CN101349572A - Method for designing electromagnetic shielding body of liquid capacitance-type sensor - Google Patents

Abstract

The invention relates to a design process of a shield for resisting electro-magnetic interference in a liquid capacitance type sensor, which comprises firstly determining the frequency spectrum of an interference source needing to be shielded, secondly determining the sensitivity characteristic of the sensor, conducting the sensitivity test for a device which is actually interfered to obtain the sensitivity characteristic curve line, thirdly calculating the round hole size of the shield according to the relative theories of cut-off waveguide, the actual shielding requirements and the working environment request of the sensor, fourthly building a simulation model in a computer, and conducting value emulation calculation, comparing the shielding results, and finally instructing to process real objects according to the calculated results. The invention solves the problem of the electro-magnetic interference of the sensor, and facilitates the sensor to work normally under the electro-magnetic radiation, and the shield which is designed has light weight and excellent oil permeability, and can not affect the normal working.

Description

The method for designing of electromagnetic shielding body of liquid capacitance-type sensor

Technical field

The present invention is a kind of method for designing of liquid capacitance-type sensor shield, belongs to the application foundation field.

Background technology

Liquid capacitance-type sensor is to measure the isoparametric a kind of sensor of liquid volume.The capacitance of capacitive transducer is very small, must convert correspondent voltage, electric current or frequency signal to by metering circuit.The kind of change-over circuit is a lot, at present normal adopt bridge diagram, resonant circuit, FM circuit and operational amplification circuit etc. are arranged.

But along with the develop rapidly of science and technology, the electromagnetic environment of electronic device works becomes increasingly complex.Liquid capacitance-type sensor is owing to its structure, and electromagnetic radiation is responsive to external world, and when promptly outside electromagnetic interference being arranged, operate as normal has been subjected to influence, and the parameter of measurement is no longer accurate, brings very big difficulty to surveying work.Fig. 3 is the simple structure synoptic diagram that is subjected to interference capacitors formula sensor, can produce stronger induction current on two pole plates of electromagnetic radiation lower sensor.Again because working sensor in liquid environment, so the present invention is directed to this situation, proposes a kind of effective shield design.

Summary of the invention

The objective of the invention is to propose a kind of method for designing of shield of the anti-electromagnetic interference (EMI) at liquid capacitance-type sensor at this technical matters.This method is dealt with problems easy to be clear, and the engineering exploitativeness is good, has certain extendability.

The objective of the invention is to realize by following technical measures:

A kind of method for designing of liquid capacitance-type sensor anti-electromagnetic interference shielding body is characterized in that may further comprise the steps:

A kind of method for designing of liquid capacitance-type sensor anti-electromagnetic interference shielding body is characterized in that may further comprise the steps:

1) definite spectral characteristic that needs the interference source of shielding;

2) determine the sensitivity characteristic of sensor, the disturbed equipment of reality is carried out the susceptibility test, draw the sensitivity characteristic curve;

3) according to the correlation theory of cut-off waveguide,, calculate the circular hole size of shield according to the shielding demand of reality and the working environment requirement of sensor;

4) in computing machine, set up realistic model, the line number of going forward side by side value simulation calculating, comparison shield effectiveness;

5), instruct processing in kind in conjunction with the aforementioned calculation result.

Further:

In described step 1),, at this moment adopt the antenna of different reception frequency ranges and frequency and the amplitudes that receiver is measured the actual interference source by the definite spectral characteristic that needs the interference source of shielding of the method for reality test.

In described step 2) in, according to the regulation of the mark RS103 of army, use emitting antenna, power amplifier, signal source, electrometer of field strength, the field intensity probe of required frequency that the disturbed equipment of reality is carried out the susceptibility test.

In described step 3), design the metal tub shield that has circular hole according to the physical features of sensor, according to the size that designs circular hole by the character of waveguide.

The invention has the advantages that:

1 has solved capacitance type sensor is subjected to electromagnetic interference problem.This shield can be resisted outside electromagnetic interference for capacitance type sensor, makes it not influence the operate as normal of liquid sensor simultaneously again in operate as normal under the electromagnetic radiation.

2 provide stronger shield effectiveness.Because this structure is taked not increase on the basis of main box wall thickness, lengthening makes shield effectiveness increase greatly by the method for the length of waveguide, has improved its engineering range of application.

3 is in light weight.Many electronic equipments all have very harsh weight requirement, so the wall thickness of this shield can do very thinly, and can adopt the synthetic surfaces copper coating, have so just reduced the weight of shield greatly.

4 exploitativenesses are good, are easy to engineering practice.

