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Tracker Handbook by Javier Vera

Metrology

The Versatile Laser Tracker

Portable, flexible, compatible

Published: Thursday, January 27, 2011 - 15:30

The technology from which the laser tracker is drawn has a diversified beginning, from the laser instrumentation to the methods of capturing the beam. Unmistakably, it is the integration of all components, along with its portability, that makes the laser tracker an appealing measurement tool.

Many of the customers that I have visited have coordinate measuring machines (CMMs). Others have fixtures or jigs that they have used in qualifying and quantifying components in a process. It seems clear to me, and to many with whom I have spoken, much of that technology is under reconsideration in favor of the benefits that come with using a laser tracker instead. Having the ability to relocate the tracker, while keeping the part static and not losing the integrity of the measurement, conjures up exciting possibilities. Here is an example of a case where these benefits made the difference to a manufacturer.

In its existing inspection plan, the manufacturer had to schedule and employ the use of a crane to position a large and extremely heavy part on a static measuring system. The part had to be picked and placed on a pallet to transfer it to the measuring device. After getting the part on the measuring device, the part was subjected to rigorous processes to ensure component stability and the measurement tool’s ability to reach all the necessary data and targets for inspection. Afterward, the part had to be returned to the original location for fitting. If any changes were made to the part, the manufacturer had to begin again with the cumbersome scheduling and employment of the crane, and repeat the tedious and time-consuming measurement process. The process time to manage the component on the measuring instrument would take from two to six hours to position the part in the “just right” orientation for inspection.

The manufacturer then changed the process to use a laser tracker instead. Now the inspection of components, assemblies, and artifacts takes approximately one hour. If re-inspection is necessary, that task is accomplished in minutes, not hours.

This real-world example demonstrates that using a laser tracker has saved both time and money, as well as opened up possibilities in measurement capability for a manufacturer. In some cases, it includes being able to make measurements that were impossible when using the static measuring system that required bringing the part to the measurement tool. The trend is to bring the precision measurement tool to the part; the laser tracker accomplishes that task.

To hold or not to hold

Another laser tracker benefit in addition to flexibility of measurement is the ability to measure components in a constrained state, or measure without taking the part from the production machine. The ability to measure in a constrained state is huge. In many instances components respond differently to measurement depending on the state in which they are held—or not held. Measuring in the manufacturing tool allows the operator to make adjustments on the fly, without reconstraining it and possibly distorting it. 

Many laser trackers work alongside or in conjunction with robotic devices. As a matter of fact, the laser tracker has its roots in the robotics field. However, the laser tracker is being used by other industries at an ever-accelerating rate. One trait that seems to set the laser tracker apart from other metrology tools is its ability to operate as an assembly tool.

Targets are placed in the components, and by using the tracker and a computer-aided design (CAD) model, the operator can move the component to another component and ensure that the assembly is accurately positioned in real time. In laser tracker lingo, this process is referred to as “dynamic build.” In some instances, the dynamic build can be done using 3-D, i.e., the X-, Y-, and Z-coordinate axes. In more complex assembly scenarios, the dynamic build can be done using 6-D, i.e., 3-D that also incorporates the rotational aspects of each axis. An assembly process that would take hours or days of painstaking measurements can be shortened considerably through this application.

Software flexibility

One of the greatest benefits that many laser trackers possess is an ability to run on numerous software platforms. Having this flexibility among different software platforms provides users with hands-down benefits. Rather than forcing users to use one method of collecting and analyzing data, laser trackers give access to various software platforms. By providing the flexibility to choose from among measurement software, laser trackers allow users to apply the software that best suits the needs of the company.

In addition, this software flexibility allows a company to integrate other measurement tools to enhance the overall measurement process. For example, a company can integrate an articulating arm with trigger probe, or an articulating arm with a red-light scanner, with a laser tracker. Another example would be the integration of a laser tracker with a white-light measuring system or a CMM. The days of one machine, one manufacturer, and dedicated software are gone. Industry and informed consumers have realized that no single company offers the only product, software, and service solution.

There are specific areas where all developers of laser tracker software agree. One such is how coordinates are managed when measuring.

Familiar coordinates

The basis for all coordinate measurement has a commonality in the definition of three mutually perpendicular axes that cross a common point from which all values are drawn. In most cases, the azimuth is branded as X, Y, and Z, respectively. From these axes, position, angular, and location are referenced.

Laser trackers are no different. True, trackers deal with angular (i.e., elevation) and radial (azimuth) coordinates as pertaining to the primary ordinate definition. Notwithstanding, the results the users work with are based on the standard X, Y, and Z coordinates. In addition to the coordinates, most tracker software supports many of the standard operation functions found in other measurement systems, including form, position, and relational functions. 

In addition to the standard functions, laser trackers have some unique functions that are software based. For example, software provides functions that allow for defining the best estimate of a measured point within the global reference frame. This includes point weighing. Putting weight on a point allows the analysis of a coordinate system where certain points influence others over the hierarchy of the best fit. In addition to determining the best fit of one coordinate system and one tracker, point weighing provides a method for minimizing uncertainty through a network topology.

Next month

Another area of consideration is maintenance of a laser tracker. In my next column, we will consider some of the more common points to keep in mind when working with laser trackers.

Discuss

About The Author

Tracker Handbook by Javier Vera’s picture

Tracker Handbook by Javier Vera

Javier Vera works in the application department of Automated Precision Inc. (API) and has held positions as product manager and metrology manager. Currently he has responsibilities over technical customer support on the West Coast. Javier has more than two decades of experience in the field of metrology, during which he has written several articles and white papers on the subject.