solution to decrease uncertainty in measurement tape calibration

National Institute of Standards and Technology:

Solution to Decrease Uncertainty in Measurement Tape Calibration.

December 14, 2007

Solution to Decrease Uncertainty in Tape Measure Calibration

Acknowledgements:

Daniel Sawyer

Bruce Borchardt

Chris Blackburn

Ed Morse

Provided by UMD Student, December 14, 2007 2


TABLE OF CONTENTS

1.0 Introduction 4

1.1 Introduction to LSCMG and Measuring Tapes 4

1.2 Introduction to Research Problem 5

1.3 Sources of Error 5

1.4 Proposed Solution 5

2.0 Background 6

2.1 NIST Organizational Structure 6

2.2 LSCMG Purpose 6

2.2.1 Tape Calibration Lab 6

2.3 Measurement System 8

2.3.1 Introduction to Measurement System Components 8

2.3.2 Measurement System – Component Functions 10

2.3.3 How the Measurement System Works 15

2.5 Calibration Procedure 16

3.0 Calibration Problems 17

3.1 Abbé Error 17

3.2 Operator Error 18

4.0 Problem Analysis 19

5.0 Possible Solution 21

5.1 Experimental Comparison 22

5.1.1 Introduction 22

5.1.2 Experiment Setup Description 23

5.1.3 Simulation 1: Manual Leveling Procedure 24

5.1.4 Simulation 2: Pitch Correction with Wyler Fowler

Level Procedure 25

6.0 Conclusion 26

6.1 Experimental Results 26

6.2 Concluding Discussion 27

6.3 Future Steps 28



Appendices



1.0 Introduction

National Institute of Standards and Technology (NIST) functions to advance U.S. innovation and industrial competitiveness. As a federal agency of the U.S.A. it achieves such goals by advancing measurement science, standards, and technology in ways that enhance economic security and improve the quality of life. The Large Scale and Coordinate Metrology Group (LSCMG) under the Manufacturing Engineering Lab at NIST provides a service to improve such measurement science.

1.1 Introduction to LSCMG and Measuring Tapes

LSCMG provides calibration services for metal measuring tapes that are used in industry such as oil gauging, surveying and telecommunications for distance measurement. Because such industries require accurate measurement, companies from these industries send in their master tapes to be calibrated. The companies then use the calibrated master tape as the basis for which the duplicates are made. The accuracy of calibration for companies is important to reduce cost. A measurement too short would cheat the customer while a measurement too long would promote money loss for the company. To meet client needs, the LSCMG would like to improve their equipment to perform a calibration within a 10 micrometer uncertainty as opposed to the current 250 micrometer uncertainty.

Typically the types of tapes calibrated by LSCMG are chrome clad tapes and chain tapes (Figure 1.0).


Figure 1.0 – Tape Types, Chrome Clad (Top right), Chain (Bottom right)


1.2 Introduction to Research Problem

Currently, the LSCMG uses outdated equipment called the microscope carriage (Figure 1.1) to create tape calibrations. I was hired to redesign the microscope carriage to reduce the measurement uncertainty during calibration. My goal was to reduce the measurement uncertainty from 250 micrometers to 10 micrometers.To reduce the uncertainty by a factor of 25 it was necessary to reconfigure the microscope carriage to decrease operator error and Abbé Error1.

1.3 Sources of Error

When a tape is received, an operator lays out the tape on a 60 meter bench in the laboratory. The operator then rolls the microscope carriage along the bench and over the tape from graduation to graduation to calibrate the tape. Because the bench surface is not completely flat, an uncertainty in displacement measurement between graduations occurs as the microscope carriage “pitches” or tilts. Due to such tilt behavior, a linear positioning error or Abbé Error is created. In the current procedure, to decrease uncertainty due to Abbé Error the operator manually re-adjusts the tilt of the carriage to eliminate calibration error. Yet, such manual tilt adjustment technique creates another source for error, or operator error. The goal is to limit the Abbé and operator error as much as possible to decrease the measurement uncertainty.

1.4 Proposed Solution

Originally it was desired by the LSCMG to reduce uncertainty due to Abbé error and operator error by incorporating an electromechanical self leveling system on the microscope carriage. Such system would automatically align the carriage during calibration to eliminate its tilt while eliminating operator error.

1 Linear positioning error due to angular displacement and dimensional offset.


Figure 1.1 – Microscope Carriage


During the research term a revolutionary idea was discovered. To eliminate the error due to tilt, I proposed an approach that would determine the tilt of the microscope carriage. By using the magnitude of tilt through mathematical calculations, the origin of the true measurement value can be found. This approach would reduce the Abbé Error while eliminating the need for manual operator adjustment.

2.0 Background

Because I conducted research within the Large Scale Coordinate Metrology Group, it is important to provide background on the LSCMG and its mission. This section provides detail on the group facility, the tape calibration laboratory, laboratory components and the process of tape calibration.

2.1 NIST Organization Structure

National Institute of Standards and Technology is divided into multiple engineering laboratories such as Materials Science Engineering Lab (MSEL), Electronics and Electrical Engineering Lab (EEEL) and Manufacturing Engineering Lab (MEL). Each lab is divided into divisions, as each division is also divided into groups of particular disciplines. The Large Scale Coordinate Metrology Group belongs to the precision engineering division, where Large Scale is considered to be a field of metrology in within the micrometer (1x10-6 meter) scale.

2.2 LSCMG Tape Calibration Lab

This section introduces the LSCMG Facility. Specifically it covers the calibration laboratory and the measurement system used to create tape calibrations.

