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Using Portable Dimensional Metrology in Remote Locations

Mining the Sky – Today’s Energy Solutions

Written by Elizabeth Engler Modic –  May 2012

SolarReserve’s first-generation solar power plant is a welcome sign of green progress. Once completion of the Crescent Dunes Solar Energy Project in 2013, it will generate roughly 480,000MW-h per year of clean, renewable electricity to power 75,000 homes during peak electricity periods. Developed by the same Rocketdyne engineers and scientists who designed Apollo rockets, Space Shuttle engines, and the solar power system for the International Space Station is the company’s unique concentrating solar power (CSP) technology and innovative energy storage.

Harnessing Sunlight

Entrance to Tonopah Test Range from Highway 6,...
Entrance to Tonopah Test Range from Highway 6, Nevada (Photo credit: Wikipedia)

Tonopah Solar Energy, a subsidiary of SolarReserve, manages the project located on 1,600 acres northwest of Tonopah, NV, a historical silver mining town near the Nevada desert. The site selection was based on key ingredients for optimum solar energy – hours of direct sunlight, altitude (more intense sunlight), and DNI (Direct Normal Irradiance) strength. The power plant design includes a tall receiver tower and power block positioned centrally in a large circular field of mirrors known as heliostats. The 100ft receiver mounts on the 553ft concrete tower. Tower height is essential for plant efficiency, ensuring the heliostat array can concentrate sunlight onto the receiver.

Comprised of thousands of tracking mirrors in a two-square mile area, the solar collection field operates whenever there is ample solar resource to collect energy. SolarReserve’s propriety motion control software lies on a central computer that sends angle location data to the heliostat field, as the mirrors move simultaneously as the sun travels across the sky. The energy is stored and delivered to the grid anytime, even after sundown. The 110MW plant will utilize an ordinary steam turbine generator to produce electricity, but integrates a sophisticated hybrid cooling system using less than 600 acre-feet/year of water, a scarce commodity in the Southwest.

The power tower receiver glows as it soaks up the sun during the day, but remains dormant at night. Its high heat flux hardware is a unique blend of liquid rocket engine heat transfer technology and molten salt handling expertise. Inside the receiver, concentrated sunlight heats molten salt to more than 1,000°F. The liquid then flows into an insulated storage tank maintaining 98% thermal efficiency. Eventually, pumps into the generator for electricity production. This process is similar to a standard coal-fired power plant, but clean and free solar energy fuels this process.

The solar power plant utilizes components primarily manufactured in the United States as opposed to competing technologies using mostly imported parts and assemblies. Bar a few one-of-a-kind components, the plant employs readily available materials (mirrors) and established technologies (steam generators and turbines). The inexpensive molten salt, made from an environmentally friendly mixture of sodium and potassium nitrate, is the same ingredients used in garden fertilizer. This configuration of materials and equipment will enable SolarReserve to provide electricity at or below prices from traditional sources such as coal or natural gas.

Challenges of Production
Project manager Gary Raczka is responsible for the design, manufacture, and installation of the solar collector system in the field, as well as the software that drives the mirrors to focus on the receiver. Heliostat design is a large team effort, and his staff includes mechanical engineers, software developers, and a growing engineering workforce to support not only the heliostat production, but also the entire solar plant.

For the build aspect of the project, the team works closely with contract manufacturers well versed in handling large mechanical components. To get a feel for the size of these heliostat assemblies, the mirror surface is approximately 28ft x 24ft, with the pedestal made from large diameter piping that is 11ft to 16ft tall. Other metal components and trusses supporting the mirror structure range from 28ft to 30ft long.

The company cut its teeth on small test facilities, and then went on to create three different designs and sizes of heliostats supported by various structural configurations. Each SolarReserve plant is roughly the same in terms of tower size, general field layout, and overall square meters of glass in the field. Use in Tonopah will be the baseline 62.5m2 heliostat that translates to 17,608 heliostats in the collection field. The project has an ambitious plan to install 70 heliostats per day until completion.

