Automobile tire pressure detection system technology and application chip
In the world today, many national highway safety associations have enforced legislation. The tire pressure monitoring system TPMS (Trie pressure monitoring system) has a decisive influence on improving vehicle safety, and its low power consumption and reliable long-term operation in harsh environments Performance, smaller pressure sensor error tolerance and longer working life are the key requirements of TPMS, so the design and chip selection of its solutions are also based on these requirements.
At present, there are three main ways to implement TPMS, namely direct TPMS system, indirect TPMS system and the hybrid TPMS being launched. However, indirect TPMS has certain limitations. Direct TPMS uses a pressure sensor fixed in each wheel to directly measure the air pressure of each tire. Then, these sensors will send the tire pressure data to the central microprocessor for analysis through the transmitter, and the analysis results will be transmitted to the display installed in the car. The type of display is different from the simple tire pressure indicator that is installed on most vehicles today. It can display the actual air pressure of each tire, and even include the air pressure of the spare tire. Therefore, including slave tasks, TPMS can be connected to the display. Tell the driver which tire is under-inflated and detect a small air pressure drop. In order to meet the requirements of multi-tire pressure detection, because the system is equipped with direct air pressure sensors, hybrid TPMS can overcome the limitations of conventional direct TPMS. They can detect two tires on the same axle or the same side of the vehicle at low pressure. When all four tires are under low pressure, the system can also detect failures.
This means that the MCU, sensor and RF transmitter are packaged together. Compared with existing products, it integrates barometric pressure sensor, acceleration sensor, 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 can also reduce the overall cost of the system because the board design is simpler and smaller in size. Another important design challenge comes from wireless control. The design of the first-generation TPMS transmitter uses the ASK modulation technology of the SAW resonator to generate the wake-up command issued by the node. This should be the proper transmission frequency. Nowadays TPMS adopts FSK modulation method based on crystal oscillator and PLL synthesizer to generate center frequency and frequency traction.
This article takes the scheme of distributed real-time tire pressure monitoring system based on LIN bus as an example for analysis, and introduces a new chip for TPMS.
1. Design scheme of distributed real-time tire pressure monitoring system based on LIN bus
In order to achieve the goal of long-term (≥10 years) service life, low-power integrated components must be used, and its power supply becomes 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 the measured data, and combining the actual situation of the vehicle (stop or running) to change the working mode of the monitoring system and efficient data collection and control algorithms.
The direct TPMS system is realized by MCU, RF and sensor. The schematic diagram of TPMS scheme based on LIN bus is shown in Figure 1.
The tire pressure monitoring system indicated by the practical TPMS is composed of four signal transmitters integrated with the tire valve, a receiving antenna, a receiver and a signal display instrument. For this reason, it is necessary to first introduce the relevant technologies of LIN bus for automotive applications.
1. LIN bus technology for automotive applications
(1) The main features of 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 TPMS, there are power doors, power windows, side mirrors, wipers, seat belt alarms, and external lighting. The maximum transmission speed of the LIN bus is 20kbps, and it can support up to 16 nodes in a single-channel bus loop. The length of the bus cable can be expanded to 40 meters at most. LIN bus is a single-wire serial communication protocol based on the universal SCI (UART) byte interface. Figure 2 (a) is a block diagram of the structure of the UN bus API to the physical layer. The main characteristics of the LIN bus are: the concept of a master node and 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; guarantee 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 includes a master node and several slave nodes (too many nodes will cause the network impedance to be too low, and the total number of nodes in a LIN network should not exceed 16). The master node includes both master tasks and slave nodes but only slave tasks. Figure 2 (b) is a schematic diagram of LIN topology. The host node can also be connected to other buses such as CAN through the gateway.
2. Design of distributed real-time tire pressure monitoring system based on LIN bus
Figure 1 describes the overall structure of TPMS based on LIN bus. Among them, the central controller has three main functions: namely, notify the LIN slave node to wake up the sending module in the corresponding tire through the LIN bus; return the tire pressure and other data received by the LIN slave node through the LIN bus; analysis, display, and sound and light report . When the LIN slave node receives the LIN master node, it will send an LF wake-up signal to the sending module to allow it to enter a large working state. The LIN master node Master sends an acquisition command frame to the LIN slave node, and the LIN slave node feeds the data back to the LIN master node (master control) through the LIN bus.
(1) Module and chip selection for synthesis of pressure sensor IC and transmitting IC in tire 1) Module selection:
Because the pressure sensor IC and the sending IC integrated module in the tire are placed in the tire, the requirements for the IC are particularly high. The general requirements are as follows: working temperature: -40 ℃ ～ 125 ℃ (up to 150 ℃ 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; small IC and light weight; with pressure, temperature and voltage detection.
The pressure sensor IC is an IC that integrates pressure, temperature, and voltage detection sensors, LF, and MCU. The sending IC is a series of RF transmitting chips.
