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Dec 31 2019

What is Laser Sensor: Working, Function and Application

I Introduction

Laser sensor is a kind of sensor which uses laser technology to measure. It is generally composed of laser, optical parts and photoelectric devices. It can convert the measured physical parameters (such as length, flow, speed, etc.) into optical signals, and then use photoelectric converter to convert the optical signals into electrical signals. Through the filtering, amplification and rectification of corresponding circuits, the output signals can be obtained, so as to calculate the measured quantity.

Laser technology has the characteristics of strong direction, high brightness and good monochromaticity. It is widely used in industrial and agricultural production, national defense and military, medical and health, scientific research and other aspects, such as distance measurement, precision detection, positioning, etc., as well as length benchmark and optical frequency benchmark.

Laser Distance Sensor Overview

Catalog

I Introduction

II What is Laser?

 2.1 The Concept of Laser

 2.2 Important Characteristics of Laser

 2.3 Types of Laser

 2.4 What can Laser Sensor Detect?

III Laser Displacement Sensor

 3.1 What is Laser Displacement Sensor

 3.2 How Does Laser Displacement Sensor Work?

 3.3 Application of Laser Displacement Sensor

 3.4 What are the Parameters to Know When Choosing a Laser Displacement Sensor?

IV Laser Distance Sensor

 4.1 Classification of Laser Distance Sensors 

 4.2 Measuring Principle of Different Laser Distance Sensors

 4.3 Application of Laser Distance Sensor

V Laser Sensor Application Case

II What is Laser?

2.1 The Concept of Laser

Laser light is different from ordinary light. (See more about light and photoelectric effect in the article introducing light sensor and photoresistor)We need to use laser to produce laser light. In the normal state, most of the atoms in the laser are in stable low energy level E1. Under the action of appropriate frequency of external light, the atoms in low energy level absorb photon energy to excite and transition to high energy level E2. The photon energy E = e2-e1 = h V, where h is the Planck constant and V is the photon frequency. On the contrary, when the frequency of light is V, the atom in level E2 will jump to the low energy level to release energy and emit light, which is called stimulated radiation. First of all, the laser makes the atoms of the working materials abnormally in the high-energy level (i.e. inversion distribution of the particle number ), which can make the stimulated radiation process dominant, so that the induced light with the frequency of V can be enhanced, and the large stimulated radiation light can be produced through the avalanche amplification of the parallel reflector, which is called laser light for short.

Laser technology

Figure1. Laser

2.2 Important Characteristics of Laser

(1)High directivity, small divergence angle of light speed, the laser beam extends only a few centimeters from a few kilometers away.

(2)High monochromaticity, the frequency width of laser light is more than 10 times smaller than that of ordinary light.

(3)High brightness, laser beam convergence can produce temperatures up to several million degrees.

2.3 Types of Laser

Laser can be divided into four types according to working substance:

(1)Solid state laser

Its working substance is solid. Ruby laser, neodymium doped yttrium aluminum garnet laser (i.e. YAG laser) and neodymium glass laser are commonly used. Their structures are basically the same, characterized by small and solid, high power. At present, neodymium glass laser is the device with the highest pulse output power, which has reached tens of megawatts.

(2)Gas laser

Its working substance is gas. Now there are various kinds of gas atoms, ions, metal vapor, gas molecular lasers. Commonly used are carbon dioxide laser, helium neon laser and carbon monoxide laser, whose shape is like a common discharge tube, characterized by stable output, good monochromaticity, long life, but small power, low conversion efficiency.

(3)Liquid laser

It can be divided into chelate laser, inorganic liquid laser and organic dye laser, the most important of which is organic dye laser. Its main feature is that the wavelength is continuously adjustable.

(4)Semiconductor laser

It is a younger laser, and the more mature one is GaAs laser. It is characterized by high efficiency, small size, light weight and simple structure, and is suitable for carrying on airplanes, warships, tanks and infantry. It can be made into range finder and sighting device. However, the output power is small, the directivity is poor, and it is greatly affected by the ambient temperature.

2.4 What can Laser Sensor Detect?

(1) Laser measurement of length

Precise measurement of length is one of the key technologies in precise machinery manufacturing industry and optical processing industry. Modern length measurement is mostly based on the interference phenomenon of light wave, and its accuracy mainly depends on the monochromaticity of light. Laser is the most ideal light source. It is 100 thousand times purer than the best monochromatic light source (krypton-86 lamp). Therefore, the laser measurement range of length is large and the accuracy is high. According to the optical principle, the relationship between the maximum measurable length L of monochromatic light and wavelength λ and spectral line width δ is L = λ 2 / δ. The maximum measurable length of krypton-86 lamp is 38.5cm. For a long object, it is necessary to measure in sections to reduce the accuracy. If He-Ne gas laser is used, it can measure tens of kilometers at most. Generally, the length within several meters can be measured with an accuracy of 0.1 μ M.

