CN114823044B - Uniform magnetic field generator with variable frequency and field intensity and control method thereof - Google Patents
Uniform magnetic field generator with variable frequency and field intensity and control method thereof Download PDFInfo
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- CN114823044B CN114823044B CN202210439539.1A CN202210439539A CN114823044B CN 114823044 B CN114823044 B CN 114823044B CN 202210439539 A CN202210439539 A CN 202210439539A CN 114823044 B CN114823044 B CN 114823044B
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Abstract
The invention relates to a uniform magnetic field generator with variable frequency and field intensity and a control method thereof, wherein the uniform magnetic field generator with variable frequency and field intensity comprises an alternating current power supply module, a coil module, a variable capacitor module and a control circuit module, wherein the alternating current power supply module, the coil module and the variable capacitor module are sequentially and electrically connected, and the control circuit module is respectively and electrically connected with the alternating current power supply module and the variable capacitor module, wherein the control circuit module determines a compensation capacitance value and a resonance current amplitude value according to a set magnetic field frequency and a set magnetic field amplitude value, drives the variable capacitor module to adjust the capacitance value, and drives the alternating current power supply module to adjust the voltage frequency and the amplitude value, so that the coil module generates a uniform magnetic field with variable frequency and field intensity. The invention utilizes the self-inductance of the compensation capacitor and the Helmholtz coil to form a resonance network to realize the steady-flow output, so that the Helmholtz coil generates a magnetic field with adjustable frequency and adjustable magnetic induction intensity.
Description
Technical Field
The invention belongs to the technical field of uniform magnetic field generators, and particularly relates to a uniform magnetic field generator with variable frequency and field intensity and a control method thereof.
Background
The Helmholtz coil is composed of a pair of mutually parallel and coaxial circular communicating coils, the distance between the two coils is the same as the radius of the circular coils, so that currents with the same direction and the same size pass through the two coils, and a wider uniform magnetic field area is generated near the common axis. On the basis of a one-dimensional magnetic field, the superposition of a two-dimensional and three-dimensional combined magnetic field can be carried out, an alternating current magnetic field or a direct current magnetic field can be provided, and the current and the magnetic field have a very stable linear relationship. Can generate extremely weak magnetic field up to hundreds of tesla (T) magnetic field, and is used for counteracting the geomagnetic field, detecting the characteristics of the permanent magnet, calibrating the Hall probe and various magnetometers, measuring and eliminating the space radiation magnetic field and the like.
Research shows that the magnetic induction intensity of the Helmholtz coil in the proximal region is distributed most uniformly, and the uniform region with a certain size is realized only by increasing the geometric dimension of the coil, so that the design, processing and manufacturing of the coil are inconvenient. In order to generate a magnetic induction with a certain intensity, a large current must be passed through the coil, and the coil must be matched with a reliable power supply device. Therefore, how to design a high-performance uniform magnetic field generator with variable frequency and field intensity is a problem to be solved urgently.
Disclosure of Invention
In view of the above, there is a need for a variable frequency and field strength uniform magnetic field generator and a control method thereof, which can overcome the problem of complicated design of the variable frequency and field strength uniform magnetic field generator in the prior art.
In order to solve the technical problem, the invention provides a uniform magnetic field generator with variable frequency and field intensity, which comprises an alternating current power supply module, a coil module, a variable capacitor module and a control circuit module, wherein the alternating current power supply module, the sampling resistor module, the coil module and the variable capacitor module are sequentially and electrically connected, and the control circuit module is respectively and electrically connected with the alternating current power supply module, the sampling resistor module and the variable capacitor module, wherein the control circuit module determines a compensation capacitance value and a resonance current amplitude value according to a set magnetic field frequency and a set magnetic field amplitude value, drives the variable capacitor module to adjust the capacitance value, and drives the alternating current power supply module to adjust the voltage amplitude value, so that the coil module generates a uniform magnetic field with variable frequency and field intensity.
Further, the ac power supply module includes a low frequency unit and a high frequency unit, wherein:
the low-frequency unit comprises a signal generator and a power amplifier which are electrically connected, wherein the signal generator is electrically connected with the control circuit module, and the power amplifier is electrically connected with the Helmholtz coil module;
the high-frequency unit comprises a preset power direct current source and a phase-shifted full-bridge circuit which are electrically connected, wherein the phase-shifted full-bridge circuit is respectively electrically connected with the control circuit and the alternating current power supply module.
Further, the helmholtz coil module includes a sampling circuit and a helmholtz coil, wherein:
the sampling circuit comprises a first sampling resistor to a third sampling resistor, wherein the first sampling resistor is electrically connected to the alternating current power supply module, the variable capacitance module and the control circuit module respectively, the second sampling resistor is electrically connected to the alternating current power supply module, the first sampling resistor and the control circuit module respectively, and the third sampling resistor is electrically connected to the alternating current power supply module, the second sampling resistor and the Helmholtz coil respectively;
the Helmholtz coil is electrically connected to the alternating current power supply module and the variable capacitance module respectively.
Further, the variable capacitance module comprises at least one regulating branch and at least one parallel plate branch connected in parallel, wherein:
the at least one adjusting branch circuit comprises capacitors with different capacitance values and a relay which are connected in series, and the relay is used for controlling the capacitors to be connected or not connected;
the at least one parallel polar plate branch comprises a parallel polar plate capacitor and a stepping motor corresponding to the parallel polar plate capacitor, and the capacitance value is adjusted by adjusting the staggered distance between the polar plates of the at least one parallel polar plate capacitor through the stepping motor.
