CN105203096B - Rotating excitation field fast tracking method and system based on 4 points of measurements - Google Patents

Abstract

The invention belongs to the technical field of electromagnetic tracking, in particular to a method and system for fast tracking of a rotating magnetic field based on four-point measurement. The method of the present invention includes: firstly, within the 180° rotation search range of any plane, the magnetic field source collects a magnetic induction intensity value for every 45° rotation of the three-axis magnetic sensor, and collects four values; then, according to certain rules, the four sampling values Divide into two groups located on both sides of the maximum magnetic induction intensity; then, connect the 2-point data of each group into a straight line, and calculate the slope of the two straight lines; finally, correct the two straight lines according to certain rules, then the intersection of the two straight lines corresponds to The rotation angle is consistent with the rotation angle corresponding to the maximum magnetic induction intensity. The invention also includes an electromagnetic tracking system for realizing the fast tracking method of the rotating magnetic field based on four-point measurement. The invention can improve the accuracy and speed of the rotating magnetic field pointing to the magnetic sensor, thereby improving the performance of the electromagnetic tracking system.

Description

Method and system for fast tracking of rotating magnetic field based on four-point measurement

technical field

The invention belongs to the technical field of electromagnetic tracking, and in particular relates to an optimization method for measuring the maximum rotation angle of magnetic induction intensity of an electromagnetic tracking system and a system thereof.

Background technique

Electromagnetic Tracking (Electromagnetic Tracking), or electromagnetic field positioning, is a method of detecting and real-time tracking the spatial position and attitude of the tracking target using electromagnetic fields. This method can be applied to the navigation of minimally invasive surgery, and can also be applied to fields such as virtual (augmented) reality and three-dimensional ultrasound imaging. The electromagnetic tracking system generally consists of three parts: a magnetic field source (such as a permanent magnet, an electromagnet coil), a magnetic field sensor, and a control processing unit. The magnetic field source generates a magnetic field at a fixed position, and then uses the magnetic induction intensity data measured by the magnetic sensor attached to the tracking target to solve the spatial position and attitude of the tracking target.

The electromagnetic tracking method based on the rotating magnetic field uses a magnetic sensor to capture the maximum value of the magnetic induction intensity, and determines that the maximum value of the magnetic induction intensity generated by the rotating magnetic field source (composed of electromagnetic coils) points to the magnetic sensor fixed on the tracking target, thereby calculating the tracking target relative to The position and attitude of the magnetic field source. Therefore, if two magnetic field sources with known relative distances are used to search alternately, and the maximum magnetic induction intensity generated by them is finally directed to the magnetic sensor, the geometric relationship between the tracking target and the two magnetic field sources can be obtained , quickly calculate the position and attitude of the six degrees of freedom of the tracking target through a non-iterative algorithm. Compared with the iterative position and attitude algorithm, the non-iterative algorithm has fast calculation speed, simple operation, stable performance, and low requirements for hardware configuration. Problems with local extremum points, etc.

However, for the above-mentioned electromagnetic tracking system based on the rotating magnetic field, how to quickly and accurately direct the maximum magnetic induction intensity generated by the magnetic field source to the sensor will directly affect the performance of the entire system.

Contents of the invention

The purpose of the present invention is to provide a rotating magnetic field fast tracking method and system based on the rotating magnetic field electromagnetic tracking system.

In the present invention, the rotating magnetic field electromagnetic tracking system includes three parts: a magnetic field source unit composed of two relatively fixed and known magnetic field sources, a three-axis magnetic sensor unit and a control processing unit. Wherein the magnetic field source can realize arbitrary orientation of the maximum value of the magnetic induction intensity by rotating and scanning; this rotation can be realized by a mechanical system or an electronic system. The three-axis magnetic sensor can collect the magnetic induction intensity values in three orthogonal directions, and obtain the magnitude and direction of the magnetic induction intensity at the location of the magnetic sensor through vector synthesis. On the one hand, the control processing unit provides suitable excitation for the electromagnetic coils that make up the magnetic field source and current controls the rotation and scanning of the magnetic field source, so that the maximum magnetic induction intensity generated by the two magnetic field sources finally points to the magnetic sensor (tracking target); on the other hand, the three The data collected by the axial magnetic sensor unit is processed and the rotation angle when the maximum magnetic induction intensity points to the magnetic sensor is calculated.