2.2.1 Tape Calibration Lab

The tape calibration laboratory is a82m x 4m x 3m concrete tunnel located approximately two meters underground (Figure 1.2). This underground arrangement creates good thermal and vibration isolation



for the facility. Two major components of the calibration lab are the tape calibration bench and the laser - interferometer measurement system.

Tape Calibration Bench

The tape calibration bench2 is a 60 meter stainless steel bench that runs three quarters of the tunnel length along the left concrete wall. The bench is a specially designed adjustable calibration bench constructed of stainless steel bars. The bench is supported on precision bearings, which allow it to contract and expand with temperature changes.

Climate Control System

The tunnel has a climate controlled environment created by an air distribution system, which originates in the ceiling in the form of vertical laminar3 flow and flows down along the sides of the room. The air is recalculated via an air return located 0.3 meters from the floor. This allows the tunnel to be kept at 20°4 Celsius ±0.1° at a single point and a ±0.25 °C maximum temperature difference over full length.

The climate control system uses calibrated thermistors5 (Figure 1.4) throughout the tunnel length to measure both the ambient temperature and the tape bench surfaces. These temperatures are then read by the calibration system control computer to

2 Tapes are laid out on the bench top for calibration.3 Fluid flows in straight, parallel layers, with no disruption between the layers.4 World wide standardized calibration temperature due to mechanical properties of metals. Metals can

compress and expand due to temperature difference, thus creating non constant calibration conditions. 5 Type of resistor used to measure temperature changes


Figure 1.2 – Calibration Laboratory

Figure 1.3 – Bench Top Surface

Figure 1.4 – Thermistors


ensure that the readings are consistent within the acceptable temperature range. If the temperature is not consistent, the climate is adjusted accordingly by the computer.

2.3 Measurement System

In the tape calibration lab the calibration measurements are performed using a laser-based displacement interferometer system. This system combines four main subsystems: Software, Laser and Interferometer, Microscope Carriage and A - Frame tensioning unit. Section 2.3.1 will introduce each subsystem and what it is, while Section 2.3.2 will introduce how each subsystem works. At the end it will shown how a combination of such subsystems is used to create a displacement measurement.

2.3.1 Introduction to Measurement System Components

A- Frame Tensioning Subsystem

During calibration, each tape must be placed under tension. The A - Frame tensioning subsystem (Figure 1.5) is used for this task. This system incorporates two pulleys with near-frictionless bearings of Grade “A” quality to allow tensioning of the tape by a suspended weight. For tensioning tapes of various lengths, this subsystem is mounted independently of the bench to allow mobility from the zero end of the tape calibration bench to the 61m end.

Laser and Interferometer Subsystem

The primary tool used for tape calibrations is a displacement measurement laser interferometer subsystem (Figure 1.6). This subsystem combines an interferometer with a two - frequency helium neon laser. With a range of 80 meters, the laser beam is used to create a displacement


Figure 2.2 - A – Frame Pulley Subsystem

Figure 1.5 – A - Frame


measurement along the bench. The interferometer is an optical device located between the laser and the microscope carriage to combine the outgoing reference beam from the laser with the reflected measurement beam [from the retroreflector on microscope carriage].

Computer Software

Before the calibration measurement can be recorded, it has to be processed and calculated through computer software (Figure 1.7). The laboratory software was derived for this purpose by a senior level engineer Daniel Sawyer using Visual Basic programming. Also called Renishaw software, it is responsible for processing multiple signals and input values to output a measurement calculation based on the laboratory environment. The environment is analyzed based on the temperature, humidity and pressure readings which are gathered through laboratory sensors. Humidity and pressure readings are attained through barometers and hydrometers located within laboratory (Figure 1.8).


Figure 1.6 – Laser Interferometer Subsystem

Figure 1.7 – Renishaw Software

LaserInterferometer



Figure 1.8 – Hygrometer (Humidity Sensor - Left) Barometer (Pressure Sensor - Right)


Microscope Carriage

The microscope carriage was the focus of this research project. This subsystem consists of a rolling carriage constructed by the NIST personnel (Figure 1.9). The carriage transports a bubble level, a retroreflector and 20X microscope. As one of the most important parts of the measurement system, the carriage is used to locate the tape graduations for calibrationby the operator.

2.3.2 Measurement System – Component Functions

Laser and Interferometer Subsystem

The Renishaw helium neon laser is used to create a displacement measurement along the bench length through the use of a light beam. As the beam travels to the location of the microscope carriage and back to the laser from the retroreflector (Figure 2.1), a displacement measurement can be calculated by the comparison of the reference and measurement beam.



Figure 2.0 – Laser Beam

Figure 2.1 – Beam Path


The outgoing beam from the laser (reference beam) and the returning beam from the retroreflector6

(measurement beam) are combined through the interferometer (Figure 2.2). The beam combination is then sent to the light sensor within the Renishaw laser system for comparison of light wavelengths between the measurement and reference beam. The sensor detects interference fringes7 found in the beam combination to create a displacement measurement signal. The Renishaw displacement signal is then sent to the Renishaw software for further possessing to calculate a displacement value in micrometers8. Computer Software

Due to multiple factors, the Renishaw laser signal must be corrected by software processing. To output a final displacement reading acceptable for recording, the software must create a calculation to correct for the temperature, pressure and humidity variances9. Humidity and pressure readings are used to correct the change in laser beam wavelength due to the refractive index of air in laboratory. Similarly, temperature monitoring is necessary for correction of tape length variance due to the thermal expansion of metal material defined by the coefficient of thermal expansion10.

In addition to the signal processing, the Renishaw software is designed to serve as an operator interface (Figure 2.3) with the procedure. Through the use of the software the operator is able to start and stop a calibration, zero the laser, record displacement values, monitor the laboratory environment and view the output measurement file.

