Laser Measuring Device

Project - 98.3

Design Team Members:

Donald Ainsworth, David Linder, Sagar Mathur, Bokah Worjoloh

[email protected]

Customer: Travis Bogetti, U.S. Army

Center for Composite Materials; [email protected]

Advisor: Dr. Ajay Prasad Spencer Lab; [email protected]

Table of Contents

I. Executive Summary

II. Background

III. Customers IV. System Benchmarking

V. Wants, Constraints, and Metrics

VI. Conceptual Design

VII. Fabrication and Assembly

VIII. Testing

IX. Results and Conclusions

X. Suggested Modifications

Appendix A: Operation Manual

Appendix B: Figures and Graphs generated in the Testing Program.

Appendix C: Engineering Drawings

I. Executive Summary

Our primary customer, the US Army, is looking for highly accurate methods to test and characterize composites. Specifically, measuring samples for hygrothermal growth (moisture absorption in high heat and humidity conditions), resin cure shrinkage, and cure warpage (warpage of a material due a resin cure on the surface). These proposed experiments would require measurements to be taken in the thickness dimension to characterize the relative growth or shrinkage.

To obtain the necessary data, the Army purchased a reflective laser measurement system and assigned us the task to design, build, and characterize a computer-controlled device, which implements these lasers, to measures relative thickness changes in composites.

To this end, we implemented a Total Quality Design approach. This method utilizes the initial steps of system benchmarking and the development of wants and constraints to provide the framework to devise metrics, generate concepts, and choose a final design. With a final design in hand, we then proceed with fabrication and assembly of a prototype. Finally, we conclude with testing to prove that our device satisfies the requirements as quantified by our metrics, which then in turn demonstrates that we have succeeded in satisfying the customer wants.

This report demonstrates each of these steps specifically to better illustrate this process and concludes with further modifications that may be incorporated.

II. Background

Composite materials are used in a wide range of commercial, industrial and military applications. There are many well-known advantages in using these materials, however their growth characteristics have not been sufficiently documented. In fact, composites may undergo significant dimensional changes as a result of their manufacture, storage environment, and in-service environment that would not be realized with the use of more traditional materials. These dimensional changes are of great interest to researchers and designers because they play an integral role in structural design and industrial applications. Currently, these dimensional changes are determined through the use of micrometers or extensometers. These manual methods are not as desirable as the proposed laser system for a few reasons. The most important issue is accuracy, since the required measurable strain is at the micron level. Specifically, most micrometers are accurate to 0.0001” (2.54 m) while the laser resolution is 0.2 m. Another limitation with manual methods is sampling rates. The experiments proposed above would require numerous thickness measurements at different locations on the samples for varying reasons (as you will see some experiments required as many as 2000 data points). The laser has a sampling capability of 50,000 Hz, while a human using a micrometer may only be able to measure and record a few measurements in a minute. Another benefit that is realized with the increased sampling rate is the ease by which data may be manipulated. Due to the fact that the laser system digitizes the thickness data, a variety of calculations can be performed by the click of a mouse to better characterize the data.

With these benefits, growth characterization experiments can be performed in ways that were previously impossible. These new procedures will increase accuracy and decrease the time needed to quantify strains. When this strain is quantified, researchers can develop theories to predict these changes. From these predictions engineers can design composite systems to function in varying environments. III. Customers

We utilized these customers (prioritized) to provide us with various wants. When these wants conflicted, design decisions were made in cooperation with our main customer.

1) US Army , Travis Bogetti

Dr. Bogetti is our primary customer. His research consists mainly of

the testing and characterization of composites. Specifically, he is

advisor of the two graduate students mentioned below. 2) Center for Composite Materials, Jack Gillespie

Dr. Gillespie is the Technical Director at CCM. He is interested in a

device that will reduce the time required to determine growth in

composites. He is also responsible for allocating funds for the project.

3) Graduate Student CCM, Tong Wang

For his research, Mr. Wang would like to use the device to measure

hygrothermal growth in composites. In a 0.1” thick sample the

projected strain is 0.5% (13m).

