Temperature sensor interface circuit

Abstract

The utility model belongs to the technical field of temperature measurement, and particularly relates to a temperature sensor interface circuit, which comprises a to-be-measured object circuit, a Zener diode, an arrangement circuit and a control room circuit which are electrically connected in sequence, two ends of the Zener diode are connected in parallel with a circuit of an object to be tested; the arrangement circuits are respectively connected in series with two ends of a circuit of an object to be tested, and lead resistors are arranged in the arrangement circuits at two ends; the control room circuit is electrically connected with a first circuit and a second circuit through a switch, a constant current source device is connected in series between the two arrangement circuits in the first circuit, a resistor and a constant voltage source device are sequentially connected in series between the two arrangement circuits in the second circuit, and the first circuit and the second circuit are connected in parallel. According to the temperature sensor interface circuit, only one Zener diode needs to be additionally arranged at the sensor end, and accurate measurement of lead resistance and RTD resistance is achieved in a time-sharing mode by means of voltage stability of reverse breakdown and tiny leakage current characteristics of reverse cut-off of the Zener diode.

Description

Technical Field

[0001] The utility model belongs to the technical field of temperature measurement, and in particular relates to a temperature sensor interface circuit. Background Art

[0002] High-precision temperature measurement provides essential data for product development and industrial automation applications, ensuring product quality and production safety. Resistance temperature sensors (RTDs) are widely used due to their excellent linearity, measurement repeatability, and stability. However, for remote measurements, the resistance of the long connecting wires between the RTD and the control room instrument can produce significant measurement errors. This unwanted error varies not only with the length of the wire but also with the ambient temperature of the wiring area. To compensate for lead resistance errors, the traditional method is to increase the number of wires, such as using a three-wire or four-wire system. This approach increases wiring complexity and sharply increases costs in applications where a large number of remote temperature measurement points are used.

[0003] In publication number CN102768078B (Patent Name: Method for Automatic Compensation of Lead Resistance in Two-Wire Thermistor Temperature Measurement), the inventors proposed a method for testing lead resistance and compensating for thermistor measurement errors using the transient characteristics of an RC circuit. This method is unaffected by the characteristics of the additional capacitor itself, while also providing real-time access to the lead resistance value, thus avoiding the adverse effects of lead resistance temperature drift. However, since each measurement cycle requires waiting for the capacitor to complete a charge and discharge cycle, the temperature measurement frequency is limited. According to the implementation scheme described in the patent, the measurement frequency is less than 5 Hz, making it unsuitable for applications requiring dynamic testing of temperature changes. Utility Model Content

[0004] The technical problem to be solved by the utility model is to compensate for the measurement error of the lead resistance of a two-wire RTD and simultaneously resist the adverse effect of the lead resistance temperature drift on the compensation circuit.

[0005] In order to solve the above technical problems, the technical solution provided by the present invention is a temperature sensor interface circuit, which is characterized in that: it includes a resistor to be measured, a Zener diode, a layout circuit and a control room circuit electrically connected in sequence; the two ends of the Zener diode are connected in parallel with the resistor to be measured, and the Zener diode is close to the resistor to be measured; the layout circuit is respectively connected in series with the two ends of the resistor to be measured, and there are lead resistors in the layout circuits at both ends; the control room circuit is electrically connected to a first circuit and a second circuit through a switch, a constant current source device is connected in series between the two layout circuits in the first circuit, and a resistor and a constant voltage source device are connected in series in sequence between the two layout circuits in the second circuit.

[0006] Furthermore, the Zener diode is a reverse breakdown diode, and the side of the Zener diode connected to the switch is a negative electrode.

[0007] Furthermore, one end of the constant current source in the first circuit connected to the switch is the positive electrode.

[0008] Furthermore, one end of the constant voltage source device in the second circuit connected to the resistor is the positive electrode.

[0009] Furthermore, the arrangement circuit is a circuit for an outdoor lead arrangement process.

[0010] Furthermore, the switch is a single-pole double-throw switch.

[0011] Furthermore, the circuit lines are arranged with equal lengths and the lead resistances are the same.

[0012] The advantages of this utility model compared with the prior art are:

[0013] This temperature sensor interface circuit only requires the addition of a Zener diode at the sensor end. Utilizing its reverse breakdown voltage stability and reverse cutoff leakage current characteristics, it achieves accurate time-sharing measurement of lead resistance and RTD resistance. The components used are simple and compact (only one Zener diode is required in parallel at the sensor end). The test time is short because the Zener diode does not require charging and discharging like a capacitor, allowing the operating point to be established quickly without affecting the sensor's sampling rate. The Zener diode has good reverse breakdown voltage consistency, with an initial accuracy of 0.05% available as standard. The lead resistance can be acquired in real time and can be used for lead fault diagnosis. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a structural diagram of a temperature sensor interface circuit of the utility model.