6 Optical device located on the carriage that reflects an outgoing beam completely parallel to the incoming beam.

7 Interference Fringes - Interference in light wave intensity from combination of independent waves due to differences in phase of light waves.

8 Micrometer = 1x10-6 meters9 Humidity and pressure readings are attained through barometers and hydrometers located within

laboratory, Figure 1.8 – Hydrometer (Humidity Sensor - Top). Barometer (Pressure Sensor - Bottom).10 Coefficient of Thermal Expansion (CTE) is a material property which reflects the materials ability to

expand or contract due to temperature difference. Different CTE values dependent on the type of tape material are logged into the software for length variance correction.


Figure 2.2 – Interferometer Beam Combination


The software incorporates the following interface features (Figure 2.3):

Software features:

Bench Temperature Profile Display – displays the temperature value for each of the thermistors along the bench length. Warning in a form of a red flash will appear next to the temperature if it exceeds the tolerance.

Pressure and Humidity Display – displays the pressure and humidity values gathered from the hygrometer and barometer.

Signal Strength Display – displays the strength of the laser beam which is dependent on the alignment of the laser and optical devices for displacement measurement.

Previous and Current Measurement Display – displays the previously recorded displacement measurement (Figure 2.3 – Bottom Readout) and the real time displacement measurement (Figure 2.3 – Top Readout) in micrometers.

Coefficient of Thermal Expansion Display – displays the coefficient of thermal expansion with respect to the temperature difference.


Figure 2.3 – Software Interface


A - Frame Pulley Subsystem

The A - Frame tensioning subsystem (Figure 2.4) is used to place the tape under tension by applying a specified tensioning force. Through the use of two near frictionless pulleys, the tape is placed under tension through a method of dead weight load. Such method places the tape under tension by suspending calibrated weights from the tape as seen in Figure 2.4. This approach is highly efficient as it creates a precise and consistent tension force for calibration.

Tapes are placed under tension during calibration to resemble the field environment. Because of the material properties of metal, the tape can elastically deform under various tensioning forces based on its modulus of elasticity E11. As the graduation spacing depends on the applied tension, the same tensioning force is applied during calibration as in the field.

Microscope Carriage

The microscope carriage is a subsystem build to allow motion along the bench surface for tape calibration. Through the use of the incorporated 20X microscope, the operator manually locates the scale graduations for calibration. The retroreflector at the front of the carriage reflects the laser beam back to the interferometer for the reference and measurement beam combination.

11 Modulus of Elasticity - property of a material which reflects the ability of such material to stretch elastically (i.e. non-permanently).



Figure 2.4 – A -Frame

Weight


Unfortunately, the carriage motion along the bench also involves angular motion or tilt (Figure 2.6) due to imperfections of the tape bench. As explained previously, such tilt is responsible for Abbé error and operator error. To eliminate Abbé error during calibration, the carriage is designed so that the tilt of the carriage can be detected by the operator through a bubble level. Using the bubble level reading, the operator then manually adjusts the pitch of the carriage by microscrew to eliminate the tilt and minimize calibration uncertainty. The following are the incorporated features of the microscope carriage:

Incorporated Features:

Retroreflector - optical component that receives the reference laser beam and reflects a measurement beam back to the interferometer for beam combination.

Microscope - used to sequentially center the microscope carriage on the tape graduation to be calibrated via cross hairs. A battery operated L.E. D. illumination system is used to illuminate the graduation.Magnification value: 20X

Bubble Level - leveling device with a precision of 5 arc seconds used to detect the tilt (pitch) of the carriage.

Microscrew - a manually adjustable screw mechanism [located in the front of the carriage] used to perform correction of the microscope carriage tilt with respect to the bubble level.

Linear Bearing - bearings arranged in a linear fashion to allow vertical travel of the carriage front12.

12 - Pivoting occurs on the axis of the rear wheels, thus tilt can be adjusted through the microscrew by lowering or raising the carriage front.


Figure 2.6 – Carriage Tilt

Figure 2.7 – Measurement System Setup


Magnet - secures the carriage against the test bench edge to prevent falling during movement.

Teflon Pads - create frictional force to stop the carriage after moment and prevent movement after aligning.

2.3.3 How the Measurement System Works

Phase 1:

The metal tape is laid out along the bench surface and placed under tension by dead weight load using the A – Frame.

Phase 2:

Computer software and laser interferometer subsystems are loaded and warmed up. Using the software, the origin of measurement (Zero Mark13) is defined for the Renishaw laser. As an uninterrupted process, the laser sends a reference beam to the retroreflector located on the carriage as it passes through an interferometer. The reference beam is then reflected back [now measurement beam] to the interferometer for beam combination.

Phase 3:

Within the interferometer, the reference beam and the measurement beam are combined for analysis by the Renishaw laser. Through detection of the interference fringes, a displacement signal from Renishaw laser is made. This signal is than processed by the Renishaw software to create a precise distance measurement value ready for recording.

13 The defined “Zero Mark” tells the laser where to begin measurement from.



2.5 Calibration Procedure

The procedure for calibrating a tape is as follows:

1. Lay out the tape on the bench and put under tension using the A- Frame tensioning system.

2. Turn on and load the system software and allow it to evaluate the environment signals (i.e. temperature, pressure and humidity conditions).

3. Enter the coefficient of thermal expansion into the software based on the material of the tape (i.e. steel, aluminum or teflon).

4. Level the microscope carriage using microscrew.

5. Center the microscope reticule on the initial tape graduation and zero the laser displacement measuring system.

6. Move the carriage to the first graduation, level the carriage and center the microscope reticule on the graduation.

7. Record the measurement displacement d1 using system software by pressing RECORD.

8. Move the carriage to the next desired graduation, level the carriage and center the microscope reticule on the graduation.

