Temperature measurement and control system composition and temperature signal generation and processing

When we developed the nitrogen oxide chemiluminescence analyzer, the entire system required temperature measurement and control in three places: the reaction chamber, the molybdenum conversion chamber, and the photon counter PMT. The temperature in the reaction chamber has a certain impact on the chemical reaction (nitric oxide and ozone reaction). We need to find the optimal temperature to maximize the reaction efficiency. The temperature of the molybdenum conversion chamber affects the efficiency of converting nitrogen dioxide into nitric oxide, so the temperature at which the efficiency is maximum is also required. The requirements for temperature measurement and control are: the measurement and control temperature range of the reaction chamber is: 30-70OC, fluctuation: ±0.5 OC; the measurement and control range of the molybdenum conversion chamber is: 250-370 OC, fluctuation: ±3 OC. The photon counter PMT is greatly affected by temperature. The higher the temperature, the higher the dark count of the photon counter PMT. While cooling the photon counter PMT, its temperature is also monitored to ensure that it is working in a low temperature (about 5OC) environment. The system requires temperature measurement accuracy of 0.05OC.

In order to ensure system requirements and shorten system development time, we used National Instruments' graphical programming software system LabVIEW and data acquisition card Lab-PC-1200 to build the entire temperature measurement and control system of the analyzer. In the process of building the system, the problems of multi-channel measurement and output control of the data acquisition card were solved. Under certain hardware conditions, the optimization program further improved the system measurement and control performance. A preliminary discussion was conducted on the construction of a multi-channel measurement and control system based on virtual instruments.

Temperature measurement and control system composition

The system combines computers, powerful graphical programming software and modular hardware to establish flexible computer-based measurement and control application solutions, and ultimately build a system that meets your own needs. The system consists of the following parts: computer, LabVIEW, data acquisition card, temperature sensing circuit, and heating control circuit. The temperature signal is converted into a voltage signal by the sensor, and then enters the computer through the data acquisition card. The LabVIEW program running on the computer analyzes and processes the input data, displays the results on the computer, and at the same time outputs control signals to the external heating through the data acquisition card. Control circuit to measure and control temperature.

Among them, the data acquisition card Lap-PC-1200 is a low-cost board used on computers. It can collect analog signals and digital signals, has the function of timer, and also has the function of analog output. This data acquisition card has high-performance data acquisition and control capabilities and can be used for laboratory testing, production testing, and industrial monitoring and control.

We mainly use the analog input and analog output functions of the card. Lab-PC-1200 data acquisition card has eight analog input channels and two analog output channels.

There are eight analog input channels ACH0-ACH7. The internal analog-to-digital converter is a 12-bit progressive approximation type. You can set it to eight single-ended signal input modes or four differential signal input modes. The card has three different analog input modes: RSE, NRSE, DIFF input mode. We set the RSE input mode. The RSE input mode means that all input signals are referenced to the common ground AGND (the common ground here refers to the analog input ground). The Lab-PC-1200's analog inputs can also be selected as unipolar or bipolar. Select unipolar, the input voltage range is 0 to 10 V, 0V corresponds to 0 hex, and 10 V corresponds to FFF hex (4095 decimal). Select bipolar and the input voltage range is -5 to +5 V. We set the analog input to be unipolar.

There are two analog output channels DAC0OUT and DAC1OUT. You can set the analog output channel to be unipolar or bipolar output. Unipolar output range is 0 to 10 V, value range is 0 to 4095 (0 to FFF hex). The bipolar output range is -5 to +5 V, and the value range is -2048 to 2047 (F800 hex to 7FF hex). We set the analog output to be unipolar. There are two ways to refresh the analog output voltage: one is called immediate update mode. When you have data written to the digital-to-analog converter (DAC), its output voltage is refreshed. The other is called delayed update mode. Its output will only start to refresh when it detects that counter A2 or EXTUPDATE is low level. We set up the immediate refresh mode. DAC0OUT corresponds to analog output channel 0, and DAC1OUT corresponds to analog output channel 1. AGND is the reference ground of these two analog output terminals [2].

