Laser Displacement Sensor Basic Guide

Introduction

The laser displacement sensor is a device that uses laser technology for measuring. It consists of a laser, a laser detector and a measuring circuit. It is a new type of measuring instrument. For example, it can accurately measure the position and displacement of a object in a non-contact method.
Laser light has the excellent characteristics of good straightness. Thus the same laser displacement sensor has higher accuracy than the known ultrasonic sensor. But the laser generating device is relatively complex and large in size. Therefore the application range of the laser displacement sensor is strict.

1 Basic Working Principle

1.1 Measurement Objects

Laser beam from the laser diode is irradiated to the surface of the measuring object, and the reflected light passes through a set of lenses and is projected onto the photosensitive element matrix. The photosensitive part can be a CCD(charge-coupled device), CMOS(complimentary metal oxide semiconductor) or a PSD(position-sensitive detector) element. And the intensity of the reflecting light depends on the surface characteristics of the object. What’s more, the laser displacement sensor can also measure more indexes. Such as thickness, vibration, distance, diameter and other geometric quantities of the detecting object.

1.2 Measurement Methods

According to the measurement principle, there are laser triangulation method and laser echo analysis method. The former method is generally suitable for high-precision and short-distance measurement, and the latter method is suitable for long-distance measurement. The following are the details of there methods.

1) Triangulation Method

The simplest triangular displacement measurement system is to emit a beam of light to the surface of the object. Then observe the position of the reflection point through imaging in the other direction. Finally detect that the displacement of the object. Since the incident and reflected light form a triangle, this method is  triangulation method. According to the relationship between the angle between the incident light and the surface of the object. It includes direct type and inclined type.

Direct Type

Light emitted by the laser, after being focused by the condenser lens, is vertically incident on the surface of the measured object. Except that, there is need to noted that the movement of the object or the surface change. Because they will cause the incident light spot to move along the optical axis. Receive the scattered light from the incident point of the lens and image it on the sensitive surface. Where is the position detector (such as PSD, CDD) of the light spot. However, the laser beam of the sensor is perpendicular to the object surface. So there is only one accurate focus position. And meanwhile, the images at the other positions are in different degrees of high focus state. In addition, high focus will cause the dispersion of the image point. It reduces the measuring accuracy.

Reflection Angle

In order to improve accuracy, θ1 and θ2 must meet: tgθ1=Utgθ2
For the formula, U is the lateral magnification. At this time, the side points within a certain depth of field can display on the detector. It ensures the accuracy.

Displacement Formula

Supposing the displacement of the light spot on the imaging surface is x’. Then use the proportional relationship between the sides of similar triangles. Calculate the displacement of the measured surface according to the following formula:

Where, α is the distance from the intersection of the laser beam optical axis and the receiving optical axis to the front main surface of the receiving lens, and b is the distance from the rear main surface of the receiving lens to the center point of the imaging surface. θ1 is the angle between the optical axis of the laser beam and the optical axis of the receiving lens, and θ2 is the angle between the measured normal line and the optical axis of the receiving lens.

Inclined Type

Laser light is incident on the measured surface at a certain angle. With the normal direction of the measuring surface, it is necessary. And the scattered light or reflected light of the measuring light spot is also received. The conditions should be:tg(θ1+θ2)=Utgθ3
Assuming the image of the light spot moves x’ on the sensitive surface of the detector. Then use the proportional relationship of similar triangles. So the moving distance of the object surface along the normal direction is:

Where,
θ1 is the angle between the optical axis of the laser beam and the normal of the measured surface.
The angle between the optical axis of the imaging lens and the normal of the measured surface is θ2.
Angle θ3 between the optical axis of the detector and the optical axis of the imaging lens.

Working Process

Laser transmitter shoots the visible red laser light to the surface of the target object through the lens. For example, the laser light scattered by the surface of the object passes through the receiver lens.  We will use the internal CCD(charge-coupled devices) linear camera. According to different distances, the camera can have different angles to trace this light spot. After got this angle and the known distance between the laser and the camera. Then the digital signal processor can calculate the target distance.

The Output

And meanwhile, analog and digital circuits process the position of the light beam at the receiving element. Then calculate the corresponding output value by the microprocessor analysis. Finally, the standard data signal outputs proportionally in the analog window set by the user. If takes the switch output, it will turn on within the set window and turn off outside the window. In addition, the analog quantity and the switch quantity output can set up the detection window independently.

Sound Features

The maximum linearity of laser displacement sensor adopting the triangulation method can reach 1um. While the resolution can reach the level of 0.1um. For example, the ZLDS100 type sensor can achieve better performance. Such as high resolution of 0.01%, high linearity of 0.1%, and high response of 9.4KHz. According to it, the sensor can adapt to harsh environments.

2) The Echo Analysis

Method Overview

Use the principle of echo analysis to measure the distance to achieve a certain degree of accuracy. The inside of the laser sensor consists of a processor unit, an echo processing unit, a transmitter, and a receiver. The laser displacement sensor emits one million laser pulses. The speed is per second through the laser transmitter to the detection object and returns to the receiver. The processor calculates the requiring time for the laser pulse. It can meet the detection object and return to the receiver to calculate the distance. The output value is the average output of thousands of measurement results. It measures by the so-called pulse time method.

Detection Distance

The laser echo analysis method is suitable for long-distance detection. However, the measurement accuracy is lower than that of the laser triangulation method.  What’s more, the farthest detection distance can reach 250m.