Metrology is the Glue
For optimum performance, the solar plant design calls for an overall beam quality and accuracy of less than 1.5mRad for each facet of the entire heliostat field. This high precision specification dictated the early need for dimensional control and verification. Raczka found specialized expertise at Hexagon Metrology Services, North Kingstown, RI, to verify designs and build confidence into all aspects of manufacture and assembly. Rina Molari, a seasoned metrologist from Hexagon, was excited to be a part of this groundbreaking endeavor.

Because the project entailed working with sizeable parts in formidable outdoor settings, Molari employed a portable Leica AT901 laser tracker for its ability to handle numerous quality assurance tasks. Due to its long-range measurement volume of 525ft when used with a standard corner cube, use of this laser tracking system is primarily for aerospace and other in-place measurement applications. Based on Leica’s Absolute interferometer (AIFM) technology, the portable CMM maintains precision measurement in all operating conditions, with multiple, built-in redundancies to ensure high accuracies.

 see more via Mining the Sky – TES – Today’s Energy Solutions.

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The Art of Aerospace

Boeing's Everett plant, the largest enclosed f...
Boeing’s Everett plant, the largest enclosed facility in the world. (Photo credit: Wikipedia)

via Boeing.com

3-D mapping technology created unique depictions of the Boeing factory in Everett, Wash., that are akin to modern art. The art of aerospaceBy Nathan A. Hulings

As Boeing researcher Charles Erignac recently walked through the company’s expansive factory in Everett, Wash., he used a camera and laser-scanning sensors that captured three-dimensional images of the plant and the large twin-aisled commercial airplanes assembled there.

It’s expected that aerospace manufacturing of the future will require increasingly detailed and timely 3-D maps to meet efficiency and productivity demands – and Boeing researchers such as Erignac are working to improve the technology that can create such maps.

The images “are quite stunning on their own.”

via The art of aerospace.

Compensating for Thermal Expansion – does size matter?

 In today’s manufacturing world the improvements in technology have allowed us to accurately and precisely design, manufacture and inspect parts and assemblies to tighter tolerances then at anytime in history.

Many applications do not require any Compensation for Thermal Expansion (CTE) due to size, type of material or design/engineering requirements. However, many applications benefit from applying CTE due to the material type and the environment. In general, the larger the part or object being measured the bigger the affect of thermal changes to the material of the part/object.

During Inspection and fabrication (particularly automotive/aerospace tooling), the temperature of the object being measured can have a major impact on the measurement results. Understanding the proper application of the Compensation for Thermal Expansion will result in more repeatable measurements regardless of the objects temperature.

Measurement Devices and the Environment

Many measurement devices that utilize laser interferometry require accurate environmental measurements of temperature, air pressure and humidity to acquire accurate measurement data. The correction for environmental factors; temperature, air pressure, humidity are not required for the operation of  some measurement systems for example; Articulating Arms, Photogrammetry and  CMM’s.

When using a measurement system with a laser interferometer or distance meter of some variety for distance measurement (Laser Trackers most common, Total Stations)  the temperature and air pressure that is entered for the initialization process of the device is used to correct the instrument measurements back to the standard 68° F or 20° C. This is particularly important to note when point to point distances will be measured on an object that is not  68° F or 20° C.

It is recommended that you refer to the Manufacturers recommendations.

When to Compensate

The type of measurement task will determine whether Compensation of Thermal Expansion (CTE) must be used and when the CTE should be applied.

