3-Huzalmérési megoldás PT100-hoz (RTD) Érzékelő

LTSpice simulation of 3-wire measurement scheme for PT100 (RTD) sensor: Pt100 is a thermal resistor temperature sensor, the full name is platinum resistor 100 ohms. It is made of pure platinum, and its resistance value increases linearly in a certain proportion when the temperature changes.

PT100, the full name of platinum thermal resistor, is a resistive temperature sensor made of platinum (Pt), and its resistance value changes with temperature. A 100 after PT means that its resistance value is 100 ohms at 0℃, and its resistance value is about 138.5 ohms at 100℃. It has the characteristics of high precision, good stability, erős anti-interferencia képesség, and the relationship between its resistance and temperature change is: R=R0(1+αT), where α =0.00392, Ro is 100Ω (resistance value at 0℃), and T is Celsius temperature.

PT100 temperature resistance corresponding change table

2. Import pt100 resistor
Since there is no pt100 in the LTspice component library, we need to import pt100 manually. Since the spice file of pt100 is not found, we import the sliding resistor here as a substitute. To import the sliding resistor, you need to add the following three files in the LTspice installation directory. Copy the three files (asc, asy and lib) separately, create files for each, and finally put them in the corresponding location of the LTSpice installation. Put asc with other schematics, put asy in sym under lib, and put lib in sub under lib. After adding, you can see potentiometer in the component in LTSpice. This potentiometer is the required sliding resistor.

potentiometer_test.asc

Version 4
SHEET 1 880 680
WIRE 272 48 0 48
WIRE 528 48 272 48
WIRE 272 80 272 48
WIRE 528 80 528 48
WIRE 0 96 0 48
WIRE 0 192 0 176
WIRE 272 208 272 176
WIRE 528 208 528 176
FLAG 272 208 0
FLAG 0 192 0
FLAG 320 128 out1
FLAG 528 208 0
FLAG 576 128 out2
SYMBOL voltage 0 80 R0
SYMATTR InstName V1
SYMATTR Value 10
SYMBOL potentiometer 272 176 M0
SYMATTR InstName U1
SYMATTR SpiceLine2 wiper=0.2
SYMBOL potentiometer 528 176 M0
SYMATTR InstName U2
SYMATTR SpiceLine R=1
SYMATTR SpiceLine2 wiper=0.8
TEXT 140 228 Left 2 !.op

potentiometer.asy

Version 4
SymbolType BLOCK
LINE Normal 16 -31 -15 -16
LINE Normal -16 -48 16 -31
LINE Normal 16 -64 -16 -48
LINE Normal 1 -9 -15 -16
LINE Normal 1 0 1 -9
LINE Normal 1 -94 1 -87
LINE Normal -24 -56 -16 -48
LINE Normal -24 -40 -15 -48
LINE Normal -47 -48 -15 -48
LINE Normal -16 -80 16 -64
LINE Normal 1 -87 -16 -80
WINDOW 0 30 -90 Left 2
WINDOW 39 30 -50 Left 2
WINDOW 40 31 -23 Left 2
SYMATTR Prefix X
SYMATTR ModelFile potentiometer.lib
SYMATTR SpiceLine R=1k
SYMATTR SpiceLine2 wiper=0.5
SYMATTR Value2 potentiometer
PIN 0 -96 NONE 8
PINATTR PinName 1
PINATTR SpiceOrder 1
PIN 0 0 NONE 8
PINATTR PinName 2
PINATTR SpiceOrder 2
PIN -48 -48 NONE 8
PINATTR PinName 3
PINATTR SpiceOrder 3

potentiometer.lib

* This is the potentiometer
* _____
* 1–|_____|–2
* |
* 3
*
.SUBCKT potentiometer 1 2 3
.param w=limit(wiper,1m,.999)
R0 1 3 {R*(1-w)}
R1 3 2 {R*(w)}
.ENDS

