Up to this point, we have defined the term “accuracy” in how it relates to laser tracker measurement systems.
We reviewed angular versus distance accuracy, as well as volumetric accuracy.
In this final section, we will continue our discussion on volumetric accuracy, as well as cover some consequences of sub-optimal angular accuracy.
In order to illustrate how much angular errors dominate for a measured object, consider Figures 1 and 2.
Figure 1 – Points on measured object in line with the laser tracker
Figure 2 – Line between two points on the object perpendicular to the laser tracker
If the laser tracker in Figures 1 and 2 is deemed to be 2 meters from the measured object, the following typical MPE performance is attained from a measurement of the 2.3 meter length.
Assume use of IFM and a Typical MPE
Figure 1 = 3 micrometers
Figure 2 = 33 micrometers
Note the relatively large difference - this is because in Figure 2 the laser tracker’s relative position to the points which require to be measured dictate that the angular or transverse measurement system errors dominate.
Objects with features which require measurement are rarely offered up as depicted in Figure 1. Objects tend to be more irregular in shape and size, and very often not all points can be viewed from an in-line position. Typical examples might be large assembly tools for the aerospace industry.
Figure 2 orientation is more common with the scenario mentioned previously (in Part 2 of our discussion) coming into play if not all points can be seen from one position. The moment that multiple laser tracker positions come into play coupled with objects positioned to the laser tracker as depicted in Figure 2, means that the angular measurement system errors dominate.
It can therefore be concluded that the specifications and actual performance of the angular or transverse measurement systems onboard the typical laser tracker play a very important role in its day-to-day performance for the average user. It’s very easy to forget this fact when confronted with the specifications and by the outstanding performance of modern IFM and ADM distance measurement systems.
Selected Consequences of Sub-Optimal Angular Accuracy
Examples of sub-optimal angular accuracy are apparent for large assembly tooling within the aerospace industry. If angular accuracy is sub-optimal, the following could ensue:
• Poor initial reference system leading to a lack of accuracy and repeatability when setting and certifying the tool
• Poorly fitting parts leading to cost issues downstream of the assembly process
• Costly rework based on a poor signal from the measurement system (costs include labor and time tying up a tool which is on critical path)
If the angular accuracy is optimized, it is more likely that the tool will not have to be reworked during its first recertification due to measurement variation at least. Reworking tools such as these described can cost several thousand dollars.
Conclusion
Understanding accuracy terms is an important aspect of selecting the best instrument for a particular application. In the case of laser trackers, distance accuracy specifications are often not achievable in the end user’s application due to the limited measurement conditions that would be required. The majority of the time the laser tracker user is most interested in measuring points and dimensions that require movement of the encoders and place the application into the volumetric accuracy case. Thus it is the volumetric performance of the instrument that is most critical when considering a laser tracker for the majority of applications.
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With lean manufacturing being such a focus in today’s world economy, it wasn’t surprising to learn that American automakers are leading the way in redesigning assembly processes. One particular company decided to implement virtual manufacturing many years ago to streamline product development. Ford Motors adopted the philosophy of simultaneous engineering. This term refers to using 3D scanned data to conduct simulations of production processes. In order to accomplish this task, the FARO Laser Scanner Photon was brought in to provide point cloud data of the production floor. The FARO Photon has a color capability that makes point measurement easier to the human eye. After several scans, the 3D meshes were then mixed with CAD models to create simulations that allowed Ford Motors to review for production enhancements.
Click Here for the complete story in laser scanning production floors.
Distance Measurement (Ranging) Accuracy
As mentioned previously, the distance measurement systems typically found in a laser tracker are an IFM and ADM or even ADM only. Independent upon whether IFM or ADM systems are being used, the ability of these systems to detect and measure displacement is well known and documented.
Interferometers were typically used to measure the displacement between two points and therefore the product would be a distance between the two points. It follows that in order for a laser tracker instrument to measure distance optimally, it needs to be positioned in line with the points to be measured. In this case there is no influence from the angle measurement system.
Angular (Transverse) Measurement Accuracy
The angular accuracy of a laser tracker describes how well the instrument discerns angle measurements from its angular measurement encoders prior to processing them together with the distance or ranging element in the form of a coordinate.