Further, control circuit includes main control board, host computer, switching power supply, relay drive module and motor drive module, wherein:
the main control board is electrically connected with the upper computer, the switching power supply, the relay driving module and the motor driving module respectively;
the upper computer is electrically connected with a signal generator in the alternating current power supply module;
the switching power supply is electrically connected with the relay;
the relay driving module is electrically connected with at least one adjusting branch in the variable capacitor module;
and the motor driving module is electrically connected with the stepping motor in the variable capacitance module.
The invention also provides a control method of the uniform magnetic field generator with variable frequency and field intensity, which is based on the uniform magnetic field generator with variable frequency and field intensity and comprises the following steps:
acquiring the magnetic field frequency and the magnetic field amplitude input by an upper computer;
determining a compensation capacitance value and a resonant current amplitude value according to the magnetic field frequency and the magnetic field amplitude value;
and driving the variable capacitance module to adjust the capacitance value according to the compensation capacitance value and the resonance current amplitude value, and driving the alternating current power supply module to adjust the voltage amplitude value, so that the coil module generates a uniform magnetic field which accords with the magnetic field frequency and the magnetic field amplitude value.
Further, determining a compensation capacitance value and a resonant current amplitude based on the magnetic field frequency and the magnetic field amplitude comprises
Determining the compensation capacitance value according to the magnetic field frequency and the magnetic field amplitude value based on a resonance relation;
determining the resonant current amplitude according to the magnetic field frequency and the magnetic field amplitude based on a PEEC method;
the open and closed state of a relay in at least one regulating branch in the variable capacitance module is determined based on a genetic algorithm.
Further, the driving the variable capacitance module to adjust the capacitance value and the ac power module to adjust the voltage amplitude according to the compensation capacitance value and the resonant current amplitude includes:
determining a phase difference between the voltage signal and the current signal according to the voltage signal and the current signal;
and judging whether the phase difference meets a preset condition, if not, driving a stepping motor to adjust the capacitance value according to the compensation capacitance value until the preset condition is met.
Further, the driving the variable capacitance module to adjust the capacitance value and the driving the ac power module to adjust the voltage amplitude according to the compensation capacitance value and the resonance current amplitude further includes:
if the phase difference meets the preset condition, comparing the current signal with the resonance current amplitude value, and determining the amplitude difference between the current signal and the resonance current amplitude value;
if the amplitude difference exceeds the error range, determining a voltage adjustment value according to the current signal, the resonance current amplitude and the resistance value;
and controlling the alternating current power supply module to adjust the voltage amplitude according to the voltage adjustment value until the amplitude difference is within an error range.
Further, the controlling the ac power module to adjust the voltage amplitude according to the voltage adjustment value until the amplitude difference is within an error range includes:
if the voltage is in the high frequency, adjusting the amplitude of the signal generator, and changing the voltage amplitude until the amplitude difference is within the error range;
if the frequency is low, the phase angle of the phase-shifted full bridge is adjusted, and the voltage amplitude is changed until the amplitude difference is within the error range.
Compared with the prior art, the invention has the beneficial effects that: the high-frequency-band-pass filter is characterized in that an alternating current power supply (a low-frequency band is composed of a signal generator and a power amplifier, and a high-frequency band is composed of a high-power direct current power supply and a phase-shifted full-bridge circuit), a voltage and current sampling resistor, a variable capacitor bank, a Helmholtz coil and a main control board are arranged, and the main control board is connected with an upper computer through a serial port to realize real-time communication; the circuit frequency variation range can be set to be 1 Hz-400kHz, the variable capacitance is divided into 17 constant value capacitances and 2 parallel polar plate adjustable capacitances, the access state of 19 relays is determined by compensating capacitance values under the given frequency of an upper computer, an adjustable capacitor with the capacitance value range of 4 pF-20 uF is realized, a compensating capacitance group and the self-inductance of a Helmholtz coil form a resonance network, when the circuit reaches a stable state, the magnetic induction intensity generated by the Helmholtz coil is calculated according to PEEC, the input end voltage is adjusted, the current value of the circuit is stabilized, and the Helmholtz coil generates a magnetic field with adjustable frequency and field intensity. In summary, the invention utilizes the self-inductance of the compensation capacitor and the Helmholtz coil to form a resonance network to realize the steady-current output, so that the Helmholtz coil generates a magnetic field with adjustable frequency and adjustable magnetic induction intensity.
Drawings
FIG. 1 is a schematic structural diagram of an embodiment of a uniform magnetic field generator of variable frequency and field strength provided by the present invention;
FIG. 2 is a schematic diagram showing the detailed structure of an embodiment of the uniform magnetic field generator with variable frequency and field strength provided by the present invention;
fig. 3 is a schematic simulation diagram of an embodiment of a simulation model of a parallel plate constant value capacitor and a parallel plate adjustable capacitor in an ANSYS according to the variable capacitance region provided in the present invention;
FIG. 4 is a schematic flow chart of an embodiment of a method of controlling a uniform magnetic field generator with variable frequency and field strength provided by the present invention;
FIG. 5 is a flowchart illustrating an embodiment of step S403 in FIG. 4 according to the present invention;
FIG. 6 is a schematic flowchart of another embodiment of step S403 in FIG. 4 according to the present invention;
FIG. 7 is a waveform diagram illustrating voltage and current waveforms under direct drive according to one embodiment of the present invention;
FIG. 8 is a schematic diagram of waveforms of one embodiment of voltage and current under a full compensation scheme provided by the present invention;
FIG. 9 is a waveform diagram illustrating an embodiment of voltage and current under compensation provided by the present invention;
FIG. 10 is a waveform diagram illustrating voltage and current waveforms under compensation conditions according to another embodiment of the present invention.
