Design of Temperature Control System Based on LM35 Temperature Sensor

This design introduces a temperature control system developed using the LM35 temperature sensor, focusing on the system structure, working principle and sampling value quantification. At the same time, the characteristics of LM35 sensor, system hardware circuit design and software design are also introduced. The system is small in size, low in cost, reliable in operation, and has high engineering application value. The system can be modified or expanded slightly to perform functions such as temperature measurement.

1, lM35 temperature sensor

The LM35 is one of NS's integrated circuit temperature sensor products. It has high operating accuracy and a wide linear operating range. The output voltage of this device is linearly proportional to Celsius temperature. Thus, from a use point of view, the LM35 is superior to the Kelvin-standard linear temperature sensor. The LM35 provides external room temperature accuracy of ±1/4°C without external calibration or fine-tuning.

1) Working voltage: DC 4~30V;

2) Working current: less than 133μA

3) Output voltage: +6V ~ -1.0V

4) Output impedance: 0.1Ω at 1mA load;

5) Accuracy: 0.5 °C accuracy (at +25 ° C);

6) Leakage current: less than 60μA;

7) Scale factor: linear +10.0mV/°C;

8) Nonlinear value: ± 1/4 ° C;

9) Calibration method: directly calibration with Celsius temperature;

10) Package: sealed TO-46 transistor package or plastic TO-92 transistor package;

11) Operating temperature range: -55 ~ +150 ° C rated range. Pin introduction:

1 positive power supply Vcc;

2 output;

3 output ground / power ground.

2, system structure and working principle

The temperature control circuit is composed of a sensor circuit, a signal conditioning circuit, an A/D sampling circuit, a single chip system, an output control circuit, and a heating circuit. The basic working principle of the circuit: the sensor circuit outputs the sensed temperature signal to the signal conditioning circuit in the form of voltage. After the signal is conditioned, it is input to the A/D sampling circuit, and the digital value is sent to the single-chip microcomputer system by the A/D converter. The system determines whether the heating circuit needs to be turned on according to the designed temperature requirements. In this paper, the design is based on 0°C. When the temperature is lower than or equal to 0°C, the heating circuit is turned on. When the temperature is higher than 0 °C, the heating circuit stops working.

It can be seen from Figure 1 that, for whatever reason, the ambient temperature is lower than 0 °C, the microcontroller system will output the corresponding logic level (in this case, designed as a low level), and the relay of the control output circuit is closed after being driven. Make the heating circuit work. This system is an open loop control system.

3, core hardware circuit design and sampling value quantification

The core component of the sensor circuit is LM35AH. When the power supply voltage is 15V DC, the operating current is 120mA. The power consumption is extremely low. When the whole temperature range is working, the current changes are small. The voltage output adopts the differential signal mode, which is directly output by 2 and 3 pins. The resistance R is 18K ordinary resistance, and D1 and D2 are 1N4148. As shown in Figure 2. This circuit is suitable for applications with a temperature range of -55 to +150 °C. If the temperature range changes, you can make some adjustments to this circuit. I have done temperature test on this circuit separately, put the sensor in the temperature change cycle box, observe and record the output voltage every 5 °C as a test point (test data and UT curve are limited to the length, omitted), test results It shows that the linearity of LM35AH is satisfactory.

The signal conditioning circuit mainly performs the functions of amplifying and limiting the sensor signal, and the output range of the sensor circuit is changed to a DC voltage of about 2V, and is regulated to ±10V DC voltage, and the operational amplifier adopts LF412. The A/D sampling circuit uses a 12-bit AD converter AD574. The one-chip computer system uses AT89C55 as CPU, external latch and output drive circuit. The output circuit uses the Panasonic PhotoMOS relay AQZ202 to control the on/off of the heating circuit. The heating circuit adopts the method of heating the power resistor to design a heating plate separately. The resistance adopts the method of “series+parallel”, the total resistance is about 14Ω, the power supply voltage is 28V DC, and the whole board heating power is 50W.

The accurate quantification of the sampled value is the key to the normal operation of the temperature control circuit. The following conversion methods are used for quantization.

It is assumed that the voltage after signal conditioning is Ui, then -10V≤Ui≤10V, the temperature corresponding to -10V is known to be -55°C, and the temperature corresponding to 10V is 125°C, and the proportional factor Kt=0.111V/°C is easily obtained.

When the temperature is 0 ° C, ΔT = 55 ° C (i.e., the amount of change with respect to -55 ° C). Ui=-10V+ΔT·Kt=-10V+55°C&TImes; 0.111V/°C=-3.895 V. After Ui is converted to digital quantity, the corresponding voltage value of each digital quantity is 4.883mV (available from 12-bit AD, full-scale 20V), and is represented by Ks. Correspondence between digital change and temperature change can be obtained:

Kt/Ks = (0.111V / °C) / (4.883mV / digital) = 22.73 digital / °C

At 0 °C, the digital output of the AD is D0 = 0 + 55 ° C & TImes; 22.73 digital / ° C = 1250 = 04 E2H. The digital quantity corresponding to other temperatures can also be calculated by the above method.

4, system software design

The software adopts PLM/51 language and ASM mixed programming, adopts modular structure, mainly consists of main module, AD sampling module, initialization module, timer module, error processing module, etc. It is very convenient to modify and maintain.

The AD is connected to the AT89C55 of the microcontroller system using an interrupt mode. After the AD conversion is completed, the CPU reads the converted digital quantity, and judges by comparison, if the digital quantity is greater than 0 ° C corresponding to the digital quantity 04E2H, the logic output port P1 is refreshed and the low level is sent. Otherwise, the P1 port is high. The software workflow is shown in Figure 3:

In order to avoid malfunction due to interference, the software takes some redundancy and fault tolerance processing. Software filtering is used when the AD module processes the sampled data to filter out spikes that may occur in the circuit. The method is to continuously sample five times, and by comparing and judging, the maximum and minimum values ​​are removed. The remaining three values ​​are summed and averaged. The average value is used as the valid data used by the CPU for discriminating, and then compared with 04E2H (0°C corresponding digital quantity). The code of the AD module is as follows:

The temperature control system developed based on LM35 has been tested and tested repeatedly, and it is stable and reliable. It has the characteristics of small volume, high sensitivity, short response time and strong anti-interference ability. The system is low in cost, the devices are conventional components, and have high engineering value, and have been applied to a certain type of UAV flight control system. With a little modification, the system can be easily expanded into a product integrating temperature measurement and control. At the same time, the small-scale nonlinearity of the sensor LM35 can be corrected by software algorithms.

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