The article is on the topic of diffuse laser triangulation proximity sensors. Click here for laser through beam proximity switches.
1. How do laser triangulation distance sensors work?
Diffuse laser triangulation proximity sensors are distance sensors that operate according to the principal of triangulation. The sensor consists of a laser diode and a photo detector. Laser light emitted by the diode, passes through a lens and is projected perpendicularly onto the target surface. A photo detector (either CMOS, CCD or PSD) similar to a digital camera image sensor, images the laser spot on the target surface. As depicted in the illustration below, the location of the laser spot on the photo sensitive device varies as a function of the distance between the target and the sensor. The sensor calculates the centroid of the laser spot. Owing to the triangulation principal, the position of the spot fully defines the distance to the target.
The above configuration is only suitable for detecting non-reflective targets (also called diffuse targets) because such targets scatter light in all directions so that the laser spot is visible at any angle. If the target is reflective (also called specular targets), the laser is reflected back into the diode and is not picked up by the detector. There are sensor configurations intended for measuring reflective surfaces. In such sensors, the laser is angled (angle of incidence) relative to the perpendicular to prevent the laser from reflecting back into the diode. Furthermore, the photo detector is placed at an equal and opposite angle (angle of reflection) so that the laser is reflected into the detector.
Do not confuse diffuse and retro-reflective sensors. Both sensor configurations illustrated above are diffuse sensors. Retro-reflective sensors, which are not the topic of the article, look similar to diffuse sensors but are in fact a type of through beam sensor in which the beam is reflected by a static mirror so that the light source and detector can be located in a single package. Through beam sensors do not measure distance, they are a type of Boolean proximity switch which detects whether the laser beam has been interrupted.
2. General characteristics of laser triangulation distance sensors
Laser triangulation proximity sensors are medium to high cost sensors capable of measuring targets at distances between 0.5 and 50 cm, with some models capable of ranges over 100 cm. Cost fluctuates significantly depending on measurement resolution. Their high sampling rate of 1-50 kHz enables them to measure at high velocities. The laser spot produced by laser triangulation sensors is of small size, enabling them to perform highly localized measurements, far beyond what is possible with most other non-contact sensors. They do not experience mechanical wear and therefore have an extremely long lifetime. Furthermore, they have excellent environmental protection (IP67). However, they are sensitive to contamination of the sensor window with dirt and are not suitable for operation in environments subject to high intensity background light.
3. Input and output signals
Laser triangulation proximity sensors operate on a 10-30 Vdc supply voltage. Unlike most sensors, which measure changes in intensity, triangulation sensors detect changes in position of the laser spot on the photo detector. The sensor then outputs either an analog (e.g. 0-10 V or 4-20 mA) or digital signal (serial communication). Some sensors include adjustable switching outputs which switch states when a threshold distance is passed.
4. Applications of laser triangulation distance sensors
Laser triangulation proximity sensors are widely used in industry for both high and low accuracy applications. Low accuracy applications include collision detection, part counting and fill level indication. High accuracy applications include measuring sheet thickness, part dimensions and surface profiling. Many of their low accuracy applications overlap with those of laser and ultrasonic time of flight sensors. However, time of flight sensors have a lower resolution meaning that laser triangulation proximity sensors have a clear advantage for high accuracy applications.
5. Typical specification
|Cost||Medium to high|
|Measurement range||5-500 mm (up to 2,500 mm available)|
|Laser spot size||20-2,000 µm|
|Ambient temperature||-10 to 50 °C|
|Supply voltage||1-30 Vdc|
|output voltage||Analog or digital|
|Passive / active||Active|
|Contact / non-contact||Non-contact|
6. Purchasing tips
- laser spot size: Depending on the model, spot size may vary from as small as 20 µm to as large as several millimeters. Although high measurement resolution and small spot size normally go hand in hand, it is important to check laser spot size when purchasing a sensor for highly localized measurements.
- Wavelength: Most laser triangulation proximity sensors emit a red or infrared pulse. However, sensors are also available with blue laser pulses, intended for applications where there is significant red or IR background light which would otherwise interfere with the sensor.
- Diffuse/retro-reflective: Do not confuse diffuse sensors with retro-reflective sensors. It is easy to spot the difference between diffuse and retro-reflective sensors because the latter include a mirror for installation opposite the sensor.
- Switching outputs: Some sensors include adjustable Boolean switching outputs which switch logical levels when the programmed distance has been crossed.
- Photo detector: The photo detectors are either CCD, CMOS or PSD devices. PSD devices are sensitive to irregular surfaces and so CCD and CMOS devices are advised if the target surface is not flat.
- Reflective targets: A reflective target causes the incoming light to be reflected back into the diode. There are specialized sensors for detecting reflective surfaces, in which the diode is angled so that the laser reflects into the detector.
7. Advantages of laser distance sensors
Laser triangulation proximity sensors:
- Have extremely high spatial resolution and accuracy, making them a great choice for high accuracy applications. Their resolution is sufficient for many surface profiling applications.
- Operate at high sampling rates (typically 1-50 kHz, though as high as 150 kHz), enabling operation at high velocities.
- Have a small laser spot size, making them suitable for extremely localized measurements, including surface profiling applications.
- Have an ingress protection rating of IP67.
- Are available in both red and blue wavelength to prevent interference (e.g. if the environment contains red/IR light, the blue wavelength can be used).
8. Disadvantages of laser distance sensors
Laser triangulation proximity sensors:
- Have a limited measurement range and are not capable of measuring targets extremely close to the sensor face.
- Are sensitive to intense background light. Most sensors can perform with background light intensities up to 50,000 lux. Direct sunlight can surpass 100,000 lux.
- May struggle to detect transparent objects.
9. Application tips
- Surface profiling: Many laser triangulation sensors have small laser spot sizes (as small as 20 µm), enabling them to perform extremely localized measurements. Their high measurement resolution, coupled with the extremely localized measurements, makes laser triangulation sensors extremely useful for surface profiling.
- Spot size and range: Laser spot size increases as a function of the distance between the target and sensor. It is therefore recommended that for highly localized measurements (e.g. for surface profiling), the sensor is operated at a short distance from the target.
- Orientation: Correct orientation is important for achieving accurate measurements. Where possible the sensor should be orientated perpendicular to the direction of motion. Furthermore, it is vital that the returning beam dose not collide with the target (e.g. because of a steep change in height).