A two-wire weak strain bridge signal transmitter

Abstract

The invention discloses a two-wire weak strain bridge signal transmitter, which is suitable for converting a resistive weak strain bridge signal with extremely small output voltage change (several millivolts) and large zero offset (equivalent to or even greater than the output voltage change) into a current signal for transmission. The transmitter consists of a weak strain bridge, a differential amplifier circuit, a zero offset adjustment circuit, a V / I conversion module, an output current adjustment circuit, and an output loop. The weak voltage signal with zero offset output by the weak strain bridge is input to the first differential amplifier for primary amplification, and its output voltage is then differentially amplified with the zero offset adjustment voltage for the second stage, and its output voltage is then input to the V / I conversion module, and converted into a 4-20mA current output under the control of the output current adjustment amount, and at the same time, the V / I module provides working power to the weak strain bridge, the differential amplifier circuit, and the zero offset adjustment circuit. The invention has a simple circuit, good linearity, high cost performance, and convenient wiring.

Description

Technical Field

[0001] The invention patent relates to the field of detection technology and automation equipment. More specifically, the invention patent relates to a transmitter instrument for weak strain bridge signal conversion. Background Art

[0002] Resistive strain sensors are widely used in the measurement of force, pressure, acceleration, mass and other parameters. The basic principle is that the elastic body (elastic element, sensitive beam, etc.) produces elastic deformation under the action of external force, causing the resistance strain gauge (conversion element) pasted on its surface to also deform. After the resistance strain gauge is deformed, its resistance will change (increase or decrease), and then the corresponding measurement circuit (Huygens bridge) converts the change in resistance into a voltage signal, thereby completing the process of converting force-related parameters into electrical signals. Its typical application is electronic weighing equipment.

[0003] However, most resistive strain bridge output voltage signals have low sensitivity, small voltage changes, and large zero offsets, which are even equivalent to the voltage changes. Subsequent signal conditioning and conversion are difficult. For example, a cantilever beam strain bridge type 1kg range weighing sensor purchased on the market has a low sensitivity. When the reference current I ref =1.6mA current bridge, after testing, when the weighing weight changes from 0 to 800g, its zero point drift reaches 1.3mV, and the maximum output change is only 1.0mV.

[0004] The most widely used method in industry is 4-20mA current to transmit analog quantity, because the current signal is not easily disturbed, and the internal resistance of the current source is infinite, and the wire resistance in series in the loop will not affect the accuracy, so that hundreds of meters of transmission can be achieved. Therefore, in industry, it is usually necessary to convert the weak strain bridge signal into 4-20mA current for transmission, and the output voltage of the above-mentioned sensor changes slightly, and the zero offset is large, which undoubtedly increases the difficulty of signal conditioning and conversion. It is usually necessary to use a high-precision ADC for sampling and a microprocessor for processing, or to equip it with a three-wire processing circuit module provided by the manufacturer for conversion. Its circuit is complex, the cost is high, and the wiring is more complicated than that of a two-wire instrument.

[0005] Searching for relevant patent documents of current type transmitters, the high temperature melt automatic zeroing pressure transmitter invented by Wu Zhenghui, Long Xia and others uses a microprocessor and software programming to realize automatic adjustment of zero point and full scale and 4-20mA current output; the force balance transmitter of strain type displacement detection element invented by Wang Kechong has a relatively complex overall structure, and the signal conditioning and conversion circuit itself cannot automatically offset the zero point of the resistance strain bridge. In addition, the invention requires that the upper and lower limits of the bridge output voltage meet the quantitative relationship of 1:5 (that is, the zero point offset and the voltage change meet the quantitative relationship of 1:4), and a negative feedback closed-loop balancing device is required to complete the conversion and ensure the measurement accuracy and linearity. The patent of this invention realizes high-multiple amplification and zero point offset of weak signals through simple signal conditioning, amplification, adjustment and V / I conversion circuits, and can obtain higher conversion accuracy and linearity without high-precision AD acquisition plus digital processing or complex feedback balancing devices.

