Short-distance wireless control system based on RFID

0 Introduction

Wired communication technologies have been widely used in various control fields. They have low power consumption, high speed, and are easy to implement. They are very suitable for occasions with few communication nodes and relatively fixed node positions. However, with the development of distributed and modular technologies, more and more control systems have begun to adopt distributed control structures with higher flexibility and scalability. Compared with conventional structures, they have the characteristics of more control nodes and uncertain positions and connection relationships between nodes. This makes the wiring of distributed control systems connected by cables cumbersome, the failure rate is high, and the flexibility and scalability are also greatly limited. Using wireless communication technology to build wireless distributed control systems can avoid the above problems, thereby effectively improving the reliability, scalability and reconfigurability of the system. At present, wireless communication technology has become a hot spot in communication technology, and various short-range wireless interconnection standards for industrial control automation and home appliance intelligence have emerged one after another. In recent years, the more popular ones include infrared (IrDA), Bluetooth (Bluetooth) and Wi-Fi (IEEES02.11). IrDA is a technology that uses infrared for point-to-point communication. It is mainly used for short-range data transmission between computers and peripherals [1]. Bluetooth can realize data communication within a short distance, has a universal radio interface and control software, and has been widely used for data communication between portable devices [2]. Wireless LAN (Wi-Fi) based on Ethernet protocol and CSMA/CA mechanism is the main standard for wireless access at present. It has a long communication distance and high communication speed [3]. The above wireless communication standards solve the problems existing in wired control systems, increase the flexibility of the system, and reduce the risk of integrating dedicated wireless communication technologies. However, IrDA has problems such as line of sight angle and short transmission distance, and is not suitable for connection between multiple devices. Although Bluetooth and Wi-Fi have the advantages of easy networking, high communication rate and long transmission distance, their software and hardware are relatively complex, the cost is high, and the power consumption is also large. Therefore, in order to meet the requirements of simple and low-cost wireless networking of some control systems, it is necessary to adopt a wireless connection technology with low complexity, low cost and low power consumption. Radio Frequency Identification (RFID) technology is a contactless automatic recognition technology that obtains relevant data of target objects through radio frequency signals [4]. It was originally used to replace barcodes in the field of logistics, and has now been applied to transportation, processing and manufacturing, production line automation, wireless sensor networks, etc. Compared with the above wireless communication technologies, it has the advantages of low power consumption, low cost, strong anti-interference ability, and long service life. Table 1 is a performance comparison of RFID communication technology and several other commonly used wireless transmission methods.

In view of the use requirements of small wireless distributed control systems and the characteristics of RFID wireless transmission technology, this paper proposes a short-range wireless control system based on RFID. The system adopts a one-master-multiple-slave communication mode. Among them, the host is composed of PC and RFID reader; the slave uses a high-speed microcontroller as the control chip, the active RFID tag as the wireless data transceiver module, and uses the general input/output interface of the microcontroller to realize the collection of sensor data and the control of the execution unit. The system circuit structure is simple and the application is flexible. It is suitable for small, small data volume, and low power consumption distributed control systems.

1 RFlD System Overview

1.1 Composition of RFID system

A basic RFID system usually consists of three parts: Tag: It consists of a chip and an antenna. Each tag has a unique electronic code and can store a certain amount of data to identify the target object. Tags can be divided into three types: active, semi-passive, and passive, depending on the way they send radio frequency signals. Active tags contain an internal power supply and can actively send radio frequency signals to the reader; semi-passive tags also contain an internal power supply, but they only power the processor in the tag, and do not provide the energy required to transmit radio frequency signals; passive tags do not have a power supply, and the energy required for their processors and signal transmission and internal processor operation comes from the electromagnetic field generated by the reader.

Reader/writer: It is used to control the RF transceiver to transmit RF signals, and receive the encoded RF signals from the tag through the transceiver, or encode information into the tag.

Application software: controls the reader or tag to send the specified data signal and performs relevant processing on the received data.

1.2 Working principle of RFID system

The working principle of the RFID system is shown in Figure 1. When a semi-passive or passive tag enters the electromagnetic field generated by the reader, it obtains energy through the induced current and sends out the specific information stored in the chip in the form of reflection modulation. After detecting the read/write request of the reader, the active tag actively sends a signal of a certain frequency. After receiving the signal from the tag, the reader decodes it and sends it to the application software for processing.

2 System overall structure and workflow

The RFID-based distributed wireless control system mainly includes a master PC with RFID reading and writing functions and multiple slaves with RFID tags. Its logical structure is shown in Figure 2, which is divided into two layers. The upper layer consists of PC and RFID reader, where the RFID reader is connected to the PC through a USB interface. The lower layer consists of multiple slaves, each of which includes five parts: a single-chip microcomputer, a sensor, an actuator, an RFID tag, and a battery. The upper structure is responsible for the coordination and management tasks of the entire system. It can obtain the sensor information and operating status of all slaves, and thus plan and coordinate the behavior of each slave.

