In order to improve the security and reliability of the smart password lock, in addition to the measures taken in the device selection (such as the device with low power consumption and wide temperature range), some key technologies are also adopted in the design.
Line multiplexing technology
The smart monitor and electronic lock are placed off-site, and the smart monitor supplies the power required by the electronic lock and receives the alarm and status information it sends. If the communication line and the power supply line are separated, it is necessary to increase the number of cable cores and increase the safety hazard. This paper adopts the line multiplexing technology, which uses only one two-core cable to realize the transmission of power and information. The schematic is shown in Figure 3.
At the transmitting end, the electronic lock boosts the modulated data signal through the pulse transformer T and sends it out; at the receiving end, the pulse transformer T steps down the received data signal and sends it to the demodulator to reduce the carrier signal during transmission. Loss in the middle. In order to reduce mutual interference between communication and power supply, the selection of choke coil L and coupling capacitor C should be considered comprehensively.
Let the carrier frequency fo=400kHz, in order to ensure that most of the signal energy is transmitted to the receiving end, take L=33.7μHC1=0.047μF.
Current monitoring technology
In order to prevent the artificial destruction of the communication line and the electromagnetic actuator from causing the current flowing through the electromagnetic coil to be excessively burnt due to some reason, the current monitoring technology is adopted in the design of the smart code lock. The current monitor uses the MAX471 current-to-voltage conversion chip from MAXIM. The chip can convert the measured current I into a ground output voltage U, and has the characteristics of large measuring range, high precision, and the output voltage U and the measured current I are proportional. The current monitor output voltage is sent to the A/D converter. The MCU reads the A/D conversion result to know the current change in the line, and finds the abnormality in time by analysis and issues an alarm signal. The principle circuit is shown in Figure 4.
Data communication and preprocessing technology
The intelligent monitor receives the status information sent by the lock (including the opening and closing of the lock, the first password error, the second password error, the third password error, etc.), the current value flowing through the electromagnetic actuator coil, and Reading the supply current value of the communication line at that time, the three combine to form a data block, wherein the operation state occupies 1 byte, the supply current occupies 2 bytes, and the coil current occupies 2 bytes. The intelligent monitor is always in the receiving state during communication with the electronic lock. In order to improve communication reliability, this paper adopts the method of repeated transmission in the communication protocol. The electronic lock repeatedly transmits 5 times for each group of data. After receiving the data, the intelligent monitor corrects the error by using the large number decoding law. The accuracy of data reception. In addition, in order to save memory, pre-processing technology is adopted for the received data, that is, after receiving one data, the data is first compared with the set threshold, and if it is greater than the threshold, an over-limit alarm is issued; if less than The threshold value is compared with the same type of data received on the same day, and the larger one is retained. In this way, the data stored every day is the maximum value in the same type of data, and the flow chart is as shown in the figure.
The intelligent analysis and prediction technology is based on each received data block, compared with the recorded value of the previous similar data, analyzes the magnitude and trend of the current change caused by the operation, finds problems in time, and reports the management personnel. Improve the reliability of the entire system.