Detailed explanation of the design process of temperature and humidity sensor tags based on low-power active RFID
RFID (Radio Frequency IdentificaTIon) is a technology that uses radio frequency signals to automatically identify a target ID to obtain object information and obtain related data. Different from traditional magnetic cards and IC cards, RFID technology solves the two major problems of passive and contact-free. At the same time, it can realize moving targets and multi-target recognition, and can be widely used in various occasions. Its outstanding advantages are strong environmental adaptability, ability to penetrate non-metallic materials, large data storage capacity, and strong anti-interference ability. According to different power supply methods, RFID can be divided into two categories: passive RFID and active RFID. When passive RFID works, the tag obtains energy through the electromagnetic field of the reader, and the tag itself does not require a battery. Active RFID is on the contrary, it needs to provide the power required for all devices to work, and the electronic tag needs its own battery. Compared with passive tags, active RFID temperature and humidity tags have the advantages of low transmission power requirements of readers and long effective reading distance. Therefore, they are used in cold chain logistics, medical systems, warehousing materials management, vaccine production logistics, and health and epidemic prevention systems. , Scientific research institutions and other aspects have a very wide range of applications. However, active RFID temperature and humidity sensor tags have higher requirements for service life, reliability, and volume. Therefore, designing an active RFID temperature and humidity sensor tag with long life, high reliability and small size is of great significance in national life. This article mainly solves the problem of low power consumption in active tag design.
1 Active RFID system composition and working principleActive RFID system consists of three parts: active tag, reader and application system, as shown in Figure 1. Active tags have a unique identification code (ie ID), and some active tags have integrated sensors inside to measure specific physical quantities. Within the effective working range of the reader, the electronic tag actively sends its ID and the measured physical quantity to the reader in the form of electromagnetic waves, and the reader stores relevant information in its own storage device and in the reader The data can be transmitted to the application system through communication interfaces such as Ethernet port, RS-232, USB, etc., for further processing of the data.
2.1 Structure
The block diagram of the active temperature and humidity sensor tag designed in this paper is shown in Figure 2. The core of the active tag is a microcontroller (MCU), the radio frequency module receives/transmits radio frequency signals through the antenna; the EEPROM stores the identification code of the tag and the attributes of the item; temperature detection and humidity detection are used to detect the tag In order to simplify the design, the temperature and humidity of the environment can use a chip integrating temperature and humidity detection; the power detection module detects the battery voltage and indirectly detects the remaining power of the battery according to the comparison relationship between the battery power and the voltage ; The battery provides power for the normal operation of each module.
2.2 Overall circuit
2.2.1 Main control module
The main control module adopts a 16-bit ultra-low power single-chip microcomputer of Microchip's model PIC24F16KA102. This series of MCUs uses nanoWatt XLP (eXtreme Low Power) ultra-low power technology, and its typical sleep current can be as low as 20 nA, the real-time clock current is as low as 490 nA, and the watchdog timer current is as low as 370 nA. MCU can run continuously for more than 20 years without changing the battery, becoming the MCU with the most outstanding low-power performance among the 8-bit and 16-bit MCUs in the industry. The microcontroller has SPI, I2C, UART, 9 analog input channels, 3 16-bit timers/counters, and 3 external interrupts, which can fully meet the needs of active tags. MCU and label carry on serial communication through SPI interface, as shown in Fig. 3. J1 in Figure 3 is the ICSP interface used by the PIC 24F16KA102 microcontroller to download and debug programs.
2.2.2 RF transceiver module
nRF24L01 is a single-chip wireless transceiver chip that works in the 2.4 GHz ~ 2.5 GHz universal ISM frequency band. nRF24L01 is mainly composed of modulator/demodulator, CRC encoder/decoder, GFSK filter, mid-band pass filter, power amplifier, low noise amplifier (LNA), first-in first-out buffer (FIFO). Carry on communication with MCU through SPI interface, its circuit diagram is shown as in Fig. 4. nRF24LOT RF transceiver chip has the following advantages:
(1) It has 125 optional working channels, which can be used for frequency hopping working mode, which can effectively reduce the interference of the surrounding environment.
(2) The package area of ​​QFN20 is only 4 mm & TImes; 4 mm, which occupies a smaller PCB area.
(3) Low power consumption. When working in transmit mode, when the transmit power is -6 dBm, the current consumption is 9.0 mA, while in receiving mode it is 12.3 mA, and the current consumption is lower in power-down mode and standby mode.
(4) With automatic answer and automatic retransmission functions.
(5) Higher data transmission rate. It is 1 Mb/s when in ShockBurstTM mode and 2 Mb/s when in enhanced ShockBurstTM mode.