Common Applications

  • Object is constructed of material that is not thermally stable.
  • Tight tolerances in environment where temperature is not controlled
  • Large objects
  • Establishing Large Measurement Networks on Large Structures

Applying CTE

The actual application of Compensation of Thermal Expansion (CTE) is unique in nearly every Measurement Software on the market. To properly apply CTE in the software that is being used the operator MUST understand the following to be successful:

  • Type of Material – the exact type of material should be known in order to accurately calculate the CTE of the object.
  • Part Temperature – depending on the required accuracy of the measurement task a variety of devices are available to read temperatures. For small parts, one reading is sufficient when the task is Large Volume Measurement a number of readings may be required and an average determined. If the object is tall, readings should be taken at several altitudes.
  • What type of ‘Part Alignment’ will be used – the sophisticated measurement software’s on the market provide the power of applying CTE through 7 parameter transformation matrixes. This provides the ability to ‘Align’ to the part using a ‘Best Fit’ algorithm, which fits Translations, Rotations and Scale, based on Nominal data. A keen understanding of how the Nominal data was established is required prior to ‘Scaling’ an Alignment based on the Nominal information. Nominal data that was not properly valued will yield inaccurate to catastrophic results.
  • If a ‘minimum fit’ Alignment will be used when should scale be applied – this varies in measurement software’s as some software’s require CTE be applied prior to this type of Alignment, others after the Alignment has been calculated. See Software documentation for best practices.
  • Does the CTE need to be applied before or after Data Collection – this is key as some software’s apply the CTE as the data comes in to the software, while others apply it to the measured data during post processing.

Verifying ‘Scale’

When applying a Compensation of Thermal Expansion the ‘Scale’ of the measurement task or dataset is changed. When using a 7-parameter ‘Best Fit’ Transformation this ‘Scale’ factor should be verified in particular on Large Scale measurement tasks. Scale Factors are typically displayed as a ratio of the active Units of Measure, this ratio needs to be considered over the entire Measurement ‘Envelope’, a common mistake is to review ONLY the X, Y, Z, 3D deviations after Scale has been applied.

To properly evaluate the CTE a calibrated ‘artifact’ made of the same type of material must be used, the ‘artifact’ must also be the same temperature as the part being measured, for these reason’s many facilities are unable to properly evaluate the application of CTE.

Using a Certified Artifact

One method for verifying Scale is to use some type of ‘Artifact’ this is typically a ‘Scale Bar’ or ‘Ball Bar’, the artifact must be the same type of material as the part being measured, and be the same temperature.

Certified Ball Bars

Scale Bars/Ball Bars can be made from a wide variety of materials with the most common available are Aluminum, Steel, Composite and Invar.

Ball Bars have highly accurate spheres fixed to each end. These Ball Bars typically have a calibrated distance between the two spheres.

Scale Bars typically have magnetic mounts at each end to accept Laser Tracker Spherically Mounted Reflectors (SMR’s) or highly accurate Sphere’s. The distance between the points is calibrated by using highly accurate Laser Interferometers.

Certified Scale Bars

How to Verify Scale – When an artifact of the same type of material is available, it must be thermally ‘soaked’ to the same temperature as the object being measured. After applying the CTE within the Measurement Software application the operator can measure Points or Spheres (depending on artifact) and simply check the point to point distance. Dependent on the accuracy of the measurement system being used, the result should typically be within 0.0020 inches or 0.050 mm. The results will vary depending on the device being used, and the accuracy of the artifact being used.

Analyzing the Results – when point to point distances vary from the Calibrated Artifact significantly, the operator must determine what the overall impact to the measurement task will be. If the artifact is significantly bigger than the part will the scale impact be acceptable within the parts envelope? If using a 7 parameter ‘Best Fit’ Transformation is the ‘nominal’ information accurate? If the ‘nominal’ information was derived from a previous measurement; was CTE accounted for correctly?

The use and application of CTE can improve the accuracy and repeatability of nearly every measurement task when applied correctly. Refer to your Metrology Software package documentation for details of use within that software.

Related articles

A Survey of Training in the Portable Dimensional Metrology Industry

This Survey was performed in 2010 for a Technical Presentation at the Coordinate Metrology Society Conference (CMSC), since this time, I have seen little change in the “priority” of training operators. At the two most recent CMSC’s there have been “Measurement Study’s” performed that directly correlate to what was seen in this survey….

