CN115755582A - Helmholtz coil magnetic field gradient control method and device based on PID algorithm - Google Patents
Helmholtz coil magnetic field gradient control method and device based on PID algorithm Download PDFInfo
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- CN115755582A CN115755582A CN202211405306.6A CN202211405306A CN115755582A CN 115755582 A CN115755582 A CN 115755582A CN 202211405306 A CN202211405306 A CN 202211405306A CN 115755582 A CN115755582 A CN 115755582A
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Abstract
The invention discloses a Helmholtz coil magnetic field gradient control method and device based on a PID algorithm. Firstly, a magnetic field gradient measurement model is established, the magnetic field gradient is measured through a three-axis fluxgate magnetometer, three axial magnetic field gradients in the current device are collected in real time, the collected signals are filtered, and the collected signals are compared with a given magnetic field gradient steady-state value to calculate the current magnetic field gradient deviation. And calculating a current control Helmholtz coil by applying a PID algorithm aiming at the deviation, and controlling the coil to generate a magnetic field to compensate the magnetic field gradient in the device. According to the method, the PID algorithm is used for automatically controlling the magnetic field gradient, so that the high stability and the high uniformity of the magnetic field environment of a target area are realized, and the extremely weak magnetic field environment is provided for the measurement of the SERF atomic magnetometer.
Description
Technical Field
The invention belongs to the field of magnetic shielding devices and magnetic shielding methods, and particularly relates to a method and a device for controlling the magnetic field gradient of a Helmholtz coil based on a PID algorithm.
Background
The high-sensitivity detection and measurement of the magnetic field have wide application and important function in the fields of biomedicine, space detection, basic physics research, military navigation and the like. The atomic magnetometer working in a Spin-Exchange Relaxation (SERF) state has the highest sensitivity among the current numerous magnetic field measurement technical devices, and can realize fT/Hz 1/2 Magnitude magnetic field measurements.
However, the realization of SERF states by atomic magnetometers has extremely high requirements on the ambient magnetic field detection environment. The SERF atomic magnetometer needs to work under the condition of a magnetic field close to zero, and the biggest challenge of the control of the ambient magnetic field environment is the control of the magnetic field gradient. During the operation of the atomic magnetometer, unpredictable time-varying magnetic fields may be generated due to the influence of geomagnetism and the surrounding environment, for example, up to 10 minutes may occur in a single measurement experiment (10-20 minutes) 2 The magnetic field gradient of nT/m varies significantly, causing significant errors in the atomic magnetometer measurements. In order to effectively reduce the magnetic field inhomogeneity in the measurement environment, the spatial magnetic field gradients have to be controlled in real time by the magnetic field gradient compensation coils. In addition, the atomic magnetometer, which is located at the center of the spatial field of the device, needs to be adjusted to have a wider gap with the surrounding magnetic compensation coil under some application conditions, which results in higher magnetic field non-uniformity in the space, so that the magnetic field gradient compensation coil needs to be used to compensate the spatial magnetic field.
Disclosure of Invention
In order to solve the problem of high magnetic field unevenness in an SERF atomic magnetometer measuring device, the invention provides a Helmholtz coil magnetic field gradient control method and device based on a PID algorithm, which can realize rapid magnetic field gradient compensation in real time based on the PID algorithm, control the spatial magnetic field gradient to a set value and establish a foundation for the SERF atomic magnetometer to realize measurement of an extremely weak magnetic field environment.
In order to achieve the purpose, the invention adopts the technical scheme that:
a Helmholtz coil magnetic field gradient control method based on a PID algorithm comprises the following steps:
step (1), establishing a triaxial magnetic field gradient measurement model, and respectively arranging a first fluxgate magnetometer and a second fluxgate magnetometer on an x axis at an interval of L by using 3 pairs of fluxgate magnetometers 1 (ii) a The third and fourth flux gate magnetometers are arranged on the y axis with a distance L between 2 The fifth fluxgate magnetometer and the sixth fluxgate magnetometer are arranged on the z axis and have a distance L between 3 (ii) a The difference of the magnetic fields measured by each pair of the fluxgate magnetometers is divided by the distance between the measuring points to obtain the axial magnetic field gradient of the pair of the fluxgate magnetometers, and the magnetic field gradient signals of the x axis, the y axis and the z axis are obtained from the magnetic field information obtained by the 3 pairs of the fluxgate magnetometers;
and (2) processing the obtained magnetic field gradient signals by a preamplifier, converting by a DA (digital-to-analog) module and recording the processed three-axis magnetic field gradient signals as G x 、G y 、G z With the desired magnetic field gradient setpoint G x0 、G y0 、G z0 Performing subtraction to obtain a difference value Err x (t)、Err y (t) and Err z (t)；
Step (3) of measuring Err measured in the step (2) x (t)、Err y (t) and Err z Err for (t) i And (t) represents and substitutes into a PID algorithm, and the PID algorithm is input into the DSP controller:
wherein q is i (t) is the output value of the PID control algorithm, K p Is in proportionCoefficient, T I As an integral coefficient, T D I = x, y, z, representing coordinate axes;
discretizing the PID, and integrating PID coefficients to obtain:
wherein q is i (k) Is an output signal Err obtained by the calculation of the controller DSP through a PID algorithm i (k) Representing this deviation after dispersion, err i (K-1) represents the last deviation, K p K i And K d Respectively represent the discrete integrated PID coefficients. After a closed loop is formed, PID parameters are adjusted by observing the output signal of the fluxgate magnetometer;
and (4) converting the obtained calculation output signal through a DA module, outputting the signal to a current source, and outputting high-precision current to the gradient coil pair through the current source, so that the control of the magnetic field gradient in the space is realized, and the required magnetic field gradient is obtained.