[0015] Figure 2 This is a specific implementation circuit diagram of a temperature sensor interface circuit of the utility model.

[0016] As shown in the figure:

[0017] A. Represents the resistance to be measured; B. Represents the Zener diode; C. Represents the layout circuit; D. Represents the control room circuit. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0019] Example 1: A temperature sensor interface circuit, comprising a resistor to be measured, a Zener diode, an arrangement circuit and a control room circuit electrically connected in sequence; the two ends of the Zener diode are connected in parallel with the resistor to be measured, and the Zener diode is close to the resistor to be measured; the arrangement circuit is respectively connected in series with the two ends of the resistor to be measured, and there are lead resistors in the arrangement circuits at both ends; the control room circuit is electrically connected to a first circuit and a second circuit through a switch, a constant current source device is connected in series between the two arrangement circuits in the first circuit, and a resistor and a constant voltage source device are connected in series in sequence in the second circuit between the two arrangement circuits.

[0020] The Zener diode is a reverse breakdown diode, and the side of the Zener diode connected to the switch is the negative pole; the end of the constant current source in the first circuit connected to the switch is the positive pole; the end of the constant voltage source device in the second circuit connected to the resistor is the positive pole; the layout circuit is a circuit of the outdoor lead layout process; the switch is a single-pole double-throw switch; the layout circuit lines are equal in length and the lead resistances are the same.

[0021] As the instruction manual Figure 1 As shown, in the technical solution proposed by the present invention, a complete (resistance temperature sensor) RTD resistance value test process includes three steps:

[0022] Step 1: Measure the lead resistance. Figure 1 The switch SW in the circuit is switched to the first circuit. The constant current source CCS provides a current I c , through the lead resistance R w 1 and R w 2. Make the Zener diode break down in reverse and stabilize the voltage across it at U d Since the resistance of the Zener diode is very small after reverse breakdown, most of the current flows through the diode. t The current is relatively small, but the adverse effect of the RTD's self-heating effect on temperature measurement must be considered. At this time, the voltage measured at position 3 is recorded as U3, and the lead resistance R can be calculated by substituting it into formula (1). w .

[0023] U3=2I c R w +U d (1)

[0024] Record U3 and R w The value will be used in the following steps.

[0025] Step 2: Measure the working current. Switch SW to position 2, RTD is powered by constant voltage source CVS, whose voltage is U c , lower than the reverse breakdown voltage U of the Zener diode dAt this time, the Zener diode works in the reverse cutoff region, the equivalent resistance is very large, and the current passing through it is several uA, recorded as I d The accurate value should be obtained through prior testing. The voltage measured at position 3 is represented by U3', which satisfies formula (2):

[0026] U′3=2IR w +(II d )R t (2)

[0027] In equation (2), I represents the output current of the voltage source. Meanwhile, the voltage measured at location 2 is represented by U2. Therefore, the current I is obtained using equation (3). The values ​​of U2 and U3' are saved for the following steps.

[0028] I=(U c -U2) / R s (3)

[0029] Step 3: Calculate the RTD resistance. Substituting equations (1) and (3) into equation (2) yields the RTD resistance, as shown in equation (4).

[0030]

[0031] Equation (4) shows that it is a simple algebraic equation that can be solved in a microcontroller to obtain the RTD resistance. The RTD resistance can then be converted to a temperature value by simply using a table lookup or directly solving the polynomial.

[0032] Error analysis:

[0033] From formula (4), we can see that the factors that affect the RTD resistance measurement accuracy are: (1) U d and I d Stability, (2) U C and I C stability, (3) measurement accuracy of U2, U3 and U3', (4) accuracy of Rs.

[0034] The first of these is the key factor in implementing this technology and is the most difficult. The other factors can be solved using conventional technologies, such as high-precision AD sampling, low-temperature drift power supplies, and sampling resistors.

[0035] Compared with the forward conduction characteristic IV curve of the diode, the reverse breakdown characteristic IV curve of the Zener diode has a narrower breakdown voltage range and a larger curve slope. Therefore, its reverse breakdown voltage U dThe variation with operating current is very small. Currently, for high-precision Zener diodes, the nominal error in the reverse breakdown voltage can reach 0.05%, demonstrating very high device consistency. Furthermore, the reverse leakage current of a Zener diode remains relatively stable within the reverse cutoff region. These characteristics of the Zener diode facilitate system calibration. To achieve higher measurement accuracy, the reverse breakdown voltage and reverse leakage current values ​​of each Zener diode in the measurement circuit must be individually calibrated and stored in non-volatile memory.

