Technology White Paper FARO Focus Mobile Mapping - Simple and affordable use of FARO technology by Tobias Möhlihs, M.Eng

Introduction

Mobile Mapping is an essential part of modern 3D documentation. A mobile mapping system is integration of remote sensors and time synchronized navigation. It is a powerful application for collection 3D information in a large scale, e.g. road mapping or 3D city models. Since available mobile mapping systems require a large investment of up to 1.000.000€, this paper shows a way to convert a FARO Focus Laser scanner with minimal investments into a mobile mapping system. For this paper the Applanix AP15 IMU was used that provides high accuracy to an affordable price. Of course any other IMU system can be used, depending on the actual needs of the user. Also, every FARO Focus is capable to operate in a helical mode and therefore is convertible into a mobile scanner.

Components

To convert a FARO Focus into a mobile mapping system there are additional parts required. The most important sub system of a mobile mapping system next to the FOCUS itself is an inertial navigation system (INS) which provides a position and orientation of the scanner. In this case, the Applanix AP15 is used but can be replaced with any Applanix POS LV or any other INS System. The needed parts are: - - - - - -

FARO Focus Laser scanner FARO helical adapter FARO Zero Point Clamp (ZPC, optional) FARO Helical App (FARO 3D App Center) Applanix INS (or others) A platform to mount all the parts

The Applanix INS contains an inertial measurement unit (IMU) and two GNSS receivers.

WORKFLOW

The basic workflow is very simple. Once all INS components are installed on a platform the Focus can be attached and removed from the mobile mapping superstructure as needed. This flexibility enables the user to still use his Focus as a regular laser scanner as the system does not require a permanent installation of the Focus.

PREPARATION

In order to install the system, there is a little preparation needed. At first, there is no cable provided by Applanix or FARO to actually connect the two separate systems. Second, all components need to be placed and fixed to a vehicle which leads to a platform.

WWW.FARO.COM

THE CABLE

In order to synchronize the Focus with the INS both systems have to be connected. For the synchronization the user needs to connect the Automation Testbox I/O Interface with the POS Event I/O. If another INS is used, please refer to the manual of the INS how to address a sync_in Event. It is important that the INS can read the TTL signal provided by the FARO Automation test box for time synchronization.

In case an Applanix INS is used the following pins have to be connected:

Automationbox Pin

POS I/O cable 1 Pin

3 (Trigger_Out) 5 or 7 (GND) -

2 (Event_In) 5 (GND)

I/O 1/2

figure 2 Automation Test box pin mapping

figure 2 Applanix POS I/O pin mapping

THE PLATFORM

To actually mount the Scanner, the IMU and the GPS Antennas a Platform is needed. It is highly recommended that at least the Scanner and the IMU are mounted as close to each other as possible. How everything is mounted onto a vehicle is up to the user and his needs. But for an easy setup and in this example the best choice is an all-in-one platform that can be attached to universal roof racks. The layout of all components is uncritical except for scanner and the IMU and specifications for the GNSS antennas (please refer to the manual of the GNSS antenna manufacturer)   As an example, the platform can look like this:

figure 3 drawing of a possible platform layout

FREE CALL: 00800 32767253

figure 4 platform with mounted devices (Applanix POS, AP15 IMU and FARO ZPC)

INSTALLATION

For an easy installation the setup is divided into stages: In a first step mount all devices to the platform. These are the IMU, the POS, and the scanner mounting. For mounting the Scanner it is recommended to use the FARO ZPC for easier attaching and removing of the Scanner. Now place the platform on the roof racks. To mount the Scanner, attach the FARO Helical Adapter as shown in the FARO Automation manual. Then attach the Applanix DMI and the GPS Antennas to the vehicle as shown in the Applanix manual and connect all devices with the provided cables. Finally connect the FARO Automation Testbox to the Helical Adapter and connect the Automation Testbox I/O Interface to the POS I/O with the custom made cable. Before the Scanner can be used as a mobile Scanner, the new retainers, which are included in the FARO Helical Kit, must be mounted and the cover of the connection pins has to be removed.

figure 5 Focus with mounted helical retainers

figure 6 FARO Helical Adapter

Please note: before the vehicle starts moving, the scanner must be fixed in its helical position with the brackets of the helical adapter! See the Automation Manual for more information.

WWW.FARO.COM

figure 7 Focus fixed into helical position

After the scanner is mounted, connect the Automation Testbox with the Applanix POS I/O Interface with the custom made cable. The scanner can now be connected with the Applanix system over the Automation Testbox.

figure 8 Automation Testbox connected with Applanix Event I/O

figure 9 Automation Testbox connected to Helical Adapter

figure 10 Applanix POS connected with Automation Testbox via a custom cable

figure 11 assembled platform mounted to universal roof racks

FREE CALL: 00800 32767253

Scanning

Once all parts are mounted and connected the Scan project can be started. Activate the Helical_grey profile for the Focus as shown in the FARO Automation manual and choose the quality and resolution settings. Good values to start with are quality 4x and resolution 1/10. Please note: the density of the resulting pointcloud depends not only on the scanner settings, but also on the speed of the vehicle! To initialize the Applanix INS refer to the training provided by Applanix or the Applanix manual. When the scanner is recording data, the event counter in POSView should display the incoming signals from the scanner. Otherwise check if the connection is correct and if the event input is enabled within the POSView software. The mobile mapping can now start.