2 Measurement Applications in Product Line

Measured targets include many physical quantities. Such as length, distance, vibration, speed, and orientation. Even the flaw detection and atmospheric pollutant monitoring.

1) Size determination

Position identification of small parts
Monitoring of the presence of parts on the conveyor belt
Detection of material overlap and coverage
Control of the robot position (tool center position)
Device status
Device position (through small holes)
Liquid level monitoring
Thickness measurement
Vibration analysis
Crash test measurement
Automobile related test

2) Thickness Measurement of Metal Flakes and Thin Plates

Measures the thickness of a thin metal sheet (thin plate). For example, detection of changes in thickness can help find wrinkles, small holes or overlaps to avoid machine malfunctions.

3) Simultaneous Measurement

Measuring more values includes angle, length, inner and outer diameter eccentricity, conicity, concentricity and surface profile.

4) Stuff Length Measurement

Put the target stuff on the conveyor belt at the designated position. And then the laser sensor detects the it and measures it simultaneously with the laser scanner. Fnally obtains the length.

5) Inspection of Uniformity

Place several laser sensors in a row in the tilt direction of the workpiece to take measurement. Then directly output the measurement value through one sensor. In addition, you can also use a software to calculate the measurement value. And finally read the result according to the signal or data.

6) Inspection of Electronic Components

First use two laser scanners to place the component under test between them. Then read the data through the sensor. Finally detect the accuracy and completeness of the component size.

7) Inspection of Filling Level on the Production Line

Integrate laser sensors into the manufacturing of the filling product. When the product passes the sensor, it can be detected whether it is full. Then the sensor uses the extension program.  It is the laser beam reflection surface that accurately identify whether the filling product is good.  Even the quantity of the product.

8) Measuring the Straightness of the Object

Installation

First, have 2-3 laser displacement sensors to perform combined measurement. Then install them on a straight line parallel to the production line. And determine the distance between the three laser displacement sensors. It is based on the measurement accuracy you need. Finally, you need to make this object move in a direction parallel to the installation line.

Measuring Process

Supposing the production line and the sensor installation line are parallel. The greater the difference between the distances measured by the three sensors. The worse the straightness of the object, and the smaller the difference among the sensors. It indicates the straightness of the object. Calculate a straightness percentage according to the length of the object and the distance between the three sensors. So as to obtain an effective  significant signal output by detecting the object straightness.

3 Laser Displacement Sensors Advantages

1) High Resolution and Repeatability

Even in a larger working range, the sensor can still maintain a higher resolution and repeatability.

2) Stable Performance

Sensors are less affected by the material and surface characteristics of the measured object. In addition, it can directly measure the highly reflective, diffuse reflective and rough surfaces of different materials. What’s more, there is no spraying developer powder (Except for a few special materials).

3) Measurement of Holes and Complex Geometric Surfaces

Coaxiality of the laser sensor enables high-precision measurement of deep holes, narrow slots, grooves and blind holes.

4) Large Measurable Angle

170° (±85°) in space is the maximum measurable angle range of the laser sensor. While this technology enables the sensor to truly restore every minute detail of the complex surface.  Cause fewer damages to the measured object.

4 Alternative Types of Laser Displacement Sensor

1) Eddy Current Displacement Sensor

Resolution: Maximum resolution of the eddy current sensor
It can reach 0.1um. And it is basically equivalent to the laser displacement sensor.
Linearity: It is generally low, about 1% of the range. Moreover, the high-end laser displacement sensor is generally 0.1%.
Measurement Conditions: Eddy current sensor requires the target object to be a conductor and non-magnetic. That is, a non-magnetic conductor, such as aluminum, copper, etc. The laser displacement sensor is suitable for whether the object is magnetic or conductive. And there are intersections between the two.

2) Capacitive Displacement Sensor

Accuracy of capacitive displacement sensors is very higher. But the range of them is very small and generally less than 1mm. And the range of laser displacement sensors can be up to 2m.

3) Optical Fiber Displacement Sensor

Working principle of the optical fiber displacement sensor is measuring the change.  They include the light flux and light intensity reflected on the surface of the object due to the displacement. And the probe consists of two parts: a transmitting fiber and a receiving fiber. For the displacement and vibration of small objects, the conventional non-contact displacement sensor doesn’t good. Because the limitation of the reflection area and the measuring result are not ideal. But the optical fiber displacement sensor can be made into a very small probe (minimum 0.2mm diameter). What’s more, it can also be made into a form of linear transmission and reception. Calculate the displacement value by measuring the degree of obstruction of the optical fiber during the displacement process. The accuracy can reach 0.01um, and the range can be up to 4mm.

5 Famous Laser Displacement Sensors Suppliers

IFM Efector, Inc.
Kinequip, Inc.
Mod-Tronic Instruments Limited
Micro-Epsilon
Wenglor sensoric LLC
Automation Products Group, Inc.
Polytec, Inc.
Limab
Baumann Machinery
Linear Measurement Instruments Corp.
Zygo Corporation
Industrial Controls
MTI Instruments, Inc.
LMI Technologies Inc.
Digi-Key
Diamond Technologies, Inc.
Ergonomic Partners
Schmitt Industries, Inc.
Baumer Ltd.
Rockwell Automation
RIEGL USA, Inc.
Steven Engineering
PICS INC
Finch Automation

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