Design of Automobile Comfort Evaluation System Based on Single Chip Microcomputer

0 Introduction
With the development of economic level, people's demand for cars is increasing. While cars bring great convenience to our travel, they also pose challenges to the health of drivers and passengers. The car compartment is a relatively small space, and its internal environment has an impact on the health of drivers and passengers (especially drivers who drive for a long time). Strengthening the
research on car comfort can provide theoretical support for improving car comfort. At present, there are still few studies on car comfort evaluation, and both theory and technology are still immature. Proposing a reasonable design method for car comfort evaluation system will be conducive to improving car comfort and improving the overall performance of cars. On this basis, we designed a car comfort evaluation system based on MSP430 microcontroller. This system uses MSP430 series microcontroller as the hardware core of the system, detects relevant parameters through three sensors (temperature sensor, humidity sensor, vibration sensor), and outputs the results on the LCD display after data fusion to show whether it is comfortable.

Working principle: To evaluate the comfort of a car's interior space, we must first determine the evaluation parameters. We use temperature, humidity and vibration levels, which are the most sensitive to the human body, as evaluation parameters. The system block diagram is shown in Figure 1. The microcontroller controls the opening and closing of the relay through the corresponding drive circuit. The low-power MSP430 microcontroller is used as the hardware core, and appropriate humidity, temperature and vibration sensors are selected as sensors for parameter measurement. The data is imported from the A/D port of the MSP430, and the imported data is processed by filtering, denoising and other means. Then, the corresponding humidity, temperature and vibration levels are calculated by relevant methods (such as table lookup method), and then a comprehensive evaluation is made on the three parameters, and they are sent to the display end at the same time, and control signals are given to adjust related equipment to improve the comfort of the car until it is comfortable and maintained, which is used to improve the entire system and match it with the whole vehicle.

1 Hardware Design
1.1 Power supply and system reset circuit
The whole system is powered by 3.3V. Considering that the hardware system requires the power supply to have voltage stabilization and small ripple, and also considering the low power consumption of the hardware, the power supply of the system hardware is implemented by TI's TPS76033 chip, which can well meet the requirements of this hardware system.

The design of the reset circuit must enable the system to work stably and reliably under various complex situations. Poor reset performance will affect the normal operation of the system. There is an RST reset pin in the MSP430 microcontroller, which is multiplexed with the non-maskable interrupt function pin. Its function can be selected by software. Under normal circumstances, it is a reset function. As long as there is a low-level input, the system will reset. The reset circuit can use an RC reset circuit or a reset circuit implemented by a reset chip. The RC reset circuit is economical, but the reliability is not high. The reset circuit implemented by the reset chip has high reliability. Therefore, in order to ensure the reliability of the reset circuit, this system uses a reset circuit implemented by a reset chip. This system uses the MAX809 chip. The reset circuit is shown in Figure 2. In order to reduce the interference of the power supply, a 0.1 μF capacitor should be added to the power input end of the reset chip to achieve filtering to reduce the interference on the input end. [page]

1.2 Selection of temperature sensor and circuit design
AD7416 is a single-chip low-power digital temperature sensor launched by Analog Devices (ADI) of the United States. It contains a bandgap temperature sensor and a 10-bit A/D converter, which can convert the sensed temperature into a digital signal with a resolution of 0.25°C. The microcontroller can read and write the internal registers through the IIC interface, and allows 8 AD7416s to be hung on the same serial bus. This temperature sensor can be widely used in ambient temperature detection, industrial process control, household appliances, battery charging, computer systems, etc. Its power consumption is extremely low. When used for heating pipe temperature measurement, it adopts a specific power management mode and works with a 20s sampling cycle. The average power consumption is only 66 (nW). It is very suitable for forming a battery-powered thermal energy calculation table with TI's MSP430 ultra-low power single-chip microcomputer, which can make the random battery work for more than 5 years. Use P1.6 to connect a 10kΩ pull-up resistor to simulate the SCL and SDA of I2C respectively. The hardware connection diagram is shown in Figure 3.

1.3 Selection of humidity sensor and circuit design
The humidity sensor uses Honeywell's integrated humidity sensor HTH3610. The sensor uses a thermosetting polyester capacitive sensor head and integrates a signal processing function circuit inside. Therefore, the sensor can complete the task of converting the relative humidity value into a capacitance value and then converting the capacitance value into a linear voltage output. At the same time, the sensor also has the advantages of high precision, fast response, good stability, low temperature drift, strong chemical corrosion resistance and good interchangeability. Because it is a linear voltage output, the peripheral circuit design can be simplified. However, the following two points should be noted when using this sensor: a. When the relative humidity changes from 0% to 100%, the voltage output of the sensor is 0.8V to 3.9V. This output voltage value cannot meet the standard input voltage (0~5V, ±5V, 0~10V) requirements of the A/D acquisition board, so voltage conversion is still required. b. The signal conditioning circuit inside IH3610 is corrected to Vsupply = 5V. At 25℃, the output voltage changes from 0% to 100% relative humidity to 0.8 to 3.9V. However, when the operating temperature of the humidity sensor changes, the output voltage value will be different for the same humidity value. Therefore, the sensor must be temperature compensated when used. The compensation formula (1) is:
RH = (sensor%RH) / (1.0546-0.0216T) (1)

