necessary to locate with better precision the LCD panelswith respect to robotic tools. As LCD glass becomes thinner, the structural rigidity of theglass goes down and the LCD panels deform more under the panels own weight duringprocess steps. Thus the need to locate LCD Glass in space to assist in the robotic handlingof glass during a manufacturing process is the motivation behind this research.

The unique characteristics of LCD glass makes optical and vision based sensingchallenging because LCD glass is optically transparent, optically smooth, thin (<lmm) andparticulate free. During the manufacturing of the large LCD glass sheets (>1 x 1 m), theglass is moving, hot (>100 °C), and contained within a dynamic processing environment.By combing the characteristics of LCD glass and the harsh manufacturing environment,the sensing of position and orientation would be challenging using any measurementmethod.

To protect the pristine nature of the LCD glass and be effective in a manufacturingenvironment, additional requirements are placed in the measurement method. The sensormust be non-contact and provide for good sensing contrast. The measurement system mustbe robust, easy to use, easy to implement, easy to maintain, and be cost effective.

This paper discusses two approaches which are currently being pursued to makeranging measurements on LCD Glass. The first approach is based off a laser confocaldisplacement sensor. This optical sensor will be used offline to measure the mechanicaldeformation of the glass sheets when supported on two edges. The second approach tolocating LCD glass is based on commercial ultrasonic ranging sensors and is intended tobe used as an in-process sensor. The knowledge gained from these measurements will

CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00

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improve the positioning of the robotic tools with respect to the LCD glass during themanufacturing process.

LASER CONFOCAL DISPLACEMENT SENSOR

At many LCD display manufacturers, the LCD glass substrates are stored incassettes that support the LCD glass at the edges. The glass substrates deform under theirown weight as shown in Figure 1. The fact that the glass substrates deforms or sags is notin of itself an issue. The true issue with sag is variability in LCD glass sheets betweenbatches and vendors. As LCD glass becomes thinner, the structural rigidity of the glassgoes down and the variability increases. Thinner glass compounds the issue.

A measurement bench has been designed to measure deformation at points throughout the glass sheet. The measurement bench will sample LCD glass sheets off line to trackvariations in LCD glass deformation with time. The sensor is based on a confocal laserdisplacement sensor. Figure 2 shows the design of the sensor and the salient specifications[1]-

The active confocal principle works in the following manner [1]. A laser beam isfocused on the glass surface through an objective lens that vibrates up and down by beingattached to a tuning fork. The surface can be specular or diffuse. If the target istransparent, either the front surface or the back surface can be used. The focussed laserlight is reflected off of the target surface and back into the sensor head. The light is thenredirected by half mirrors to converge on a pinhole in front of a photodiode. Themaximum voltage output from the photodiode determines the tuning fork's exact positionwhen the laser beam is focused on the target surface. The distance to the target surface isthen calculated.

By placing the sensor in a 3D gantry system, the out of plane deformation can beimaged as shown in Figure 3. The LCD sheets can then be sampled as they aremanufactured and the deformation of the LCD sheets can be tracked. When the variabilityof the sheets reaches a threshold, this information can be fed back into the LCD glassmanufacturing process to reduce the deformation variability of the LCD glass.

FIGURE 1. When LCD glass is supported by the edges, the glass sheet deforms due to gravity forces.

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• 7 pm beam spot size• Resolution of 0.2pm• Range ±1 mm• Stand off-28 mm• Angular Acceptance ±7°

FIGURE 2. Diagram showing the laser confocal sensor and the salient specifications.

Sag Example

Sag (mm)

LO 0

CM 8

XSample Dimension (mm)

150Y Sam pie

500 Dimension (m m)

FIGURE 3. Graph showing the measured deformation of a LCD glass plate supported along two edgesfrom the laser confocal displacement sensor.

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ULTRASONIC RANGING

Over time during the manufacture of LCD Glass, the LCD glass can significantly"walk" from its original starting position and robotic tools can loose their alignment. Thusa sensing system that can locate the LCD glass in space will be useful to maintain therobotic alignment to the LCD glass. The work presented in this section discusses thedevelopment of a non-contacting ultrasonic ranger to locate LCD glass in a harshmanufacturing environment.

