阅读说明：本技术 适用于朗缪尔探针的非线性微电流采集装置及方法 (Nonlinear micro-current acquisition device and method suitable for Langmuir probe ) 是由 杜清府 王进 郭怀龙 张清和 李建泉 邢赞阳 于 2021-01-25 设计创作，主要内容包括：本发明公开了适用于朗缪尔探针的非线性微电流采集装置及方法,包括：若干个并列的朗缪尔探针；每个朗缪尔探针的第一端布设在电离层的等离子体中；每个朗缪尔探针的第二端均与非线性采集放大电路连接；非线性采集放大电路还与扫描电压加载电路的输出端连接；扫描电压加载电路的输入端还与微控制单元连接；非线性采集放大电路还与低通滤波电路的输入端连接；低通滤波电路的输出端与微控制单元MCU连接；微控制单元MCU与卫星中心机通讯接口连接。(The invention discloses a nonlinear micro-current acquisition device and method suitable for Langmuir probes, which comprises the following steps: a plurality of juxtaposed Langmuir probes; a first end of each Langmuir probe is disposed in the plasma in the ionosphere; the second end of each Langmuir probe is connected with a nonlinear acquisition amplifying circuit; the nonlinear acquisition amplifying circuit is also connected with the output end of the scanning voltage loading circuit; the input end of the scanning voltage loading circuit is also connected with the micro control unit; the nonlinear acquisition amplifying circuit is also connected with the input end of the low-pass filter circuit; the output end of the low-pass filter circuit is connected with the MCU; and the micro control unit MCU is connected with the communication interface of the satellite central unit.)

1. Nonlinear micro-current acquisition device suitable for Langmuir probe, characterized by includes:

a plurality of juxtaposed Langmuir probes;

a first end of each Langmuir probe is disposed in the plasma in the ionosphere;

the second end of each Langmuir probe is connected with a nonlinear acquisition amplifying circuit;

the nonlinear acquisition amplifying circuit is also connected with the output end of the scanning voltage loading circuit; the input end of the scanning voltage loading circuit is also connected with the micro control unit;

the nonlinear acquisition amplifying circuit is also connected with the input end of the low-pass filter circuit;

the output end of the low-pass filter circuit is connected with the MCU;

and the micro control unit MCU is connected with the communication interface of the satellite central unit.

2. The non-linear micro-current collection device suitable for use with a langmuir probe as claimed in claim 1, wherein the operating principle comprises:

the voltage output by a digital-to-analog conversion circuit of the micro control unit MCU is amplified by a scanning voltage loading circuit and then is applied to the Langmuir probe, and the scanning voltage loading circuit applies scanning voltage to the probe;

the potential presented on the langmuir probe attracts the charged particles to strike the surface of the langmuir probe, which causes the langmuir probe potential to be non-uniform, causing a current to flow, the magnitude and direction of the current on the langmuir probe reflecting the physical properties of the plasma;

the nonlinear acquisition amplifying circuit acquires current signals of the corresponding Langmuir probes and converts the current signals into voltage signals;

the nonlinear acquisition amplifying circuit transmits the voltage signal to a low-pass filter circuit for filtering;

the low-pass filter circuit sends the filtered signals to an analog-to-digital conversion circuit for analog-to-digital conversion;

and the micro control unit MCU sends the signals converted by the analog-to-digital conversion circuit to a satellite central machine communication interface.

3. The non-linear microcurrent collection device for langmuir probes as claimed in claim 1, wherein the non-linear collection amplifying circuit comprises:

the first logarithm operation circuit and the second logarithm operation circuit are connected in parallel;

the output end of the first logarithmic operation circuit and the output end of the second logarithmic operation circuit are both connected with the subtraction proportional operation circuit;

the input end of the first logarithmic operation circuit is connected with the Langmuir probe;

the input end of the second logarithmic operation circuit is connected with the reference voltage V through a resistor_REFConnected to generate a standard current.

4. The nonlinear microcurrent collection apparatus for langmuir probes as claimed in claim 3, wherein the first logarithmic operation circuit comprises:

a first operational amplifier, an inverting input of the first operational amplifier being connected to the Langmuir probe;

the positive input end of the first operational amplifier is connected with the output end of the scanning voltage loading circuit;

the positive input end of the first operational amplifier is grounded through a capacitor;

the output end of the first operational amplifier is connected with the emitting electrode of the first bipolar PNP triode; the collector of the first bipolar PNP triode is connected with the reverse input end of the first operational amplifier through a resistor; the base electrode of the first bipolar PNP triode is connected with the scanning voltage loading circuit;

the inverting input end of the first operational amplifier is connected with a reference voltage V through a compensation resistor_REFAnd (4) connecting.

5. The nonlinear microcurrent collection apparatus for langmuir probes as claimed in claim 3, wherein the second logarithmic operation circuit comprises:

the inverting input end of the second operational amplifier is connected with the reference voltage source through a series resistor;

the positive input end of the second operational amplifier is connected with the output end of the scanning voltage loading circuit;

the positive input end of the second operational amplifier is grounded through a capacitor;

the output end of the second operational amplifier is connected with the emitter of the second bipolar PNP triode; the collector of the second bipolar PNP triode is connected with the reverse input end of the second operational amplifier through a resistor; and the base electrode of the second bipolar PNP triode is connected with the output end of the scanning voltage loading circuit.

