Design of a direct TPMS tire pressure monitoring system

This paper focuses on the circuit design and related technical issues of realizing tire pressure monitoring system (TPMS). The TPMS system consists of a pressure sensor module and a central receiver. The pressure sensor module, which consists of a pressure sensor SPl2, a microcontroller and a 433 MHz transceiver integrated radio frequency IC (nRF401), completes the task of collecting and transmitting tire pressure. The receiver receives the pressure information and processes it.

With the continuous development of urban transportation technology, while people enjoy the efficient and convenient way of highways, the rate of traffic accidents caused by them is also increasing. Among them, the proportion caused by tire pressure is as high as 80%. This phenomenon requires people to monitor the tire pressure during driving. Some developed countries have successively introduced relevant bills (TREAD regulations in the United States), requiring that new cars must be equipped with safety electronic devices that can monitor the tire pressure in real time during driving. Therefore, installing a tire pressure monitoring system (TPMS) on cars in the future will also be an inevitable development trend, just like ABS and airbags. The tire pressure monitoring system monitors the pressure in the tire all day long, and alarms for tire leakage, low pressure, and high pressure, so that the vehicle is always in a safe operating state.

1 System Design

The TPMS system consists of a tire pressure sensor and a receiver. The tire pressure sensor uses Microchip's PIC16F628A low-power 8-bit MCU, the pressure sensor uses Infineon's SPl2, and the RF IC uses Nordic's nRF401. The

pressure sensor installed in the tire transmits the pressure and temperature information in the tire to the receiver in the cab, so that the driver can know the air pressure in the tire at any time, as shown in Figure 1.




1.1 RF Circuit

In the RF signal circuit shown in Figure 2, the nRF401 device of Nordic, Norway, is used. This device is a single-chip UHF wireless transceiver IC in the 433 MHz ISM band. It uses FSK modulation and demodulation technology, with a maximum operating rate of 20 kb and adjustable transmission power, up to 10 dBm. The basic technical indicators are as follows:




the center carrier frequency is 433.92/434.23 MHz;

the maximum transmission power is 10 dBm and

the operating voltage is 2.7"5.25V;

the current consumption during reception is 250μA, the maximum during transmission is 28 mA, and the standby current is 8/μA.

ANT1 and ANT2 of NRF401 are the input/output multiplexed pins of the antenna. In input mode, the RF signal is demodulated after connecting this pin to the low noise amplifier; similarly, in output mode, the modulated signal is output through this pin after power amplification.

Pin 4 is the input of the PLL phase-locked loop filter of nRF401, and the voltage of this pin during normal operation is 1.1V0.2V.

Pins 5 and 6 are peripheral circuits controlled by piezoelectric crystal oscillators, and these two pins are connected to a 22μH inductor with a Q value of >45@433 MHz.

Pin 7 and Pin 8 are data input and output pins respectively. The information is sent and received in coordination with the potential timing on Pin 18 PWR_UP and Pin 19 TXEN. Figure 3 is a timing diagram for controlling the operation of each pin when transmitting and receiving data.





Pin 11 is connected to an external RF power control resistor. As shown in Figure 2, R3 has a value between 22"100kΩ. Generally, 7 dBm is achieved at around 30kΩ.

1.2 Structure of the helical antenna

Since there are different types of antennas, they should be selected according to specific application requirements. In this application, the antenna is located in the wheel hub of the car. Close to the valve. During high-speed driving, the antenna constantly changes direction. In order to maximize the receiving angle, a helical antenna is selected.

The number of turns (N), diameter (D) and turn distance (S) of the helical antenna determine the gain and directivity of the antenna.

The total length of the antenna is LN=NLO=Nsqit(S2+C2) where C=xD is the circumference of the spiral and Lo=sqrt(S2+C2) is the length of a circle of wire.

Another important parameter is the helix angle α, which is the angle between the tangent of the helix and the plane perpendicular to the helix axis. The definition of the helix angle is α=tan-1(S/C). The helical antenna has the following two working modes.

① Normal mode. In the normal working mode, the antenna radiation field has a maximum value in the normal plane relative to the helix axis. For this mode: NLo〈〈λ.

② Axial (end-fire) mode. This working mode has only one main lobe, its maximum radiation intensity is along the helix axis, and there is an inclination angle between the secondary lobe and the axis. To excite this mode, its diameter D and space 3 must be a large fraction of the wavelength.

1.3 Sensor circuit

SPl2 is a tire pressure sensor IC of Infineon, as shown in Figure 4, it integrates the detection circuits of pressure, temperature, acceleration, and voltage through MEMS technology, directly outputs the indication of each physical quantity in digital form, and interacts with the periphery using the SPI protocol.







At the same time, there are two clock signals inside, WAKEUP and RESET. WAKEUP outputs a pulse signal every 6s, and RESET outputs a pulse signal every about 54min, which is low electronic effective.

1.4 System power consumption

Since the overall static current of the TPMS system is required to be less than 20μA, the selected devices must be low-power or ultra-low-power chips. The static current of PICl6F628A is 0.1μA, the static current of sensor SPl2 is 0.6μA, and the standby current of RF NRF401 is 8μA.

After actual measurement, the static power consumption Ist=Ist_mcu+Ist_sensor+ist_rf+Ist_cap=15μA. Ist_cap is the leakage current of the tantalum capacitor. The dynamic power consumption is 25mA (maximum value) when the radio frequency is in continuous transmission. [page]

2 Software Design

2.1 Overall Software Design

The software is generally divided into four operating modes: debugging mode (for factory diagnosis), initialization mode, sleep mode (hibernation) and measurement mode.

(1) Debugging mode

Mainly performs some internal tests and diagnostic functions of the product before leaving the factory. It includes two parts: sensor test and radio frequency test.

(2) Initialization mode

Receive the ID code and level judgment threshold given by the host.

(3) Sleep mode

Run with the lowest power consumption and respond to the system's operating requirements.

(4) Measurement mode

The measurement mode mainly completes the following functions:

① Regularly measure tire pressure and temperature;

② When there is an alarm in different pressure levels, the same level is reported only once;

③ When there is no change in pressure, the pressure and temperature are reported once at a fixed interval;

④ Ensure system reliability. The system resets every 1 hour.

⑤ When different levels of alarms appear at the same time, the high temperature, high pressure and low pressure levels are used as the priority alarms.

Their mutual conversion relationship is shown in Figure 5.

The system power-on reset workflow is shown in Figure 6.







2.2 Processing of sensor data

SP12 is a pressure sensing IC, which directly processes the digital quantity internally. By sending the corresponding command words and instructions to it through the SPI physical interface, the current tire pressure, temperature, acceleration and other values ​​can be obtained. Since PICl6F628A has no SPI module, the SPI driver can be simulated by software here to achieve the purpose of communicating with SP12.

2.3 Processing of RF data

In order to ensure the reliability of data transmission and reception in harsh environments, and based on the characteristics of small information volume and simple data in this application, the information redundancy method is used to ensure reliable data reception, that is, to continuously send information with the same content. The

data transmission process uses the USART module inside the MCU.

The data frame format is preamble + synchronization character + ID code + pressure + temperature + alarm bit + check code.

In the reception mode of nRF40l, there is a continuous random signal input on the DOUT pin. How to efficiently filter out the transmitted information in the byte stream? Here, because the transmission format is fixed, the finite state machine method is used in the interrupt processing process.

Conclusion

The TPMS system has the characteristics of small size, low cost, and two-way golden time, and is widely used. For China with tens of millions of cars, it will have a very broad market prospect. This will set off a new round of climax in the application of automotive pressure sensors.