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

Laser Triangulation Sensors
1) Introduction
APPLICATION NOTE:
Laser triangulation sensors can be divided into two categories based upon their
performance and intended use. High resolution lasers are typically used in displacement
and position monitoring applications where high accuracy, stability and low temperature
drift are required. Quite frequently these sensors are used in process monitoring and
closed-loop feedback control systems. Proximity type laser triangulation sensors are
much less expensive and are typically used to detect the presence of a part, or used in
counting applications. The following paper describes characteristics of high resolution
systems, their operating principle, advantages/disadvantages and how to successfully
apply them.
Operating Principle
Laser triangulation sensors contain a solid-state laser light source and a PSD or
CMOS/CCD detector. A laser beam is projected on the target being measured and a
portion of the beam is reflected through focusing optics onto a detector. As the target
moves, the laser beam proportionally moves on the detector as shown in Figure 1.
Figure 1: Laser Triangulation Principle
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The signal from the detector is used to determine the relative distance to the target. This
information is then typically available through an analog output, a digital (binary)
interface or a digital display for processing.
CMOS and CCD type sensors detect the peak distribution of light quantity on a sensor
pixel array to identify target position, whereas, PSD type sensors calculate the beam
centroid based upon the entire reflected spot on an array. Because of this, PSD type
sensors are more susceptible to spurious reflections from changing surface conditions,
which can reduce their accuracy. However, when measuring to ideal matte finishes or
specular targets their resolution is unmatched. CCD and CMOS systems are typically
more accurate over a wider variety of surfaces because only the highest charged pixels
from the reflected beam are used to calculate position. The lower charged pixels are
usually energized by unwanted reflections from changing optical properties of the surface
being measured and can easily be ignored during signal processing. This allows them to
be used in a wider variety of applications. Figure 2 show the signal distribution difference
between CMOS and PSD technology, highlighting the potential accuracy problem
associated with PSD type sensors.
Figure 2: Potential Errors Induced by PSD Type Laser Sensor
Laser triangulation sensors can also be used on highly reflective or mirror surfaces,
commonly referred to as specular. With these surfaces the typical triangulation sensor, as
shown in Figure 1, can’t be used because the laser light would bounce directly back into
itself. For these cases it is necessary to direct the beam to the target at an angle. The beam
will reflect from the target at an equal but opposite angle and focus onto the detector.
MTII manufactures laser heads specifically designed for specular surfaces or any of our
lasers can be mounted at an angle and operated in the “specular mode” if necessary.
Figure 3 shows the operating principle of a specular laser head.

Figure 3: Specular Laser Triangulation Principle
2) Characteristics of Laser Sensors
i.) Non Contact
Laser displacement sensors are non contact by design. That is, they are able to precisely
measure the position or displacement of an object without touching it. Because of this the
object being measured will not be distorted or damaged and target motions will not be
dampened. Additionally, they can measure high frequency motions because no part of the
sensor needs to stay in contact with the object, making them ideal for vibration
measurements or high speed production line applications.
ii.) Range/Standoff Distance
As shown in Figure 1, laser triangulation systems have an ideal operating point which is
sometimes referred to as the standoff distance. At this point the laser is at its sharpest
focal point and the reflected spot is in the center of the detector. As the target moves the
spot will move toward the ends of the detector allowing for measurements over a specific
range. Both the range and standoff of a sensor are determined by its optical design.
Optimal performance is obtained at the standoff distance because the spot is smallest at
its focal point and highly concentrated on the detector. Detection algorithms correct for

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: Laser 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

