Choose the Right Measuring Tool

Six questions to ask before you buy

Published: Wednesday, May 18, 2011 - 06:00

Today’s research and development laboratories, engineering facilities, and manufacturers are under increasing pressure to design and produce parts that meet ever-more stringent performance specifications and cost-efficiency standards. Having the right measuring tool can help streamline development and production, and ensure that parts meet performance requirements. There are many effective measuring tools that support research and industrial measurement applications: interferometers, profilometers, laser confocal microscopes, scanning electron microscopes, Raman spectroscopy equipment, stereomicroscopes, remote visual inspection equipment, among others.

Selecting the right instrument for a given measuring application can be a challenge. By considering the answers to six core questions, industrial research and engineering professionals can find the ideal tool to provide cost-effective, accurate measurement data to support their research, engineering, and production goals. These questions address the samples to be measured along with the goals and purpose of the measurement process. After considering sample and feature size, measuring challenges, contact, application standards, output, and overall throughput, you can help narrow down the instruments that best match your application. Then you can compare the features and benefits of your top-choice instruments, reviewing what they deliver against your budget to come up with the best instrument for your needs.

Question 1: Size

How large are my samples? How large are the features I’m measuring?

This may sound obvious, but you will need an entirely different scale of instrument for measuring the curvature of an entire airplane wing, for example, than for measuring the coating on a pill. For the largest samples, consider contact measuring arms, laser trackers, photogrammetry, or laser radar. For smaller ones, a microscope or other equipment that involves placing the sample on an instrument for measurement may be best.

The size of the features you are measuring is also important. Even large-scale products sometimes require microscopic evaluation. For instance, huge solar panels can require micron-level examination of their surface layer to ensure they can absorb maximum sunlight. If these features cannot be examined apart from the large-scale finished product, your instrument choices may be somewhat limited.

Question 2: Challenges and constraints

What features am I trying to measure, what measurements must be taken, and what obstacles or special issues exist regarding fixturing or measurement?

Every measuring application has its challenges and constraints. Some features are remotely located or hard to reach. Many samples are difficult to fixture. Still others are smooth, monochromatic, or clear, making them hard to visualize. Some have steep angles and deep trenches, making accuracy difficult. Some parts must be measured under water; others require complex mathematical algorithms for accurate quantification. Still others must be imaged using particular wavelengths of light. The instrument you select must be able to overcome the limitations and meet the challenges of the specific application.

Question 3: Contact

Can my measuring instrument come in contact with my sample?

The answer to this will tell you whether you can use any kind of measuring instrument with a probe or touch mechanism. If so, contact measuring arms or similar measurement equipment can work on a larger scale, and scanning probe microscopy or atomic force microscopy are options for microscope measurement. Often, however, such as in many medical device and semiconductor manufacturing applications, it is vital that the entire measurement process happen optically, with no physical contact.

Question 4: Standards

How accurate and repeatable must my measurements be? Must the measurement be traceable?

One of the most important aspects of selecting an instrument is making certain that the instrument can provide accurate and reliable data. There is no single standard for this. Before buying a system, make sure it can meet your accuracy and repeatability standards, based on the specifications of your own application. The traditional Rule of Ten indicates that the measuring ability of an instrument should be 10 times better than the part specification indicates. In addition to accuracy and repeatability, there are other factors to consider here as well, such as linearity (accuracy over its range), resolution (the closest that two elements can be to one another and still be distinguishable as separate elements), and stability (the ability of an instrument to maintain its calibration over time without drift).

For example, a microscope application might require the use of a compound microscope, which can provide much higher magnification and resolution (down to ~0.35 microns) than stereomicroscopes. Compound microscopes also provide a single optical path that is orthogonal to the part. Using a compound scope provides higher certainty with regard to the edge determination of the measured part and eliminates parallax issues. Additionally, compound microscopes provide fixed magnifications, each of which can be certified to provide a superior level of measurement certainty.

More advanced integrated digital solutions employ high dynamic range (HDR) imaging to optimize contrast. They acquire a series of successive images and then select the one that displays the best contrast. Such imaging algorithms provide images of higher quality, contrast, and homogeneity, streamlining measurement and enhancing ease of use. In addition, they offer a higher level of accuracy because the measured surface can be seen more clearly.

