CN111879847A - Magnetic flux leakage detection method and detection device - Google Patents
Magnetic flux leakage detection method and detection device Download PDFInfo
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Abstract
The invention discloses a magnetic flux leakage detection method and a detection device, wherein the detection method comprises the following steps: step one, an excitation source is set, and parameter setting is carried out on the excitation source; step two, exciting a magnetic flux leakage detection module through the excitation source in the step one to generate an excitation magnetic field and generate a magnetic flux leakage detection signal; and step three, conditioning, collecting and shaping the signals generated in the step two, and uploading the signals to an upper computer. The invention has the beneficial effects that: the technical scheme has the advantages of high regulation speed of the constant current source, high precision, stable output, good linearity and perfect multiple protection functions. The precision and the stability of low-frequency alternating-current magnetic flux leakage detection can be improved.
Description
The scheme is a divisional application, and the original application name is as follows: a quick constant current control method is disclosed, the application date of the original application is as follows: 2017-07-18, and the application number of the original application is as follows: 201710586514.3.
Technical Field
The invention relates to the technical field of magnetic flux leakage detection, in particular to a magnetic flux leakage detection method and a magnetic flux leakage detection device suitable for a steel plate.
Background
Ferromagnetic materials have been widely used in the fields of petroleum, petrochemicals, railways, etc. because of their high strength, impact resistance, low cost, etc. However, due to the comprehensive factors of internal medium corrosion, stress corrosion and external environment corrosion, various defects such as cracks, pitting corrosion, wall thickness reduction and the like inevitably occur in the practical process. The storage tank mostly adopts a direct buried mode, and the bottom is easy to age and corrode, so that leakage accidents are caused.
In order to discover defects in the early stage and reduce the occurrence of disaster accidents, in recent years, scientific researchers have developed nondestructive testing by adopting various technologies such as acoustic emission, guided wave, magnetic flux leakage and the like. The acoustic emission detection technology can be used for evaluating the dynamic activity of various defects in a use state, can realize the long-term activity monitoring of the defects, and has the advantages of large material adaptation range and wide temperature adaptation range of online monitoring. But the method has the defects of easy interference of electrical noise, dependence on field detection experience for data interpretation, uncertain detection result and further confirmation by other nondestructive detection methods. The high-frequency guided wave detection technology is used as a rapid base metal detection technology with a point surface, and plays a certain role in improving the reliability of online detection and monitoring of equipment under the current conditions that the sulfur content of crude oil is increased and the corrosion of the equipment is increasingly serious in China. The presence of bulk defects and planar defects was mainly found. But is not sensitive to uniform corrosion, pitting, axial cracking, etc.
Magnetic leakage detection is an electromagnetic nondestructive detection technology which is most widely applied at present. In recent years, as the leakage magnetic field detection technology has been studied more and more, a new low-frequency magnetization technology has been developed in addition to the ac magnetization. The low-frequency magnetization penetration depth is large, so that the detection thickness can be increased; the method is used for measuring the corrosion condition and the thickness change of the workpiece by extracting the phase and amplitude information of the signal, and has high reliability. The method is characterized in that a worship product and the like detect and quantitatively evaluate the damage of the inner surface and the outer surface of a ferromagnetic component by using amplitude and phase information of a low-frequency magnetic leakage signal [ a low-frequency alternating current magnetic leakage detection method for detecting cracks on the inner wall of a ferromagnetic pipeline [ D ]. Beijing university of industry 2015 ]. Populus, et al, have studied on low-frequency electromagnetic detection of metal plate defects, and have studied the influence of excitation current and excitation frequency on magnetic induction intensity and the change law between defect size and leakage magnetic field intensity [ low-frequency electromagnetic detection of metal plate defects [ J ]. shenyang industrial university, 2015 ].
At present, a voltage source is widely adopted as an excitation source in the low-frequency magnetic flux leakage detection technology, and the excitation current is changed by adjusting the output voltage, so that the excitation intensity is changed. The exciting device for low-frequency magnetic leakage is a coil with a magnetic core, is inductive, and has impedance related to frequency when excited by alternating voltage. Therefore, when the frequency of the excitation signal changes under the action of the constant excitation voltage, the current flowing through the coil changes, so that the excitation intensity changes, and further the intensity of the leakage magnetic field changes. According to the principle of magnetic leakage detection, the defect position will cause the magnetic leakage intensity to change. Thus, it is impossible to distinguish which factor causes the change in the leakage magnetic strength. When a constant voltage source is used for researching the influence of the frequency change of an excitation signal on the magnetic flux leakage detection effect, the amplitude of the output voltage of the excitation source needs to be manually adjusted after the excitation frequency is changed every time so as to realize constant current excitation, and the constant current excitation device is complex in operation, large in error and low in precision.
When the storage tank bottom plate with the anticorrosive coating is detected, the lift-off value has a large influence on a detection result, and the uneven spraying of the anticorrosive coating causes large fluctuation of the lift-off value of the detection coil in the detection process. In the voltage source excitation mode, the fluctuation brings the fluctuation of the excitation current, and further influences the stability of the excitation magnetic field, resulting in the reduction of the stability of the detection result.