Description of drawings

Fig. 1 is the surveying instrument arrangenent diagram;

Fig. 2 is the shield structure master drawing of design;

Fig. 3 is the simple structure synoptic diagram that is subjected to interference capacitors formula sensor;

Fig. 4 represents the preceding several TE moulds of circular waveguide and the cutoff frequency of TM mould;

Fig. 5 is illustrated in the surface field distribution situation in the shield under the electromagnetic radiation.

Embodiment

The invention will be further described below in conjunction with the drawings and specific embodiments.

The present invention is a kind of method for designing of anti-electromagnetic interference shielding body of liquid capacitance-type sensor, based on the correlation theory of cut-off waveguide, comprises mathematical modeling and numerical operation, carries out the appropriate design of shield according to result of calculation.The concrete steps of this method are:

(1) determines the spectral characteristic of interference source: spectral characteristic that need to determine the interference source of shielding, adopt actual method of testing to determine the characteristic of airborne interference source, method of testing has many kinds, can adopt the different antenna of frequency range and frequency and the amplitudes that receiver is measured the actual interference source of receiving;

(2) determine the sensitivity characteristic of capacitance type sensor.The disturbed equipment of reality is carried out the susceptibility test, simulate the sensitivity characteristic curve; During test, generally can carry out according to the target regulation RS103 of army.Promptly use the formation such as emitting antenna, power amplifier, signal source, electrometer of field strength, field intensity probe of required frequency, the instrument layout when specifically measuring as shown in Figure 1.

According to the discrete point of test gained, simulate the sensitivity curve of capacitance type sensor system.

(3) according to the sensitivity curve of reality test, determine the shield effectiveness value that designed shield should provide, promptly should reach the shield effectiveness value that difference between the standard value is determined shield according to the minimum of responsive thresholding and system are actual.

Because liquid capacitance-type sensor works in the liquid environment, so design the square shield that has circular hole according to the physical features of sensor.Circular hole on the metal cylinder can better make liquid smooth and easy by and can not influence the work of sensor.

The size of circular hole mainly is according to the character by waveguide.Now correlation theory is presented below:

Suppose that electromagnetic wave is along e ^{-j β z}Propagate, wherein ${\mathrm{<figure-callout\; id="2"\; label="k\; c"\; filenames="A20081011936800082.png"\; state="\{\{state\}\}">k</figure-callout>}}_c^{}=k^2-{\mathrm{\&beta;}}^2.$ We know circular waveguide support TE, TM type electromagnetic wave, for TE mould, E _z=0, and H _zBe wave equation:

${\&dtri;}^2{H}_z+k^2{H}_z=0---\left(1\right)$

Separate.If H _z(ρ, φ, z)=h _z(ρ, φ) e ^{-j β z}, then formula (1) can be expressed as with cylindrical-coordinate system:

$(\frac{{\&PartialD;}^2}{{\&PartialD;\mathrm{\&rho;}}^2}+\frac{1}{\mathrm{\&rho;}}\frac{\&PartialD;}{\&PartialD;\mathrm{\&rho;}}+\frac{1}{{\mathrm{\&rho;}}^2}\frac{{\&PartialD;}^2}{{\&PartialD;\mathrm{\&phi;}}^2}+k_c^2)h_z(\mathrm{\&rho;},\mathrm{\&phi;})=0---\left(2\right)$

Can get like this:

h _z(ρ，φ)＝[Asin(nφ)+Bcos(nφ)]J _n(k _cρ)(3)

By the boundary condition E on the wave guide wall _Tan=0, and the relational expression of the transverse field of circular waveguide in cylindrical-coordinate system can get:

$E_{\mathrm{\&phi;}}(\mathrm{\&rho;},\mathrm{\&phi;},z)=\frac{\mathrm{j\&omega;\&mu;}}{k_c}[A\sin \left(\mathrm{n\&phi;}\right)+B\cos \left(\mathrm{n\&phi;}\right)]J_n^{\&prime;}{\left(k_c\mathrm{\&rho;}\right)e}^{-\mathrm{j\&beta;z}}---\left(4\right)$

J ' _n(k _cρ) be J _nTo the difference quotient of its independent variable, if J ' _n(x) root is defined as p ' _Nm, then can end wave number $k_c=\frac{p_{\mathrm{nm}}^{\&prime;}}{a},$ A is the radius of circular waveguide.Table 1 is the p ' of circular waveguide TE mould first few items _NmValue.