Detailed Description
The accompanying drawings, which are incorporated in and constitute a part of this application, illustrate preferred embodiments of the invention and together with the description, serve to explain the principles of the invention and not to limit the scope of the invention.
In the description of the present invention, the terms "first" and "second" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implying any number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. Further, "plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.
Reference throughout this specification to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present invention. The appearances of the phrase in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. It is explicitly and implicitly understood by one skilled in the art that the described embodiments can be combined with other embodiments.
The invention provides a uniform magnetic field generator with variable frequency and field intensity and a control method thereof.
Before the description of the embodiments, the related words are paraphrased:
a magnetic field generator: the electromagnetic induction principle is utilized, current with certain frequency and certain intensity passes through the coil, and corresponding magnetic field intensity is generated around the coil. The method is widely applied to the fields of element analyzers, nuclear magnetic resonance, environment detectors and the like. The types of magnetic field generators commonly used at present generally include fixed magnetic field generators, helmholtz coils, solenoid coils, electromagnets, and other magnetic field generators. The standard magnetic field is a constant magnetic field generated in the gap by a plurality of permanent magnets through a magnetic field loop, the size of the magnetic field is generally hundreds of gausses and thousands of gausses, and the standard magnetic field can be used for gauss meter calibration and the like; the solenoid coil magnetic field is formed by a single solenoid coil, a long cylindrical uniform magnetic field is generated in the coil, the size of the magnetic field in a uniform area is changed by changing the current, but the uniformity is slightly lower than that of a Helmholtz coil; the electromagnet is composed of two flat coaxial single coils, a magnetic field in each coil is led out by pure iron, a uniform magnetic field is generated in a gap between the two pure irons, the magnetic field is changed by changing the current, and the electromagnet has the characteristics of adjustable magnetic field, large uniform area, strong magnetic field and the like, and the highest magnetic field is up to 10T;
helmholtz coil: the magnetic field is generally below 1000GS, and according to the direction of the magnetic field, the magnitude of the magnetic field can be designed into a one-dimensional Helmholtz coil, a one-dimensional compensation Helmholtz coil, a three-dimensional Helmholtz coil and the like, so that a direct-current magnetic field and an alternating-current magnetic field can be generated.
Based on the description of above-mentioned technical noun, among the prior art, one-dimensional Helmholtz coil central magnetic field distribution is similar to the quadrangle, and the magnetic field degree of consistency is lower, will realize the even district of certain size, can only satisfy through the geometric dimensions who increases the coil, brings inconvenience for coil design, processing and preparation. The present invention is therefore directed to a high performance novel uniform magnetic field generator of variable frequency and field strength.
Specific examples are described in detail below, respectively:
the embodiment of the invention provides a uniform magnetic field generator with variable frequency and field intensity, and as seen in fig. 1, fig. 1 is a schematic structural diagram of an embodiment of the uniform magnetic field generator with variable frequency and field intensity, which is provided by the invention, and comprises an alternating current power supply module 101, a coil module 102, a variable capacitor module 103 and a control circuit module 104, wherein the alternating current power supply module 101, the coil module 102 and the variable capacitor module 103 are sequentially electrically connected, and the control circuit module 104 is respectively electrically connected with the alternating current power supply module 101 and the variable capacitor module 103, wherein the control circuit module 104 determines a compensation capacitance value and a resonance current amplitude value according to a set magnetic field frequency and a set magnetic field amplitude value, drives the variable capacitor module to adjust the capacitance value, and drives the alternating current power supply module to adjust the voltage amplitude value, so that the coil module generates a uniform magnetic field with variable frequency and field intensity.
In the embodiment of the invention, an alternating current power supply (a low-frequency section is composed of a signal generator and a power amplifier, and a high-frequency section is composed of a high-power direct current power supply and a phase-shifted full bridge circuit), a voltage and current sampling resistor, a variable capacitor bank, a Helmholtz coil and a control circuit are arranged, and the control circuit is integrated on a main control board. The main control board is connected with the upper computer through a serial port to realize real-time communication; the circuit frequency variation range can be set to be 1 Hz-400kHz, the variable capacitance is divided into 17 constant value capacitances and 2 parallel polar plate adjustable capacitances, the access state of 19 relays is determined by compensating capacitance values under the given frequency of an upper computer, an adjustable capacitor with the capacitance value range of 4 pF-20 uF is realized, a compensating capacitance group and the self-inductance of a Helmholtz coil form a resonance network, when the circuit reaches a stable state, the magnetic induction intensity generated by the Helmholtz coil is calculated according to PEEC, the input end voltage is adjusted, the current value of the circuit is stabilized, and the Helmholtz coil generates a magnetic field with adjustable frequency and field intensity.
It should be noted that when the frequency of the power supply in the circuit changes, the inductive reactance and the capacitive reactance in the circuit will follow the frequency change, so the voltage and current responses in the circuit also change with the frequency, and the operating state of the circuit also changes with the frequency. The circuit is realized by adopting an RLC series resonance circuit, the impedance of L and C can be completely or partially counteracted by utilizing a resonance network consisting of a capacitor and an inductor, constant current output is realized by matching with feedback control of a power supply on the premise that the output voltage of an alternating current power supply does not change much, and meanwhile, a higher power factor can be ensured. The input impedance Z (j ω) of the circuit can be expressed as
When the circuit reaches a resonant state, at this time
The angular frequency ω 0 and the frequency f0 at which the circuit resonates are known as
In the embodiment of the invention, the large Helmholtz coil is driven to generate the magnetic field with adjustable frequency and field intensity, and the topological structure is simple.