Angular Measurement Accuracy versus Distance
With some laser trackers specified out at ranges of 50 meters or more, the pure ability for the unit to accurately measure angles is very important for respectable performance in the field. This becomes more or less important depending upon the volume of the object and where it is practical and economical to position the instrument in order to measure the points of interest.
Volumetric Accuracy
Volumetric accuracy is often used as a term to describe how accurate the instrument is for a particular measured volume. Where they exist, illustrated concepts of volumetric accuracy from manufacturers have to be aligned with the ability of the user to practically position the instrument in the real-life situation for their particular measured object.
Where measurement volumes are large it is sometimes more efficient for the user to reposition the instrument in strategic and practical positions to enable the viewing of all the required points of interest. The use of more instrument stations will have the effect of reducing the distance from the instrument to the points of interest which will also tend to weight the contribution of the angle measurement. This is especially true if there are practical limitations for the positioning of the instrument, placing more emphasis on the angular measurement capability in the quest to achieve an accurate set of coordinates.
In this scenario the following is relevant:
• The laser trackers have limited room to maneuver in the Z direction
• All points of interest cannot be seen from a single position
• Multiple laser tracker positions are required to achieve the required accuracy
• The highest possible accuracy is required
• The measurement volume dictates that the laser tracker angle measurement capability is exercised, especially in the Y (vertical) direction
• Measurement distances have been cut, but laser tracker angles are exercised more severely
To be continued…
Metrology technology has constantly evolved as the needs of manufacturing have become more stringent. Recent developments have allowed portable CMMs to become more prominent in the marketplace since they can be integrated into the manufacturing process. This has turned what has traditionally been an inspection device into a value added option for cutting-edge manufacturers.
There are several different tools available for the measurement and inspection of parts and products. The specific application often determines the best choice as each tool has its own benefits and drawbacks. Over the years, these tools have become more advanced in order to keep up with improved quality standards.Today’s manufacturing demands often require that processes adhere to the best possible practices to maximize value. One clear way to do this is to improve production times and to minimize waste. Implementing portable CMM technology at every aspect of the manufacturing cycle can achieve this goal – improving both time and cost savings.
There are two main types of portable CMMs: articulated arms and laser trackers. Articulated arms determine and record the location of a probe in 3D space and report the results through software. To calculate this location, the rotational angle of each joint and the length of each segment in the arm must be know. The rotational angle is determined using optical rotary encoders that count rotations incrementally, and software is used to convert those counts into angle changes.
Laser trackers operate by measuring two angles and a distance. The tracker sends a laser beam to a retroreflective target held against the object being measured. As light is reflected off the target, it bounced back and re-enters the tracker at the same position it left and is measured by a distance meter, measuring the distance between the tracker and target.Portable CMM technology can be used for many different applications and continue to grow in popularity. Companies implementing this technology are getting the accuracy results they need in addition to flexibility to use the unit wherever and whenever it's most convenient.Watch a webinar on portable CMM technology.Download a white paper on portable CMMs.
As we celebrate Veterans Day, it’s also important to remember that there are many companies that provide high quality material for the military that helps protect our soldiers. One such company is Ultra Machine & Fabrication in Shelby, North Carolina. With over 20 years of fabrication and machining experience, Ultra Machine has earned a reputation for quality and expertise. They are proud to provide precision armor parts and weldments for military vehicles such as the Mine Resistant Ambush Protected (MRAP) vehicle.
In fact, most of the parts Ultra checks are machine and fabricated parts for the military. Older tools like gauges and calipers lacked several functional capabilities – GD&T, angles, parallel, and flatness, for example. Ultra was often left second guessing themselves on these 3D measurements and they also lacked a needed reverse engineering ability.
To solve these issues, Ultra turned to the FaroArm® – a precision instrument capable of providing accurate measurements of 3D features that quickly confirms parts are meeting or exceeding expectations. The deciding factor in their decision though was the FaroArm’s accuracy of up to .001” on an 8-foot Platinum.
Ultra now has several FaroArms that enable them to measure and document parts. They can view highly accurate, 3D product files in real time and provide statistical data to their suppliers and customers – an added benefit that helps Ultra remain a world-class provided of fabricated armor parts.
By using their FaroArms, Ultra has reduced scrap by tens of thousands of dollars monthly. “Just one mistake can cost us $10,000 fast,” said Quality Director James Shelf. “With FARO, we can prevent that mistake before it happens.”