N	p′ n1	p′ n2	p′ n3
				0	3.832	7.016	10.174
1	1.841	5.331	8.536
				2	3.054	6.706	9.970

Table 1

Can get cutoff frequency is:

$f_{\mathrm{Cmn}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="k\; c"\; filenames="A20081011936800082.png"\; state="\{\{state\}\}">k</figure-callout>}}_c}{2\mathrm{\&pi;}\sqrt{\mathrm{\&mu;\&epsiv;}}}=\frac{p_{\mathrm{nm}}^{\&prime;}}{2\mathrm{\&pi;a}\sqrt{\mathrm{\&mu;\&epsiv;}}}---\left(5\right)$

In like manner for the TM mould, its cutoff frequency is:

$f_{\mathrm{Cmn}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="k\; c"\; filenames="A20081011936800082.png"\; state="\{\{state\}\}">k</figure-callout>}}_c}{2\mathrm{\&pi;}\sqrt{\mathrm{\&mu;\&epsiv;}}}=\frac{{\mathrm{<figure-callout\; id="2"\; label="p\; nm"\; filenames="A20081011936800082.png"\; state="\{\{state\}\}">p</figure-callout>}}_{\mathrm{nm}}}{2\mathrm{\&pi;a}\sqrt{\mathrm{\&mu;\&epsiv;}}}---\left(6\right)$

Fig. 4 is the relative value of the cutoff frequency of TE and TM mould.As seen from the figure, TE ₁₁The cutoff frequency of mould is minimum.Because TE ₁₁The cutoff frequency of mould is minimum, is benchmark with its cutoff frequency when designing then.After trying to achieve cutoff frequency, calculate shield effectiveness, can judge according to the shield effectiveness that this step calculates whether designed shield satisfies actual shielding demand according to following formula.

$\mathrm{SE}=1.823\&times;f_{\mathrm{Cmn}}\&times;L\&times;{10}^{-9}\sqrt{1-{\left(\frac{f}{f_{\mathrm{Cmn}}}\right)}^2}---\left(7\right)$

Wherein L is the length of waveguide, and f is the undesired signal frequency.

Except that above-mentioned principle, also have following three principles during the metal cylinder perforate:

A, when opening the hole with homalographic, replace square opening with circular hole as far as possible;

Rule is wanted in the arrangement of b, hole array;

C, metal wire try not to pass from the hole.

For reducing the thickness of shield, only need the thickness of elongated pore seam, and do not spend the thickness that increases whole shield, such structure has but reduced the weight and the volume of shield after satisfying same shielding requirements.The structure of the shield of designing promptly has certain thickness rectangular parallelepiped cavity as shown in Figure 2, has circular apertures in cavity sidewalls, and the length in hole can be given prominence to sidewall surfaces, thereby increases the shield effectiveness of shield.

(4) in computing machine, set up realistic model, the line number of going forward side by side value simulation calculating, the comparison shield effectiveness, Fig. 5 is shield effectiveness figure.Shown among the figure under electromagnetic radiation, the distribution plan of shield internal electric field, wherein light areas is the encirclement space of shield, shield effectiveness is very remarkable as can be seen.Used radiation source is a plane wave among the figure, frequency 300M, and horizontal polarization, amplitude are 1v/m.

(5), instruct processing in kind in conjunction with the aforementioned calculation result.

Claims (5)

1. the method for designing of a liquid capacitance-type sensor anti-electromagnetic interference shielding body is characterized in that may further comprise the steps:

1) definite spectral characteristic that needs the interference source of shielding;

2) determine the sensitivity characteristic of sensor, the disturbed equipment of reality is carried out the susceptibility test, draw the sensitivity characteristic curve;

3) according to the correlation theory of cut-off waveguide, according to the shielding demand of reality and the working environment requirement of sensor, calculate shield the circular hole size;

4) in computing machine, set up realistic model, the line number of going forward side by side value simulation calculating, comparison shield effectiveness;

5), instruct processing in kind in conjunction with the aforementioned calculation result.

2. the method for designing of liquid capacitance-type sensor anti-electromagnetic interference shielding body as claimed in claim 1 is characterized in that:

In described step 1), by the definite spectral characteristic that needs the interference source of shielding of the method for reality test.

3. the method for designing of liquid capacitance-type sensor anti-electromagnetic interference shielding body as claimed in claim 2 is characterized in that:

In described step 1), adopt the different antenna of frequency range and frequency and the amplitudes that receiver is measured the actual interference source of receiving.

4. the method for designing of liquid capacitance-type sensor anti-electromagnetic interference shielding body as claimed in claim 1 is characterized in that:

In described step 2) in, according to the regulation of the mark RS103 of army, use emitting antenna, power amplifier, signal source, electrometer of field strength, the field intensity probe of required frequency that the disturbed equipment of reality is carried out the susceptibility test.

5. the method for designing of liquid capacitance-type sensor anti-electromagnetic interference shielding body as claimed in claim 1 is characterized in that:

In described step 3), design the metal tub shield that has circular hole according to the physical features of sensor, design the size of circular hole and the length in hole according to character by waveguide.

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