The realization of the present invention is also based on the following prior knowledge and conditions: 1. Observed at a fixed point, the magnetic induction intensity of the rotating magnetic field in the plane has a periodicity of 180°, and the maximum/minimum value in the cycle is unique; The two max/min values are 180° apart. ② According to the working principle of the electromagnetic tracking system based on the rotating magnetic field applicable to the present invention, the magnetic field source can be rotated and searched within an arbitrary range of 180°, so that a maximum value of magnetic induction can be directed to the sensor; Corresponds to two rotation angles. When two relative positions are fixed and the maximum magnetic induction intensity produced by known magnetic field sources all point to the sensor, since the sensor can only be located at the intersection of four possible rotation angles (2 for each magnetic field source), the uniqueness can be obtained by the method of exclusion Determine the angle of rotation of the two magnetic field sources. In order to simplify the problem, the present invention limits the rotation search range of the magnetic field source to 180°.

The key to improving the positioning speed of the present invention is that according to the distribution characteristics of the magnetic induction intensity generated by the rotating magnetic field source, the rotation angle of the maximum magnetic induction intensity can be determined only by collecting the magnetic induction intensity signals of four angles with a difference of 45° in sequence.

The method for fast tracking of the rotating magnetic field based on four-point measurement proposed by the present invention, the specific steps are:

First, data acquisition is carried out, the magnetic field source is within the 180° search range, and the rotation scan is performed with a 45° step (the step is the angle of each rotation);

The magnetic sensor collects a magnetic induction intensity data every 45° rotation; then data is grouped, and according to the size of the collected four data, they are divided into two groups located on both sides of the maximum magnetic induction intensity according to certain rules;

Then, calculate the slope of the two sets of data, and get two straight lines passing through 2 points of each set of data;

Finally, since the collected four points are not necessarily symmetrical about the maximum value, in order to reduce the estimation error, on the basis of obtaining the mean value of the absolute value of the two slopes, the two obtained straight lines are corrected according to certain rules, so that the intersection point The corresponding angle is consistent with the rotation angle corresponding to the maximum value of the magnetic induction. This method can be used in a non-iterative electromagnetic tracking system that determines the six-degree-of-freedom position and attitude of the tracking target (the magnetic sensor is fixed on the tracking target) based on the rotating magnetic field.

In the present invention, the data acquisition refers to that the magnetic sensor collects the magnetic induction intensity at its position once every time the magnetic field source rotates 45°; when the magnetic field source rotates in the range of 180°, the magnetic sensor can collect four magnetic induction intensity data, They are recorded as Ba, Bb, Bc, Bd in turn. Since the magnetic induction intensity of the rotating magnetic field in the plane has a periodicity of 180°, this rotation process can be expressed by a closed circle as shown in Figure 1.

In the present invention, the data grouping is based on the four collected magnetic induction intensities. As shown in Figure 2, due to the 180° periodicity of the rotating magnetic field, there is always a maximum and/or minimum within the 180° search range. And the maximum value of the magnetic induction intensity is always the closest to the maximum value among the four sampling points; and the minimum value of the magnetic induction intensity is always the closest to the minimum value among the four sampling points. Such four sampling points can always be divided into two groups (two points in each group) located on both sides of the maximum magnetic induction intensity. Table 1 lists all possible situations of the four collected magnetic induction values and gives the corresponding grouping rules. The first column lists all the possible situations of the collected four magnetic induction intensities, and the second column shows the range of the maximum magnetic induction intensity in each case in combination with Figure 1, and accordingly the collected four magnetic induction The intensities are divided into two groups located on either side of the magnetic induction maximum (column 3 of Table 1).

