"1. Introduction
The prison has high security, stability and real-time requirements for the design of its security system. At present, the video monitoring system, alarm system and access control system used in the prison system are based on wired communication, which makes some areas inconvenient for wiring become the blind area of security monitoring. In addition, the invariable monitoring area also provides opportunities for criminals. In order to prevent criminals from fighting, self mutilation, suicide, prison escape, assaulting the police and other illegal and criminal activities, timely and accurately control and access the prison violence scene, prison escape behavior and other information, and build a trinity security system of civil air defense, material defense and technical defense, a design scheme of prison security system based on wireless sensor network is proposed. Using wireless instead of wired can bring many benefits, such as reducing deployment difficulty, reducing maintenance cost and increasing flexibility, especially in areas where cabling is impossible or inconvenient. As Helmut Macht, chief technical expert of Siemens Building Technology Group, said: "the innovation of wireless communication will replace more and more wired communication". However, due to the particularity of the internal environment of the prison, the use of wireless technology in the prison will face many problems. Firstly, the adoption of wireless technology will face the problems of multipath transmission interference, transmission conflict and obstacle reflection, which will affect the reliability and scalability of wireless network and the data throughput of broadband communication; Secondly, because most of the existing monitoring systems are designed based on wired network, their original high-level applications and protocols must be improved if they are to be applied in wireless network; In addition, if wireless technology is to be widely used in prison security system, the cost of equipment is also an important factor. The wireless communication module must meet the low-cost requirements of the monitoring system. The design of prison security system based on wireless sensor network studied in this paper does not blindly use wireless instead of wired, but retains its original wired monitoring means such as video and threshold with large data transmission volume on the basis of the existing prison wired security system, and adds static temperature vibration sensitive wireless sensor network and dynamic real-time positioning wireless sensor network, So as to further improve the security system, eliminate the monitoring blind area, and make the functions of each subsystem complement each other.
2. Wireless network technology of prison security system
Wireless sensor network is a wired or wireless network composed of a group of sensors in the form of ad hoc. Its purpose is to collaboratively perceive, collect and process the information of perceived objects in the geographical area covered by the sensor network and transmit it to the observer. This sensor network integrates sensor technology, embedded computer technology and wireless communication technology. It can collaboratively perceive, monitor and collect the information of the perceived objects in various environments. Through the collaborative information processing of these information, the accurate information of the perceived objects can be obtained, and then transmitted to the users who need these information through ad hoc. The sensor used in sensor network is different from the general concept sensor. It not only can sense the change of the measured physical quantity and output the corresponding change information, but also has the function of remote communication. It must be an intelligent sensor. In addition, in order to complete the communication function, in addition to sensitive elements, it must also have power supply, embedded microprocessor, memory, data transmission and communication components, etc. Fig. 1 is a typical sensor network structure diagram, which includes sensor, sink, base station, etc.
In sensor networks, a large number of nodes are deployed inside or near the sensing objects by means of aircraft distribution and manual arrangement. These nodes form a wireless network through self-organization, perceive, collect and process specific information in the network coverage area in a cooperative way, and can collect, process and analyze information at any place at any time. This self-organizing network transmits the data back to the sink node (transit node) through multi hop relay. Finally, the data in the whole area is transmitted to the remote control center for centralized processing with the help of sink link.
The basic element of wireless sensor networks is nodes. In different applications, the design of sensor nodes is different, but their basic structure is the same. The typical hardware structure of the node is shown in Figure 2, which mainly includes the following units: sensing unit (composed of sensor and analog-to-digital conversion function module), processing unit (composed of embedded system, including microprocessor, memory, signal conditioning circuit, etc.), communication unit (composed of wireless RF module), and power supply. In addition, other functional units that can be selected include: positioning system, mobile system, power self power supply system, etc.
3. Design scheme of on-site executive layer
3.1 design scheme of static monitoring system
The wireless sensor network nodes are arranged in the prison room, production area, commanding point, confinement room, teaching area, infirmary, etc., so as to realize the comprehensive coverage of the prison. The introduction of wireless sensor network technology into prison security system is mainly because there are many sensor nodes in the prison, the wiring engineering is very large, and many areas are not suitable for wired connection. If wireless access is adopted, the deployment of sensors becomes very convenient, and the sensor nodes can be easily added or changed. By periodically or irregularly changing the position of the monitoring sensor node, the blind area of the existing wired monitoring system can be eliminated, and the target can be located through the sensor array. Moreover, because the location of monitoring nodes often changes, prisoners can not find monitoring vulnerabilities, so as to improve the security system.
3.2 design scheme of dynamic real-time monitoring system
By wearing the node on the prisoner, the positioning mechanism is used to grasp the prisoner's activities in real time. However, due to the large number of prisoners, only two types of data need to be transmitted:
1) When the prisoner's range of activities exceeds the specified range;
2) When a node fails due to external force or energy depletion, the remote center must detect the missing node.
