3.1 Positioning realization of the mobile node
The method based on the received signal strength is selected to realize the positioning of the beacon node to the mobile node. Combined with the positioning engine in the mobile node core processor CC2431, it can achieve efficient and low-energy wireless positioning . The formation algorithm between each mobile node can be processed. Keep a certain formation moving toward the target point, and at the same time, if a command to change the formation is received, the current formation will be adjusted according to its own position until it is adjusted to the desired formation.
In the positioning experiment, it is necessary to select an appropriate value of N to achieve accurate positioning. However, since the value of N is very susceptible to interference from environmental factors, it is relatively difficult to obtain its value. The most common method is to test all networks at the test site. The nodes inside are tested in turn. Although this method is cumbersome, it can select an accurate N value for positioning. Since the value of the propagation coefficient N is greatly affected by the environment, in actual positioning, the parameter N_index is often used instead of the N value to perform positioning calculations to reduce errors caused by environmental interference. For the CC2431 positioning engine, the expected value of parameter A is between 30 and 50, and the value of N_index is between 15 and 25. Finally, the current position is displayed on the host computer, and each packet of data transmitted wirelessly contains the short address of the node itself, so that the node information will not be confused.
Realization of mobile node positioning based on RSSI
In wireless sensor networks, beacon nodes provide blind nodes with position coordinates and RSSI values, and the received signal strength is easily affected by environmental factors. Therefore, a reasonable layout of beacon nodes is required to ensure that blind nodes can use the beacon. The information provided by the target node calculates its accurate position. In this subject, 16 beacon nodes are selected in a 20 × 20 m experimental site to locate the mobile nodes, and the distance between adjacent beacon nodes is 5 m. The beacon nodes are all distributed at the outer contour of the area, and no beacon nodes are placed in the movement area of the mobile node. This layout makes it unnecessary for the mobile node to consider obstacle avoidance during the travel process, which simplifies the overall complexity of the system.
After the core processor CC2431 of the mobile node receives the data packet sent by each beacon node, it arranges the RSSI value in it in descending order and selects the top 8 data. If it is not due to external interference or other reasons, if the data information of more than 8 beacon nodes can be obtained, all current beacon nodes will be sorted and processed. At the same time, the program is designed such that the mobile node receives 6 data packets within 300 ms, and then uses the average filtering method to smooth the RSSI value to ensure positioning accuracy.
3.2 Z-Location positioning monitoring
When the preliminary work of positioning is ready, the system can be started for wireless positioning, and the monitoring of the upper computer and analysis of positioning errors can be performed with the help of Z-Location Engine software. You can change the position coordinates of the beacon node in the coordinate system in Reference Node Setup on this interface. After the setting is completed, the beacon node will remember its position, which can be used directly in future experiments. After the software is turned on, you can observe the coordinates (X, Y) of the current mobile node at BlindNodeSetup, compare this coordinate with the actual position coordinate, calculate the error between the true value, and change these two parameters through the A and N input fields the value of will get the new coordinates of the mobile node. Repeat this operation until the values of A and N are changed to values suitable for the current experimental site. This not only finds the appropriate A and N values, but also improves the accuracy of positioning.
3.3 Coordinated control of multiple mobile nodes
The mobile node obtains its own pose information from the beacon node through the communication mechanism, and at the same time, each mobile node exchanges information to realize information sharing. When a node in the system receives the status information of other members in the system, it calculates and processes it, and combines its own perception of the environment to make corresponding behavior plans, so that each member in the system can achieve expectations through collaboration of the goal Multiple mobile nodes coordinate with each other to control communication between each node in a wireless manner, and sense their own heading angle information through the geomagnetic sensor, so as to make judgments and adjustments on the current formation and transformation conditions.