In addition, the excitation current is influenced by factors such as heating of the excitation coil and temperature change of a detection field environment, and the excitation intensity is changed due to the fact that the current is not constant under the constant-voltage excitation condition, so that the stability and the precision of a detection result are influenced.
In order to meet the low-frequency magnetic flux leakage detection requirement, stabilize the excitation magnetic field and improve the detection stability, a special constant-current excitation source needs to be designed. The traditional alternating current constant current source mostly adopts SPWM waves, generates alternating voltage in an inversion mode, has large harmonic component and is not suitable for the application requirement of low-frequency magnetic flux leakage detection. Another widely used constant-interest source design scheme is to obtain a high-precision sine wave by a direct digital frequency synthesis technology, and then realize constant-current control by using a closed-loop control scheme based on current feedback. The scheme has the advantages of high adjusting speed and small steady-state error, and is suitable for most application places.
However, the frequency of the excitation signal used for low-frequency leakage flux detection is low, and an excitation signal below 100Hz is often used, and the period of the excitation signal is more than 10 ms. In practical application, the current sampling value lags behind load current change by more than one period, which has little influence on high-frequency signals but has great influence on the design of a low-frequency constant current source. Therefore, the constant current source designed based on the two current sampling modes has low constant current speed and large output current fluctuation. The higher application requirement of the low-frequency magnetic flux leakage detection on the constant-current excitation source cannot be met.
Disclosure of Invention
In order to achieve the aim, the invention provides a design scheme of a low-frequency alternating-current rapid constant current source suitable for steel plate magnetic flux leakage detection and a method for improving detection precision and stability by applying the constant current source to the steel plate magnetic flux leakage detection.
A rapid constant current control method controls a signal source through an upper computer to generate a sinusoidal signal loaded on a load,
step one, sampling is respectively carried out from output ends of a signal source and a load, and the output voltage of a sinusoidal signal is as follows:
wherein Us is the sinusoidal signal output voltage, A is the sinusoidal amplitude, ω is the angular frequency,
is the initial phase;
the load sampling voltage is denoted as Ur and the sampling voltage is
Wherein, the sampling time is t,
is the phase at the instant Us of time t,
phase of Ur and Ir, A1Is the sampling voltage amplitude;
step two, comparing the sampling results of the step one;
will taThe voltage sampling value at the time is denoted Ur (t)a) Equation of the meridian
To obtain:
then, obtaining a sampling voltage amplitude value:
according to the formula, the voltage amplitude A at the sampling moment can be obtained1Then obtaining the current amplitude in the load at the sampling moment;
step three, obtaining a current error by taking the current amplitude in the step two and a set value of an upper computer as a difference value;
and step four, feeding the current error obtained in the step three back to an upper computer, and adjusting the output state of the sinusoidal signal by the upper computer according to the error.
Preferably, the constant current source system for implementing the rapid constant current control method is as follows:
the controller 2 is connected with the DA conversion unit 3 to generate a sine signal; the sinusoidal signal is modulated through the low-pass filtering unit 4, the amplitude adjusting unit 5 and the power amplifying unit 6 in sequence to obtain an excitation signal, and the excitation signal is loaded on a load 7; the sampling unit in the feedback circuit comprises a current sampling unit 8 and a phase difference sampling unit 9, and the load 7 is connected with the controller 2 through the current sampling unit 8 and an AD conversion unit 10; the input end of the phase difference sampling unit 9 is respectively connected with the output ends of the current sampling unit 8 and the low-pass filtering unit 4, and the output end of the phase difference sampling unit 9 is connected with the controller 2.
Preferably, the load 7 is an excitation coil; the exciting coil is an inductive load with an iron core, and generates an exciting electromagnetic field under the action of a constant-current exciting signal.
Preferably, the current sampling unit 8 includes an operational amplifier and comparator circuit connected to the load 7 through a sampling resistor, and an output terminal of the operational amplifier and comparator circuit is connected to the controller 2 through an AD conversion unit 10;
the phase difference sampling unit 9 comprises two in-phase voltage zero-crossing comparison circuits, and the input end of one of the two in-phase voltage zero-crossing comparison circuits is connected with the low-pass filtering unit 4; the input end of the other in-phase voltage zero-crossing comparison circuit is connected with the output end of the current sampling unit 8; the output ends of the two in-phase voltage zero-crossing comparison circuits are connected with the controller 2.
Preferably, the power amplifying unit 6 includes a three-stage amplifying circuit, wherein the first-stage amplifying circuit is an in-phase proportional operation circuit, the second-stage amplifying circuit is an anti-phase proportional operation circuit, the first-stage amplifying circuit and the second-stage amplifying circuit are connected in series, the three-stage amplifying circuit is a symmetrical power amplifying circuit, and output ends of the first-stage in-phase proportional operation circuit and the second-stage anti-phase proportional operation circuit are respectively connected with an input end of the three-stage amplifying circuit.
