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.
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.
This simplified diagram visualizes the difference between two different acceptance angle sensors
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 (depicted with a lower number) the better in most cases because greater resolution may be obtained. To achieve the highest sensitivity, it’s ideal to have the laser beam traverse across the complete detector length over the application measurement range. Sensitivity is determined by the slope of the sensor output response.
The output of two sensors with different sensitivities is depicted in the graph. Please note that the slope of each curve represents the respective sensitivity factor with Curve A being twice as sensitive.
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 a specific formula:
Resolution = Sensitivity x Noise
Based on the formula, it’s evident that for a fixed sensitivity the resolution is solely dependent upon the noise of the system. The lower the noise the better the resolution.
The amount of noise depends on the system’s 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.
Our 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 figures below show the difference in the output of two identical systems with different low pass filters. All of our laser triangulation systems have software adjustable low pass filters for easy adjustment in the field.
Amplifier output noise with 20kHz low pass filter
Amplifier output noise with 100Hz low pass filter
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 our laser sensors have adjustable filter settings. The appropriate filter should be selected for the application to prevent any attenuation of the output. Our application engineers can assist in selecting the appropriate filter settings.
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 repeatedly 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. Our 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 major contributor. The linear response of our sensors is very repeatable. Calibration reports provide data that can be used to correct for the non-linearity of a system with inexpensive computers and correction software, resulting in improved accuracy if needed.