MTII’s laser triangulation systems have software adjustable low pass filters for easy
adjustment in the field.
Figure 6: Amplifier Output Noise with 20kHz Low Pass Filter
Figure 7: Amplifier Output Noise with 100Hz Low Pass Filter
MTII’s laser sensors also provide displacement values in digital formats. Digital output
resolution is calculated by dividing the displacement range by the processor bit rate. For
example, a sensor with a 2000 micron range would have a resolution of 2000/2E16, or
0.03 microns for a 16 bit system. If using a 12 bit converter the resolution would be
worse at 2000/2E12, or 0.5 microns.
v.) Bandwidth
The bandwidth, or cutoff frequency, of a system is typically defined as the point where
the output is dampened by -3dB. This is approximately equal to an output voltage drop of
30% of the actual value. In other words, if a target is vibrating with an amplitude of 1mm
at 5kHz, and the bandwidth of the laser sensor is set at 5 kHz, the actual output would be
1mm X 70% = 0.70mm. So, it is important to set the system’s frequency response higher
than the expected target motion. All of MTII’s laser sensors have adjustable filter
settings. The appropriate filter should be selected for the application to prevent any
attenuation of the output. MTII’s Application Engineers can assist in selecting
appropriate filter settings.

vi.) Spatial Resolution
When taking measurements, laser sensors provide a distance approximately equal to the
average surface location within the laser spot. They are not capable of accurately
detecting the position of features smaller than the size of the spot, however, they can
repeatably measure to rough surfaces. Because of this the laser spot should always be
approximately 25% smaller than the smallest feature you are trying to measure. Smaller
spots can distinguish smaller features on an object.
vii.) Linearity
In an ideal world the output from any sensor would be perfectly linear and not deviate
from a straight line at any point. However, in reality there will be slight deviations from
this line which define the system linearity. Typically, linearity is specified as a
percentage of the Full Scale Measurement Range (FSR). During calibration the output
from the laser head is compared to the output of a highly precise standard and differences
are noted. These differences are automatically corrected for; through the use of look up
tables. MTII’s Microtrak II laser sensors offer the highest linearity available today. Most
systems exceed +/-0.05% FSR with some achieving +/-0.01% or better.
Accuracy is a function of linearity, resolution, temperature stability and drift, with
linearity being the majority contributor. Fortunately, the linear response of MTII’s
sensors is very repeatable. Calibration reports provide data that can be used to correct
additionally for the non-linearity of a system with inexpensive computers and correction
software, resulting in improved accuracy if needed.
3) Applying Laser Sensors
i.) Material and Finish
In Section 1 we briefly mentioned the difference between a specular and diffuse laser
head. When applying a laser sensor it is first necessary to determine the surface
reflectivity. A consistent matte finish is desirable for best performance when using
diffuse heads. If a highly polished or mirror finish will be used it is strongly
recommended to use a specular laser head.
ii.) Target Shape
For ideal performance the target should be positioned normal (90 degrees) to the laser
head to prevent tilt errors. The influence from tilt will be dependent on the surface
reflective properties. An ideally diffuse target will allow proper operation on surfaces
tilted 30 degrees or more from normal. However, a mirror target will produce errors if the
tilt changes by as little as 1 degree. Care should be taken during fixture design and
operation to minimize any target tilt.
Laser sensors can also be used to measure curved targets. For best results the beam
should be positioned facing directly toward the center of curvature. This will virtually
eliminate any “tilt” seen by the laser. In addition, the orientation of the head should be

such that the curved surface does not skew the laser triangulation angle. Figure 8 shows
the proper orientation for a system to reduce tilt effects.
Figure 8: Note how the Laser Beam may be Deflected by Target Shape
Care should also be taken to ensure the laser return light is not blocked by some feature
on the target being measured. Figure 9 shows the right and wrong way to orientate a laser
sensor.
Figure 9: Note how the Laser Beam may be Obstructed by Target Features

iii.) Environmental Conditions
Because laser triangulation systems are optical type sensors it is important to keep the
optical path clean and free from obstructions or foreign materials. Dirt, dust and smoke
can affect the measurement results or even render sensors completely useless. Care
should be taken to eliminate such contamination and clean air purge systems should be
used when required. If this type of system is not possible it is important to regularly clean
the outer lenses to avoid problems.
The most common environmental problem that can affect the accuracy of a laser sensor is
temperature. Not only do the electronics exhibit temperature drift, but also expansion and
contraction of mechanical components and fixturing can physically change the sensor
gap. All of MTII’s Microtrak II sensors have a temperature stability of less than +/-0.05%
of the full scale measurement range over a temperature change of 0 to 40ºC.
iv.) Fixturing
It is important that the fixture holding a laser triangulation sensor is stable. As mentioned
above, temperature changes can cause expansion and contraction, resulting in a distance
change to the target. Fixtures should be made of the appropriate material to minimize this
effect. The fixture supports should also be as short as possible and long cantilevers
should be avoided to minimize not only temperature issues but to also reduce vibration.

