A three-wire resistance measurement circuit and method based on dual current sources

Abstract

A kind of three-wire resistance measurement circuit and method based on double current source belong to resistance sensor measurement technical field.The present application is to solve the problem that existing three-wire measurement scheme cannot completely eliminate the influence of lead resistance and its temperature drift.The circuit of the present application includes a first current source, a second current source, a first measurement point, a second measurement point, a resistance temperature detector (RTD) and a measurement circuit. The first current source generates adjustable current and is connected to the first lead, the second current source generates adjustable current and is connected to the second lead, one end of the RTD is connected to the two current sources through the first and second leads respectively, and the other end is grounded through the third lead. The present application completely eliminates the influence of lead resistance and its temperature drift on measurement accuracy through double current source excitation and specific calculation, while retaining the advantage of simple three-wire connection, suitable for high-precision temperature monitoring.

Description

Technical Field

[0001] This invention relates to the field of sensor measurement technology, and more specifically, to a measurement circuit and method for a three-wire resistive detector, particularly a measurement scheme that can completely eliminate the influence of lead resistance, suitable for applications requiring high-precision temperature monitoring such as industrial process control, laboratory measurement, and medical equipment. Background Technology

[0002] Resistance temperature detectors (RTDs) are widely used in industrial process control, laboratory measurement, medical equipment, and other applications requiring high-precision temperature monitoring due to their high accuracy and good stability. In practical applications, the sensor is usually located far from the measurement circuit and needs to be connected via long leads. However, the resistance of the leads themselves and their drift effect with temperature changes become the main factors affecting the final measurement accuracy.

[0003] To reduce the impact of lead resistance, various solutions have been developed in existing technologies. Among them, the traditional three-wire system uses a single current source excitation and differential measurement to partially cancel the resistance on the two leads. This method theoretically assumes that the resistances of the two leads are completely equal. However, in practical engineering, due to factors such as inconsistent lead lengths, uneven ambient temperature distribution, and differences in contact resistance at connection points, this assumption is difficult to strictly hold, leading to residual measurement errors. Especially in industrial environments with drastic temperature changes, the temperature coefficient difference in lead resistance further exacerbates this error.

[0004] The four-wire scheme, with its independent excitation and measurement leads, can theoretically eliminate the influence of lead resistance. However, it requires four leads, resulting in high system cost and complex wiring, making it unsuitable for large-scale distributed systems. Other schemes, such as time-domain switching and digital adaptive compensation schemes, either require complex time-series control and digital signal processing, leading to slow response speeds; or they rely on high-performance processors and complex compensation algorithms, increasing the difficulty and cost of system implementation.

[0005] Therefore, there is an urgent need in the existing technology for a high-precision, low-cost measurement solution that can maintain the simplicity of three-wire connection and completely eliminate the influence of lead resistance like four-wire connection. Summary of the Invention

[0006] The present invention aims to provide a three-wire resistance measurement circuit and method based on dual current sources, so as to solve the technical problem that existing three-wire measurement schemes cannot completely eliminate the influence of lead resistance and its temperature drift.

[0007] To achieve the above objectives, according to one aspect of the present invention, a three-wire resistance measurement circuit based on a dual current source is provided. The circuit includes: a first current source for generating an adjustable first direct current. A second current source is used to generate an adjustable second direct current. The first measurement point is connected to the output terminal of the first current source and is used to measure the first voltage. The second measurement point is connected to the output terminal of the second current source and is used to measure the second voltage. A resistive current detector (RTD), wherein a first end of the RTD is connected to the first measurement point via a first lead, the first end of the RTD is also connected to the second measurement point via a second lead, and the second end of the RTD is connected to a common ground via a third lead; and a measurement and calculation unit for measuring the first DC current. Second DC current First voltage Second voltage The resistance value of the RTD is calculated according to a specific formula.

[0008] Furthermore, the measurement and calculation unit calculates the resistance value R_rtd of the RTD according to the following formula:

[0009] .

[0010] Preferably, both the first current source and the second current source are adjustable precision current sources, and the first DC current... With the second DC current The ratio is adjustable from 0.5 to 2.0. This setting optimizes the measurement signal-to-noise ratio and controls the total current flowing through the RTD, avoiding self-heating effects.

[0011] Preferably, the circuit further includes a lead resistance monitoring module, used to detect whether the resistances of the first lead, the second lead and the third lead meet the condition of equality, and to issue an alarm or use an extended compensation algorithm when the condition is not met.

[0012] According to another aspect of the present invention, a three-wire resistance measurement method based on dual current sources is provided. The method includes the following steps:

[0013] (a) Applying a first DC current to the first terminal of the RTD through a first current source ;

[0014] (b) Apply a second DC current to the first terminal of the RTD through a second current source. ;

[0015] (c) Measure the first voltage at the first measurement point The second voltage at the second measurement point ;

[0016] (d) According to the formula Calculate the resistance value R_rtd of the RTD;

[0017] (e) Calculate the corresponding temperature value based on the calculated resistance value R_rtd using a standard calibration table.