The uniform magnetic field generator system mainly comprises an alternating current power supply device, a sampling resistor area, a Helmholtz coil, a variable capacitance area and a control circuit area. In fig. 2, R1 (i.e., the first sampling resistor) is a non-inductive resistor for current sampling, and R2 (i.e., the second sampling resistor) and R3 (i.e., the third sampling resistor) are high-impedance resistors for voltage sampling. The control circuit comprises a signal processing circuit, a driving circuit and a communication circuit. In order to reduce the loss caused by the skin effect of the lead at high frequency, litz wires are selected as main circuit connecting wires, and twisted pair shielding wires are selected as communication wires.
As a preferred embodiment, referring to fig. 2, fig. 2 is a schematic structural diagram of an embodiment of a uniform magnetic field generator with variable frequency and field intensity provided by the present invention, where the alternating current power supply module includes a low frequency unit and a high frequency unit, where:
the low-frequency unit comprises a signal generator and a power amplifier which are electrically connected, wherein the signal generator is electrically connected with the control circuit module, and the power amplifier is electrically connected with the Helmholtz coil module;
the high-frequency unit comprises a high-power direct current source and a phase-shifted full-bridge circuit which are electrically connected, wherein the phase-shifted full-bridge circuit is respectively electrically connected with the control circuit and the Helmholtz coil module.
In the embodiment of the invention, the alternating current power supply part is divided according to low frequency (1 Hz-100 kHz) and high frequency (100 kHz-400 kHz), so that the frequency variation range of the circuit can be set to be 1 Hz-400kHz, the frequency adjustable range is wide, and the frequency precision is high.
As a preferred embodiment, the helmholtz coil module includes a sampling circuit and a helmholtz coil, wherein:
the sampling circuit comprises a first sampling resistor to a third sampling resistor, wherein the first sampling resistor is electrically connected to the alternating current power supply module, the variable capacitance module and the control circuit module respectively, the second sampling resistor is electrically connected to the alternating current power supply module, the first sampling resistor and the control circuit module respectively, and the third sampling resistor is electrically connected to the alternating current power supply module, the second sampling resistor and the Helmholtz coil respectively;
the Helmholtz coil is electrically connected to the alternating current power supply module and the variable capacitance module respectively.
In a specific embodiment of the present invention, the helmholtz coil is a double-layer coil with a spiral structure, and the parameters of the coil are as follows: the wire diameter is 6mm, the radius of the first layer is 0.285m, the radius of the second layer is 0.315m, the number of turns is 8, and the turn interval is 0.024m. The self-inductance of the coil in a low frequency band is 2.97mH by the PEEC method, and the size and distribution of a uniform magnetic field region under the excitation of unit current are obtained. The invention can be suitable for the calibration of the probe with the block size of 100mm multiplied by 100mm, but is not limited to the function;
according to the principle of the manufacturing process of the Helmholtz coil, the turn-to-turn capacitance of the coil can be known, so that the equivalent self-inductance values of two ends of the Helmholtz coil can be changed along with the increase of the frequency, and meanwhile, due to the existence of the skin effect and the proximity effect, the equivalent resistance of two ends of the coil can be increased along with the increase of the frequency of the circuit. In order to ensure the accuracy of the calculation of the compensation capacitance, the self-inductance and self-resistance measurement of the built Helmholtz coil is required before the experiment, and the experimental data is used for calculating the resonance capacitance at a given frequency.
As a preferred embodiment, the variable capacitance module comprises at least one regulating branch and at least one parallel plate branch connected in parallel, wherein:
the at least one adjusting branch circuit comprises capacitors with different capacitance values and a relay which are connected in series, and the relay is used for controlling the capacitors to be connected or not connected;
the at least one parallel polar plate branch comprises a parallel polar plate capacitor and a stepping motor corresponding to the parallel polar plate capacitor, and the capacitance value is adjusted by adjusting the staggered distance between the polar plates of the at least one parallel polar plate capacitor through the stepping motor.
In the embodiment of the invention, the stepping motor is used for positioning and adjusting, the accuracy of adjusting the capacitor is only related to the position accuracy of motor adjustment, the stepless adjustment of the capacitor in a set range can be realized, and the parallel polar plate capacitor can ensure the withstand voltage of the system.
As a more specific example, the inductance value substituted in the compensation capacitance calculation is the actual measurement value of the helmholtz coil at different frequencies, and an experimental table is embedded in the program, so that the final compensation capacitance calculation can be more accurate.
As a more specific example, in the variable capacitance region, a part of the fixed capacitance is commercially preferred, and a part of the fixed capacitance is stacked parallel plate capacitance to ensure a withstand voltage level.
As a more specific embodiment, in the variable capacitance area, two-gear adjustable capacitors with parallel plates are finally arranged, the positions of the plates of the adjustable capacitors are dragged by a 42-step motor, the stepping precision is high, and the precision of the capacitance value is ensured when compensation is carried out in a high-frequency band.
As a more specific embodiment, the state of the compensation capacitance value is obtained by a genetic algorithm, and the final relay state and the motor motion position are determined by judging the current-voltage phase difference and the current value in combination with historical data.