MTII’s laser sensors have through holes that can be used to mount and secure the laser
heads. Fixtures should be made to match the location of these holes and maintain the
laser head perpendicular to the target of interest.
v.) Synchronization
When making differential thickness measurements with 2 laser heads it is important to
take and process measurements from both heads at the exact same time. This is to
eliminate the effects due to vibration. If the target is moving, and measurements are taken
at slightly different times, the processed results may report a slightly thinner or thicker
target. MTII’s Microtrak II line of laser sensors have provisions to synchronize heads
eliminating this problem.
4) Advantages and Disadvantages
i.) Advantages
As with any sensing technology, laser systems have both advantages and disadvantages.
Perhaps their greatest attribute is their ability to resolve measurements below one micron
at a fraction of the cost of other high performance technologies. In addition, their
measurement range is large allowing them to fulfill a variety of application requirements.
The large operating distance provides sufficient standoff to reduce possible damage from
contacting the moving target.
ii.) Disdvantages
As mentioned above, laser sensors should be kept clean. Dirt or other foreign debris can
affect accuracy so frequent cleaning may be required. Because laser heads have sensitive
electronic components their operating temperature is limited and vacuum installations are
not recommended without external cooling.
5) Applications
i.) Position Sensing
General positioning is probably the most common application for laser sensors. Their
fast, highly linear response makes them ideally suited to be applied in both static and
active feedback positioning applications. Large operating distance and measurement
range provides the flexibility for process and quality control monitoring. Typical
applications include:
• Pavement and concrete road profiling
• Railroad track alignment
• Robot location
• Welding head position

Figure 10: Lead Position and Pitch on Integrated Circuits
Figure 11: Closed Loop Control of Robotic and Positioning Systems
ii.) Dynamic Measurements
Non-contact sensors are ideal for measuring moving targets because they have high
frequency response and do not dampen target motions by adding mass. MTII’s laser
sensors are designed with a 40 kHz sampling frequency and a true 20 kHz frequency
response, making them ideal for high speed applications such as:
• Spindle runout analysis
• Piezoelectric characterization
• Ultrasonic vibration measurements
• In-line process monitoring

Figure 12: Vacuum Seal Integrity for Canning Industry
Figure 13: Surface Profile of a Wide Variety of Materials
iii.) Thickness and Dimensional Measurements
On-line production thickness measurements have conventionally been made using direct
contact type measurement systems. Sensors, such as LVDT’s, are positioned above and
below the material being measured to track surface position. The sensor outputs are
combined through software or a summing device and thickness is determined.
Unfortunately, contact type methods cause measurement problems. Not only can the
material being measured be damaged but sensor wear also occurs. In addition, contact
sensors are slow and may not properly track targets that may move or vibrate, making
these applications ideal for MTII’s laser systems.
Single sided thickness measurements are possible if one side of the material can be held
constant against a fixed reference plane, however, for best results, two sided
measurements are preferred. This is because a two sided approach eliminates any errors
that might be introduced from the material moving or vibrating. MTII’s two sensor
approach synchronizes the data sampling for both sensors which ensures a correct

thickness reading. This type of system provides both analog (0-10V), (4-20 ma) and
digital outputs (RS-485 binary format). Either can be used to provide thickness results,
but analog is the preferred choice if high frequency (>100Hz) thickness is required.
Successful applications include:
• Process monitoring of wood thickness
• Quality control during concrete block manufacturing
• Separation distance between rollers
• Brake rotor thickness
Figure 14: Sheet and Web Thickness
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