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The article introduces the resistance change of PT100 and PT1000 metal thermal resistor sensor probes at different temperatures, as well as a variety of temperature acquisition circuit solutions. Including resistance voltage division, bridge measurement, constant current source and AD623, AD620 acquisition circuit. In order to resist interference, especially electromagnetic interference in the aerospace field, an airborne PT1000 temperature sensor acquisition circuit design is proposed, including a T-type filter for filtering and improving measurement accuracy.
Abstract generated by CSDN through intelligent technology
PT100/PT1000 temperature acquisition circuit solution
1. Temperature resistance change table of PT100 and PT1000 sensors
Metal thermal resistors such as nickel, copper and platinum resistors have a positive correlation with the change of temperature. Platinum has the most stable physical and chemical properties and is the most widely used. The temperature measurement range of the commonly used platinum resistance Pt100 sensor probes is -200～850℃, and the temperature measurement ranges of Pt500, Pt1000 sensor probes, etc. are successively reduced. Pt1000, temperature measurement range is -200～420℃. According to the IEC751 international standard, the temperature characteristics of the platinum resistor Pt1000 meet the following requirements:
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According to the Pt1000 temperature characteristic curve, the slope of the resistance characteristic curve changes slightly within the normal operating temperature range (as shown in Figure 1). The approximate relationship between resistance and temperature can be obtained through linear fitting:
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1.1 PT100 temperature resistance change table
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1.2 PT1000 temperature resistance change table
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2. Commonly used acquisition circuit solutions
2. 1 Resistor voltage divider output 0~3.3V/3V analog voltage single chip AD port direct acquisition
The temperature measurement circuit voltage output range is 0~3.3V, PT1000 (PT1000 resistance value changes greatly, and the temperature measurement sensitivity is higher than PT100; PT100 is more suitable for large-scale temperature measurement).
The simplest way is to use the voltage division method. The voltage is generated by the TL431 voltage reference source chip, which is a 4V voltage reference source. Alternatively, REF3140 can be used to generate 4.096V as a reference source. Reference source chips also include REF3120, 3125, 3130, 3133, and 3140. The chip uses a SOT-32 package and a 5V input voltage. The output voltage can be selected according to the required reference voltage. Of course, according to the normal voltage input range of the AD port of the microcontroller, it cannot exceed 3V/3.3V.
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2.2 Resistor voltage division output 0~5V analog voltage, and the AD port of the microcontroller directly collects it.
Of course, some circuits are powered by a 5V microcontroller, and the maximum operating current of the PT1000 is 0.5mA, so an appropriate resistance value must be used to ensure the normal operation of the component.
For example, the 3.3V in the voltage division schematic diagram above is replaced by 5V. The advantage of this is that the 5V voltage division is more sensitive than the 3.3V voltage, and the collection is more accurate. Remember, the theoretical calculated output voltage cannot exceed +5V. Otherwise, the microcontroller will be damaged.
2.3 The most commonly used bridge measurement
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Use R11, R12, R13 and Pt1000 to form a measurement bridge, where R11=R13=10k, R12=1000R precision resistor. When the resistance value of Pt1000 is not equal to the resistance value of R12, the bridge will output a mV level voltage difference signal. This voltage difference signal is amplified by the instrument amplifier circuit and outputs the desired voltage signal, which can be directly connected to the AD conversion chip or the AD port of the microcontroller.
The principle of resistance measurement of this circuit:
1) PT1000 is a thermistor, and its resistance changes basically linearly with the change of temperature.
2) At 0 degrees, the resistance of PT1000 is 1kΩ, then Ub and Ua are equal, that is, Uba = Ub – Ua = 0.
3) Assuming that at a certain temperature, the resistance of PT1000 is 1.5kΩ, then Ub and Ua are not equal. According to the voltage divider principle, we can find Uba = Ub – Ua > 0.
4) OP07 is an operational amplifier, and its voltage amplification factor A depends on the external circuit, where A = R2/R1 = 17.5.
5) The output voltage Uo of OP07 = Uba * A. So if we use a voltmeter to measure the output voltage of OP07, we can infer the value of Uab. Since Ua is a known value, we can further calculate the value of Ub. Then, using the voltage divider principle, we can calculate the specific resistance value of PT1000. This process can be achieved through software calculation.
6) If we know the resistance value of PT1000 at any temperature, we only need to look up the table according to the resistance value to know the current temperature.
2.4 Constant current source
Due to the self-heating effect of the thermal resistor, it is necessary to ensure that the current flowing through the resistor is as small as possible, and generally the current is expected to be less than 10mA. It has been verified that the self-heating of the platinum resistor PT100 of 1 mW will cause a temperature change of 0.02 to 0.75℃, so reducing the current of the platinum resistor PT100 can also reduce its temperature change. However, if the current is too small, it is susceptible to noise interference, so it is generally taken at 0.5 to 2 mA, so the constant current source current is selected as a 1mA constant current source.
The chip selected is the constant voltage source chip TL431, and then the current negative feedback is used to convert it into a constant current source. The circuit is shown in the figure:
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The operational amplifier CA3140 is used to improve the load capacity of the current source, and the calculation formula for the output current is:
Insert picture description here The resistor should be a 0.1% precision resistor. The final output current is 0.996mA, that is, the accuracy is 0.4%.
The constant current source circuit should have the following characteristics:
Temperature stability: Since our temperature measurement environment is 0-100℃, the output of the current source should not be sensitive to temperature. And TL431 has an extremely low temperature coefficient and low temperature drift.
