3D Laser Scanner

Lütfi Mutlu Dokuz Eylül Üniversitesi 02.01.2012

1/7

MEC 5015 - Sensors and Interfacing Techniques

Using Laser Measurement Sensor for 3D Scanning

by Lütfi Mutlu

Purpose: Using laser measurement sensor (LMS) for three dimensional scanning, also methods

for calibration of the laser sensor will be studied.

1. ABSTRACT

There are many needs for the ability to fast acquire 3D data from environmental surroundings,

such as navigation, mapping, localisation and robot mobility, fire and police planning, urban

planning, but the technology for acquiring dense, wide ranging, accurate 3D data is too

expensive to be used widely [1]. In this study laser measurement sensor is used for three

dimensional scanning of the objects that passing through a conveyor line (Fig. 1). A Visual Basic

program is used for interface and collect data from the sensor. Then data are projected in

MATLAB with a M-file.

Fig. 1: Sensor setup view

2. MATERIAL

Windows PC

LMS100 laser measurement sensor

Visual Basic and MATLAB programs

Conveyor line and sample objects

12V battery for LMS100 and adjustable DC power supply for conveyor line.

RS232 serial and Ethernet connection cables


2/7

3. LASER MEASUREMENT SENSOR (LMS)

A laser measurement sensor (Fig. 2) is a device which uses a laser beam to determine the distance to an object. The most common form of laser measurement sensor operates on the time of flight principle by sending a laser pulse in a narrow beam towards the object and measuring the time taken by the pulse to be reflected off the target and returned to the sender. Due to the high speed of light, this technique is not appropriate for high precision sub-millimetre measurements, where triangulation and other techniques are often used [2].

Fig. 2: Laser measurement sensor (LMS100)

3.1. Operating principle of the LMS

The LMS is an electro-optical laser measurement system that electro-sensitively scans the

perimeter of its surroundings in a plane with the aid of laser beams (Fig. 3). The LMS measures

its surroundings in two-dimensional polar coordinates. If a laser beam is incident on an object,

the position is determined in the form of distance and direction.

Fig. 3: Measuring principle of the LMS

Scanning takes place in a sector of 270°. The scanning range of the LMS is maximum 20 m on

light, natural surfaces with an object remission > 13% (e.g. a white house wall).


3/7

3.2. Distance measurement

The LMS emits pulsed laser beams using a laser diode. If such a laser pulse is incident on an

object or a person, it is reflected at its surface. The reflection is detected in the laser

measurement system’s receiver using a photodiode. The distance to the object is calculated from

the propagation time that the light requires from emission to reception of the reflection at the

sensor. This principle of “pulse propagation time measurement” is used by radar systems in a

similar manner.

Fig. 4: Principle of operation for pulse propagation time measurement

3.3. Direction measurement

The emitted laser beams are deflected using a mirror and scan the surroundings in a circular

manner. The measurements are triggered at regular angular steps using an angular encoder. The

LMS scans with a scanning frequency of 25 or 50 Hz. During this process, a laser pulse and

therefore a measurement is triggered after an angular step of 0.25° or 0.50°.

3.4. Influences of object surfaces on the measurement

The signal received from a perfectly diffuse reflecting white surface corresponds to the

definition of a remission of 100%. As a result of this definition, the remissions for surfaces that

reflect the light bundled (mirrored surfaces, reflectors), are more than 100%. The reflection of

the laser beam will vary as a function of the surface structure and colour. Light surfaces reflect

the laser beam better than dark surfaces and can be detected by the LMS over larger distances.

Brilliant white plaster reflects approx. 100% of the incident light, black foam rubber approx.

2.4%. On very rough surfaces, part of the energy is lost due to shading. The scanning range of

the LMS will be reduced as a result.

Fig. 5: Reflection of the laser beam at the surface of an object


4/7

3.5. Scanning range of the LMS

The scanning range of the LMS is dependent on the remission of the objects to be detected. The

better a surface reflects the incident radiation, the greater the scanning range of the LMS. The

diagram in Fig. 6 indicate the relationship between remission and detectability.

Fig. 6: Scanning range of the LMS100 as a function of the target remission

3.6. Data interfaces

The LMS has different data interfaces for the configuration and the transmission of measured

values.

It is only possible to output all measured values of a scan in real-time using the Ethernet

interface.

The data transmission rate of the RS-232 interfaces is limited. Therefore these interfaces

are not suitable for transmitting scan data in real time.

