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Phase-Sensitive Detector (PSD) measurement
PSD method based on the same method which is used for focusing in SLR cameras. The light is divided with beam splitter in two rays and then by comparing the two rays' phase the distance can be measured. There is movable lens and its position is known so when two beams are perfectly aligned the distance can be determined by current lens position. In figure 2 you can see how this method works. Purple ball in left is object being measured and red and green lines are light rays.
Figure 2: Distance measurement with PSD. Figures 1 to 4 represent conditions where the lens is focused (1) too near, (2) correctly, (3) too far and (4) much too far.
PSD method is similar to CMOS but PSD is can be more accurate though only with matte or specular surfaces. While with CMOS one can choose the brightest pixel to be the center of the laser beam but with PSD the beam centroid is determined by whole reflected dot. If reflective surface is rough, the beam spreads quite large area and distance is calculated inaccurately. So PSD method suites well for example surveying where one need to use some specular stick anyway at the other end. However, that method is not so good for universal laser meter which is usually used with various materials which are often quite rough.
Figure 2 3 illustrates how different methods differ from each other. Of course this is the worst case scenario what can happen in certain conditions.
Figure 23: Accuracy comparison between PSD and CMOS
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Pulsed time-of-flight measurement
Figure 34: Basic principle of time-of-flight (TOF) measurement
Distance of remote object can also be calculated when the speed of light and the time is known. Figure 3 4 shows the basic principle of the measurement method. A very short laser pulse is emitted from laser diode and it immediately partially reflects from front glass of the device. The reflection is captured and the system starts to measure the time for the second incoming pulse of light. When the second reflection from the obstacle reaches the sensor, TDC (time-to-digital converter) returns elapsed time in digital format.
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The amount of noise is dependant of meters bandwidth. The bandwith is defined usually by the point where output voltage is dropped 3 dB or 30%. The noise is distributed over all frequencies but it can be reduced by filtering which limits bandwidth.
Presentation
References
- Webcam Based DIY Laser Rangefinder. Todd Danko. Accessed: 10.4.2014
https://sites.google.com/site/todddanko/home/webcam_laser_ranger High Precision Laser Sensors in Robotics. Dragos George Calin, Intorobotics. Accessed 16.4.2014.
http://www.intorobotics.com/high-precision-laser-sensors-in-robotics/- Parallax Laser Range Finder, datasheet. Accessed 10.4.2014
http://www.parallax.com/sites/default/files/downloads/28044-LaserRangeFinder-v1.0.pdf - Scanning Laser Range Finder URG-04LX-UG01, specifications. Acessed 14.4.2014http://www.hokuyo-aut.jp/02sensor/07scanner/download/products/urg-04lx-ug01/data/URG-04LX_UG01_spec_en.pdf
- A receiver – TDC chip set for accurate pulsed time-of-flight laser ranging. Juha Kostamovaara, Sami Kurtti, Jussi-Pekka Jansson University of Oulu, Department of Electrical Engineering, Electronics Laboratory, FINLAND. Accessed 17.4.2014
http://www.minifaros.eu/files/PaperAC11_KostamovaaraJ_OuluUniversity.pdf - Range Finding Using Pulse Lasers, application note. Stefan Morgott, OSRAM. Accessed 17.4.2014
http://www.osram-os.com/Graphics/XPic2/00054201_0.pdf/Range%20Finding%20using%20Pulsed%20Laser%20Diodes.pdf - MTI instruments, triangulation. Accessed 16.4.2014
http://www.mtiinstruments.com/technology/triangulation.aspx
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