**2. Exploring and mapping underground mines**

The underlying idea in the UPNS4D+ project is to deploy two kinds of vehicles in an underground mining facility. There is an exploration vehicle that periodically drives around in the underground mine when no regular work is taking place to initially record and then update a map. The second vehicle is a regular processing vehicle that is performing the daily work in the mine. It used the map that is periodically updated by the exploration vehicle. While the exploration vehicle needs to have more sophisticated sensory equipment for recording the map, for the processing vehicle, a stripped-down equipment suffices, since it only needs to localize within a given map.

With our project partners, we developed the exploration robot shown in **Figure 1a**. It is a skid-steered tracked robot based on a mini excavator platform. It carries the modular sensor platform shown in **Figure 1b**. The robot can drive up to 3 ms<sup>1</sup> and is controlled via the ROS [17] Movebase. For navigation, collision avoidance and terrain classification, two Velodyne VLP-16 Puck LiDARs are mounted at the front. They acquire environment information with 16 scan lines with an opening angle of 30° and with 20 Hz. They are mounted on a 20° slope to be able to get information in the close vicinity of the robot. With a horizontal opening angle of 360°, they can acquire 3D data from the front and the sides of the robot. For safety reasons, additional 2D laser range finders have been mounted at two corners of the sensor platform which are also used for collision avoidance.

platform consists of a Velodyne VLP-16 PUCK LiDAR and a Hokuyo UTM-30LX-EW range scanner which are both mounted opposite to each other on a disk which rotates both scanning devices around the centre of the disk. The disk and the upper part of the scanner are driven by a motor which is equipped with absolute encoders. Both scanners transfer their data via Ethernet which is connected by a slip ring which connects the revolving part to the rest of the scanning device. The combination of motor and gear head provides us with 3 Nm of torque and allows for a maximum rotation speed of 2.6 Hz. However, a reasonable azimuth resolution can only be achieved with a scanning speed of up to 1.67 Hz, while the full-sphere point clouds are then captured with a half revolution which equals 3.34 Hz for this. We deploy a 14 bit industrial grade absolute SSI encoder which is mounted on the drive shaft. The resolution provides a maximum error of 1.32<sup>0</sup> or 0.022°. In a distance of 10, this corresponds to 3.8. The second part of the platform is the rotating sensor mount. It houses a gigabit Ethernet switch, the interface box of the Velodyne VLP-16 PUCK and the Hokuyo UTM-30LX-EW, the power distribution for the sensors and several mounting rails for different sensors. The raw data of the deployed Velodyne VLP-16 PUCK and the attached Hokuyo UTM-30LX-EW are registered making use of the SSI absolute encoder. Besides the absolute encoders, there is another incremental encoder attached to the motor shaft. Then, based on the readings of the absolute encoder, the raw data is collected and integrated into a point cloud for the device. This is done with a best-effort time-stamping on the data and where one UDP package of the Velodyne VLP-16 PUCK is transformed altogether. The time difference between the laser readings within one UDP package is about 1.33 ms. For the rectification of the Hokuyo UTM-30LX-EW measurements, the recording time for one sweep is taken into account. As a final ingredient, our SWAP platform is equipped with an IMU (*μ*IMU

*A System for Continuous Underground Site Mapping and Exploration*

*DOI: http://dx.doi.org/10.5772/intechopen.85859*

) for providing the orientation of the platform w.r.t. the ground. **Figure 2**

<sup>1</sup> http://www.northropgrumman.litef.com/en/products-services/industrial-applications/product-ove

*The components and a photo of our rotating sensor platform. (a) Components of the platform and (b) Photo of*

shows a CAD drawing as well as a photo of the device.

from NG1

rview/mems-imu/.

**Figure 2.**

**67**

*the platform*

#### **Figure 1.**

*Exploration robot developed for mapping underground mining sites. (a) Exploration vehicle and (b) Sensor setup of the exploration robot.*


#### **Table 1.**

*Comparison between SWAP and FARO Focus3D X 130.*

The mapping operation is not run autonomously in the mine environment for now. At the front of the robot, an Allied Vision GT6600C high-resolution camera with a wide-angle lens is mounted. The camera can be used for teleoperation.

As an additional safety feature, we mounted a FLIR A315 thermal camera at the front of the robot in order to be able to detect persons even when not sufficient light is available.

For mapping the mine, the platform is equipped with a rotating 3D LiDAR system, the SWAP platform, which we will describe in detail in the next section. For reference, we mounted a FARO Focus3D X 130 LiDAR, which can be used in a stopand-go fashion. Scanning times of the Focus LiDAR lie between 1 and 30 min. To remotely operate the LiDAR, we developed a ROS driver based on the FARO SDK.

As part of our project contribution, we developed a rotating sensor platform for the swift acquisition of dense point clouds as reported in [16]. The main goal was to find a compromise between acquiring accurate and dense point clouds which usually takes much time and having available data for online use in a robotic system for tasks such as localization which has to be updated more frequently. For instance, taking the FARO LiDAR with an angular resolution of 0.0035°, very dense and accurate point clouds can be recorded. However, the robot needs to stand still, and the scanning time of a single scan can take up to 30 min. **Table 1** shows a comparison of the two scanners.

## **3. The 3D LiDAR system SWAP**

In this section, following previous work in [16], we present the 3D LiDAR platform SWAP which was developed during the mine mapping project. The SWAP