[0036] Sampling frequency analysis:

[0037] In principle, the update rate of the measured value depends on the sum of the Zener diode's response time (200µs) to a step voltage input, the analog switch's single switching time (18ns), the time it takes to sample two AD samples, and the controller's calculation time. The values ​​in parentheses are from a standard device manual. A typical 16-bit ADC chip has a conversion time of 2µs, and a typical 32-bit microcontroller with a 72MHz main frequency should take no more than 30µs to calculate Equation (4). Therefore, this total time can generally be controlled within 250µs, meaning the update rate can reach 4kHz.

[0038] Example 2:

[0039] As the instruction manual Figure 2 As shown, the RTD1 and RTD2 terminals of the RTD to be measured are connected in parallel with the Zener diode D2, and the two ends are connected to R W 1 and R W 2Electrical connection indicates wiring resistance;

[0040] R W 1 and R W 2 are respectively connected to the digital signal input terminal D and GND corresponding to the analog switch ADG719;

[0041] There is a 0.1uF capacitor between the power supply terminal and GND of the analog switch ADG719;

[0042] The analog switch ADG719 is connected to the OUT of the constant voltage source chip REF3212 through the output terminal S1, with a 200Ω resistor in the middle of the connection. A 0.1uF capacitor is connected in parallel between OUT and GND. At the same time, the constant voltage source chip REF3212 is powered by an external 5V power supply, and a 0.1uF capacitor is connected in parallel between the power supply terminal and GND.

[0043] The analog switch ADG719 is connected to the two OUT terminals of the constant current source chip LT3902 via the output terminal S2, and a 125Ω resistor is present on the connection line. The connection line is also connected to the voltage sampling point terminal SET of the constant current source chip LT3902 via the Zener diode D1, so that the Zener diode D1 is connected in parallel with the two OUT terminals and the 125Ω resistor on the connection line. The input terminal of the constant current source chip LT3902 is connected to an external 5V power supply.

[0044] The analog switch ADG719, constant voltage source chip REF3212, and constant current source chip LT3902 are all powered by +5V, and RW1 and RW2 are powered by GND to ensure safety.

[0045] During use, the constant current source chip LT3902 provides current I C , through the lead resistance R w 1 and R w 2. Make the Zener diode reverse breakdown and stabilize the voltage at both ends at U d , and the voltage U3' is measured at U3, and the voltage at the analog switch ADG719 is measured as U2 for calculation.

[0046] The working process is as follows:

[0047] 1. Make the port SW input high level to turn on the current source REF3012 and cut off the voltage source REF3012;

[0048] 2. Measure and record the voltage value of port U3, which is recorded as U3;

[0049] 3. Make the port SW input low level to turn on the voltage source REF3012 and cut off the current source REF3012;

[0050] 4. Measure and record the voltage value of port U3, record it as U3', and record the voltage value of port U2, record it as U2;

[0051] 5. Calculate the temperature sensor resistance Rt according to Formula 4 in Example 1;

[0052] 6. According to the resistance-temperature relationship table of the temperature sensor, the difference is the corresponding temperature.

[0053] The above description of the present invention and its embodiments is non-limiting. The specific embodiments shown in the specific embodiments are only one embodiment of the present invention, and the actual structure is not limited thereto. In short, if a person skilled in the art is inspired by the above, and does not deviate from the purpose of the present invention, without creatively designing a structure and embodiment similar to the technical solution, they shall fall within the scope of protection of the present invention.

Claims

1. A temperature sensor interface circuit, characterized in that: It includes a resistor to be measured, a Zener diode, a layout circuit and a control room circuit which are electrically connected in sequence; The two ends of the Zener diode are connected in parallel with the resistor to be measured, and the Zener diode is close to the resistor to be measured; The arrangement circuit is connected in series with both ends of the resistor to be measured, and lead resistance exists in the arrangement circuit at both ends; The control room circuit is electrically connected to a first circuit and a second circuit through a switch. A constant current source device is connected in series between the two arrangement circuits in the first circuit, and a resistor and a constant voltage source device are connected in series in sequence between the two arrangement circuits in the second circuit.

2. A temperature sensor interface circuit according to claim 1, characterized in that: The Zener diode is a reverse breakdown diode, and the side of the Zener diode connected to the switch is a negative electrode.

3. The temperature sensor interface circuit according to claim 1, wherein: One end of the constant current source in the first circuit connected to the switch is the positive electrode.

4. The temperature sensor interface circuit according to claim 1, wherein: The end of the constant voltage source device in the second circuit connected to the resistor is the positive electrode.

5. The temperature sensor interface circuit according to claim 1, wherein: The layout circuit is a circuit for the outdoor lead layout process.

6. The temperature sensor interface circuit according to claim 1, wherein: The switch is a single-pole double-throw switch.

7. The temperature sensor interface circuit according to claim 1, characterized in that: The circuit lines are arranged to have equal lengths and the same lead resistances.

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