Since the HIH-3610 humidity sensor has a linear output, its interface circuit with the MSP430F149 is very simple. As shown in Figure 4.
1.4 Selection of vibration sensor and circuit design
During the driving process of the car, the vibration involved is generally three-dimensional, so the collection of vibration parameters requires the collection of vibration levels in the three directions of x, y, and z. An acceleration sensor is required to measure the vibration level. This system uses the EGAXT3 series acceleration sensor of Precision Instruments.
The acceleration sensor uses a bridge measurement. The sensor has a total of 12 wires, each axis has a power line and a ground line, and two output lines. The three axes can be powered independently. As shown in Figure 5.

1.5 Selection of display devices and auxiliary circuit design
The system selects the liquid display module LCMl2864ZK with Chinese character library. Its ROM contains 8192 16×16 dot Chinese fonts and 128 16×8 half-width letter symbol fonts; in addition, the drawing display screen provides a 64×256 dot drawing area GDRAM; and the built-in CGRAM provides 4 sets of software-programmable 16×16 dot matrix character creation functions. The power supply operation range is wide (2.7V to 5.5V); the low power consumption design can meet the power saving requirements of the product.
At the same time, the interface with microcontrollers such as single-chip microcomputers is flexible (three modes: parallel 8-bit/4-bit, serial 3-wire/2-wire). This system uses parallel 8-bit mode because of the rich ports of MSP430F149.
LCMl2864ZK can realize the same-screen display of Chinese characters, ASCII codes, and dot matrix graphics, and is widely used as a display device in various instruments, household appliances and information products.

LCMl2864ZK has commands to move the current display screen up/down/left/right and clear the screen, cursor display/flicker control commands and LCD sleep/wake-up/display off commands. A variety of control lines (reset/serial-parallel selection/brightness adjustment) are reserved for users to use flexibly. The interface circuit between LCMl2864ZK and MSP430F149 is shown in Figure 6. [page]

2 Software Programming
2.1 Temperature Parameter Collection
Combining the structural characteristics of the temperature sensor and MSP430F149, the temperature collection needs to first set P1.6 and P1.7 connected to the temperature sensor as output, configure the temperature sensor parameters, and then change P1.6 and P1.7 to input after the configuration is completed. Read the temperature value in the temperature sensor data register and send it to the memory of the microcontroller for comfort evaluation, and then return to execute the next module program.
2.2 Humidity Parameter Collection
Humidity parameter collection should be performed after temperature collection is completed. Because the humidity sensor needs temperature compensation. The humidity sensor HIH3610 is an analog voltage output with a voltage range of 0.8V to 3.9V. Through voltage adjustment, the analog voltage is converted to AD through a channel P6.0 of the ADC port (i.e. P6 port) of the MSP430Fll49. After calculation by formula (1), the humidity value can be obtained.
2.3 Collection of vibration parameters
The collection of vibration acceleration parameters is done by using the three channels P6.3, P6.4, and P6.5 in the ADC port (P6 port) of the MSP430F149. Therefore, to realize the collection function, the three channels should be set to ADC mode first, and then the corresponding voltages should be read from the three channels in turn, and then stored in the corresponding addresses, and then transferred to the main program.
Using the ADC channel for parameter collection requires time coordination because the ADC requires conversion time. Time coordination can be achieved by software delay method, timer interrupt method, and software query method.
2.4 Output of LCD display
The LCD display should be initialized after power-on. First, delay more than 40ms to set the function (8-bit/4-bit, basic instructions/extended instructions); then delay 100 μs, which is also the function setting; then delay 37 μs, switch display setting; delay 10 μs, clear the screen (clear display RAM); finally delay 10ms to set the cursor movement direction when reading and writing.
2.5 Comprehensive Processing Program
The comprehensive processing program is also the main program. The main program is responsible for starting the microcontroller and calling each module subroutine in turn to realize the system function. The flow chart of the comprehensive processing program is shown in Figure 7.

3 Conclusion
This paper designs a car comfort evaluation system based on the ultra-low power microcontroller MSP430F149. This system can collect the temperature, humidity and vibration acceleration of the car compartment in real time, divide the car comfort into 5 levels, and give the comfort level of the car compartment.