Abstract: This paper presents a tire temperature acquisition system based on wireless communication, sensor technology, and microcontroller technology. The system reliably, accurately, and in real-time acquires temperature data inside the tire and has data storage capabilities, providing reliable data for research on vehicle safety and tire temperature fields. Unlike traditional tire temperature monitoring systems, this system features multi-point acquisition of tire internal temperature. The temperature acquisition unit mainly consists of a sensor module, a controller module, and an RF module. The RF module uses the nRF905 wireless transmission chip to achieve short-range wireless data transmission.
Keywords : wireless communication; tire temperature; temperature field; NRF905
Intermediate Classification Number : TP 9 Document Identification Code: B
0 Introduction
Tire performance directly impacts vehicle safety and operational efficiency. Since tires are primarily made of rubber and reinforcing materials (such as nylon cords and steel cords), excessively high tire temperatures cause the nylon cords to shrink and the rubber to age faster. Sustained high temperatures significantly reduce tire lifespan. Currently, heat damage is the most common type of tire damage, severely affecting tire lifespan and vehicle safety.
Tires are a crucial component of automobiles. Aside from aerodynamic forces, almost all external forces acting on a vehicle are generated through the contact between the tires and the road surface. Measuring tire temperature using experimental methods allows for the study of temperature distribution during tire rolling, exploring the impact of operating conditions (tire speed, tire pressure, load, and environment, etc.) on tire temperature. It also provides a reference for optimizing tire design and formulation, and offers necessary reference conditions for numerical analysis.
The experimental method can be used for tires of any type and size, and it includes contact and non-contact methods. The non-contact method mainly refers to determining the tire's surface temperature by measuring the amount of infrared radiation emitted by the tire using an infrared thermometer. However, since infrared thermometry can only measure the surface temperature of the tire, this method has significant limitations when measuring the internal temperature of the tire. Oh used an infrared imaging system to measure the temperature distribution on the outer surface of the tire. Wang Qingnian used a two-way infrared thermometry system to measure the temperature of the rolling tire surface, obtaining the temperature distribution on the rolling tire surface. The system performance was reliable and met the surface temperature measurement requirements. Using tire-embedded sensors and wireless transmission to collect the internal temperature of the tire is currently a hot topic, and the system development varies depending on different functional requirements. Developing a wireless tire temperature acquisition system capable of multi-point measurement is the research direction of this design.
1 System Overview
During tire rotation, the temperature varies at different points inside the tire, necessitating the collection of temperature data from multiple points. Given the characteristics of this system—numerous measurement points, harsh construction site environment, difficult wiring, and relatively short communication distance—this research focuses on short-range wireless data transmission technology. By comparing several short-range wireless transmission technologies, a short-range radio frequency technology meeting system requirements was adopted to achieve high-precision temperature data collection and fully digital transmission. At the on-site monitoring center, human-computer interaction technology is used, employing computer monitoring software to collect real-time changes in the tire's internal temperature, providing accurate and abundant data for subsequent quantitative and qualitative analysis.
1.1 Overall System Structure
The wireless tire temperature acquisition system mainly consists of a temperature collector, a temperature receiver, and a host computer. The temperature collector and the temperature receiver exchange data wirelessly, and the host computer connects to the temperature receiver via an RS232 bus to read and process the data.
Tire temperature measurement is a crucial part of this system, and the accuracy and real-time measurement of temperature data directly impacts the operation of the entire system. The temperature acquisition unit mainly consists of a temperature sensor, a signal processing module, a microcontroller, and a wireless transceiver module. The temperature sensor is directly connected to the microprocessor, and the connection is convenient and easily detachable. The acquisition unit is battery-powered, and its aluminum casing provides strong anti-interference capabilities and is very easy to install. The overall structure of the system is shown in Figure 1-1.
Figure 1-1 Overall structure diagram of the system
Fig 1-1 Frame picture of the system
1.2 System Working Principle
The system collects temperatures at different points inside the tire; therefore, the temperature collector includes multiple temperature sensors. The signals from the temperature sensors are analog signals. A signal processing circuit is dedicated to amplifying and converting these analog signals to digital (A/D) values. A microcontroller performs software filtering and visualization of the temperature data, and the final data is transmitted wirelessly by a transceiver module. The temperature collector exchanges data with the receiver wirelessly, and temperature data acquisition only begins upon receiving a collection command from the receiver.