Commercial transducers have been tested for this application and work well whenthe LCD glass is at room temperature. When the glass is being measured during theforming process, the glass is moving, vibrating and is at temperatures in excess of 200°C.Under these conditions, the commercial sensors become unreliable. This section discussesthe initial development of a robust ultrasonic ranger.

Prior internal results show that the major causes of ultrasonic ranging instabilitiesin commercial ultrasonic rangers are due to dynamic thermal gradients that causeamplitude fluctuations in the return ultrasonic signal. The commercial sensors use a signalthreshold to determine the time of arrival of the ultrasonic pulse. If the amplitude of theultrasonic signal changes, the relative location of the threshold within the returningultrasonic signal will change. The change in the threshold location relative to theultrasonic waveform will cause an apparent change in target range. Thermal effects can bereduced by having a smaller beam size and shorter stand off distances. However,occasional spiking still can occur. To further reduce the thermal effects and enable largerstand off distances, a more sophisticated algorithm that continuously tracks the reflectedultrasonic signal will be discussed.

Figure 4 shows the experimental setup to develop an ultrasonic ranger that is robustwith respect to thermal gradients. The LCD glass is heated by laboratory hot plate. Thehot plate is mounted such that the "hot plate" is vertical and normal to the ultrasonicpropagation direction. A thermo-couple and volt meter is used to measure temperatures.

The LCD Glass (305 X 305 X 0.7 mm) is mounted vertically and also normal to theultrasonic propagation direction. Temperature is measured by placing the thermo-couplebetween the glass and the hot plate. The thermo-couple is held in place by the pressurebetween the glass and hot plate on the lead wire insulation. It should be noted that thethermo-couple is not in direct contact with the glass or the hot plate.

The ultrasonic transducer is mounted to a translation stage. The mounting fixture isdesigned to ensure that the ultrasonic beam intersects the glass plate and hot platenormally. The beam is centered on the targets. The stand off distance is nominally 1094mm. To minimize ultrasonic side lobe [2] reflections off of the optical table between thetransducer and the target, ultrasonic absorbing foam is placed on the table between thetransducer and the target.

Figure 5 shows a typical return ultrasonic signal. The return signal consists of apulsed 450 kHz sinusoid with an amplitude envelop that is approximately Gaussian inshape. As a first step to improve the reliability of the ultrasonic ranger, a Gaussian is fit tothe return signal. The center location in time of the Gaussian envelope should beindependent of ultrasonic amplitude.

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LCD Glass305 X 305 X 0.7mm

UltrasonicRanger

Translation Stagewith Micrometer

67.5 mm1094mm

AcousticAbsorber

HotPlate

67.5 mm

FIGURE 4. Diagram of the setup for the ultrasonic ranging experiments on LCD glass.

Ultrasonic Ranging: Return Echo, 23°C

2.14 2.16 2.26 2.28 2.3 2.322.18 2.2 2.22 2.24Time (ms)

FIGURE 5. Plot of the return ultrasonic pulse that consists of a 450 kHz carrier frequency modulated witha near Gaussian envelope and the resulting Gaussian fit.

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The ranging results for the LCD plate at room temperature for variousdisplacements from the nominal standoff distance of 1094 mm is shown in Figure 6. Thereturn echo timing is shown to be a linear relationship with travel distance. The measuredvelocity of 346.6 m/s in air is in good agreement with the theoretical value of 345.1 m/s at23 °C in dry air [3]. The 0.4% difference might be due to higher humidity in the lab.Table 1 lists the salient statistics from the room temperature testing. The displacementstandard of deviation is an indication of the resolution of the ranger. At room temperatureusing a Gaussian envelope to determine return echo time, the resolution of the ultrasonicranger is 0.2 mm.

Ranging results for the LCD plate at 331°C for various displacements from thenominal standoff distance is shown in Figure 7. The return echo timing is shown to bemuch noisier than the room temperature timing. The linear relationship with traveldistance is not as compelling but the measured velocity of 343 m/s in air is still in goodagreement with the theoretical value of 345.1 m/s at 23 °C in dry air. Since the transduceris being moved within room temperature air, the measured velocity should match thetheoretical at room temperature. Table 2 lists the salient statistics from the 331°Ctemperature testing. At a 331°C temperature, the resolution of the ultrasonic ranger is 0.7mm. The displacement resolution at high temperature is a factor of three worse than theroom temperature resolution.