9. Record the measurement displacement d2 using system software by pressing RECORD.

10. Repeat steps 8 and 9 until all desired graduations have been sampled.

At the end of calibration, system software creates a .txt file with recorded time stamped measurement, temperature, humidity and pressure values. The measurement data consists of the measured displacements d1,d2, …, dn between the tape graduations defining nominal calibrated lengths L1, L2… Ln. Using these values a calibration report is assembled for the customer. Based on the calibration data the companies are able to adjust tape manufacturing for better measurement accuracy.



3.0 Calibration Problems

Multiple problems are present with the current design of the microscope carriage and the technology on board. Both of these factors contribute to the magnitude of uncertainty involved with tape calibration. Two main sources of such uncertainty are the Abbé error and operator error.

3.1 Abbé Error

Abbé error is a significant source of error in positioning applications. It refers to a linear error caused by the combination of an underlying angular motion and a dimensional offset (Abbé Offset) between the object being measured and the accuracy determining element. In this case the Abbé error is present due to the Abbé offset and the angular motion of the carriage.

Considering a sixty meter tape bench it can be understood that its assembly cannot be performed without possible deflections on a micrometer scale. Such deflections or small waves are responsible for tilting of the carriage in the pitch or yaw direction as it is rolled. Calibration uncertainty due to Abbé errors are generated from an angular displacement under Yaw and Pitch conditions.

As shown in Figure 2.8 above:

*Yaw – Rotation of carriage about the vertical axis.

* Pitch – Rotation of carriage about the horizontal axis.


Figure 2.8 - Carriage Pitch and Yaw Behavior


From previously gathered data it was found that pitch behavior of the carriage is responsible for the largest source of uncertainty (≈ 250 µm) in comparison to the yaw related behavior. Any pitch motion of the microscope carriage will cause an Abbé length measurement error.

Figure 2.9 provides the description of the source of Abbé Error due to carriage pitch. The top diagram shows the carriage in parallel with the bench top, which is the desired position for proper calibration as no angular related error is present. In the top diagram, the microscope is focused directly on the desired graduation. Yet, due to the geometry of the carriage and the imperfections of the bench top, the carriage is able to tilt or “pitch” in a clockwise directionof angle Ө as seen at the bottom of Figure 2.9. If the tilt angle is not corrected (as done manually in the current design by microscrew adjustment) a displacement error is present as the actual reading is displaced from the desired measurement. The displacement error seen in Figure 2.9 is what is responsible for calibration uncertainty.

3.2 Operator Error

Operator error due to manual pitch adjustment is another large source of measurement uncertainty. In the current design the pitch of the carriage is adjusted and maintained approximately constant using a bubble level (for pitch detection) and microscrew. As the pitch is corrected manually by monitoring the bubble level, the accuracy of pitch adjustment can be done by eye with minimal error of 5 arc - seconds.


Figure 2.9 – Abbé Error due to Abbé Offset


Operator error also occurs during the centering of the microscope reticule on the measurement graduation. After the reticule is aligned and centered on the graduation, the operator must eliminate the carriage pitch by manually adjusting it back to zero pitch. After this, the graduation alignment must be checked to make sure that it is still centered. In multiple instances, the last step is forgotten as the carriage may have shifted after pitch adjustment. Therefore, an error due to operator negligence is also present. Because such adjustments cannot be done perfectly, operator error contributes greatly to the measurement uncertainty.

4.0 Problem Analysis

Before attempting to find a possible solution, it was necessary to evaluate the problem parameters. The first step was to evaluate the pitch deflection of bench along its full length to define the scale of pitch. The evaluation of the bench pitch was performed by using the Wyle Fowler Digital Levelmeter shown on the carriagein Figure 3.0. This Levelmeter was chosen for this experiment as itallowed me to obtain a digital read out and direction of the pitch within a milliradian scale.

The pitch evaluation was performed by taking pitch measurements in 100 and 400 millimeter increments to check for consistency. The results from this evaluation can be seen in on Page 20. The data plotted in the Microsoft Excel spread sheet allowed me to see how much the bench fluctuates in pitch along its length. It can be noted that in some cases an amplitude of up to 7 Arc-Minutes is attained. With Ө = 7 Arc-Minutes


Figure 3.0 – Wyler Fowler Digital Level


(≈0.117 degrees), an error of approximately 101 micrometers14 may be present.

14 Tan(Ө) = (Error/Abbe Offset), Error = Tan(0.117)(0.05) = 101x10-6 meters


Bench Length Comparison

-8

-6

-4

-2

0

2

4

6

8

0 10 20 30 40 50 60

Disp. (m)

Pitch (A

rc-Minut

es)

100mm Bench Length

400mm Bench Length




One of the most important factors of performing this experiment was to gain an understanding of the bench behavior with respect to pitch. The scale of the pitch can define the type of design and technological sensitivity necessary to create pitch measurement. In this case, it is necessary to be able to measure pitch within an arc-minute scale with up 7 arc minute maximum.

5.0 Possible Solution

After considering an electromechanical self leveling system as a possible solution for pitch correction, I have come to a conclusion that it is not the best approach. This approach would be difficult to assemble as multiple technological problems could potentially arise. Particularly, incorporating this system would involve a major carriage design overhaul to provide a structure that would work for this leveling system.