4) Graduate Student CCM, Florella Flores

For her research, Ms. Flores would like to use the Device to measure

cure shrinkage of resin (variable growth based on thickness of resin) as

well as the warpage of a material due to resin cure ( 3mm of

deflection in 5”).

IV. System Benchmarking

System Description Key Strength Laser Flaw Measuring Uses translation stages and a - X-Y motion Device (Physics Dept.) laser to create oscillations capability that can be measured to - Motor control and characterize flaws. data acquisition using LabView

Universal Length Measures various dimensions - Measurement of large Measuring Instrument using an adjustable air range of sample system dimensions Topometrix Atomic Force Measures dimensions in the - Accuracy of +/- 1.5 Microscope micron range micron. 5-D Laser Measuring Measures linearity horizontal - Accuracy of +/- 1 System and vertical straightness, micron pitch and yaw.

V. Wants/Constraints/Metrics

WANTS METRICS - Measure 0.5% strain in a 0.1 inch thick Obtain an overall accuracy of < +/- 5 m. sample (13 m strain) - Measure various composite materials

Measure average thickness of sample. Obtain > 1725 samples when measuring the average.

Measure cure-shrinkage of resin. Sample orientation zero (+/- 0.5) degrees from horizontal.

Measure warpage of aluminum Ability of device to translate sample for at least 10 inches while measuring. Measure samples of various sizes Ability of device to accommodate samples (thickness/width) that are 10x10 inches and up to 5/8 inches thick. Store and Manipulate data Have 10 bytes of disk space per test. Easy to Use Take no more than 20 minutes to measure a sample. Use in Lab Setting Operating Conditions: Temperature- 0 to 40 Celsius. Humidity- 35% to 85%, no condensation.

CONSTRAINTS METRICS

Cost $3,000 Use of laser system already purchased Implementation of Keyence LC-2440 Time for completion This project must be completed by 4/23/98

VI. Conceptual Design

The two primary concepts, named below, were generated to accurately measure

strain in the thickness direction.

1) Exact Relocation

2) Bulk Average Note: due to the nature of the experiments either concept would be sufficient for warpage or resin cure

The first concept was to measure growth at specific points. This entailed exactly

repositioning the laser heads to measure thickness at the exact point where a previous

measurement had already been obtained. Exact Relocation required a precise servomotor

motion mechanism with a feedback control system. This would provide strain data at

specific points as function of time. The data could then be compared to each other, and

perhaps averaged, to get an overall growth. In addition, a transparent casing would be designed to allow a high temperature/humidity environment to flow over the sample during the measurement procedure. Therefore, analyzing the data would yield the overall growth as well as a growth rate.

To determine the precision of relocation that this concept would require we obtained profileometer data (see Appendix B, Figure 9). This data yielded a topographical map of the sample roughness, which we were able to model as a two superimposed sinusoidal waves (Appendix B, Figure 10). Using the slopes of the sine waves, we calculated the error in thickness measurement as a result of the resolution of the servo motor system. The system that was specified to the customer had a resolution of 3.5 m, which created a measurement error of 0.7 m. By contacting the vendors, we determined that a system of this type would cost the customer approximately $15,000.

The customer decided that the cost wasn’t worth the benefit of the accuracy and growth rate and opted that we develop a less expensive concept to determine strain, which is

Concept 2.

Concept 2 utilizes the simpler method of bulk averages. Initially, the sample would be randomly scanned to acquire enough thickness measurements for the running average of the thickness to converge. This average would then represent the original average thickness. Then the sample would be removed and conditioned. Finally, the above mentioned measuring procedure would be repeated to determine an average thickness after growth. These two averages would then be compared to yield the average strain.

This method could be implemented inexpensively, however it was not clear if the running mean would converge well enough to fall within our accuracy requirements. The only way to determine this was to design, test, and characterize a device that measured with this random bulk average method. Working with our customer, we selected concept two as an initial cost effective solution to the wants.