2) Chip selection and characteristics
The dashed frame on the left in FIG. 3 is the pressure sensor IC and the transmission IC in the tire. They are Freeseale's Mpxy8020A6 (or Mpxy8040A) chip and MC68HC90RF2 chip. Mpxy8020A6 contains pressure sensor, temperature sensor, power supply control and battery voltage detection, timer with wake-up function (belongs to surface micromechanical CMOS processing technology, SSOP package); and MC68HC90RF2 of UHF transmitter + MCU (Flash) contains 2kB User FLashROM, timer, integrated radio frequency (Rn transmitter, low-voltage detection and RAM, and internal clock generator. The whole dashed left frame of Figure 3 is a schematic diagram of the telemetry module synthesized by Mpxy8020A6A and MC68HC90RF2. MC33591 on the right of Figure 3 is the UHF receiver, It contains a phase-locked loop (PLL) ultra-high frequency open key control (OOK) receiver; 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 suitable for RFID and RF applications. ory Access) mode. All of these provide the greatest possibility for reducing power consumption at the transmitter and extending battery life. In terms of transmitting IC. Melexis has different frequency and modulation of IC and automotive-grade IC (1 operating temperature -40 ℃ ~ 125 ℃), such as 315MHz, 433MHz, 868MHz and 915MHz and other ISM band frequency bands: IC FSK, ASK and FM and other different modulation IC, can Work in a wide voltage range of 1.85V ~ 5.5V, and the transmit power can be adjusted in the range of -12dBm ~ + 10dBm.
(2) Receiver module and chip selection
For the main node of the receiving module in the LIN network, that is, the central controller, see Figure 1. You can also choose MLX82001, an MCU designed specifically for UN bus applications.
3. TPMS system software design ideas
The first factor to consider when designing a TPMS system with stable operation and high efficiency is software. Because the wheel module usually uses a microcontroller to execute commands. Therefore, an intelligent algorithm should be used to achieve the desired efficacy. Secondly, using the low frequency function is a very effective method of controlling TPMS. When using the low-frequency interface, the sensor module can always be in the power-off mode. Only after receiving the wake-up signal will the sensor perform measurement and data transmission. In addition to reducing power consumption, the low-frequency interface also has flexibility and other advantages. For example, low-frequency communication allows the system to send specific commands to a microcontroller via a low-frequency interface. To recalibrate and position the tires. Here is a description of the design scheme of the sending module software with MLX90603 with LF (Low Frequency) interface as an example. MLX90603 has LF (LowFrequeney) interface. Therefore, the transmitter can be put into sleep mode for most of the time, and it can be awakened by a low-frequency signal when necessary, and then the measurement can be performed and the measured data can be sent to the corresponding LIN slave node through the TH720x transmitter chip. Fig. 4 is a partial flowchart of the transmitting end.
In this scheme, the TDMA (Tag. Direct Memory Access) module integrated in MLX90603 is fully utilized. After the MLX90603 collects the data, configure the registers required by TDMA and RF to put the MLX90603 into Sleep mode, and use the TDMA module to automatically transmit the data to be transmitted to the RF to fully save power consumption. Because the transmission works in a harsh environment, in order to ensure reliable data transmission at the sending end and the receiving end. Considering the small amount of information and simple data in this application, we use the information redundancy method to ensure the reliable reception of data (that is, one frame of data is sent N times). According to the collected data, the number of transmissions N is dynamically adjusted.
2. Selection of several chips in the new transmitter (remote control key) and receiver
1. Selection of MAX1473 receiver and MAX7044 transmitter
The RF receiver device (MAX1473) is the latest 300MHz to 450MHz ASK (Amplitude Conversion Modulation) radio frequency receiver (average sensitivity is -114dBm) and consumes only 5.5mA (typical) current during normal operation. Built-in image frequency suppression eliminates the need for commonly used front-end SAW filters. In sleep mode, the MAX1473 can start and send data in less than 250ps to ensure a deeper sleep cycle and longer battery life. MAXl473 can work at a supply voltage of 3V to 5V.
The MAX7044 device in the transmitter is a transmitter that can output a + 3dBm ASK signal, and is packaged in a miniature 8-pin SOT. When the coding method with a duty ratio of 50% (such as Manchester code) is used, only 7.7 mA of current is consumed. The MAX7044 can be powered by a single lithium battery with a voltage as low as 2.1V.
MAX7044 data sheet: http: //
2. MAX1471 block diagram and application of dual-channel receiver capturing two signals simultaneously
Use the MAX1471 dual-channel receiver (see Figure 5) to capture two signals at the same time, that is, to receive ASK and FSK (frequency conversion data) at the same time, and the switching time between modes is zero. For low-cost system design that requires decoding of ASK and FSK at the same time, the MAX1471 dual-mode receiver can also perform self-polling, and the device can maintain a sleep mode for up to 8 minutes and wake up the microprocessor to further save energy. The MAX1471 has a built-in voltage regulator for 3.3V or 5V, so it can work at 2.4V.
It can be seen from the figure that the MAX1471 can also be used as a receiver in a car tire pressure monitoring system.
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