[0006] Invention patent content

[0007] The technical problem to be solved by the patent of this invention is to provide a two-wire current type transmitter, which can process, transform and output weak strain bridge measurement signals with voltage changes as small as several millivolts and zero offset voltage equivalent to the change (even the voltage change is less than the zero offset).

[0008] In order to solve the technical problem, the present invention provides a two-wire weak strain bridge signal transmitter, which is characterized in that a pure analog circuit is used to process and transform the signal. The transmitter is composed of a weak strain bridge, a differential amplifier circuit, a zero offset adjustment circuit, an output current adjustment circuit, a V / I conversion module, and an output circuit. The weak strain bridge is a resistive strain bridge, and the differential amplifier circuit is divided into two stages, both of which are composed of a low-power instrumentation amplifier AD627. The zero offset adjustment circuit is composed of a 5MΩ fixed resistor and a 1MΩ potentiometer in series, the output current adjustment circuit is composed of an adjustment resistor, and the V / I conversion module is composed of a single-core integrated 4-20mA current source chip XTR105. The output circuit is composed of an NPN transistor, a rectifier bridge to prevent reverse connection of the power supply, an overvoltage protection diode, and a voltage-stabilizing capacitor.

[0009] The output voltage of the weak strain bridge is connected to the first differential amplifier, the output end of the first differential amplifier and the output end of the zero offset adjustment circuit are both connected to the second differential amplifier, the output end of the second differential amplifier is connected to the V / I conversion module, and the output current adjustment circuit is connected to the V / I conversion module, and the output signal of the V / I conversion module is connected to the output loop.

[0010] The weak voltage signal with zero offset output by the weak strain bridge is input to the first differential amplifier for primary amplification, and its output voltage is then differentially amplified with the zero offset adjustment voltage in the second stage, and its output voltage is then input to the V / I conversion module, and converted into 4-20mA current under the control of the output current adjustment amount, and finally the power supply, rectifier bridge, V / I conversion module XTR105 and transistor BU406, load R L A current of 4 to 20 mA flows through the circuit; the zero offset and the output amplification factor can be easily adjusted through the zero offset adjustment potentiometer and the output current adjustment potentiometer, and the influence of the offset voltage and offset current of the operational amplifier on the conversion accuracy of weak signals can be eliminated.

[0011] The working power required by the weak strain bridge, differential amplifier circuit, and zero offset adjustment circuit in the transmitter is provided by the V / I conversion module, and no additional power supply is required, so pure two-wire power supply and output can be achieved; the system also has a weak strain bridge output nonlinear automatic compensation function, that is, by inputting the nonlinear compensation current provided by the V / I conversion module into one arm of the weak strain bridge through the compensation resistor, the system nonlinear automatic compensation is achieved, and the output linearity and conversion accuracy are improved; through the optimized design of the scheme, the total power consumption of the aforementioned weak strain bridge, differential amplifier circuit, and zero offset adjustment circuit is extremely low, and the total current is less than 4mA, so that a two-wire power supply and current output with 4mA as the zero point can be achieved.

[0012] Preferably, the transmitter also includes an output circuit, in which a current signal can be output as long as a 24V DC power supply is connected, and the output circuit has a diode reverse connection protection circuit, and the power supply can be connected to the circuit in any direction.

[0013] The present invention patent includes at least the following beneficial effects:

[0014] (1) For weak strain bridge measurement signals whose voltage variation is as small as several millivolts and whose zero offset voltage is equivalent to the variation (even the voltage variation is smaller than the zero offset), the present invention can effectively and accurately convert them into 4-20mA current signals for output.

[0015] (2) Pure analog circuits are used to process and transform signals, without the need for high-precision AD acquisition plus digital processing or complex feedback balancing devices. The structure is simple and the cost is low.