The data obtained by the sensor node can be simply processed locally, and then aggregated and transmitted to the base station (PC or PDA). The remote server and monitoring system of the prison can analyze the indicators of each monitored prisoner in real time, and judge the real-time situation of the prisoner through the expert system. If the prisoner is found to be abnormal or dangerous, call the police and take measures quickly. At the same time, the system can also provide other auxiliary functions, such as prisoner positioning and tracking. The sensor network not only monitors the necessary vibration and position indicators of prisoners in real time, but also allows prisoners to move freely within a certain range, which helps to keep patients in good mood and is very helpful for prisoners' transformation. Since each prison room is equipped with corresponding sensors, prison guards can also fully grasp the rest of prisoners. When prisoners are active in the prison or outside the prison (sensor networks are required to be deployed outside the ward), prison guards can still locate and track them, and obtain their vibration and position index parameters in time.
Wear the node on the prison guard. Using the vibration sensor and positioning system configured on the node, the working area of prison guards can be monitored in real time and their personal safety can be guaranteed. On the one hand, when an emergency occurs, such as a prisoner attacking the police or a fight between prisoners, the monitoring system and the remote service center will respond quickly and notify the police force to quickly arrive at the scene of the incident for support; On the other hand, through the positioning and tracking of prison guards, we can master the work of prison guards, which is conducive to improving work efficiency.
4. System software and hardware design scheme
4.1 design of sensor nodes
Each sensor node includes one or more sensor elements, mote and battery pack. The sensor mainly detects vibration and temperature data. Mote is responsible for processing these data and transmitting them to the base station through the network.
The RF adopts nRF9E5, which is a system level RF chip launched by Nordic company in Norway. It adopts GFSK modulation and has strong anti-interference ability; Support multipoint communication, with high data transmission rate up to 0.1 MB / S; It has a unique shock burst signal transmission mode and transmission signal carrier monitoring function, which can effectively reduce power consumption current and avoid data conflict. The internal register provides the basic communication protocol for users, which is convenient for users to expand. Therefore, it is very suitable for the design of wireless data transmission system. The transceiver of nRF9E5 has three working modes: shock burst receiving (Rx) mode, shock burst transmitting (TX) mode and idle mode. The operation mode of nRF9E5 transceiver is determined by the special function register TRX_ CE and TX_ EN decision.
Digital vibration temperature sensor DS181320 is a miniaturized, low-power programmable single bus digital vibration temperature sensor developed by Dallas company, which can directly convert the measured temperature value into digital signal output. DS18B20 follows strict single bus protocol, that is, initialize first, then send ROM command, and finally send function command. Initialization consists of the host sending a reset pulse (by pulling the bus down at least 480) μ S), the host waits for the presence pulse sent back by DS18B20. DS18B20 waits 15 ~ 16 from detecting the rising edge of reset pulse μ After s, pull down the single bus by 60 ~ 240 μ s. Realize the transmission of existing pulse. The read-write operation of DS18B20 is realized through read-write timing. Therefore, using DS18B20 can simplify the circuit and improve the node integration and vibration temperature measurement accuracy.
4.2 software design
The system is a point to multipoint communication mode, so the communication protocol is divided into three layers: the first layer is the physical layer, which is implemented by nRF9E5 module hardware; The second layer is the network layer; The third layer is the application layer.
Tee (threshold sensitive energy efficient sensor network protocol) protocol is adopted in the network layer. The protocol is a responsive sensor network protocol, which transmits data only when the observed variable changes suddenly. Because the energy of sensor nodes is limited and inconvenient to supplement, the use of this protocol can effectively save energy and prolong the network life. In addition, in the prison security system, only when there is a sudden change in vibration and temperature data, it is necessary to transmit data, and it is not necessary for the prison security system to continuously send monitoring data to sink nodes at a constant rate. Figure 3 and Figure 4 are the software flow chart of host module and node module respectively.
In specific applications, two thresholds, hard and soft, are defined in teen to determine whether monitoring data needs to be sent. Setting of hard threshold. When the temperature and vibration exceed this value, the node sends monitoring data to sink node; Setting of soft threshold. When the vibration and temperature change range exceed this value, data will be sent.
5. Simulation and result analysis
In order to verify the feasibility and effectiveness of the algorithm, the influence of wind speed, wind direction, temperature, humidity, noise and other factors on the positioning accuracy is not considered temporarily. It is assumed that the propagation speed of sound in the air is 340 m / s.
As shown in Figure 5, it is assumed that three sensor nodes are distributed in an area of 100 m100m. In the figure, O represents the actual position of the target, * represents the position of the sensor node, and + represents the calculated target position.
The target can be located by statically distributed sensor array. The simulation results show that the error between the target position calculated by the target positioning system and the actual target position is within the acceptable range. In case of emergency, according to the calculation results, the police force can take measures quickly and effectively.
In the 100m square simulation area, a certain number of nodes, 10, 20,..., 100, are randomly and evenly distributed. Each node has the same communication distance of 10m, including 5 cluster head nodes. Fig. 6 shows the positioning error of DV hop positioning algorithm in the case of randomly selecting anchor nodes. The simulation results show that the localization error decreases with the increase of the number of sensor nodes. Therefore, in practical application, the number of sensor nodes in the dynamic monitoring system can be appropriately increased on the basis of comprehensive consideration of cost.
6. Conclusion
The fundamental purpose of prison security system is to bring higher reliability to management. The development of wireless network technology brings a wider space for the integration of prison security system, which not only overcomes the inconvenience caused by cable limitation in the process of security system integration, but also enables the security system to realize many new applications. The wireless sensor networking mode and the integration framework of wireless sensor and control network can make the wireless network technology effectively applied in the prison security system.
Reprinted from - viku electronic market network“
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