Detection circuit of 4-20mA signal

Abstract

The invention discloses a 4-20 mA signal detection circuit, and relates to the technical field of sampling current detection, and the circuit comprises a current sampling module which is configured to collect a sampling current and convert the sampling current into a voltage signal; the operational amplifier module is configured to amplify the voltage signal and input the voltage signal into a voltage sampling circuit at the rear end; the operational amplifier negative feedback module is configured to add negative feedback to the operational amplifier module; the voltage bias module is configured to provide bias voltage for the reverse input end of the operational amplifier module; and the voltage isolation module is configured to be respectively connected with the voltage bias module, the operational amplifier negative feedback module and the reverse input end of the operational amplifier module. A group of operational amplifier negative feedback circuits, a group of voltage bias circuits and a group of voltage isolation circuits are added on the basis of a traditional resistance sampling circuit, so that 4-20mA standard signals are accurately converted into voltage signals, and reliable detection of zero signals is realized.

Description

Technical Field

[0001] The present invention relates to the field of industrial control technology, in particular to the field of signal detection technology in industrial control, specifically, to a detection circuit for 4-20mA signals. Background Art

[0002] In industrial control, how to transmit the physical signals collected by various sensors over long distances is still an important issue faced by the field of industrial control. At the same time, accurate and reliable transmission of physical signals to controllers is one of the key factors to ensure the reliable operation of industrial equipment.

[0003] The 4-20mA signal system is an analog signal standard for process control systems developed by the International Electrotechnical Commission, where 4mA represents zero signal and 20mA represents full scale. The 4-20mA output interface is an important communication signal. How to accurately sample zero signal has always been a problem to be solved in the field of industrial control. Summary of the invention

[0004] The object of the present invention is to provide a 4-20mA signal detection circuit for realizing accurate sampling of zero signal in industrial control.

[0005] The present invention solves the above problems through the following technical solutions:

[0006] A 4-20mA signal detection circuit, comprising:

[0007] A current sampling module is configured to collect sampled current and convert it into a voltage signal;

[0008] An operational amplifier module, configured to amplify the voltage signal and input the amplified voltage signal into a voltage sampling circuit at the rear end;

[0009] an operational amplifier negative feedback module, configured to add negative feedback to the operational amplifier module;

[0010] A voltage bias module, configured to provide a bias voltage to the reverse input terminal of the operational amplifier module;

[0011] The voltage isolation module is configured to be connected to the voltage bias module, the operational amplifier negative feedback module and the reverse input terminal of the operational amplifier module respectively.

[0012] Furthermore, the current sampling module includes a sampling resistor RS, and two ends of the resistor RS are respectively connected to an inflow end and an outflow end of the sampling current.

[0013] Furthermore, the operational amplifier module includes an operational amplifier, a positive input terminal of the operational amplifier is connected to the inflow terminal of the sampling current through a balancing resistor R1, and a negative input terminal of the operational amplifier is respectively connected to the voltage isolation module and the operational amplifier negative feedback module.

[0014] Furthermore, the operational amplifier negative feedback module includes a resistor Rf and a resistor R2, the first end of the resistor Rf is connected to the output end of the operational amplifier, the second end of the resistor Rf is connected to the first end of the resistor R2, the reverse input end of the operational amplifier and the voltage isolation module, and the second end of the resistor R2 is connected to the outflow end of the sampling current and is grounded.

[0015] Furthermore, the voltage isolation module is a resistor R5, a first end of the resistor R5 is connected to the inverting input end of the operational amplifier, and a second end of the resistor R5 is connected to the voltage bias module.

[0016] Furthermore, the voltage bias module includes a resistor R3 and a resistor R4, a first end of the resistor R3 is connected to a power supply, a second end of the resistor R3 is connected to a second end of the resistor R5 and a first end of the resistor R4, and a second end of the resistor R4 is grounded.

[0017] Furthermore, the power supply is a positive input power supply of the operational amplifier.

[0018] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0019] (1) The present invention provides a safe and reliable 4-20mA signal detection circuit. On the basis of the traditional resistance sampling circuit, a group of operational amplifier negative feedback circuits, a group of voltage bias circuits and a group of voltage isolation circuits are added to achieve accurate conversion of 4-20mA standard signals into voltage signals and reliable detection of zero signals.

[0020] (2) The present invention adopts a balancing resistor to eliminate zero drift; adds negative feedback to improve the stability of signal conversion; constructs a functional relationship to achieve accurate conversion of standard signals, and realizes it through a voltage bias module, an operational amplifier module, an operational amplifier negative feedback module and a voltage isolation module.

[0021] (3) The circuit structure of the present invention is simple, reliable, and has high practicality and promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a principle block diagram of the present invention;

[0023] Figure 2 It is the circuit principle diagram of the present invention;

[0024] Among them, U1-operational amplifier. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto.

