Sensors

This section provides information about the proposed sensors for the docking system.

11.2.1 Laser scanner

The laser scanner that is proposed for the detection concept is the Hokuyo UTM-30LX-EW 2D laser scanner. The laser is chosen because it is already available, and because it has proven to provide sufficient data when detecting reflective objects based on intensity measurements through small-scale experiments.

11.2.2 Laser scanner specifications

Table 11-1 on the next page shows the specifications for the Hokuyo UTM-30LX-EW

laser given by its manufacturer that are relevant for the docking system. Scans from more than one laser can be merged and provide a greater scan angle [90].

Table 11-1: An overview of relevant specifications for the Hokuyo laser scanner.

Hokuyo UTM-30LX-EW Laser Scanner

Scan angle (Horizontal) 270°

Angular resolution 0.25°

Range Operating range: 0.1 – 30𝑚

Accuracy At range 0.1 – 10m: ±30𝑚𝑚

At range 10 – 30m: ±50𝑚𝑚 Detection Min. detectable width at 10m: 130𝑚𝑚

A range experiment has been conducted for the laser to determine the minimum and maximum range that can be used for detection with intensity data. A description of the test, alongside results, is located in Appendix I. The range experiment was conducted outdoors with the laser being directly exposed to sunlight to mimic a worst-case scenario.

Table 11-2: An overview of detection specifications for the Hokuyo laser.

Specification Minimum Maximum

11.2.3 RGB-D camera

The Intel RealSense D415 depth camera has been proposed as a camera for the function.

This camera is already available and has proven satisfactory performance with the AprilTag detector. However, other cameras may also be used as long as they provide adequate data for the AprilTag detector.

11.2.4 RGB-D camera specifications

Table 11-3 provides an overview of relevant specifications for the Intel RealSense D415 camera given by its producer [95].

Table 11-3: An overview of relevant specifications for the RealSense D415 RGB-D camera.

Intel RealSense D415 Depth Camera

Depth FOV 65° ± 2° × 40° ± 1° × 72° ± 2°

Depth Resolution 1280 × 720

Depth fps 90 fps

Depth range ~ 0.16 – 10 𝑚

A simple range test has also been conducted for the depth camera to determine the minimum and maximum range for AprilTag detection. Quadratic tags with height and width set to 160mm were used for the experiment, and a complete overview of the results is provided in Appendix I. Based on the test, the following specifications have been set for AprilTag detection with the Intel RealSense D415 camera:

Table 11-4: An overview of range specifications for AprilTag detection with the RGB-D camera.

Specification Minimum Maximum

Detection of tags that are

maximum 2.4 meters apart 2.5 𝑚 4 𝑚

Ranging based on detection

of one tag 0.5 𝑚 5 𝑚

Figure 11-3: An image of the components in the Intel RealSense D415.

11.2.5 Sensor operation

As two sensor technologies will be used to complete the docking, Figure 11-4 is made to depict the areas for which the camera and laser each are expected to operate. The distance 𝑑 is given by the maximum range values in Table 11-4, whereas 𝜇 is given by the camera specifications for the field of view in Table 11-3. The rotating laser’s operational area is set by its field of view of 270° and a radius 𝑟 equal to the maximum range in Table 11-2.

11.2.6 Sensor detection prerequisites

The proposed detection system has a few requirements for the physical properties of the dock and the station. A mock-up design of a dock has been made to describe what is necessary to include for the docking system to work. The design is shown in Figure 11-5, where the center of the two poles defines the desired position for the robot's geometric center when successfully docked. In the design, the poles are depicted with reflective metal.

In reality, these poles will need to be covered with reflective tape to provide satisfactory reflection. The pictured design is not necessarily how the dock will look, as the physical design goes beyond the scope of this thesis.

Figure 11-5: An image of a possible design of a charging dock which is made in SolidWorks for illustrative purposes. AprilTags with ID 1 and 2 from tag family 36h11 has been mounted on each of the poles.

Figure 11-4: An illustration of the operating areas for the two sensor technologies.

11.2.6.1 Reflective markers

To distinguish objects of interest with the laser, it is proposed to cover the surface of the objects with reflective markers. Small-scale experi-ments have indicated that reflective markers with prism structure provide the best intensity measurements, thus making them more appropriate for the laser detection concept.

Examples of reflective markers that are commonly used for laser surveying are shown in Figure 11-6.

11.2.6.2 AprilTags

It is proposed to use two pairs of AprilTag IDs to mark both the charging dock and the entrance of the charging station. By doing so, the task of entering the docking station can easily be distinguished from the task of the actual docking. Furthermore, as a supplement to the function when only using cameras, a fifth tag ID can be used for range determination.

A range may become necessary to ensure correct positioning during the docking. The following set of AprilTags from tag family 36h11 should be used for the functions, but IDs may be substituted if desired.

Tag ID: 0 This tag will be used to measure specific distances when a laser is unavailable.

Tag ID: 1 This tag will be used to mark the left side of the charging dock.

Tag ID: 2 This tag will be used to mark the right side of the charging dock.

Tag ID: 3 This tag will be used to mark the left side of the charging station gate.

Tag ID: 4 This tag will be used to mark the right side of the charging station gate.

Figure 11-6: An image of reflective markers used for surveying. (Leica Geosystems)