The cooperative control of multiple mobile nodes is mainly embodied in that when the mobile node perceives the external environment information and receives the pose information of other nodes, it influences the behavior of other mobile nodes through its own decision-making and planning. The coordinated control of multiple mobile nodes mainly studies the two-party recognition results
In the process of collaborating to complete a certain task, how to make each node maintain a consistent goal;
How to avoid conflicts and deadlocks in behavior planning between nodes when running to a consistent target point and what measures should be taken to effectively eliminate conflicts or deadlocks.
The multi-mobile node coordinated control system established with the above two aspects as the core will be able to complete the overall task of the system efficiently with good dynamics, adaptability, and flexibility.
3.4 Formation reference points
In order for the mobile node to have a corresponding adjustment mechanism for the maintenance and transformation of the formation in the process of moving to the target point, it is necessary to select suitable reference points for these mobile nodes. In this way, each mobile node can determine its own position and the position of the reference point. After calculation and processing, reasonable adjustments can be made to maintain the formation and make corresponding changes. In general, there are three methods for selecting reference nodes: taking the navigator as the reference point, taking the adjacent node as the reference point, and taking the geometric center as the reference point.
The selection method based on the navigator as a reference point can provide reference for other nodes without resorting to a large amount of communication. The communication is simple and the control is convenient. This method is used to realize the formation maintenance and formation transformation of the mobile nodes in the wireless sensor network. Taking the navigator as a reference, each mobile node adjusts its position autonomously to achieve the purpose of maintaining and changing the formation.
3.5 Improved Leader-Follower method design
This paper chooses the Leader-Follower method to study the formation transformation and formation maintenance of mobile nodes. This paper designs an improved Leader-Follower method. In this research, the traditional three types of nodes are divided into four types according to the different formation responsibilities in the research work of the formation transformation of mobile nodes in the wireless sensor network environment, namely, the coordinator, beacon node, master mobile node (Leader), and slave mobile node (Follower). These four types of nodes have a clear division of labor in the network.
Since the functions of the beacon node and the coordinator have not changed, this article focuses on the analysis of the functions of the master mobile node and the slave mobile node. First of all, for the master mobile node, as the leader of other nodes in the network, its fundamental task is to provide references for its followers.
After it accurately locates itself with the help of the information of the beacon node, the information should be sent out in the form of broadcast, and the coordinator sends the information to the host computer through the serial port for tracking display. The pose information sent by the master mobile node to the slave mobile node in real time includes three types of information, namely, the current position coordinates of the master mobile node, the current speed, and the current formation to be maintained or formed.
When the mobile node is traveling to the target point, it receives the formation change command from the coordinator. This command is a point-to-point communication between the coordinator and the master mobile node, and the slave mobile nodes in the network will not receive the command. The slave mobile node can obtain the related transformation of the formation from the pose information sent to it in real time by the master mobile node, because the slave mobile node needs to make real-time adjustments to the path during the formation transformation. After a series of positioning calculations, the mobile node transmits its position information to the coordinator in a point-to-point manner. At the same time, it needs to receive the leader’s pose information in real time, calculate its distance and angle, and extract the information transmitted by the main mobile node. Determine whether the current formation information is consistent with the formation at the previous moment. If it is inconsistent, the formation needs to be changed. According to the formation information, the corresponding expectation matrix is selected as the target to adjust its position.
3.6 Formation maintenance of mobile nodes
In order to enable the mobile nodes in the network to maintain the current formation and move toward the target point after completing the formation change, but in the process of travel, due to the interference of various factors in the road conditions and the environment, one or the other will inevitably appear. Thousands of mobile nodes are left behind. Therefore, this paper adopts the l-φ closed-loop control method to correct the formation in real time, and maintain the formation of the mobile node group during the walking process, so that the current follower will be between the current leader and the leader. Adjust the distance and included angle to the desired value l and φ. As long as the follower successfully receives the pose information sent by the leader, the l-φ controller can maintain a certain distance and angle with the leader, so that the formation can be maintained. The slave mobile node compares its own pose with the expected value and realizes the correction of the formation through the feedback of the current position and angle error, which ensures the stability of the formation. Closed loop control is shown in Fig. 2.