From time to time there are advances in technology that merit recognition. From the invention of the measurement ruler, to the first electric light bulb, to the creation of the first transistor, which led to the invention of the computer—such devices have advanced our abilities to manufacture better products. Technological innovation has also been key to the advancement of coordinate measurement systems. One of fastest-growing class of technologies is portable measurement systems, specifically the laser tracker.
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For years, precise length measurement was dedicated to fixed, structured systems, such as coordinate measurement machines (CMMs). There was a common belief that to be accurate, precise, and repeatable, a rigid structure like a CMM was the only way to get acceptable measurements. That way of thought was radically changed in 1986 when Dr. Kam Lau, then a research engineer at the National Bureau of Standards (now NIST), developed the modern laser tracker. A year later he founded Automated Precision Inc. (API) and the laser tracker soon became a commercially available product that freed manufacturers from the proverbial “box” of a structured CMM.
So how did the laser tracker evolve to what we have today? What are the components of a laser tracker? What sets the tracker technology apart?
The best place to start learning the operations and capabilities of a laser tracker is by becoming familiar with key terms and components. Some terms may vary from one manufacturer to the next, but laser-tracker operation methods are common the world over.
Early laser tracking
Laser tracker technology goes back a couple of decades. The first trackers were capable of measuring in the ranges of 20 µm within 30 m. Even though the trackers were extremely accurate, they were much bulkier and required more setup time than today’s models. Also, to obtain acceptable measurement results, the work environment had to be more optimized than today’s work environments.
The functionality and flexibility of the first trackers provide the perfect fit for robotic alignments. Today the tracker is found in almost all industries that require measurements outside the realm of traditional measuring systems—i.e., those that are static or where the operator takes the part to the machine. The demand for on-the-fly measurement and bringing the measurement tool to the product has prompted companies to reduce size and weight. API answered consumers’ demands by producing the smallest trackers on the market today. The T3 is such a unit, more robust and more easy to use than its earlier predecessors.
The unit itself
Like any other measuring system, laser trackers have key components that define the system and are common to all manufacturers. For example, the majority of trackers are laser-based systems using highly reliable light sources. The lasers are generally two types: interferometric (IFM) or absolute distance measurement (ADM).
IFM is one of the most accurate photonic sources. Accuracy, stability, and repeatability of the laser is so precise that currently the world standard for measurement—one meter—is measured using such a laser in vacuum condition.
The ADM laser system uses differing technology to obtain a measurement. It is also a stable laser for measurement. The IFM and ADM lasers have different strengths and weaknesses that are best understood when evaluating the environment in which it will be situated.
In addition to laser type, an end-user should consider the methods used to shoot and capture the laser beam and how the laser beam affects measurements. For example, some trackers have the IFM and ADM lasers mounted on a common shaft. Others have the laser beam in the body away from the primary laser beam, and use prisms and mirrors to shoot the laser out of the measuring head. Yet others have the laser light transferred via fiber-optic conduit from the body. Each of these technologies has benefits and challenges associated with it. For the purpose of this Laser Handbook series, or unless stated otherwise, the referenced technology relating to laser tracking will be considered in its most standard format—i.e., the laser is emitted through the tracker head. A more in-depth look at this aspect of laser technology will be considered in upcoming columns.
Lighting the way
Other components common to most laser trackers include controllers, motors, the head of the tracker, and the tools used to reflect back the laser beam and produce the measurement from the laser tracker. The measuring tool is most commonly known as a “spherically mounted retro-reflector” (SMR). An SMR is typically a spherical ball with reflective surfaces precisely placed within a machined-out section of a spherical steel ball. These reflective properties provide the laser beam capture, which is then returned to the laser tracker as a signal. Thus, SMRs are “line of sight” tools.
SMRs are commonly manufactured of magnetic steel, made either as standard or high-accuracy reflectors. High-accuracy reflectors have glass or gold-plated mirrors. Less-expensive SMRs are made entirely of steel and in most cases have a reflective coating to provide reflectivity back to the laser tracker. These are suitable for standard applications.
Beyond SMRs, other measuring tools include wired and wireless probes, scanners, and combination scanner probes. We’ll discuss these in more detail in future columns.
Future discussions
Having discussed the basic history and components of the laser tracker, including its robustness in meeting today’s measurement environment, we are ready to take an in-depth look at how these devices are used. As the discussion continues, I’ll delve deeper into some of the technology trade-offs, benefits and methods to make them work in many types of applications.
I encourage you to write me at Javier.Vera@apisensor.com with your questions, application concerns, challenges, and experience.
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