Automotive tire pressure detection system technology and application chip

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In today's world, many national highway safety associations have enacted legislation. The tire pressure monitoring system TPMS (Trie pressuremonitoring system) has a significant impact on improving vehicle safety, while its low-power, long-term operation in harsh environments is reliable. Sexuality, small pressure sensor error tolerance and longer working life are the key requirements of TPMS, so the design and chip selection of the solution also surround these requirements.

At present, there are three main implementations of TPMS, namely direct TPMS system, indirect TPMS system and hybrid TPMS being introduced. However, indirect TPMS has certain limitations. The direct TPMS directly measures the air pressure of each tire using a pressure sensor fixed in each wheel. These sensors then send the tire pressure data to the central microprocessor for analysis via the transmitter, and the results of the analysis are transmitted to the display mounted in the vehicle. The type of display differs from the simple tire pressure indicator assembled on most vehicles today, which shows the actual air pressure of each tire and even the air pressure of the spare tire. Therefore, directly including the slave task, the TPMS can be connected to the display. Tell the driver which tire is under-inflated and can detect a small air pressure drop. In order to meet the multi-tire pressure detection requirements, the hybrid TPMS can overcome the limitations of conventional direct TPMS due to the installation of direct air pressure sensors. They can detect two tires in the low pressure state on the same axle or on the same side of the vehicle. The system can also detect faults when all four tires are in low pressure.

This means that the MCU, sensor and RF transmitter are all packaged together. Compared to existing products, it integrates air pressure sensor, accelerometer, temperature sensor, 8051 microprocessor with on-chip flash memory, low frequency receiver interface and 315/433/868/91.5MHz RF transmitter. In addition to reducing the number of components, it also reduces overall system cost because the board design is simpler and smaller. Another important design challenge comes from wireless control. The first generation of TPMS transmitters were designed using the ASK modulation technique of the SAW resonator to produce the wake-up command issued by the node. The proper emission frequency of the student. Today's TPMS uses a crystal oscillator based FSK modulation method and a PLL synthesizer to generate center frequency and frequency pull.

This paper takes the scheme of distributed real-time tire pressure monitoring system based on LIN bus as an example, and introduces the new chip for TPMS.

I. Design of real-time tire pressure monitoring system based on distributed LIN bus

In order to achieve long-term (≥10 years) life expectancy, low-power integrated components must be used, and their power supply is the primary challenge (longer life under limited energy). These can reduce the power consumption of the entire system by using low-power pressure sensors, analyzing measured data and changing the working mode of the monitoring system and efficient data acquisition control algorithms in combination with the actual situation of the vehicle (blackout or operation).

The direct TPMS system is realized by MCU, RF and sensor. The schematic diagram of the TPMS scheme based on LIN bus is shown in Fig. 1.

The tire pressure monitoring system indicated by the practical TPMS is composed of four signal transmitters, a receiving antenna, a receiver and a signal display unit integrated with the tire valve. For this reason, it is necessary to introduce the relevant technology of the LIN bus for automotive applications.

1. LIN bus technology for automotive applications

(1) The main features of the LIN bus. For LIN1.0 (local interconnect network) and LIN2.0 bus systems for automotive applications, its goal is low-cost applications. In addition to the TPMS, there are electric doors, power windows, side mirrors, wipers, seat belt alarms, and external lighting. The LIN bus has a maximum transfer speed of 20kbps and supports up to 16 nodes in a single-channel bus loop. The bus cable can be extended up to 40 meters in length. The LIN bus is a single-wire serial communication protocol based on the general-purpose SCI (UART) byte interface. Figure 2 (a) is a block diagram of the UN bus API to the physical layer. The main features of the LIN bus are: a point of a main node, the concept of multiple slave nodes, without bus arbitration; low cost: based on ordinary UART/SCI interface hardware; self-synchronization, no crystal oscillator or ceramic oscillation in the slave node Clock; guarantees the delay time of signal transmission; low-cost single-wire connection; speed up to 20kbps; signal based on application interaction: LIN bus driver/receiver specification complies with IS09141 standard.

(2) LIN topology. LIN adopts single-master and multi-slave modes. A LIN network consists of one host node and several slave nodes (since too many nodes will cause the network impedance to be too low, the total number of nodes in a LIN network should not exceed 16). A host node includes both a master task and a slave node but only a slave task. Figure 2(b) is a schematic diagram of the LIN topology. The host node can also be connected via a gateway to other buses such as CAN.

2. Design of Real-time Tire Pressure Monitoring System Based on LIN Bus Distributed

Figure 1 depicts the overall structure of a TPMS based on the LIN bus. Among them, the central controller has three main functions: that is, the LIN bus is used to notify the LIN slave node to wake up the transmitting module in the corresponding tire; the LIN bus returns the data of the tire pressure received from the node; the analysis, display, and sound and light report . When the LIN receives the LIN master node from the node, it sends an LF wake-up signal to the transmitting module to let it enter the working state. The LIN master node sends a get command frame to the LIN slave node, and the LIN slave node feeds the data back to the LIN master node (master) through the LIN bus.

(1) Module and chip selection of tire internal pressure sensing IC and transmission IC synthesis 1) Module selection:

Since the pressure sensing IC in the tire and the module that synthesizes the IC are placed in the tire, the requirements on the IC are particularly high. Generally have the following requirements: operating temperature: -40 ° C ~ 125 ° C (150 ° C in a short time); low power consumption to maintain battery life; can withstand the centrifugal force of 2000G (250km / h) tire rotation; the sensor can maintain long-term Stable; IC small size, light weight; with pressure and temperature and voltage detection.