The invention relates to a control device for realizing the Helmholtz coil magnetic field gradient control method based on the PID algorithm, which comprises the following steps:
a magnetic field gradient signal acquisition unit which acquires a spatial magnetic field gradient by using 3 pairs of symmetrically placed three-axis fluxgate magnetometers;
the control unit comprises a DSP controller and a DA module;
and the execution unit comprises a high-precision current source and a gradient compensation coil driven and controlled by the high-precision current source.
Further, the 3 pairs of symmetrically placed three-axis fluxgate magnetometers are respectively placed on an x axis, a y axis and a z axis which are symmetrical about a spatial center position, and each pair of fluxgate magnetometers has the same distance from the spatial center position for measurement; adjusting the position of the fluxgate magnetometer and the position of the coil according to the required measurement space requirement; the three-axis fluxgate magnetometer is arranged around the central position in an axial symmetry manner, so that the measurement error caused by the interference of crossed axes is effectively eliminated.
Compared with the prior art, the invention has the advantages that:
(1) The invention is based on PID algorithm, and can realize real-time accurate magnetic field gradient control. In addition to being able to control the initially static spatial magnetic field gradient to a desired value, the magnetic field gradient can be quickly controlled and compensated for by the helmholtz coil pair when the external environment creates magnetic field gradient disturbances to the device.
(2) The magnetic field sensor for measuring the magnetic field gradient is a three-axis fluxgate magnetometer with 3 pairs, which is respectively positioned on an x axis, a y axis and a z axis which are symmetrical about the spatial central position, and the distances from the magnetic field sensor to the spatial central position are the same. The measuring device is arranged around the central position in an axisymmetric manner, so that the measuring error caused by the cross-axis interference can be effectively eliminated.
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FIG. 1 is a flow chart of a method for controlling the magnetic field gradient of a Helmholtz coil based on a PID algorithm according to the invention;
FIG. 2 is a system block diagram of a Helmholtz coil magnetic field gradient control device based on a PID algorithm according to the present invention;
fig. 3 is a schematic system structure diagram of the helmholtz coil magnetic field gradient control apparatus according to the present invention.
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. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
As shown in fig. 2, the control system of the helmholtz coil magnetic field gradient control device based on the PID algorithm of the present invention includes a magnetic field gradient signal acquisition unit, a control unit, and an execution unit. The control unit comprises a DSP controller and a DA module; the execution unit comprises a high-precision current source and a magnetic field gradient compensation coil driven and controlled by the high-precision current source; the magnetic field gradient signal acquisition unit acquires the spatial magnetic field gradient by using 3 pairs of symmetrically placed three-axis fluxgate magnetometers. After being processed by the preamplifier, the AD module and the filter circuit, the signal is input into the DSP control module, and is subjected to difference with a magnetic field gradient set value to obtain a deviation value, and the DSP performs calculation to achieve the purpose of eliminating the deviation value. The control signal output by the DSP is converted by the DA module and output to the gradient compensation coil through the voltage-controlled current source, and further the space magnetic field gradient is controlled.
As shown in fig. 1, the helmholtz coil magnetic field gradient control method based on the PID algorithm of the present invention comprises the following steps:
and (1) establishing a triaxial magnetic field gradient measurement model. As shown in fig. 3, according to the measurement requirement, 3 pairs of three-axis fluxgate measurement distances and placement positions are determined. Respectively arranging a first fluxgate magnetometer 1 and a second fluxgate magnetometer 2 on an x axis with a distance L between 1 (ii) a The third fluxgate magnetometer 3 and the fourth fluxgate magnetometer 4 are arranged on the y axis with a distance L between them 2 A fifth fluxgate magnetometer 5 and a sixth fluxgate magnetometer 6 are arranged on the z-axis with a distance L therebetween 3 . The difference of the magnetic fields measured by the pair of fluxgate magnetometers is divided by the distance between the measuring points to obtain the axial magnetic field gradient of the pair of fluxgate magnetometers, and the magnetic field gradient signals of the three axes of x, y and z are obtained from the magnetic field information obtained by the 3 pairs of fluxgate magnetometers.