After brainstorming possible solutions, I have come up with a pitch correction approach that may have great potential. From a simple calculation, measurement error (Figure 3.1) can be calculated by knowing the magnitude of the carriage pitch. Therefore, the final measurement may be corrected by subtracting the error out. This approach would eliminate the necessity to manually adjust pitch, thus eliminating operator error while correcting for the Abbé error. The approach would incorporate a digital inclinometer15 on board of the carriage to detect the magnitude of carriage pitch. The pitch value can also be easily sent wirelessly to the Renishaw software by the use of a wireless modem transmitter and receiver. By knowing the value of the pitch (Ө), the software could be adjusted to create a calculation of error [Error = Abbé Offset*Tan(Ө)] seen in Figure 3.1. By knowing the error involved in the measurement, the measurement could be corrected within software to eliminate the Abbé error and the operator error.

15 Instrument or sensor used for measuring angles of tilt or inclination with respect to gravity. Also known as levelmeter.


Figure 3.1 – Error Diagram


This approach would be the simplest and most affective way to decrease measurement uncertainty by correcting for Abbé and operator errors. It would provide the benefit of not having to change the current carriage design, higher precision in pitch measurement (no bubble scale) and no manual operation.

5.1 Experimental Comparison

5.1.1 Introduction

Before I could begin the search for possible inclinometers and wireless modems compatible for this task, it was necessary to evaluate whether the proposed approach would work and would it work better then the current approach to calibration. To create a comparison between the two methods of calibration I performed an experiment consisting of two calibration simulations. One simulation involved the current method of calibration, or manual pitch adjustment with the bubble level. Second simulation involved the use of the Wyler Fowler Digital Levelmeter16 or inclinometer (conveniently owned by the LSCMG) to record pitch without manual adjustment for resulting measurement correction. This experiment would allow me understand whether the use of digital inclinometer and the process of displacement correction by software (through pitch) is better then the use of the bubble level and manual pitch adjustment by the operator.

To check for consistency of data, the comparison experiment was performed three times by using manual pitch adjustment and then twice by using pitch measurement. Using statistical analysis of the gathered data, the best approach would be defined by a comparison of standard deviations of error from both data sets to understand which method of calibration is more efficient. Both data sets can be seen in Appendix A.

16 The Wyler Fowler Levelmeter was used only for experimental purposes to define is the new approach of pitch measurement is feasible. If it is, LSCMG would pursue this approach and build a customize inclinometer for the carriage.



5.1.2 Experimental Setup Description

Wyler Fowler Digital Levelmeter 2000

The Wyler Fowler Digital Levelmeter is a digital instrument that is used to measure angles of pitch or tilt. This particular version, Levelmeter 2000, can measure angles of pitch within ±1.9 milliradian range17.

This instrument incorporates a tilt sensor, a signal transmitter and a receiver. In Figure 3.2, the combination of the tilt sensor and signal transmitter can be seen on the carriage. As the tilt sensor defines the pitch, the signal is then sent wirelessly to the receiver (Figure 3.2 – Top Left Corner) which displays the reading of the pitch in milliradians. The receiver was also connected to the Renishaw software to directly output a pitch reading with respect to a calibration displacement value from Renishas software.

17 The milliradian scale was sufficient enough to measure arc-minute scale pitch. ±1.9 milliradian range also covered the maximum and minimum range of the bench pitch evaluated earlier.


Figure 3.2 – Level Setup


5.1.3 Simulation 1: Manual Leveling Procedure(3 Trials Total)

Purpose:

- To simulate tape calibration process by an operator with the use of the bubble level and manual pitch correction.

- To gain the standard deviation of error that is involved in such process.

Procedure:

- Using a ruler or a tape on a tape bench, define a zero mark.

- Level the carriage at a zero mark by adjusting the micro-screw to attain the zero mark reading (zero pitch) on the bubble level. Zero the Renishaw laser. Through the use of the Renishaw tape calibration software, record the displacement and the pitch values respectively.

- Reset the position and the pitch of the carriage by adjusting the displacement off the zero mark and pitching the carriage off the zero pitch. Now re-adjust the carriage to the zero mark for both, the pitch and the displacement using the bubble level and the microscope. Again, through the use of the Renishaw tape calibration software, record the displacement and the pitch values respectively.

- Repeat for any number of trials.



5.1.4 Simulation 2: Pitch Correction with Wyler Fowler Level Procedure(2 Trials Total)

Purpose:

- To simulate a newly proposed tape calibration process through the use of the Wyler Fowler digital level and technique of displacement correction through pitch.

- To gain the standard deviation of error that is involved in such process.

Procedure:

- Using a ruler or a tape on a tape bench, define a zero mark.

- Level the carriage at a zero mark by adjusting the micro-screw to attain the zero mark reading (zero pitch) for a digital read-out on the Level Meter. Zero the Renishaw laser. Through the use of the Renishaw tape calibration software, record the displacement and the pitch values respectively (as a zero measurement).

- Pitch the carriage manually through the use of the micro-screw to a desired increment in the desired + or - direction, the microscope cross hairs will have moved from zero mark. Align the carriage with the zero mark and then record the displacement and the pitch values respectively.

- Repeat at pitch increments until the minimum and maximum values of the Level Meter are reached (≈±1.9 mRad).

- The following Pitch increments for data collection were used:


Upscale (mRad)0.00.20.40.60.81.01.21.41.61.81.9

Downscale (mRad)0.0-0.2-0.4-0.6-0.8-1.0-1.2-1.4-1.6-1.8-1.9


6.0 Conclusion

This section will provide the experimental result and discussion along with concluding decisions and future plans for lowering uncertainty in calibration.

6.1 Experimental Results

After performing the experimental comparison between the two methods, it became evident that displacement correction through pitch detection has some benefits. Experimental data shows that both methods are rather equivalent in calibration accuracy. By taking a look at the plotted standard deviation of error in Figure 3.3, it can be seen that the


Deviations: Manual Leveling Vs. Wyler Fowler Digital Level

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Trials

Pit

ch

De

via

tio

ns

Wyler Fowler Level 1Manual Pitch Adjustment 1Manual Pitch Adjustment 2Wyler Fowler Level 2Manual Pitch Adjustment 3


error deviations between both techniques overlap. The trend line between the two methods also suggests close consistency in pitch correction for both. Considering these results it can be seen that the measurement uncertainty in both methods is nearly equivalent.