The Ethernet interface has a data transmission rate of 10/100 MBit. The interface is a TCP/IP

interface. Full duplex and half duplex are supported. The Ethernet interface allows the

configuration of the LMS as well as the output of measured values. The factory setting for the

Ethernet interface is as follows:

• IP address: 192.168.0.1

• Subnet mask: 255.255.255.0

• TCP port: 2111

3.7. Data communication using messages

The LMS sends messages over the interfaces to communicate with a connected host. The

following functions can be run using messages:

• Request for measured values by the host and subsequent output of the measured values

by the LMS

• Parameter setting by the host for the configuration of the LMS

• Parameters and status log querying by the host


5/7

Measured value message can be requested using the “sRN LMDscandata” command for

each scan.

PC LMS (Sensor Input)

ASCII String: <STX>sRN{SPC}LMDscandata<ETX>

Hex String : “02 73 52 4E 20 4C 4D 44 73 63 61 6E 64 61 74 61 03“

LMS PC (Sensor Output)

ASCII Answer: No ASCII Answer possible

HEX Answer: “sRA LMDscandata 1 1 89A27F 0 0 343 347 27477BA9 2747813B 0 0 7 0 0 1388 168 0 1 DIST1 3F800000 00000000 186A0 1388 15 8A1 *...

BF6 0 0 0 0 0 0“ [3]

*... Measured distance values in HEX format, 540 values for 0.25° angular step and 1080

values for 0.5° angular step.

3.8. Laser Radiation Safety

The LMS corresponds to laser class 1 (eye safe) as per EN 60825-1. The laser beam cannot be

seen with the human eye. The laser operates at a wavelength λ= 905 nm (invisible infrared

light). The radiation emitted in normal operation is not harmful to the eyes and human skin. The

laser warning is on the LMS on the right side of the housing (Fig. 7).

Fig. 7: Laser warning label on the LMS

4. PROCEDURE

4.1. Collecting Data with Visual Basic Interface Program Program can communicate with LMS by TCP/IP or RS-232 protocol. TCP/IP is used for better

communication performance in 0.25° angular step scanning mode. Program sends data request in

every 10 ms via Ethernet with the command line below:

“Winsock1.SendData Chr(2) + "sRN LMDscandata" + Chr(3)“

(<STX> (start of text) send by “Chr(2) “ and <ETX> (end of text) send by “Chr(3)” in

HEX format)


6/7

The object on the conveyor is scanned and output values of each scan are converted hexadecimal

to decimal form. Measured values can be viewed and drawn in Visual Basic program on request,

but this decreases 3D scanning performance. For 3D scanning, values of each scan is recorded to

a string variable and at the end of scanning process, recorded to a text file. Source codes of the

interface program can be seen in Appendix A.

Fig. 8: Visual Basic Interface Program.

4.2. Three Dimensional Data Display in MATLAB

Recorded scanning data is read from a text file (vv.txt) with the M-file below. Text file is read as

string and assigned to a integer as a matrix. Data can be filtered with Butterworth low pass filter

on demand. Scanned data matrix is displayed three dimensional with mesh or surf command in

MATLAB.

M-file: clc, clear; a=str2num(fileread('vv.txt')) mesh(-a(:,10:120)) %Unfiltered Data

w=0;z=0;had=0;n=5; Wn=[10/100]; [w,z]=butter(n,Wn,'low'); had=filter(w,z,a); uz=length(a(:,1)) mesh(-had(0.2*uz:uz,10:120)) %Filtered Data


7/7

5. RESULTS

Scanned sample objects can be seen in Fig. 9 and Fig. 10. Results show that LMS100 can be

used for 3D measurements with under 2 mm error rate for length and width measurement and

under 0.1 mm error rate for depth measurement on light and flat surfaces. Error rate increases if

the surface is dark, rough or far away 20 cm. Scanning performance can be increased by

choosing proper LMS according to the purpose and changing scanning method or improving a

filter.

Fig. 9: Scanned USB Flash Box Foam in 0.25° angular step.

Fig. 10: Scanned Hook Wrench in 0.5° angular step.

REFERENCES

[1] Aiwu Zhang a, Shaoxing Hu b, Yulin Chen a, Haiyun Liu b, Fan Yang a, Jia Liu a, Fast

Continuous 360 Degree Color 3d Laser Scanner

[2] LMS100/111/120/151 Laser Measurement Systems, Operating Instructions

[3] SICK Laser Measurement Sensor, Developer’s Guide LMS1xx

3D Laser Scanner

Documents

Transcript of 3D Laser Scanner