Table 1 Grouping of sampling points in various situations after four measurements

In the present invention, the determination principle of the maximum rotation angle of the magnetic induction intensity is as shown in Figure 3: if two symmetrical points are respectively taken on the left and right sides of the maximum value, then the line connecting the two points on the left and the line connecting the two points on the right The intersection point must be in the direction of the maximum value of the magnetic induction intensity curve, and the abscissa of the intersection point is the estimated value of the maximum rotation angle of the magnetic induction intensity. According to the data collection and grouping steps, without loss of generality, the coordinates of the four sampling points located on both sides of the maximum magnetic induction intensity in Figure 3 are defined as:

, then the slopes of the two straight lines are calculated by formulas (1) and (2) respectively (where ):

(1)

(2)

In the present invention, the correction of the two straight lines and the determination of the maximum magnetic induction rotation angle, the four magnetic induction values obtained by sampling are not necessarily symmetrical about the maximum value, resulting in The absolute values of are not equal, resulting in an error in the final maximum magnetic induction intensity. For this, first calculate by formula (3) mean:

(3)

Then, according to the four methods of determining the straight line in Table 2, the maximum magnetic induction rotation angle is calculated by formulas (4)-(7) (where Indicates the rotation angle corresponding to the maximum value of magnetic induction calculated according to the first method, and * takes 1, 2, 3, 4).

Table 2 Four methods of determining a straight line

(4)

(5)

(6)

(7)

The above four methods for determining straight lines are equivalent, and one of them can be selected for implementation in the actual system.

The present invention also relates to a rotating magnetic field electromagnetic tracking system based on the above rotating magnetic field fast tracking method, the structure of which is shown in FIG. 5 . It includes three parts: a magnetic field source unit 10 composed of two relatively fixed and known magnetic field sources, a three-axis magnetic sensor unit 20 and a control processing unit 30 . Wherein, the magnetic field source unit 10 is made up of the coil wound on the magnetic rod, and the magnetic rod is fixed on the platform that can rotate in horizontal and vertical two planes; Three-axis magnetic sensor unit 20 includes three-axis magnetic sensor 21, signal conditioning Circuit 22 and analog-to-digital conversion (ADC) circuit 23; control processing unit 30 includes; data sampling and storage unit 31, sampling control unit 32, magnetic field source rotation/excitation control unit 33, data grouping module 34, slope calculation module 35, correction Module 36, the magnetic rod rotation angle determination module 37 corresponding to the maximum value of the magnetic induction intensity; the rotation of the pan/tilt is controlled by the magnetic field source rotation/excitation control unit 33, and the arbitrary orientation of the magnetic rod (that is, the magnetic field source 10) in space is realized; In addition, The magnetic field source rotation/excitation control unit 33 also provides an appropriate excitation current for the coil. The three-axis magnetic sensor 21 perceives the magnetic induction intensity in three orthogonal directions at the spatial position of the sensor; the signal conditioning circuit 22 filters and amplifies the signal sensed by the three-axis magnetic sensor 21, and the analog-to-digital conversion circuit 23 is in the sampling control unit 32. Under control, the analog signal output by the signal conditioning circuit 22 is converted into a digital signal and sent to the control processing unit 30 for further processing; the control processing unit 30 sends the data output by the sensor to the data sampling and storage unit 31 for storage, and in the After the scanning of the control magnetic field source is completed, the data is grouped by the data grouping module 34, the slope is calculated by the slope calculation module 35, and the correction process is performed by the correction module 36. Finally, according to the angle corresponding to the intersection point of the two straight lines, the maximum value of the magnetic induction intensity The corresponding magnetic rod rotation angle determination module 37 obtains the magnetic rod rotation angle corresponding to the maximum magnetic induction intensity.