Two years later…. economic downturn have created further cuts in Training in many industries, Training is always the first to go, and seems to be undervalued in the Metrology industry, which seems very odd based on the fact that our industry is based on “Accuracy and Precision“…..Company’s seem to find the a way to purchase new hardware and software, but finding the $$ and time to train new operators to be efficient seems to be nearly impossible in many area’s, does this not cost them more in money in the long run? Could it be that our industry itself does not put enough emphasis on Training? How can we train new people if new customers and user’s are told during the Sales process “it doesn’t take any time to learn it, you can pick it up in a couple days”. We need the “ease of use” to make the sale, but is that hurting the customer and the industry over the long term as we now try to initiate some type of industry wide “Certification” process??

Here is the Survey and Technical Paper posted on Quality Digest.

Breaking down assumptions about the knowledge base of metrology operators

Expensive, highly technical metrology equipment, which is used worldwide to measure critical parts in aerospace, automotive, nuclear, and communications industries (to name a few), has increased productivity and quality of virtually every product manufactured today. There’s an accompanying assumption that users of portable dimensional metrology equipment are highly trained in the field of dimensional metrology. To validate this assumption, I developed a survey using Surveymonkey.com and distributed the link though my network of metrology professionals. I also asked those individuals to pass the survey on to their constituents.

via A Survey of Training in the Portable Dimensional Metrology Industry | Quality Digest.

Top Read – Compensating for Thermal Expansion

The following page has received the most visits since JustMetrology.com came online in 2009
 
Thermometer with Fahrenheit units on the outer...
Image via Wikipedia

For any individual working in the Dimensional Metrology Industry the subject of Compensating for the Thermal Expansion of Materials has been an issue at one time or another. For accurate measurements consideration must be given to the type of material and the temperature of that material especially in environments where the ambient temperature is NOT 20° C or 68° F and the material is not within those temperature ranges. Granted, there are materials that are not affected by temperature change, therefore it is imperative for the Measurement Technician to be familiar with the proper application of Thermal Expansion.

In the Dimensional Metrology industry it is common for the Compensation of Thermal Expansion procedure to be called “Scale” as the Metrology Software typically applies a “Scale Factor” to adjust the data being measured and analyzed for the actual part temperature. This Scale Factor can be applied in various fashions, from “Scaling” the Measured Data, “Scaling” the Measurement Equipment, to “Scaling” a Least Squares Transformation thus creating the 7-parameter Transformation. Each Metrology Software uses specific processes for applying “Scale” the Technician should be familiar with the best process for the Measurement application to make the best decision for how to apply the “Compensation of Thermal Expansion”, incorrect application of “Scale” (Compensation of Thermal Expansion) can yield disastrous results great care should be taken when measuring part temperature, determining type of Material and the method utilized to apply the “Scale”.

Why Scale Matters…..

During the year the ambient temperature in many manufacturing areas throughout the world change with the seasons, thus Winter months are much cooler in the Manufacturing area then it is in the Summer months. Although this change in temperature may only be 6-8° F the change in the materials depending on the size of the object can be significant. Consider this: In simple terms, Aluminum changes 0.001″ inches in length for EVERY degree in Fahrenheit above or below 68° F. What this means is that a structure that is 240.000 inches in length, changes 0.0024″ for EVERY degree it varies from 68° Fahrenheit. Given that many of the Accuracy requirements for the Dimensional Metrology industry these days is in the neighborhood of 0.0020″ inches the requirement to account for that change in temperature of the material should be easily understood. This values are general in nature and vary depending on the exact type of Aluminum being used.

With correct application of “Scale” the temperature of the part becomes the “Same” by using the Metrology Software.

But my Equipment Compensates for the Temperature Automatically…..