In a specific embodiment of the present invention, referring to fig. 3, fig. 3 is a simulation schematic diagram of an embodiment of a simulation model of the parallel plate constant value capacitor and the parallel plate adjustable capacitor in the variable capacitor region in ANSYS provided by the present invention, and the present invention only shows a cross-sectional view in the x direction. According to the formula of parallel plate capacitance
Wherein epsilon is dielectric constant in vacuum, S is the opposite area of the parallel polar plates, and d is the distance between the parallel polar plates. In the invention, the equivalent capacitance value between the anode 2 and the cathode 1 is changed by changing the size and the number of the parallel polar plates, so that the three-gear pF-level constant value capacitor is realized; and simultaneously, the staggered distance 5 between the parallel polar plates is changed to realize the part of the adjustable capacitor in the variable capacitor area. In fig. 3, 1 denotes a negative electrode, 2 denotes a positive electrode, 3 denotes a negative parallel plate, 4 denotes a positive parallel plate, 5 denotes a parallel plate offset distance, and 6 denotes a distance between parallel plates.
In one specific embodiment of the invention, the ULN2803A is used as a relayThe driving chip comprises eight independent Darlington tube driving circuits inside the chip, a freewheeling diode is integrated and can be used for driving a relay, the type of the relay is 855AP-1A-C, and 12V is selected for coil driving voltage. The capacitors with different capacitance values are connected in series with the relay and then connected into the main circuit in parallel, and the single chip microcomputer controls the relay to determine the connection or non-connection of a certain capacitor, so that the capacitance value required when the circuit resonates is combined. According to experimental data, the self-inductance of the Helmholtz coil under each frequency is obtained, the frequency ranges from 1Hz to 400kHz, and the quality factor of the circuit
When (when Q)<1, the circuit does not need a compensation capacitor), and the capacitance value range of the compensation capacitor needed by the circuit is designed;
in the invention, three groups of capacitors of uF, nF and pF are selected, and the maximum compensation capacitance value is expected to be 20uF, namely all the capacitors are connected. According to the dichotomy principle, the desired capacitance values are 10uF, 5uF, 2.5uF, 1.5uF, 625nF, 312.5nF, 156.25nF, 78.13nF, 39.06nF, 19.5nF, 9.77nF, 4.88nF, 2.44nF, 1.22nF, 610.5pF, 305.18pF, 152.59pF, for 17 gears. According to the actual requirements of the capacitor on voltage resistance and overcurrent, C4AF7BW5100A3OK, C4AFBBW4500T3MK, C4ASPBW4250A3MJ, C4BSNBX4150ZAJJ, CRA6500154J54F6 and B32642B0393K6 types of capacitors are selected, the first 14 capacitors are obtained by combining the capacitors of the same type in series and parallel, and the capacitors are the market optimized constant value capacitors in the variable capacitor area; and the final 3 grades of fixed value capacitors are composed of stacked parallel plate capacitors and are parallel plate fixed value capacitors in the variable capacitor area. The actual value of the final fixed-value capacitor is as follows: 10uF, 5uF, 2.5uF, 1.5uF, 600nF, 300nF, 150nF, 78nF, 39nF, 19.5nF, 9.75nF, 4.875nF, 2.4375nF, 1.211875nF, 615pF, 264pF, 153pF. Considering the possibility that other stray capacitances exist in a hardware system, such as capacitances among wires, and the capacitance actually connected into the system is larger or smaller, a group of capacitance ranges of the variable capacitance group is set as follows: 13.842pF-155.98pF, and the other group is set to be 4.025pF-23.027pF, which is a parallel plate adjustable capacitor of the variable capacitor area. The final adjustable capacitance ranges from 4pF to 20uF.
As the preferred embodiment, control circuit includes main control board, host computer, switching power supply, relay drive module and motor drive module, wherein:
the main control board is electrically connected with the upper computer, the switching power supply, the relay driving module and the motor driving module respectively;
the upper computer is electrically connected with a signal generator in the alternating current power supply module;
the switching power supply is electrically connected with the relay;
the relay driving module is electrically connected with at least one adjusting branch in the variable capacitor module;
and the motor driving module is electrically connected with the stepping motor in the variable capacitance module.
In the embodiment of the invention, the voltage and the current values of the main circuit are subjected to signal processing by the sampling resistor through attenuation, second-order filtering, lifting, amplification and the like, the frequency, the size and the phase difference of the voltage and the current values are calculated by an FFT program in an STM32F4, the calculation speed of the STM32F4 is high, and the calculation result is accurate.
It should be noted that the sampling measurement circuit is composed of an attenuation circuit, an isolation circuit, an amplification filter circuit, and a second-order active low-pass filter circuit. In addition, in order to isolate the analog ground from the digital ground, a +2.5V direct current circuit is added and serves as the digital ground of signal processing. In the invention, because the measured frequency span is large, a high-low frequency band measurement mode is adopted, and two measurement circuits only differ in a filtering parameter part. For a signal to be detected, firstly attenuation processing is carried out, isolation buffering is realized through a signal amplifier, then the signal to be detected is amplified, the amplitude value of the signal to be detected is matched with the input signal range of an A/D converter, the low-frequency band filtering cut-off frequency is 400Hz, and the high-frequency band filtering cut-off frequency is 400kHz. The amplified and filtered signals are sent to a single chip microcomputer, ADC data acquisition is carried out on the detected signals, and FFT conversion is carried out to obtain the frequency and the amplitude of the signals. The FFT measures the signal frequency and has the advantages of high amplitude and frequency measurement precision, and the phase difference between the voltage and the current is measured, so that the requirements of the invention can be completely met.
As a more specific embodiment, when the resonance of the circuit reaches a stable state, the magnetic induction intensity generated by the helmholtz coil is calculated according to the PEEC, the voltage at the input end is adjusted, and the current value of the circuit is changed, so that the magnetic induction intensity generated by the helmholtz coil can be changed, and a feedback adjustment circuit is formed.