6. The nonlinear microcurrent collection apparatus for langmuir probes as claimed in claim 3, wherein said subtraction scaling circuit comprises:

the inverting input end of the third operational amplifier is connected with the output end of the first operational amplifier through a resistor R;

the positive input end of the third operational amplifier is connected with the output end of the second operational amplifier through a resistor R;

the positive input end of the third operational amplifier passes through a resistor R_fGround, the resistor R_fIs connected with a capacitor C in parallel;

the output end of the third operational amplifier is connected with the input end of the low-pass filter circuit;

the output end of the third operational amplifier passes through a resistor R_fThe inverting input end of the third operational amplifier is connected;

and the output end of the third operational amplifier is connected with the inverting input end of the third operational amplifier through a capacitor C.

7. The nonlinear microcurrent collection apparatus for langmuir probes as claimed in claim 6, wherein said low pass filter circuit is a third order low pass filter circuit;

the third-order low-pass filter circuit specifically comprises: a fourth operational amplifier;

the inverting input end of the fourth operational amplifier is connected with the input end of the low-pass filter through resistors R4, R2 and R1 which are connected in series;

the positive input end of the fourth operational amplifier is grounded through a resistor R5;

the connection point between the R2 and the R1 is grounded through a capacitor C1;

the connection point between the R2 and the R4 is grounded through a capacitor C2;

the output end of the fourth operational amplifier is connected with the inverting input end of the fourth operational amplifier through a capacitor C3;

the output terminal of the fourth operational amplifier is connected to the connection point between R2 and R4 through a resistor R3.

8. The nonlinear micro-current collection device for langmuir probe as claimed in claim 1, wherein the nonlinear collection amplifying circuit operates according to a principle comprising:

the first operational amplifier converts the micro-current into a voltage signal, wherein the logarithmic conversion of a PN junction and the linear conversion superposition of a resistor are utilized to play a role in nonlinear amplification; in a smaller signal area, the current amplification factor is large, and the signal-to-noise ratio of a tiny signal is increased; in a larger current region, the amplification factor is relatively smaller, and the acquisition end is ensured to have a larger dynamic range.

9. The device for acquiring nonlinear microcurrent for langmuir probe as claimed in claim 6, wherein the low pass filter circuit operates on the principle comprising:

an active three-order low-pass filter network is formed by basic electronic devices such as a resistor, a capacitor and an operational amplifier, and the power consumption of the system is reduced by adopting a low-power operational amplifier; the proper cut-off frequency is ensured by setting the resistance-capacitance parameters through analyzing the circuit principle.

10. The nonlinear micro-current acquisition method suitable for the Langmuir probe is characterized by comprising the following steps of:

the micro control unit MCU outputs small-amplitude scanning voltage through the built-in DA module; the small-amplitude scanning voltage is 0-3.3V;

the small-amplitude scanning voltage is processed by the scanning voltage loading circuit and then is changed into a scanning voltage of-10V to 10V; applying a scanning voltage of-10V to the Langmuir probe;

the potential presented on the langmuir probe attracts the charged particles to strike the surface of the langmuir probe, which causes the langmuir probe potential to be non-uniform, causing a current to flow, the magnitude and direction of the current on the langmuir probe reflecting the physical properties of the plasma;

the nonlinear acquisition amplifying circuit acquires current signals of all Langmuir probes and converts the current signals into voltage signals;

the nonlinear acquisition amplifying circuit transmits the voltage signal to a low-pass filter circuit for filtering;

the low-pass filter circuit sends the filtered signals to an analog-to-digital conversion circuit for analog-to-digital conversion;

and the micro control unit MCU sends the signals converted by the analog-to-digital conversion circuit to a satellite central machine communication interface.

Technical Field

The application relates to the technical field of ionospheric plasma diagnosis, in particular to a nonlinear micro-current acquisition device and method suitable for Langmuir probes.

Background

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

With the increase of altitude, the atmosphere becomes thinner gradually, and gas molecules are partially ionized by the extreme ultraviolet radiation to form an ionized layer in a plasma state. Due to the difference of the solar radiation intensity and the influence of other various environmental factors, plasmas in the ionized layer present irregular shapes in a space scale and physical characteristic changes in a time scale. The plasma cloud in the ionosphere varies in spatial scale from about a few meters to several kilometers. The change of the space weather is reflected by the change of the scale of the plasma cloud and the change of the physical parameters of the plasma, and the change of the space weather has great significance to the production and the life of the modern society; in addition, the ionosphere influences satellite-to-ground radio wave communication; therefore, the method has important significance for the research of plasma in the ionized layer.

