Measuring arms
OnyxAceMeasuring arms
with 3D scanners
Onyx SkylineAce Skylinewith 3D scanners
In metrology, achieving precise and accurate 3D measurements is crucial for maintaining high quality in engineering and manufacturing. However, uncertainty is inherent in any measurement process, even with advanced systems. Proper uncertainty management is essential to ensure measurement results are reliable and accurate.
Measurement uncertainty in 3D metrology represents the deviation of the nominal measurement when compared to the measured object. It arises from imperfections related to the measurement process, including equipment limitations, operator experience and environmental factors.
In other words, it is the repeatability error, which is the deviation observed during successive measurements of the same object under identical conditions. Uncertainty affects measurement accuracy across domains such as engineering design, product development and scientific research.
Evaluating and quantifying these uncertainties is vital. Mastering and understanding measurement uncertainties is essential for quality control, ensuring compliance with high industrial standards such as those in the automotive and aerospace industries.
For instance, imagine you’re measuring the diameter of a small metal rod using a calliper. The true diameter of the rod is 5.00 mm. However, due to the limitations in the caliper's precision and how tightly you’re holding the rod, your measurement could be slightly off, resulting in a reading of 4.92 mm or 5.08 mm instead of the exact 5.00 mm.
In this case, the measurement uncertainty comes from factors like the precision of the calliper and the consistency of your grip. So, rather than saying the diameter is exactly 5.00 mm, you would report it as (5.00 ± 0.08) mm, which means the true value of the diameter is likely within the range of 4.92 mm to 5.08 mm.
Therefore, this measuring device cannot control a diameter whose acceptable manufacturing tolerance is equal to or less than a measurement uncertainty of 5 mm ± 0.08.
The standard indicates that the measurement process must have an uncertainty of at least 4 times lower than the tolerance interval, and even some metrology applications have it closer to the 10th factor. For example, to measure a diameter of 51mm ± 0.4 mm, whose tolerance interval is 0.8 mm in total, a measurement process with a measurement uncertainty of less than 0.8/4 = 0.2 mm will be required.
In metrology, errors and uncertainty are often misunderstood as the same concept, but they represent different aspects of measurement quality. Therefore, it is crucial to understand the difference between errors and uncertainty.
Errors in the measurements denote how much the measured value deviates from an object's true value. It is a known and correctable difference, meaning that once identified,steps can be taken to reduce or eliminate it.
In contrast, uncertainty expresses the degree of doubt or confidence associated with measurement results. It is unavoidable and accounts for all possible unknowns affecting the results. Instead of being corrected, uncertainty is quantified and reported to understand how reliable the measurement is.
Engineers strive to minimise errors through precise tools and techniques, while metrological experts recommend using professional equipment and software to enhance measurement accuracy.
Deviations in equipment geometry and calibration errors, such as probe length errors, laser scanner alignment issues and sensor noise, can affect measurement reliability. Equipment that hasn't undergone rigorous testing and thorough qualification according to industry standards may perform poorly or function inadequately, regardless of the user's expertise.
Conditions such as temperature, humidity, and vibrations can impact measurement performance. For instance, quality control of large parts often takes place directly in the production environment rather than in a controlled metrology lab. In such settings, vibrations, temperature and fluctuations can destabilise validation jigs and interfere with measurements. Temperature fluctuations may cause materials to expand or contract, introducing measurement errors.
Human errors remain a significant cause of mistakes in measurement results, including inconsistent handling, improper setup, or incorrect data analysis, despite technological advancements. This misuse is often due to insufficient guidance or incorrect use of equipment, resulting in incomplete and skewed data quality. Inexperienced operators may overlook the importance of preparing measurement arrangements in advance, which can result in lost time,part scrappage or wasted resources.
Measurement uncertainty can also arise from the algorithm used in metrology software. Different software solutions may process the same input data differently, leading to variations in results. Factors such as data filtering, point cloud processing, and alignment algorithms can introduce inconsistencies.
To minimise uncertainty, it is essential to use reliable software, especially for highly important metrological applications in industries like automobiles and aerospace.
Regular calibration of measurement instruments is essential to minimise uncertainty. Calibration should be performed using traceable standards to ensure that the equipment settings are well-calibrated with its reference measuring point and that the device complies with its stated specifications. For the scanner, the right calibration process involves setting parameters to accurately locate the laser line. For the arm, it allows you to define the arm parameters to locate the end of the arm. A well-calibrated device enhances measurement precision, which is vital for controlling engineering design specifications, leading to better decision-making and higher overall product quality.
Maintaining stable temperature, humidity and vibration levels can minimise external influences on measurements. Using climate-controlled rooms and vibration-dampening setups helps create a stable environment, improving measurement reliability.
Investing in comprehensive training for operators ensures they understand the intricacies of the measurement process and are proficient in using the equipment. Skilled operators are less likely to introduce errors during critical quality control tasks. An experienced technician can also prepare the 3D measuring program beforehand to streamline the complete inspection process by automating measurement activities and guiding the operator.
Advanced metrology software can correct known sources of error, such as thermal expansion or instrument drift. These tools help align measurements with CAD models, organise report data and provide real-time feedback.
Relying on international standards like ISO is more trustworthy than the accuracy stated on a manufacturer’s specification sheet. These standards have an impeccable reputation in the industry and a proven track record in quality control processes, making them the best way to achieve confidence in your measurement results.
ISO 10360-08 certified 3D laser scanners, and measuring arm validated by the ISO 10360-12, deliver the expected specifications for rigorous measurement results.
Documenting all aspects of the measurement process, including conditions, settings, and operator actions, helps identify and address sources of uncertainty. Documenting all the steps involved in creating a product, from design to manufacturing, can help track its evolution, and the history can contribute to some critical conclusions during the production processes.
Routine maintenance of measurement devices prevents performance degradation over time. Moreover, conducting peer reviews or audits of the measurement process can provide an additional layer of scrutiny, helping to identify overlooked sources of uncertainty.
Kreon offers solutions like the Onyx Skyline and Ace Skyline, which combine measuring arms with 3D scanners for contact (probing) and non-contact measurements (laser scanning). The Kreon arms come with temperature sensors to ensure that the temperature is constantly monitored and compensated so that any expansion of the arm components associated with temperature variations does not affect measurement accuracy.
ISO-compliant, like all Kreon scanners, Zephyr III is a versatile 3D scanner that can be mounted on the CMM to achieve an exceptional accuracy of 5 µm. The highly precise and reliable data collected from the 3D measurement is then employed for analysis to reduce errors and boost production reliability.
Uncertainty in 3D measurements involves deviations from the true value, stemming from equipment limitations, environmental conditions and operator errors. Addressing these uncertainties through regular calibration, environmental control, ISO-certified technologies and proper training ensures accurate and reliable results. By implementing best practices, manufacturers can enhance the precision and quality of their metrology processes.