VII. Fabrication and Assembly (Note: All Drawing referenced in this section are contained in Appendix C)

Before discussing our fabrication, we must describe the device that was previously designed for the customer by Ragunath Seetharam, of the Electrical

Engineering Department at the University of Delaware. The previous configuration is shown in Drawing 1. As shown this device consisted of a base table, a gripper, stands for the lasers, slide bearing and a lead screw to move the sample. Besides the mechanical setup, there was a Labview program designed to write raw data to a file. This device was useless to the customer be cause it did not satisfy any of the metrics. For example, the Labview file did not write the measurements to a file, and therefore the accuracy could not be quantified and also no analyses could be performed on the data. However, with exception of the gripper, the other mechanical components were functional. In fabricating our design we chose to retain some items from the original setup, which were the slide bearing, the lead screw, and the base table. In addition, we fabricated the following to complete our device:

1) Back Support

2) Setscrew Mechanism

3) Gripper

4) A Working Labview Program.

See Drawing 2 for the newly designed device.

The first step in the fabrication of the device was to design and build the Back

Support (Drawing 3). This component essentially inverted the original system and satisfied the metric of horizontal orientation. As shown, there are a pair of tapped holes that anchor the Back Support to the Base. Additionally, the Back Support contains holes for the attachment of the Lead Screw and Bearing.

The Setscrew Mechanism (Drawing 4) is functionally tied to the Back Support, in that, once the bearing is inverted something must prevent the laser heads from sliding.

We designed the Setscrew mechanism based on two considerations that are directly tied to the metrics. First, it is critical to the accuracy of the device that the heads do not move during the measurement process. Secondly, the laser heads must retain their ability to slide in a direction perpendicular to the sample face for the device to have the ability to accommodate samples of various thicknesses. This is due to the fact that the lasers can only operate in a certain range of focal lengths and significant increases in sample thickness’, through sample changeover, would require that the laser heads be adjusted. The bracket fits over the bearing and contains a nylon setscrew that drives directly into the bearing shaft. This design prevents movement while measuring, yet is quick and easy to adjust the laser heads for the measurement of thicker samples.

Designing another gripper was also essential to the correct performance of our device as dictated by our metrics. The previous holding fixture was inadequate for three major reasons. First, it could only accommodate samples of up to 0.3 in. in thickness, which was not large enough to be satisfied by our metrics. Secondly, it could not hold the sample completely horizontal without some degree of tilt, which could cause measurement error. Lastly, it held the sample about 6 inches above the lead-screw, which made it more susceptible to the effects of vibration. Initially, we benchmarked clamping devices such as the Peterson Vice to develop concepts. We designed the new gripper to be flexible, shorter, and have truer edges to reduce tilt. The three components of the gripper can be seen in the Drawings 5-7.

Specifically, the Base of the gripper (Drawing 5) is attached to the platform of the lead screw by four screws. Two screws then attach the Upright (Drawing 6), to the Base.

Finally, there is a Clamper (Drawing 7) that slides on the base and, through the use of two nuts and bolts, clamps the sample.

The last step in our design was to modify the existing Labview file to (a) take the running mean and standard deviation, (b) output the number of data points per trial, and

(c) write all data to a file. In order for the Labview program to take the running average and standard deviation, we had to build an array to store all the raw data so that it could averaged after each data point was obtained. This required that we placed a shift register at the loop to hold the previous values. Additionally, we attached an “array size” indicator to output the size of the array. Once this was accomplished we set up another set of functions to format and write the data to a file so that we could import it into Excel.

These new characteristics allowed us to use the mean and standard deviation to calculate the errors in measurement data, as will be explained in the testing section.

VIII. Testing (Unless otherwise noted all Figures referenced in this section are contained in Appendix B)

Four samples were used in the testing program, two different fiberglass composites, one entab composite, and sheet aluminum with a surface resin cure. These samples were chosen based on their broad range of color and roughness, to simulate the variation in samples that the device would be required to measure. We designed this program to yield the following four pieces of information that were critical to demonstrate that the prototype satisfied the metrics.

1) The overall accuracy of our device such that it falls within the range as dictated by our first metric.

2) When measuring the relative average thickness of a sample, the number of measurements required before the average converges sufficiently

3) The verification of laser data and possible additional calibration methods. 4) Measurement the warpage of aluminum with a surface resin cure.

