APPLICATION NOTE: Laser Triangulation Sensors - MTI Instruments ...

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any inaccuracies caused when operating slightly out of focus and most manufacturers specify performance over the complete measurement range. For a given length detector a smaller acceptance angle offers a larger measurement range and operating distance. A larger angle provides the opposite, however, higher sensitivity can be obtained because of optical leveraging. Figure 4 is a simplified diagram that visualizes the difference between two different acceptance angle sensors. Figure 4: <strong>Laser</strong> Acceptance Angle Dictates the Sensitivity and Measurement Range iii.) Sensitivity In measurement systems sensitivity is usually defined by how much displacement occurs per unit of measurement, typically expressed in microns/milli-volt. The “higher” the sensitivity (actually the smaller the number) the better in most cases because greater resolution may be obtained. To achieve the highest sensitivity it is desired to have the laser beam traverse across the complete detector length over the application measurement
ange. Figure 5 shows the output of two sensors with different sensitivities. Please note that the slope of each curve represents the respective sensitivity factor with Curve A being twice as sensitive. Figure 5: Sensitivity is Determined by the Slope of the Sensor Output Response iv.) Resolution The resolution of a displacement sensor is defined as the smallest amount of distance change that can be reliably measured. When properly designed, laser triangulation sensors offer extremely high resolution and stability, often approaching that of expensive and complex laser interferometer systems. Because of their ability to detect such small motions they have been successfully used in many demanding, high-precision measurement applications. The primary factor in determining resolution is the system’s electrical noise. If the distance between the sensor and target is constant, the output will still fluctuate slightly due to the “white” noise of the system. It is assumed that, without external signal processing, one cannot detect a shift in the output of less than the random noise of the instrument. Because of this most resolution values are presented based on the peak-topeak value of noise and can be represented by the following formula: Resolution = Sensitivity X Noise From the formula you can see that for a fixed sensitivity the resolution is solely dependent upon the noise of the system. The lower the noise the better the resolution! It is important to note that some manufacturers specify resolution based on peak or rms noise, resulting in claims that are 2x and 6x respectively better than peak-to-peak. Although an acceptable method, it is somewhat misleading as most users do not have the ability to decipher voltages changes less than the peak-to-peak noise value. The amount of noise depends on the system bandwidth. This is because noise is generally randomly distributed over a wide range of frequencies and limiting the bandwidth with filtering will remove some unwanted higher frequency fluctuations. Figures 6 and 7 show the difference in the output of two identical systems with different low pass filters. All of
Page 1 and 2: Laser Triangulation Sensors 1) Intr
Page 3: Figure 3: Specular Laser Triangulat
Page 7 and 8: vi.) Spatial Resolution When taking
Page 9 and 10: iii.) Environmental Conditions Beca
Page 11 and 12: Figure 10: Lead Position and Pitch
Page 13: thickness reading. This type of sys

APPLICATION NOTE: Laser Triangulation Sensors - MTI Instruments ...

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