In the system of the present invention, the data grouping module 34 is used for the grouping calculation of the present invention (ie, the calculation of Table 1); the slope calculation module 35 is used for the slope calculation of the present invention (ie, the calculation of formulas (1), (2) ); the correction module 36 is used for the correction calculation of the present invention (that is, the calculation of formula (3)); the magnetic bar rotation angle determination module 37 corresponding to the maximum value of the magnetic induction intensity is used for the corresponding value of the maximum value of the magnetic induction intensity The calculation of the rotation angle of the magnetic bar (that is, the calculation of the formula (4)-(7)).

In order to make the maximum magnetic induction intensity generated by the rotating magnetic field point to the sensor, in the traditional method, the magnetic field source needs to search in the range of 180° with a small step size (when the step size is 1°, the angle error may be 0.5°) (sampling per rotation step Once), only 180 sampling data can be obtained reliably, which seriously affects the real-time performance of system positioning. The rotating magnetic field fast tracking method based on four-point measurement proposed by the present invention, according to the characteristics of the magnetic induction intensity distribution of the rotating magnetic field source, uses a 45° large-step search method to determine the magnetic induction generated by the rotating magnetic field with only 4 sampling data. The angle of rotation at which the intensity maximum is directed to the sensor makes the positioning/tracking speed of the system greatly increased.

Description of drawings

Figure 1 is a schematic diagram of four-point measurement area division.

Figure 2 shows the possible range of determining the maximum or minimum value through four-point sparse sampling (T1\T2\T3 is the range of the maximum value, and T3\T4\T1 is the range of the minimum value).

Fig. 3 is a schematic diagram of determining the maximum rotation angle of magnetic induction based on four-point measurements.

Fig. 4 is a block diagram of the rotating magnetic field electromagnetic tracking system based on the rotating magnetic field fast tracking method of the present invention based on four-point measurement.

Fig. 5 is a structure diagram of the rotating magnetic field electromagnetic tracking system based on the rotating magnetic field fast tracking method based on four-point measurement in the present invention.

Figure 6 is a flowchart of magnetic field source scanning and data acquisition.

Fig. 7 is a flow chart for realizing the fast tracking method of the rotating magnetic field based on the four-point measurement.

detailed description

Below in conjunction with accompanying drawing 4～7 the present invention will be further described.

FIG. 4 shows the composition of an electromagnetic tracking system applicable to the fast tracking method of the rotating magnetic field based on four-point measurement in the present invention, including a magnetic field source unit 10 , a magnetic sensor unit 20 and a control processing unit 30 . Wherein the control processing unit 30 controls the magnetic field source 10 to scan and search in the range of 180 °; the magnetic field source 10 rotates every 45 °, and the magnetic sensor 20 collects the magnetic induction data once, and stores the data in the control processing unit 30; the control processing unit 30 passes through The rotation angle corresponding to the maximum value of the magnetic induction intensity data is determined by the fast tracking method of the rotating magnetic field based on the four-point measurement.

Figure 5 is a detailed block diagram of the magnetic positioning system. The magnetic field source 10 of the present embodiment is made up of the coil that is wound on the magnetic bar, and the magnetic bar is fixed on the platform that can rotate in horizontal and vertical two planes; Three-axis magnetic sensor unit 20 comprises three-axis magnetic sensor 21, signal Conditioning circuit 22 and analog-to-digital conversion (ADC) circuit 23; control processing unit 30 includes; data sampling and storage unit 31, sampling control unit 32, magnetic field source rotation/excitation control unit 33, data grouping module 34, slope calculation module 35, Correction module 36, magnetic rod rotation angle determination module 37 corresponding to the maximum value of magnetic induction intensity; the rotation of the pan/tilt is controlled by the magnetic field source rotation/excitation control unit 33, and the arbitrary orientation of the magnetic rod (that is, the magnetic field source 10) in space is realized; in addition , the magnetic field source rotation/excitation control unit 33 also provides an appropriate excitation current for the coil. The three-axis magnetic sensor 21 perceives the magnetic induction intensity in three orthogonal directions at the spatial position of the sensor; the signal conditioning circuit 22 filters and amplifies the signal sensed by the three-axis magnetic sensor 21, and the analog-to-digital conversion circuit 23 is in the sampling control unit 32. Under control, the analog signal output by the signal conditioning circuit 22 is converted into a digital signal and sent to the control processing unit 30 for further processing. The control processing unit 30 stores the data output by the sensor, and after the scanning of the control magnetic field source is completed, the data is grouped by the data grouping module 34, the slope is calculated by the slope calculation module 35, and the correction process is performed by the correction module 36. Finally, according to the obtained The angle corresponding to the intersection of the two straight lines is obtained by the magnet rod rotation angle determination module 37 corresponding to the maximum magnetic induction intensity corresponding to the magnetic rod rotation angle corresponding to the maximum magnetic induction intensity.

