27-12-2012, 01:01 PM
LASER TRIANGULATION SENSOR
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INTRODUCTION
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 determine the position of a target by measuring reflected from the target surface. A 'transmitter' (laser diode) projects a spot of light to the target, and its reflection is focused via an optical lens on a light sensitive device or 'receiver'. If the target changes its position from the reference point the position of the reflected spot of light on the detector changes as well. The signal conditioning electronics of the laser detects the spot position on the receiving element and, following linearization and additional digital or analogue signal conditioning, provides an output signal proportional to target position
The CCD element is a digital pixelised array detector, with 1,024 discrete voltages representing the amount of light falling on each pixel of the detector. A CCD element detector can carry 1,024 x 1,024 pieces of light intensity information. The intensity distribution of the imaged spot is completely 'viewed' with the help of a powerful DSP device, and image processing is then incorporated for the linear triangulation measurement. The post data processing of the intensity distribution enables almost all of the problems posed by non-ideal targets to be overcome.
WORKING 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 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.
CHARACTERISTICS OF LASER SENSORS
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.
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.
The DSP finds the one single pixel with the highest light intensity and uses an algorithm to perform sub-pixel resolution by interpreting the light intensity of adjacent pixels. The technology of thresholding is used to discard unwanted information pertaining to stray and secondary reflections, which would cause a PSD receiver to change its output. Smart CCD sensors also use closed loop control to adjust the power of the transmitting laser, according to the amount of reflected light received from the target. An optimum light intensity for the sensing element is achieved, regardless of the target colour or its surface texture.
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 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.
RESOLUTION
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.
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.
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.
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.
[attachment=45398]
INTRODUCTION
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 determine the position of a target by measuring reflected from the target surface. A 'transmitter' (laser diode) projects a spot of light to the target, and its reflection is focused via an optical lens on a light sensitive device or 'receiver'. If the target changes its position from the reference point the position of the reflected spot of light on the detector changes as well. The signal conditioning electronics of the laser detects the spot position on the receiving element and, following linearization and additional digital or analogue signal conditioning, provides an output signal proportional to target position
The CCD element is a digital pixelised array detector, with 1,024 discrete voltages representing the amount of light falling on each pixel of the detector. A CCD element detector can carry 1,024 x 1,024 pieces of light intensity information. The intensity distribution of the imaged spot is completely 'viewed' with the help of a powerful DSP device, and image processing is then incorporated for the linear triangulation measurement. The post data processing of the intensity distribution enables almost all of the problems posed by non-ideal targets to be overcome.
WORKING 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 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.
CHARACTERISTICS OF LASER SENSORS
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.
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.
The DSP finds the one single pixel with the highest light intensity and uses an algorithm to perform sub-pixel resolution by interpreting the light intensity of adjacent pixels. The technology of thresholding is used to discard unwanted information pertaining to stray and secondary reflections, which would cause a PSD receiver to change its output. Smart CCD sensors also use closed loop control to adjust the power of the transmitting laser, according to the amount of reflected light received from the target. An optimum light intensity for the sensing element is achieved, regardless of the target colour or its surface texture.
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 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.
RESOLUTION
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.
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.
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.
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.