[0016] (3) The zero offset and output gain are easy to adjust. The amplifier offset voltage and offset current have no effect on the conversion accuracy of weak signals. It has automatic nonlinear compensation and high measurement accuracy and linearity.

[0017] (4) Two-wire power supply and output, the transmitter has no positive and negative polarity distinction, the wiring is convenient and simple, and it is convenient for remote transmission.

[0018] Other advantages, objectives and features of the present invention will be reflected in part through the following description, and in part will be understood by those skilled in the art through research and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a structural schematic diagram of the two-wire weak strain bridge signal transmitter described in the patent of this invention.

[0020] Figure 2 This is a circuit diagram of the two-wire weak strain bridge signal transmitter described in the patent of this invention.

[0021] Figure 3 (a) and (b) are the design examples without compensation resistor R LIN and compensation resistor R LIN =9.33kΩ, the relationship between the measured and output currents. DETAILED DESCRIPTION

[0022] The present invention is further described in detail below in conjunction with the accompanying drawings so that those skilled in the art can implement it according to the description.

[0023] It should be understood that the terms such as “having”, “including” and “comprising” used herein do not exclude the existence or addition of one or more other elements or combinations thereof.

[0024] The present invention provides a two-wire weak strain bridge signal transmitter, which uses a pure analog circuit to process and transform the signal. Its composition structure is as follows: Figure 1 As shown, it is composed of a weak strain bridge 1, a first differential amplifier 2, a zero offset adjustment circuit 3, a second differential amplifier 4, an output current adjustment circuit 5, a V / I conversion module 6, and an output loop 7; wherein the weak strain bridge 1 is a resistive strain bridge, the first differential amplifier 2 and the second differential amplifier 4 are both composed of a low-power instrumentation amplifier AD627, the zero offset adjustment circuit 3 is composed of a 5MΩ fixed resistor and a 1MΩ potentiometer in series, the output current adjustment circuit 5 is composed of an adjustment resistor, the V / I conversion module 6 is composed of a single-core integrated 4-20mA current source chip XTR105, and the output loop 7 is composed of an NPN transistor, a rectifier bridge to prevent reverse connection of the power supply, an overvoltage protection diode, and a voltage-stabilizing capacitor.

[0025] The output voltage of the weak strain bridge 1 is connected to the first differential amplifier 2, the output end of the first differential amplifier 2 and the output end of the zero offset adjustment circuit 3 are both connected to the second differential amplifier 4, the output end of the second differential amplifier 4 is connected to the V / I conversion module 6, and the output current adjustment circuit 5 is connected to the V / I conversion module 6, and the output signal of the V / I conversion module 6 is connected to the output loop 7.

[0026] The working principle of the transmitter is as follows: the weak voltage signal with zero offset output by the weak strain bridge 1 is input to the first differential amplifier 2 for primary amplification, and its output voltage is input to the second differential amplifier 4 together with the voltage output by the zero offset adjustment circuit 3 for second differential amplification, and its output voltage is input to the V / I conversion module 6, and is converted into 4-20mA current under the control of the output current adjustment circuit 5 and output through the output circuit 7, that is, finally, the power supply, rectifier bridge, V / I conversion module XTR105 and transistor BU406, load R L A current of 4 to 20 mA flows through the circuit; the zero offset and the output amplification factor can be conveniently adjusted through the zero offset adjustment potentiometer and the output current adjustment potentiometer.

[0027] like Figure 2 The example circuit is a two-wire transmitter for converting 4-20mA current of a cantilever beam strain bridge weighing sensor.

[0028] The maximum range of the cantilever beam strain bridge load cell used in the example is 1kg. When the reference current I ref When the bridge is powered by 1.6 mA, a weight of 0 to 800 g is applied to the cantilever beam, and the output voltage changes in the range of 1.3 to 2.3 mV, which is consistent with the characteristics of large zero offset and the measured voltage change being less than the zero offset.