[0026] Embodiment 1:

[0027] Combined with Figure 1 As shown, a 4-20mA signal detection circuit includes:

[0028] The current sampling module is configured to collect the sampling current and convert it into a voltage signal; the sampling current (4-20mA) is generated by the current generating module.

[0029] An operational amplifier module, configured to amplify the voltage signal and input the amplified voltage signal into a voltage sampling circuit at the rear end;

[0030] an operational amplifier negative feedback module, configured to add negative feedback to the operational amplifier module;

[0031] A voltage bias module, configured to provide a bias voltage to the reverse input terminal of the operational amplifier module;

[0032] The voltage isolation module is configured to be connected to the voltage bias module, the operational amplifier negative feedback module and the reverse input terminal of the operational amplifier module respectively.

[0033] Embodiment 2:

[0034] Based on Example 1, Figure 2 As shown, the current sampling module includes a sampling resistor RS, and the two ends of the resistor RS are respectively connected to the inflow end and the outflow end of the sampling current. The sampling current flows through the resistor to generate a sampling voltage Ui.

[0035] In this embodiment, in order to convert the current signal (4-20mA) generated by the current generating module into a voltage signal, the simplest and most effective resistance sampling is adopted, that is, 4-20mA current is passed through a resistor to generate voltage. The maximum value of this resistor (sampling resistor RS) can be 5V / 20mA=250Ω, where 5V is the maximum sampling voltage of the back-end voltage sampling circuit. Here, 5V is used as an example, but it is not limited to 5V, and other maximum voltages are also applicable. In order to make the conversion accuracy high enough, a 1% precision resistor is used, and RS in this application takes a value of 249Ω.

[0036] Furthermore, the operational amplifier module includes an operational amplifier U1, a positive input terminal of the operational amplifier U1 is connected to the inflow terminal of the sampling current through a balancing resistor R1, and a negative input terminal of the operational amplifier U1 is respectively connected to the voltage isolation module and the operational amplifier negative feedback module.

[0037] In order to solve the problem of no high impedance isolation between input and output and poor stability, a high impedance circuit is introduced between input and output, that is, an amplifier circuit based on an operational amplifier is used to achieve input and output isolation. The function of the balancing resistor R1 is to eliminate zero drift, and its value is generally tens of kilo-ohms.

[0038] Furthermore, the operational amplifier negative feedback module includes a resistor Rf and a resistor R2, wherein the first end of the resistor Rf is connected to the output end of the operational amplifier U1, the second end of the resistor Rf is connected to the first end of the resistor R2, the reverse input end of the operational amplifier U1 and the voltage isolation module, and the second end of the resistor R2 is connected to the outflow end of the sampling current and grounded. Adding a negative feedback link improves the stability of signal conversion.

[0039] Furthermore, the voltage isolation module is a resistor R5, a first end of the resistor R5 is connected to the inverting input end of the operational amplifier U1, and a second end of the resistor R5 is connected to the voltage bias module.

[0040] In order to solve the problem that the unfixed voltages at pins 2 and 3 of the operational amplifier U1 are affected by the fixed voltage division of the positive input power supply +15V, a voltage isolation circuit is introduced. In this embodiment, a resistor R5 is added between the two circuits for isolation.

[0041] Furthermore, the voltage bias module includes a resistor R3 and a resistor R4, a first end of the resistor R3 is connected to a power supply, a second end of the resistor R3 is connected to a second end of the resistor R5 and a first end of the resistor R4, and a second end of the resistor R4 is grounded.

[0042] Furthermore, in order to make 4mA correspond to 0V, the function relationship can be constructed: U0 = k*(Iin-4). In this function, when Iin = 4mA, U0 = 0V; in order to make U0 = 5V when Iin = 20mA, k must be set to 5 / (20-4) = 0.3125, so the function structure is: U0 = 0.3125*(Iin-4) = 0.3125Iin-1.25. U0 is the output voltage of the operational amplifier U1. In order to realize this functional relationship, a common-mode proportional amplifier (the ultra-low offset OP07 operational amplifier is selected) is constructed, and the dual power supply is ±15V.

[0043] Furthermore, in order to solve the problem of "-1.25V" in the constructor U0=0.3125Iin-1.25, a +1.25V signal is input from the inverting terminal of the operational amplifier OP07. The +1.25V voltage can be obtained by dividing the +15V voltage.

[0044] To verify that the circuit in the present invention can achieve 4-20mA to 0-5V, that is, U0 = 0.3125Iin-1.25, we made the following calculations:

[0045] According to the virtual open and virtual short characteristics of the operational amplifier, Iin*RS=Ui=V2=V3;

[0046] Wherein, Iin is the sampling current, V2 is the reverse input terminal voltage of the operational amplifier U1, and V3 is the forward input terminal voltage of the operational amplifier U1.