More advanced than typical compound and digital microscopes are toolmakers' microscopes, the most advanced and most traceable measuring microscopes available. They combine compound microscopy with precision stands and measuring stages, and are manufactured to provide the utmost in rigidity and temperature tolerance. Further, the microscope frames are manufactured to high tolerances for orthogonality to the stage. The Z-axis of the toolmakers' microscope is often equipped with linear encoders matched to the stage system so that equal measuring capability is obtained in all three axes. The measuring microscope stage provides the highest degree of flatness throughout its travel.

Of course, not every microscope measuring application requires this level of performance. By understanding your exact needs, you can select an instrument that meets the standards without having to pay for output that is far beyond your requirements.

Traceability is key for many quality control procedures. Having equipment traceable to Japan Quality Assurance (JQA) or the National Institute of Standards and Technology (NIST) can provide a level of confidence in the measurements. Even the most basic measuring systems can be purchased with traceable standards.

Question 5: Output

What happens after the measurements are made? What kind of output do I need to create?

All measurements are made with the idea of affecting some future outcome. Whether in engineering or production, the results of the measurement are vital for directing the course of further development, research, or determining if manufactured products are viable and if manufacturing processes can continue. The format in which this information is communicated is vitally important (Figure 1). Some applications require that raw data are shared immediately with the operator. Other applications require extensive image analysis or other intermediate steps before output can be reported. Still other applications might demand an automated feedback loop affecting go/no go decisions in real time during the manufacturing process.

Figure 1: Sample output from the Olympus Stream system illustrates one way that software systems can offer live and archived images and reliable statistical data. Such systems streamline the process—from initial image capture, analysis, and measurement through reporting, archiving, and database management.

Reporting may either be automated or handled manually, and reports can be output in hard copy or electronic format, depending on the application. Some output must be shared with users who are remote from the instrument, perhaps in another room or even on another continent. In that case communication between the instrument capturing measurements and remote instruments via network interface or the Internet may be a vital part of the measurement and decision-making process. Some output may include only data, while other output includes images captured during the measurement process.

Question 6: Throughput

How many samples do I need to review and in what time frame? Is this a low-, medium- or high-throughput situation?

Consider whether you are reviewing your samples one by one, reviewing results before moving on, or in high volumes. Higher throughput requirements may require more automation of the sample handling or measurement processes, and also may create different analysis and reporting requirements for the software.

Following are three brief scenarios to illustrate decision making for three different measurement challenges:
• A company manufactures carbon nanotubes for applications as varied as electronic components, optics, and even body armor. The specifications require extraordinary depth (Z-axis) resolution, a wide field of view and the ability to image a variety of feature shapes and sizes. XY measurement also is required, but at less exacting resolutions. This application requires moderately high levels of throughput. In this scenario, an interferometer may provide the best choice. The strength of many interferometers is in superior Z-axis measurement. Though their XY dimensional measurement sometimes is not as strong, interferometers offer a very wide field of view, output similar to a microscope, and a reasonable throughput rate.
• A company makes soft polymer films for both the medical device and photovoltaic fields. The company requires analysis of the films' surfaces to help understand adherence, performance and other issues. Measuring some surfaces can be done with a touch stylus, but in this case, since the surface is soft, a stylus could cause damage. In this case, a noncontact measuring instrument, such as an optical profilometer or laser scanning confocal microscope, might be selected, depending on the surface structure and the measurement and analysis requirements.
• A third example might be in residue analysis. Examples include a supplier in the petrochemical field that must ensure oils and fuels do not cause subsequent contamination in an automobile, or a pharmaceutical manufacturer that must characterize every particle captured in a filter during the manufacturing process. For such applications, which usually require extensive image analysis, a microscope with inspection software is often the best option.

Conclusion

Selecting the right instrument for your application involves considering the sample size, feature size, any special measuring challenges; whether your process can involve actual contact with the sample; what precision and accuracy specifications your application requires; what output you must produce; and how high your level of throughput will be. Once you have these answers, consider the variety of measuring instruments, from gauges to microscopes to automated measuring systems, with your budget and requirements in mind. Measurement and inspection requirements are as varied as the applications they are designed for, and technology is advancing rapidly. Take the time to do a thorough analysis to determine the best system for your needs.

About The Author

David Rideout