Why Choose 3D Laser Scanning over Touch Probes in Manufacturing Quality Control
Ian Wright posted on June 03, 2016 | 16344 views

The mass metrology migration is upon us!

Whether you’re manufacturing your six-millionth crankshaft or a first next-generation fuel nozzle, your QA/QC system is only as good as its inspectors and analysts—and those professionals are only as good as their tools.

Laser scanners are used for both reverse engineering and inspection applications. This article concentrates on the latter. (Image courtesy of Creaform.)
Laser scanners are used for both reverse engineering and inspection applications. This article concentrates on the latter. (Image courtesy of Creaform.)

Laser scanners have come a long way in the past 20 years. Now, they can form a vital part of the quality professional’s toolkit for first article inspection, part-to-CAD and particularly in the growing area of in-process measurement. This is true for both high-accuracy applications like aerospace engineering and in the high-volume world of automotive manufacturing.

 

The Most Important Laser Scanner Features

A poll conducted on The Laser Scanning Forum asked members to identify the three most important features to consider when choosing a laser scanner. Here are the results:

Responses ranked as a percentage of total votes. n = 220. Source: Laser Scanning Forum.
Responses ranked as a percentage of total votes. n = 220. Source: Laser Scanning Forum.

Not surprisingly, accuracy, speed and portability/size ranked highest. Although the forum’s members are made up of more than just manufacturing professionals—including surveyors, archaeologists and general metrology enthusiasts—these three features are worthy of consideration by any metrology professional.

It’s also worth noting that cost ranks lower than color capture/high-definition resolution as well as the top three features, suggesting that a laser scanner should not be purchased on price alone. That might seem like an obvious piece of advice, but when it comes to selecting a tool that can make the difference between shipping out-of-spec parts and not, the extra up-front cost could be well worth it.

 

Categories and Types of Laser Scanners

3D laser scanners can be divided into two broad categories based on their method of capturing data:

  • White-light and structured-light scanners take single snapshots or scans of objects
  • Scan arms and portable handheld scanners capture multiple images continuously

These two categories can be broken down further based on their method of positioning. 

The MetraSCAN 3D. (Image courtesy of Creaform.)
The MetraSCAN 3D. (Image courtesy of Creaform.)

Within white-light and structured light scanners, tracked 3D scanners such as Creaform’s Metrascan 3D and Hexagon’s Leica Absolute Scanner are positioned via an external optical tracking device using markers such as passive or active targets. These targets bind to the scanner, enabling it to position itself within a coordinate system.

The Leica Absolute Scanner LAS-20-8. (Image courtesy of Hexagon.)
The Leica Absolute Scanner LAS-20-8. (Image courtesy of Hexagon.)

These scanners offer good accuracy and excellent precision as well as freedom of movement, but they also require the tracker to have a clear and direct line of sight to the scanner. They are also generally more expensive than portable 3D scanners.

FARO Cobalt 3D. (Image courtesy of FARO.)
FARO Cobalt 3D. (Image courtesy of FARO.)

In contrast, structured-light 3D scanners, such as FARO’s Cobalt 3D, use one or more cameras to position themselves via offline targeting or part geometry. In this case, a single camera requires the position of the projector relative to the camera to be determined in advance, while in a dual-camera setup the stereoscopic pair must be calibrated together.

Structured-light 3D scanners have the advantage of offering excellent resolution, but they also typically need to perform multiple scans to cover all the angles on complex parts. This can be time-consuming.

The ROMER Absolute Arm with external scanner. (Image courtesy of Hexagon.)
The ROMER Absolute Arm with external scanner. (Image courtesy of Hexagon.)

Falling into the second broad category, scan arms, such as Hexagon’s ROMER Absolute Arm, are positioned via mechanical encoders within the arm.

The primary advantage of using a scan arm is that it can accommodate both scanners and touch probes, enabling a variety of measurements to be performed with a single instrument. On the other hand, because the arm needs to be bolted to a stable surface, scan arms tend to be susceptible to vibrations and other environmental factors.

The mechanical connection between the positioning system and the scanner also makes them impractical for scanning hard-to-reach areas.

The HandySCAN 700. (Image courtesy of Creaform.)
The HandySCAN 700. (Image courtesy of Creaform.)

Finally, portable 3D scanners, such as Creaform’s HandySCAN 3D, are positioned using real-time self-positioning via targets, object geometry or both. This combination is known as hybrid positioning. Two cameras in the scanner create stereoscopic vision and enable the device to determine the scanner position in relation to specific points on the object.

The main advantage of portable scanners is their ease of transport and use, with some high-end models acquiring more than 500,000 points per second. Some can even rebuild the triangle mesh during the scanning process.

Unfortunately, portable 3D scanners are limited in scanning volume, with errors increasing as the volume grows. Users can compensate for this issue using photogrammetry and positioning targets, but these increase setup time and limit scanning volume.

