The two-wire temperature transmitter is matched with a thermocouple and a thermal resistor, respectively, and can linearly convert the temperature signal into a 4 ~ 20mA DC standard output signal. The two-wire temperature transmitter should have the following main features:
The two wires complete the input of the power supply and the 4-20 mA DC current output, that is, the two wires are both the power line and the 4-20 mA standard signal output line.
Since the two-wire integrated transmitter is installed in the sensor junction box, it must have good reliability, stability, a wide temperature operating range (0 ~ 85 ° C), and a small temperature drift, while requiring the volume to be as small as possible .
A linearization circuit is used in the thermocouple and thermal resistance temperature transmitter, so that the 4-20mA output signal of the transmitter has a linear relationship with the measured temperature.
In the thermocouple temperature transmitter, cold junction compensation is required, and the cold compensation range is 0 ~ 100 ℃.
The transmitter is divided into a range unit and an amplification unit on the line structure. The amplification unit is universal, and the range unit varies with the variety and measurement range. Design the circuit structure shown in Figure 1.
The thick line in the picture is the power line, and the thin line is the signal flow. The two external wires are both the power line and the signal line. The 4 ~ 20mA signal system provides the possibility for a two-wire design. When the measured signal changes from the lower range to the upper range (0% ~ 100%), the current on the two transmission lines corresponds to the 4 ~ 20mA change; 4mA is used as a transmitter The circuit working consumes current, and it is easy to identify the disconnection and power failure. RL is the signal sampling load resistance (RL≤250W). V (AB) must be greater than 12V to ensure the normal operation of the system. Under the premise that the power supply is normal (17 ~ 30V), the loop 4 ~ 20mA current I is determined by the input thermal resistance R or the thermocouple mV signal.
From the block diagram, we can see that, first, the signal generated by the signal source needs to be collected, and then the collected signal is amplified, linearized, and adjusted to zero and full. Finally, the linearity of the The voltage signal is converted into a current signal I1 (0 ~ 16mA), and the 4mA static working current I2 of the circuit is added to form a 4 ~ 20mA current signal through a two-wire power supply line. For thermocouple transmitters, a small CU50 thermal resistor is used to measure the cold junction temperature and perform cold junction compensation. Both transmitters use LM124 integrated op amps, which are four independent sets of high-gain internal frequency-compensated operational amplifiers. It can meet the requirements of single circuit operation of this circuit. The power supply has a wide range of voltage, good temperature characteristics, and high cost performance. The op amps used in the subsequent circuits are all LM124.
Second, the design of the two-wire thermal resistance transmitter
The detailed circuit diagram of the thermal resistance two-wire transmitter is shown in Figure 2 (pt100 as an example). The following describes the working principles of each part.
1.Signal acquisition circuit
Thermal resistance measures the temperature by using the characteristics of the resistance of the conductor as the temperature changes. Commonly used are platinum resistance Pt100, Pt10 copper resistance Cu50, Cu100 and so on. The relationship between its resistance and temperature can be queried through the index number table.
In the figure, Pt100 thermal resistance is taken as an example (here, other thermal resistances, such as Cu50, Cu100, etc.) can be used. TL431 is a 2.5V Zener diode, and D2 is a protection diode to prevent reverse input voltage. Impact or damage to the circuit. R1 is a current-limiting resistor. R2, R3, R4 and R5 (Pt100) are used together to form a resistance measurement bridge. Because the integrated two-wire heating resistance transmitter is installed in the junction box, the lead resistance is ignored. R1, R2, R3, and R4 can be determined (the values are shown in Figure 2), where the thermal resistance R5 changes with temperature. R4 takes different values according to the thermal resistance graduation number used. For example, when measuring Pt100, take R4 as 100W, and for Cu50, take R4 as 50W. The voltage at the two points in the middle of the bridge is used as the input signal of the subsequent differential amplifier. They are:
Because R2 = R3 >> R4 and R5, therefore:
2.Amplification circuit and linearization adjustment circuit
One of the functions of this circuit is to amplify the weak signals collected. Differential amplification is adopted in this level of circuit. At the same time, a positive feedback non-linear adjustment circuit is connected to the amplifier circuit. Its main function is to correct the non-linearity between the thermal resistance and the temperature resistance to ensure that the measured temperature of the output voltage of the amplifier is linear. R7, R8, R9, and LM124 form the amplifier circuit. For this local circuit, the input signals come from the collected signals V and V ', and the input signals respectively enter the first set of operational amplifiers of LM124 through R7 and R8, and the output voltage V1 is obtained (the non-linear adjustment circuit is not considered here, that is, the feedback loop Effect of R6 on the circuit input).