Data exchange between the temperature receiver and the temperature acquisition unit is wireless. To improve transmission efficiency and ensure communication reliability, wireless transmission follows a self-developed wireless communication protocol, and communication coordination is controlled by the temperature receiver according to the protocol. Communication between the temperature receiver and the acquisition unit must not exceed the effective distance of the communication module; otherwise, communication failure will occur. The temperature receiver exchanges data with the host computer via a serial port, and their communication also follows a specific communication protocol. The host computer sends control commands to the temperature receiver, and the temperature receiver transmits temperature data back to the host computer. The entire acquisition system operates under the control of the host computer management software, which initiates the control commands. The receiver not only transmits and converts data but also displays the temperature at different points inside the tire via an LCD screen. Under the control of the host computer management software, the host computer performs functions such as displaying temperature data and recording historical data. The system software can open multiple display windows on the monitor, each displaying real-time temperature data from different points simultaneously, and the host computer's temperature data acquisition cycle is adjustable.
2 Temperature acquisition device hardware design
2.1 Microcontroller Unit
The design of the microcontroller unit is the core of the entire temperature collector. Considering the requirements of small size and low power consumption in this system, the microcontroller selected is the AT89S52 microcontroller from Atmel. The main function of the microcontroller is to manage all peripheral devices of the system and complete the tasks of temperature data acquisition, processing and transmission.
The AT89S52 is a low-power, high-performance CMOS 8-bit microcontroller with 8KB of in-system programmable Flash memory. Manufactured using Atmel's high-density non-volatile memory technology, it is fully instruction and pin-compatible with industrial 80C51 products. The on-chip Flash memory allows for in-system programmability of the program memory and is also compatible with conventional programmers.
2.2 Temperature Signal Conversion Circuit
This wireless tire temperature acquisition system collects temperatures at different points inside the tire, unlike traditional tire temperature monitoring systems. Thermocouples are commonly used temperature sensing elements in industry. To address the aforementioned issues such as linearization, thermocouple cold junction compensation, and analog-to-digital conversion, this design employs a single-chip K-type thermocouple-to-digital converter, the MAX6675.
(1) Main performance of MAX6675
The MAX6675 is a thermocouple amplifier and digital converter launched by Maxim Integrated, which integrates a thermocouple amplifier, cold junction compensation, A/D converter and SPI serial port [8]. The MAX6675 is packaged in an 8-pin surface mount package, and the pin arrangement is shown in Figure 2-1. The pin functions are listed in Table 2-1.
Figure 2-1 Pin diagram of MAX6675
Fig.2-1 Pins picture of MAX6675
Table 2-1 MAX6675 Pin Functions
Tab.2-1 Explain of MAX6675's pin defunction
pin | name | Function |
1 | GND | grounding terminal |
2 | T- | K-type thermocouple negative electrode |
3 | T+ | K-type thermocouple positive electrode |
4 | VCC | Positive power supply terminal |
5 | SCK | Serial clock input |
6 | CS | When the chip select pin (CS) is low, the serial interface is started. |
7 | SO | Serial data output |
8 | NC | Empty pin |
The MAX6675 operates at 3.3V and has an operating temperature range of -20 to +85℃. It features on-chip cold junction compensation, high-impedance differential input, and thermocouple disconnection detection. It also boasts a temperature measurement range of 0 to 1024℃, a 12-bit resolution of 0.25℃, a 2000V ESD signal, and low power consumption.
(2) Circuit design of MAX6675
The interface circuit between the MAX6675 chip and the microcontroller is shown in Figure 2-2. The SO, CS, and SCK pins of the chip are connected to P1.0, P1.1, and P1.2 of the microcontroller, respectively. T+ is connected to the positive terminal of the thermocouple, and T- is connected to the negative terminal of the thermocouple. A 0.1μF ceramic capacitor is connected in parallel between the VCC and GND pins of the MAX6675 to reduce power supply coupling noise.
Figure 2-2 Interface circuit between MAX6675 and microcontroller
Fig.2-2 Interface description of MAX6775 and MCU
2.3 Wireless Communication Unit
Considering the design requirements of miniaturization, integration, high precision, and low power consumption for this temperature acquisition system, the nRF905 single-chip transceiver chip, which integrates transmission and reception, was selected as the system's wireless transceiver chip after comparing wireless transceiver chips.
(1) Main electrical performance indicators of nRF905
The main electrical performance specifications of the nRF905 are shown in Table 2-2. The pin functions are shown in Table 2-3.
