Everyone seems to be talking about laser trackers in the precision measurement industry – and there are many good reasons why.
Laser trackers are extremely appealing metrology instruments for numerous reasons including size, portability, repeatability and accuracy. Add to that the ability to capture large volumes of 3-dimensional coordinate data quickly and in real-time, well it is no wonder they are so popular. We here at OASIS are certainly big fans and have added sixteen laser trackers to our arsenal of precision measurement tooling.
In this post, we will provide an overview of how a laser tracker works as well as provide manufacturing applications where the use of a laser tracker excels.
How do Laser Trackers Work?
Laser trackers measure 3-dimensional coordinates with impressive, and highly repeatable, accuracy in their operating volume. The range of a modern laser tracker is as much as 390′ and in most cases, when working within reasonable range to a measurement, laser trackers are accurate to 0.001”. Like most measurement instruments that measure by accounting for angles, its accuracy diminishes as the distance to the target increases. Generally, for laser trackers, this error is 1 arcsecond.
Laser tracker technology is based on the combination of two techniques: a distance meter to measure absolute distance and angular encoders to measure the laser tracker’s two mechanical axes – the azimuth axis and the elevation (or zenith) axis. Laser trackers emit a low power laser beam to a retro-reflective target that is either held or mounted on the object to be measured. Light is reflected off of the target and re-enters the tracker at the same position from which it left. The distance meter (either a laser interferometer or absolute distance meter, ADM) analyzes the light as it re-enters the tracker and measures the distance from the tracker to the retro-reflector. The laser tracker also accounts for Earth’s level, which is a critical component of many alignments.
In a typical measurement sequence, the metrology engineer moves the retro-reflector to the desired location as the laser tracker follows with the “laser beam” remaining fixed to the center of the retro-reflector. As the retro-reflector is moved, the laser tracker records the distance, azimuth and elevation. These polar coordinates are transformed to Cartesian coordinates, which are sent to a metrology software application to process.
There are a number of very good 3D metrology software solutions available for laser trackers. With the use of any one of the software solutions, the coordinate data collected can be converted to geometrical features such as points, planes, spheres or cylinders. These features can then be referenced to defined datums for position, form, parallelism, perpendicularity, concentricity, circularity and cylindricity. Typically, the data can be presented instantly – enabling CAD-to-part comparison in real-time.
Laser Trackers Ideal for Many Manufacturing Applications
In the not so distant past, manufacturers were forced to perform tests on parts and components using stationary coordinate measurement machines (CMMs). This was extremely difficult if trying to measure large parts or parts still on the production machine. With the portability of laser trackers, 3D measurements can now be completed with unprecedented speed and minimum disruption. The applications where laser tracker capabilities excel are many, here we describe just a few.
The aerospace industry was one of the early adopters of laser tracker technology. In an industry where there are stringent quality standards and regulations, laser trackers are ideal not only because of their accuracy, but also because of their portability, reliability and repeatability. Using a laser tracker (or sometimes more than one laser tracker) metrology engineers can quickly and accurately deliver 3D measurements and process improvement in the following areas:
♦ Jig and assembly tool geometry, periodic dimensional inspections and calibration
♦ Tooling inspection and certification
♦ Die inspection
♦ Detailed part inspection
♦ First article inspections
♦ Sub and final-assembled product inspection
♦ Machine tool accuracy and repeatability diagnostics
Whether on an automotive production line or the shop floor of a custom car builder, laser trackers provide a precision measurement solution at all points of the design and production process. Costly machine rework, production line downtime and misaligned components can be avoided with the use of a laser tracker system directly on the production floor. Areas in automobile manufacturing that benefit from the laser tracker’s capabilities include:
♦ Measurement of panels and assembled body panels
♦ Surface measurement for component verification
♦ Deformation measurement and dynamic measurement
♦ Repeatability check of individual components
♦ Alignment of hinge lines and body components
♦ Verification of shape analysis
♦ Adjustment of robotic equipment
♦ Production line measurements
Metal Extrusion Presses
Traditional methods of machinery alignment typically cannot resolve most extrusion press issues such as broken stems or out-of-spec wall thickness, however with laser tracker technology, the inspection and alignment of extrusion presses is much more efficient and cost effective. With the ability to obtain high tolerance measurements quickly and accurately – including conventional level and square measurements – plus capture data between pushes, when changing dies or piercers or when cleaning the stem, laser trackers work exceptionally well for inspecting a metal extrusion press. With a laser tracker, a metrology engineer can inspect for:
♦ Platen-to-platen parallelism
♦ Rod length
♦ Rod stretch
♦ Crosshead, stem, container and die alignment
♦ Uneven wear on ways
♦ Unusual wear on dummy blocks
Plant downtime is especially costly in the power generation industry. With laser trackers, the inspection of power generating turbo machinery – whether nuclear, wind, hydro, steam or gas – is much faster, more extensive and flexible than ever before. Benefits of portable laser tracker systems include:
♦ Quick setup and reverse engineering measurement of existing installations
♦ Precise and accurate measurements of turbines and generators
♦ Turbo machinery alignment
♦ 3-dimensional analysis (penstock, draft tube, piping, etc.)
♦ Dynamic load and thermal growth studies and verification
♦ Foundation stability studies and pipe strain effects analysis
Pulp & Paper
While the more traditional optical alignment methods and tooling are still considered the best option for many areas of the paper machine, the capabilities of the laser tracker excel in several areas. With the laser tracker, metrology engineers can quickly inspect or setup 3D geometry, measure exact distances and/or angles between components, and profile rolls and aprons. Additionally, the movement of components such as frames, nipping rolls, pickup rolls or shoes can be tracked in real-time. The paper machine applications that benefit most from laser tracker technology include:
♦ Winder inspection
♦ Press geometry
♦ Fourdrinier planizing
♦ Forming zones geometry inspection
♦ Layout rebuilds, retrofits and erection
The industries listed here are just a sampling of industries where laser trackers are quickly becoming the tool of choice for precision measurement. In future posts, we will discuss additional manufacturing applications where 3D metrology tools shine and we will provide more information on the various 3D metrology tools that are available today.