[0018] Furthermore, the method also includes dynamically adjusting before or during the application of current. and / or The steps are to optimize system power consumption and self-heating effects while ensuring measurement accuracy.

[0019] Key features of this invention include dual current source excitation, four-parameter measurement, analytical calculation, and complete elimination of lead resistance. Dual current source excitation provides additional degrees of freedom, enabling the system to achieve more accurate compensation without increasing the number of leads. Four-parameter measurement requires simultaneous measurement of two current values ​​and two voltage values, providing sufficient information for subsequent calculations. Analytical calculation means directly calculating the RTD resistance using a closed-form formula, without relying on iterative or approximate algorithms, ensuring real-time and deterministic calculations. Complete elimination of lead resistance is the greatest advantage of this invention; the formula does not include a lead resistance term, theoretically eliminating its influence, including changes caused by temperature drift.

[0020] The beneficial effects of this invention are as follows:

[0021] 1. High precision: Through dual current sources and specific calculation formulas, the influence of lead resistance and its temperature drift, contact resistance changes and other factors on the measurement results are theoretically completely eliminated.

[0022] 2. Good real-time performance: The compensation process is completed in real time through hardware measurement and analytical formula calculation, without the need for complex iterative algorithms or additional calibration time, resulting in fast response speed.

[0023] 3. Simple connection: It retains the traditional three-wire connection method, is compatible with existing RTD interfaces and cables, and has a lower cabling cost than the four-wire system.

[0024] 4. Strong robustness: It is not sensitive to the absolute value of the lead resistance. It only requires that the resistance of the three leads be approximately equal to achieve excellent compensation effect, and the engineering implementation conditions are relaxed.

[0025] 5. Easy to implement: It mainly relies on analog circuits and basic digital computing, without the need for high-performance digital signal processors or complex algorithms, and the system cost and development difficulty are controllable.

[0026] 6. It has good scalability and is easy to implement multi-channel synchronous measurement, making it suitable for distributed temperature monitoring systems. Attached Figure Description

[0027] Figure 1 This is a basic circuit schematic diagram provided in an embodiment of the present invention.

[0028] Figure 2 This is an implementation circuit block diagram provided in the embodiments of the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0030] Example 1: Basic Simulation Implementation Scheme

[0031] like Figure 1 The basic circuit diagram shown illustrates a typical implementation structure: the power supply is located at the top of the circuit, with two adjustable current sources connected downwards. and A voltage measurement point is connected below each current source. and ), and then through the lead resistor ( and The junctions converge at the upper end of the RTD sensor. The lower end of the RTD sensor is connected to the third lead resistor ( Grounding forms a complete current loop. Voltage measurement point. and It is used to acquire differential voltage signals to provide data for subsequent calculations.

[0032] This embodiment provides a three-wire resistance measurement circuit based on dual current sources. The circuit includes a first current source (generating...) ), second current source (generating) First voltage measurement point Second voltage measurement point The RTD sensor and the leads connecting them. The upper end (first end) of the RTD is connected to the first lead (resistor). ) and second lead (resistor) Connect to and Point, the lower end (second end) of the RTD passes through the third lead (resistor) Grounding. and All are adjustable DC current sources, implemented by a voltage-to-current conversion circuit consisting of a precision operational amplifier and matching resistors. Their temperature drift coefficient is controlled within 10ppm / ℃, and the output current range is adjustable from 0.1mA to 5mA.

[0033] The voltage measurement section uses an instrumentation amplifier. and For differential or single-ended measurements, the instrumentation amplifier provides high input impedance (>1GΩ) and a common-mode rejection ratio (CMRR) greater than 100dB to reduce measurement-introduced errors. At the heart of the measurement and computation unit is a microcontroller integrating a 24-bit delta-sigma analog-to-digital converter (ADC) and a floating-point computing unit. The ADC synchronously samples the feedback value from the current source (representing...). , ) and voltage measurement value ( , After reading these four digital values, the microcontroller immediately executes the formula. The floating-point operation yields the RTD resistance value. Finally, the microcontroller linearly interpolates the resistance value into a temperature value based on the internal platinum resistance (e.g., PT100) calibration table and outputs it via RS-485 or 4-20mA interface.

[0034] In this embodiment, the initial settings =1.1mA, =0.9mA, to ensure and There is a sufficient difference to improve the numerical stability of the denominator of the formula, and the maximum total current flowing through the RTD is 2mA, which avoids the self-heating effect.

