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Technology of Laser Sensors for Distance Measurement

Laser distance sensors measure positions and distances contactlessly with laser light. They are precise and can be used over long distances, as well as in close range. These sensors are ideal for precise position and distance measurement or for detecting objects regardless of color and surface.

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

Precise determination of distances in the close range up to 1 m

Detection of very small objects or differences in distance

Linearity deviation < 1 mm

Very fast measurements

Measurement on different shapes and surfaces

High precision down to the micron range

Transit Time Sensors for Long Distances

Determination of long distances up to 100 m with reflectors

Working range up to 10 m on objects

Linearity deviation > 10 mm

Resistant to interfering influences

Very high ambient light resistance

Reproducible measurement over 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

Woodworking industry

Boards must be milled to the correct width when making parquet floors. The wood surfaces vary from light to dark and from shiny to matt. The nominal width must be maintained so that the boards can be laid cleanly.

Laser distance sensor application image parquet plank measurement

Rail industry

Overhead lines of rail networks require regular maintenance. Special maintenance vehicles with lifting platforms are used for this purpose. To prevent excessive tilting or unintentional tipping of the vehicle when the work platform is extended, four laser distance sensors per carriage measure the exact distance from the vehicle to the rail head using triangulation with precision in the low micrometer range.

Battery industry

Coated, high-gloss films are used to manufacture batteries. In order to detect their level on winding devices, triangulation laser distance sensors measure the exact distance – regardless of shape, color or surface.

Machinery manufacturing

Welding nuts are used in car body construction, which are automatically welded to sheet metal parts to screw assemblies together. They are placed in a welding fixture on a mandrel before a robot positions and centers the body part.

Electronics industry

In the electronics industry, PCBs are fixed and conveyed laterally in transport modules, which can lead to bending. In order for the solder robot to compensate for this bend and create the ideal distance between the plate and the solder head, the bending of the PCBs must be measured.

Laser distance sensor application image for conductor track measurement

Automotive industry

During the automatic removal of flat metal sheets from a supply by a robot, several sheets can stick together due to the magnetic charge. The thickness of the sheets must be detected to ensure that only one sheet per assembly process is gripped.

Transit Time Sensors

Logistics

In logistics centers, shuttle systems must automatically deliver goods from the warehouse to production. Time of flight laser distance sensors with wintec integrated on the front detect in advance end positions or forward-moving shuttles in the field of vision up to ten meters ahead so that shuttles can slow down or stop.

Beverages industry

In the beverages industry, individual bottles and bottle containers must be placed in Pick-and-Place applications with gripper arms during the automated filling and packaging process.

Food industry

In the production of hard cheese, it is important to ensure that the blocks of cheese are placed exactly on the conveyor belt. Time-of-flight laser distance sensors with wintec are able to detect the cheese blocks in a fresh, still shiny condition regardless of the positioned angle.

Packaging industry

When filling and sealing transparent food trays, their position and presence on a multi-lane conveyor must be reliably recorded.

Metalworking industry

In the steel industry, red-hot tubes are transported for cooling on conveyor lines after being manufactured in casting furnaces. In order to control this process, the presence of hot blanks between 700 and 1,000 °C must be reliably detected.

Building materials industry

After production, fired and coated clay tiles are temporarily stored in magazines for cooling and drying. They are then removed and fed to quality control before the packaging plant.

The Triangulation Principle

The triangulation principle is a geometric measurement method that uses the triangular relationship. In this method, a light point is projected onto the object to be measured. The object reflects the light and hits a light-sensitive CMOS receiving element in the sensor at a certain angle. The position of the light spot on the CMOS line changes depending on the distance of the object. In this way, the distance to the object to be measured can be precisely determined even at small distances.

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.

Example CP24MHT80 laser distance sensor triangulation:
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.

The CMOS line enables highly accurate distance measurement and is typically used in laser distance sensors based on the triangulation principle.

How to Install Triangulation Sensors Properly

To achieve the most stable object detection and measurement possible, the following instructions must be observed when adjusting the sensor.