Or does it? Typically, the Measurement Systems make adjustments to internal distance measurement components due to the ambient temperature they are being utilized in. For instance; Laser Trackers, Total Stations, Scanners may require the Measurement Technician to enter a Temperature, this temperature is utilized to adjust the Measurement Equipment to take accurate measurements however it makes no adjustment for the material being measured. When a system compensates for the Material the Technician is prompted for both the Type of Material and the Part Temperature without this information it is NOT possible to accurately calculate and apply a Scale Factor.

If the system being utilized requires the use of a Certified Scale Bar to determine Scale the equipment can provide accurate “Scale” ONLY when the Scale Bar Material and Temperature are the SAME as the part being measured.

When do I apply the “Scale” Factor?

The answer to this question is that each software uses different processes to perform the Scale process. It is best to consultant an Expert on the Software to answer this question as the wrong information can have catastrophic results.

Scaling during a Least Squares Transformation

This use of “Scale” requires that the operator have X, Y, Z values for previously measured points that had the Compensation of Thermal Expansion applied correctly during the initial measurement process.

This type of Transformation is common when building large Tooling structures in the Aerospace industry as it allows the Measurement Technician to apply the “Scale Factor” one time when valuing the points and then Exporting point data to be used as Nominal Points throughout the life of the Tool. As the temperature changes the relationship of the points to themselves changes, the Least Squares Transformation process accounts for these changes automatically when using a 7 parameter transformation.

If the Measurement Technician is faced with a measurement task that includes limited constraints for the available points the operator would typically “Fix” the Scale Factor. This method varies between Metrology Software’s, it is advised that prior to utilizing this type of Transformation which requires “weighting” of points that the Technician consult a Software Expert.

How do I know if Scale is right?

A simple check that has become a standard in the Aircraft Manufacturing Industry is to measure a Certified Scale Bar that is the same material and temperature of the part being measured. When Scale has been applied correctly the measurement equipment should be able to measure the Certified Scale Bar accurately (some shops allow up to +/-0.0030 inches of deviation). Dependent on the type of equipment, measurement volume and the measurement task requirements you deviations will vary. CAUTION:Your certified Scale Bar should measure within the expected accuracy expectations for the Measurement task.

If you do not have a Certified Scale Bar of the same material, it is only possible to verify that the Measurement Equipment is measuring “Scale” correctly. By measuring the points on the Certified Scale Bar in several different positions the Technician can assess the accuracy of the Measurement System. It is critical that prior to checking point to point distances the operator correctly account for the material type and material temperature of the Certified Scale Bar.

Collaborating Colleagues – Collaboration is not an option, it’s a necessity.

From the editor of Quality Magazine –
When is the last time you thought of your competitors as collaborators? For many of us, sharing information with the “enemy” is unthinkable. But for others, it’s a way of life.

I recently attended the CMSC in Phoenix. For those of you not familiar with the Coordinate Metrology Society, it is a membership of users, service providers and OEM manufacturers of close-tolerance industrial coordinate measurement systems, software and peripherals. The society gathers each year to gain knowledge of the advancements and applications of any measurement system or software solution that produces and uses 3-D coordinate data.

 

Metrology?

Growing up in the “jet city” I have had the advantage of being exposed to airplanes from every conceivable angle; watching 707’s fly 150 feet above me while I laid on the grass in my grandparents backyard, so loud you had to cover your ears to helping assemble 737 aircraft as a young man. It is this very aircraft industry that has provided me with so many opportunities in my life I would have otherwise not had. Growing up in what was then a small farming town (now a booming metropolis), I never dreamed that the skills I would learn as a young man building aircraft jigs and fixtures would end up taking me all around the world and meeting some of the finest people in the world.