Temperature measurement of PMT

The temperature of the photon counter PMT is about 5OC under the action of the semiconductor cooling chip. Its temperature measurement circuit is shown in Figure 2

In the circuit, AD590 integrates a temperature sensor, which is a two-terminal temperature device with constant current output. Inside it is a corrected and calibrated control current source, and its output current is proportional to the absolute temperature, that is

(1)

In the formula [i]k［/i] is the temperature coefficient

(2)

The operating voltage range of AD590 is DC4-30V. Within this voltage range, the output current and temperature have a good linear relationship when the ambient temperature changes between -55-150［i］℃［/i］. The reference voltage source of MC1403 has an output of 2.5V. Adjust the variable resistor W1 to make I2= 273.2［i］m［/i］A, then (3)

The input voltage of DAQ analog input terminal ACH1 (AI2) is

(4)

We set the analog input to RSE mode, unipolar, then its input voltage range is 0 to 10 V, and set its internal amplification factor to 10, then its input voltage range becomes 0 to 999.756 mV.

(5)

U is the analog input quantity read in the LabVIEW program, which can be deduced from equation (5):

(6)

Since the temperature sensor, amplifier, reference voltage source and resistor will all have certain deviations, we use a standard thermometer for calibration, and finally adjust equation (6) to:

(7)

Since the resolution of the analog-to-digital conversion of the analog input is 12-bit, the minimum resolution value of the temperature can be derived from equation (7): △t=8.06%26;#215;△U=8.06%26;#215;( 10/4095)≈0.02, 0.02OC meets the system requirements. Temperature measurement and control of reaction chamber

The reaction chamber is equipped with a heating rod and a temperature sensor (thermistor), which are connected to the temperature measurement and control circuit and controlled by a computer to achieve precise temperature control. The temperature measurement and control circuit is shown in Figure 3.

In the circuit, RL is the heating rod; Rt is the negative temperature coefficient thermistor; MOC3021 is the optocoupler; Z0409MF is the thyristor; Date Acquisition Board (DAQ) is the data acquisition card Lab-PC-1200, AO, AI and GND are respectively are its analog output, analog input and ground terminals. The configuration of the data acquisition card Lab-PC-1200 is as follows: the analog input is in RSE mode and unipolar, so its input voltage range is 0 to 10 V; the analog output is unipolar and in immediate refresh mode. Select ACH0 for the analog input channel, DAC0OUT for the analog output channel, and AGND as the analog ground.

The working process of the circuit can be divided into: temperature signal generation and processing, temperature control signal generation and output.

1. Temperature signal generation and processing

The temperature signal is generated by the temperature measurement circuit. The temperature measurement circuit is composed of R3, Rt, and DC+5V power supply. Rt is a negative temperature coefficient thermistor, and the relationship between its resistance and absolute temperature is

(8)

In the above formula, B and C are constants. The thermistor resistance was measured experimentally at different temperatures, and lnRT was linearly fitted to the absolute temperature T, and B=4206.96 C=-10.23 was obtained. From equation (8), the relationship between absolute temperature and thermistor resistance is:

(9)

Convert absolute temperature to degrees Celsius

(10)

At 50%26;#176;C, the thermistor resistance is 16.20KW. To obtain the maximum sensitivity around 50%26;#176;C, select the voltage dividing resistor R3 to be 16.20KW. From the circuit, it is known that the thermistor is

(11)

In the above formula [i]U[/i] is the thermistor voltage divider. The temperature t voltage signal U can be obtained from equations (10) and (11). The voltage signal U is read into the computer from the analog input channel ACH0 (pin 1) of Lab-PC-1200, and then the temperature value t is calculated by the LabVIEW program.

2. Temperature control signal generation and output

This part of the function is realized by program-controlled data acquisition card and computer. The thermal voltage U is introduced into the data acquisition card from the analog input channel ACH0 (pin 1). In the program, the temperature t can be calculated through formulas (10) (11), and t is compared with the set temperature t0, where a and b is the percentage coefficient

(12)

If the duty cycle is greater than or equal to 1, it means that the temperature is not close to the set temperature and heating is required throughout. The AO output of the analog output terminal of the data acquisition card is all high level (voltage 5V). If the duty cycle is less than 1, the ratio of the high level time in the square wave output by the analog output terminal AO of the data acquisition card to the square wave period is equal to the duty cycle. According to the heating capacity of the heating rod and the heat dissipation of the reaction chamber, the percentage coefficients a and b can be appropriately adjusted so that when the temperature reaches the set temperature, the heat absorbed by the reaction chamber is equal to the heat dissipated, so that the temperature of the reaction chamber is in a dynamic state. balance.