Laser Triangulation Sensors
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 laser 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.
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
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, laser displacement sensors 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
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 range. 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
The resolution of a laser 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-to-peak 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.
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.
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.
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.
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 has provisions to synchronize heads eliminating this problem.
4) Advantages and Disadvantages
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.
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.
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 run-out 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