In order to calculate the level of accuracy of our device, we had to first determine the source and magnitude of all possible errors. We determined the total error to be comprised of the following.

1) Error due to incomplete convergence of the mean.

2) Error due to vibration.

3) Error due to the failure to calibrate the system (linearity).

To characterize the effects of vibration we designed an experiment to measure relative thickness at a stationary point for three of the samples (Figure 1). As shown, the variations in measurements, which represent the error, range from 12 to 40m. This error is obviously not tolerable as illustrated by our first metric of 5 m accuracy. Fortunately, the laser’s control box is equipped with an averaging function to solve such a problem.

Therefore, the same experiment is performed with the number of averaging measurements set to 131,000 i.e. one measurement output is the average of 131,000 samples (Figure 2). Here the variance is approximately 0.4 m, which can be characterized as the error due to vibration. Note: All subsequent experiments were run in this averaging mode.

Another source of error is due to the incomplete convergence of the mean; that is, once the average has reached a point of convergence small fluctuations will remain mainly due to the roughness of the sample. To determine this error, an experiment was designed to output the running mean of the relative thickness along a scan line for 2000 data points (Figures 3-5), this took the maximum time specified by our metrics, which was 20 minutes. At the bottom portion of each graph a “blown up” view can be seen of the converging region. As shown in Figure 8, the last few hundred samples follow a

Gaussian distribution, and therefore we may implement a standard deviation approach to give the measure of accuracy that occurs with this incomplete convergence. For our calculations, m we let the accuracy be +/- two standard deviations, which then yields over 99% of all possibilities. Our results show a range of accuracy between +/- 0.6 to

+/- 3.6 m, again these are errors due to incomplete convergence and can be accounted for.

Finally there is an error due to non-correction of the factory calibration of the lasers. Calibration entails moving a target a known distance and then imputing this distance to the laser as the measuring distance. The system is calibrated at the factory using a white diffuse target, however the laser should be re-calibrated after sample changeover to reduce linearity for the reasons explained below.

Calibration is performed to compensate for the measurement

deviation from the actual displacement that is caused by the

difference in target color, material and surface condition. This

deviation results from the differences in the in the target surface,

since the laser beam reflects differently depending on the target

surface. This is a characteristic of sensors using the principal of

laser beam reflection. By calibrating, the measurement deviation

between two measurement points can be corrected. The amount of

deviation between two measurement points varies depending on the linearity for each target. (Source: Keyence LC-2400 Series

Instruction Manual)

The maximum linearity for our device is 0.05% of full scale, or +/- 1.5m, which again, is an error that can accounted for.

The second piece of information to be determined from our testing program was how many measurements are needed before the average converges. From the top portion of Figures 3-5 we see a range of 1550 to 1725 data points till convergence for the various samples used. We determined when convergence occurs by merely inspecting the graphs.

The third set of tests to be performed was a verification and possible calibration method experiment. Basically, we slid a piece of shim stock of known thickness between the setscrew and the sliding portion of the bearing (see Appendix C, Drawing 2) for each of the three samples. Then we compared the laser measurement to a micrometer measurement of the shim stock to verify that both measurements were the same order of magnitude. Results are shown in Figure 6.

Our final test was to use the designed device to measure warpage of aluminum.

To accomplish this we used a 10” scan line and output the raw measurement data for 267 data points. The results can be seen in Figure 7 with a maximum deflection of 2.5 mm. IX. Results and Conclusions of Testing Program

An important first conclusion is to quantify how the laser performs with varying sample characteristics. We note that all experiments using entab produced data with increased variance and higher standard deviations, as shown by the stationary-not averaged and running mean data. This is most likely due to the fact that the top coating of entab is fairly transparent which makes for a poor reflective surface. It is therefore concluded, that measuring a sample with the same surface properties, as entab material will cause significant error and is not recommended. Similarly, a correlation can be made between the roughness and the accuracy of the average. For the rougher black fiberglass sample, the running mean converged with a larger standard deviation and therefore produced less accuracy than the smoother yellow fiberglass sample.

