Design of vehicle-mounted compaction tester based on AT89S52

1 Introduction

Compaction refers to the ratio of the dry density of the compacted soil to the maximum dry density obtained by the standard compaction test. In highway construction, the roadbed, base layer, base layer and surface layer all need to be compacted well to achieve a certain density, improve the road's bearing capacity, and prevent subsidence, water infiltration, etc. An important indicator for measuring the quality of roadbed compaction is the compaction of the roadbed. Generally, for every 1% increase in compaction, the basic bearing capacity will increase by 10%. For asphalt concrete pavement, for every 1% increase in compaction, the bearing capacity and life can be increased by 10-15%. The importance of compaction work is obvious.

At present, the construction or testing department has been using the traditional manual sampling method to ensure the compaction quality. This method is not only time-consuming and labor-intensive, but also will cause damage to the roadbed. With the rapid increase in transportation volume and the further development of highway construction, the construction department has put forward higher requirements for compaction detection. The traditional compaction measurement method can no longer meet the requirements of modern construction. Therefore, it is urgent to develop a fast, non-destructive, and quantitative compaction detection instrument.

2 Hardware design of vehicle-mounted compaction tester

As the compaction work of the vibratory roller proceeds, the density and elastic modulus of the soil continue to increase, and the interaction force between the soil and the compaction wheel also continues to increase, so the acceleration amplitude of the compaction wheel is also increasing. Therefore, the compaction condition of the roadbed can be indirectly measured by detecting the change in acceleration amplitude. The vehicle-mounted compaction tester is an online real-time compaction tester. It mainly

The system consists of the following parts: acceleration sensor, charge amplifier, A/D converter, single chip microcomputer, LCD and external PC connected to the vehicle-mounted compaction tester through RS-232 serial port. The system hardware composition is shown in Figure 1.

2.1 Selection of MCU

The AT89S52 microcontroller produced by Atmel is selected here, and its CPU adopts a structure and instruction system compatible with MCS-51.

AT89S52 is a low-power, high-performance 8-bit CMOS microcontroller with 8 kB in-circuit programmable Flash memory. It uses Atmel's high-density non-volatile storage manufacturing technology and is compatible with the industry-standard 80C51 instruction set and pin structure. The on-chip Flash memory allows repeated in-circuit programming or programming through a traditional non-volatile programmer, eliminating the trouble of pulling out and burning the chip every time. AT89S52 also provides the following standard functions: 8 KB Flash memory, 256 bytes of RAM, 32 I/O pins, watchdog timer, two data pointers, three 16-bit timers/counters, a six-vector two-level interrupt structure, a full-duplex communication port, on-chip oscillator and clock circuit. The above functions can well meet the design requirements of the system hardware part.

2.2 Selection of A/D Converter

ADC0809 is an 8-bit A/D converter. For a vibratory roller, its vibration frequency is between 30 Hz and 50 Hz. According to the sampling theorem, if the continuous signal xa(t) is of limited bandwidth, that is, fa is bounded, then the discrete signal x(n) when the sampling frequency fs>2fa uniquely determines xa(t). The highest frequency of xa(t) is fa, and x(n) represents the discrete signal of xa(t) with a sampling interval of T. Referring to the hardware resources of the actual engineering application system, fs=10fa is generally taken. The vibration frequency range of the vibratory roller in this system is between 30 Hz and 50 Hz, so fa=500 Hz, that is, the sampling period T=2 ms. It takes about 100μs for ADC0809 to complete a conversion, so it can meet the requirements of the system. For an A/D converter that responds to instantaneous values, the required conversion rate depends on the required conversion accuracy and the frequency of the converted signal. In this system, only one acceleration signal of the vibration wheel is collected, so ADC0809 can fully meet the requirements.

2.3 LCD Module

Among FPD (Flat panel display) devices, LCD stands out for its advantages of being light, thin, small, and low in power consumption. By configuring a driving circuit for LCD, a liquid crystal display module (LCM) can be formed. In addition to LCD and driving circuit, its basic components generally include connectors, backlight, etc. The liquid crystal display module, like other circuits in the system, has a simple interface and provides a rich command system for controlling the display, saving time for the development of the entire machine.

Since the acceleration change curve needs to be displayed, the LCD dot matrix graphic liquid crystal display module is selected here. The control and drive of the dot matrix LCD is relatively complex. With the development of large-scale integrated circuit technology, there are now drive circuit chips and control circuit chips, and some are made on the circuit board on the back of the LCD screen, making LCD application more convenient. The FYD12864-0402B liquid crystal display module provided by Chengdu Feiyuda Industrial Co., Ltd. is selected here. Its interface circuit with AT89S52 is shown in Figure 2.

2.4 AT89S52 and PC serial interface

The output of the microcontroller is TTL level. To interface with RS-232, the level conversion should be performed between the microcontroller AT89S52 and the interface of the PC. Generally, the corresponding TTL level converter can be used to achieve this. The most commonly used RS-232 level conversion device is MAX232 from MAXIM. The level conversion circuit of MAX232 is shown in Figure 3. Among them, the 11th pin of MAX232 is connected to the 11th pin of AT89S52, the 12th pin of MAX232 is connected to the 10th pin of AT89S52, and the 15th pin of MAX232 is connected to the 20th pin of AT89S52. [page]

3. Software design of vehicle-mounted compaction tester

3.1 Data collection and transmission program design

In the system, the AT89S52 serial port controller SCON is set to 50H, SM0 and SM1 are 0 and 1 respectively, which is serial working mode 1, and REN is 1, which allows serial port reception. In addition, the AT89S52 interrupt enable register IE should also enable or disable all interrupt bits EA to 1, and enable or disable serial channel interrupt bits ES to 1, which allows serial port interrupts. The parallel ports P0 and P2 of the AT89S52 are used as the input and output ports of 8-bit data respectively. The output of the P3 port is used as the data latch and the disable and enable of the A/D. The flow chart of the lower computer data acquisition and transmission program is shown in Figure 4.

3.2 LCD display program design

The vehicle-mounted compaction tester finally displays the acceleration change curve through the LCD screen. FYD12864-0402B has a built-in ST7920 drive control module. Therefore, users do not have to worry about writing complex driver programs. They only need to connect the circuit correctly according to the interface instructions and write the main program according to the requirements. The LCD display program flow chart is shown in Figure 5.

4 Conclusion

Before the instrument is put into actual operation, the vibration acceleration must be calibrated. Experiments have shown that the vehicle-mounted compaction tester has incomparable advantages over traditional compaction testers. This is reflected in the following aspects:

(1) Speed: The vehicle-mounted compaction tester can reflect the compaction status of the roadbed online in real time. The traditional compaction test method often requires manual analysis after sampling to determine the current compaction status of the road surface, which often causes delays in construction.

(2) Non-destructive: Traditional compaction testing methods require digging pits to collect samples, so it is inevitable to

Causes damage to the base.

(3) Accuracy: The traditional compaction detection method can only measure the compaction value at a limited point, and has a large randomness, which cannot well reflect the compaction status of the entire compaction area. However, the vehicle-mounted compaction detection method can show the compaction status of the entire compaction area.

(4) Ease of operation: Traditional compaction testing methods require skilled manual operations, including digging, weighing, and measuring moisture content, while vehicle-mounted compaction testing methods do not require manual sampling.

It is believed that with the continuous expansion of the scale of modern high-grade highway construction and the improvement of the requirements for roadbed compaction quality, vehicle-mounted compaction testers will definitely be more widely and deeply used in future highway construction.