PROCESSING THE DATA

After the data recording is finished the navigation data from the Applanix System must be post processed with the Applanix POSPac software. The data has to be exported as a standard ACSII file with the “Event 1” data selected. The scanproject has to be saved into a .fws Workspace file. To convert the helical scan into a 3D pointcloud start the helical App. The Helical App will first ask for the navigation file and then for the workspace file. The Helical App will now merge the data and transforms the helical scan into a 3D pointcloud in the .xyz format.

Figure 12 Helical App

In addition, values for offset and rotation in x, y and z for the center of the scanner according to the center of the IMU must be entered. How to get these values is explained in the calibration chapter of this paper. Please note: all points are georeferenced and therefore it is necessary to enter a display offset into the Helical App, otherwise the values for x, y and z would be too big to be displayed in Scene! E.g. a point has UTM coordinates N 5403085.456, E (32) 512731.294 and H 301.516 enter a display offset for N with 5403000, E 512700 and H 300. Now the point can be displayed in Scene with X =31.294 Y =85.456 z =1.516

WWW.FARO.COM

Calibration

This calibration is an easy way to calibrate the system without a laboratory or special signalized reference points. It can be done every time and everywhere. This ensures that the FARO Focus can always be detached to perform a regular scan if needed because no permanent installation is required. A simple way to calibrate the system is to scan an area from two different directions. While the INS determines the position and orientation of the vehicle, the position and orientation of the scanner relative to the INS is unknown. Because the origin of the scanner is never the same as the origin of the inertial navigation system (INS) both scans are different from each other. As shown in figure 13 the real object is displayed as the black line. The green and the red lines are the result of the two mobile scans from different directions. The difference between the green and the red scan is now two times the difference between one scan and the real object.

Figure 13 calibration concept The real object (black profile in figure 13) is unknown since there are only the two scans (figure 14). Each scan has an offset in X,Y,Z and a rotation offset around all axis because the position and orientation of the scanner relative to the INS is unknown.

Figure 14 profile view of two scans Because the real object is not available, one of the two scans is considered as the “real” and error-free scan and therefore as a reference. With registering the second scan to the reference scan the transformation parameters are two – times the parameters for offset and rotation of the scanner relative to the INS. To determine the values process both scans with an offset and rotation of 0 (or a rotation of 180° as a start value if the scanner is mounted upside down). If a custom scanner mounting is used (e.g. 45° rotation around the y-axis) one has to enter the values for the rotation given by the manual or the drawing of the mounting as a start value. Now both, IMU and Scanner have the same center. Please note that the registration must not be performed with the whole point cloud but with reference points along a profile line. As shown in figure 15 and 16 a rotation of the scanner towards the IMU doesn’t lead to a rotated point cloud (figure 15), instead, each scan point (line) is rotated. This leads to a deformed point cloud (figure 16). Place your reference points for a registration (figure 17) along a profile as shown in figure 14!

FREE CALL: 00800 32767253

Figure 15 rotated pointcloud

Figure 16 deformed pointcloud

figure 17 selection of reference points

figure 18 point cloud before calibration

figure 19 after calibration

This is essential since a pointcloud from a helical scan has no single origin like a normal scan. Instead, every single point (or scanline for a simplified view) has its own origin. Therefore a regular, global (best fit) registration can’t be used. As an example, figure 18 and 19 show the result of a scanner that was installed with a 45° rotation around the y-axis. As a result each individual scanline is rotated, but not the pointcloud itself!

GPS INFLUENCE

It can be assumed that the general GPS conditions are the same for both scans if they were recorded consecutively. Is this case, position errors are the same for both scans. While this way of calibrating can be used to determine the rotation of the scanner relative to the INS, it cannot be used to determine the X,Y and Z offset between the two systems. While the calculation of the relative position of each scanline is supported by all sensors of the INS, the absolute position is affected by all GPS influences and possible errors. As shown in figure 20 the positioning error ranges from < 5cm up to 50cm. Therefore the calculated X,Y,Z offset between both scans is depending directly on the GPS positioning and the results can’t be used as calibration parameters. To determine the offset between the INS and the scanner simply measure the distance by hand or take the values of the drawings of the mounting.

Figure 20 position error under changing GPS conditions

WWW.FARO.COM

Summary

Do it yourself mobile mapping can add a huge amount of extra value to a FARO Focus laser scanner. The Focus can be converted very easily and with an, in comparison, low investment for the new Applanix AP15 INS. This enables the user to gather 3D data in a large scale and to work very efficient in an outdoor environment. In addition, the Focus is not attached to the system in a permanent way and can still be used as a regular laser scanner.

figure 21 a scanned city block

Faro Europe GmbH & Co. KG Lingwiesenstraße 11/2 70825 Korntal-Münchingen Deutschland

WWW.FARO.COM

( +49 (0)7150 9797 0 Ê +49 (0)7150 9797 44 E-Mail: [email protected]