Langmuir probe, as an immersion type plasma diagnostic method, is widely used for plasma parameter diagnosis because of its advantages of simple design and reliable result. Langmuir probe theory, supported by Orbital-Motion-Limited (Orbital-Motion-Limited) theory proposed by Moth-Smith and Langmuir in 1926, is widely used in laboratory plasma and spatial plasma diagnostics. The OML theory elaborates upon the collision-free motion trajectories of electrons and ions around cylindrical and spherical probes. Langmuir probes have also been successfully used in multiple acquisition satellites, especially in microsatellites, as the main load for spatial plasma diagnostics. A multi-Langmuir probe system (m-NLP) developed by Norwegian Oslo university and loaded with a first scientific exploration satellite NorSat-1 in Norwegian to successfully launch in 2017, wherein the system works in a near-earth orbit at a distance of 600KM from the earth and can realize current sampling of at most 1 kHz; in the QB50 international cooperation project dominated by the European Union, an m-NLP improved version developed by Oslo university carries EX-Alta 1 and Hoopoe cube star to work at a lower orbit of 380KM, and the improved version system can realize micro-current collection of 1nA-2.2 muA at a sampling frequency of at most 255 Hz. In space plasmaOn the aspect of detection, the development in China is relatively late, but in recent years, some happy achievements are obtained. A scientific research test satellite 'Zhang Heng I' independently developed by China is loaded with a Long March Ding launch vehicle in 2018, 2 months and 2 days, and a Langmuir probe system serving as one of effective loads of the Zhang Heng satellite I can realize 5 multiplied by 10⁸-1×10¹³m^-3Electron density measurement in a range, but because the Zhang Heng I satellite does not belong to a micro satellite column and column, the probe surface area is large, and the acquisition current is large, the acquisition system is not suitable for a micro satellite Langmuir probe system; in addition, taiwan "central university" team integrates a Planar Langmuir Probe (PLP), a Retarding Potential Analyzer (RPA), an Ion Drift Meter (IDM) and an Ion Trap (IT) into a set of detection system, so that the whole system can measure electron density and temperature, ion density and temperature and ion drift velocity, and the whole set of system achieves better results. The Langmuir probe system can also carry a sounding rocket to detect the vertical structure of the ionosphere. In China, the Langmuir probe system which is developed by Chinese academy teams and carries a meridian engineering sounding rocket makes outstanding contribution to the detection of the vertical and high fine structure of the low latitude ionosphere in China.

With the increasing launching plans of microsatellites, the loading gap applicable to microsatellites is large. The plasma detection system developed and suitable for the microsatellite can be used for ionospheric plasma high-spatial-resolution sampling, provides scientific data for ionospheric scientific research and space weather forecast, and meets the increasing microsatellite emission requirements.

Disclosure of Invention

In order to overcome the defects of the prior art, the application provides a nonlinear micro-current acquisition device and method suitable for Langmuir probes;

in a first aspect, the present application provides a nonlinear microcurrent collection device suitable for use with langmuir probes;

nonlinear micro-current collection device suitable for Langmuir probe includes:

a plurality of juxtaposed Langmuir probes;

a first end of each Langmuir probe is disposed in the plasma in the ionosphere;

the second end of each Langmuir probe is connected with a nonlinear acquisition amplifying circuit;

the nonlinear acquisition amplifying circuit is also connected with the output end of the scanning voltage loading circuit; the input end of the scanning voltage loading circuit is also connected with a Micro Control Unit (MCU);

the nonlinear acquisition amplifying circuit is also connected with the input end of the low-pass filter circuit;

the output end of the low-pass filter circuit is connected with the MCU;

and the micro control unit MCU is connected with the communication interface of the satellite central unit.

In a second aspect, the present application provides a nonlinear microcurrent acquisition method suitable for langmuir probes;

the micro control unit MCU outputs small-amplitude scanning voltage through the built-in DA module; the small-amplitude scanning voltage is 0-3.3V;

the small-amplitude scanning voltage is processed by the scanning voltage loading circuit and then is changed into a scanning voltage of-10V to 10V; applying a scanning voltage of-10V to the Langmuir probe;

the potential presented on the langmuir probe attracts the charged particles to strike the surface of the langmuir probe, which causes the langmuir probe potential to be non-uniform, causing a current to flow, the magnitude and direction of the current on the langmuir probe reflecting the physical properties of the plasma;

the nonlinear acquisition amplifying circuit acquires current signals of all Langmuir probes and converts the current signals into voltage signals;

the nonlinear acquisition amplifying circuit transmits the voltage signal to a low-pass filter circuit for filtering;

the low-pass filter circuit sends the filtered signals to an analog-to-digital conversion circuit for analog-to-digital conversion;

and the micro control unit MCU sends the signals converted by the analog-to-digital conversion circuit to a satellite central machine communication interface.

Compared with the prior art, the beneficial effects of this application are:

the satellite running in the ionized layer has higher movement speed and different specification sizes, and in addition, because the plasma density of the ionized layer has larger difference along with the altitude, the longitude and latitude change and the day and night difference, the current acquired by utilizing the Langmuir probe has two characteristics of large dynamic range and small current amplitude. Aiming at the characteristics of the Langmuir probe, the invention develops the nonlinear micro-current acquisition system suitable for the Langmuir probe, can realize high-precision acquisition of micro-current signals, has a larger dynamic range, has the working power consumption of the whole system less than 1W, has light weight, and meets the requirement of micro-satellite load. The system is matched with a high-speed A/D conversion system, so that the measurement of high spatial resolution can be realized; and the signal is subjected to third-order low-pass filtering, so that the signal-to-noise ratio of the acquired signal is obviously improved. Data calibration experiments prove that the collection and processing of the whole set of system on the tiny signals meet the requirements of theoretical design, and the 10-degree-of-freedom micro-scale micro-⁸m^-3-10¹⁴m^-3High precision diagnosis of density range plasma. The system performs experiments of various density ranges in the space plasma simulation vacuum chamber, and the result proves that the developed plasma diagnosis system can realize the diagnosis of the plasma with large dynamic density range and can meet the requirements on ionosphere plasma diagnosis.

Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate embodiments of the application and, together with the description, serve to explain the application and are not intended to limit the application.

Figure 1 is a typical I-V curve acquired with the langmuir probe of the first embodiment;

figure 2 is a block diagram of a cylindrical langmuir probe of the first embodiment;

FIG. 3 is a schematic diagram of the electron density electron temperature of the ionosphere at an altitude of 500KM at 21: 00 at 6/21/2020;

FIG. 4(a) is a comparison of an ideal I-V curve and a measured I-V curve across a PN junction of a first embodiment;

FIG. 4(b) is a comparison of the acquisition end range of the first embodiment under linear amplification and logarithmic amplification at the same analog-to-digital conversion voltage;

FIG. 5 is a standard logarithmic operation circuit of the first embodiment;

FIG. 6 is a schematic diagram of a first embodiment of a non-linear amplification circuit based on logarithm operation;

FIG. 7 is a schematic diagram of a third order low pass filter circuit of the first embodiment;

FIG. 8(a) is a log amplitude-frequency curve of the filter of the first embodiment;

FIG. 8(b) is a logarithmic phase-frequency curve of the filter of the first embodiment;

FIG. 8(c) is a graph showing the effect of the input signal and the output signal of the filter according to the first embodiment;

FIG. 9 is a functional block diagram of the system of the first embodiment;

FIG. 10 is a calibration comparison diagram of the front-end circuit of the first embodiment;

FIG. 11 is a schematic diagram of an experimental environment of the first embodiment;

FIGS. 12(a) -12 (c) are the voltage acquisition at the A/D terminal of the first embodiment;

fig. 12(d) -fig. 12(f) are comparison graphs of the acquired data of the first embodiment.

Detailed Description

It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments according to the present application. As used herein, the singular forms "a", "an", and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise, and it should be understood that the terms "comprises" and "comprising", and any variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.

The embodiments and features of the embodiments of the present invention may be combined with each other without conflict.

Example one

The embodiment provides a nonlinear micro-current acquisition device suitable for Langmuir probes;

as shown in fig. 9, the nonlinear microcurrent collection apparatus suitable for langmuir probe comprises:

a plurality of juxtaposed Langmuir probes;

a first end of each Langmuir probe is disposed in the plasma in the ionosphere;

the second end of each Langmuir probe is connected with a nonlinear acquisition amplifying circuit;

the nonlinear acquisition amplifying circuit is also connected with the output end of the scanning voltage loading circuit; the input end of the scanning voltage loading circuit is also connected with a Micro Control Unit (MCU);

the nonlinear acquisition amplifying circuit is also connected with the input end of the low-pass filter circuit;

the output end of the low-pass filter circuit is connected with the MCU;

and the micro control unit MCU is connected with the communication interface of the satellite central unit.

Further, the nonlinear micro-current acquisition device suitable for the Langmuir probe has the working principle that:

the voltage output by a digital-to-analog conversion circuit of the micro control unit MCU is amplified by a scanning voltage loading circuit and then is applied to the Langmuir probe, and the scanning voltage loading circuit applies scanning voltage to the probe;

the potential presented on the langmuir probe attracts the charged particles to strike the surface of the langmuir probe, which causes the langmuir probe potential to be non-uniform, causing a current to flow, the magnitude and direction of the current on the langmuir probe reflecting the physical properties of the plasma;

the nonlinear acquisition amplifying circuit acquires current signals of the corresponding Langmuir probes and converts the current signals into voltage signals;

the nonlinear acquisition amplifying circuit transmits the voltage signal to a low-pass filter circuit for filtering;

the low-pass filter circuit sends the filtered signals to an analog-to-digital conversion circuit for analog-to-digital conversion;

and the micro control unit MCU sends the signals converted by the analog-to-digital conversion circuit to a satellite central machine communication interface.

Further, a non-linear acquisition amplification circuit comprising:

the first logarithm operation circuit and the second logarithm operation circuit are connected in parallel;

the output end of the first logarithmic operation circuit and the output end of the second logarithmic operation circuit are both connected with the subtraction proportional operation circuit;

the input end of the first logarithmic operation circuit is connected with the Langmuir probe;

the input end of the second logarithmic operation circuit is connected with the reference voltage V through a resistor_REFConnected to generate a standard current.

Further, the first logarithmic operation circuit specifically includes:

a first operational amplifier, an inverting input of the first operational amplifier being connected to the Langmuir probe;

the positive input end of the first operational amplifier is connected with the output end of the scanning voltage loading circuit;

the positive input end of the first operational amplifier is grounded through a capacitor;

the output end of the first operational amplifier is connected with the emitting electrode of the first bipolar PNP triode; the collector of the first bipolar PNP triode is connected with the reverse input end of the first operational amplifier through a resistor; the base electrode of the first bipolar PNP triode is connected with the scanning voltage loading circuit.