Measured Relative Displacements on 23 C LCD Glass166

140

120

100

80

60

40

20

0

-20

-40

y = - 346.6*x + 2226

6 6.05 6.1 6.15 6.2 6.25 6.3 6.35 6.4 6.45 6.5Time of Echo (ms)

FIGURE 6. Plot showing the ultrasonic ranging data taken from LCD glass at room temperature. Thespeed of sound in air is determined by the slope of the line fitted to the measured data points.

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TABLE 1. Table showing the measurement statistics for ultrasonic ranging on LCD glass at roomtemperature.

ParameterStandoff distanceRound trip return timeStandard of deviation (s)Measured velocityStandard of deviation (m)Gaussian Fit

Measured Value1094mm6.42 ms (average from 5 trials)0.9 jis346.6 m/s (theoretical 345.1 m/s)0.2 mm0.5 Max RF envelope value

TABLE 2. Table showing the measurement statistics for ultrasonic ranging on LCD glass at room 331°C.

ParameterStandoff distanceRound trip return timeStandard of deviation (s)Measured velocityStandard of deviation (m)Gaussian Fit

Measured Value1094mm6.45 ms (average from 5 trials)4.0 {is343 m/s (theoretical 345.1 m/s)0.7mm0.5 Max RF envelope value

oMeasured Relative Displacements on 331 C LCD Glass20

15

105

5

i.-5

-10

UT RangingLinear FitError BarsMax RF Signal >1.0V

6.4 6.41 6.42 6.43 6.44Time of Echo (ms)

6.45 6.46 6.47

FIGURE 7. Plot showing the ultrasonic ranging data taken from LCD glass at 331°C. The speed of soundin air is determined by the slope of the line fitted to the measured data points.

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SUMMARY

To help achieve better yields in the manufacture of Liquid Crystal Display glass, itis necessary to locate with better precision the LCD panels with respect to robotic tools.As LCD glass becomes thinner, the structural rigidity of the glass goes down and the LCDpanels deform more under the panels own weight and during processing steps. Twomeasurement systems have been presented in this paper to sense the position of LCD glass.Both measurement sensors are non-contact to protect the pristine nature of the LCD glass.

The first approach discussed is based off a laser confocal displacement sensor.This optical sensor will be used offline to measure the mechanical deformation of the glasssheet when supported on two edges. The sensor has been integrated into a 3D gantry toimage the out of plane deformation with a resolution less than ten micrometer. Theadvantages of non-contact measurement are three-fold: 1) for laser systems, nothing, suchas a sticker, needs to be placed on the sample - this made 3D measurements a possibility -in the same vein, we do not add any mass whatsoever and change the sag magnitude; 2) wedo not deflect the sample and change the sag magnitude as would happen with a contactprobe; 3) the measurement process is more efficient since there is no sample prep otherthan cutting the glass to the desired size.

The second approach discussed is based on ultrasonic ranging sensors to locateLCD glass and is intended to be used as an in-process sensor. A commercial ultrasonicranger in the presence of temperature gradients has been modified to reliably determine thereturn echo time by fitting a Gaussian envelope to the return ultrasonic signal. Thismodified transducer has been shown to have a resolution of 0.7 mm with a LCD glasstemperature of 331°C at a standoff distance of 1 m. The knowledge gained from thesemeasurement systems will be used to improve the positioning of the robotic tools withrespect to the LCD glass during the manufacturing process.

ACKNOWLEDGEMENTS

The author would like to thank Brian Strines and Monroe Marlowe for developingthe sag measurement bench and for providing the sag data. The author would also like tothank Lori Hamilton, Display Technologies and Corning Incorporated for proving thesupport necessary to write this paper.

REFERENCES

1. Keyence, Laser Confocal Displacement Meters, LT Series, Cat. No. LT-2.5M-699,Keyence Corporation of America, 50 Tice Blvd., Woodcliff Lake, NJ 07675, 1999.

2. Kinsler, L. E. , Frey, A. R., Coppens, A. B., Fundamentals of Acoustics, JohnWiley & Sons, New York, 1999.

3. Nave, C. R., HyperPhysics, Georgia State University, http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe3 .htmlffcL 2000.

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