Preferably, the amplifier in the operational amplifier and comparator circuit is a low power consumption instrumentation amplifier AD 620.
Preferably, the in-phase voltage zero-crossing voltage comparator is an LM311D voltage comparator.
A method for detecting magnetic flux leakage comprises detecting magnetic flux leakage,
step one, an excitation source is set, and parameter setting is carried out on the excitation source;
step two, exciting a magnetic flux leakage detection module through the excitation source in the step one to generate an excitation magnetic field and generate a magnetic flux leakage detection signal;
step three, conditioning, collecting and shaping the signals generated in the step two, and uploading the signals to an upper computer;
the excitation source is a constant current source generated by the rapid constant current control method.
A magnetic flux leakage detection device comprises a parameter setting module, an excitation source module, a magnetic flux leakage detection module, a signal processing module, a signal acquisition module and an upper computer which are sequentially arranged; the magnetic flux leakage detection module comprises an excitation coil, and the excitation source module is an excitation source generation module based on the rapid constant current control method.
The technical scheme provided by the embodiment of the invention has the following beneficial effects: the constant current source of the technical scheme has the advantages of high adjusting speed, high precision, stable output, good linearity and perfect multiple protection functions. The low-frequency alternating-current magnetic leakage detection excitation source is used as a low-frequency alternating-current magnetic leakage detection excitation source, the excitation current change caused by the factors such as excitation frequency change, lift-off value change, coil heating and environment is overcome, the stability of an excitation magnetic field is effectively ensured, and therefore the precision and the stability of low-frequency alternating-current magnetic leakage detection can be improved.
Drawings
FIG. 1 is a schematic block diagram of an embodiment of the present invention.
Fig. 2 is a circuit diagram of a current sampling and phase sampling unit according to an embodiment of the invention.
Fig. 3 is a waveform diagram of a current sampling and phase sampling unit according to an embodiment of the invention.
Fig. 4 is a circuit diagram of a power amplifying unit according to an embodiment of the invention.
Fig. 5 is a block diagram of a magnetic flux leakage detection method according to an embodiment of the present invention.
FIG. 6 is a graph of an experiment of an embodiment of the present invention.
Wherein the reference numerals are: 1. a parameter setting unit; 2. a controller; 3. a DA conversion unit; 4. a low-pass filtering unit; 5. an amplitude adjustment unit; 6. a power amplifying unit; 7. a load; 8. a current sampling unit; 9. a phase difference sampling unit; 10. an AD conversion unit; 11. a power supply detection and control unit; 12. a multi-channel power supply conversion unit; 13. 12V storage battery.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. Of course, the specific embodiments described herein are merely illustrative of the invention and are not intended to be limiting.
Example 1
A rapid constant current control method controls a signal source through an upper computer to generate a sinusoidal signal loaded on a load,
step one, sampling is respectively carried out from output ends of a signal source and a load, and the output voltage of a sinusoidal signal is as follows:
wherein Us is the sinusoidal signal output voltage, A is the sinusoidal amplitude, ω is the angular frequency,
is the initial phase;
the load sampling voltage is denoted as Ur and the sampling voltage is
Wherein, the sampling time is t,
is the phase at the instant Us of time t,
phase of Ur and Ir, A1Is the sampling voltage amplitude;
step two, comparing the sampling results of the step one;
will taThe voltage sampling value at the time is denoted Ur (t)a) Equation of the meridian
To obtain:
then, obtaining a sampling voltage amplitude value:
according to the formula, the voltage amplitude A at the sampling moment can be obtained1Then obtaining the current amplitude in the load at the sampling moment;
step three, obtaining a current error by taking the current amplitude in the step two and a set value of an upper computer as a difference value;
and step four, feeding the current error obtained in the step three back to an upper computer, and adjusting the output state of the sinusoidal signal by the upper computer according to the error.
Example 2
Referring to fig. 1 to 4, a constant current source system for implementing the fast constant current control method is:
the controller 2 is connected with the DA conversion unit 3 to generate a sine signal; the sinusoidal signal is modulated through the low-pass filtering unit 4, the amplitude adjusting unit 5 and the power amplifying unit 6 in sequence to obtain an excitation signal, and the excitation signal is loaded on a load 7; the sampling unit in the feedback circuit comprises a current sampling unit 8 and a phase difference sampling unit 9, and the load 7 is connected with the controller 2 through the current sampling unit 8 and the AD conversion unit 10; the input end of the phase difference sampling unit 9 is connected with the output ends of the current sampling unit 8 and the low-pass filtering unit 4 respectively, and the output end of the phase difference sampling unit 9 is connected with the controller 2.
The load 7 is an exciting coil; the exciting coil is an inductive load with an iron core, and generates an exciting electromagnetic field under the action of a constant-current exciting signal.