V1 = V '+ R9 (V-V') / R8
In addition, there is a very important part in this circuit, that is the linearization adjustment circuit, which is R6 in this circuit. For the process and principle of linearization adjustment, we can use Figure 3 to explain.
The dotted line in the figure shows the curve when the output voltage changes with the source temperature when no linearization adjustment is performed. The solid curve in the figure shows the specific process of R6 non-linear adjustment. As the temperature increases, the output voltage increases and positive feedback The effect is enhanced. As long as the resistance of R6 is appropriate, it can just offset the non-linear effect of the thermal resistance itself, so that the output voltage and temperature are linear, as shown in the straight line in Figure 3. According to the linearization adjustment principle, the feedback voltage V of the linear adjustment resistor R6 is inversely:
Then the actual output:
Due to the good linearity of the thermal resistance, the R6 = 8.2k in this circuit is adjusted by calculation, and the nonlinear correction of the thermal resistance can reach an accuracy of two thousandths.
3.Zero adjustment, power balance and secondary amplifier circuit
The circuit that adjusts the zero point is essentially adjusting the magnitude of the amplified voltage output of this stage to ensure that the entire loop current I1 = 4mA when the signal source is zero degrees (R5 = 100W, the output of the first-stage amplifier is zero). It consists of R10, R16, R13, and W1. In essence, a zero-adjusting voltage is superimposed on the positive end of the voltage input of this stage, so that the static working current of less than 4mA reaches 4mA. In addition, there is another part in this circuit, which is to reduce the impact of power supply fluctuations on the circuit output, that is, R15 in the circuit, which can suppress the impact of power supply fluctuations. When the external voltage source fluctuates greatly (or the load resistance RL changes), the static working current of the circuit will change slightly. We can use R15 to stabilize the output current. Its working principle is that on the one hand, the increase of the power supply brings an increase in quiescent current. , It can ensure the stability of the output current when the power supply fluctuates within the allowable range. R17 determines the secondary magnification.
4, full circuit and V / I conversion circuit
The full-regulation circuit is composed of R18, R20, and W2 and divides the upper-level voltage output v2. By adjusting W2, the final output (the output of the entire circuit when the signal source is at its highest input) reaches the required output result V (W2 middle tap voltage). R21, R22, R23, R24, R25 and op amp form a V / I conversion circuit. Since R22, R23, R24 are all 200k large resistors, R25 is a small 100W resistor, and the entire circuit current output I2 ≈ V / R25. R26 is a load resistor.
Third, the thermocouple two-wire transmitter circuit design
The main difference between the thermocouple two-wire transmitter circuit and the thermal resistance two-wire transmitter is the signal acquisition and non-linear correction part. We will introduce the two parts separately below.
1.Signal acquisition and primary amplification circuit
The output of the thermocouple is a mV signal that varies with the measured temperature. The local circuit design is shown in Figure 4. In the circuit, the role of TL431 is to output a stable 2.5V. D0 is a protection diode, which can protect the hazard of the positive and negative phases of the power input. By dividing the voltage between R3 and TL431, the working voltage across TL431 is maintained at 2.5V, and the cold junction compensation is provided later, which provides DC power for the correction circuit and the zeroing circuit. In this circuit, a copper-wired thermal resistor Cu50 acts as a cold junction compensation. When the thermoelectric potential E12 of the thermocouple changes with the change of the cold junction temperature, the voltage across the copper resistor Cu50 also changes in the opposite direction. If the resistance of the voltage dividing resistor R2 is selected properly, the voltage change across Cu50 can be automatically changed. The effect of the cold junction temperature change on the thermocouple thermoelectric potential. According to the definition of cold junction compensation, the voltage difference between Cu50 at 50 ° C and 0 ° C should be equal to the thermoelectromotive force of the thermocouple at 50 ° C. When the cold junction temperature is zero, the voltage is 50 / (R2 + 50) × 2500mV Solved by the following zero-adjusting circuit. Taking a nickel-chromium-nickel-silicon (nickel-aluminum) thermocouple (index K) measurement transmission range 0 ~ 1300 ℃ as an example,
The output thermoelectric potential is equal to 2.022mV at 50 degrees K.
