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Oct 16 2020

AD590 Based Digital Temperature Control Device Design

I. Introduction

For large systems such as missile weapons and equipment, their performance is often affected by the external environment and their own operating conditions. Among them, the influence of temperature often plays a very important role. Therefore, temperature detection and control has always been the focus of many researchers.

 

However, some temperature measurement and control devices have low accuracy and inaccurate temperature control, and some new instruments are expensive and difficult to promote. It should be particularly pointed out that the temperature measurement and control system developed in the past is usually an independent system, one thing for one use, it is difficult to be adopted by other systems, and there are problems such as maintenance difficulties and inconvenience.

 

To this end, the author developed a high-precision temperature measurement and control device suitable for research and development under laboratory conditions based on the currently popular modular design principle.The device uses a new integrated temperature sensor AD590 as the temperature measurement element, and provides two control units for experimental comparison. By measuring and controlling the temperature in the thermostat, satisfactory results have been obtained.

AD590Catalog

I. Introduction

II.Working Principle

III. Integrated Temperature Sensor AD590

IV. Temperature Measuring Bridge

V. PID Regulator

VI. Program Design

VII. Experimental Analysis and Conclusion

FAQ

II. Working Principle

Figure 1 is the electrical schematic diagram of the WCZ-98 temperature measurement and control device. Its working principle is: the temperature signal taken by the temperature measuring bridge with AD590 as a bridge arm is differentially amplified and buffered and then sent all the way to the digital display for digital temperature display, and the other is compared with the set value . The compared difference is controlled by switch K and can choose to send to two-way adjustment controller. One route is composed of a comparison amplifier and a relay, which can be used as an adjustment controller to form an independent temperature measurement and control equipment without connecting to a computer; the other route is a PID regulator (composed of A/D, D/A and Computer composition of PID adjustment software) and SCR composition. The signal from the regulating controller realizes temperature control through the temperature control actuator.

Figure 1 Electrical schematic diagram of temperature measurement and control device

Figure 1 Electrical schematic diagram of temperature measurement and control device

 

III. Integrated Temperature Sensor AD590

 

AD590 is a dedicated integrated temperature sensor produced by American AD company, which belongs to the current output type. Figure 2 shows the current-voltage characteristic curve of AD590 at three different temperatures. In a certain temperature range, it is equivalent to a high resistance current source, and its current temperature sensitivity is lμA/K. It is not susceptible to interference from contact resistance, lead resistance, voltage noise, etc. In addition, it also has the characteristics of small size, high temperature measurement accuracy, good linearity and strong interchangeability. It is very suitable for long-distance measurement and control. It is also suitable for the characteristics of modular and split structure required by this article. The main technical indicators are:

 

  • Temperature measurement range:h
  • Current output (calibration factor): lμA/K;
  • Power supply voltage: DC 4-30V;
  • Linearity: less than ±0.5℃ in the full scale range;
  • Repeatability: ±0.1℃;
  • Output impedance: about 10MQ
  • Long-term drift: ±0.1℃/month

Figure 2 I-V curve of AD590

Figure 2 I-V curve of AD590

The current Ir flowing through the AD590 is a single-valued function of the absolute temperature of its environment, and the microampere of Ir is equal to the absolute temperature T, namely:

 

Ir=T×10-6A=TμA (1)

 

IV. Temperature Measuring Bridge

Figure 3 is the schematic diagram of the temperature measurement bridge. The voltage formed on the current IiR2 and Rw2 flowing through the AD590 is:

 

Ul=Ii×(R2+Rw2) (2)


Figure 3 Schematic diagram of temperature measuring bridge

By adjusting Rw2 to make (R2+Rw2) equal to 10K, substituting formula (1) into formula (2), we can get:

 

  U1=Ii×(R2+Rw2)=T×10-2V (3)

 

  U2=2.732V by adjusting Rwl. Then the output of the bridge is:

 

  UAB=U1﹣U2=T×10-2﹣2.732=(T﹣273.2)×10-2V (4)

 

Because T is the absolute ambient temperature measured by AD590, after subtracting 273.2 from it, the Celsius temperature t can be obtained, namely:

 

  UAB=t×10-2V (5)

 

At this point, the temperature measuring bridge converts the ambient temperature into a voltage value that is proportional to the temperature in Celsius.

 

V. PID Regulator

 

One of the adjustment controllers of the temperature measurement and control device uses a PID regulator (proportional integral derivative regulator), which can determine the size of the control quantity according to the proportional value, integral value, and derivative value of the difference between the temperature set value and the actual value . The temperature measurement and control device adopts the output feedback type control. Extracting this part from the general principle diagram, you can get the PID control principle diagram as shown in Figure 4. In the figure, Ud and U are the set value and actual value of the thermostat respectively, the error e=Ud﹣KT, K is the magnification of the measuring transducer, and Y is the adjustment value of the PID output.

