"In recent years, Internet of things technology has developed rapidly, and the informatization level of the whole society has been continuously improved. Smart home is one of the main applications of the Internet of things. It has become a hot research field and the development direction of home life in the future. It can provide users with a comfortable and convenient living environment. However, because most of the relevant products in the market are expensive, the penetration rate is still low. Previous exploration and development often focused on the transformation of electrical equipment itself, which once made smart home products a luxury. This paper introduces an electric energy control system. As an important part of smart home, it realizes the remote automatic control function without changing the original electrical equipment.
1 system structure
The electric energy control system is composed of remote controller and socket node, and its working principle is shown in Figure 1. When the user is at home, the socket is controlled by radio frequency through the remote control. After receiving the signal, the socket node is decoded by the microcontroller, and the socket with a specific number is powered on and off according to the obtained results, so that the electrical appliances connected to it are started or closed. When the user is far away from his residence, he can send a mobile phone short message to the remote controller through the GSM network. After reading the information, the microcontroller transmits the information to the indoor wireless network through the RF chip, so as to control the socket on the corresponding address. It can be seen that the remote controller belongs to the gateway node in the whole smart home system. On the one hand, it forms an indoor RF LAN with the socket node. On the other hand, it is connected with the GSM network to extend the remote control distance. The internal structure of the remote controller is shown in Figure 2, including temperature and humidity detection circuit, clock module, nRF905 RF transceiver module, GSM module and other functional circuits. These modules are connected with the control core LM3S811. The microcontroller adopts arm coaex-m3 architecture. Due to relying on the high-density thumb-2 instruction set, the memory overhead is greatly reduced and the transplantation of the operating system is more convenient.
Fig. 1 working principle of remote control socket
Figure 2 structure block diagram of remote controller
The socket node mainly realizes RF communication with the remote controller and on-off control of the relay. Its structural block diagram is shown in Figure 3. Smoke sensors in sockets are used to prevent fire hazards. Once smoke or combustible gas is detected, the corresponding relay on the socket will be disconnected and reported to the remote controller through the RF transceiver module. After receiving the information, the remote controller will timely remind the user to take corresponding measures through the SMS function of GSM module to prevent the occurrence of danger or the further expansion of property loss. Due to less workload at the socket end, STC12c5620AD microcontroller is used as the main control chip at the socket end in terms of cost and performance.
Figure 3 socket node structure block diagram
2 hardware circuit design
2.1 temperature and humidity detection circuit
The system uses temperature sensor LM35 and humidity measurement module chm-02 for environmental monitoring. The voltage output of LM35 has a linear relationship with Celsius temperature, and the measurement accuracy of ± L / 4 ℃ can be achieved in normal temperature environment without calibration. Chm-02 module can detect the humidity in the range of 20 ~ 95% RH at the temperature of 0 ~ 70 ℃, and the measurement accuracy at room temperature is 5% RH. The interface diagram between temperature and humidity sensor and MCU is shown in Figure 4. Since the analog signals output by the two sensors are within the detection range of the A / D sampling circuit in the MCU chip, the outputs of the two sensors are directly connected with the two ADC pins of the MCU. The use of analog sensor not only makes full use of the on-chip resources of the controller, but also improves the utilization of subroutines.
Fig. 4 Schematic diagram of interface between temperature and humidity sensor and MCU
2.2 smoke detection circuit
Based on the electrochemical characteristics of SnO:, the smoke sensor mq-2 has good detection sensitivity for combustible gases and soot. The schematic diagram of smoke detection circuit is shown in Figure 5. Mq.2 before normal operation, the H.H poles of the internal heating wire need to be powered on for preheating. In order to prevent the temperature of the internal signal line from being too high due to excessive heating current, the heating wire is connected in series with 100 Q resistance here. When the smoke or combustible gas in the environment exceeds the warning threshold, the conductivity between the two poles of sensor A.B increases rapidly, and the voltage obtained by its load resistance m in series also increases accordingly. After the voltage signal is amplified by the low-power operational amplifier tlc27m2, the analog output corresponding to the smoke or combustible gas concentration is obtained, and finally connected to the ADC module of the controller for quantization.
Fig. 5 Schematic diagram of smoke detection circuit
2.3 clock module
In addition to displaying the system time, the clock module can also conduct on-off timing for a single socket. The clock circuit schematic diagram is shown in Figure 6. DS1302 communicates with MCU through serial mode. In order to ensure the stability of signal transmission, the interface has been pulled up. The chip adopts dual power supply, and the main power supply can charge the standby power supply with trickle current when it works normally; In case of power failure, start the standby power supply to avoid clock stagnation caused by sudden power failure. Considering the convenience of use, the remote controller is powered by lithium battery. The main power pin VCC2 of the DS1302 is connected to the 3.3 V output of the integrated regulator, while the standby power pins VCC1 and 4700 μ The capacitance of F is connected in series, and the two power pins are isolated by diodes. Due to the low power consumption of the chip, the discharge of the capacitor can temporarily maintain the operation of the chip in the process of replacing the battery.
Fig. 6 schematic diagram of clock circuit
2.4 RF transceiver module nRF905
RF transceiver module is a bridge connecting socket and remote controller. NRF905 integrated transceiver can be configured and used in three ism bands with low power consumption. All nodes in the system are set in the 433 MHz frequency band. The schematic diagram of RF transceiver circuit is shown in Figure 7. The SMA interface is used to connect the single ended antenna with characteristic impedance of 5O Ω, which is conducive to the omni-directional radiation of the signal. Single ended antenna is also called unbalanced antenna, and its main reference point is signal ground, while the antenna interface (pins Ant1 and ant2) of nRF905 is differential RF output port. In order to maintain the signal balance and ensure the impedance matching of the two ports, a balun circuit is added between them to adjust the characteristics of the chip output.