The inverting input end of the first operational amplifier is connected with a reference voltage V through a compensation resistor_REFAnd (4) connecting.

Reference voltage:

V_REF＝V_R-V_DA，

wherein, V_DAThe output voltage of one channel of the digital-to-analog conversion module is controlled by the MCU.

Further, the second logarithm operation circuit specifically includes:

a second operational amplifier having an inverting input terminal connected to the reference voltage V via a series resistor_REFConnecting;

the positive input end of the second operational amplifier is connected with the output end of the scanning voltage loading circuit;

the positive input end of the second operational amplifier is grounded through a capacitor;

the output end of the second operational amplifier is connected with the emitter of the second bipolar PNP triode; the collector of the second bipolar PNP triode is connected with the reverse input end of the second operational amplifier through a resistor; and the base electrode of the second bipolar PNP triode is connected with the output end of the scanning voltage loading circuit.

Further, the subtraction proportion operation circuit specifically includes:

the inverting input end of the third operational amplifier is connected with the output end of the first operational amplifier through a resistor R;

the positive input end of the third operational amplifier is connected with the output end of the second operational amplifier through a resistor R;

the positive input end of the third operational amplifier passes through a resistor R_fGround, the resistor R_fIs connected with a capacitor C in parallel;

the output end of the third operational amplifier is connected with the input end of the low-pass filter circuit;

the output end of the third operational amplifier passes through a resistor R_fThe inverting input end of the third operational amplifier is connected;

and the output end of the third operational amplifier is connected with the inverting input end of the third operational amplifier through a capacitor C.

Further, the nonlinear acquisition amplifying circuit has the working principle that:

the first operational amplifier converts the micro-current into a voltage signal, wherein the function of nonlinear amplification is achieved by the superposition of logarithmic conversion of PN junction and linear conversion of resistance. In a smaller signal area, the current amplification factor is large, and the signal-to-noise ratio of a tiny signal is increased; in a larger current region, the amplification factor is relatively smaller, and the acquisition end is ensured to have a larger dynamic range.

In order to eliminate the obvious influence of temperature on PN junction reverse saturation current, a standard current is converted through an I-V conversion circuit with the same structure consisting of a second operational amplifier, and I is eliminated through the operation of a subtraction proportional circuit_SAnd the acquisition precision of the circuit under different temperature conditions is improved.

Furthermore, the low-pass filter circuit adopts a third-order low-pass filter circuit;

the third-order low-pass filter circuit specifically comprises: a fourth operational amplifier;

the inverting input end of the fourth operational amplifier is connected with the input end of the low-pass filter through resistors R4, R2 and R1 which are connected in series;

the positive input end of the fourth operational amplifier is grounded through a resistor R5;

the connection point between the R2 and the R1 is grounded through a capacitor C1;

the connection point between the R2 and the R4 is grounded through a capacitor C2;

the output end of the fourth operational amplifier is connected with the inverting input end of the fourth operational amplifier through a capacitor C3;

the output terminal of the fourth operational amplifier is connected to the connection point between R2 and R4 through a resistor R3.

Further, the low-pass filter circuit works according to the principle that:

an active three-order low-pass filter network is formed by basic electronic devices such as a resistor, a capacitor and an operational amplifier, and the power consumption of the system is reduced by adopting a low-power operational amplifier; the proper cut-off frequency is ensured by setting the resistance-capacitance parameters through analyzing the circuit principle.

The invention mainly introduces the realization of a micro-current acquisition system applied to Langmuir probes and experimental test results thereof.

Langmuir probe diagnostic: the specific method for plasma diagnosis by using the Langmuir probe is to apply a scanning voltage on the probe through a control circuit, wherein the potential on the probe attracts charged particles to impact the surface of the probe, the potential of a metal body is uneven due to the impact of the charged particles, current flows, and the magnitude of the current on the probe reflects the physical properties of the plasma. Therefore, the accuracy of the probe system for collecting the current directly influences the accuracy of the calculated plasma physical parameters.

The I-V characteristic of a typical langmuir probe with current varying with scan voltage is shown in figure 1. The ion current is magnified for clarity of display.

The curve can be divided by two dividing points V_f,V_PThe ion saturation area, the transition area and the electron saturation area are sequentially arranged from left to right. Demarcation point V_fThe probe current is a point with 0, the ion current and the electron current collected by the point are equal in magnitude and opposite in direction, and the sum is 0; demarcation point V_PIs the inflection point of the I-V curve, and the voltage corresponding to this point is the plasma potential.

In the transition region of the curve, there is an exponential increasing trend between the probe current and the scanning voltage, as shown in formula (1).

Wherein I_PIs probe current, I_eThe current generated for the probe to absorb electrons, I_iThe current generated for the probe to absorb ions, a, represents the probe surface area. In the curve transition region, the collected electron current is exponentially and rapidly increased along with the increase of the applied voltage of the probe, and the collected ion current is extremely small compared with the electron current, so that the ion current is in the curve transition regionThe curve transition region may be thought of as the probe current is entirely caused by the attraction of electrons striking the probe.