Figure 2 Structure of wireless control system based on RFID

The system workflow is as follows: When the system is running, the PC starts to send commands to query sensor information and actuator status to all slaves at regular intervals. The slaves that receive the query commands are activated and, after completing the corresponding data collection tasks, send the information to the RFID tag in a specified format. After the RFID reader receives the information from the RFID tag, it sends it to the PCE through the USB interface for processing. Subsequently, the processed control commands are sent to each RFID tag. Finally, the microcontroller completes the operation of the actuator according to the requirements of the control commands. [page]

3 Hardware Circuit Design

The system hardware consists of a PC, an RFID reader and several slaves. Both the PC and the RFID reader are widely used commercial components, so the hardware design of the system is also the design of the slave. As shown in Figure 2, each slave consists of four parts: a microcontroller, an RFID tag, a sensor, and an actuator. The selection of sensors and actuators is closely related to the specific application background. Therefore, according to the system characteristics, choosing the right microcontroller and RFID tag becomes the most critical part of the design.

3.1 Selection of MCU

Since the slave is powered by a battery, in order to extend the battery life and simplify the design of the power supply circuit, the microcontroller needs to have low power consumption and a wide operating voltage range. Atmel's AVR series microcontroller ATmega8L is a low-power 8-bit CMOS microcontroller based on RISC structure [5]. It has an 8KByte flash, 512Byte EEPROM and 1KByte SRAM, an advanced instruction set and single-clock cycle instruction execution time, and a data throughput of 1MIPs/MHz; the operating voltage range is 2.7V to 5.5V, the operating clock range is 0 to 8MHz, and the current is only 3.6mA when running at 4MHz. It can meet the system's requirements in terms of power consumption, voltage range and processing speed. In addition, it has a rich high-level language programming environment, and software development is also relatively convenient.

3.2 Selection of RFID tags

The RFID tag is the core of the entire system and is directly related to the communication rate, communication distance and power consumption of the system. IDS-SL900A is a UHF semi-passive RFID tag launched by IDSMicrochip [6]. Its voltage range is 1.1~3.3V and it operates at 860MHz~960MHz. IDS-SL900A has built-in functional modules such as temperature sensor, real-time clock, ADC and EEPROM. It only needs an external battery and antenna to work. The system parameters can also be read and configured through the SPI interface, so it is very convenient to use.

The power consumption of IDS-SL900A is very low. It has three working modes: shutdown mode, idle mode and recording mode. In shutdown mode, the system has the minimum working current, which is generally 0.1uA; in idle mode, the current is about 2uA, at which time only the crystal oscillator circuit and clock circuit in the chip are working; and in recording mode, all functional modules such as the temperature sensor and EEPROM inside the IDS-SL900A are working, and the current is about 200uA. As can be seen from the above, IDS-SL900A has many functions, low power consumption, and simple peripheral circuits, which is very suitable for battery-powered applications.

3.3 Design Schematic Diagram

The hardware connection of the slave is shown in Figure 3. The main control microcontroller is ATmega8L; the active RFID tag is IDS-SL900A; and the actuator is the S3110 micro servo in the Futaba series of servos. Among them, ATmega8L is connected to the RFID tag through the SPI interface and two general input/output interfaces to read the data in the tag and configure the tag operation parameters. The control signal of the S3110 servo is the PPM (Pulse Position Modulation) signal, which is a pulse width modulation signal with a period of 20ms. The width of the positive pulse determines the rotation angle of the servo, ranging from 1ms to 2ms, corresponding to the minimum and maximum values ​​of the rated rotation angle of the servo. Figure 4 shows the format of the PPM signal, which is generated by the internal timing counter l of ATmega8L and output to S3110 through the output comparison interface 0ClA.

4 Software Design

The system adopts a one-master-many upper and lower layer structure. All slaves work under the supervision of the PC and receive user instructions through the graphical interface running on the PC. According to the structural characteristics of the system and the functions implemented by each part of the system, the system software can be divided into three parts: human-computer interaction program, data communication program, and sensor and actuator control program, as shown in Figure 5.

The human-computer interaction program runs on the PC. On the one hand, it receives the control parameters input by the user and converts them into control commands for each slave machine. On the other hand, it also sends query instructions to each slave machine at regular intervals and feeds back the sensor information and operating status information sent back by the slave machine to the user.

The data communication program is independent of the application program. It realizes bidirectional data transmission between the slave and the PC through the data buffer and synchronization signal. It includes the upper communication subroutine running on the PC and the lower communication subroutine running on the ATmega8L.

The control programs of sensors and actuators run on the lower layer ATmega8L. They are responsible for collecting sensor signals and slave operation status. At the same time, after receiving the control command, they can also complete the control operation of the actuator according to the control parameters specified by the control command.

5 Summary

In recent years, distributed control systems have received more and more attention. According to the usage characteristics of this control system and on the basis of analyzing the characteristics of commonly used communication technologies, this paper proposes a distributed wireless control system based on RFID. The system adopts a master-slave communication mode and has the advantages of simple structure, flexible application, and strong practicality. It can be flexibly configured according to various needs, and also has obvious advantages in cost and power consumption.