Linearization Processing of WZP Platinum Resistance Temperature Sensor Pt1000 Signal

The WZP platinum resistance temperature sensor Pt1000 is a device that uses the characteristics of the electromagnetic parameters of the component to change with temperature to detect temperature and temperature-related parameters. Because of its relatively good linearity, strong oxidation resistance, and wide temperature range, its temperature measurement range is from -200℃ to +650℃, and it is currently widely used in industrial production and scientific research. The signal processing circuit of the sensor needs to complete the conversion of the temperature-related resistance change signal into a unified voltage signal.

1 Output characteristic curve of Pt1000

Put the Pt1000 into a high and low temperature test box, set the temperature to -30℃～+70℃, and measure the resistance value of the temperature sensor at different temperatures. The measured data is shown in Figure 1. Within the measurement temperature range, the output resistance value of Pt1000 is proportional to the temperature. However, under high and low temperature conditions, there is a certain deviation, and temperature compensation needs to be performed in the conditioning circuit.

2 Basic constant current source circuit

The signal processing circuit of the platinum resistance temperature sensor can use a constant voltage source or a constant current source. Through the study of the constant voltage source, it is found that there are problems such as instability and low precision in practical applications. The reason is that when the constant voltage works, in addition to the nonlinear error of the platinum resistance itself, the inherent error of the constant voltage working circuit will also be generated, which will increase the system error of the entire circuit. Therefore, this paper designs the method of constant current source.

The basic constant current source circuit is shown in Figure 2. The platinum resistor RT is used to replace the Rf of the inverting amplifier. According to the formula of the inverting amplifier, we can get:

After Vi and R1 are fixed, the current flowing through RT is constant, Vo is proportional to RT, and the corresponding voltage change can be obtained from the change of RT, thereby realizing voltage output and the linearity remains unchanged.

3 Temperature processing circuit with common-phase input

The ideal temperature sensing circuit has an output voltage of 0 V at 0°C, but in Figure 1, RT = 1 000.8 Ω. Substituting Vo into equation (1) is not 0, so the voltage needs to be adjusted to zero. The implementation method is to add an input voltage to the non-inverting terminal of the operational amplifier in Figure 2 for adjustment, as shown in Figure 3.

Therefore, as long as k can be adjusted to a suitable value, the signal can be zeroed. However, this is only a theoretical calculation, the actual op amp is not ideal, and the resistance of each resistor will not completely meet the nominal resistance due to the influence of temperature, etc. Therefore, the size of R2 and R3 is not fixed, and variable resistors are used for fine-tuning in practice.

The addition of the adjustment voltage at the in-phase input terminal makes the output voltage decrease even if the temperature rises. Therefore, in order to ensure that the signal outputs linearly between -300 and 700 mV at -30℃ and +70℃, an amplifier circuit is used to adjust the amplification factor after zero adjustment, as shown in Figure 4.

Figure 4 shows an inverting amplifier circuit, which can not only achieve an amplification effect of an amplification signal of A=R5/R4, but also invert the voltage Vo1 inverted by the previous operational amplifier, that is, it becomes a positive voltage that meets the requirements.

4 Linearization Supplementary Processing

The relationship after amplifier 1 is shown in equation (2). After linear amplification by amplifier 2, if RT is constant, Vo is proportional to Vi. Therefore, to maintain linearity, Vi is preferably a constant value. Otherwise, two function values, Vi and RT, will appear in the function relationship of Vo, and the temperature measurement cannot be accurately achieved. The voltage fluctuation range given by the design is ±10% VCC. If RT is a constant value at a certain temperature, then the final Vo will also have a ±10% fluctuation. Using a unified RT relationship to judge, the temperature error obtained within the measurement range becomes ±10°C, which is intolerable. The circuit must have a reliable and high-precision voltage regulator. Therefore, in the actual operation of the circuit, instead of using self-power supply, the high-precision voltage reference MAX6025 can be used to provide a standard 2.5 V voltage reference to power components and circuits.

In addition, from the relationship between the Pt1000 platinum resistance value R and temperature T, it can be seen that within the measurement temperature range, Pt1000 has a high sensitivity of 3.786 59 Ω/℃, so only a general-purpose operational amplifier is needed. When the temperature coefficient of the resistors used is matched, nonlinear errors can also be ignored.

5 Actual temperature sensing signal processing circuit

The actual temperature sensing signal processing circuit is shown in Figure 5. The capacitor C in the circuit diagram is a noise reduction capacitor with an actual value of 1μF. The 2.5 V voltage reference is provided by MAX6025. According to the given parameters in the figure, before the actual temperature test, it must first be zeroed and fully adjusted. Use a precision adjustable resistor to replace Pt1000 and connect it to the circuit. Change the resistance value to make it equal to the equivalent resistance of 1 000.8 Ω at 0℃, adjust the variable resistor R5 to make the output voltage 0, then change the precision adjustable resistor value to 1 265.8 Ω at 70℃, adjust the variable resistor R6 to make the output 700 mV, and complete the zeroing and fully adjusting. The circuit has been tested experimentally and achieved a good temperature sensing effect.