Taking logarithm to both ends of the equation:

the simplification is as follows:

it can be seen that the natural logarithm of the current collected in the transition region has a linear relationship with the voltage, and the slopeIs the electron temperature T_eThe reciprocal of (c). Thus, the electron temperature can be derived from the transition zone data.

At the boundary point V between the transition region and the electron saturation region_pFrom the OML theory, the electron density can be calculated from the current and voltage at the inflection point, according to equation (4), in combination with the electron temperature.

Langmuir probe design: the plasma macroscopically exhibits electrical neutrality, but there is an electric field in the vicinity of each charged particle, which electric field, when shielded by the fields of surrounding particles, exhibits electrical neutrality in a certain space. This shielding is called the Debye shielding, and the size of the space occupied by the particle shielding field is called the Debye length λ_B. Within the shielding field generated by the particles, i.e. the radius around the particles is lambda_BThe three-dimensional sphere space of (2) does not satisfy the condition of plasma electric neutrality.

The plasma boundary has a sheath layer, which can be divided into three parts from near to far from the wall (plasma boundary), a pre-sheath layer and a plasma, the inside of the sheath layer does not satisfy the condition of electric neutrality, and the thickness r of the sheath layer_sAnd debye length λ_BOf the same order of magnitude.

To reduce the effect of the debye mask on the probe's absorption of particles, the characteristic dimension of the probe should be as small as possible below the debye length according to the limited orbital motion theory proposed by langmuir, and the characteristic dimension of the probe is calculated as shown in equation (5).

Wherein r is_pDenotes the radius of the cylindrical probe,/_pThe probe length is indicated.

Determining the length l of the probe by taking the probe size one order of magnitude smaller than the Debye length as a design condition_p25mm and radius r_pUnder the condition of 0.25mm, the characteristic size of the cylindrical probe can be calculated to be about 1.1mm, and the requirement of the limited orbit theory is met.

A schematic of the cylindrical langmuir probe design is shown in figure 2.

Electronic performance requirements: the ionospheric plasma density and electron temperature distribution over the earth, as shown in figure 3, has a conventional variation in plasma density ranging from 10 when the satellite is operating in a near earth orbit above 300km¹⁰m^-3To 10¹³m^-3The range of change in probe current is estimated to be, in combination with the range of change in electron temperature and the characteristic size of the probe: 1nA-6 uA.

When a langmuir probe system is mounted with a microsatellite, the satellite surface conductor serves as a counter electrode of the langmuir probe. It is generally believed that when the conductor area is greater than 100 times the probe surface area, a relatively accurate electron temperature can be obtained, and that when the conductor area is greater than 1000 times the probe surface area, an accurate electron density measurement can only be obtained. Thus, the surface area of the satellite limits the probe surface area from being too large, resulting in a small current being collected.

In order to realize accurate acquisition and amplification processing of the current in such a large dynamic range, a common linear amplification circuit is difficult to complete tasks, and a set of nonlinear micro-current acquisition system is developed by comprehensively considering the characteristics of various signal processing circuits and the characteristics of plasma parameters.

Designing a circuit system: aiming at the two characteristics that the Langmuir probe acquires very small current under the condition of low-density plasma and has large dynamic range along with the change of the plasma density, a nonlinear amplification circuit based on logarithmic operation is designed to carry out pre-processing before analog-to-digital conversion on the current.

Current-voltage characteristic of diode: different from the volt-ampere characteristic of a resistor, the voltage at two ends of the diode shows logarithmic change along with the change of current, and logarithmic operation has the effect of obviously compressing the dynamic range and is sensitive to the change of tiny current, so that the logarithmic amplification circuit is applied to facilitate the compression of the dynamic range of the collected current, and the sensitivity of the system to the collection of the tiny current is improved. The relationship between the current of the diode and the voltage across the diode is shown in equation (6).

In the formula I_SIs reverse saturation current of diode, U_TU is the voltage equivalent of temperature at normal temperature (T300K)_T26 mV. When u is_D>>U_TThen, the above equation can be approximated as:

namely:

as shown by equation (8), there is a logarithmic relationship between the voltage and current of the diode. Therefore, a logarithmic operation circuit can be constituted by using diodes.

The standard current-voltage characteristic curve of the diode and the current-voltage characteristic curve obtained by applying a scanning voltage to the PN junction of the 9012 triode are shown in fig. 4 (a).

As shown in fig. 4(b), when the analog-to-digital conversion chip with the acquisition range of 0-5V is used for voltage acquisition, assuming that the amplification factor is 100,000, the range of the range that can be involved in logarithmic amplification is 1pA-1.5mA, and the range that can be reached by linear amplification is 1nA-50 μ a, so that a larger dynamic range of the acquisition end and better acquisition accuracy can be obtained by using the logarithmic amplification processing circuit compared with the linear amplification.

Designing a nonlinear micro-current amplifying circuit: a standard logarithmic op-amp applicable to practical circuits is shown in figure 5. The basic logarithmic operation circuit composed of diodes can only accord with logarithmic relation in a small range, and a bipolar triode is connected into a diode form to be used as a feedback branch circuit, so that a large working range can be obtained.

The output u of the circuit_oAnd an input current i_DThe relationship between them is:

wherein I_SIs the reverse saturation current of the diode to overcome the temperature pair I_SCan design a complementary compensation circuit to remove I by operation_SThe influence of (c).

