How Do Laser Distance Sensors Work?
Laser sensors are photoelectronic sensors and, thanks to their contactless measuring principle and high accuracy, are well-suited to object detection, path, position and distance measurement. wenglor laser distance sensors work according to the transit time measurement principle and use laser triangulation. In both procedures, distances are measured with laser light and output as a distance value.
When to Use a Triangulation Sensor and When to Use a Transit Time Sensor
Triangulation Sensors for Short Distances
Transit Time Sensors for Long Distances
Possible Uses of Laser Sensors for Distance Measurement
Presence check
Thickness measurement
Diameter control
Edge counting
Positioning
Robot positioning
Stacking height monitoring
Parts measurement
Differential Measurement
Contrast Recognition
Monitoring of Two-Layer Materials
Sectors and Industries which Use Laser Distance Sensors
Triangulation Sensors
Transit Time Sensors
The Triangulation Principle
This technology enables distance sensors to detect very small details. The triangulation principle is used by the distance sensors CP, OCP, YP, P3 series and PNBC.
Do Triangulation Sensors Have a Blind Spot?
Sensors that operate according to the triangulation principle have a so-called blind spot. This is dependent on the distance from which the reflected light meets the receiving element (CMOS line). If the reflected light does not hit the CMOS line, no measurement can be taken. The blind spot is below the working range and means that objects located in this area are not detected and no measured values are output.
Working range: 40…160 mm
Blind apot: 0…40 mm
The CMOS Receiving Line
The CMOS line is a light-sensitive receiving element with a large number of pixels. It is used to evaluate the position at which the laser light hits the line. The electrical charge in the pixels of the CMOS sensors (Complementary Metal-Oxide Semiconductor) is converted into a voltage. The position of the object can be determined based on the light distribution on the CMOS line.
How to Install Triangulation Sensors Properly
Round, Glossy, Reflective Objects
When measuring shiny or round surfaces, it should be ensured during sensor installation that no direct reflections fall on the receiving element.
Tip: Align the sensor so that it is positioned in an axis with the round object.
Steps, Edges, Recesses
For all distance sensors, it should be ensured that the receiving beam has a direct line of sight and is not covered by an obstacle such as an edge, step, hole or gap.
Tip: Align the sensor orthogonally to the gap course!
Moving Objects
One example of moving objects to be measured is conveyor belts. It is important that the object moves orthogonally to the sensor. This prevents direct reflections to the receiver.
Tip: Install the sensor orthogonally!
Color Edges
When measuring objects with color transitions, so-called color edges, it is important that the color edge runs orthogonally to the sensor. This prevents color errors.
Tip: Install the sensor orthogonally!
Differences Between Spherical and Aspherical Lenses
Spherical Lens
The lens has a spherical surface
Incoming light on the edge area is more strongly refracted than in the central area
Bundling of the light beams leads to a loss of precision
Aspheric Lens
The lens has an uneven curvature
The light beam is evenly broken over the entire surface
Lens shape reduces imaging errors
Focus point is mapped precisely on the line
Very high measuring accuracy
Time of Flight Principle (ToF)
The ToF (Time-of-Flight) laser sensors for distance measurement combine reproducible measurement results, reliability and a large measuring range. This makes them suitable for a variety of applications at distances of up to one hundred meters with reflectors or ten meters to objects.
The Time-of-Flight measuring principle, also known as transit time measurement, calculates the distance L to the object via light pulses. The diode in the sensor emits laser pulses that are reflected by the object. The time interval from the emission of the light pulse to the object and back again is measured. The time T and the light speed C then provide the corresponding distance to the object.
The following physical formula is used to determine the distance:
The Time-of-Flight measuring principle is used by the distance sensors P1PY, P2PY, P1KY and OY.
The Most Important Facts About the Speed of Light at a Glance
Do ToF Sensors Have a Blind Spot?
ToF sensors have no blind spot. Objects can be detected below the setting range and the sensor switches, but cannot provide any measurement results.
At What Coverage of the Light Spot Does the Sensor Switch?
If ambient light, such as sunlight or illumination, increases, the object appears to become darker for the sensor. In such cases, a larger area of the light spot must hit the object to ensure reliable detection.
Due to the optics of the sensor, there is also a small proportion of scattered light that occurs outside the actual light spot. With highly reflective, glossy surfaces, this can lead to the object being detected before the light spot actually reaches it. It is therefore important to avoid disturbing, shiny structures near the light beam.