Figure 7 RF transceiver circuit
2.5 GSM communication module
The combination of short-range RF network and GSM technology not only gives play to the flexible configuration of short-range RF network, but also gives play to the advantages of GSM technology in communication distance. The core of GSM communication circuit is SIM300 module, and its peripheral circuit is shown in Figure 8.
Figure 8 schematic diagram of GSM communication circuit
Schematic diagram SIM300 communicates with MCU through serial port, and 22 Ω resistance in series between module and SIM card is used for impedance matching. In order to ensure the signal transmission quality, the SIM card data line is pulled up, and the smf05c electrostatic suppressor connected in parallel with the pin is used for electrostatic protection. 100 in parallel between power supply and ground μ F tantalum capacitor and 1 μ F ceramic capacitor is used to remove low-frequency burrs and takes into account the high-frequency characteristics to a certain extent. Press the key S1 to lower the potential of pwrkey pin for about 2s to complete the power on and power off of the module. The current state is connected in series with VDD_ LED indication on ext pin. In order to facilitate program control, a triode switching circuit is added on the basis of the original key. When the module works abnormally, the state of PWR port can be rewritten by software to realize the automatic reset of SIM300.
3 software design
The remote controller and socket are the nodes of the whole RF wireless network, but the differences in hardware structure determine their functions and status, and also make them different in the way of software design.
3.1 remote controller node program design
The remote controller is not only the control core of the system, but also the link between the user and the socket. Therefore, there are many concurrent modules and heavy tasks in the program. Considering that the arm processor used in the remote controller can provide comprehensive support for the operating system, using μ C / OS - Ⅱ operating system schedules multiple tasks in the node, which can effectively ensure the real-time and stability of the system, and is also conducive to the expansion of functions. Before operating system transplantation, tasks need to be divided, and each function corresponds to a system task. At the same time, the problem of frequent scheduling caused by too fine division should be avoided. The program flow of the remote control node is shown in Figure 9, which includes 7 tasks. Synchronization and communication between tasks are realized through semaphores, message queues, message mailboxes, etc. From the user's point of view, these tasks are executed concurrently.
Figure 9 program flow chart of remote control node
The priority of key scanning task is the highest among all user tasks. Read the key value input by the user through the interrupt mode, and store the data in the message mailbox keymbox. If the number keys 1-6 are pressed, notify the RF transmission task processing; If the clock setting key is pressed, adjust the clock or set the timer. The clock timing task is used to obtain the clock output value of DS1302. After the timing time arrives, send a message to notify the RF transmission task processing, and automatically hang up after completion. The RF transmission task is to send the power on / off control signal to the socket with corresponding number in RF mode according to the control code obtained in other tasks, and then wait for the socket end to return the action information. If there is no feedback after timeout, resend it once, and suspend the task after resending it three times. The danger alarm task needs to go through the same frequency carrier detection and address matching confirmation before receiving the RF signal, and then send the information to the mailbox. After decoding and confirming the danger alarm identification, it will notify the user in the form of short message through the GSM module. The short message receiving task is responsible for receiving the user's short messages and storing them in the message mailbox gsmmbox. Through the at command "at + cmgr = I", only the short message with sequence number 1 is read each time. After successfully extracting the control code (including socket ID number and switch action code), delete the message and send a task transmission message to the RF. The environmental monitoring task is responsible for cyclic sampling of indoor temperature and humidity information. Although the linearity of the temperature sensor is good, the external environment has a great impact on the humidity sensor, so it is necessary to linearize its output voltage. The data is stored in the message queue, and the final result is the arithmetic mean of the three measurements. LCD display has the lowest priority. After the above tasks are completed, it is responsible for displaying the final status of each socket, clock information and indoor temperature and humidity measurement results.
3.2 socket node program design
The socket node program flow is shown in Figure 10, in which the most important work is to realize the reception and transmission of RF signals. When there is no smoke alarm, nRF905 enters the receiving mode and listens to the channel at the same time; If the same frequency carrier is detected and the packet address is valid, start receiving; When the CRC check result is correct, the hardware will remove the preamble, check code and address code of the data packet by itself, notify the MCU that the data is ready, and then the MCU reads the received information through the SPI serial bus.
Fig. 1o socket node program flow chart
RF signal transmission is essentially the inverse process of reception. When the nRF905 enters the standby mode, the MCU transmits the address and data information to the transmission register of the RF chip, starts the chip to enter the RF transmission mode, and then the on-chip hardware automatically completes the tasks of data packaging, coding, modulation and transmission. After one frame of data is sent, the RF chip turns into standby mode and waits for the next activation. Every receiving or transmitting process of RF circuit is accompanied by the on or off action of relay. By default, the smoke sensor is enabled. In order to prevent users from misjudging the system by smoking indoors, the smoke detection function can also be set to fail.
4 test and analysis
The field test was carried out in the house shown in Figure 11. Six sockets and one remote controller were placed in seven areas a to g. In order to evaluate the anti-interference ability of the system, two interference sources with frequencies of 432 MHz and 434 MHz are placed at the junction of each area. Change the location of the remote controller, remotely control each of the six sockets for 200 times, and record the feedback signal at the socket end. If an error message is returned or no feedback is given, it will be recorded as a packet loss.
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