Abstract: To address the problem of industrial process monitoring in the presence of strong electric and magnetic field interference and where close-range measurement is inconvenient, this paper proposes a novel wireless sensor telemetry system design. Based on the given system architecture, the system utilizes the CC2420 wireless communication chip and the AT89S53 microcontroller to achieve timed acquisition, calculation, error processing, storage, and data transmission control of the required measured parameters. An encoder is used in the transmitting circuit, and a decoder is added to the receiving circuit, effectively controlling errors caused by wireless channel noise or interference, while also expanding the data transmission capacity of the RF circuit. The system features a simple circuit design, high information transmission reliability, high data transmission accuracy, and strong anti-interference capability.
Keywords: data acquisition; wireless transceiver; encoder; decoder; telemetry system
Design of a New type of Wireless Sensor Telemetry System
Zhang-Xiujian1, Wang-Junben2
1. Institute of Electrical Engineering,Yanshan University,Qinhuangdao 066004,China.
2. Northwestern Polytechnical University,Xi'an 710129, China)
Abstract: A new type of wireless sensor telemetry system design method is proposed in this paper for the strong electric field, the strong magnetic field disturbance and without the industrial process to facilitate close monitoring. Based on the proposed system framework, we realize the timing parameters for measurement of the collection, calculation, error handling, storage, and data transmission control by using wireless communication chip CC2420 and MCU AT89S53. The encoder is used in send circuit and the decoder is used in receiving circuit accordingly, thus the mistakes which is caused by the wireless channel noise or the interference can be controlled effectively, and expanding the RF circuit of the transmission data capacity. Experimental result shows that the design of the system circuit is simple, good reliability of data transmission with high precision, and strong anti-jamming ability.
Keywords: data acquisition; wireless transceiver; encoder; decoder; telemetry system
1 Introduction
In recent years, with the maturity of wireless communication technology, wireless sensor telemetry systems have shown great advantages. Telemetry systems enable the measurement of parameters such as temperature over long distances to solve the problem of the distance between the measured location and the instrument room or control room, or the location of the measured parameter being harmful to personal safety [1-2]. For example, parameter telemetry is required in fields such as stress measurement of large dams, temperature measurement of blast furnace walls in ironmaking, parameter measurement of large automated production equipment, and parameter measurement of nuclear reaction zones.
There are many ways to transmit information over long distances, such as wireless transmission via radio frequency and infrared, and wired transmission via cable and fiber optic. Using cable for information transmission is the simplest method, but the information is lost during transmission, has poor anti-interference performance, and is heavy; using fiber optic transmission is too expensive and difficult to maintain; using infrared transmission is greatly affected by the weather, and requires a large transmission power when the distance is long [3]. This system uses a wireless sensor hardware node based on the radio frequency chip CC2420 and the microcontroller AT89S53 as the core to design a new type of telemetry system that is simple, has high data transmission accuracy, and strong anti-interference capability. This system is mainly used in situations where there is strong electric field and strong magnetic field interference and it is not convenient to measure at close range. This paper takes remote temperature measurement as an example to illustrate the design idea.
2 System Overall Scheme
The system mainly consists of four parts: a data acquisition section; a data processing section; a wireless communication section; and a receiving and display section. The data acquisition section comprises sensors, signal conditioning circuits, opto-isolation devices, and an A/D converter. The receiving and display circuit consists of a decoder, a seven-segment decoder, a decimal counter, a driver, and a digital display circuit. An encoder is used in the transmitting circuit, and a decoder is added to the receiving circuit to effectively control errors caused by wireless channel noise or interference, while also expanding the data transmission capacity of the RF circuit. The components selected for the system design all have strong anti-interference capabilities and are suitable for industrial environments with strong electric and magnetic field interference.
This system is a general telemetry system. By changing different sensors, adjusting the conditioning circuit structure and parameters, and modifying the calculation and processing software program, the purpose of telemetry of various measurement parameters can be achieved [4]. In this design, the wireless communication chip selected is CC2420, and the microcontroller is AT89S53. The overall system flowchart is shown in Figure 1.
Figure 1 System Overall Scheme Flowchart
3 System Hardware and Software Design
3.1 Hardware Design
3.1.1 Data Acquisition Unit Design
This system uses the HD01 series temperature transmitter. It is an instrument that converts temperature sensor signals (resistance temperature detector (RTD) and thermoelectric field signals) into standard DC signals through fully isolated amplification, thereby enabling accurate measurement of the measured signal. This transmitter features full isolation between input, output, and power supply, providing strong anti-interference capabilities. It also offers a wide input and output selection range, corresponding to various temperature ranges from -200 to 1600℃, high accuracy, selectable power supply, and DIN rail mounting for easy inspection and maintenance.