Figure 4 PID control principle diagram

Figure 4 PID control principle diagram

The simulation expression of PID algorithm is:

In the formula,

 

Y(t): regulator output value;

E(t): input deviation;

KP: regulator proportional coefficient;

Tl, TD: verse unit integral, derivative time

After discretizing equation (6), the PID incremental control equation is obtained:

In the formula, the integral coefficient Kl=KPT/T1, the differential coefficient KD=KPTD/r, and T is the sampling period.

then apply (7) to Z-transform, and get:

In the experiment, the author used a step signal to roughly measure the response in the open-loop state. From the step response curve, it is known that the thermostat is a first-order inertia link plus a pure time delay link, namely:

The lag time r of the system is determined to be approximately 20 seconds, and the target time constant TP is approximately 50 seconds. Select the control degree to be 1.5, according to the step response curve tuning parameter method (refer to literature [1]), obtain:

 

T=0.34z=6.8s

Kr=0.85Tr, /r=2.125

T1=1.62r=32.4s

TD=0.65r=13s

 

Substituting the above value for equation (9), we can obtain:

 

Q0=6.41

Q1=-5.96

Q2=4.06

 

The equation of PID regulator is:

 

VI. Program Design

 

The PID control program flow of the WCZ-98 temperature measurement and control device is shown in Figure 5. The basic idea is the same as the general PID control flow.It’s no need to repeat here.

Figure 5 PID control program flow chart

Figure 5 PID control program flow chart

 

VII. Experimental Analysis and Conclusion

 

Put the temperature measurement and control device into a thermostat with an external dimension of 248×208×262 (mm). The thermostat uses 50mm thick polystyrene as the heat insulation material and water as the medium. The heating device is composed of 2 SRS3-220/0.5 heating tubes and auxiliary parts to prevent leakage. The temperature can be preset outside the thermostat and there is a switch to select the type of controller.

 

Through experiments, comparing the control effects of the two adjustment control methods, we found that the temperature of the adjustment controller composed of a comparison amplifier and a relay is not stable during the temperature control process and always fluctuates within a certain error range. The temperature control performance of the regulating controller composed of PID regulator and thyristor is very good. Taking temperature control of 60°C as an example, the temperature change curve obtained by the experiment is shown in Figure 6.

Figure 6 PID temperature control experiment result curve

Figure 6 PID temperature control experiment result curve

It can be seen that the use of analog circuits for adjustment and control is beneficial to make the measurement and control device an independent instrument (no need to connect to a computer), and its temperature measurement and control accuracy can meet the general requirements; and through PID control, its precision of temperature measurement and control is very high. It is used in conjunction with the thermostat and the self-developed SYZJX-2 experimental adapter box. The analog input board PCL-818L and the analog output board PCL-726 are connected to the computer to achieve high-precision temperature control.


FAQ

  • What is AD590?

AD590 is a temperature sensor, the current output sensitivity is 1μA/℃, the standard output value is 298.2μA at 25℃, and the working voltage range is 4~30V.

  • What are the characteristics of AD590 temperature sensor?

Single function (only temperature measurement), small temperature measurement error, low price, fast response speed, long transmission distance, small size, micro power consumption, etc. It is suitable for remote temperature measurement and temperature control without non-linear calibration. The peripheral circuit is simple.

  • How to detect the quality of AD590?

AD590 has a current of 273 mA at 0°. Because 2113 is a Wen sensitive resistor 5261, it means that it is greatly affected by the surrounding temperature 4102. It is very difficult to measure without relying on 1653 other tools. Give you some suggestions.

  1. When the ambient temperature rises by one degree, the current of AD590 increases by 1uA. What you have to do is to work with AD590 simultaneously with the help of a high-precision temperature test instrument. After AD590 series 10K resistance, measure its voltage, that is to say, it should be 2.73V at 0°, and 2.98V at room temperature 25°.
  2. For higher accuracy, it is recommended that you use the electronic building block software Ardunio for measurement, and put the corresponding data into MATLAB for linear regression. The better the linearity, the more stable the measurement.
  3. AD590 is not a high-precision temperature testing device. If high-precision testing is required, other components are recommended.
  • What is the difference between AD590 and PT100?

AD590 is a current-type temperature sensor. It converts temperature changes into current conversion. The simplest processing is to pass a resistor (10K) after the output to convert the current into a voltage, and then through the detection voltage, the current at this time can be deduced. Use the relationship between current and temperature in the sensor data to calculate the current temperature.

PT100 is a resistance type temperature sensor, which converts temperature changes into resistance changes. The simplest process is to place Pt100 in a bridge, use the voltage difference at the midpoint of the bridge arm, and use a differential amplifier circuit (instrument amplifier circuit) Amplify the voltage, use the amplifier gain and bridge structure data, and use the detected voltage to inversely calculate the current resistance value, and use the relationship between resistance and temperature in the PT100 data sheet to calculate the current temperature.

  • Is AD590 a thermocouple or a thermal resistance?

It is neither a thermocouple nor a thermal resistance. The main principle is to detect the temperature according to the temperature change, the output current change, and the current size.

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