The current sampling unit 8 realizes the circuit including, through sampling the operational amplification and comparison circuit that the resistance is connected with load 7, the output end of the operational amplification and comparison circuit connects the controller 2 through AD converting unit 10;
the phase difference sampling unit 9 comprises two in-phase voltage zero-crossing comparison circuits, and the input end of one of the two in-phase voltage zero-crossing comparison circuits is connected with the low-pass filtering unit 4; the input end of the other in-phase voltage zero-crossing comparison circuit is connected with the output end of the current sampling unit 8; the output ends of the two in-phase voltage zero-crossing comparison circuits are both connected with the controller 2.
The power amplifying unit 6 comprises a three-stage amplifying circuit, wherein the first-stage amplifying circuit is an in-phase proportional operation circuit, the second-stage amplifying circuit is an anti-phase proportional operation circuit, the first-stage amplifying circuit and the second-stage amplifying circuit are connected in series, the third-stage amplifying circuit is a symmetrical power amplifying circuit, and output ends of the first-stage in-phase proportional operation circuit and the second-stage anti-phase proportional operation circuit are respectively connected with an input end of the third-stage amplifying circuit.
The amplifier in the operational amplification and comparison circuit is a low-power consumption instrument amplifier AD 620.
The in-phase voltage zero-crossing voltage comparator is an LM311D voltage comparator.
The parameter setting unit 1 provides a flexible setting interface for a user, can set parameters such as excitation source frequency, voltage, current and the like and control commands, and realizes interaction with the FPGA unit through a serial communication interface.
The controller 2 is an FPGA unit and comprises a system controller and a DDS module which take an NIOS soft-core processor as a core, wherein the system controller is a core component of a constant current source and coordinates and manages the orderly work of each module, and the FPGA unit specifically comprises the following components: the parameter setting unit is connected with the control unit and used for realizing the interaction between the parameters and the control commands; the power supply detection and control unit is connected with the power supply detection and control unit to realize power supply monitoring and fault processing; the AD conversion unit is connected with the current sampling value and converts the current sampling value into a digital value; and the DA conversion unit is connected to convert the waveform data in a digital form into an analog voltage. The DDS module produces sine wave data according to preset parameters.
DA conversion unit 3: a high-speed DA converter, a current-voltage conversion circuit and a voltage boost circuit are designed. The high-speed DA converter realizes conversion from digital quantity output to analog quantity, the current-voltage conversion circuit converts the output of the current type parallel DA into unipolar analog voltage signals, and the voltage lifting circuit converts the unipolar analog voltage signals into bipolar analog voltage signals.
Low-pass filtering unit 4: the high-order elliptical low-pass filter is designed, has good passband and stopband approximation characteristics, and is used for filtering high-frequency noise caused by direct frequency synthesis technology and DA conversion.
The amplitude adjusting unit 5: a digital voltage division type amplitude adjusting unit is designed, is connected with a control unit and is used for accurately adjusting the amplitude of a signal, and the amplitude adjusting precision reaches 0.1%.
Power amplification unit 6: a voltage amplifying circuit, a voltage inverting circuit and a symmetrical power amplifying circuit are designed. The voltage amplification circuit is connected with the amplitude adjusting unit, so that the voltage amplitude of the sinusoidal signal can be raised, and the requirement of post-stage power amplification is met. The voltage inverting circuit inverts the output signal of the voltage amplifying circuit to provide two groups of sinusoidal signals with equal magnitude and opposite phase for the power amplifying circuit. The symmetrical power amplifying circuit consists of an upper part and a lower part with completely consistent parameters, and both the upper part and the lower part form an independent power amplifying circuit by a linear power amplifier and a feedback resistor. The input ends of the two parts of circuits are respectively connected to the front end and the rear end of the voltage inverting circuit, so that symmetrical signals with equal signal magnitude and opposite phases are obtained at the output ends of the two parts of circuits. The load is connected in a differential output mode, and the amplitude of the load voltage is doubled.
The load 7 is an excitation coil: the inductive load with the iron core generates an excitation electromagnetic field under the action of a constant current excitation signal, and is a key device of a magnetic flux leakage detection system.
Current sampling unit 8: the sampling circuit is connected in series in an output loop, samples the output current according to a fixed proportion, and superposes the sampled alternating current signal on the direct current voltage with a fixed amplitude value.
Phase difference sampling unit 9: and the phase difference sampling circuit is designed by being connected with the current sampling unit 8, the low-pass filtering unit 4 and the controller 2, so that the sampling of voltage phase and current phase difference is realized.
AD conversion unit 10: and the output voltage of the current sampling unit 8 is converted into digital quantity and transmitted to the controller 2 through a high-speed parallel data line, wherein the output voltage is connected with the current sampling unit 8 and the controller 2.
Power detection and control 11: and the multi-path power generation unit 12 is connected with the controller 2 and comprises a voltage division sampling circuit and a power supply control circuit. The controller 2 collects voltage values of multiple paths of power supplies in real time through a voltage division sampling circuit formed by precise resistors, and timely alarms and performs power failure processing when abnormality is found. The power control circuit takes the relay as an actuating mechanism, provides a power supply for each unit under the control of the controller 2, and cuts off power supply voltage and input signals in time when the controller 2 detects that faults such as overcurrent and undervoltage occur in the hardware circuit, so that the purpose of protecting the hardware circuit and load is achieved.
