Distance metering using laser
There are two main tecniques to measure the distance with laser beam. One way is to pulse the laser beam and measure the time between emitted and reflected pulse. Second way is to beam laser constantly and then shoot the camera with special filter to measure how much the dot of the laser has moved on obstacle. The latter is the cheaper method as it does not need that special processor that is capable of sampling at nanoscale timeframe.
Laser sensor characteristics
|Model||URG-04LX-UG01 (Pulse measurement)|
Parallax Laser Range Finder (CMOS measurement)
|Typical accuracy||< 3 %||< 5%|
Interface electronic circuits
Input signal handling depends on the technology how the measurement is implemented.
Figure 1: Distance measurement with angle detection method
If laser beam is projected inside camera focal area then it is possible to resolve the distance using simple math. The optical axis of camera and laser beam should ideally be aligned perfectly. However, it is possible to add constant angle correction into math if angle is not perfectly aligned. A relatively simple algoritm can be used to find brightest pixels. This method assumes that the brightest pixels are laser dot which is the case usually indoors but not necessarely outdoors. Thats why red pass filter should be used to lessen disturbance of other light sources. When laser dot is resolved from image we can calcute the distance because the dot position in the image frame is known.
Distance D can be calculated with formula:
In this formula the h is constant distance between laser and camera. Theta is calculated from:
θ = pfc*rpc+ro
pfc = Number of Pixels From Center of focal plane
rpc = Radians per pixel pitch
ro = Radian Offset (compensates for alignment errors)
So the final equation would be:
To cut out the amount of data that needs to be processed the image frame can be cropped to save only pixel area where the laser dot can be.
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 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 3 illustrates how different methods differ from each other. Of course this is the worst case scenario what can happen in certain conditions.
Figure 3: Accuracy comparison between PSD and CMOS
Both CMOS and PSD method can be used with highly reflective surfaces by sending laser beam at a angle that is right with the collector. There are devices available where the mode is user selectable. Without that mode the laser beam will reflect straight back where it came from but on the other hand if device is designed to be used only with specular materials the beam will not reflect to collector from matte surface.
Pulsed time-of-flight measurement
Figure 4: 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 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.
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.
- Webcam Based DIY Laser Rangefinder. Todd Danko. Accessed: 10.4.2014
High Precision Laser Sensors in Robotics. Dragos George Calin, Intorobotics. Accessed 16.4.2014.
- Parallax Laser Range Finder, datasheet. Accessed 10.4.2014
- 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
- Range Finding Using Pulse Lasers, application note. Stefan Morgott, OSRAM. Accessed 17.4.2014
- MTI instruments, triangulation. Accessed 16.4.2014