The compensated circuit is shown in fig. 6.

The output u of the circuit_oAnd an input current i_DThe relationship between them is:

wherein the content of the first and second substances,R₁＝R₂，V_Rfor the scan voltage applied to the probe, C10 nF is to cancel high frequency noise, u_o2For the second operational amplifier output voltage u_o1Is the output voltage of the first operational amplifier, k is the linear amplification factor, V_RFor the scanning voltage applied to the probe, i_TZTo collectTo the probe current, R₁A resistor R connected in series with the triode for the first operational amplifier feedback loop₂The same process is carried out; u shape_TIs equivalent of PN junction voltage, I_SIs a reverse saturation current; i.e. i_REF1The current flowing through the two feedback loops for the operational amplifier can be V_REFSetting; r₂And a resistor is connected in series with the feedback loop of the second operational amplifier.

As can be seen from the expression, the I pair caused by temperature change can be counteracted by the operation of the compensation circuit_SThe influence of (c). As shown in fig. 4(a), when the current passing through the diode is greater than 2 μ a, the slope of the voltage change at two ends of the diode is small, which indicates that the voltage change is too gentle with the increase of the collecting current, and the problem of poor collecting precision for a large current is possibly caused.

When scanning voltage who applys on the probe is the negative value, can attract ion striking probe, gather ionic current, because ion and electron electrical property are opposite, probe acquisition current flow direction is opposite, needs the system to measure reverse current, in order to solve when the current flow direction changes, the PN junction of triode ends and leads to the problem that can not effectively measure, increases compensating circuit to the realization is gathered ionic current. OP at acquisition terminal₁The reverse input end of the voltage regulator is connected with a compensation resistor R₀₂To V_REFBy adjusting V_REFThe effect of moving the acquisition zero in the negative direction can be realized. By setting R₀₁＝R₀₂So that i_REF1＝i_REF2＝i_REFCompensated output voltage u_oAnd an input current i_TZThe relationship between them is:

design of the low-pass filter: the langmuir probe operates in a space plasma or vacuum chamber, while the acquisition circuitry is typically under normal conditions inside the satellite or outside the vacuum chamber. Therefore, a long connecting wire is required to connect the micro-current collected by the probe to the collecting circuit board. Due to interference of radio waves in the environment, high-frequency noise is superimposed on the acquired direct current by a long connecting line, so that the amplified signal has large noise, and low-pass filtering needs to be performed on the amplified signal in order to improve the signal-to-noise ratio of the acquired signal.

The integrated low-pass filter circuit chip has better performance, the cut-off frequency can be configured by a simple peripheral circuit, but the integrated low-pass filter circuit has the defect of higher power consumption, a set of single-operational-amplifier third-order low-pass filters is used in consideration of the strict requirement of satellite payload on the power consumption, and a circuit schematic diagram is shown in fig. 7.

Establishing a differential equation for each node of the filter circuit, and obtaining a transfer function by taking Laplace transform:

the filter inverts the input and output in addition to attenuating high frequency noise. WhereinBy configuring R₁,R₂,R₃Let k equal to 1.

The logarithmic amplitude-frequency characteristic and the logarithmic phase-frequency characteristic of the filter circuit after the configuration are shown in fig. 8(a) and 8 (b). In order to perform performance test on the oscilloscope, gaussian white noise, uniform white noise and high-frequency sinusoidal noise are sequentially superimposed on the low-frequency sinusoidal signal of 9000 data points, and the pair before and after the signal passes through the low-pass filter is shown in fig. 8(c), so that the visible filter has the capability of remarkably limiting the high-frequency noise.

The relation between the output signal after passing through the filter and the input current is as follows:

wherein the content of the first and second substances,the maximum value of the ion current can be collected. As can be seen from formula (13), when i_TZ→-i_REFWhen u is turned on_o→ infinity. V can be adjusted by the microcontroller of the acquisition system_REFAnd carrying out dynamic adjustment to adapt to the acquisition of plasma ion currents with different concentrations. When collecting ionic current, i_TZ<At 0, u_o<0, to accommodate an analog to digital converter that can only convert positive voltages, the output voltage can be raised by V using an addition operation_bAnd then analog-to-digital conversion is carried out.

The relationship between the output and input current of the whole system is shown as equation (14).

The overall design of the system is as follows: the probe current signal is converted into a voltage signal by the nonlinear conditioning circuit and is directly input into the analog-to-digital conversion module for voltage acquisition.

Data calibration: and manufacturing a printed circuit board by using the designed principle circuit, and calibrating the acquired data of the circuit board after the selected devices are welded.

And calibrating data of the acquisition circuit by using a German technology B2912A Source table (Source/Measure Unit-SMU). The SMU supports dual-channel output acquisition and can carry out current measurement of a flying safety level. Setting a channel I of the SMU into a current source mode, acquiring the output voltage of the front-end circuit by the channel II, carrying out scanning calibration on the channel I from-50 nA to 50 mu A, and acquiring the output voltage V of the circuit_OAnd a scanning current I_TZThe relationship pair of (c) is shown in fig. 10.

It can be seen that the nonlinear acquisition circuit has higher sensitivity when acquiring weak signals, and has a larger acquisition range, and the calibration result has stronger consistency with theoretical calculation.