The multi-path power generation unit 12: the 12V voltage of the storage battery is converted into multi-path voltage output, and the multi-path voltage output comprises low-voltage logic power supply voltage, AD and DA reference voltage, a +/-12V power supply required by an operational amplifier and a +/-36V power supply required by a power amplification unit.
12V battery 13: and a 12V high-capacity lead acid salt battery is selected to meet the portable design requirement.
Secondly, the working process of the constant current source is as follows:
after the constant current source is electrified, the controller 2 detects whether the power supply of each power supply is normal through power supply detection and the controller, and if the power supply of each power supply is normal, an alarm is started to prompt that the system has a fault. If the detection is normal, parameters such as excitation frequency, current and the like can be set through the parameter setting unit 1, and then waveform parameters and control commands are downloaded through the serial port. The controller 2 sets parameters such as frequency, waveform and current according to the received data, starts a direct digital frequency synthesizer to generate waveform data, outputs the waveform data to the high-speed DA conversion unit in a parallel mode, and obtains a unipolar sinusoidal signal at the output end of the DA conversion unit 3. High-frequency interference is filtered by a high-order elliptical filter, and a bipolar sinusoidal signal is obtained through a voltage lifting circuit. Under the coordination of the controller, the voltage amplitude of the signal can be adjusted by the amplitude adjustment unit 5. And then the excitation signal with controllable current amplitude is obtained through the power amplifying unit 6. The controller 2 calculates the amplitude of the output current by collecting two parameters of the current amplitude and the phase in real time, and obtains a current error as the input quantity of the controller by making a difference value with a set value. The controller 2 adjusts the amplitude of the output voltage through the amplitude adjusting unit 5, thereby realizing the adjustment of the amplitude of the output current. The sampling and control method greatly improves the regulation speed of the constant current source and improves the stability and the rapidity of constant current control.
A fast constant current control method based on current amplitude and phase double-parameter feedback is disclosed.
The rapid constant current control method based on current amplitude and phase double-parameter feedback acquires current amplitude by acquiring sampling resistor voltage, acquires current phase by acquiring phase difference between sampling voltage and excitation voltage, and acquires current amplitude at sampling time by using instantaneous current value and phase value at the same acquisition time. The current sampling speed is only related to the sampling rate and is not related to the signal frequency, compared with the conventional effective value sampling and peak value sampling method, the current sampling speed under the method is greatly improved, and particularly for constant current control of low-frequency signals, the advantage of fast response to current change is more obvious.
Specifically, first, a sinusoidal signal Us is generated by the controller (2) and the DA conversion unit (3):
where A is the sine wave amplitude, ω is the angular frequency,
is the initial phase.
The phase and frequency of Us can be accurately adjusted by a controller. The Us is amplified by an amplitude control unit 5 and a power amplification unit 6 to obtain the high-voltage and high-power electricityAn excitation signal of the current driving capability, which is applied to the load 7, i.e. the excitation coil, drives it to operate, generating an excitation magnetic field for magnetic flux leakage detection. The exciting coil for low-frequency magnetic leakage detection has iron core and forms a closed magnetic circuit with the tested piece, and the phase difference between current and voltage in the coil is recorded as
And the magnetic permeability, the electric conductivity, the internal defects and other factors of the tested piece. The output voltage of the current sampling unit 8 is denoted as Ur, the current is denoted as Ir, and the current sampling is performed by adopting a sampling resistor in the design, which is a pure resistive element, so that Ur and Ir have the same phase. By measuring the phase difference of the voltages Us and Ur, the phase difference of Us and Ir can be obtained
The phase difference between Us and Ur can be measured by the phase sampling unit 9.
From equation (a), the phase at time t US is
Ur and Ir have a phase of
If the sampled voltage amplitude is denoted as A1, the sampled voltage can be expressed as:
the controller carries out real-time acquisition to Ur through AD converting unit 10, and the voltage sampling value of the time ta is taken as Ur (ta), and for the time ta, the following are provided:
after transformation, the following can be obtained:
from the formula (d), ω and ta、
In order for the parameters to be known,
measured by the phase sampling unit 9, Ur (t)a) Can be measured by the AD conversion unit 10, so that the sampling voltage amplitude A of the current sampling moment can be obtained by a group of sampling values1. From ohm's law, t can be obtainedaThe magnitude of the current in the load at that moment.
By collecting two parameters of the current amplitude and the phase, the amplitude of the output current can be obtained by using less sampling data, and the current error is obtained by taking the difference between the current and a set value as the input quantity of the controller 2. The controller 2 adjusts the output current by using the voltage amplitude as an adjustment amount. The sampling and control method greatly improves the regulation speed of the constant current source and improves the stability and the rapidity of constant current control.