[0047] Apply Kirchhoff's first law to list the node formed by resistors R2, R5 and Rf:

[0048] (U0-V2) / Rf=V2 / R2+(V2-V1) / R5 (1)

[0049] Wherein, V1 is the voltage of the node between the resistor R4 and the resistor R3 in the voltage bias module.

[0050] According to Ohm's law:

[0051] V2=Iin*RS (2)

[0052] The voltage division principle of series resistors is:

[0053] V1=15*R4 / (R3+R4) (3)

[0054] Combining formulas (1) to (3): we get:

[0055] U0=[1+Rf / R2+Rf / R5]*RS*Iin-15Rf*R4 / [R5*(R3+R4)] (4)

[0056] To make (1+Rf / R2+Rf / R5)*RS=0.3125 in formula (4), and to minimize the influence of R2 and Rf on the voltage divider resistors R3 and R4, R2 and R5 take the largest possible positive value, take R5=200kΩ, the inverting terminal resistor R2=100kΩ, and the maximum input current of the operational amplifier U1 does not exceed 5 / 100000=50uA. Substituting R2=100kΩ, R5=200kΩ and RS=249Ω into formula (4), we get:

[0057] Rf=17KΩ (5)

[0058] Substituting equation (5) into 15Rf*R4 / [R5*(R3+R4)] in equation (4), we get:

[0059] 15Rf*R4 / [R5*(R3+R4)]=1.25, so R4=50.3Ω.

[0060] From the above, it can be concluded that the component parameters involved in this invention patent are all calculable.

[0061] The present invention adds a group of operational amplifier negative feedback circuits, a group of voltage bias circuits and a group of voltage isolation circuits on the basis of the traditional resistance sampling circuit, so as to accurately convert the 4-20mA standard signal into a voltage signal and realize the reliable detection of the zero signal.

[0062] Furthermore, the power supply is the positive input power supply of the operational amplifier U1, so that the voltage bias module directly uses the power supply of the operational amplifier U1.

[0063] Although the present invention is described herein with reference to the illustrative embodiments of the present invention, the above embodiments are only preferred embodiments of the present invention, and the embodiments of the present invention are not limited to the above embodiments. It should be understood that those skilled in the art can design many other modifications and embodiments, which will fall within the scope and spirit of the principles disclosed in this application.

Claims

1. A 4-20mA signal detection circuit, characterized in that: include: A current sampling module is configured to collect sampled current and convert it into a voltage signal; An operational amplifier module, configured to amplify the voltage signal and input the amplified voltage signal into a voltage sampling circuit at the rear end; an operational amplifier negative feedback module, configured to add negative feedback to the operational amplifier module; A voltage bias module, configured to provide a bias voltage to the reverse input terminal of the operational amplifier module; The voltage isolation module is configured to be connected to the voltage bias module, the operational amplifier negative feedback module and the reverse input terminal of the operational amplifier module respectively.

2. A 4-20mA signal detection circuit according to claim 1, characterized in that: The current sampling module includes a sampling resistor RS, and two ends of the resistor RS are respectively connected to an inflow end and an outflow end of the sampling current.

3. A 4-20mA signal detection circuit according to claim 2, characterized in that: The operational amplifier module includes an operational amplifier, a positive input terminal of the operational amplifier is connected to the inflow terminal of the sampling current through a balancing resistor R1, and a negative input terminal of the operational amplifier is respectively connected to the voltage isolation module and the operational amplifier negative feedback module.

4. A 4-20mA signal detection circuit according to claim 3, characterized in that: The operational amplifier negative feedback module includes a resistor Rf and a resistor R2, wherein the first end of the resistor Rf is connected to the output end of the operational amplifier, the second end of the resistor Rf is connected to the first end of the resistor R2, the reverse input end of the operational amplifier and the voltage isolation module, and the second end of the resistor R2 is connected to the outflow end of the sampling current and is grounded.

5. A 4-20mA signal detection circuit according to claim 4, characterized in that: The voltage isolation module is a resistor R5, a first end of the resistor R5 is connected to the inverting input end of the operational amplifier, and a second end of the resistor R5 is connected to the voltage bias module.

6. A 4-20mA signal detection circuit according to claim 5, characterized in that: The voltage bias module includes a resistor R3 and a resistor R4, a first end of the resistor R3 is connected to a power supply, a second end of the resistor R3 is connected to a second end of the resistor R5 and a first end of the resistor R4, and a second end of the resistor R4 is grounded.

7. A 4-20mA signal detection circuit according to claim 6, characterized in that: The power supply is a positive input power supply of the operational amplifier.

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