6.2 Concluding Discussion

Although the experimental results suggest that uncertainty in calibration is equivalent, the displacement correction method has multiple benefits over the current calibration process.

One benefit is that the manual adjustment by the operator is no longer necessary. The absence of manual pitch adjustment eliminates sources of error that may lie within this technique. The possibility of error due to shifting of carriage before or after the graduation alignment and imprecision of alignment based on a bubble level scale is eliminated. This method avoids room for human error due to manual bubble level style pitch control. In addition, this technique makes the process of calibration more automated. With the new technique, it is only necessary to locate the gradation through the microscope and then record the measurement value. There are no steps to graduation alignment, then pitch adjustment, then checking for realignment. By a more automated process, the required time for calibration is lowered, thus creating a shorter return time to the customer and lower use of the laboratory utilities.

A second benefit may be found by a suggested further research. Considering the experimental comparison of both methods it can be understood that uncertainty through displacement correction may be improved drastically through the use of more accurate and precise equipment. As this was a preliminary experiment in search of a possible solution, the Wyler Fowler Digital Levelmeter had the necessary specifications for the experimental comparison, but it was not an optimized piece of equipment. For this reason, uncertainty equivalency between both methods suggests that displacement correction technique can be used. In fact, it can be further optimized to lower the measurement uncertainty in calibration.


Figure 3.3 – Standard Deviation of Error Plot


6.3 Future Steps

Currently, the LSCMG is taking this approach to improve their calibration method. For the future, the group plans to take this approach of calibration as they will try to optimize this method by incorporating a more precise digital system for pitch measurement.

Before leaving my position at NIST, I spent time researching a possible system that would accomplish this task. After speaking with the representative of the Applied Geomechanics Inc., an order had been placed for a customized level system designed for the microscope carriage and laboratory use.The new level system will incorporate the Applied Geomechanics IRIS Tilt Controller (Figure 3.4) with two precision ceramic sensors for pitch detection, wireless modems for wireless signal transfer and other necessary equipment for functioning. Due to the time constraints, I was not able to work with the new inclinometer system. But LSCMG will follow this approach to lower the measurement uncertainty through a more precise pitch measurement. In the future, to reach the 10 micrometer uncertainty goal, the group plans to combine the new approach to pitch correction with a new microscope carriage design of minimal Abbé Offset.


Figure 3.4 – IRIS Tilt Controller


Appendix A

Data: Pitch Correction with Wyler Fowler Level – Trial 1

Abbe Offset 106.8 mm

TimeRenishaw

(mm) Time Pitch (mRad) Error (mm)2:43:34 -0.00032 14:43:36 0.001 -0.00042682:45:47 -0.03035 14:45:50 -0.206 -0.00834922:47:19 -0.0432 14:47:22 -0.41 0.0005882:48:17 -0.06867 14:48:19 -0.623 -0.00213362:49:23 -0.07935 14:49:26 -0.822 0.00843962:50:01 -0.10179 14:50:04 -1.036 0.00885482:50:53 -0.12403 14:50:56 -1.233 0.00765442:51:46 -0.14156 14:51:48 -1.444 0.01265922:52:54 -0.16432 14:52:58 -1.616 0.00826882:53:46 -0.18817 14:53:49 -1.822 0.00641962:54:39 -0.19889 14:54:44 -1.917 0.00584562:55:29 -0.18892 14:55:32 -1.828 0.00631042:57:00 -0.1624 14:57:04 -1.621 0.01072282:57:40 -0.15024 14:57:43 -1.439 0.00344522:58:30 -0.12703 14:58:32 -1.227 0.00401362:59:36 -0.1048 14:59:39 -1.038 0.00605843:01:23 -0.07677 15:01:26 -0.833 0.01219443:02:06 -0.06029 15:02:08 -0.604 0.00421723:02:37 -0.04283 15:02:40 -0.44 0.0041623:03:37 -0.01243 15:03:40 -0.207 0.00967763:04:30 0.00876 15:04:34 0.002 0.00854643:05:28 0.02911 15:05:33 0.232 0.00433243:06:20 0.05073 15:06:22 0.424 0.00544683:07:12 0.06939 15:07:14 0.633 0.00178563:07:52 0.09698 15:07:56 0.845 0.0067343:08:35 0.12209 15:08:38 1.055 0.0094163:09:07 0.13603 15:09:10 1.217 0.00605443:09:38 0.15076 15:09:41 1.41 0.0001723:10:24 0.18316 15:10:26 1.618 0.01035763:11:02 0.20108 15:11:05 1.816 0.00713123:11:42 0.20618 15:11:44 1.933 -0.00026443:12:58 0.20283 15:13:01 1.817 0.00877443:13:29 0.18184 15:13:32 1.628 0.00796963:14:01 0.16384 15:14:04 1.428 0.01132963:14:24 0.13393 15:14:27 1.198 0.00598363:16:34 0.11681 15:16:37 1.033 0.00648563:17:10 0.09577 15:17:12 0.816 0.00862123:18:20 0.07812 15:18:22 0.633 0.01051563:19:14 0.05856 15:19:16 0.433 0.01231563:20:01 0.03166 15:20:04 0.228 0.00730963:20:32 0.00467 15:20:35 -0.003 0.0049904



Error (mm) Vs. Pitch (mRad)

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Pitch (mRad)

Err

or

(mm

)