The feedback adjusting circuit has two selection modes, the first mode is to directly measure the magnetic induction intensity generated by the helmholtz coil and adjust the input voltage according to the field intensity to form closed-loop control. However, the field intensity measurement is very complicated, which makes the circuit more complicated and is not suitable for selection. The second way is to measure the current and voltage at two ends of the main circuit, calculate the magnetic induction intensity generated by the helmholtz coil according to the PEEC when the circuit resonance reaches a stable state, adjust the voltage at the input end of the power supply, change the current value of the circuit, and enable the helmholtz coil to generate a magnetic field with adjustable frequency and field intensity.
The embodiment of the present invention provides a control method for a uniform magnetic field generator with variable frequency and field strength, and with reference to fig. 4, fig. 4 is a schematic flow chart of an embodiment of the control method for a uniform magnetic field generator with variable frequency and field strength provided by the present invention, and based on the uniform magnetic field generator with variable frequency and field strength, the method includes steps S401 to S403, where:
in step S401, a magnetic field frequency and a magnetic field amplitude input by the upper computer are acquired;
in step S402, determining a compensation capacitance value and a resonant current amplitude value according to the magnetic field frequency and the magnetic field amplitude value;
in step S403, the variable capacitor module is driven to adjust the capacitance value and the ac power module is driven to adjust the voltage amplitude according to the compensation capacitance value and the resonance current amplitude, so that the coil module generates a uniform magnetic field corresponding to the magnetic field frequency and the magnetic field amplitude.
In the embodiment of the invention, for the main circuit, frequency-division output is realized in the alternating current power supply part, the voltage current sampling resistor is used for replacing the traditional voltage current sensor, the Helmholtz coil is independently designed by adopting a PEEC method, the low-frequency-band self-inductance can be accurately calculated, and the fixed-value capacitor and the variable parallel plate capacitor are combined by the variable capacitor area. For the main control board and the control part, voltage and current sampling values are subjected to links such as attenuation, isolation, filtering amplification and the like, the frequency and the amplitude of the sampling values can be accurately calculated by a single chip microcomputer FFT program through ADC conversion, the state of the variable capacitor bank is preliminarily determined by given frequency, the feedback link determines the position of the final parallel polar plate variable capacitor through judging information such as current voltage amplitude, phase difference and the like, the output of the alternating current power supply is further adjusted, and the stable current states of the alternating current power supply under different frequencies are guaranteed. The upper computer and the single chip microcomputer are in real-time communication through the serial port, the circuit state is reflected at any time, and good interaction is achieved.
First, the upper computer issues a command to input the frequency f set by the user and the desired magnetic field magnitude B h The program calculates the compensation capacitor C according to the resonance relation S Then, the expected resonance current value I is obtained by the PEEC method s0 . At the moment, the upper computer can determine the on-off state at the moment according to the genetic algorithm and drive a signal instruction to the single chip microcomputer relay through serial port communication. The lower computer performs FFT calculation on the sampled voltage and current signals and judges the phase difference of the voltage and current signals at the moment
Whether the capacitance is less than 10 degrees, or else, calculating the capacitance C needing fine adjustment at the moment z And adjusting by a stepping motor until the phase difference is less than 10 degrees. With desired value of resonant current I s0 Comparing with the sampling current value Is, if the difference value of the two amplitude values Is s -I s0 Out of error range, by calculating (I) s -I s0 ) And judging the amplitude of the power supply voltage needing to be increased or decreased by the xR, increasing or decreasing the amplitude of the signal generator at high frequency, and changing the amplitude of the voltage by changing the phase angle of the phase-shifted full bridge at low frequency until an error range is met, and waiting for the next instruction. Finally, the Helmholtz coil generates a uniform magnetic field with variable frequency and field intensity.
As a preferred embodiment, the step S402 includes:
determining the compensation capacitance value according to the magnetic field frequency and the magnetic field amplitude value based on a resonance relation;
determining the resonant current amplitude according to the magnetic field frequency and the magnetic field amplitude based on a PEEC method;
the open and closed state of a relay in at least one regulating branch in the variable capacitance module is determined based on a genetic algorithm.
In the embodiment of the invention, the upper computer sends out an instruction and inputs the frequency f set by a user and the expected magnetic field size B h The program calculates the compensation capacitor C according to the resonance relation S Then, the expected resonance current value I is obtained by the PEEC method s0 。
As a preferred embodiment, referring to fig. 5, fig. 5 is a schematic flowchart of an embodiment of step S403 in fig. 4 provided by the present invention, where step S403 specifically includes step S501 to step S502, where:
in step S501, a phase difference between the voltage signal and the current signal is determined according to the two signals;
in step S502, it is determined whether the phase difference satisfies a preset condition, and if not, the step motor is driven to adjust the capacitance value according to the compensation capacitance value until the preset condition is satisfied.
In the embodiment of the invention, the lower computer carries out FFT calculation on the sampled voltage and current signals and judges the phase difference of the voltage and current signals at the moment
Whether the capacitance is less than 10 degrees, or else, calculating the capacitance C needing fine adjustment at the moment z And adjusting by a stepping motor until the phase difference is less than 10 degrees.
As a preferred embodiment, referring to fig. 6, fig. 6 is a schematic flowchart of an embodiment of step S403 in fig. 4 provided by the present invention, where step S403 specifically includes step S601 to step S603, where:
in step S601, if the phase difference satisfies the preset condition, comparing the current signal with the resonant current amplitude to determine an amplitude difference therebetween;
in step S602, if the amplitude difference exceeds the error range, determining a voltage adjustment value according to the current signal, the resonant current amplitude, and the resistance value;
in step S603, the ac power module is controlled to adjust the voltage amplitude according to the voltage adjustment value until the amplitude difference is within the error range.
