PT100/PT1000溫度採集電路解決方案

1. PT100 and PT1000 temperature resistance change table
鎳等金屬熱電阻器, copper and platinum resistors have a positive correlation with the change in resistance with temperature. 鉑具有最穩定的物理化學性質，應用最廣泛. The temperature measurement range of the commonly used platinum resistor Pt100 is -200～850 ℃. 另外, the temperature measurement ranges of Pt500, 鉑1000, 等. 依次減少. 鉑1000, temperature measurement range -200～420 ℃. 根據IEC751國際標準, 鉑電阻Pt1000嘅溫度特性滿足以下要求:

Pt1000溫度特性曲線

根據Pt1000溫度特性曲線, the slope of the resistance characteristic curve changes little within the normal operating temperature range (如圖 1). Through linear fitting, the approximate relationship between resistance and temperature is:

1.1 PT100耐溫變化表

PT100耐溫變化表

1.2 PT1000 temperature resistance change table

PT1000 Temperature Resistance Change Table

2. 常用嘅採集電路解決方案

2.1 Resistor voltage division output 0~3.3V/3V analog voltage

Single-chip AD port direct acquisition
測溫電路電壓輸出範圍0~3.3V, PT1000系列 (PT1000電阻值變化好大, temperature measurement sensitivity is higher than PT100; PT100更適合大規模溫度測量).

Resistor voltage divider outputs 0~3.3V 3V analog voltage

最簡單嘅方法係使用分壓法. The voltage is the voltage reference source 4V generated by the TL431 voltage reference source chip, or REF3140 can be used to generate 4.096V as the reference source. The reference source chips also include REF3120, 3125, 3130, 3133, 和 3140. The chip uses SOT-32 package and 5V input voltage. 輸出電壓可根據所需的參考電壓進行選擇. 答案係肯定嘅, according to the normal voltage input range of the MCU AD port, 它不能超過3V/3.3V.

2.2 Resistor voltage division output 0~5V analog voltage MCU AD port direct acquisition.
答案係肯定嘅, some circuits use 5V MCU power supply, and the maximum operating current of PT1000 is 0.5mA, so appropriate resistance value should be used to ensure the normal operation of the components.
例如, the 3.3V in the voltage division schematic diagram above is replaced with 5V. The advantage of this is that the 5V voltage division is more sensitive than 3.3V, and the acquisition is more accurate. 記得, 理論計算輸出電壓不能超過+5V. 否則, it will cause damage to the MCU.

2.3 最常用嘅電橋測量
R11, R12系列, R13 and Pt1000 are used to form a measuring bridge, 其中R11=R13=10k, R12=1000R precision resistors. 当Pt1000嘅電阻值不等於R12嘅電阻值時, the bridge will output a mV-level voltage difference signal. 該電壓差信號由儀表放大器電路放大並輸出所需嘅電壓信號. This signal can be directly connected to the AD conversion chip or the AD port of the microcontroller.

R11, R12系列, R13 and Pt1000 are used to form a measurement bridge

該電路嘅電阻測量原理:
1) PT1000係熱敏電阻. 隨著溫度嘅變化, the resistance changes basically linearly.
2) 係 0 度, PT1000嘅電阻為1kó, 則Ub同Ua相等, 噉係, Uba = Ub – Ua = 0.
3) 假設在一定溫度下, PT1000嘅電阻為1.5kó, 則Ub同Ua唔相等. According to the voltage division principle, we can find out that Uba = Ub – UA > 0.
4) OP07係一款運算放大器, and its voltage gain A depends on the external circuit, 其中A = R2/R1 = 17.5.
5) OP07嘅輸出電壓Uo = Uba * 一個. 所以如果我哋用電壓表嚟測量OP07嘅輸出電壓, 我哋可以推斷出Uab嘅值. 由於Ua係已知值, 我哋可以進一步計算Ub嘅值. 然之後, using the voltage division principle, 我哋可以計算出PT1000嘅比電阻值. 呢個過程可以透過軟件計算嚟實現.
6) 如果我哋知PT1000喺任何溫度下嘅電阻值, we only need to look up the table based on the resistance value to know the current temperature.

2.4 恆流源
由於熱敏電阻嘅自熱效應, the current flowing through the resistor should be as small as possible. 麻麻, the current is expected to be less than 10mA. 經驗證，鉑電阻PT100嘅自發熱 1 mW will cause a temperature change of 0.02-0.75℃. 因此, reducing the current of the platinum resistor PT100 can also reduce its temperature change. 然而, 如果電流太細, 易受噪聲干擾, so the value is generally 0.5-2 馬, 因此，恒流源電流被選為1mA恒流源.

The chip is selected as the constant voltage source chip TL431, and then converted into a constant current source using current negative feedback. 電路如圖所示

其中, the operational amplifier CA3140 is used to improve the load capacity of the current source, 輸出電流的計算公式為:

The resistor should be a 0.1% 精密電阻器. 最終輸出電流為0.996mA, 噉係, 精度為 0.4%.

恆流源電路應具有以下特點

Select the constant voltage source chip TL431

溫度穩定性: 由於我哋嘅溫度測量環境為0-100°C, 電流源嘅輸出不應對溫度敏感. The TL431 has an extremely low temperature coefficient and low temperature drift.