Join FARO in honoring all of our military veterans – not only today, but everyday. And we’d also like to say thank you to people like those at Ultra Machine that help keep our veterans safe.
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FARO’s Laser Scanner Photon has proven to be a critical technological solution to the world of accident investigations. It is an industry that depends on the ability to make present observations from events that happened in the past. So much time and money is being tied up in court cases, therefore, forensic engineers look to laser scanning for dependable data that can provide them with tangible evidence.
Recently, Spar Point Research LLC posted an article which discusses how three companies use FARO Laser Scanners in their daily forensic operation. Each company describes how 3D scanning has revolutionized their surveying and applied measurement techniques. Packer Engineering, Arnold & O’Sheridan, and Gilbert Engineering are all experts in the accident reconstruction industry. They all have a unique story in how laser scanning has provided speed, safety, accuracy and portability.
For the full story in laser scanning for accident investigations CLICK HERE.
Measurement instruments and systems evolve constantly, enabling the user to take advantage of a whole host of features. The technical specifications of most measurement systems may include reference to such entities as resolution and repeatability but will always include reference to the term accuracy.
Let’s explore the meaning of accuracy in the context of metrology in general and in particular for the use of a laser tracker instrument.
We’ll also explain the term and distinguish between in-line (or radial accuracy) and angular accuracy. Also described is the tendency for the angular errors to dominate during the measurement of a typical object and examples are offered to illustrate this.
Finally, we’ll illustrate the consequences of poor angular accuracy in particular.
Accuracy
Accuracy, or bias as it is sometimes called, is a term which can be described as the closeness of the agreement between the result of a measurement and a true value of the particular quantity being measured. In reality, we will never know the true value of the measured quantity which is why in cases where more certainty of the measured quantity is important, there tends to be an emphasis on ensuring that at least the accuracy of the measurement instrument is optimized.
Laser Tracking Instruments and Accuracy
Laser tracking measurement instruments are very versatile by nature, although their use is frequently for applications where there is a demand for the highest possible performance from the measurement system to characterize the object being measured.
One of the reasons why laser trackers have evolved into this high accuracy sector is because of their outstanding ability to accurately measure distances. This was initially due to the use of laser interferometers (IFM) followed later by Absolute Distance Meters (ADM).
Distance measurement capability on its own is not enough if a set of coordinates is required. In order to achieve this, the laser tracker instrument in its basic form is equipped with an angular measurement system to enable the distance measurement to be processed alongside two measured angles to arrive at the required coordinates.
It is clear then that in order for the ever-increasing customer specifications to be met, there has to be an emphasis upon the accuracy of both the distance and angular measurements of the instrument.
Specifications for Distance and Angular Measurement Accuracy
Dependent upon the manufacturer of the laser tracker, specifications over the last few years have varied in the sense of their presentation, usefulness and practicality to the user.
The introduction of the ASME B89.4.19-2006 standard has helped manufacturers to standardize the approach toward specifying, although there is some way to go in order that complete agreement is reached across continents.
The ASME standard offers the concept of Maximum Permissible Error (MPE) to the manufacturer and subsequently the user. MPE is useful in the sense that it encompasses the extreme values of error that are permitted by a specification.
The following is a typical distance measurement specification offered by a manufacturer. Please note that actual numbers are for reference only:
For the IFM system:
4 micrometers + 0.8 micrometers/meter
For the ADM system:
20 micrometers + 0.8 micrometers/meter
For the Angular or more often called the Transverse system
36 micrometers + 0.6 micrometers/meter
Note that in all cases there are two terms - the first term is the offset, while the second term is the slope or scale factor. The slope or scale factor is predominantly a function of distance; therefore it should encapsulate the range of specified environmental conditions together with the specified range of the instrument.
The MPE specifications can sometimes be offered as “Typical”, in which case they constitute a halving of the full MPE value for the purposes of portraying a value which will be typically achieved the majority of the time.
As for the relevance of the MPE specs to the user, it’s clear that the declared MPE values can be compared to assess the relevance of the purchase across manufacturers. If Point-to-Point accuracy is of interest to the user, some manufacturers publish the formulae used to calculate the MPE, making it possible for the user to calculate his or her own situation.
Some manufacturers also offer to certify the instrument with respect to the B89.4.19-2006 standard. If this is available, together with processes to protect or guard-band the MPE specifications, it can only be of benefit to the user.
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