Laser Trackers – IFM vs ADM Technology

There is a lot of talk today about the different types of technology used in laser trackers to determine the distance measurement. Let’s briefly discuss the two main technologies and how they work.

Distance measurement, an important function of a laser tracker, can be either incremental or absolute. Incremental distance measurement is made with an interferometer (IFM) and a frequency-stabilized, helium-neon laser. The laser light splits into two beams. One travels directly into the interferometer. The other beam travels out of the laser tracker, reflects off the spherically mounted retroreflector (SMR) and, on the return path, passes into the interferometer. Inside the interferometer, the two beams of light interfere, resulting in a cyclic change each time the SMR moves closer to or farther from the laser tracker by a distance equal to one quarter of the light’s wavelength (~0.0158 micron). Electronic circuitry counts the cyclic changes (known as “fringe counts”) to determine the distance traveled.

In a typical measurement sequence, the operator places the SMR in the laser tracker’s home position and resets the interferometer to the known (home) distance. As the operator moves the SMR to the desired location, the laser tracks along, remaining fixed to the center of the SMR. This procedure works well as long as the beam from the laser tracker to the SMR isn’t broken by an obstruction in the beam path. If the beam is broken, however, the number of counts is no longer valid and the distance isn’t known. When this happens, the laser tracker signals that an error has occurred. The operator must then return the SMR to a reference point, such as the laser tracker’s home position.

Interferometer-based measurement can trace its origins to the late 1800s – it was not until the invention of Absolute Distance Meter (ADM) that the laser tracker made its move from a laboratory instrument to a real-world measurement system. ADM represents the latest in laser tracker technology.

Absolute distance measurement capability has been around for a long time. Within the last several years, however, ADM systems have undergone dramatic improvement, offering accuracy comparable to interferometers. The advantage of ADM measurement over incremental distance measurement is the ability simply to point the beam at the target and shoot. The ADM system measures the distance to the target automatically, even if the beam has previously been broken. In a laser tracker with ADM, infrared light from a semiconductor laser reflects off the SMR and re-enters the laser tracker, where it’s converted into an electrical signal. Electronic circuitry analyzes the signal to determine its time of flight, multiplying this value by the speed of light in air to determine the distance from the laser tracker to the SMR.

Absolute distance meters first appeared in laser trackers in the mid-1990s. At that time, ADM units measured too slowly to permit scanning of surfaces. Because of this, all early laser trackers contained either an interferometer alone or an interferometer and an ADM. Today some absolute distance meters have been made fast enough to permit high speed scanning without negligible loss in accuracy. Hence some modern laser trackers contain only an ADM with no interferometer.

An example of this is the latest laser tracker from FARO, called the FARO Laser Tracker ION™. It contains the fastest, most sophisticated distance measuring system: Agile Absolute Distance Meter (aADM). This patented technology provides the ability to acquire a handheld target even if the target is moving.

The ION’s Agile ADM measuring system is the only ADM system on the market that is fast enough to allow for high density scanning without relying on an interferometer. Agile ADM modulates its laser beam at three slightly different frequencies. By comparing the phase of the three modulated frequencies received by the Laser Tracker, aADM eliminates any ambiguity and calculates the position of the target.

With Agile ADM comes great simplification of the system – there is no need to switch between ADM and IFM-based systems – aADM does it all.

Learn more about the FARO Laser Tracker ION