For easy reference, the above information on categories and types of 3D scanners is compiled in the following table:

 

Laser Scanners: Does Color Matter?

It might be tempting to classify laser scanners in terms of the color laser they employ—typically red, blue or green, but Simon Côté, product manager at Creaform, suggested that this would be a mistake. “To be honest, all of that is more marketing than anything,” Côté said. “It all depends on the image processing and filter that you use. You can achieve good results with any color.”

A moment’s reflection explains why this is true.

The Electromagnetic Spectrum.
The Electromagnetic Spectrum.

On the electromagnetic spectrum, visible light occupies the 400- to 700-nm range. Hence, the maximum difference between two different laser colors (blue and red) is approximately 300 nm. That’s two orders of magnitude smaller than the tightest conceivable tolerance in the manufacturing world today, outside of nanotech applications.

The upshot is that when it comes to color, you can treat laser scanners like lightsabers: just choose your favorite color.

 

5 Advantages of Laser Scanning

There are several advantages to laser scanners in comparison with both more sophisticated metrology tools like Coordinate Measuring Machines (CMMs) and more basic tools like calipers and gauges. More specifically, compared to these other tools, laser scanners have advantages in terms of the following attributes:

  • Measurement Coverage
  • Measuring Speed
  • Automation
  • Required Training
  • Cost

1. Measurement Coverage

Although touch probes are highly accurate, they are limited in the number of points they can generate from a given object. Each data point generated by a touch probe requires a corresponding touch. In contrast, the width of a laser scanning stripe can range from 50 mm to 200 mm (1.97” to 7.87”), enabling the scanner to capture thousands of data points in a single pass.

One of the main advantages of laser scanning over touch probing is its ability to capture the entire surface of the part.

Touch probe measurements of highly curved or free-flowing areas yield relatively few useful data points. What’s more, laser scanners can pass over homogenous surfaces where tolerances are higher much more quickly than touch probes.

 

2. Laser Scanner Speed

“In the last ten years, because of faster computing processes, we can scan something and reverse-engineer it and then adjust the computer program in a matter of minutes, whereas in the past it would take weeks to do something like that,” said Rina Molari-Korgel, chairman of the executive committee of the Coordinate Metrology Society.

Even the fastest touch probes are unlikely to exceed a few thousand points per second. Although that’s still a significant gain in speed compared to even a decade ago, it doesn’t come close to the hundreds of thousands if not millions of points per second that can be captured by a laser scanner.

In terms of the sheer number of data points collected, nothing beats a laser scanner. However, from an inspection standpoint, speed isn’t measured simply in terms of the number of data points collected. Ultimately, it’s a question of the total time needed to assess the quality of the part—and that depends on your particular inspection application.

You can get a sense of the speed of 3D scanning in this video featuring Creaform’s HandySCAN:



“If you have just a few features to measure, a CMM will be faster, since you can build the program and repeat it for the same part,” said Côté. “If you have to inspect multiple features, like multiple holes and surface profiles, most of the time a scanner will be much faster.”

In addition to the number of features, surface reflectance is also a significant factor. “Typically, if you have prismatic features then probing is faster and easier and if you have organic shapes then scanning is faster,” said Steve Young, owner of Exact Metrology, Inc.


3. Automated Laser Scanning

While it’s certainly possible to integrate an automated coordinate measuring machine into an assembly line, such setups typically require large bridge- or gantry-style CMMs that have been specifically designed to withstand the shop floor environment. Even horizontal arm CMMs are sensitive and cumbersome compared to industrial robots.

An array of Cobalt 3D imagers mounted on a robot. (Image courtesy of FARO.)
An array of Cobalt 3D imagers mounted on a robot. (Image courtesy of FARO.)

Mounting a laser scanner onto such a robot is often the more efficient option, especially given the robot’s ability to reach features and surfaces that would be inaccessible to a CMM.

In addition, integrated software packages can be used to handle measurements, data gathering, comparing point clouds to CAD models and generating visual representations and tabular data in an automated process.

 

4. Laser Scanner Training

“Scanning is the absolute easiest thing you can do,” said Doug Sutphin, east coast regional sales manager for laser trackers at Hexagon Manufacturing Intelligence. “It’s basically like spray painting, so if you can teach someone to spray a can of paint, you can teach them to laser scan. All you have to do is give them a little bit of guidance on aiming the laser beam and how fast to scan. It’s very simple.”

Côté agrees: “3D scanners are generally pretty easy to use. Typically, it takes two days to be trained to use a scanner. For scanning an object, I can just show you the scanner and after ten minutes, you’ll be able to scan almost anything.”

“Learning to operate the scanner is very easy and almost anyone can be trained to do it. The harder part,” added Young, “is setting up the software to manipulate the point cloud and present the results that you want. Scanning is the easy part.”

