Asphalt intelligent softening point tester based on single chip microcomputer

introduction

The asphalt ball sample with a diameter of 10mm is placed on a bracket immersed in a heat transfer medium. When heated to a certain temperature, the asphalt melts. At this time, asphalt solution will drip from the sample through the holes on the bracket and be detected by the photoelectric detection device installed below. The temperature at this time is the softening temperature of the asphalt. The whole process is carried out in a 1000mL beaker.

1 System Hardware Structure

The whole system is a controller with AT89C51 single-chip microcomputer as the core. The hardware structure block diagram is shown in Figure 1 and can be divided into the following parts.

(1) The single-chip microcomputer and peripheral circuits such as keyboard and display use the AT89C51 single-chip microcomputer chip as the processor. It has a 4KB E2PROM on the chip, and the instruction system is fully compatible with Intel MCS-51. This system is programmed in assembly language, and the control program capacity is about 2KB, so there is no need to expand the program memory, which simplifies the control circuit. In addition, the 128-byte user RAM in the chip is sufficient for the system.

According to the needs of instrument operation, two buttons are designed on the instrument panel, one is the "test" key, and the other is the "result" key, which are controlled by P1.0 and P1.1 respectively. After power on, press the "test" key, P1.1 inputs a low level, and the instrument starts, otherwise it does not start; press the "result" key at any time, P1.2 inputs a low level, at this time if the test is not over, the display result becomes zero, if the test has ended, the softening point temperature is displayed.

In order to avoid the system expansion of parallel interface chips, the output mode of the single-chip microcomputer serial interface mode 0 is adopted, and two 74LS164 shift registers are connected to form an eight-bit LED display interface. The output end of one of them is used as the position control of the display through a PNP transistor and connected to the word port of the display; the other output controls the data code to be displayed, and in order to improve the driving ability, a 74LS245 is connected to the output end of the chip to output data to the word port of the display, so that the output current is expanded to 20mA. The left four digits of the eight-bit display are time display, the unit is minutes, and the right four digits are temperature display, the unit is ℃, both accurate to one decimal place.

(2) The heating control circuit uses a thyristor to control the triggering of the heating circuit. A heating indicator light is connected in the circuit. During the test, the heating and stopping of the resistor wire can be known by observing the on and off of the indicator light on the instrument panel. When the monitoring program determines that heating is required, the P1.4 port of the single-chip microcomputer sends a low level to turn on the PNP transistor 8850. After being photoelectrically isolated by MOC3041, the trigger control signal of the thyristor is output at its 4 pins, so that the thyristor trigger end is triggered and turned on, and the heating resistor wire is heated by 220 volt AC power; when the control program determines that heating needs to be stopped, the P1.4 port of the single-chip microcomputer sends a high level to turn off the 8850 and stop heating the resistor wire. The circuit has strong anti-interference ability, complete isolation of input and output, and good insulation performance.

(3) The temperature signal acquisition, amplification and conversion circuit uses a P?N junction temperature sensor - transistor 9013 to obtain temperature data. This sensor has high sensitivity and good linearity. Its temperature measurement range is 0℃～+125℃. As the temperature rises, the voltage between its base and emitter decreases, and the rate of change (temperature coefficient) is 2mV/℃. In order to obtain higher measurement accuracy, the software compensation method can be used to calculate the voltage value that needs to be compensated for each temperature data based on the temperature-voltage characteristic curve of 9013, and compensate it in the A/D conversion program. The voltage signal measured by 9013 is amplified by the general integrated operational amplifier LM324 with differential input.

In order to convert the analog voltage output by the amplifier into a digital signal and input it to the single-chip microcomputer, the LM331 voltage-frequency converter is used. The input signal of the LM331 is the output voltage of the amplifier, which is converted into a pulse of a certain frequency by the LM331. The pulse signal is directly connected to the T1 terminal of the AT89C51 (the external counting input terminal of the on-chip timer/counter T1), and the number of the pulse sequence is counted by T1, thereby realizing the analog-to-digital conversion. The value of the voltage signal input terminal of the LM331 is 0~10V, and the corresponding frequency signal of the output terminal is 10~10kHz. Since the LM331 outputs a frequency signal, it has strong anti-interference ability, high linearity, and simple and convenient transmission. Only one output signal line is needed to be directly connected to the single-chip microcomputer, which greatly saves system port resources and simplifies the control circuit design.

(4) Detection circuit In order to ensure reliable detection of asphalt softening, two photoelectric detection devices are installed. When both detection devices detect asphalt softening ( bitumen solution drips, blocking the light at the light emitting end of the detection device, so that the receiving end cannot receive light), their output ends respectively send high-level signals. In order to prevent the detection device from malfunctioning due to weak signals caused by impurities in the water and bubbles generated by boiling water, an operational amplifier LM324 is used as a voltage comparator. Only when the output signal has a certain intensity can the voltage comparator output a high level; the signal is connected through transistors to make P1.6 and P1.5 obtain low-level input signals; when P1.6 and P1.5 detect low levels at the same time, the asphalt is considered to have softened. At this time, the heating circuit is cut off. At the end of the experiment, the temperature at that moment is recorded, and the temperature value is the softening temperature of the asphalt.

2 System Software Design

The software adopts a modular structure, consisting of a main program and an interrupt service program. T0 of AT89C51 is used to control the time of heating and stopping, with a time base of 2ms, and works in interrupt mode 0; T1 works in mode 0 to count the number of pulses output by LM331. Since the maximum value of LM331 output is 10kHz, T1 cannot overflow. The main program includes the system initialization module, temperature control module, A/D conversion and data processing module, standard data storage module, etc. The interrupt service program includes keyboard processing and data display, etc. The initialization content includes the setting of stack pointer, interrupt enable register, timer working mode register, etc., the setting of timer initial value, the start of timer, the clearing of internal RAM unit, etc. In the interrupt service program, the keyboard program is scanned every 0.5 seconds, and the A/D converted temperature data is read every 1.2 seconds. Since the heating process is not a linear process, real-time control must be performed. In the temperature control program, the read temperature data is compared with the standard temperature data. If it exceeds the standard data, over-temperature treatment is performed, otherwise under-temperature treatment is performed.

The main program flow chart is shown in Figure 2, and the temperature control program flow chart is shown in Figure 3.

3 Conclusion

After actual use, the asphalt softening point instrument has accurate test, stable performance and strong anti-interference ability. In actual use, in order to make the temperature field uniform, magnetic stirring is added to the container to further improve the accuracy of the test.