Laser Measure

Figure2. Laser Measure

(2) Laser measurement of distance

Its principle is the same as that of the radio radar. After the laser is aimed at the target, the round-trip time is measured, and then the round-trip distance is obtained by multiplying the speed of light. Because of the advantages of laser, such as high directivity, high monochromaticity and high power, these are very important for the measurement of long distance, the determination of target orientation, the improvement of signal-to-noise ratio of the receiving system, and the guarantee of measurement accuracy, so the laser rangefinder is paid more and more attention. The lidar developed on the basis of the laser rangefinder can not only measure the distance, but also the azimuth, velocity and acceleration of the target. It has been successfully used in the ranging and tracking of the artificial satellite. For example, the lidar using ruby laser has a distance measuring range of 500-2000 km with an error of only a few meters. At present, ruby laser, neodymium glass laser, carbon dioxide laser and Gas laser are often used as the light source of laser rangefinder.

Measuring Distance with Laser Sensor

Figure3. Measuring Distance with Laser Sensor

(3) Laser measurement of thickness

Based on the principle of triangle ranging, a precise laser ranging sensor is divided at the upper and lower part of the C-frame. The modulated laser emitted by the laser hits the surface of the measured object. By sampling the signal of the linear CCD, the distance between the measured object and the C-frame is synchronously obtained by the linear CCD camera under the control of the control circuit. The thickness of the middle measured object is calculated by the data fed back by the sensor. Because the detection is continuous, the continuous dynamic thickness of the measured object can be obtained.

Thickness measuring with laser sensor

Figure4. Thickness Measuring with Laser Sensor

  • Thickness measurement by single laser displacement sensor

Put the measured body on the measuring platform, measure the distance from the sensor to the platform surface, then measure the distance from the sensor to the measured body surface, and measure the thickness after calculation. It is required that there is no air gap between the measured body and the measuring platform, and the measured body is not cocked. These strict requirements can only be achieved offline.

  • Thickness measurement by double laser displacement sensor

A laser displacement sensor is installed above and below the measured body respectively, and the thickness of the measured body is d = C - (a + b). Among them, C is the distance between two sensors, a is the distance between the upper sensor and the measured body, and B is the distance between the lower sensor and the measured body. The advantage of this method for on-line thickness measurement is that it can eliminate the influence of the vibration of the measured body on the measurement results. But at the same time, there are requirements for sensor installation and performance. The conditions to ensure the accuracy of measurement are that two sensor beams must be coaxial and that two sensor scans must be synchronous. Coaxiality is realized by installation, and synchronization depends on the selection of laser sensor with synchronization end.

Thickness Measurement

Figure5. Thickness Measurement

III Laser Displacement Sensor

3.1 What is Laser Displacement Sensor

The laser displacement sensor is called the eyes of the robot and machine, and has an irreplaceable role in welding, blank manufacturing, mechanical processing, heat treatment, loading and unloading, assembly and other operations. So, what is a laser displacement sensor?

The laser displacement sensor is a sensor that uses laser technology for measurement, and is composed of a laser, a laser detector, and a measurement circuit. As a new type of measuring equipment, the laser displacement sensor can accurately measure the position, displacement and other changes of the measured object, and can also measure precise geometric measurements such as displacement, thickness, vibration, distance, and diameter.

3.2 How Does Laser Displacement Sensor Work?

The laser displacement sensor can accurately and non-contactly measure the position, displacement and other changes of the measured object, and is mainly used to measure the displacement, thickness, vibration, distance, diameter and other geometric quantities of the object.

According to the measurement principle, the principle of laser displacement sensor is divided into laser triangulation method and laser echo analysis method. Laser triangulation method is generally suitable for high-precision and short-distance measurement, while laser echo analysis method is used for long-distance measurement.

The following is the introduction to two measurement methods of laser displacement sensor principle.

  • Triangulation

Laser Displacement Sensor

Figure6. Laser Displacement Sensor

The laser emitter shoots the visible red laser to the object surface through the lens, and the laser reflected by the object passes through the receiver lens, which is accepted by the internal CCD linear camera. According to different distances, the CCD linear camera can "see" this light point at different angles. According to the distance between the laser and the camera known from this angle, the digital signal processor can calculate the distance between the sensor and the measured object.

At the same time, the position of the beam in the receiving element is processed by analog and digital circuits, and the corresponding output value is calculated by microprocessor analysis, and the standard data signal is output in proportion in the analog quantity window set by the user. If switching value output is used, it will be conducted in the set window and cut off outside the window. In addition, independent detection window can be set for analog quantity and switch quantity output.