Implementation of phase difference measurement
The current sampling and phase sampling unit circuit is shown in FIG. 2, U4And U7The output end of the power amplifying unit is connected, the CON2 terminal is connected to a load, Rs is a sampling resistor, and the waveform of the sampling current is shown in fig. 3 (d). R23-R26The resistor is a voltage dividing resistor which divides the high-voltage common-mode voltage at two ends of Rs and limits the high-voltage common-mode voltage within the range of operation abandoning power supply voltage. A. the8Is an operational amplifier for instruments, the voltage at the output end of which is recorded as U8The waveform is shown in FIG. 3 (e). A. the8Can be amplified by a resistor R50Adjusting the magnification factor Au to be 1+49.9k/R50。D7And D8Is a bidirectional voltage stabilizing diode which can absorb the peak spike pulse in the circuit and protect A8Is not damaged. A. the10And the peripheral circuit forms an in-phase voltage zero-crossing comparison circuit which can input a bipolar sinusoidal signal U8Common-frequency unipolar square wave U converted into 0-3.3V10As shown in fig. 3 (f). Us waveforms are shown in FIG. 3(a), R35And R36Form a voltage divider circuitThe input signal Us is divided to obtain a voltage U12As shown in fig. 3 (b). A. the11Similar to the peripheral circuit structure, the output waveform is shown in fig. 3(c), which is not repeated. It can be seen from fig. 3(c) and 3(f) that the falling edge of the square wave pulse has a time t1And t2Will U is10And U11Input to FPGA, and easily obtain the time difference t of its falling edge1-t2. This time difference is the zero-crossing time difference Δ t between the voltage Us and the current Ir. Knowing the period T of the excitation signal, the phase difference between Us and Ir can be calculated
Similarly, another phase difference may be measured at the rising edge.
The symmetrical power amplifier unit circuit shown in FIG. 4 comprises three stages, the first stage is composed of an integrated operational amplifier A1Resistance R1、R2And R3The in-phase proportional operation circuit is formed, and the amplification factor is as follows:
the voltage pre-stage amplification can be realized according to the voltage amplification requirement. The second stage circuit is composed of an integrated operational amplifier A2Resistance R4、R5And R6The inverse proportion operation circuit is formed, and the amplification factor is as follows:
get R5＝R4When the magnification is-1, U is known5＝-U2So that A is1And A2The output voltages of (1) are equal in magnitude and opposite in phase.
The third stage circuit is a high-voltage high-power linear operational amplifier A3、A4And the peripheral resistor form a symmetrical power amplifying circuit. Specifically, let the input voltage of the circuit in part A be U2Output voltage of U4An operational amplifier A3The same-phase input terminal voltage is U3。RaIs A3The current limiting resistor of (1) samples the output current as A3The current-limited sampling terminal CL of (1) provides a sampled voltage.In practical use RaThe values are small (less than 0.1 ohm), so the resistance values can be ignored when analyzing the part of the operational relationship. Deducing U according to the concept that the operational amplifier works in linear working regions of ' virtual short ' and ' virtual break2、U3、U4The relationship with the resistances R7-R9 yields the following relationships:
determining R according to the load and the requirement of constant utilization on the output current8And R9The resistance value of (c). The working principle of the circuit of the part B is completely the same as that of the part A, and the description is omitted here. The parameters of the upper circuit and the lower circuit of the dotted line frames A and B are consistent, the amplitudes of the input signals of the dotted line frames A and B are equal, the phases of the input signals of the dotted line frames B are opposite, and the output ends of the two circuits are connected with a load in a floating difference mode, so that the multiplication of the voltage output amplitude is realized.
Example 3
Referring to fig. 5 and 6, a magnetic flux leakage detecting method,
step one, an excitation source is set, and parameter setting is carried out on the excitation source;
step two, exciting a magnetic flux leakage detection module through the excitation source in the step one to generate an excitation magnetic field and generate a magnetic flux leakage detection signal;
step three, conditioning, collecting and shaping the signals generated in the step two, and uploading the signals to an upper computer;
the excitation source is a constant current source generated by the rapid constant current control method.
By designing multiple protections such as hardware current limiting, software overcurrent automatic protection, power supply undervoltage automatic turn-off and the like, the working reliability of the constant current source is improved.
A constant current excitation source is utilized to generate a constant low-frequency alternating current excitation signal, the constant low-frequency alternating current excitation signal is applied to an excitation coil, and an alternating magnetic field is generated in a closed loop formed by the excitation coil and a steel plate test piece. And a signal conditioning module is used for receiving the output signals of the plurality of detection coils and carrying out filtering and amplification processing. And acquiring the conditioned multi-channel detection signals by using a data acquisition module card, and sending the conditioned multi-channel detection signals to upper computer acquisition software, wherein the upper computer software is used for carrying out data processing and interpretation.