My day today was spent working a booth at a trade show in a local Airplane museum, many of the attendee’s had little knowledge of precision measurement or “metrology” so again I spent a good part of the day educating people on what precision measurement does for industries such as the aircraft industry. As I prepared to leave and was touring the museum I found this display no more than 50 feet from where I spent my day….coincidence?? I think not……there are many of us around that look at this picture and smile, and say to ourselves….yep, that’s where I started…….(apologize for the picture quality…).

Also see this classic post; Metrologist??

Measurement: The Metrology Training Dilemma

This derivation of the Vitruvian Man by Leonar...
Image via Wikipedia

by Stephen Kyle

May 1, 2011

Verifying the position of aerodynamic parts on race cars was a difficult task before Andretti Autosport began using a precision mechanical arm. Source: Faro Technologies

To make effective use of a high precision metrology system, one needs to understand how it works, how to squeeze the best performance out of it, how to choose the optimal one for the application and what to do if something goes wrong.

Editor’s note: This article is an excerpt from Stephen Kyle’s white paper presentation given at the 2010 Coordinate Metrology Systems Conference (CMSC).

In the realm of metrology, there is a wide variety of instrumentation used for portable coordinate measurement (PCM) and large volume metrology (LVM). A precision mechanical arm captures accurate 3-D points and surfaces from a race car. A laser beam tracks a wireless probe, held by the operator and used to check critical dimensions on an automobile body. A photogrammetry system inspects the three-dimensional features of an aircraft door. A handheld laser line scanner is tracked by an imaging system as it builds up the surface shape of a car door. These high-precision dimensional measurement systems can be brought on site to help build, check and reverse engineer a wide range of large, manufactured objects.

These measurement technologies are not new. Many mobile optical systems delivering 3-D shape were developed by shrinking down mapping systems configured to measure large land masses and optimizing them for car bodies and aircraft wings. This shrinks down the error, too, and produces systems which can accurately measure to a few tenths of millimeters or a few tens of micrometers. Early applications of industrial photogrammetry (precise measurement from multiple images) or industrial surveying (precise measurement from multiple theodolites) date from around 1970.

Developments in electronic imaging and processing have since turned industrial photogrammetry into a range of sophisticated vision metrology tools and extend the multiple imaging methods into structured light triangulation systems for detailed measurement of surface form. Surveying systems have moved from multiple theodolite intersection systems to widespread use of single instrument systems based on industrial total stations (theodolites with built-in laser distance measurement) and the more recent developments, in the 1990s, of laser trackers and large volume spherical scanning systems.

The 1970s and 1980s also saw the development of articulated arm coordinate measuring machines (AACMMs), often called more simply CMM arms. The very high accuracy of conventional 3-axis coordinate measuring machines (CMMs), housed in specially built,

See the complete story via Measurement: The Metrology Training Dilemma – Feature Article – Quality.

Impact of Training on Measurement Quality

using the vernier caliper to measure a nut
Image via Wikipedia

Having just finished reading a great Blog Post by Michael L. Ryan of Mitutoyo (link is below), I am happy to see such an extensive article on the importance of proper measurement practices to achieve the best results. Much too frequently today the emphasis is on how fast and easy the measurement task “should be” and not on the skills required to achieve accurate and repeatable results.

In the past on this blog I have written about Training and the importance of it in the Dimensional Metrology industry in particular (that is my expertise, share below your experiences) to the legitimacy of the industry. When untrained and unskilled people perform measurement work not only are the products they are measuring in jeopardy of being in error, but the downstream affect of an automotive or aircraft part being wrong can have devasting consequences to the end user.

This blog post by Michael L. Ryan drives home the point by demonstrating how the use of a relatively simple tool (Calipers) can deliver far different results just based on the user and how they use it. While many of us have made millions of caliper readings, since long before Computer Aided Measurement Systems were the norm, many of our younger generation “dimensional metrologists” lack these simple skills. How much did these simple measurement skills teach you? I know for me, those skills have not been forgotten, they are part of how I view measurement work, the basics. Enjoy this blog by Michael too: http://www.qualitymag.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000001022800.

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