Good load regulation: If the current ripple is too large, it will cause reading errors. According to theoretical analysis. Since the input voltage varies between 100-138.5mV, and the temperature measurement range is 0-100℃, the temperature measurement accuracy is ±1 degree Celsius, so the output voltage should change by 38.5/100=0.385mV for every 1℃ increase in ambient temperature. In order to ensure that the current fluctuation does not affect the accuracy, consider the most extreme case, at 100 degrees Celsius, the resistance value of PT100 should be 138.5R. Then the current ripple should be less than 0.385/138.5=0.000278mA, that is, the change in current during the load change should be less than 0.000278mA. In the actual simulation, the current source remains basically unchanged.
3. AD623 acquisition circuit solution
The principle can refer to the above bridge measurement principle.
Low temperature acquisition:
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High temperature acquisition
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4. AD620 acquisition circuit solution
AD620 PT100 acquisition solution for high temperature (150°):
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AD620 PT100 acquisition solution for low temperature (-40°):
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AD620 PT100 acquisition solution for room temperature (20°):
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5. Anti-interference filtering analysis of PT100 and PT1000 sensors
Temperature acquisition in some complex, harsh or special environments will be subject to great interference, mainly including EMI and REI. For example, in the application of motor temperature acquisition, high-frequency disturbances caused by motor control and high-speed rotation of the motor.
There are also a large number of temperature control scenarios inside aviation and aerospace vehicles, which measure and control the power system and environmental control system. The core of temperature control is temperature measurement. Since the resistance of thermistor can change linearly with temperature, using platinum resistance to measure temperature is an effective high-precision temperature measurement method. The main problems are as follows:
1. The resistance on the lead wire is easily introduced, thus affecting the measurement accuracy of the sensor;
2. In certain strong electromagnetic interference environments, the interference may be converted into DC output offset error after being rectified by the instrument amplifier, affecting the measurement accuracy.
5.1 Aerospace airborne PT1000 acquisition circuit
Refer to the design of an airborne PT1000 acquisition circuit for anti-electromagnetic interference in a certain aviation.
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A filter is set at the outermost end of the acquisition circuit. The PT1000 acquisition preprocessing circuit is suitable for anti-electromagnetic interference preprocessing of airborne electronic equipment interfaces; the specific circuit is:
The +15V input voltage is converted into a +5V high-precision voltage source through a voltage regulator. The +5V high-precision voltage source is directly connected to the resistor R1, and the other end of the resistor R1 is divided into two paths. One is connected to the in-phase input end of the op amp, and the other is connected to the PT1000 resistor A end through the T-type filter S1. The output of the op amp is connected to the inverting input to form a voltage follower, and the inverting input is connected to the ground port of the voltage regulator to ensure that the voltage at the in-phase input is always zero. After passing through the S2 filter, one end A of the PT1000 resistor is divided into two paths, one through resistor R4 as the differential voltage input D, and one through resistor R2 to AGND. After passing through the S3 filter, the other end B of the PT1000 resistor is divided into two paths, one through resistor R5 as the differential voltage input E, and one through resistor R3 to AGND. D and E are connected through capacitor C3, D is connected to AGND through capacitor C1, and E is connected to AGND through capacitor C2. The precise resistance value of PT1000 can be calculated by measuring the differential voltage across D and E.
The +15V input voltage is converted into a +5V high-precision voltage source through a voltage regulator. The +5V is directly connected to R1. The other end of R1 is divided into two paths, one connected to the in-phase input of the op amp, and the other connected to the A end of the PT1000 resistor through the T-type filter S1. The output of the op amp is connected to the inverting input to form a voltage follower, and the inverting input is connected to the ground port of the voltage regulator to ensure that the voltage at the inverting input is always zero. At this time, the current flowing through R1 is a constant 0.5mA. The voltage regulator uses AD586TQ/883B, and the op amp uses OP467A.
After passing through the S2 filter, one end A of the PT1000 resistor is divided into two paths, one through resistor R4 as the differential voltage input end D, and one through resistor R2 to AGND. After passing through the S3 filter, the other end B of the PT1000 resistor is divided into two paths, one through resistor R5 as the differential voltage input end E, and one through resistor R3 to AGND. D and E are connected through capacitor C3, D is connected to AGND through capacitor C1, and E is connected to AGND through capacitor C2.
The resistance of R4 and R5 is 4.02k ohms, the resistance of R1 and R2 is 1M ohms, the capacitance of C1 and C2 is 1000pF, and the capacitance of C3 is 0.047uF. R4, R5, C1, C2, and C3 together form an RFI filter network. The RFI filter completes the low-pass filtering of the input signal, and the objects filtered out include the differential mode interference and common mode interference carried in the input differential signal. The calculation of the ‑3dB cutoff frequency of the common mode interference and differential mode interference carried in the input signal is shown in the formula:
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Substituting the resistance value into the calculation, the common mode cutoff frequency is 40kHZ, and the differential mode cutoff frequency is 2.6KHZ.
End point B is connected to AGND through the S4 filter. Among them, the filter ground terminals from S1 to S4 are all connected to the aircraft shielding ground. Since the current flowing through PT1000 is a known 0.05mA, the precise resistance value of PT1000 can be calculated by measuring the differential voltage at both ends of D and E.
S1 to S4 use T-type filters, model GTL2012X‑103T801, with a cutoff frequency of M±20%. This circuit introduces low-pass filters to the external interface lines and performs RFI filtering on the differential voltage. As a preprocessing circuit for PT1000, it effectively eliminates electromagnetic and RFI radiation interference, which greatly improves the reliability of the collected values. In addition, the voltage is directly measured from both ends of the PT1000 resistor, eliminating the error caused by the lead resistance and improving the accuracy of the resistance value.
5.2 T-type filter
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The T-type filter consists of two inductors and capacitors. Both ends of it have high impedance, and its insertion loss performance is similar to that of the π-type filter, but it is not prone to “ringing” and can be used in switching circuits.