[0035] Example 2: Digital Enhancement Implementation Scheme

[0036] Based on Example 1, this example adds lead resistance monitoring and adaptive compensation functions. While calculating R_rtd, the measurement and calculation unit obtains an estimated value r of the lead resistance. By analyzing the magnitude and stability of the r value, the lead connection status (e.g., whether it is open-circuited or short-circuited) and the consistency of the three lead resistances can be determined. When the three lead resistances are not completely equal (e.g., the calculated r value deviates from a preset threshold by more than 5%), the system can switch to a more advanced compensation algorithm, which utilizes the measured values... , , , By solving for... , , The non-homogeneous linear equations are used to obtain a corrected value for R_rtd. This scheme further enhances the robustness of the system under non-ideal conditions.

[0037] Example 3: Multi-channel temperature measurement system

[0038] like Figure 2 The implementation circuit block diagram shows the complete system architecture, including an RTD resistance sensor, a constant current source, a voltage detection module, and a computing unit. The RTD resistance sensor uses a three-wire connection, with typical sensors being PT100 / PT1000 type resistance temperature sensors. The constant current source provides two adjustable currents I1 / I2 to drive the RTD resistor, generating two voltages V1 / V2, which are input to the voltage detection module. The computing unit then outputs the detection result.

[0039] This embodiment provides a multi-channel temperature measurement system comprising eight measurement circuits as described in Embodiment 1. Each channel is independently connected to a three-wire PT100 sensor and has its own dual current source and voltage measurement front end. The measurement and calculation functions of all channels are centrally implemented by a central ADC with a multiplexer and a main control microcontroller. The system adopts a time-division multiplexing method: the main control microcontroller sequentially selects each channel, configures the current source for that channel, reads the voltage and current values, and calculates the resistance and temperature. The measurement results of all channels are finally uploaded to the host computer via an Ethernet interface. This embodiment effectively reduces the overall cost of the multi-channel system by sharing the high-cost ADC and controller while ensuring the measurement accuracy of each channel.

[0040] Example 4: Integrated Chip Implementation Scheme

[0041] This embodiment integrates all the circuitry from Embodiment 1 except for the RTD sensor and leads (including dual adjustable current sources, instrumentation amplifiers, ADCs, calculation and control logic, and communication interfaces) into a single CMOS chip. The chip integrates a high-precision matching resistor network and a low-offset, low-drift operational amplifier to generate a stable excitation current. An on-chip temperature sensor monitors the chip's own temperature and digitally compensates for temperature drift in the current source and measurement link. The chip provides a standard SPI / I²C digital interface; the external host only needs to write configuration parameters (such as...) , The system can then read the final temperature data. This integrated solution greatly simplifies the design process for users, reduces system size, and is suitable for large-scale applications that are sensitive to space and cost.

[0042] The basic analog implementation uses a precision operational amplifier and precision resistors to construct a voltage-to-current conversion circuit, generating an adjustable excitation current. The current range is typically between 0.1mA and 5mA, with the specific value determined based on the RTD type and measurement range. The stability of the current source is a critical indicator; temperature drift should be less than 10ppm / ℃ to ensure long-term measurement accuracy. The voltage measurement section uses an instrumentation amplifier, providing high input impedance and excellent common-mode rejection ratio to avoid loading effects affecting measurement accuracy. The ADC conversion uses a 24-bit delta-sigma converter, supporting synchronous sampling to ensure... , , , The measurement time is consistent. The control and calculation section uses a microcontroller with floating-point arithmetic capabilities to execute the calculation formula in real time and convert the resistance value into a temperature value output.

[0043] The workflow includes initialization, measurement, calculation, and output phases. Initialization is configured during this process. and The initial value is usually chosen to be a current with a certain difference, such as... =1.1mA, =0.9mA, providing good numerical stability. The measurement phase simultaneously acquires four physical quantities and performs digital filtering to remove noise. The calculation phase calculates the RTD resistance value according to the formula and converts it to a temperature value according to a standard calibration table. The output phase provides multiple interface options, including digital and analog outputs, to adapt to different application requirements.

[0044] Current source optimization strategies are key to improving measurement accuracy. Following the differential principle ensures... and Sufficient differential values ​​improve the numerical stability of the denominator in the formula. Simultaneously, the sum of the current and its value are controlled to not exceed the maximum allowable current of the RTD, avoiding the impact of self-heating effects on measurement accuracy. The adaptive adjustment algorithm dynamically optimizes the current value based on the measurement results, reducing power consumption while maintaining accuracy.

[0045] The digitally enhanced implementation adds lead resistance monitoring, fault diagnosis, and adaptive compensation functions to the basic scheme. Additional measurements determine whether the lead resistances meet the equality condition; if not, an extended formula is used for compensation. The fault diagnosis function can detect abnormal states such as open circuits and short circuits in the leads, improving system reliability. The adaptive compensation algorithm maintains measurement accuracy even when lead resistances are not completely equal, enhancing the system's applicability.