From this we can find: R2 = 13k.
In the circuit, the mV signal of the thermocouple and the voltage across the cold-filled copper resistor are added. After R4 is input to the first-stage amplifier of the LM124, according to the working principle of the amplifier, we can obtain the output voltage (set to include the sum of the thermocouple and the cold-fill). The input signal is V) V1 = V (1 + R6 / R5). Design considerations enable the output voltage of the amplifier to be 2.5V when the temperature of the thermocouple reaches a maximum value (the corresponding thermoelectric potential of 1300 ℃ is 52.398mV). In other words, the voltage at the cold junction temperature of the thermocouple is 0 ° C plus the maximum thermoelectric potential of the thermocouple, and then multiplied by the magnification should be equal to 2.5V, that is:
Among them, K is the magnification of LM324, from which K = 40 can be calculated. If R4 = R5 = 5.1k, R6 should be 180k.
2.Linearization adjustment circuit and secondary amplifier circuit
The local circuit (the output of this stage V2) is a very important link in this circuit, and it is also a more difficult link. Because it involves linear regulation of the entire circuit. The enlarged part has been described previously, and now the problem of linear adjustment will be explained. The specific circuit is shown in Figure 5 (the circuit connected to several diodes in the figure is a linear correction circuit). R9, 10, in the circuit
R11, R13, R14, R15, R16 are all disconnected, and we only add this resistor when needed.
This circuit uses a non-linear amplifier circuit to correct the non-linear characteristics of the measured parameter. The principle is that the broken-line parallel branches composed of diode compensation resistors successively function at different positions of the input signal, so that the amplifier is in different positions of the signal size. Different amplification factors, its non-linear characteristics are just opposite to the non-linear characteristics of the measured thermocouple. In this circuit, six vertices are used (three are positive and three are negative). The position of the vertices can change the on-state voltage adjustment of the branch diode. Adjusting the resistance of the branch line can change the slope of the line compensation. In the actual design process, several points can be taken for correction. For K division (detection range 0 ~ 1300 ℃), it can be assumed that the range of 0 ~ 100 ℃ is approximately linear, and the non-linear error is ignored. In addition, 500 ℃, 900 ° C and 1300 ° C are the correction detection points. When the detection point value is above the required linear value, it means that the output value is too large. This requires lowering the output. The specific measure is to connect a level adjustment circuit in D7 ~ D12; otherwise, connect A certain level adjustment circuit in D1 ~ D6. The inflection point selection diode in the circuit can select silicon tube or germanium tube according to the needs of the correction. The adjustment method is as follows: first zero adjustment at 0 ℃ and full adjustment at 1000 ℃, and then repeatedly adjust in the following order:
A. For the non-linear adjustment in the range of 100 ℃ ~ 500 ℃, we can connect D1 or D12, and then adjust the resistance of R9 or R16 to change the amplification of the amplifier to achieve the specified output value. If it is detected that the output value is too small, select R9 D1, calculate and adjust the resistance value of R9, and make the amplifier of this section increase in amplification factor until the output voltage increases to the required linear value. If we detect that the output value is too large, we need to choose R16 and D12. And adjust the resistance value of R16, make the amplifier of this section reduce the magnification and reduce the output voltage to the required linear value.
B. When adjusting the non-linear adjustment between 500 ℃ and 900 ℃, we can connect D2, D3 or D10, D11, and then adjust the size of R10 or R15.
C. For the non-linear adjustment between 900 ℃ and 1300 ℃, the output value of 1300 ℃ at the detection point is too large or too small to decide which one of the remaining two polyline compensation branches (three diodes) is connected. The method is the same as above. .
Same as the thermal resistance transmitter, the function of R12 in this circuit is to correct the impact on the entire circuit when the power supply fluctuates. Prevent 4 ~ 20mA fluctuation caused by unstable voltage source. The zero-adjust and full-adjustment and V / I conversion circuit are also the same as the thermal resistance, so I will not repeat them here.