Improving vehicle braking performance based on LIN bus and MAX6675 temperature sensor

1. Introduction

Large and heavy vehicles in China generally use drum brakes. Every year, heavy vehicles frequently have accidents due to brake failure when driving downhill on mountain roads. The main factor leading to brake failure is the overheating of the brake drum, which causes the brake heat dissipation to fail to meet the requirements. The best solution to improve the braking performance of heavy vehicles is to install ABS, although the country has issued standards GB7258 and GB12676 to stipulate that buses and trucks must be equipped with anti-lock brakes that meet the requirements to enhance road safety. However, the actual configuration is not ideal, because replacing drum brakes with disc brakes and equipping ABS will increase the manufacturer's costs, so in the next few years, most large vehicles will still use water tanks to cool the brake drums.

Based on the current domestic actual situation, the design proposed in this paper to monitor the brake drum temperature, automatically control the spray system and add a safety alarm function based on the LIN bus is a low-cost and easy-to-implement solution to improve the braking performance of existing heavy-duty vehicles, and provides a reference solution for the transition of large vehicles to be equipped with ABS as a whole.

2. Overview

This design uses the MAX6675 K-type thermocouple temperature sensor to monitor the temperature of each brake drum, sprays water to cool down the overheated brake drum, and directly alarms when the temperature is too high to take corresponding measures such as emergency stop. Compared with ABS, this design is much cheaper and is a compromise solution before ABS is truly implemented. It is particularly suitable for large and heavy vehicles. The system composition block diagram is shown in Figure 1.

Nowadays, heavy vehicles are equipped with water tanks. When the brake drum temperature exceeds the temperature threshold 1, the spray system is automatically turned on to cool the brake system; when it exceeds the threshold 2, the warning light is turned on to prompt the driver to stop. Some other auto parts manufacturers provide brake drums with automatic spraying function. Combined with this system, their products will be more competitive.

3. Connection between MAX6675, MCU and LIN

The MAX6675 is connected to a microcontroller that supports the LIN bus and transmits the collected digital temperature signal to the microcontroller. According to the actual application, two threshold temperatures are set: the automatic spray temperature threshold (such as 120°C) and the temperature alarm output threshold (selected in the range of 200°C to 300°C).

3.1 K-type thermocouple temperature sensor MAX6675

In the field of high temperature detection, thermocouples are the main temperature measurement components. The MAX6675 has a signal conditioning amplifier, a 12-bit ADC, a cold-end compensation diode, an analog switch, and a digital controller. The measured ambient temperature is converted into a voltage. By processing the thermocouple voltage and the diode detection voltage, the compensated hot-end temperature is calculated. The digital output is the result of the compensation of the thermocouple test temperature. In the temperature range of 0°C to +700°C, the device temperature error is kept within ±9 LSB.

As shown in Figure 2, T+ is connected to the positive pole of the K-type thermocouple, and T- is connected to the negative pole of the K-type thermocouple; the serial interface is started when the chip select input CS is pulled low, the clock input SCK and the data output SO are connected to the PTA5 and PTA4 pins of the MCU (MC90SQL4) respectively, and the 12-bit data after temperature conversion is output to the MCU via the SPI serial port.

The positive electrode of the K-type thermocouple can be pre-buried in the brake drum. The negative electrode is connected near the circuit board. The system is connected to the LIN bus through the LIN transceiver. Combined with the LIN master controller, more functions can be realized.

3.2 8-bit Microcontroller

The 8-bit microcontroller can be Freescale's LIN bus microcontroller MC908QL4. The microcontroller processes the temperature data, compares it with the two set temperature thresholds (such as 120°C), and correspondingly turns on the automatic spray system, or lights up the warning light and buzzer to remind the driver that the current brake drum temperature is too high and needs to stop to cool down.

LIN is a low-speed serial bus with simple structure, flexible configuration and low cost. In situations where bandwidth requirements are not high, functions are simple and real-time requirements are low, such as the control of body electrical appliances, the use of LIN controllers can effectively simplify network wiring harnesses, reduce costs, and improve network communication efficiency and reliability.

3.3 MAX13021

The MAX13021 provides an interface between LIN master/slave protocol control. It complies with the LIN 2.0 specification and is used in vehicle-mounted sub-networks with a single master and multiple slave devices. The system is connected to the LIN bus through a LIN transceiver and combined with a LIN master controller to achieve more functions.