The pressure sensor IC is an IC that integrates pressure, temperature, voltage detection sensors, LF, and MCU. The transmitting IC is a series of RF transmitting chips.

2) Chip selection and characteristics

The left dotted line in Figure 3 shows the pressure sensor IC and the transmitting IC in the tire. They are freeseale's Mpxy8020A6 (or Mpxy8040A) chip and MC68HC90RF2 chip. Mpxy8020A6 contains pressure sensor, temperature sensor, power control and battery voltage detection, wake-up timer (surface micro-machined CMOS processing, SSOP package); and UHF transmitter + MCU (Flash) MC68HC90RF2 contains 2kB User FLashROM, timer, integrated RF (Rn transmitter, low voltage detection and RAM, and internal clock generator. The left dashed box in Figure 3 is a schematic diagram of the telemetry module synthesized by Mpxy8020A6A and MC68HC90RF2. The MC33591 on the right side of Figure 3 is a UHF receiver. It contains a phase-locked loop (PLL) UHF open-key control (OOK) receiver; the MC912DP256 receiver controller contains 256kB Flash, 12kB RAM, 4kB EEP ROM, up to 5CAN, 1xJ1850, 256MHz operating frequency.

Of course, the sensor IC in the tire can also choose Melexis MLx90603 chip. The biggest feature of MLx90603 is that it has different working modes: Shelfmode, Sleep mode, Runmode, Idlemode and TDMArrag Direct Mem for RFID and RF applications. Ory Access) mode. These offer the greatest possibility to reduce the power consumption of the transmitter and extend the battery life. In terms of transmitting ICs. Melexis has different frequency and modulation of lC and automotive grade ICs (1 for temperatures -40 ° C ~ 125 ° C), such as 315 MHz, 433 MHz, 868 MHz and 915 MHz and other ISM band bands: IC FSK, ASK and FM and other modulation ICs, It operates over a wide voltage range of 1.85V to 5.5V, and the transmit power can be adjusted from -12dBm to +10dBm.

(2) Receiver module and chip selection

For the central controller of the receiving node in the LIN network, as shown in Figure 1. Also available is the MLX82001, an MCU specifically designed for UN bus applications.

3. TPMS system software design ideas

The first factor to consider when designing a stable, efficient TPMS system is software. Because the wheel module usually uses a microcontroller to execute commands. Therefore, an intelligent algorithm should be used to achieve the expected effect. Second, the use of low frequency functions is a very effective way to control TPMS. When using the low frequency interface, the sensing module can always be in the power off mode, and the sensor will only perform measurement and data transmission after receiving the wakeup signal. In addition to reducing power consumption, the low-frequency interface has the flexibility and other advantages. For example, low frequency communication allows the system to send specific commands to the microcontroller via the low frequency interface. To recalibrate and position the tire. Here, the MLX90603 with LF (Low Frequency) interface as an example of the transmission module software design is described. The MLX90603 has an LF (LowFrequeney) interface. Therefore, the transmitting end can be in the sleep mode for most of the time, and wake up by the low frequency signal when needed, and then the measurement is performed and the measured data is sent to the corresponding LIN slave node through the TH720x transmitting chip. Figure 4 is a partial flow chart of the transmitting end.

In this solution, the TDMA (Tag. Direct Memory Access) module integrated in the MLX90603 is fully utilized. After the MLX90603 has collected the data, configure the registers required for TDMA and RF to enter the Sleep mode, and use the TDMA module to automatically transmit the data to be transmitted to the RF to fully save power. Since the transmitting work is in a harsh environment, in order to ensure reliable data transmission between the transmitting end and the receiving end. Considering the small amount of information and simple data in this application, we use information redundancy to ensure reliable reception of data (that is, one frame of data is sent N times). The number of transmissions N is dynamically adjusted according to the collected data.

Second, the selection of several types of chips in the new transmitter (remote control key) and receiver

1. Selection of the MAX1473 Receiver and the MAX7044 Transmitter

The RF receiver device (MAX1473) is the latest 300MHz to 450MHz ASK (Amplitude Modulated Modulation) RF receiver (average sensitivity -114dBm), which consumes only 5.5mA (typ) of current during normal operation. Built-in image rejection, eliminating the need for a front-end SAW filter. In sleep mode, the MAX1473 can start and send data in less than 250ps to ensure deeper sleep cycles and longer battery life. The MAXl473 operates from a 3V to 5V supply voltage.

The MAX7044 device in the transmitter is a transmitter that can output +3dBm ASK signals in a tiny 8-pin SOT package. When using a 50% duty cycle (such as Manchester code), only 7.7mA is required. The MAX7044 can be powered from a single lithium battery with voltages as low as 2.1V.
2. The MAX1471 block diagram and application of a two-channel receiver that captures both signals simultaneously

Using the MAX1471 dual-channel receiver (see Figure 5) to capture both signals simultaneously, it is possible to receive both ASK and FSK (frequency-converted data) with zero switching time between modes. For low-cost system designs that require both ASK and FSK decoding, the MAX1471 dual-mode receiver can also perform self-polling, and the device can maintain up to 8 minutes of sleep mode and wake up the microprocessor for further energy savings. The MAX1471 has a built-in voltage regulator for 3.3V or 5V, so it can operate at 2.4V.

It can be seen from the figure that the MAX1471 can also be used in receivers in automotive tire pressure monitoring systems.

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