Resistance-temperature measurement system of carbon black composite conductive material based on single chip microcomputer

0 Introduction
With the rapid development of the electronics industry and information technology, the demand for polymer materials with conductive functions is becoming more and more urgent. Conductive composite materials have the characteristics of light weight, no corrosion, easy to process into various complex shapes, good dimensional stability, adjustable conductivity in a large range, easy to mass produce and cheap, so they are widely used in antistatic, microwave absorption, self-controlled temperature heating materials, electromagnetic wave shielding and other fields. Among them, carbon black composite conductive material is currently the most widely used and the largest amount. Here, with AT89S51 single-chip microcomputer as the core, a simple system for measuring the resistance and temperature of carbon black composite conductive materials is designed. The system block diagram is shown in Figure 1.

The single-chip microcomputer collects the current temperature value through the thermocouple amplifier chip; the resistance value is collected through the voltage conversion circuit and sent to FM24C02 for query. The LCD displays the current resistance and temperature. The user can easily read the changes in resistance and temperature, and can also communicate with the host computer through the serial port.

l Working principle and application of AD595
AD595 is a thermocouple amplifier produced by AD company. It integrates instrument amplifier and thermocouple cold joint compensator on a single chip and produces an output of 10 mV/℃. The selectivity of the pin makes it possible to use it as a linear amplifier compensator or a switch output for setting the operating point controller.
AD595 includes a thermocouple fault alarm. If one or both legs of the thermocouple are open, an alarm signal can be displayed. There are many flexible ways of alarm output, including TTL form.
AD595 can be powered by a single-ended +5 V voltage. If a negative voltage is used, temperatures below 0℃ can be measured. In order to minimize its own heating, the total current of an unloaded AD594/AD595 is 160μA, and of course, more than 5 mA of current can be delivered to the load.
1.1 Temperature stability
Each AD595 has been tested for the temperature error when measuring the zero point at different temperatures. The error of the cold joint compensator, the offset of the amplifier, and the gain error combine to determine the output stability of the AD595 within the rated ambient temperature range. Figure 2 shows the measurement error distribution range of AD595. The coordinate unit in Figure 2 is ℃.
1.2 Thermal environment effect
The inherent low energy dissipation of AD595 and the low thermal impedance package make the error caused by its own heating negligible. For example, in still air, the environmental thermal impedance of the chip is about 80℃/W (D-type package). At the rated dissipation of 800μW, the self-heating error in free air is less than 0.065℃. Immersed in liquid fluorine, the thermal impedance is about 40℃/W, making the self-heating error about 0.032℃. Figure 2 shows the measurement error distribution of AD595.

When the single-chip microcomputer collects temperature values, the first foot of AD595 senses the temperature change of the carbon black conductive material; the eighth foot outputs the voltage change corresponding to the temperature, and the coefficient of change is 10 mV/℃. The voltage is amplified by LM358 and sent to the V/F unit for conversion. After that, the single-chip microcomputer stores the collected information in FM24C04.

2 V/F conversion device LM331
LM331 is a cost-effective integrated chip produced by NS Company in the United States, which can be used as a precision frequency voltage converter. LM331 adopts a new temperature-compensated bandgap reference circuit, which has extremely high accuracy in the entire operating temperature range and at a power supply voltage as low as 4.0V; at the same time, it has a wide dynamic range of up to 100 dB; good linearity, with a maximum nonlinear distortion of less than 0.01%; low operating frequency, with good linearity at 0.1 Hz; high conversion accuracy, with a digital resolution of up to 12 bits; simple external circuit, only a few external components need to be connected to easily form a V/F or F/V conversion circuit, and it is easy to ensure conversion accuracy. The circuit is shown in Figure 4.

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3 AT89S51 microcontroller and ISP interface
The AT89S51 microcontroller is compatible with the MCS51 microcontroller, but it has been improved over the earlier AT89C51. It has a built-in watchdog timer, and does not require an external monitoring chip. The system can be set up to work reliably through software settings, and it supports in-system programming. Blank devices can be programmed without removing the device from the circuit board.
When the microcontroller collects resistance signals, the resistance of carbon black is converted into a voltage value through a constant current source circuit, which is then amplified and converted into a frequency signal and sent to the AT89S51. The microcontroller stores the collected information in FM24C04 for query.

AT89S51 has two ISP modes: serial port and parallel port. The parallel download interface mode is adopted in the design. Its interface circuit with the microcontroller is shown in Figure 4. The circuit has fast download speed and stable operation. Online programming can be completed using Easy51Prov2.0 software.

4 Display Circuit T6963C
Among the medium-scale graphic LCD display modules, the LCD display module with built-in T6963C controller is currently a more commonly used graphic LCD display module. The drive control system of the LCD display module with built-in T6963C controller is composed of the LCD display controller T6963C and its peripheral circuits, row driver group, column driver group and LCD driver bias circuit.
The biggest feature of T6963C is that it has a unique hardware initial value setting function. The parameters required for display driving, such as duty cycle coefficient, number of bytes/row driven transmission and font selection of characters, are all set by pin level. In this way, the initialization of T6963C is basically set when powered on, and the main focus of software operation can be fully used for the design of display screen. T6963C uses hardware initialization settings, so its instruction function is concentrated on the setting of display function, thereby strengthening the display control ability of T6963C.
Some of the questions when T6963C instructions are running are uncertain, because the operation of some instructions is affected by the state of the control part at that time. Some instructions in T6963C require parameter supplement, such as address pointer setting. The input of T6963C instruction parameters is before the instruction code is written. The flow chart of T6963C instruction writing is shown in Figure 6.

5 Software Design
The system software is designed according to the module structure, mainly including setting initial values, calling subroutines, setting the status of each interface, read/write operation control, etc. The microcontroller control software mainly completes the following functions, namely V/F conversion control, resistance value and temperature display, and computer serial port communication. The software uses C language program (omitted), and the program flow is shown in Figure 7.

6 Debugging process and results
The measured signal is amplified, and the analog quantity is converted into frequency through the V/F converter (LM331), and the frequency value is read by the CPU. Through conversion calculation, the value of the measured signal is obtained. Resistance
measurement: Use a constant current source to flow a constant current through the sample to be tested and convert the voltage value into a frequency value. The current cannot be too large. A large current will cause temperature changes and thus affect the resistance value.
Temperature measurement: Use a K-type thermocouple to measure the temperature, and use AD595 to convert the temperature value into an analog voltage. AD595 has its own cold end automatic compensation. The test system tests the R-T change as shown in Table 1. It can be seen from the data changes in the table that the carbon black composite conductive material has PTC characteristics.

7 Conclusion
Such a resistance-temperature measurement system built using a single-chip microcomputer can be easily applied to the measurement of various conductive materials with temperature sensor characteristics. It is relatively convenient to use and can communicate with the host computer using the serial port, making the measurement results more intuitive and convenient.