[0029] The first differential amplifier 2 and the second differential amplifier 4 both use low-cost, high-precision, low-power instrumentation amplifier AD627, whose gain range is 1 to 1000 and only requires one external resistor to set the gain. The first differential amplifier performs primary amplification on the 1.3 to 2.3 mV differential voltage output by the bridge, R c As the gain adjustment resistor, a 1kΩ potentiometer in 3296 package is used. Adjust the potentiometer R c The resistance value is adjusted to about 500, so that the output voltage range to ground is about 0.65~1.15V.

[0030] The V / I conversion module 6 adopts a single-core integrated 4-20mA current source chip XTR105, which has a very low adjustment error, two 0.8mA high-precision mirror current sources and a 5.1V, 1mA output capacity reference voltage source. The two current sources are designed to be connected in parallel to provide a 1.6mA current reference to the weak strain bridge to form a current-type bridge, which can output a 1.3-2.3mV measurement signal, and the 5.1V, 1mA capacity reference voltage source is used to power two AD627 instrument amplifier circuits and zero offset adjustment circuits. In addition, the total current provided by XTR105 for the above weak strain bridge, two instrument amplifier circuits, and zero offset adjustment is less than 4mA, so that a two-wire power supply and current output with 4mA as the zero point can be achieved.

[0031] The zero offset adjustment circuit 3 is composed of a 5MΩ fixed resistor and a 1MΩ potentiometer connected in series and connected to a 5.1V reference voltage source. The center tap of the potentiometer outputs an adjustment voltage, and the adjustment voltage can vary in the range of 0 to 0.85V. This voltage can offset the 0.65V zero offset voltage with high accuracy, and the current of the loop is only 0.85μA, with extremely low power consumption.

[0032] The second differential amplifier 4 (AD627) differentially amplifies the voltage output by the first differential amplifier and the zero offset adjustment voltage, without connecting a gain setting resistor, and the amplification factor is 1. Figure 2 , 1kΩ potentiometer R CM The 0.01μF capacitor in parallel with it is connected between the bridge reference ground terminal and the 6th pin of XTR105 to form a bridge power supply loop. At the same time, the bridge reference ground terminal (i.e. R CM The left end of the bridge is connected to the 5th terminal (output reference terminal) of the second differential amplifier 4 and the 2nd terminal (input voltage reference terminal) of the V / I conversion module XTR105. The 6th terminal of the second differential amplifier 4 outputs a voltage of about 1.25 to 1.75 V to ground, that is, the 1.6 mA current passing through the bridge is at R CM The voltage drop formed on the MOSFET increases the output voltage starting point but at the same time cleverly provides the XTR105 chip with the required minimum input common-mode voltage of 1.25V.

[0033] The voltage amplified by the second differential amplifier 4 is input to the 13 terminal (+V IN ) and 2-terminal (-V IN ), XTR105 automatically converts the voltage in the common mode range of 1.25V and the differential mode range of 0 to 0.5V into a current signal starting at 4mA. The input and output relationship is:

[0034] I o =4+40·V IN / R G(1)

[0035] In the formula, I o is the output current in mA; V IN is the input differential voltage, in V; R G is the gain adjustment resistor value of the output current adjustment circuit 5, that is, XTR105, in kΩ, which can be adjusted within the range of 0 to 2 kΩ. G Adjusting it to about 1.25kΩ can set the output current range to within 4 to 20mA.

[0036] The output circuit 7 is connected to the NPN transistor Q1 (BU406) to improve its output capacity and reduce the heating of the XTR105 chip. The base of the transistor Q1 is connected to the 9th terminal of the XTR105, the emitter of Q1 is connected to the 8th terminal of the XTR105, and the collector of Q1 is connected to the 10th terminal of the XTR105 and connected to a vertex of the diode rectifier bridge. The 24V DC power supply supplies power to the chip through the current loop. At the same time, D1~D4 set in the circuit uses 1N4148 diodes to form a reverse protection rectifier bridge, which can prevent the power supply from being reversed, so that the wiring direction of the transmitter power supply can be arbitrarily exchanged, and D5 (using 36V Zener diode 1N4753) is connected to absorb surge current and play an overvoltage protection role. The 0.1μF capacitor connected to the output circuit plays a voltage stabilizing role. Finally, the power supply, rectifier bridge, XTR105 and BU406, load R L A current of 4 to 20 mA flows through the circuit composed above.