07/09/07



Pitch Correction with Wyler Fowler Level – Trial 2

Abbe Offset 106.8

TimeRenishaw

(mm) TimePitch

(mRad) Error10:24:10 -0.00001 10:24:17 -0.001 010:27:14 0.02476 10:27:17 0.227 0.000419610:28:11 0.04021 10:28:08 0.42 -0.004742810:29:07 0.06573 10:29:11 0.617 -0.000262410:30:02 0.08094 10:30:06 0.802 -0.004810410:31:10 0.10342 10:31:12 1 -0.003476810:35:10 0.12266 10:35:13 1.202 -0.005810410:36:39 0.14386 10:36:42 1.409 -0.00671810:38:10 0.16765 10:38:13 1.601 -0.003433610:40:07 0.18933 10:40:10 1.805 -0.003540810:42:12 0.19861 10:42:14 1.934 -0.00803810:43:26 0.18752 10:43:29 1.801 -0.004923610:44:42 0.16737 10:44:45 1.606 -0.004247610:46:11 0.13929 10:46:14 1.4 -0.010326810:48:21 0.1202 10:48:25 1.206 -0.008697610:51:18 0.0949 10:51:20 1.007 -0.012744411:06:19 0.08339 11:06:21 0.802 -0.002360411:09:32 0.06119 11:09:34 0.602 -0.003200411:11:40 0.04181 11:11:43 0.4 -0.001006811:14:30 0.01608 11:14:32 0.199 -0.0052711:15:24 -0.00071 11:15:26 0.002 -0.001020411:17:12 -0.02277 11:17:17 -0.21 -0.000438811:18:37 -0.04733 11:18:41 -0.411 -0.00353211:21:53 -0.06921 11:21:55 -0.609 -0.004265611:24:21 -0.08448 11:24:24 -0.798 0.000649611:25:28 -0.10916 11:25:30 -1.001 -0.0023511:30:09 -0.13388 11:30:10 -1.207 -0.005069211:32:14 -0.16181 11:32:16 -1.446 -0.00747411:33:07 -0.182 11:33:10 -1.607 -0.010469211:34:40 -0.2021 11:34:41 -1.8 -0.009956811:50:12 -0.20877 11:50:15 -1.938 -0.001888411:51:45 -0.20001 11:51:47 -1.834 -0.004235611:53:22 -0.17084 11:53:25 -1.605 0.000477212:00:06 -0.144 12:00:08 -1.405 0.005957212:01:36 -0.12428 12:01:39 -1.201 0.0038912:02:26 -0.10963 12:02:28 -1.008 -0.002072412:03:49 -0.08974 12:03:51 -0.842 8.88E-0512:04:41 -0.06056 12:04:42 -0.61 0.004491212:06:43 -0.04123 12:06:45 -0.407 0.002140812:07:33 -0.02623 12:07:35 -0.205 -0.004432812:09:09 0.00129 12:09:13 0.006 0.0005524



Error Vs. Pitch

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Error Vs. Pitch



Renishaw Disp. Vs. Pitch

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0.25

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

Pitch (mRad)

Ren

ish

aw D

isp

(m

m)

0 to -1.9

-1.8 to 0

0 to +1.9

+1.8 to 0

Manual (Bubble) Leveling - Trial 1

Time Pitch (mRad) Change In Angle Time Renishaw (mm)9:47:55 1.249 0 9:47:48 -0.000029:49:20 1.243 -0.0060 9:49:11 -0.008679:52:45 1.241 -0.0080 9:52:42 -0.002129:54:04 1.242 -0.0070 9:54:01 -0.006999:55:30 1.244 -0.0050 9:55:27 -0.005929:57:38 1.237 -0.0120 9:57:35 -0.008129:59:04 1.234 -0.0150 9:59:00 -0.0041810:01:46 1.241 -0.0080 10:01:43 -0.0099810:02:34 1.24 -0.0090 10:02:30 -0.0136110:04:29 1.241 -0.0080 10:04:26 -0.0130410:06:35 1.238 -0.0110 10:06:32 -0.0038610:08:09 1.242 -0.0070 10:08:04 -0.0090610:10:14 1.238 -0.0110 10:10:11 -0.0090510:11:53 1.242 -0.0070 10:11:50 -0.0141510:15:12 1.244 -0.0050 10:15:10 -0.0129710:16:28 1.241 -0.0080 10:16:26 -0.0153510:18:20 1.234 -0.0150 10:18:18 -0.0161310:20:10 1.243 -0.0060 10:20:07 -0.0083210:21:56 1.243 -0.0060 10:21:54 -0.0114610:25:06 1.243 -0.0060 10:25:03 -0.0143110:27:43 1.242 -0.0070 10:27:41 -0.01409



10:29:52 1.245 -0.0040 10:29:50 -0.0024610:32:03 1.244 -0.0050 10:32:01 -0.0096610:50:57 1.252 0.0030 10:50:55 -0.0113710:52:42 1.251 0.0020 10:52:39 -0.0117510:55:44 1.249 0.0000 10:55:41 -0.0135210:58:37 1.249 0.0000 10:58:30 -0.0072511:01:16 1.25 0.0010 11:01:14 -0.0091911:02:53 1.251 0.0020 11:02:46 -0.010311:04:59 1.251 0.0020 11:04:55 -0.0077211:09:02 1.245 -0.0040 11:08:54 -0.01098