良好的負載調節: 如果電流紋波太大, 會導致讀取錯誤. 根據理論分析, since the input voltage varies between 100-138.5mV, 溫度測量範圍為0-100°C, 測溫精度為±1攝氏度, 因此，環境溫度每升高1°C、輸出電壓應變化38.5/100=0.385mV. 以保證電流波動不影響精度, 考慮最極端嘅情況, 係 100 攝氏度, PT100嘅電阻值應為138.5R. 咁電流紋波應小於0.385/138.5=0.000278mA, 噉係, the current change during the load change should be less than 0.000278mA. 在實際模擬中, 當前源基本保持不變.
3. AD623採集電路解決方案

AD623 acquisition PT1000 circuit solution

原理可參考上述電橋測量原理.
低溫採集:

高溫採集

4. AD620採集電路解決方案

AD620 PT100 acquisition solution

AD620 PT100 acquisition solution high temperature (150°):

AD620 PT100 acquisition solution low temperature (-40°):

AD620 PT100 acquisition solution room temperature (20°):

5. PT100 and PT1000 anti-interference filtering analysis

在一些複雜嘅環境中進行溫度採集, 惡劣或特殊嘅環境會受到好大嘅干擾, 主要包括EMI同REI.

例如, 在電機溫度採集的應用中, motor control and high-speed rotation of the motor cause high-frequency disturbances.

航空航天飛行器內部都有大量嘅溫度控制場景, 測量和控制電力系統和環境控制系統. 溫度控制嘅核心係溫度測量. 由於熱敏電阻嘅電阻可以隨溫度線性變化, 使用鉑電阻測量溫度是一種有效的高精度溫度測量方法. 主要問題如下:
1. 引線上嘅電阻好易引入, 從而影響傳感器嘅測量精度;
2. In some strong electromagnetic interference environments, the interference may be converted into DC output after rectification by the instrument amplifier
Offset error, 影響測量精度.
5.1 航空航天機載PT1000採集電路

航空航天機載PT1000採集電路

參考某航空中用于抗電磁干擾嘅機載PT1000採集電路嘅設計.

濾波器設置喺採集電路嘅最外端. The PT1000 acquisition preprocessing circuit is suitable for anti-electromagnetic interference preprocessing of airborne electronic equipment interface;
The specific circuit is:
+15V輸入電壓通過穩壓器轉換為+5V高精度電壓源, and the +5V high-precision voltage source is directly connected to the resistor R1.
The other end of the resistor R1 is divided into two paths, 一個連接到運算放大器嘅同相輸入, and the other connected to the PT1000 resistor A end through the T-type filter S1. 運算放大器的輸出連接到反相輸入以形成電壓跟隨器, 並且反相輸入連接到穩壓器的接地端口，以確保同相輸入端的電壓始終為零. 透過S2過濾器之後, PT1000電阻器嘅一耑A分為兩條路徑, one path is used as the differential voltage input terminal D through resistor R4, and the other path is connected to AGND through resistor R2. 透過S3篩選條件之後, PT1000電阻嘅另一耑B分為兩條路徑, one path is used as the differential voltage input terminal E through resistor R5, and the other path is connected to AGND through resistor R3. D同E透過電容器C3連接, D透過電容器C1連接到AGND, E透過電容器C2連接到AGND; the precise resistance value of PT1000 can be calculated by measuring the differential voltage between D and E.

+15V輸入電壓通過穩壓器轉換為+5V高精度電壓源. +5V直接連接到R1. R1嘅另一耑分為兩條路徑, one is connected to the in-phase input terminal of the op amp, and the other is connected to the PT1000 resistor A through the T-type filter S1. 運算放大器的輸出連接到反相輸入以形成電壓跟隨器, 並且反相輸入連接到穩壓器的接地端口，以確保反相輸入處的電壓始終為零. 此時, 流經R1嘅電流為常數0.5mA. 穩壓器採用AD586TQ/883B, 運算放大器使用OP467A.

透過S2過濾器之後, PT1000電阻器嘅一耑A分為兩條路徑, 一個透過電阻R4作為差分電壓輸入端的, 一個透過電阻器R2到AGND; after passing through the S3 filter, PT1000電阻嘅另一耑B分為兩條路徑, 一個透過電阻R5作為差分電壓輸入端E, 一個透過電阻器R3到AGND. D同E透過電容器C3連接, D透過電容器C1連接到AGND, E透過電容器C2連接到AGND.
R4同R5嘅電阻為4.02k歐姆, R1同R2嘅電阻為1M歐姆, C1同C2嘅電容為1000pF, C3嘅電容為0.047uF. R4系列, R5系列, C1, C2型, 同C3一起構成 RFI過濾器網絡, which completes the low-pass filtering of the input signal, and the objects to be filtered out include the differential mode interference and common mode interference carried in the input differential signal. 輸入信號中攜帶的共模干擾和差模干擾的-3dB截止頻率的計算公式如下:

將電阻值代入計算, 共模截止頻率為40kHZ, 差模截止頻率為2.6KHZ.
端點B透過S4濾波器連接到AGND. 其中, 由S1到S4嘅濾波器接地端子都連接到飛機屏蔽接地. 由於流經PT1000嘅電流為已知嘅0.05mA, PT1000嘅精確電阻值可以透過測量的同E兩端嘅差分電壓嚟計算.
S1到S4使用T型濾波器, 型號GTL2012X-103T801, with a cutoff frequency of 1M±20%. 該電路喺外部接口線路中引入低通濾波器，並對差分電壓執行 RFI濾波. 作為PT1000嘅預處理電路, 它有效地消除了電磁和 RFI輻射干擾, 大大提高咗收集值嘅可靠性. 另外, 電壓直接由PT1000電阻嘅兩端測量, 消除咗引綫電阻引起嘅誤差，提高咗電阻值嘅精度.

5.2 T型過濾器
T型濾波器由兩個電感器和電容器組成. 它的兩端都有高阻抗, 其插入損耗性能與π型濾波器相似, 但它唔易 “響” 並可用于開關電路.