To select system components for the docking system that provide satisfactory performance, a set of requirements and specifications are presented in this chapter.
7.1 System goals and requirements
This section presents the main and intermediate system goals and the key system requirements for the docking system.
7.1.1 System goal
The main goal for the docking system is to enable Thorvald to autonomously navigate to its charger. Therefore, the system needs to include components for perception, path planning, and motion. In the next section, the system goal is broken down into sub-goals and system requirements.
7.1.2 Requirements
Table 7-1 provides an overview of system requirements and goals that have been set for the docking system. The requirements have also been assigned weight coefficients that represent their importance with respect to the system goal. A scale of 1 to 5 has been used, for which 1 represents non-critical, and 5 represents crucial requirements.
Table 7-1: An overview of the key qualifications for system and their corresponding goals.
Key system
requirement Goal Weight
Coefficient Robustness Make sure that the docking system is robust and that
it can deal with disturbances. 2
Sensor technology
Make sure that the sensor technology selected for the docking system is appropriate, and that it provides adequate data for its given task.
4
Functionality Make sure that the docking system provides the desired functionality, which, from the main goal, is to enable the robot to localize and navigate to the charger and align with it.
5
Transferability Make sure that the docking system can be transferred
to various configurations of Thorvald. 1 Integration Make sure that the function can be easily integrated in
Thorvald’s system. 2
Feasibility Make sure that the function can be developed within
the given timeframe. 3
Table 7-2 provides an overview of the criteria that have been formulated for each system requirement. These criteria are formulated with must, should and could and to indicate their degree of importance in the development.
Table 7-2: An overview of the key system requirements for the docking system and their
The function should be robust, thus not significantly affected by external disturbances such as incline, bumps or environmental forces.
Sensor technology
The selected sensor technology must be able to detect and track important objects or points.
The selected sensor technology must be reliable for its operating environment.
Functionality
The function must be able to localize the charger.
The function must navigate Thorvald to the charger.
The function must align Thorvald with the charger within the tolerances that have been set.
The function must navigate Thorvald through the gate of the charging station.
The function could include a signal to open the gate of the charging station in cases where this is possible.
Transferability
The function must be transferable to different robot operating environments.
The function should be transferable between different configurations of Thorvald with minimal adjustments.
The function could be useable regardless of the charging station design.
Integration
The function must be integrable in Thorvald’s existing system architecture.
The function should be tested through simulation before deployed on the robot.
The function could be made as an ActionServer that can be executed by Thorvald when necessary.
Feasibility The construction of the function must be realistic within given timeframe.
7.2 Metric specifications
The following metric specifications hold for the Thorvald concept in general:
Table 7-3: Metric ranges for the dimensions of the Thorvald concept.
Specifications Minimum Maximum
Velocity 0 𝑚/𝑠 1.50 𝑚/𝑠
Acceleration 0 𝑚/𝑠 2.0 𝑚/𝑠!
Dimension, length 1500 𝑚𝑚 1750 𝑚𝑚
Dimension, width 1000 𝑚𝑚 >3000 𝑚𝑚
Dimension, height 825 𝑚𝑚 ~2100 𝑚𝑚
It will be challenging to consider all the robot configurations in the design process.
Therefore, it is decided to only focus on the standard configuration, but to maintain focus on transferability. This standard configuration of Thorvald holds the following specifications:
Table 7-4: Metric dimensions for the standard configuration of Thorvald.
Specification Range/dimension
Longitudinal velocity (min – max) 0 − 0.40 𝑚/𝑠
Angular velocity (min – max) 0.1 − 0.5 𝑟𝑎𝑑/𝑠
Acceleration (min – max) 0 − 2.0 𝑚/𝑠!
Dimension, length 1500 𝑚𝑚
Dimension, width 1500 𝑚𝑚
Dimension, height 825 𝑚𝑚
Specifications are also needed for the operational environment to define metric tolerances for the system. The charging station that will be used as a reference when designing the system has the following metric specifications:
Table 7-5: An overview of relevant dimensions for the charging station.
Specification Dimension
Gate width 2300 𝑚𝑚
Gate height 2000 𝑚𝑚
Inside minimum width 2250 𝑚𝑚
Figure 7-1 depicts a potential design for a charging dock. This dock is constructed with pillars that, for instance, can be marked with reflective tape to be distinguished by a laser scanner. The specifications for the illustrated dock are given by Table 7-6.
Table 7-6: An overview of relevant dimensions for the charging dock.
Specification Dimension
Distance between pillars 2200 𝑚𝑚
Pillar height 1200 𝑚𝑚
7.3 Tolerances
It is hard to design functionality with high accuracy and precision. Because of this, it is necessary to set acceptable tolerances for the system. Tolerances have been set for Thorvald's position and heading to ensure that a goal pose can be reached without the need for minimal and unnecessary final adjustments. The tolerances are provided in Table 7-7.
Offset errors are given as radii of circles to define a tolerance field surrounding the desired or actual position.
Table 7-7: An overview of the tolerances set for the system.
Specification Error
Heading error ±10 °
Position offset 200 𝑚𝑚
Detection offset 100 𝑚𝑚
The next chapter presents a functional analysis for the docking system that has been conducted to obtain an overview of the necessary functions that the system must include.
Figure 7-1: An illustration of the charging dock without the charging mechanism. The yellow fields on the pillars represent reflective tape.