The temperature transmitter processes sensor data, including signal conditioning (such as filtering and amplification), data display, automatic calibration, and automatic compensation. It converts the electrical signal detected by the sensor into a 4–20mA DC signal. The signal conditioning circuit converts the temperature sensor's standard 4–20mA current signal into a voltage signal. The simplest method is to connect a resistor in series at the output, but this results in a non-zero voltage signal. Therefore, an I/V converter is typically used; this system employs an RCV420 converter.
Linear transmission of analog signals between field sensors and A/D converters can be achieved by isolating the analog signals using the linear region of an optocoupler. Electrical couplers offer excellent isolation performance, achieving electrical isolation between the input and output terminals; optical signals are transmitted unidirectionally, with no feedback from the output signal to the input, effectively blocking electrical connections between circuits or systems without severing signal transmission; optical signals are unaffected by electromagnetic interference, ensuring stable and reliable operation; they also possess strong common-mode interference immunity, effectively suppressing interference and eliminating noise. Due to their unique principles and structural characteristics, couplers are particularly suitable for applications with strong electric and magnetic field interference. This system utilizes the Agilent HCNR201 optocoupler.
The A/D chip is one of the core components of the system, and the measurement accuracy mainly depends on the accuracy of the A/D converter. This system uses the MC14433, a 3 1/2-bit monolithic dual-slope A/D converter with zero drift compensation and manufactured using CMOS technology. It features few external components, high input impedance, low power consumption, strong anti-interference capability, wide power supply voltage range, and high accuracy, and also has automatic zeroing and automatic polarity reversal functions. Due to its low conversion speed, it can only convert some slowly changing physical parameters such as temperature, continuous pressure, and tension. It adopts a dynamic scanning BCD code output mode, that is, the thousands, hundreds, tens, and units digits of the BCD code are output in turn at Q0 to Q3 in a time-division multiplexing manner, while synchronous word selection pulses are output at DS1 to DS4 terminals, making the reading intuitive and applicable to various instruments.
3.1.2 Encoder and Decoder Design
This system uses Motorola's MC145026 encoder and MC145027 decoder. MC145026/27 are pairing chips for communication produced by Motorola. They are low-voltage CMOS codec devices with strong anti-interference capabilities and are widely used in remote control and telemetry circuits [5]. Their basic characteristics are:
(1) When the encoder transmit pin TE (pin 14) is grounded (low level), the encoder will output the 5-bit address and 4-bit data serially in different pulse encoding methods. Each time it is transmitted, the encoder will automatically send out two identical address and data pulse strings (output from pin 15).
(2) After the decoder receives the first series of pulse signals sent by the encoder, if the 5-bit address of the decoder is exactly the same as the 5-bit address of the encoder, the 4-bit data transmitted will be sent into the register. After receiving the second series of pulse signals, a second check will be performed. If the address is correct and the data matches the first time, the 4-bit data will be sent to the output terminal and latched. The VT pin changes from low level to high level to indicate that a signal is received. This high level will be maintained until there is new data input or no new data input after a interval of 4 data transmissions.
(3) The time required for each transmission depends on the operating frequency of the internal oscillator, which is determined by the external RC parameters. The oscillation frequency can be selected in the range of 1.71~362KHz.
(4) The static current is very small, less than 1uA for encoders and generally less than 100uA for decoders.
3.1.3 Data Processing Unit Design
The system utilizes the AT89S53 microcontroller to perform timed acquisition, calculation, error processing, storage, and data transmission control of the required measured parameters. The AT89S53 is a low-power, high-performance CMOS 8-bit microcontroller with 12k bytes of on-chip ISPD serially programmable Flash read-only memory capable of being erased and rewritten 1000 times. Integrating a general-purpose 8-bit central processing unit and ISP Flash memory, the powerful AT89S53 microcomputer provides a cost-effective solution for many embedded control applications.
The AT89S53 microcontroller is designed and configured with an oscillation frequency that can be set to 0Hz and a power-saving mode that can be set via software. In idle mode, the CPU suspends operation, while the RAM timer/counter, serial port, and external interrupt system continue to operate. In power-down mode, the oscillator is frozen while the RAM data is preserved, and other chip functions are stopped until an external interrupt is activated or a hardware reset is performed.
The outputs of the A/D converter, Q0-Q3 and DS1-DS4, use a polling method to input the thousands, hundreds, tens, and units digits into the microcontroller, and then store these four digits in RAM. The input data can be positive or negative, determined by the thousands digit, Q2: Q2=1 indicates positive data; Q2=0 indicates negative data. The sign bit is determined by software checking if Q2 is 1. Controlled by the microcontroller program, the sign bit, thousands digit, hundreds digit, tens digit, and units digit are sequentially output to the wireless communication chip. In addition to the above functions, the microcontroller can also perform calculations and data processing using software, calibrate nonlinear sensors, or automatically compensate for certain regular errors. This fully utilizes the microcontroller's capabilities and improves measurement accuracy.