Table 2-2 Main Electrical Performance Indicators of nRF905
Tab.2-2 Chief electric performance index of nRF905
parameter | numerical values | unit |
Operating voltage | 1.9~3.6 | V |
Maximum transmit power | 10 | dBm |
Maximum numerical transfer rate | 100 | Kbps |
Current at output power of -10dBm | 11 | mA |
Operating current in receive mode | 12.5 | mA |
Typical sensitivity | -100 | dBm |
Operating current in power-down mode | 2.5 | μA |
Table 2-3 nRF905 Pin Functions
Tab.2-3 Explain of nRF905's pin defunction
pin | name | Pin Functions | illustrate |
17 | VDD | power supply | 3.3~3.6V (DC) |
32 | TX_EN | Numeric input | TX_EN= 1 TX mode TX_EN= 0 RX mode |
1 | TRX_CE | Numeric input | Enable chip to transmit or receive |
2 | PWR_UP | Numeric input | Chip power-on |
6 | CD | Digital output | Carrier detection |
7 | AM | Digital output | Address matching |
8 | DR | Digital output | Data reception or transmission completed |
10 | MISO | SPI interface | SPI output |
11 | MOSI | SPI interface | SPI Input |
13 | CSN | SPI Enable | SPI Enable |
14 | SCK | SPI clock | SPI clock |
The design of the carrier detection pin CD, address matching pin AM, and data ready pin DR of the nRF905 facilitates software programming of the nRF905 and also reflects the characteristics of the nRF905.
The most commonly used antennas in short-range communication are monopole antennas, helical antennas, sheet antennas, and loop antennas.
To achieve a solution for low power consumption and fast startup time in the crystal oscillator, this design uses a low-value crystal load capacitor. A 16MHz crystal oscillator was selected, and resistor R1 is used as the bias resistor for the crystal oscillator. The ANT1 and ANT2 output pins provide a stable RF output to the antenna. Capacitors C5, C6, and O8 connected to the VDD pin are high-quality RF filter capacitors, and their other ends are connected to ground to form a decoupling circuit. The nRF905 chip circuit design is shown in Figure 2-3.
Figure 2-3 Circuit design of nRF905
Fig.2-3 Circuit design of nRP905
The interface circuit between the microcontroller and the nRF905 chip is shown in Figure 2-4. The microcontroller's P2.6, P2.5, P3.0, and P3.6 interfaces are connected to the nRF905's SPI port. Communication between the microcontroller and the nRF905 uses simulated SPI timing.
Figure 2-4 Interface between nRF905 and microcontroller
Fig.2-4 InterfacedescriptionofnRF905andMCU
2.4 Power Supply Circuit
The temperature data logger is battery powered, and high-capacity batteries are generally used to extend its lifespan. Considering the power requirements of the entire system, this design uses a single 9V lithium battery to power the main control chip AT89S52, the wireless transceiver chip nRF905, and its peripheral modules.
Figure 2-5 Pin diagram of AS1117
Fig.2-5 Pins picture of AS1117
Since the AT89S52 and peripheral module circuits require a 5V power supply, while the nRF905 wireless transceiver chip requires a 3.3V power supply, it is necessary to convert the 9V power supply to two different supply voltages. The power supply specifications not only have voltage requirements but also power tolerance and other specifications must meet system requirements. The system uses the AS1117 series power converter chip. The AS1117 is a low-power power converter chip with a three-pin SOTT-23 surface mount package and stable power conversion. The chip pin diagram is shown in Figure 2-5. Pin 3 is the external voltage input pin, pin 2 is the converted voltage output pin, and pin 1 is the ground pin.
The system uses the AS1117-5 chip to convert the power supply from 9V to 5V, and its power conversion circuit is shown in Figure 2-6.
Figure 2-6 AS1117-5 power supply circuit
Fig.2-6 Power source circuit diagram of AS1117-5
The system uses AS1117-3.3 to convert the power supply from 5V to 3.3V, and its power conversion circuit is shown in Figure 2-7.
Figure 2-7 AS1117-3.3 power supply circuit
Fig.2-7 Power source circuit diagram of AS1117-3.3
4. Conclusion
Based on the analysis of smart tires, this paper proposes a wireless tire temperature acquisition system that can collect temperature data at multiple points using wireless transmission technology. The system has strong anti-interference capabilities, high accuracy, and a user-friendly interface. It can collect temperature data at different points inside the tire in real time during the rolling process, providing important data support for the study of tire temperature field and has great practical value.
This design focuses on the real-time acquisition of tire internal temperature, and analyzes and designs a wireless temperature acquisition system from both software and hardware perspectives. The hardware design of the temperature collector and receiver includes determining the processor and temperature sensor, selecting the wireless transceiver chip, and designing the power module's peripheral circuitry. Practical testing of the system was conducted, and the experimental results meet the expected design requirements, achieving good results.
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