Transit Time Sensors with Reflector
This functional principle is particularly advantageous if incorrect measurements due to background objects are to be avoided. A typical application example is the control of overhead conveyor systems, where the distance to the vehicle in front must always be reliably recorded. Especially when cornering, measurements are prevented from being taken erroneously on objects in the background, as these could lead to incorrect control commands.
This technology is also ideal for applications requiring large working ranges.
Comparison of Transit Time and Triangulation Working Ranges
Key
Red area: Blind spot (objects are not reliably detected)
Green area: Working range (objects are reliably detected)
Yellow range: Setting range/measuring range (set switching points/measured values are displayed)
Output of Distance Values
Digital Switching Output
IO-Link
What Is Accuracy?
Precision | Precision, also known as repetition accuracy, can be determined by successive measurements under consistent conditions. A very precise value therefore delivers almost constant measurements. The precision of a sensor is quantified by reproducibility. |
|---|---|
Correctness | Correctness is a qualitative value. It is defined by linearity deviation, temperature drift, switch-on drift and switching distance deviation. |
The figure shows how correctness, precision and accuracy are related. The red dots represent successive measurements from a sensor, while the target indicates the correct value. If the measured values are far apart and far away from the target, this indicates a low precision and correctness. Ideally, measurements should be correct and accurate, meaning they are close together and within the target range.
Reproducibility and Linearity in Comparison: When is Which Value Used?
absolute measurement
positioning tasks
Starting point
A distance measurement is carried out and the maximum possible deviation is determined. It is always measured on the same object to rule out color errors. The ambient temperature may vary by 10 °C.
Values from the data sheet:
- Reproducibility: 3 mm
- Linearity deviation: 10 mm
- Temperature drift: 0.4 mm/K
Calculation
Precision (reproducibility) + correctness (linearity deviation, temperature drift) = accuracy
3 mm + 10 mm + (0.4 mm * 10 °C) = 17 mm
What Determines the Accuracy of the Measurement Results?
Time-of-flight laser distance sensors achieve high measuring ranges of up to 10 m on objects and 100 m on reflectors. Triangulation laser distance sensors, on the other hand, is very accurate. However, the measuring range is restricted to max. 1,000 mm. Various settings can be made to optimize the accuracy of the sensors for distance measurement depending on the application. This means that the accuracy can be further increased by filter functions.
Laser Classes and Their Modes of Action
Laser classes provide information on the potential hazard of the laser to humans. Sensors with laser light are divided into different laser classes according to EN 60825-1 depending on the degree of danger. A distinction is made between the common laser classes 1, 2, 2M, 3R and 3B. wenglor laser distance sensors only use laser classes 1 and 2, which are safe for the human eye.
| Description | |
|---|---|
| Laser class 1 | Class 1 laser devices are completely harmless to the human eye and no protective measures are required. |
| Laser class 2 | Class 2 laser devices are more powerful, but are also safe for short-term exposure. However, warnings must be affixed in this case. |
| Laser class 2M | Devices with laser class 2M are also harmless for short-term exposure. The difference to laser class 2 is that a hazard may arise with optical devices, such as a magnifying glass. |
| Laser class 3R | Class 3R laser devices can be hazardous when looking directly into the laser beam. For this reason, protective measures are required. |
| Laser class 3B | Class 3B laser devices are hazardous to the eyes and frequently also to the skin. For this reason, protective measures are required. |
Use of Red and Blue Lasers
wenglor’s laser distance sensors work with red or blue laser light. Whether red or blue light is used depends on the application. Red laser light has a wavelength of 650 nm. Blue lasers work with a wavelength of 405 nm and therefore have a shorter wavelength. This means that the blue laser beam penetrates less deeply into the object to be measured and delivers precise and stable results. Glowing surfaces in particular are not affected by the blue laser. Laser distance sensors with blue diode are very well suited for organic surfaces, polished metals, shiny plastic surfaces or dark paints.
What Is the Difference Between Normal Light and Laser Light?
Normal Light
| Dispersion direction | Light waves are dispersed in all directions |
|---|---|
| Wavelengths | Consist of many different wavelengths |
| Phase equivalence | Waves oscillate out of phase |
Laser Light
| Light waves are strongly directed |
| Consists of one wavelength (monochromaticity) |
| Waves oscillate synchronously |
Why Is There Red and Blue Laser Light?
Wavelength red: 640 – 675 nm
What Is Light?
What Is Color?
What Is a Laser?
The term laser stands for Light Amplification by Stimulated Emission of Radiation. A laser beam can be generated over a wide range of the optical spectrum. In simple terms, this means that directed light waves are bundled into a beam in high concentration.
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