Space plasma simulation vacuum chamber: the vacuum chamber arranged in the plasma detection laboratory can reach 10^-5And Pa, utilizing the cathode filament to load current to emit electrons, and filling high-concentration argon into the cabin to excite the plasma. The whole system can simulate the ionized layer low-density plasma, and the lowest stable plasma density which can be maintained is about 1 x 10¹¹/m³. The experimental environment is schematically shown in fig. 11.

Experimental results and analysis: experiment the experiment was carried out at a pressure of about 0.09Pa, and the plasma density was adjusted by changing the current of the discharge lamp filament while keeping the gas pressure constant, respectively at a plasma density of 10¹¹/m³,10¹²/m³,10¹³/m³Multiple sets of I-V curve acquisition experiments were performed over the range and the results were compared in the same experimental environment, with the comparison data shown in fig. 12(a) -12 (f).

Fig. 12(a) is a graph showing the collected a/D port voltage, and fig. 12(b) is a graph showing a comparison between the SMU collected data and the collected Current obtained by nca (nonlinear Current Acquisition system). From top to bottom, respectively, at a plasma density of 10¹³/m³,10¹²/m³,10¹¹/m³The results of the following 9 experimental groups. The data obtained by the comparative experiment can be used for obtaining the plasma concentration of 10¹³/m³,10¹²/m³In magnitude, the data collected by the NCA is slightly larger than the data collected by the SMU at low plasma density (10)¹¹/m³) The method shows opposite results, but the overall error is not large, and the error change is small in the range of high, medium and low densities. The correlation and normalized sum of the squared residuals of the two curves were calculated under different experimental conditions, as shown in table 1.

The method introduces evaluation index residual square sum of fitting curves to evaluate consistency errors of the two curves, and the calculation method of the residual square sum comprises the following steps:wherein x_iAnd y_iThe two groups of comparison data correspond to data points respectively, and the smaller the sum of the squares of the residual errors is, the better the consistency of the two curves is proved. However, because the data dimensions of each group are different, the magnitude of the residual error cannot be visually represented by simply comparing the sum of the squares of the residual errors, and therefore the data are normalized:whereinIs the mean of the set of data points.

TABLE 1 evaluation values of contrast data collected at different densities

Aiming at satellite-borne ionospheric plasma diagnosis, the invention designs a novel large-dynamic-range nonlinear Langmuir probe acquisition system. From theoretical design to experimental verification, the system is proved to be capable of well utilizing Langmuir probe pair 10⁸m^-3-10¹⁴m^-3Plasma in the density range is diagnosed. Particularly, a complementary compensation circuit is added to realize the reverse saturation current I of the PN junction of the triode_STo achieve high-precision acquisition of the micro-current. In order to obtain a complete I-V curve, a compensation circuit is designed to realize the collection of ion current. The three-order low-pass filter circuit formed by the single operational amplifier and the resistance-capacitance device can effectively attenuate high-frequency noise doped in the direct-current signal of the probe, and the signal-to-noise ratio of the acquired signal is greatly improved. A printed circuit board is designed through circuit CAD software, and data calibration is carried out on an acquisition circuit through a high-precision source meter. Finally, experiments are carried out in a space plasma simulation vacuum chamber of a plasma detection laboratory, and the experiments are respectively carried out at 10¹¹m^-3-10¹³m^-3Experiments are carried out in the density, and better experimental results are obtained by analyzing and comparing the density with SMU (sampled measured data) collected data.

At present, the launching cost of the microsatellite is gradually reduced, and the launching quantity is increased year by year. The size is small, the power consumption is low, and the function diversification is the basic requirement of the microsatellite payload. The plasma diagnosis system developed by the invention can realize high-precision diagnosis of space plasma, is designed aiming at the payload of a microsatellite, can realize various space plasma detection tasks and meet the increasing space detection requirements in China.

Example two

The present example provides a non-linear micro-current collection method suitable for langmuir probes;

a nonlinear micro-current acquisition method suitable for Langmuir probes, comprising:

the micro control unit MCU outputs small-amplitude scanning voltage through the built-in DA module; the small-amplitude scanning voltage is 0-3.3V;

the small-amplitude scanning voltage is processed by the scanning voltage loading circuit and then is changed into a scanning voltage of-10V to 10V; applying a scanning voltage of-10V to the Langmuir probe;

the potential presented on the langmuir probe attracts the charged particles to strike the surface of the langmuir probe, which causes the langmuir probe potential to be non-uniform, causing a current to flow, the magnitude and direction of the current on the langmuir probe reflecting the physical properties of the plasma;

the nonlinear acquisition amplifying circuit acquires current signals of all Langmuir probes and converts the current signals into voltage signals;

the nonlinear acquisition amplifying circuit transmits the voltage signal to a low-pass filter circuit for filtering;

the low-pass filter circuit sends the filtered signals to an analog-to-digital conversion circuit for analog-to-digital conversion;

and the micro control unit MCU sends the signals converted by the analog-to-digital conversion circuit to a satellite central machine communication interface.

The above description is only a preferred embodiment of the present application and is not intended to limit the present application, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like made within the spirit and principle of the present application shall be included in the protection scope of the present application.