In the embodiment of the invention, a desired resonance current value I is used s0 And the sampled current value I s Comparing if the two have a difference I in magnitude s -I s0 Out of error range, by calculating (I) s -I s0 ) Xr to determine the magnitude by which the supply voltage needs to be increased or decreased.
As a preferred embodiment, the step S603 specifically includes:
if the voltage is in the high frequency, adjusting the amplitude of the signal generator, and changing the voltage amplitude until the amplitude difference is within the error range;
if the voltage is in the low frequency, adjusting the phase angle of the phase-shifted full bridge, and changing the voltage amplitude until the amplitude difference is within the error range.
In the embodiment of the invention, the amplitude of the signal generator is increased or decreased at high frequency, and the amplitude of the voltage is changed at low frequency by changing the phase angle of the phase-shifted full bridge until an error range is met, and the next command is waited. Finally, the Helmholtz coil generates a uniform magnetic field with variable frequency and field intensity.
Referring to fig. 7 to 10, fig. 7 is a schematic waveform diagram of an embodiment of a voltage and a current under direct driving provided by the present invention, fig. 8 is a schematic waveform diagram of an embodiment of a voltage and a current under a full compensation scheme provided by the present invention, fig. 9 is a schematic waveform diagram of an embodiment of a voltage and a current under a compensation state provided by the present invention, fig. 10 is a schematic waveform diagram of another embodiment of a voltage and a current under a compensation state provided by the present invention, and a technical solution of the present invention is better described with specific application examples:
in a low frequency band, an uncompensated strategy is selected, in the first example, the frequency is set to be 100Hz, the variable capacitance region is disconnected, the helmholtz coil is directly driven by an alternating current power supply, the expected magnetic field size is 177uT, the corresponding current amplitude is 3A at the moment, as shown in fig. 7, the waveform diagram of voltage and current under direct driving is shown, the voltage amplitude is 6.2V, the current amplitude is 3.06A, and the voltage-current phase difference is 63 degrees;
when the frequency is less than a certain frequency, the capacitance state values calculated by the program are the same, at this time, a full compensation scheme is adopted, in example two, the frequency is set to 650Hz, the capacitances in the variable capacitance region are all connected, the modulation of the capacitances in the two gears in the adjustable capacitance region is the maximum, the expected magnetic field size is 177uT, the corresponding current amplitude is 3A at this time, as shown in fig. 8, the waveform diagrams of the voltage and the current under the full compensation scheme are shown, the voltage amplitude is 2.22V, the current amplitude is 3.1A, the voltage-current phase difference is 7.956 °, and a good compensation effect can be seen at this time.
The random setting frequency is 856Hz, the input expected magnetic field size is 177uT, the command is input to the upper computer, the corresponding current amplitude is 3A at the moment, and the resonance capacitance value calculated by the program is 11.64uF. Starting a program, wherein the calculated switching state of the relay is 1001010010101110, the adjustable capacitor is connected to a first gear, the staggered distance of the parallel polar plates is 88.4mm, as shown in fig. 9, the waveform diagram of the voltage and the current in the state at the moment is shown, the amplitude of the voltage is 2.3V, the amplitude of the current is 3.1A, and the phase difference between the voltage and the current is 1.93 degrees, so that an expected effect can be achieved;
the frequency was set at 100kHz, the desired field magnitude was input at 29.5uT, and the command was input to the host computer, where the calculated current amplitude was 0.5A and the programmed calculated resonant capacitance was 719.996pF. And starting a program, wherein the calculated relay switch state is 0000000000000001010, the adjustable capacitor is connected to the first gear, the staggered distance of the parallel polar plates is 34.2mm, as shown in fig. 10, the waveform diagram of the voltage and the current in the state is shown, the voltage amplitude is 42.8V, the current amplitude is 0.54A, and the voltage and current phase difference is 0.576 degrees, so that the expected effect can be achieved.
The invention discloses a uniform magnetic field generator with variable frequency and field intensity and a control method thereof.A voltage and current sampling resistor, a variable capacitor bank, a Helmholtz coil and a main control board are arranged by arranging an alternating current power supply (a low frequency section consists of a signal generator and a power amplifier, and a high frequency section consists of a high-power direct current power supply and a phase-shifted full bridge circuit), and the main control board is connected with an upper computer through a serial port to realize real-time communication; the circuit frequency variation range can be set at 1-400 kHz, the variable capacitance is divided into 17 constant value capacitors and 2 parallel pole plate adjustable capacitors, the access state of 19 relays is determined by the compensation capacitance value under the given frequency of an upper computer, an adjustable capacitor with the capacitance value range of 4 pF-20 uF is realized, the compensation capacitance group and the self-inductance of a Helmholtz coil form a resonance network, when the circuit reaches a stable state, the magnetic induction intensity generated by the Helmholtz coil is calculated according to PEEC, the input end voltage is adjusted, the current value of the circuit is stabilized, and the Helmholtz coil generates a magnetic field with adjustable frequency and field intensity.
