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Technology
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Optical Triangulation
Electro-optical triangulation has been around in practical form for 25 years. The basic method uses a light source usually striking the surface at a perpendicular angle. The light spot on the surface is imaged with a viewing lens onto a position sensitive  photodetector (PSD). Because of parallax, changes in the distance between the surface and the sensor cause a change in the position of the imaged light spot on the position detector. The figure shows a block diagram of a typical optical triangulation sensor. The two photocurrents from the PSD are converted to voltages by a pair of opamps. These voltages are sent to the A/D converters and the data is used to produce a range signal. Sometimes, as shown here, the microprocessor also controls the power emitted by the laser. In this way the signals from the detector are always optimized independent of the reflectivity of the surface.
Today, optical triangulation range sensors employ microprocessors, high quality optics and small laser diode sources. The PSD is either a lateral effect photodiode (LEP) or a linear charge coupled device (CCD) or charge integrated device (CID) array. There are advantages and disadvantages in using either detector. The LEP method results in a fast sample rate and a relatively inexpensive way to measure range. However, the LEP detector is only capable of determining the centroid of the light imaged onto it and not any structure (patterns). In the vast majority of surfaces this is sufficient. The LEP can be fooled by transparent surfaces, and will give a range result somewhere in between the two surfaces. The CCD approach is more expensive and usually slower. However, if the sensor is "smart" enough, it can measure the distance to both surfaces of a window for example. Many variables govern all optical triangulation sensors design and ultimate performance:
System Variables
Measurement Range
Standoff Distance
Temperature
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Optical Variables
Laser Power
Laser Wavelength
Detector Tilt Angle
Viewing optics magnification
Blur radius at detector
Viewing optics diameter
Laser beam diameter at waist
Laser beam diameter at laser source
Surface statistics (i.e. roughness, reflectivity) |
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Electronic Variables
Noise (Shot, Johnson, D-star)
Length of detector
Area of Detector
Responsivity of Detector
Resistance of Detector
Capacitance of Detector
Sampling Frequency |
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