DE3512180C2 - Vibration magnetometer - Google Patents

Description

The invention relates to a vibration magnetometer a frame-shaped measuring coil for placement in a measuring magnetic field connected to a vibrator is, the vibrations in the measuring coil in the magnetic field according to the changes in each of the Measuring coil detected magnetic field induced measuring voltage evaluated according to size and phase to the vibrations becomes.

Such a swing frame sensor is from DE-AS 12 94 550 known. With this vibration magnetometer a rigid measuring coil and connected to it Excitation coil oscillates elastically in a permanent arranged magnetic field. With their electrical vibration Excitation creates a detectable in the measuring coil Measurement signal that the tangential component of the magnetic field strength in the center of vibration that corresponds to the surface of magnetic materials or in air gaps there are magnets that are to be measured.

DD 93601 is also a transversal one Vibration magnetometer known to consist of a moving and one on a magnetic sample to be measured fastening measuring coil, the measuring signals in one Compensation circuit can be evaluated.

Furthermore, a measuring arrangement is known from US Pat. No. 4,298,772 known a magnetostrictive plate, on the sides  Excitation windings arranged for magnetostriction waves are their arrival times at any place the plate is determined with a measuring coil, whereby the respective local position of the measuring coil on the plate is determined.

It is an object of the invention to provide a vibration magnetometer to create a high sensitivity with a simple structure Lichkeit and directionality, so that it is also vectorial earth field measurement is suitable.

This task is based on the in the preamble of Claim 1 specified features by the in the characterizing Part of claim 1 specified features solved.

Special embodiments of the invention are Subject of the subclaims.  

In principle there are two ways of measuring.

First, the measurement with a transducer, one after the other brought into three perpendicular positions which measures three directional vectors that then using a computer to calculate the result serving field vector.

Second, the measurement with three arranged perpendicular to each other neten converters, which simultaneously the three directional vector measure the other, also by means of the computer of the other Calculate the resulting field vector.

The purpose of this invention was originally the vectorial le detection of the earth's magnetic field in a grid arrangement over land, so z. B. biologically effective Störun conditions such as underground water courses or strong fracture zones in the underground etc.

The storage of larger quantities compared to the Um different minerals, the other magnetic Characteristics can cause local disturbances of the otherwise homogeneous earth's magnetic field and thus measure be cash.

It was important to do it quickly and with as much effort as possible  low expenditure of technical equipment and energy to enable vectorial magnetic field measurement.

The measured values should be prepared so that immediately or later a clear grid analysis by computer is possible.

These requirements are met with my arrangement, large areas even through systematic explora tion z. B. by flying over or underwater towing could be detected.

It is also possible with this swing frame sensor Lich to build an electronic compass.

Such a compass would have the advantage of being practical no time needed to level, its time constant can be set as required using software in the computer program bar. Another important advantage would be that interference e.g. B. by ship magnetism through suitable programs can be corrected on the software side so that magnetic corrections are eliminated.

After all, software can still do many services such as B. calculation of standard deviations of the course, Calculation and evaluation of the roll and yaw movements or even the automatic, depending on the position Consideration of geological discrepancies in An be spoken. All measured values can be auto retrieved and processed mathematically from a computer will.

Of course, the arrangement is also a technical magnet fields measurable. The magnetic field to be measured does not have to be be rational, its permissible rate of change but must not exceed a maximum size that in general, however, mainly due to the data rate of the computer is limited.

Magnetic field sensors based on the Hall principle are known,  Field plates (resistance principle) and rotating coils (Dynamo principle).

The procedure presented here is basically dyna mixed, with high frequencies and with the decisive Advantage that no mechanically moving parts and none Sliding contacts are necessary.

The procedure described below is in a test device fully realized, the function is confirmed.

The device was also tested in the field, for operation only a small NC accumulator is required. In addition is a Program created with the raster measurement values from rows processed measurements and then viewed graphically be displayed.

It is important that you can use the Ge advises to determine the field direction and field strength very quickly can, depending on the direction, the maximum of the meas looking for tension.

The test device consists of two units, the measuring head with the dimensions 5 × 10 × 12 cm³ and the and the electronics with similar volume.

However, the device can be constructed much smaller.

The operation of the swing frame sensor follows so.

As is known in one, a magnetic flux turns around closing coil induces an electrical voltage the number of turns and the change over time is proportional to the magnetic flux, or mathema shown table:

N = number of turns
Φ = magnetic flux
e = induction voltage

If one carries out the magnetic induction "B" and the area "A" wrapped around the coil, so one can write with Φ = B · A.

As you can see, the tension depends on the time Lichen change in magnetic induction and of Change in the wrapped area.

Is the magnetic field stationary or quasi sta tionary, so dB / dt = 0 and it will

d. H. "e" is proportional to the magnetic induction "B" and proportional to the rate of area change "dA / dt".

With my principle, the surfaces are now according to the invention Change achieved in that a closed swing frame made of non-ferrous metal for periodic bending speed is excited, which is one of its mechanical reso corresponds to the frequency and is characterized by a suitable return coupling procedure in connection with an electronic Oscillator circuit and an electromechanical converter independently excited.  