3. Wheatstone bridge to measure PT100 resistance

Wheatstone híd kapcsolat és LTspice szimulációs modell

Single-arm bridge or Wheatstone circuit

Wheatstone híd kapcsolat és LTspice szimulációs modell:
When the bridge is balanced, the voltage meter measurement value eq?%5CbigtriangleupU=0

I1*Rt=I2*R2

I1*R3=I2*R4

From this, it can be deduced that: Rt/R3=R2/R4

Azaz: Rt*R4=R2*R3

The resistance measurement result in this way has nothing to do with the accuracy of the voltage meter, the accuracy of the resistance, and the electromotive force. It avoids the error caused by the change of the power supply over time, and avoids the problem of ammeter voltage division, voltage meter shunt, and too many wire voltage division.

Different measurement methods of PT100:

Several Leading Methods of P Thermal Resistor

When the temperature point to be measured on site is far away from the instrument, it is necessary to connect the thermal resistor with a lead wire. The lead resistance is r. The two-wire system cannot avoid the error caused by the wire resistance during calculation, and the actual resistance value measured will be smaller.

The resistance of the thermal resistor plus the lead wire is r

In order to offset the error, a four-wire connection is introduced. When Rt increases by 2r, R2 also increases by 2r. No matter how long the wire is, the bridge can be balanced. Four wires need to be drawn. Since the voltages at points p and q are equal, they can be equivalent to one point, which is the three-wire connection method, vagyis, the three-wire connection method simulated in this experiment. In practice, three-wire is also mostly used, taking into account both economy and accuracy.

4. Three-wire measurement LTSpice simulation

3-wire measurement, and connect the op amp circuit at the output

This experiment uses three-wire measurement, and connects the op amp circuit to the output part to amplify the output signal for easy measurement.
Uo= (V1-V2)*(R17/R15)=20*(V1-V2)

Azaz, V1=(Uo+20*V2)/20

According to the resistor voltage division:

V1 = Vs*(Rt/(R2+Rt))

V2 = Vs*(R10/(R9+R10))

The input voltage of this simulation is 3V. After calculation, V2≈108.434mV
V1=(Uo+2168.68)/20
V1=Rt/(R7+Rpt) *3000
Így: Rt=2000V1/(3000-V1)
Rt is the corresponding resistance value of PT100. The corresponding temperature value can be obtained by looking up the table.
Set the resistance of the sliding rheostat (Rt) hogy 130.6 ohms for the temperature of 78 degrees Celsius, read V1, V2, and Uo to calculate Rt.

Rt is the corresponding resistance value of PT100, corresponding temperature value

V1 is about 182.82mV, V2 is about 118.46mV, and U0 is about 1.39V. The calculated Rpt is about 129.78V. The table shows that the temperature read is 76 degrees Celsius, which is close.

Set the resistance of the sliding rheostat (Rt) hogy 200.05 ohms for the temperature of 266.5 degrees Celsius, read V1, V2, and Uo to calculate Rt.

V1 is about 270.45mV, V2 is about 118.46mV, and U0 is about 3.0257V. The calculated Rpt is about 198.16V, and the error value is about 1%. The table shows that the temperature read is 261.3 degrees Celsius, with an error of about 1%.

The temperature measurement principle of the three-wire PT100 is mainly based on the bridge method. The measurement circuit is usually an unbalanced bridge, and the PT100 is used as a bridge arm resistor of the bridge. When current passes through the PT100, the change in its resistance value will cause the change in the output voltage of the bridge. By measuring this output voltage, the resistance value of the PT100 can be calculated, and then the measured temperature can be obtained.
In order to eliminate the influence of lead resistance, the three-wire PT100 adopts a special design, connecting one wire to the power supply end of the bridge, and the other two wires are connected to the bridge arm where the PT100 is located and the bridge arm adjacent to it. In this way, both bridge arms introduce lead resistances of the same resistance value, so that the bridge is in a balanced state. Therefore, the change in lead resistance has no effect on the measurement result. Viszont, there will still be influences such as devices in actual measurement. The measured resistance value is not accurate. In order to eliminate this error, some compensation can be added when reading.