  • Echo analysis

The laser displacement sensor can achieve a certain degree of accuracy by using the echo analysis principle to measure the distance. The sensor is composed of processor unit, echo processing unit, laser transmitter and laser receiver. The laser displacement sensor emits one million pulses per second through the laser transmitter to the detector and returns to the receiver. The processor calculates the time required for the laser pulse to meet the detector and return to the receiver, so as to calculate the distance value. The output value is the average output of thousands of measurement results. It is the so-called pulse time method. The laser echo analysis method is suitable for long-distance detection, but the measurement accuracy is lower than the laser triangulation method, and the longest detection distance can reach 250m.

3.3 Application of Laser Displacement Sensor

(1) Dimension measurement: position identification of small parts; monitoring of whether there are parts on the conveyor belt; detection of material overlapping and covering; control of manipulator position (tool center position); device state detection; detection of device position (through the small hole); monitoring of liquid level; thickness measurement; vibration analysis; collision test measurement; automobile related test, etc.

(2) Thickness measurement of sheet metal: laser sensor measures the thickness of sheet metal. Thickness change detection can help to detect wrinkles, small holes or overlaps to avoid machine failure.

(3) Cylinder measurement: angle, length, eccentricity of inner and outer diameter, conicity, concentricity and surface profile.

Application of Laser Displacement Sensor

Figure7. Application of Laser Displacement Sensor

(4) Length measurement: place the measured component on the conveyor belt at the designated position, the laser sensor detects the component and simultaneously measures it with the triggered laser scanner, and finally obtains the length of the component.

(5) Uniformity check: place several laser sensors in a row in the tilt direction of the workpiece movement to be measured, and directly output the measurement value through one sensor. In addition, a software can be used to calculate the measurement value and read out the result according to the signal or data.

(6) Inspection of electronic components: two laser scanners are used to place the tested components between them. Finally, the data is read out by the sensor, so as to detect the accuracy and integrity of the component size.

(7) Inspection of filling level in production line: laser sensor is integrated into the production and manufacturing of filling products. When the filling products pass through the sensor, it can detect whether the filling is full. The sensor can accurately identify whether the filling product is qualified and the quantity of the product by using the extended program of laser beam reflecting surface.

3.4 What are the Parameters to Know When Choosing a Laser Displacement Sensor?

Some parameters that must be understood when selecting a laser displacement sensor are very important.

(1) Resolution: generally refers to the minimum range of the sensor, that is, the maximum recognition rate of the sensor. If the parameter is marked as 1mm, then the resolution is equal to 1mm.

(2) Repeatability: We must know that even if the measured object is at rest, the measured value will fluctuate slightly. The error margin of repeated measurement of the measured object at the same position in the static state is the repeat accuracy. For example, if the parameter is marked as 1μm, the repeat accuracy of the sensor is 1μm.

(3) Full range (effective range): the rated effective range of the sensor. When selecting a sensor, we must select the sensor that contains the effective range according to the required detection distance.

(4) Linear accuracy: the error between the measured value and the actual displacement. Linear accuracy is expressed as a percentage, but since the range is a range and the measurement accuracy is more difficult to reach the apex of the range, most sensors will mark the linear accuracy of the apex of the range to intuitively reflect the performance of the sensor.

(5) Sampling frequency/sampling period: frequency refers to the number of measurements per second. The higher the frequency, the shorter the time it takes to make a measurement. The shorter the measurement time, the more suitable it is for the detection of high-speed moving objects.

(6) Average sampling times: even in the static state, there will be slight measurement fluctuations. At this time, multiple measurements are required to calculate the average number to make the measured value stable and accurate.

Laser Sensor

Figure8. Laser Sensor

IV Laser Distance Sensor

Laser ranging is one of the earliest applications of laser. This is because the laser has many advantages such as strong directivity, high brightness, and good monochromaticity. Before 1965, the Soviet Union used a laser to measure the distance between the earth and the moon (384401km) with an error of only 250m. In 1969, the Americans landed on the moon with a retro-reflector on the lunar surface. They also used a laser to measure the distance between the earth and the moon, with an error of only 15cm.

The basic principle of using laser transmission time to measure the distance is to determine the target distance by measuring the time required for the laser to travel to and from the target.

Laser Distance Sensor

Figure9. Laser Distance Sensor

4.1 Classification of Laser Distance Sensors 

Laser distance sensor technology is divided into absolute distance measurement method and micro displacement measurement method according to the measurement range. Subdivided according to the measuring method, the absolute distance ranging method mainly includes pulse laser ranging and phase laser ranging, and the micro displacement measuring method mainly includes triangulation laser ranging and interferometric laser ranging.