The working condition of uneven coating in the detection of the steel plate with the anticorrosive coating is simulated by changing the coil lift-off value in a small amplitude. With the excitation source, for a crack defect test piece [ defect description: 5cm in length, 2mm in width and 4mm in depth, and developing a low-frequency alternating-current magnetic flux leakage detection experiment with two excitation modes of a constant-voltage source and a constant-current source. The influence of small amplitude change of the lift-off value on the detection precision of the two detection modes is researched and compared in an important mode.
The low-frequency magnetic flux leakage experiment implementation step:
1. setting the initial lift-off value of the coil to be 3mm, setting the output frequency of the excitation source to be 25Hz and the current to be 2A, and starting a constant current output mode.
2. And at the defect-free position, acquiring the peak value of the detection signal and the phase difference between the excitation signal and the detection signal by using the detection coil, and storing data.
3. And moving the excitation coil to a position right above the crack defect along the vertical direction of the crack defect, acquiring the peak value of the detection signal and the phase difference between the excitation signal and the detection signal by using the detection coil, and storing data.
4. The coil lift-off values (1.5mm, 2.0mm, 2.5mm, 3.5mm, 4.0mm, 4.5mm) were varied in sequence and the experimental steps 2-3 were repeated to obtain six additional sets of experimental data.
5. And repeatedly setting the initial lift-off value of the coil to be 3mm, setting the output frequency of the excitation source to be 25Hz, setting the peak value of the voltage to be 60V, and starting a constant voltage output mode.
6. And repeating the experiment steps 2-4 to obtain seven groups of experimental data under the constant-pressure excitation mode.
Experimental treatment and conclusion:
the data measured in the above experiment were subjected to calculation processing with the 3mm lift-off value as a reference (assuming that the thickness of the anticorrosive coating at a uniform position was 3mm), to obtain a curve as shown in fig. 6. The abscissa of the graph describes the change in lift-off value and the ordinate describes the rate of change of voltage of the detected signal at the defect compared to the measured value at the defect-free 3mm lift-off value. From top to bottom, three curves respectively describe the voltage change rate of the defect under the constant voltage excitation condition, the voltage change rate of the defect under the constant current excitation condition and the improvement rate of the detection voltage change rate under the constant current excitation condition.
Under the constant current source excitation mode, within a small amplitude variation range of a lift-off value, the detection voltage amplitude value at the defect position is changed more obviously, namely, the constant current excitation source effectively improves the detection precision and stability of low-frequency alternating current magnetic flux leakage detection.
Example 4
A magnetic flux leakage detection device comprises a parameter setting module, an excitation source module, a magnetic flux leakage detection module, a signal processing module, a signal acquisition module and an upper computer which are sequentially arranged; the magnetic flux leakage detection module comprises an excitation coil, and the excitation source module is an excitation source generation module based on the rapid constant current control method.
In the low-frequency magnetic leakage detection, the detection sensitivity can be improved by increasing the excitation current, but the excitation loop is subjected to magnetic saturation along with the increase of the excitation current, the equivalent resistance of the loop is very small at the moment, and the current is increased sharply. If the effective protection measures are lacked, the exciting coil and the exciting source are easily damaged. Therefore, multi-stage protection is realized by combining software and hardware. The basic current-limiting protection is realized by arranging a current-limiting resistor on the power amplification unit; the output current information is collected in real time through the current feedback network, and the power amplification input signal can be directly turned off by using the controller, so that overcurrent turn-off protection is realized. In addition, a power supply detection and controller is designed, the controller detects the power supply voltage in real time through the unit, and when faults such as power supply undervoltage and voltage loss are detected, the power supply voltage is cut off in time, so that power elements of the power amplification and constant current controller are prevented from being damaged due to unbalanced power supply.
The high-power constant current source is used as a low-frequency magnetic flux leakage detection excitation source, the stability of an excitation magnetic field is ensured by stabilizing the excitation current, and the detection precision and sensitivity are improved.
The low-frequency magnetic leakage detection experiment is carried out by adopting a constant-current excitation mode, the excitation current change caused by the factors such as excitation frequency change, lift-off value change, coil heating and environment is overcome, the stability of an excitation magnetic field is effectively ensured, and the precision and the stability of low-frequency alternating-current magnetic leakage detection can be improved.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (9)
1. A magnetic flux leakage detection method is characterized by comprising the following steps:
step one, an excitation source is set, and parameter setting is carried out on the excitation source;
step two, exciting a magnetic flux leakage detection module through the excitation source in the step one to generate an excitation magnetic field and generate a magnetic flux leakage detection signal;
and step three, conditioning, collecting and shaping the signals generated in the step two, and uploading the signals to an upper computer.