And (2) processing the obtained magnetic field gradient signals by a preamplifier, converting by a DA (digital-to-analog) module and recording the processed three-axis magnetic field gradient signals as G x 、G y 、G z With the desired magnetic field gradient setpoint G x0 、G y0 、G z0 Performing a difference operation to obtain a difference value Err x (t)、Err y (t) and Err z (t)。
And (3) respectively using 3 paths of DSP controllers to control the 3 pairs of coils.
Err measured in step (2) x (t)、Err y (t) and Err z Err for (t) i (t) represents and substitutes into PID algorithm, and inputs into DSP controller:
wherein q is i (t) is the output value of the PID control algorithm, K p Is a proportionality coefficient, T I As an integral coefficient, T D Is a differential coefficient; i = x, y, z, representing the coordinate axes of the established coordinate system.
Discretizing the PID, and integrating PID coefficients to obtain:
wherein q is i (k) Is an output signal Err obtained by the calculation of the controller DSP through a PID algorithm i (k) Representing this deviation after dispersion, err i (K-1) represents the last deviation, K p K i And K d Respectively representing discrete integrated PID coefficients. And after a closed loop is formed, the PID parameters are adjusted by observing the output signal of the fluxgate magnetometer.
And (4) converting the obtained calculation output signal through a DA module, outputting the signal to a current source, and outputting high-precision current to the gradient coil pair through the current source to realize the control of the magnetic field gradient in the space and obtain the required magnetic field gradient.
Those matters not described in detail in the present specification are well known in the art to which the skilled person pertains. It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (3)
1. A Helmholtz coil magnetic field gradient control method based on a PID algorithm is characterized by comprising the following steps:
step (1), establishing a triaxial magnetic field gradient measurement model, and respectively using 3 pairs of fluxgate magnetometers to respectively provide a first fluxgate magnetometer and a second fluxgate magnetometerArranged on the x-axis with a spacing L between 1 (ii) a The third and fourth flux gate magnetometers are arranged on the y axis with a distance L between 2 The fifth fluxgate magnetometer and the sixth fluxgate magnetometer are arranged on the z axis and have a distance L between 3 (ii) a The difference of the magnetic fields measured by each pair of the fluxgate magnetometers is divided by the distance between the measuring points to obtain the axial magnetic field gradient of the pair of the fluxgate magnetometers, and the magnetic field gradient signals of the x axis, the y axis and the z axis are obtained from the magnetic field information obtained by the 3 pairs of the fluxgate magnetometers;
and (2) processing the obtained magnetic field gradient signals by a preamplifier, converting by a DA (digital-to-analog) module and recording the processed three-axis magnetic field gradient signals as G x 、G y 、G z With the desired magnetic field gradient set point G x0 、G y0 、G z0 Performing a difference operation to obtain a difference value Err x (t)、Err y (t) and Err z (t)；
Step (3) of measuring Err measured in the step (2) x (t)、Err y (t) and Err z Err for (t) i (t) is expressed and substituted into a PID algorithm, and is input into the DSP controller:
wherein q is i (t) is the output value of the PID control algorithm for the i-axis magnetic field gradient controller, K p Is a proportionality coefficient, T I As an integral coefficient, T D Is a differential coefficient, i = x, y, z, representing a coordinate axis;
discretizing the PID, and integrating PID coefficients to obtain:
wherein q is i (k) Is an output signal Err obtained by the calculation of the controller DSP through a PID algorithm i (k) Representing this deviation after dispersion, err i (k-1) represents the last deviation,K p K i and K d Respectively represent the discrete integrated PID coefficients. After a closed loop is formed, PID parameters are adjusted by observing output signals of the fluxgate magnetometer;
and (4) converting the obtained calculation output signal through a DA module, outputting the signal to a current source, and outputting high-precision current to the gradient coil pair through the current source to realize the control of the magnetic field gradient in the space and obtain the required magnetic field gradient.
2. A control apparatus for implementing the method of controlling magnetic field gradient of a helmholtz coil based on PID algorithm according to claim 1, comprising:
a magnetic field gradient signal acquisition unit which acquires a spatial magnetic field gradient by using 3 pairs of symmetrically placed three-axis fluxgate magnetometers;
the control unit comprises a DSP controller and a DA module;
and the execution unit comprises a high-precision current source and a gradient compensation coil driven and controlled by the high-precision current source.
3. The control device according to claim 2, characterized in that: the three-axis fluxgate magnetometers which are symmetrically arranged are respectively arranged on an x axis, a y axis and a z axis which are symmetrical about the central position of the space, and the distance between each pair of fluxgate magnetometers and the central position of the measurement space is the same; adjusting the position of the fluxgate magnetometer and the position of the coil according to the required measurement space requirement; the three-axis fluxgate magnetometer is arranged around the central position in an axial symmetry manner, so that the measurement error caused by the interference of crossed axes is effectively eliminated.
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