In my sample construction is an electromechanical wall A piezo oxide is used.

The bending vibrations of the swing frame are so excited that they are in the direction of the spanned by the swing frame Area, which affects the effective area the area spanned by the oscillating frame periodically changes in sync with the exciting frequency.

The swing frame alone now acts as a coil a turn, which is also the primary coil of a coreless transformer acts.

A second located within the swing frame surface Coil with "N" turns acts as a secondary coil on wel cher the induced voltage is removed. This indu graced voltage is first ver in an amplifier strengthens and then fed to a synchronous detector, where the Amount and the polarity of the voltage can be determined. This signed voltage is already the input size for the computer, it can be queried automatically or can also be entered manually.

To get all the data for the calculation of the vectorial mag To get netfeldes, only the three need to be vertical directional components measured to each other the.

The arrangement with a mechanically vibrating frame is particularly suitable because, firstly, no mechanical Wear occurs and therefore no maintenance is required is such. The transducer can be completely non-egg can be encapsulated in metal, making it resistant to bedding is protected against environmental influences.

Second, the resonant frequency of the vibrator is through its geometrical dimensions definable, relative big. This is a trouble-free simple reinforcement of the induced voltage possible.

In my model construction there is a rectangular swing frame  with the external dimensions 80 mm × 50 mm and a width of 9.3 mm and a material thickness of 4 mm made of aluminum det. The excited natural frequency is practically accurate 3000 Hz.

The type of vibration excited is a little more complicated however, the theoretical description agrees very well the measured values.

Figure 1 shows the basic arrangement of my vibrator and dashed lines the extreme vibration conditions shown greatly enlarged.

However, many other modes are possible and useful. Other swing frame shapes are also possible it is also possible to isolate the swing frame from an isolie renden material z. B. to produce quartz glass and on ner surface to apply the secondary coil z. B. on vaping. In this form, the swing frame and the Piezoelectric transducers can be created integrated.

The theoretical description of the swing frame leads to the vibration behavior of a bar with a length of "L", which is fixed on both sides, as shown in Figure 2.

The rod is z. B. excited with its fourth eigenvalue condition. You have to imagine this rod folded into a rectangle, with the piezo oxide transducer engaging in the middle of the rod and the middle piece forming a short side of the rectangle according to Figure 1. In the middle of the opposite side of the rectangle, the two firmly clamped ends of the straight rod collide. This then results in the vibration curve according to Figure 1.

The equation of motion of the undamped, fixed on both sides clamped rod is the differential equation

ω = deflection
x = location
t = time
γ = specific weight of the rod material
F = area of the bar
E = dynamic modulus of elasticity of the rod material
I = area moment of inertia
g = gravitational acceleration

The solution to the deflection "ω" of this equation can be written in product form, with only one factor from time "t" and the second factor only from location "x" is dependent.

Eq. 5 ω (x, t) = W (x) · T (t)

are in it

Eq. 6 W (x) = C₁ · cos (ax) + C₂ · sin (ax) + C₃ · cosh (ax) + C₄ · sinh (ax)

and

Eq. 7 T (t) = A · cosωt + B · sinωt

with the coefficient "a" where

is.

Using the boundary condition W = 0 and W ′ = 0 on fixed clamped ends results.

Eq. 8 W (O) = 0 = C₁ + C₃

Eq. 9 W ′ (O) = 0 = C₂ + C₄

Eq. 10 W (L) = 0 = C₁ · (cosa · l-cosha · l) + C₂ · (sina · L-sinha · L)

Eq. 11 W ′ (L) = 0 = -C₁ · a · (sina · L + sinha · L) + C₂ · a · (cosa · L-cosha · L)

this ultimately results in the condition equation

respectively

This equation has as many solutions as we like is the fifth solution

Eq. 14 Ω₅ = a₅L = 14.137165

Now the fifth natural frequency of the transducer can be used be determined to

In this specific case, this leads to

F = 9.3 x 4 mm²
g = 9810 mm / s²
E = 7000 kg / mm³
I = 9.3 x 4 mm⁴ / 12
γ = 2.85 kg / 10⁶ mm³
L = 244 mm.

The measured value of the excited frequency of 3000 Hz deviates from the theoretically expected value f = 3028 Hz only insignificantly.

In order to determine the change in area of the swing frame men, the local function of the deflection W (x) must be considered will.

The equation is a determination equation for C₂ if the deflection by the piezo oxide transducer on the Place x = L / 2 with C₀.

C₀ in turn is due to the control voltage on the piezo oxide and determined its electrodynamic properties.

So that will

which results in

Eq. 21 C₂ = C₀ · 1.1764 · 10³

and thus

As can be seen in Figure 2, the area under the stretched vibrating rod is characterized by three positive maxima and two negative maxima or two positive maxima and three negative maxima, depending on the phase position.