2. The method of claim 1, wherein the excitation source is a constant current source generated by a fast constant current control method; the rapid constant current control method controls a signal source through an upper computer to enable the signal source to generate a sinusoidal signal loaded on a load, and the rapid constant current control method specifically comprises the following steps:
step one, sampling is respectively carried out from output ends of a signal source and a load, and the output voltage of a sinusoidal signal is as follows:
wherein Us is the sinusoidal signal output voltage, A is the sinusoidal amplitude, ω is the angular frequency,
is the initial phase;
the load sampling voltage is denoted as Ur and the sampling voltage is
Wherein, the sampling time is t,
is the phase at the instant Us of time t,
phase of Ur and Ir, A1Is the sampling voltage amplitude;
step two, comparing the sampling results of the step one;
will taThe voltage sampling value at the time is denoted Ur (t)a) Equation of the meridian
To obtain:
then, obtaining a sampling voltage amplitude value:
according to the formula, the voltage amplitude A at the sampling moment can be obtained1Then obtaining the current amplitude in the load at the sampling moment;
step three, obtaining a current error by taking the current amplitude in the step two and a set value of an upper computer as a difference value;
and step four, feeding the current error obtained in the step three back to an upper computer, and adjusting the output state of the sinusoidal signal by the upper computer according to the error.
3. The method according to claim 2, wherein the constant current source system for implementing the rapid constant current control method is:
the controller (2) is connected with the DA conversion unit (3) to generate a sine signal; the sinusoidal signal is modulated sequentially through a low-pass filtering unit (4), an amplitude adjusting unit (5) and a power amplifying unit (6) to obtain an excitation signal, and the excitation signal is loaded on a load (7); the sampling unit in the feedback circuit comprises a current sampling unit (8) and a phase difference sampling unit (9), and the load (7) is connected with the controller (2) through the current sampling unit (8) and an AD conversion unit (10); the input end of the phase difference sampling unit (9) is respectively connected with the output ends of the current sampling unit (8) and the low-pass filtering unit (4), and the output end of the phase difference sampling unit (9) is connected with the controller (2).
4. A method according to claim 3, characterized in that the load (7) is an excitation coil; the exciting coil is an inductive load with an iron core, and generates an exciting electromagnetic field under the action of a constant-current exciting signal.
5. The method according to claim 3 or 4, characterized in that the current sampling unit (8) implements a circuit comprising an operational amplification and comparison circuit connected to the load (7) through a sampling resistor, the output of the operational amplification and comparison circuit being connected to the controller (2) through an AD conversion unit (10);
the phase difference sampling unit (9) comprises two in-phase voltage zero-crossing comparison circuits, and the input end of one of the two in-phase voltage zero-crossing comparison circuits is connected with the low-pass filtering unit (4); the input end of the other in-phase voltage zero-crossing comparison circuit is connected with the output end of the current sampling unit (8); the output ends of the two in-phase voltage zero-crossing comparison circuits are connected with the controller (2).
6. The method according to any one of claims 3 to 5, wherein the power amplification unit (6) comprises a three-stage amplification circuit, wherein the first-stage amplification circuit is an in-phase proportional operation circuit, the second-stage amplification circuit is an anti-phase proportional operation circuit, the first-stage amplification circuit and the second-stage amplification circuit are connected in series, the third-stage amplification circuit is a symmetrical power amplification circuit, and output ends of the first-stage in-phase proportional operation circuit and the second-stage anti-phase proportional operation circuit are respectively connected with input ends of the third-stage amplification circuit.
7. The method according to any one of claims 3-6, wherein the amplifier in the operational amplifier and comparator circuit is a low power consumption instrumentation amplifier (AD 620).
8. The method of any of claims 3-6, wherein the in-phase voltage zero crossing voltage comparator is a LM311D voltage comparator.
9. A magnetic flux leakage detection device comprises a parameter setting module, an excitation source module, a magnetic flux leakage detection module, a signal processing module, a signal acquisition module and an upper computer which are sequentially arranged; the magnetic flux leakage detection module comprises an excitation coil and is characterized in that the excitation source module is used for generating a constant current source by a rapid constant current control method; the rapid constant current control method is characterized in that a signal source is controlled by an upper computer to generate a sinusoidal signal loaded on a load, and the rapid constant current control method is characterized in that: the rapid constant current control method specifically comprises the following steps:
step one, sampling is respectively carried out from output ends of a signal source and a load, and the output voltage of a sinusoidal signal is as follows: us — Asin (ω t + Φ);
wherein Us is a sinusoidal signal output voltage, a is a sinusoidal wave amplitude, ω is an angular frequency, and Φ is an initial phase;
the load sampling voltage is recorded as Ur, and the sampling voltage is Ur ═ A1sin (ω t + φ +. DELTA φ);
wherein, the sampling time is t,
is the phase at the instant Us of time t,
phase of Ur and Ir, A1Is the sampling voltage amplitude;
step two, comparing the sampling results of the step one;
will taThe voltage sampling value at the time is denoted Ur (t)a) Equation of the meridian
To obtain:
then, obtaining a sampling voltage amplitude value:
according to the formula, the voltage amplitude A at the sampling moment can be obtained1Then obtaining the current amplitude in the load at the sampling moment;
step three, obtaining a current error by taking the current amplitude in the step two and a set value of an upper computer as a difference value;
and step four, feeding the current error obtained in the step three back to an upper computer, and adjusting the output state of the sinusoidal signal by the upper computer according to the error.
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