The primary objective of our testing program was to prove that our prototype satisfied the accuracy requirement as imposed by the customer. Of course, the accuracy will differ depending on the nature of the experiment that is being performed. For example, if there is no need to obtain an average thickness then the error due to incomplete convergence will not be an issue. To determine the net accuracy for each experiment we merely sum the appropriate errors.

Sample Accuracy for Accuracy for Accuracy for resin hygrothermal [m] warpage [m] cure [m] Yellow Fiberglass 2.5 1.9 1.9 Black Fiberglass 3.6 1.9 1.9 Entab 5.5 1.9 1.9

Our next objective was to determine the number of data points necessary for the running mean to converge. Our data yielded a range of 1550 to 1725 data points required for convergence, however for samples with greater thickness variations, such as roughness, this number may increase. It is recommended that prior to the measuring of a new type of sample, an experiment be conducted to determine the number of points required to attain convergence.

The third goal was to verify the laser output and determine the possibility of the shim stock experiment being a method of calibration. From the data, we conclude that the laser output is indeed verified to be the same order of magnitude as seen by the percent differences. However, this method would be an unacceptable form of calibration due to these percent differences. The calibration mode requires that the new value entered be no more that +/- 10 % of the laser output. As shown the experimental percent differences fall well outside of this range. These differences may be explained due to imprecise nature of this experiment, such as the following.

1. The shim stock material was Brass, which may slightly deform when placed between

the bearing slide and the setscrew. 2. Surface and thickness variations of the shim stock.

3. Erroneous micrometer measurement

4. Vibration.

5. Linearity.

6. Any burs or other irregularities the may alter how the shim stock is seated between

the bearing slide and the setscrew.

Finally, we conclude that the customer wants are satisfied through our testing program, which proves that our device satisfies the requirements as quantified by our metrics.

X. Suggested Modifications

Various modifications could be made to the current design to make it a better solution to the problem.

1) A calibration system could be developed to reduce linearity due to changes in

target color and roughness. This system would consist of a micrometer and

slide attached to the sample. Calibration would entail moving a target a known

distance as measured by the micrometer and then imputing this distance to the

laser as the measuring distance.

2) The device could be further automated by adding a stepper motor to translate

the sample in either the X or Y direction (perpendicular to the laser beam).

This will allow the scanning velocity to be a known value for a specific trial.

Now the scan velocity can be adjusted according to the sample roughness to

ensure that at least two measurements are taken per wavelength on the surface of the sample. This modification should reduce variations in the data and

increase the accuracy of the average. In addition, this modification would

increase the ease of use.

3) An pneumatic or marble table could be added to reduce the effects of

vibration.

4) The Labview program can be further modified to have the ability to vary the

sampling frequency of the program to match that of the laser.

5) For improved accuracy, an algorithm should be used to statistically prove

convergence.

Appendix A

Operator’s Manual, Measurement Procedure

1) Turn on Laser and Computer (allow 60 minutes for laser to warm up).

2) Call up Labview folder, a:\lmdfiles on the included disk, to access the program vi.’s

that manipulate and write data to file. 3) Open appropriate file depending on what type of information that the user wants to be

written to a file, i.e., mean, standard deviation, or raw data.

4) Place sample you wish to measure in the gripper and tighten holding screws securely

with pliers and Allen wrench.

5) Using a flat-head screwdriver adjust the both laser head positions so that the LED on

the laser heads lights orange.

6) Set number of averaging measurements on control box to 131,000.

7) In the Labview panel, enter the filename and the directory that you would like the

data to be stored in.

8) Click on the “run continuously” button located on the panel.

9) If your experiment requires a scanning scheme, turn the lead screw handle while

measuring. Otherwise the sample will remain stationary.

10) The number of acquired data points appears in the lower right digital indicator,

labeled “data points”. Once you have achieved the desired number of measurement

samples click on the stop button.

11) Copy the output file to a disk to be plotted on any standard spreadsheet software, or to

be viewed as a text file in a program such as MS notebook.

Appendix B

Figures and Graphs generated in the Testing Program  Drawings  Grid1

 Grid2  Isofinal  Grip1  Isoview  Imdlock