[0037] Furthermore, the XTR105 12 terminal (V LIN ) and the positive voltage terminal of the weak strain bridge output through a 20kΩ potentiometer R LIN Connect together, and the nonlinear compensation current provided by XTR105 is passed through the compensation resistor R LIN Input to the lower left arm of the weak strain bridge to automatically compensate for the nonlinear output variation of the weak strain bridge. LIN Adjusting the resistance to a suitable value can significantly improve the output linearity and output accuracy. Figure 3 (a) and (b) are the design examples (i.e., 800g range cantilever beam strain bridge weighing transmitter) without compensation resistor R LIN and adjust the compensation resistor R LIN =9.33kΩ, the changing relationship curve between the measured and output current.

[0038] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and the embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily realized. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and the illustrations shown and described here.

Claims

1. A two-wire weak strain bridge signal transmitter, characterized in that: The transmitter is composed of a weak strain bridge, a differential amplifier circuit, a zero offset adjustment circuit, an output current adjustment circuit, a V / I conversion module, and an output circuit. The weak strain bridge is a resistive strain bridge, the differential amplifier circuit is divided into two stages, both of which are composed of a low-power instrumentation amplifier AD627, the zero offset adjustment circuit is composed of a 5MΩ fixed resistor and a 1MΩ potentiometer in series, the V / I conversion module is composed of a single-core integrated 4-20mA current source chip XTR105, and the output circuit is composed of an NPN transistor BU406, a diode-formed rectifier bridge to prevent reverse connection of the power supply, an overvoltage protection diode 1N4753, and a 0.1μF voltage-stabilizing capacitor. The output voltage of the weak strain bridge is connected to the first differential amplifier, the output end of the first differential amplifier and the output end of the zero offset adjustment circuit are both connected to the second differential amplifier, the output end of the second differential amplifier is connected to the V / I conversion module, and the output current adjustment circuit is connected to the V / I conversion module, and the output signal of the V / I conversion module is connected to the output loop; The weak voltage signal with zero offset output by the weak strain bridge is input to the first differential amplifier for primary amplification, and its output voltage is then differentially amplified with the zero offset adjustment voltage in the second stage, and its output voltage is then input to the V / I conversion module, and converted into 4-20mA current under the control of the output current adjustment amount, and finally the power supply, rectifier bridge, V / I conversion module XTR105 and transistor BU406, load R L A current of 4 to 20 mA flows through the circuit.

2. The two-wire weak strain bridge signal transmitter according to claim 1 is characterized in that: The working power required by the weak strain bridge, differential amplifier circuit, and zero offset adjustment circuit is provided by the V / I conversion module. No additional power supply is required, and pure two-wire power supply and output can be achieved. In addition, the nonlinear compensation current provided by the V / I conversion module is input into one arm of the weak strain bridge through the compensation resistor, thereby achieving automatic nonlinear compensation of the system and improving output linearity and conversion accuracy.

3. The two-wire weak strain bridge signal transmitter according to claim 2 is characterized in that: The total power consumption of the weak strain bridge, differential amplifier circuit, and zero offset adjustment circuit is extremely low, and the total current is less than 4mA, thereby realizing a two-wire power supply and current output with 4mA as the zero point.

4. The two-wire weak strain bridge signal transmitter according to claim 1 is characterized in that: As long as a 24V DC power supply is connected to the output circuit, a current signal can be output. The output circuit has a diode reverse connection protection circuit, and the power supply can be connected to the circuit in any direction.

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