Time Pitch (mRad) Change In Angle Time Renishaw

(mm)9:59:15 1.283 0 9:59:12 010:00:58 1.277 -0.006 10:00:55 0.0009710:03:38 1.275 -0.008 10:03:36 -0.0065510:06:12 1.274 -0.009 10:06:09 -0.0074110:07:47 1.27 -0.013 10:07:45 -0.005710:10:44 1.264 -0.019 10:10:42 -0.005110:12:09 1.263 -0.02 10:12:07 -0.0037610:15:12 1.271 -0.012 10:15:11 -0.0054410:16:56 1.27 -0.013 10:16:54 -0.0090410:18:52 1.268 -0.015 10:18:51 -0.0075110:23:33 1.269 -0.014 10:23:31 -0.0122510:52:21 1.271 -0.012 10:52:17 -0.0077610:53:59 1.274 -0.009 10:53:57 -0.0075510:55:41 1.272 -0.011 10:55:40 -0.0141210:57:50 1.276 -0.007 10:57:47 -0.0102810:59:36 1.27 -0.013 10:59:34 -0.0112611:00:58 1.279 -0.004 11:00:55 -0.01242


Time Pitch (mRad) Change In Angle Time Renishaw

(mm)10:17:10 1.757 0 10:16:58 -0.0000210:19:05 1.755 -0.002 10:19:03 -0.0024910:22:34 1.759 0.002 10:22:31 -0.0002210:24:33 1.759 0.002 10:24:31 -0.0051810:28:54 1.766 0.009 10:28:51 -0.006410:31:03 1.771 0.014 10:31:01 -0.0046710:34:58 1.775 0.018 10:34:56 -0.0113210:40:27 1.779 0.022 10:40:25 -0.0110110:41:57 1.784 0.027 10:41:55 -0.0100810:44:18 1.787 0.03 10:44:16 -0.0087910:47:47 1.79 0.033 10:47:45 -0.012910:52:31 1.791 0.034 10:52:28 -0.013910:59:05 1.799 0.042 10:59:02 -0.0140211:21:58 1.802 0.045 11:21:56 -0.0058211:25:08 1.798 0.041 11:25:06 -0.0127911:26:52 1.801 0.044 11:26:49 -0.0017811:30:21 1.801 0.044 11:30:20 -0.0018611:34:25 1.803 0.046 11:34:22 0.0057511:38:03 1.804 0.047 11:38:02 0.0051



Final Comparison

Abbe Offset 106.8

Renishaw (mm) Pitch (mRad)Error #1

(mm)Error #2

(mm)

Renishaw Bubble Trial 1(mm)

Renishaw Bubble Trial 2(mm)

RenishawBubbleTrial 3(mm)

-0.00032 0.001 -0.0004268 0 0.00002 0 0.00002-0.03035 -0.206 -0.0083492 0.0004196 0.00867 0.00097 0.00249-0.0432 -0.41 0.000588 0.0047428 0.00212 0.00655 0.00022-0.06867 -0.623 -0.0021336 0.0002624 0.00699 0.00741 0.00518-0.07935 -0.822 0.0084396 0.0048104 0.00592 0.0057 0.0064-0.10179 -1.036 0.0088548 0.0034768 0.00812 0.0051 0.00467-0.12403 -1.233 0.0076544 0.0058104 0.00418 0.00376 0.01132-0.14156 -1.444 0.0126592 0.006718 0.00998 0.00544 0.01101-0.16432 -1.616 0.0082688 0.0034336 0.01361 0.00904 0.01008-0.18817 -1.822 0.0064196 0.0035408 0.01304 0.00751 0.00879-0.19889 -1.917 0.0058456 0.008038 0.00386 0.01225 0.0129-0.18892 -1.828 0.0063104 0.0049236 0.00906 0.00776 0.0139-0.1624 -1.621 0.0107228 0.0042476 0.00905 0.00755 0.01402-0.15024 -1.439 0.0034452 0.0103268 0.01415 0.01412 0.00582-0.12703 -1.227 0.0040136 0.0086976 0.01297 0.01028 0.01279-0.1048 -1.038 0.0060584 0.0127444 0.01535 0.01126 0.00178-0.07677 -0.833 0.0121944 0.0023604 0.01613 0.01242 0.00186-0.06029 -0.604 0.0042172 0.0032004 0.00832 0.00575-0.04283 -0.44 0.004162 0.0010068 0.01146 0.0051-0.01243 -0.207 0.0096776 0.00527 0.014310.00876 0.002 0.0085464 0.0010204 0.014090.02911 0.232 0.0043324 0.0004388 0.002460.05073 0.424 0.0054468 0.003532 0.009660.06939 0.633 0.0017856 0.0042656 0.011370.09698 0.845 0.006734 0.0006496 0.011750.12209 1.055 0.009416 0.00235 0.013520.13603 1.217 0.0060544 0.0050692 0.007250.15076 1.41 0.000172 0.007474 0.009190.18316 1.618 0.0103576 0.0104692 0.01030.20108 1.816 0.0071312 0.0099568 0.007720.20618 1.933 -0.0002644 0.0018884 0.010980.20283 1.817 0.0087744 0.00423560.18184 1.628 0.0079696 0.00047720.16384 1.428 0.0113296 0.00595720.13393 1.198 0.0059836 0.003890.11681 1.033 0.0064856 0.00207240.09577 0.816 0.0086212 8.88E-050.07812 0.633 0.0105156 0.00449120.05856 0.433 0.0123156 0.00214080.03166 0.228 0.0073096 0.00443280.00467 -0.003 0.0049904 0.0005524



Deviations from Using a Bubble Level Vs. Wyler Level

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Trials

Pitch

Devia

tions

(mm)

Error With Wyler 1Renishaw With BubbleRenishaw With Bubble Trial 2Error With Wyler 2Renishaw With Bubble Trial 3


solution to decrease uncertainty in measurement tape calibration

Documents

calibration error

measurement uncertainty

measurement tape calibration

tape calibration lab

calibration procedure

operator error figure

micrometer uncertainty

calibration services