4.2 Measuring Principle of Different Laser Distance Sensors

(1) Pulse Laser Distance Sensor

A pulse laser with a very short duration is emitted by a pulsed laser, and after reaching the target to be measured after the distance to be measured, part of the energy will be reflected back. The reflected pulsed laser is called an echo. The echo returns to the rangefinder and is received by a photoelectric detector. According to the interval between the main wave signal and the echo signal, that is, when the laser pulse travels from the laser to the target to be measured, the distance of the target to be measured can be calculated. 

(2) Phase laser Distance Sensor

The emitted laser light is emphasized, and the phase change of the modulated signal is used when the laser is propagated in space. According to the wavelength of the modulated wave, the distance represented by the phase delay is calculated. That is, the indirect method of phase delay measurement is used instead of directly measuring the time required for the round trip of the laser to achieve distance measurement. The accuracy of this method can reach the millimeter level.

Working principle of laser distance measuring device

Figure10. Working Principle of Laser Distance Measuring Device

(3) Triangulation Laser Distance Sensor

As mentioned above, this measurement principle is that the light emitted by the laser is focused on the surface of the measured object after being focused by the condensing lens, and the receiving lens receives the scattered light from the incident light spot and images it on the photoelectric  position detector On the sensitive side. When the object moves, the relative distance of the object movement is calculated by the displacement of the light spot on the imaging surface. The resolution of triangulation laser ranging is very high, which can reach the order of microns.   

Triangulation Principle

Figure11. Triangulation Principle

(4) Interferometric Laser Distance Sensor

By moving the measured target and measuring the coherence, the distance increment measurement is completed by counting, so the sensitivity of the interferometric measurement is very high, which can reach the nanometer level.

4.3 Application of Laser Distance Sensor

The laser distance sensor is mainly used for: monitoring the position of moving objects; measuring the railway contact network, measuring the boundary of buildings; measuring unsuitable objects; industrial automation and intelligent production management; vehicle speed and flow statistics; industrial monitoring signal trigger control; tower crane XY positioning of crane; automatic target distance control; monitoring of ship's safe docking position; positioning of container; measurement of vehicle's safe distance; measurement of overhead cable and height limitation; measurement of width of boxes on conveyor belt.

V Laser Sensor Application Case

(1) Over-limit detection of vehicle width and height

The laser sensor is used for rapid measurement, the network core of the PC industrial control computer and the visual programming software VB are used for real-time data transmission and processing, and the friendly interface control software is also designed. Field test data shows that the system has good real-time performance and high measurement accuracy, and has certain practical value.

(2) Expressway toll station

Used in highway toll stations to count and protect vehicles. Malaysian Teras has applied hundreds of BEA laser sensors to its manual and automatic toll station systems. The laser sensor uses the time-of-flight (TOF) measurement principle, which can form 4 planes in the detection area to detect the vehicle. At the same time, the product also has functions such as anti-collision and vehicle safety protection. Compared with the traditional light curtain, the laser sensor has the advantages of high sensitivity, high accuracy, easy installation, high cost performance and strong stability.

(3) Google's second-generation unmanned vehicle

In addition to the laser sensor on the top, Google’s second-generation driverless car prototype is still quite obvious, and the other sensors are set very concealed.

The front, rear and sides of the vehicle are clearly marked with the Google unmanned vehicle logo. The driving principle of Google's unmanned vehicle is to continuously collect various accurate data of the vehicle itself and the surroundings through many sensors installed around the car, analyze and calculate it through the processor in the car, and then control the driving of the car according to the calculation results . Unmanned vehicles will use GPS equipment and sensors to accurately locate the vehicle's position and speed, and judge pedestrians, vehicles, bicycles, signal lights and many other objects around it.

Google's Self-drving CarFigure12. Google's Self-drving Car

The roof of this Lexus is equipped with a 360° rotating laser holographic sensor, which can sense the front, side and rear conditions of the car almost simultaneously. The data collected by the sensor will be input to the processor located on the right rear side of the vehicle through the green data line. This laser sensor can also allow unmanned vehicles to be accurately positioned globally. The original L-shaped Lexus logo on the front of the car was also removed and replaced with a radar sensor; it was used to measure the distance ahead and the speed of the vehicle in order to determine the condition of the vehicle ahead and control the safe acceleration and deceleration of the vehicle.

The wheel hub of the tire is also equipped with a position sensor, which is used to detect wheel rotation and help the vehicle to locate. The heart of Google's unmanned vehicles-the processor is located on the right rear side of the vehicle, the data information from each sensor will be transmitted here through the data wire, and analyzed and processed through the software in order to accurately sense and judge the difference between the unmanned vehicles object. In addition to analyzing and judging the current position of objects around the unmanned vehicle, the unmanned vehicle also needs to be calculated by software to accurately predict the possible next position of each object. Finally, the unmanned car will make safe driving decisions based on all the collected data, including controlling the speed of the car and the surrounding distance.

 

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