Concepts for detection

This section presents concepts that will enable Thorvald to localize the charging station gate and the dock itself. The aim is to use the same localization concept to localize the gate of the charging station and the charger. Multiple techniques can be used to solve the main problem, but due to the strict timeframe, only four concepts are considered.

8.2.1 2D laser scanner and reflective tape This concept uses a 2D laser scanner to recognize two poles that mark the charging dock, and possibly the gate for safe navigation into the station. The points are made distinguishable by using reflective tape as passive beacons. The reflective tape increases the intensity of the reflection measured by the lidar, and only the measurements above a certain threshold are used to define two points for the poles. From these points, the baseline between them is calculated to generate the orthogonal from its middle point. The reference line will be used to determine the orientation of waypoints and a goal pose to ensure safe navigation and alignment. The concept is depicted by the illustration in Figure 8-4, where A is the 2D lidar, B1 and B2 represent the pillars with reflective tape that mark the dock,

𝑚 is the middle point of the baseline between the pillars, and the line ref represents reference path to align with the dock. Calculations will involve trilateration and triangulation based on distance and angle information from the laser.

Pros

• Two Hokuyo UTM LX30-EW lasers are available and compatible Thorvald, which also means that the technology can be tested.

• Reflective tape is a rather mobile option and can be applied to various charging dock designs, and station gate sizes.

• The concept can easily be implemented.

• The Hokuyo laser provides very accurate measurements [90].

Figure 8-4: An illustration of the concept that considers the use of a 2D lidar and reflective tape beacons for recognition of the dock's boundaries. A: Hokuyo UTM 30LX-EW, B: Pillar with reflective tape that marks the docking area.

Cons

• The intensity of the relevant tape needs to be tested to ensure correct thresholds and robustness to environmental conditions.

• It is necessary to take into account the effects of outliers when detecting the pillars.

• Lasers are rather expensive and can easily be affected by, for instance, sunlight, rain, and fog.

8.2.2 RGB-D camera and visible landmarks

Similar to the previous concept, RGB-D cameras can also be used to recognize the pillars and the gates. This concept consists of using a depth camera to recognize either colored geometries to extract the position of the points of interest. Figure 8-5 illustrates the concept with yellow spheres that are recognized by an RGB-D camera. The pixels that represent the center of the circles are extracted from the image and used to calculate the position from the robot.

As the RGB-D camera can measure depth, the reference path or orientation of the charger can be calculated the same way as for the laser concept, given that the transformation between the camera frame and the body-fixed frame of the robot is known.

Pros

• Existing computer vision algorithms can be used to achieve the desired result.

• A RealSense RGB-D camera is available and is compatible with both ROS and Thorvald’s system.

• Python has several libraries that can be used to perform the computer vision task.

Cons

• Computer vision is highly subject to light conditions.

• The camera has a limited field of view.

• The camera can easily be affected by dirt and rain.

Figure 8-5: An illustration of the concept that considers the use of an RGB-D camera and colored geometries for recognition of the dock's boundaries (camera: realsense.com).

8.2.3 Signaling beacons

This concept uses active beacons to provide information about the position of the two pillars by the dock. Depicted in Figure 8-6, are two beacons, B1 and B2, located at the pillars of the charging dock. These active beacons transmit signals to the receiver, A, that allows Thorvald to localize them and generate a reference trajectory similar to the previous concepts.

The reference trajectory intersects the middle point of the baseline, which is marked with ref. Waypoints and a goal pose can be deter-mined by using trilateration and triangulation on the detected pillar locations.

Pros

• Inexpensive and easy to set up.

Cons

• Noisy signals can pose challenges for accurate and precise localization.

• Bad precision increases the need for robust filtering.

8.2.4 Wire guidance

This concept involves using magnetic tape to set up the desired path for Thorvald to follow. The magnetic tape is attached to the charging station floor from the gate to the dock. A hall-effect sensor is mounted on the robot and tracks lateral errors between the robot and the path. Based on these errors, lateral motion commands are generated to guide the robot along the path. An illustration of the concept is shown by Figure 8-7.

Pros

• Immune to dirt and lighting conditions.

• Inexpensive and easy to set up. that considers the use of infrared beacons for recognition of the dock's boundaries. B1 and B2 represent beacons located at the docking boundaries. “A” represents a receiver that is mounted on Thorvald.

A

This chapter has presented a function analysis for the docking system where the primary and secondary functions were revealed, and ways to obtain them were mapped. Also, concepts for detection and localization of the station gate and the dock were generated and described. In the next two chapters, selection processes will be conducted to select one concept for detection and an appropriate motion controller.