The intregal via the location function W (x) in the two ex Tremen vibration states is depending on the phase position once greater than "0" thus positive and once less than "0" is negative. So the maximum area change is equal to twice the value of the integral over W (x).

in the concrete structure with L = 244mm

Eq. 24 A = C₀ · 4.06 · 10⁴mm

The used piezo oxide from Valvo of the material type PXE 5 with a thickness of 3 mm works practically mecha nisch unloaded on a high-resistance system in resonance and thus follows the equation

Eq. 25 S = d · E₁

the relationship between electrical and mechani sizes.

"S" here is the relative change in thickness C₀ / D,
"d" is the piezoelectric charge constant and
"E₁" is the electric field strength U / D

with the control voltage "U".

With

can be written

"C₀" in equation Eq. 24 used and "U" through U₀ · sinω₅ · t replaced leads to

with a peak drive voltage of approx. 300V towards A (t)

Eq. 29 A (t) = 4.677 mm² · sinω₅ · t

and

Eq. 30 in Eq. 3 used finally results in the by the  Vibrating frame induced voltage "e" that the magneti rule induction "B" is proportional. "N" is in the model construction with 100 realized.

The magnetic field strength "H" is near earth driving

The magnetic induction "B" and the field strength "H" are linked by the equation

Eq. 32 B = µ _r · µ _o · H

where µr for air = 1 and the field constant µo is

The sizes in Eq. 32 used result

d. H. it becomes a voltage amount of approximately 0.168 mV testifies when the magnetic field passes through the perpendicular the area spanned by the swing frame.

The induced voltage "e" is opposite the frame vibration shifted by 90 ° in phase.

If the passage angle deviates by "β" from 90 °, then the effective induction

Eq. 34 B (β) = B · cosβ

At Eq. 34 you can see that the generated voltage "e" finally sign of the projected size B (β) is dependent, ie represents the vector "B".

The voltage "e" proportional to the vector "B" is ver strengthens and synchronously demodulates, so that with earth field measurements relative output voltage values in my model Range -5V U 5V can be generated. These tensions are suitable with the help of analog-digital converters auto to operate data processing.

It should be noted that e.g. B. a triangular swing frame, which is excited in the 3rd harmonic at the same Surface works about 15 times more effectively than the described rectangular swing frame.

In conclusion it should be said that the swing frame also in a reverse effect can be operated.

For this purpose, the coil is supplied with an alternating voltage strikes, causing an alternating magnet in the oscillating frame field is now generated by its retroactive effect (Mutual inductance) the oscillating frame to mechanical Stimulates bending vibrations.

Claims (12)

1. Vibration magnetometer with a frame-shaped measuring coil for placement in a magnetic field to be measured, which is connected to a vibrator, the measuring voltage induced in the measuring coil by its vibrations in the magnetic field according to the changes in the magnetic field detected by the measuring coil depending on size and phase position the vibrations are evaluated, characterized in that the frame measuring coil is capable of such frame bending vibrations in the frame plane, in which the surface area of the frame changes periodically in accordance with vibrations, and in that the vibrator is vibrationally coupled to the frame for such frame bending vibrations and with the resonance frequency the frame bending vibration is excited. 2. Vibration magnetometer according to claim 1, characterized characterized in that the frame measuring coil self-supporting or connected to an electrically non-conductive support frame is. 3. Vibration magnetometer according to claim 1 or 2, characterized characterized in that the frame measuring coil with a voltage is fed, the frequency of the frame bending vibration resonance corresponds, and this itself as a vibrator works. 4. Vibration magnetometer according to one of the preceding Claims, characterized in that the frame measuring coil so dimensioned and so excited that their Frame bending vibration an odd number, in particular five, has vibration nodes.   5. Vibration magnetometer according to claim 4, characterized characterized in that the frame measuring coil with a rectangular support frame is connected, the 4 mm thick Aluminum with side dimensions of 80 mm and 50 mm and one Width of 9.3 mm. 6. Vibration magnetometer according to claim 5, characterized characterized in that the support frame in one Vibration antinode with a piezoelectric transducer as Vibrator is coupled, which is excited at about 3000 Hz. 7. Vibration magnetometer according to one of claims 1 to 4, characterized in that the frame measuring coil on a Carrier made of electrically insulating material, in particular Quartz glass. 8. Vibration magnetometer according to claim 7, characterized characterized in that the carrier as a piezoelectric Transducer for the formation of vibrations in the frame level is trained. 9. Vibration magnetometer according to claim 4, characterized characterized in that the frame measuring coil and / or her Carrier frame is triangular and in the 3rd Harmonics to the bending vibrations is excited. 10. Vibration magnetometer according to one of the preceding Claims, characterized in that the frame measuring coil with an electrically conductive magnetically non-conductive Shield is encapsulated. 11. Vibration magnetometer according to claim 1, characterized characterized in that the vibrator from an electrical Generator is acted upon, the input side with the Frame measuring coil is fed back.   12. Vibration magnetometer according to one of the preceding Claims, characterized in that two or three of the Frame measuring coils each aligned orthogonally to each other are arranged and their signal evaluation for magnetic field The vector is displayed in terms of phase and size.