2.2.3 Temperature and humidity detection module
SHT21S is a temperature and humidity sensor from Sensirion, Switzerland, with small size, low power consumption and good stability. The temperature and humidity sensor has a temperature measurement accuracy of ±0.3°C at 25°C, a temperature response time of 5 s~30 s (τ63%); a humidity measurement accuracy of ±2.0% RH, and a humidity response time of 8 s (τ63%) . The chip communicates with the MCU through the SDM interface. The temperature and humidity measurement is selected by SCL (3 feet). When SCL is input high level, the humidity is measured; when SCL is low level, the temperature is measured. A low-pass RC filter can also be connected to the SDA pin to convert the SDM signal into an analog voltage output. The temperature and humidity detection module is shown in Figure 5.
2.2.4 EEPROM
There is 512 B of EEPROM inside the PIC24F16KA102 microcontroller. Therefore, this design uses the EEPROM inside the single-chip microcomputer to avoid external EEPROM, reduce the power consumption (generally in mA level) caused by external EEPROM, save components, reduce the area of ​​the circuit board, and reduce costs.
2.2.5 Battery detection
The power detection uses the MCU's internal high and low voltage detection HLVD (High/Low-Voltage Detect) function. The voltage value that generates the interrupt can be set through programming. This not only solves the problem of using A/D detection voltage without internal reference source, but also Reduce power consumption to a certain extent.
3 software design3.1 Format of sending data packet
The format of the sent data packet is shown in Figure 6. The preamble is used for synchronization and is only used in the sending mode; the flag bit is used for packet identification, only 2 of the 9 bits are used, and the remaining 7 bits are reserved; the data is 1 B~32 to be transmitted/received The item identification information of B width, for this design, refers to the temperature and humidity to be detected and the remaining battery capacity information; the CRC check selection generating polynomial is 16 bit CRC check of X16+X12+X5+X1.
3.2 Label workflow
In order to achieve the purpose of ultra-low power consumption, the label has two workflows: (1) Normal workflow, detecting the required physical quantities and sending them in packages, the time interval is 10 s (can be set in the program) once, Enter the deep sleep mode after every transmission, and wake up by the timer after 10 s. After waking up, the program will re-execute from the reset vector; (2) Enter the deep sleep state, through external interrupt 0 (that is, INT0, external interrupt request of nRF24L01 IRQ) to wake up, and execute again from the reset vector after wake up. The main program flowchart of the label is shown as in Fig. 7.
4.1 Power consumption test and estimation
First, the software simulator of MPLAB IDE V8.46, an integrated development environment for PIC microcontrollers, is used to determine the time required by the microcontroller in the initialization and temperature and humidity testing processes; second, the oscilloscope is used to test the duration and duration of the nRF24L01 in each process. The current consumed; then enter the data measured above into Microchip's extremely low-power battery life estimation software (Microchip XLP Battery Life EsTImator), as shown in Figure 8.
The calculation of tag life is based on the average current, that is, the theoretical life of the tag is equal to the battery capacity (mAh) divided by the average current consumed by the tag (mA). The average current is defined as follows:
It should be noted that the calculation of the actual life should consider the self-discharge rate of the battery used in the label (the estimated time given by the software of this design has taken into account the self-discharge rate of the selected battery). In the estimation, a 225 mAh LiMnO2 battery is used. The calculated battery life is 2 years, 263 days and 19 hours. The actual battery used is a 750 mAh manganese lithium battery. The label life of a 750 mAh manganese lithium battery is about 9.08. year. If you consider the actual self-discharge rate of the battery (slightly greater than the one given in this software), the actual battery life will be shorter. The calculated life is the life of the tag in continuous working day and night (that is, the temperature, humidity and battery voltage are detected every 10 s and then sent), that is, the worst possible is considered. The actual tag may only work during a certain period of the day, and enter a deep sleep state when it is not working, and the power consumed by the processor is only nA.
4.2 Whole machine test
When debugging, use the MCU serial communication method to display the received tag ID, temperature, humidity, and battery power information in the upper computer software in real time. When the sender sends ID and related data every 10 s, the receiving tag can be in accordance with the established time Receive the data correctly at intervals and display it in the upper computer software in real time. Compared with standard instruments, the accuracy of the measured temperature and humidity data can meet the requirements.
When the transmit power is set to 0 dBm in the software, the communication distance measured in the open test site is about 80 m; the communication distance tested in the enclosed corridor is 30 m-40 m.
Active electronic tags have extremely high requirements for low power consumption performance indicators, because even 1? The quiescent current of ZiA will consume more power for a long time, so special attention should be paid to this point in the hardware selection. If you need to design a system with outstanding low-power performance, you need to carefully consider both hardware and software. The low-power performance of hardware is essential, and low-power software measures are also indispensable, especially for systems that need to work for a long time. , The use of the device's sleep or standby state can greatly reduce the power consumption of the system. The active temperature and humidity sensor tag has been used in the temperature and humidity monitoring system of a company's production workshop, and has achieved a good low power consumption effect.
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