The technical scheme of the invention mainly comprises an alternating current power supply, a voltage and current sampling circuit, a variable capacitance regulating circuit, an RLC series resonance circuit and other control circuits, drives the large Helmholtz coil to generate a magnetic field with adjustable frequency and field intensity, and has the advantages of simple topological structure, wide frequency range, good compensation effect and low requirement on the power of the alternating current power supply. The variable capacitor is automatically adjusted according to given frequency and inductance value to enable the circuit to reach a resonance state, at the moment, a uniform magnetic field with corresponding required frequency and field intensity can be obtained only by adjusting circuit voltage, a resonance network is formed by the compensation capacitor and self-inductance of the Helmholtz coil to achieve steady-current output, and the Helmholtz coil generates a magnetic field with adjustable frequency and adjustable magnetic induction intensity.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Claims (9)
1. A uniform magnetic field generator with variable frequency and field intensity is characterized by comprising an alternating current power supply module, a coil module, a variable capacitor module and a control circuit module, wherein the alternating current power supply module, the coil module and the variable capacitor module are sequentially and electrically connected, and the control circuit module is respectively and electrically connected with the alternating current power supply module, the coil module and the variable capacitor module;
the coil module includes sampling circuit and helmholtz coil, wherein:
the sampling circuit comprises a first sampling resistor to a third sampling resistor, wherein the first sampling resistor is electrically connected to the alternating current power supply module, the variable capacitance module and the control circuit module respectively, the second sampling resistor is electrically connected to the alternating current power supply module, the first sampling resistor and the control circuit module respectively, and the third sampling resistor is electrically connected to the alternating current power supply module, the second sampling resistor and the Helmholtz coil respectively;
the Helmholtz coil is electrically connected to the alternating current power supply module and the variable capacitance module respectively.
2. The uniform magnetic field generator of variable frequency and field strength of claim 1 wherein said ac power module comprises a low frequency unit and a high frequency unit, wherein:
the low-frequency unit comprises a signal generator and a power amplifier which are electrically connected, wherein the signal generator is electrically connected with the control circuit module, and the power amplifier is electrically connected with the coil module;
the high-frequency unit comprises a power direct current source and a phase-shifted full-bridge circuit which are electrically connected, wherein the phase-shifted full-bridge circuit is respectively electrically connected with the control circuit and the coil module.
3. The variable frequency and field strength uniform magnetic field generator of claim 1 wherein said variable capacitance module comprises at least one tuning branch and at least one parallel plate branch connected in parallel, wherein:
the at least one adjusting branch circuit comprises capacitors with different capacitance values and a relay which are connected in series, and the relay is used for controlling the capacitors to be connected or not connected;
the at least one parallel polar plate branch comprises a parallel polar plate capacitor and a stepping motor corresponding to the parallel polar plate capacitor, and the capacitance value is adjusted by adjusting the staggered distance between the polar plates of the at least one parallel polar plate capacitor through the stepping motor.
4. The uniform magnetic field generator with variable frequency and field strength according to claim 1 wherein said control circuit comprises a main control board, an upper computer, a switching power supply, a relay drive module and a motor drive module, wherein:
the main control board is electrically connected with the upper computer, the switching power supply, the relay driving module and the motor driving module respectively;
the upper computer is electrically connected with a signal generator in the alternating current power supply module;
the switching power supply is electrically connected with the relay;
the relay driving module is electrically connected with at least one adjusting branch in the variable capacitor module;
and the motor driving module is electrically connected with the stepping motor in the variable capacitance module.
5. A method of controlling a variable frequency and field strength uniform magnetic field generator, based on the variable frequency and field strength uniform magnetic field generator according to any one of claims 1 to 4, the method comprising:
acquiring the magnetic field frequency and the magnetic field amplitude input by an upper computer;
determining a compensation capacitance value and a resonant current amplitude value according to the magnetic field frequency and the magnetic field amplitude value;
and driving the variable capacitance module to adjust the capacitance value according to the compensation capacitance value and the resonance current amplitude value, and driving the alternating current power supply module to adjust the voltage frequency and the amplitude value, so that the coil module generates a uniform magnetic field which accords with the magnetic field frequency and the magnetic field amplitude value.
6. The method of claim 5, wherein said determining a compensation capacitance value and a resonant current amplitude from said magnetic field frequency and said magnetic field amplitude comprises
Determining the compensation capacitance value according to the magnetic field frequency and the magnetic field amplitude value based on a resonance relation;
determining the resonant current amplitude according to the magnetic field frequency and the magnetic field amplitude based on a PEEC method;
the open and closed state of a relay in at least one regulating branch in the variable capacitance module is determined based on a genetic algorithm.
7. The method of claim 6, wherein said driving said variable capacitance module to adjust capacitance and said ac power module to adjust voltage frequency and amplitude based on said compensation capacitance and said resonant current amplitude comprises:
determining a phase difference between the voltage signal and the current signal according to the voltage signal and the current signal;
and judging whether the phase difference meets a preset condition, if not, driving a stepping motor to adjust the capacitance value according to the compensation capacitance value until the preset condition is met.
8. The method of claim 7, wherein said driving said variable capacitance module to adjust capacitance and said ac power module to adjust voltage amplitude based on said compensation capacitance and said resonant current amplitude, further comprises:
if the phase difference meets the preset condition, comparing the current signal with the resonance current amplitude value, and determining the amplitude difference between the current signal and the resonance current amplitude value;
if the amplitude difference exceeds the error range, determining a voltage adjustment value according to the current signal, the resonance current amplitude and the resistance value;
and controlling the alternating current power supply module to adjust the voltage amplitude according to the voltage adjustment value until the amplitude difference is within an error range.
9. The method of claim 8, wherein said controlling said ac power module to adjust the voltage amplitude until said amplitude difference is within an error range based on said voltage adjustment value comprises:
if the voltage is in the high frequency, adjusting the amplitude of the signal generator, and changing the voltage amplitude until the amplitude difference is within the error range;
if the frequency is low, the phase angle of the phase-shifted full bridge is adjusted, and the voltage amplitude is changed until the amplitude difference is within the error range.
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