[0046] The multi-channel system replicates the circuitry of this invention across multiple channels to achieve synchronous temperature monitoring. The system architecture can employ centralized control or distributed processing, selected based on application requirements. Synchronization strategies include time-division multiplexing, frequency-division multiplexing, and master-slave synchronization, balancing system complexity and performance requirements. The time-division multiplexing scheme allows each channel to take turns measuring, sharing ADC resources and reducing costs. The frequency-division multiplexing scheme uses different frequency excitations for different channels, achieving true synchronous measurement. The master-slave synchronization scheme allows each channel to measure independently, with results processed centrally, suitable for large-scale distributed systems.

[0047] Integrated circuits integrate the entire circuitry into a single chip, offering higher performance and a smaller size. The chip architecture comprises a digital core and an analog front-end. The digital core handles control logic, computation, and communication, while the analog front-end includes dual current sources, voltage measurement circuitry, and a reference source. Standard CMOS technology is used, integrating high-precision matching resistors and low-offset operational amplifiers. An on-chip temperature sensor compensates for temperature drift. Integrated circuits significantly reduce system costs, improve reliability, and are suitable for large-scale applications.

[0048] Compared to traditional technologies, this invention exhibits significant advantages in several aspects. Regarding the number of leads, this invention maintains a three-wire connection, reducing the number of leads compared to a four-wire system, thus lowering wiring complexity and cost. In terms of theoretical accuracy, this invention completely eliminates the influence of lead resistance, while the traditional three-wire system relies on lead resistance matching, the four-wire system offers high accuracy but is costly, and the time-domain switching scheme offers moderate accuracy. Regarding temperature effects, this invention is insensitive to temperature drift of lead resistance, the traditional three-wire system is sensitive, the time-domain switching scheme is moderately sensitive, and the four-wire system is insensitive. In terms of response speed, this invention, along with the traditional three-wire and four-wire systems, offers fast response, while the time-domain switching scheme is slower due to the need for time sequences. Regarding system complexity, this invention has moderate complexity, the traditional three-wire system is simple, the four-wire system is moderate, and the time-domain switching scheme is complex. In terms of cost, this invention has moderate cost, the traditional three-wire system is low cost, the four-wire system is high cost, and the time-domain switching scheme has moderate cost. Regarding industrial compatibility, this invention has good compatibility with the traditional three-wire system, moderate compatibility with the four-wire system, and poor compatibility with the time-domain switching scheme.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A three-wire resistance measurement circuit based on dual current sources, characterized in that, include: The first current source is used to generate an adjustable first direct current. ; A second current source is used to generate an adjustable second direct current. ; The first measurement point is connected to the output terminal of the first current source and is used to measure the first voltage. ; The second measurement point is connected to the output terminal of the second current source and is used to measure the second voltage. ; A resistive detector (RTD) has a first end connected to the first measurement point via a first lead, a second end connected to the second measurement point via a second lead, and a second end connected to a common ground via a third lead. as well as The measurement and calculation unit is used to measure the first DC current. Second DC current First voltage Second voltage The resistance value R_rtd of the RTD is calculated according to the following formula: 。 2. The circuit according to claim 1, characterized in that, Both the first current source and the second current source are adjustable precision current sources, and the first DC current... With the second DC current The ratio is adjustable within the range of 0.5 to 2.

0.

3. The circuit according to claim 1, characterized in that, It also includes a lead resistance monitoring module, used to detect whether the resistances of the first lead, the second lead, and the third lead meet the condition of equality.

4. A three-wire resistance measurement method based on dual current sources, applied to the circuit according to any one of claims 1 to 3, characterized in that, Includes the following steps: (a) Applying the first DC current to the first terminal of the RTD through the first current source. ; (b) Applying the second DC current to the first terminal of the RTD through the second current source. ; (c) Measure the first voltage V1 at the first measurement point and the second voltage at the second measurement point. ; (d) According to the formula Calculate the resistance value R_rtd of the RTD; (e) Calculate the corresponding temperature value using the standard calibration table based on the calculated resistance value R_rtd.

5. The method according to claim 4, characterized in that, Prior to steps (a) and (b), the first DC current is adjusted. and / or the second DC current The steps are to optimize the measurement signal-to-noise ratio or control the self-heating effect of the RTD.

6. A multi-channel temperature measurement system, characterized in that, It includes multiple three-wire resistance measurement circuits based on dual current sources as described in any one of claims 1 to 3, wherein each measurement circuit operates in parallel or in a time-sharing manner, and its measurement results are centrally processed and output by a central processing unit.

7. An integrated circuit chip, characterized in that, The chip integrates all or part of the functional modules of the three-wire resistance measurement circuit based on dual current sources as described in any one of claims 1 to 3.

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