The Two Keys to Metrology: Accuracy and Repeatability

by Brian Winters on April 4, 2018 From Ask The Experts

When it comes to evaluating metrology instruments, consumers should understand the difference between two key features: accuracy and repeatability. Both are essential to the performance of metrology tools, but they are not interchangeable. Accuracy describes how precisely an absolute measured result compares to a reference result. Repeatability describes the degree in which multiple measurements compare to each other.

To visualize the difference between accuracy and repeatability, imagine shooting an arrow at a bullseye target. Hitting the bullseye right in the center scores a high mark for accuracy. Consistently hitting the target off-center but in a very tight grouping demonstrates high repeatability, but low accuracy. Hitting the bullseye right in the center over and over again, however, scores high marks for both accuracy and repeatability – the ideal combination for a metrology tool.

Oftentimes users will refer to the accuracy of their laser scanner, but the important thing to consider is the accuracy of the system as a whole. The combination of the laser scanner and the device it is attached to is what is important to understand when discussing accuracy and repeatability as they work hand-in-hand. 

When it comes to the color of the laser line used in laser scanners, there are differences between red light and blue light. A blue light features a shorter wavelength and can generate a little less speckle. However, this does not guarantee more accurate or more repeatable scanning. The determining factor controlling accuracy and repeatability is not the color of the laser; it is predominantly determined by the device the laser is attached to, for example a portable measuring arm CMM. Only the ROMER Absolute Arm range supplies length and sphere standards that allow you to test not only the repeatability but also the accuracy of the total system. 

NA18020-Accuracy-GraphicTo test the accuracy of a scanning system, measure a qualified sphere several times with the elbow in several positions. Position the spheres around the machine and measure the sphere at each position once with the elbow left and parallel to the table, once with the elbow right and parallel to the table, and once with the elbow up or perpendicular to the table while modifying wrist positions with each measurement. Compare the diameter and the center location of each sphere measured. If the diameter of the sphere, or location of the sphere center differs from the system accuracy specification, then the measurements should raise concern.

When evaluating scanning systems, it is also important to consider the standoff distance. A larger standoff distance allows scanning into deeper cavities without concern of laser beam hitting the edge of the part.

Hexagon’s ROMER Absolute Arm with integrated scanner is an all-purpose portable measuring arm system designed to deliver accurate and repeatable results that are hard to match. While a counterbalance relieves operator fatigue it can also be a source inducing torque into the system, which can severely impact the accuracy of measuring instruments. Only the ROMER Absolute Arm utilizes an innovative Zero-G counterbalance that mitigates torque in the tubes and the base of the machine, yielding high and consistent accuracy and repeatability.
 
The ROMER Absolute Arm is designed to give users confidence in the exactness of measurements, while also decreasing the time required to perform measurements tasks. Utilizing an ultra-wide laser stripe of up to 150 mm, the current RS4 laser scanner can capture 752,000 points per second, reducing the number of passes required to capture a full scan. The permanently attached scanner does not require calibration or warm-up time, allowing for longer machine uptime. To alleviate the need for time-consuming maintenance interventions, arms are rigorously tested for enhanced durability using drop tests, vibration tests, and thermal testing.

Providing accurate and repeatable results is the function of any measurement tool. The ROMER Absolute Arm delivers fully verifiable and traceable accuracy with each and every measurement, reducing costly errors, while also economizing time to drive a more efficient measurement process. Explore all of the features and benefits of Hexagon’s ROMER Absolute Arm with integrated scanner by visiting the product webpage today or ask for an onsite demonstration.

Brian Winters

Brian Winters is the Product Manager for the ROMER Absolute Arm at Hexagon Manufacturing Intelligence North America. Winters holds a bachelor’s degree in Mechanical Engineering from the University of Texas – Austin, and has held various roles focused on process development, robotic and assembly design and manufacturing engineering. After advancing into a Product Line Manager position for Interconnect, Brian has the rare ability to identify with both the technical and commercial sides of the customer applications.

Brian has certificates in Advanced SolidWorks Design, Pragmatic Marketing, 6Sigma Green Belt, LEAN Manufacturing, Leadership Management Certification; is an Eagle Scout, and is also fluent in Spanish.