Technology White Paper

Transcription

1 Technology White Paper Phase Shift Measurement and Time of Flight Measurement Phase Shift Measurement Executive Summary The FARO Laser Scanner Focus3D and FARO Laser Tracker both make use of Phase Shift Measurement technology to measure distance. The Laser Scanner Focus3D uses Phase Shift technology to measure the distance to a surface and the Laser tracker uses it to measure the distance to a reflecting mirror called a retro-reflector. In both cases, an infrared laser is sent out and reflected back to the system. The distance is measured by analyzing the shift in the wavelength of the return beam. Overview

2 The laser beam of a known sinusoidal wave is emitted from a laser source light Emitted. Some of this laser beam is reflected back from the target to the source Light Returned. The phase of this light Returned is then compared to that of the known emitted light History of Emitted Light. The difference between the two peaks is the Phase Shift (as seen in Fig. 1). The obtained Phase Shift is 2π x the time of flight x the modulation frequency. However there is some ambiguity for the measured distance, because for increasing distance the phase will vary periodically. I.e. once the ambiguity interval for the emitted sinusoidal wave is reached the phase shift will return to 0 therefore the distance measurement will return to 0. This can be removed by measuring two or more different modulations. For the Laser Scanner LS, 3 different modulations are use ~1.2m, 9.6m, and 76m (as seen in Fig. 2) allowing for a total ambiguity interval of ~76m. The larger wavelength is used to determine the location of the next smaller wave. The accuracy of the ~1.2m wave is 0.07mm (as seen in Fig. 3).

3 For the Laser Tracker, two modulations are used resulting in two modes of operation: coarse and fine. The coarse mode has a resolution of 52 mm, and the fine mode has a resolution of mm. When a tracker locks onto a target it immediately measures the coarse distance and then switches to fine and then continuously measures the fine distance. The system stays in fine mode unless the beam is interrupted in which case is switches back to coarse mode. Variations on the Technology Time of Flight Measurement; (or pulse measurements) are based on measuring the time of flight of a laser pulse from the measurement device to some target and back again. Such methods are typically used for large distances like hundreds of meters or many kilometers. Typical accuracies of simple devices for short distances are a few millimeters or centimeters. The Leica Laser tracker uses pulsed time of flight ADM. This system has limitations such as not being able to be used at very close range (under 1.5 meters) and also longer sample times for high accuracy. Because Leica uses a pulsed time of flight technique they cannot provide an ADM only tracker, and must rely on the interferometer(ifm) for high speed measurement. In fact all measurement with a Leica laser tracker is done with IFM, and the pulsed time of flight ADM is only used to set the IFM distance. Time of Flight Measurement (or Pulse Measurement) Executive Summary 3D laser scanners typically use two different kinds of distance measurements. The HE distance modules of the FARO Laser Scanner Focus3D use the so-called Phase Shift Measurement. Some of our competitors, like Leica and Riegl, use the so-called Time of Flight Measurement to measure long range distances. In both cases a laser beam (wavelength differs depending on the vendor) is sent out and reflected back to the system. Time of flight systems emit pulsed laser light and measure the time needed by the reflected light to come back to the distance sensor.

4 Overview The time-of-flight 3D laser scanner is an active scanner that uses laser light to probe the subject. At the heart of this type of scanner is a time-of-flight laser range finder. The laser range finder finds the distance of a surface by timing the round-trip time of a pulse of light (as seen in Fig. 4). A laser is used to emit a pulse of light and the amount of time before the reflected light is seen by a detector is timed (as seen in Fig. 5). Since the speed of light c is a known, the round-trip time determines the travel distance of the light, which is twice the distance between the scanner and the surface. If t is the round-trip time, then distance is equal to (c*t)/2. Clearly the accuracy of a time-of-flight 3D laser scanner depends on how precisely we can measure the time: 3.3 picoseconds (approx.) is the time taken for light to travel 1 millimeter. The time-of-flight method is typically used for large distances like hundreds of meters or many kilometers. Using advanced techniques (involving high quality telescopes, highly sensitive photo detection, etc.), the time-of-flight method allows to measure e.g. the distance between earth and the moon with an accuracy of a few centimeters, or to obtain a precise profile of a dam. Typical accuracies of simple devices for short distances are a few millimeters or centimeters.

5 The used pulse duration is usually between 100 picoseconds and a few tens of nanoseconds, as achieved with a Q-switched laser. For large distances one requires a high pulse energy, which can raise laser safety issues, particularly if the laser wavelength is not in the eye-safe region. For nanojoule to microjoule pulse energies (as required for moderate distances), one can use e.g. a passively Q-switched microchip Er:Yb:glass laser, which can generate rather short pulses (duration of the order of 1 ns) with pulse energies around 10 μj in the eye-safe spectral region. The laser range finder only detects the distance of one point in its direction of view. Thus, the scanner scans its entire field of view one point at a time by changing the range finder s direction of view to scan different points. The view direction of the laser range finder can be changed by either rotating the range finder itself, or by using a system of rotating mirrors. The latter method is commonly used because mirrors are much lighter and can thus be rotated much faster and with greater accuracy. Typical time-of-flight 3D laser scanners can measure the distance of 100~15,000 points every second. References Encyclopedia of Laser Physics and Technology FARO and the FARO Logo are registered trademarks and trademarks of FARO Technologies, Inc FARO Technologies, Inc. All Rights Reserved. SFDC_04MKT_0245.pdf Created: 1/18/11