Côté put it this way: “You need more experience with GD&T (geometric dimensioning and tolerancing) to know how to extract the right features. The learning curve for the software—extracting measurements from the scan—is much steeper than the scanning.”

For this reason, larger companies often divide the laser scanning work between technicians, who perform the actual scanning, and metrologists, who have experience with inspection software. In smaller companies, these two jobs tend to be done by the same person. However, if you’re looking to free up your metrologist’s valuable time, passing their scanning duties onto properly trained technicians may be a good option.

 

5. Laser Scanner Cost

In terms of cost, laser scanners occupy a sort of middle-ground between CMMs and hand measuring tools. Of course, that only narrows the price of laser scanners down to somewhere between a few hundred dollars and upwards of seventy thousand.

“In terms of cost, it depends on the manufacturer and the product line you’re talking about,” said Sutphin. “Laser scanners for arms may be less expensive than laser scanners for a tracker. There aren’t too many laser scanners that work for a tracker or laser trackers that can work with laser scanner technology. Leica is one of those.”

One of the primary factors in the cost of laser scanners is the novelty of their technology. “Usually the cost is higher because it is relatively new technology and there is a lot of R&D going into hardware and software,” said Young.

Even so, compared to a CMM, 3D scanners are practically a bargain. Despite being less accurate than CMMs, for manufacturers who don’t need CMM-levels of accuracy, 3D scanners are an attractive proposition. “That’s why 3D scanners have been getting more popular in the past five or ten years,” Côté observed.

 

Laser Scanning in Automotive Applications

One particularly impressive example of 3D laser scanning in the automotive industry comes from the Laser Scanning Forum, where user Andy Goldstraw showed off some pictures from a project he carried out a few years ago.

“The task was to scan the entire underside of a car, the engine bay with no engine, the [hood] in the closed position and then to scan the engine as a separate unit,” Goldstraw wrote. “Align all of the data together and then cut sections through the data for clearance and fit between objects. Then separate parts of the suspension and engine into smaller files.”

You can see the impressive results of Goldstraw’s project below:

The car scan looking from above. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)
The car scan looking from above. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)
The car scan looking from underneath with the engine removed. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)
The car scan looking from underneath with the engine removed. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)
The engine scan. Note the parts numbers on the cast components. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)
The engine scan. Note the parts numbers on the cast components. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)
Another angle on the engine. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)
Another angle on the engine. (Image courtesy of Andy Goldstraw/The Laser Scanning Forum.)

According to Goldstraw, the data for these scans was captured in a single day and then processed off-site. “Car was held on a two-post ramp and lifted up in the air via chassis mounting points ensuring the suspension etc. is allowed to rest,” Goldstraw wrote. “This allowed me to move under the car with a 3.6m Arm on a tripod. I also did a scan set while the car was on the ground to get the [hood] and engine position to help with the alignment of the two.”


Laser Scanning in Aerospace Applications

The aerospace sector has some of the tightest tolerances in manufacturing, so you might wonder whether laser scanners can offer the necessary accuracy for quality assurance. Are CMMs always the better solution?

If you’re after high accuracy for a single point, then CMMs are obviously the way to go. 

Using a Leica T-Scan to inspect an aerospace component. (Image courtesy of Hexagon.)
Using a Leica T-Scan to inspect an aerospace component. (Image courtesy of Hexagon.)

That being said, laser scanners can work well for aircraft interiors, where tolerances are not so tight. However, when it comes to the external envelope and anything relating to the aircraft’s engine (turbine blades, fuel injectors, etc.), tolerances are typically too tight for laser scanners to be practical.

Of course, laser scanners and CMMs need not be seen as competing technologies. On that topic, Côté offered the following anecdote:

“We recently sold multiple scanners to an aerospace company for 3D inspection and they’re using them to offload their CMM. It’s in the casting industry and they use laser scanners to inspect the rough casting before machining it and then do a final inspection on the CMM, since it needs an accuracy of one or two microns.”

“But they use the laser scanner to make sure the part has enough material to be CNC-machined, because if the part needs eight hours on the CNC and after that you realize ‘Oh, we’re missing material somewhere,’ you just lost a lot of hours and a lot of money. So they’re using the scanner for pre-qualification. That way, they’re sure they have enough material when they machine the part.”

 

Laser Scanners: Accurate, Portable and Fast

Accuracy, portability and speed are the defining attributes and the primary advantages of 3D scanners. These key tools in the quality professional’s toolkit can find a role in both high-accuracy applications like aerospace engineering and in the high-volume world of automotive manufacturing.

If you need more accuracy than 3D scanners can offer, check out our 3 Tips for Choosing the Best Coordinate Measuring Machine for Your Quality Process. If you’re looking for a less expensive metrology tool, keep an eye out for our next quality feature on hand tools.

For more information on 3D scanners, visit the websites for Creaform, Exact Metrology, FARO and Hexagon.


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