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:

L = ½ × C × T

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

The speed of light is a fundamental constant of physics. In vacuum, it is 299,792,458 m/s. Nothing moves as fast as light.

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?

The surface finish of the object plays a decisive role in determining which coverage of the light spot the sensor switches. Bright surfaces lead to switching of the ToF sensor even with low coverage of the light spot, as the number of photons required for detection of the light pulse is reached faster. Dark surfaces, on the other hand, require greater coverage to achieve the same effect.

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

The use of reflectors can significantly extend the area over which the time-of-flight sensors can be used. The ToF sensors focus exclusively on the light reflected by the reflector and effectively suppress all other signals. This ensures that measurements are only taken on reflectors, while reflective objects and other shiny surfaces are not detected as reflectors and ignored accordingly.

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

The sensor at the top of the image is a transit time sensor, while the sensor below operates according to the triangulation principle.

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

Distances can be taught in via digital switching outputs. As soon as the taught-in distance is reached, the sensor outputs a switching signal at the output. This enables objects to be detected and positions to be detected.

Analog Output

The distance value is output as a linearly proportional current (4…20 mA) or voltage value (0…10 V) via an analog output. The characteristic curve can be set within the entire measuring range by teaching in.

IO-Link

IO-Link technology is used around the world for standardized communication with sensors and actuators. This is point-to-point communication.

Industrial Ethernet

Industrial Ethernet is a generic term for all Ethernet standards for real-time data transmission between the control and sensor. Industrial Ethernet protocols include, for example, EtherCAT, Ethernet/IP or PROFINET.

What Is Accuracy?

High accuracy means that the anticipated measurement results are achieved. This term is only used for qualitative statements. This means that it is not a technical parameter. Accuracy is made up of precision and correctness. The accuracy always depends on the measuring principle used.

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

Linearity and reproducibility values are important for absolute measurements, such as determining the actual distance of an object or a diameter. A good reproducibility value provides repeatable values. High linearity ensures correct measured values. Overall, both linearity and reproducibility are important factors when it comes to obtaining correct and accurate measured values in absolute measurements.

positioning tasks

The sensor delivers reproducible measured values for repeated measurements. It always hits the same point or position, i.e. it is repeatable. This is crucial to ensure accurate and reliable positioning of an object. The main goal is to always place the object in the same place. Repetition accuracy is of great importance, while linearity is less important for positioning tasks. High precision is crucial here, and the correctness can be neglected.

Example Calculation for Accuracy with the P1PY101 Time-of-Flight Laser Distance Sensor

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.

Laser class 1
Description Class 1 laser devices are completely harmless to the human eye and no protective measures are required.
Laser class 2
Description 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
Description 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
Description Class 3R laser devices can be hazardous when looking directly into the laser beam. For this reason, protective measures are required.
Laser class 3B
Description 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

Divergent light beam with large spot diameter

Laser Light

Light waves are strongly directed

Consists of one wavelength (monochromaticity)

Waves oscillate synchronously

-> Strong bundling enables small light spot diameters at great distances.

Why Is There Red and Blue Laser Light?

The light spectrum consists of different wavelengths. Each has a different color. A color can be assigned to each wave in the color spectrum. Red light differs from blue light in its wavelength and energy density.

Wavelength blue: 380 – 500 nm
Wavelength red: 640 – 675 nm

What Is Light?

Light is the part of electromagnetic radiation visible to the human eye. The radiation propagates in different wavelength ranges when emitted by a light source, for example a light bulb. The wavelength range lies between UV radiation (shorter wavelengths) and infrared radiation (longer wavelengths).

What Is Color?

The color of objects is a subjective impression created by objects absorbing different wavelengths and reflecting others. These wavelengths represent different colors. The color reflected by the object can be perceived by the human eye.

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.

Differences Between Laser Distance Sensors and Ultrasonic Sensors

Distance sensors and ultrasonic sensors differ in the size of the detection range
Ultrasonic sensors work with a wide sonic cone
Laser distance sensors work with a fine laser beam

Product Comparison

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