3.1.4 Wireless Communication Unit Design
The wireless communication module uses Chipcon's CC2420 as its core device. This chip is a radio frequency transceiver that conforms to the IEEE 802.15.4 standard and operates on the 2.4 GHz ISM public channel [6]. This transceiver has low power consumption, strong anti-interference ability, and features programmable output strength and transmission/reception frequency. The communication distance between two nearby nodes is generally 10 to 100 m, which can be increased to 1 to 3 km by increasing the wireless transmission power. Its maximum transmission/reception rate is 250 kbps. This chip also features hardware encryption, strong anti-interference ability of adjacent channels, security and reliability, and strong anti-destruction capability.
The CC2420 is used for physical layer data transmission and reception and low-level control. It uses four pins—SFD, FIFO, FIFOP, and CCA—to indicate the data transmission and reception status. The processor exchanges data and sends commands to the CC2420 via the SPI interface. The CC2420 is configured with an SPI-compatible serial interface using a simple four-wire configuration (SI, SO, SCLK, CSn), in which case the CC2420 is controlled. The AT89S53's SPI operates in master mode, acting as the controller for SPI data transmission, while the CC2420 is configured as a slave. The pin connection diagram between the AT89S53 and CC2420 is shown in Figure 2.
Figure 2 Pin connection diagram between AT89S53 and CC2420
The CC2420's peripheral circuitry includes a crystal clock circuit, an RF input/output matching circuit, and a microcontroller interface circuit. The CC2420 can be configured to operate via a 4-wire SPI bus, enabling read/write operations on buffered data and status registers. The transmit/receive buffers can be configured by controlling the states of the FIFO and FIFOP pin interfaces. The CC2420 receives data signals from the AT89S53 microcontroller via the SI pin and transmits data via the SO pin.
3.1.5 Receiver and Display Circuit Design
The sign bit and 4-bit data output from the decoder are decoded by a BCD-to-seven-segment decoder and then used in conjunction with a decimal counter (bit control) for dynamic scanning display. This system uses a 4511 seven-segment decoder and a CD4017 decimal counter. Upon power-up, when no signal is being transmitted, the 4-bit output of the MC145027 decoder is 0000. A sharp pulse is input to the 4017's clear input R, clearing the 4017 and selecting the units digit, which displays 0, indicating readiness to receive signals. When signal transmission is in progress, the communication program controls the input, first the sign bit, then the thousands, hundreds, tens, and units digit data are input sequentially. After the MC145027 receives the data of the sign bit, VT changes from low level to high level. It is connected to the input terminal CL of 4017. The signal of VT changing from low level to high level causes 4017 to carry, changing from Q0 high level to Q1 high level. BG3 is turned on, so that the sign bit is displayed on the digital tube. The rest are similar.
The novel wireless sensor telemetry system designed in this paper is designed for situations where there is strong electric field and strong magnetic field interference and it is inconvenient to measure at close range. If conventional battery power is used, the battery capacity is limited and the presence of battery leakage current will greatly shorten the battery life. Especially when used in a humid environment without certain protective measures, or due to the quality problems of the battery itself, the battery will self-discharge and the node will quickly fail due to energy depletion [7]. Considering that there is always direct or reflected light in the environment and meteorological environment of nuclear and chemical contamination areas, solar energy is the best way to power the system.
3.2 Software Design
The system's software program adopts a modular design approach. The microcontroller communication program is written in assembly language, mainly including the initialization of the sensor program, such as setting interrupts, timers, and serial port initialization, as well as the structural configuration of the CC2420, such as receive/transmit mode, RF output power, power-on/low-power mode, etc., for data acquisition and transmission. The flowchart of the A/D conversion and transmission program is not detailed here.
4. Conclusion
The system employs a CC2420 wireless communication chip and an AT89S53 microcontroller to perform timed acquisition, calculation, error processing, storage, and data transmission control of the required measured parameters. The CC2420 wireless communication chip is used for data transmission and reception, with an encoder in the transmitting circuit and a decoder added to the receiving circuit. This encodes and transmits the digital signals of the measured parameters and decodes and receives the original signals, effectively controlling errors caused by wireless channel noise or interference. The selected components offer high data transmission accuracy and strong anti-interference capabilities, making this system of significant reference value and promising for applications in situations with strong electric or magnetic field interference where close-range measurements are inconvenient.
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