Figure 6 is a schematic diagram of the magnetic field source scanning and data acquisition process, and the specific steps are:

Step 41: First, the control processing unit resets the magnetic field source to the initial position (at this time, the direction of the magnetic bar is specified as zero degrees);

Step 42: Then, the control processing unit controls the magnetic field source to rotate at an angle of 45° in the horizontal plane, and excites the magnetic field source coil with a certain current to generate a magnetic field;

Step 43: The three-axis magnetic sensor unit collects the magnetic induction intensity at the location and stores the data in the memory unit 30 of the control processing unit;

Step 44: Determine whether the horizontal plane scanning is completed (the scanning range of the magnetic field source in this embodiment is set to 180°), if the scanning is not completed, repeat the above process, otherwise,

Step 45: Complete a set of data collection, and determine the rotation angle corresponding to the maximum value of the magnetic induction intensity on the horizontal plane through the method for quickly determining the rotation angle of the maximum magnetic induction intensity based on the four-point measurement shown in Figure 7; Starting from the corner, the same data collection as that on the horizontal plane is carried out on the vertical plane.

Fig. 7 is a flow diagram of a method for fast tracking of a rotating magnetic field based on four-point measurement. The specific steps are:

Step 51: Collect four data:

Step 52: First, group the four data collected during the data collection process according to Table 1;

Step 53: Then, connect the two data of each group to obtain two straight lines, and calculate their slopes respectively (according to formulas (1), (2));

Step 54: Then, calculate the mean value of the absolute values of the slopes of the two straight lines (according to formula (3)), and correct the two straight lines according to method 2 in Table 2;

Step 55: Finally, since the angle corresponding to the intersection point of the two straight lines is consistent with the rotation angle corresponding to the maximum magnetic induction intensity, the formula The rotation angle corresponding to the maximum value of the magnetic induction intensity is calculated.

The above are only preferred embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention are all should be covered within the protection scope of the invention. Therefore, the protection scope of the present invention should be based on the protection scope defined by the claims.

Claims (2)

1. A rotating magnetic field fast tracking method based on four-point measurement, wherein the rotating magnetic field electromagnetic tracking system includes a magnetic field source unit, a three-axis magnetic sensor unit and a control processing unit composed of two relatively fixed and known magnetic field sources Three parts; the magnetic field source achieves any orientation of the maximum magnetic induction intensity through rotating scanning; the three-axis magnetic sensor is used to collect the magnetic induction intensity values in three orthogonal directions and obtain the magnetic induction intensity and the magnetic induction intensity at the position of the magnetic sensor through vector synthesis. Direction; on the one hand, the control processing unit provides suitable excitation for the electromagnetic coils that make up the magnetic field source and current controls the rotation scan of the magnetic field source, so that the maximum magnetic induction intensity generated by the two magnetic field sources finally points to the magnetic sensor, that is, the tracking target; The data collected by the three-axis magnetic sensor unit is processed and the rotation angle when the maximum magnetic induction intensity points to the magnetic sensor is calculated; it is characterized by solving the rotation angle of the maximum magnetic induction intensity through four sampling values with a spacing of 45° to efficiently track objects. Six degrees of freedom position and attitude, the specific steps are: (1) First, collect data; (2) Then, group the collected data into two groups located on both sides of the maximum magnetic induction intensity; (3) Then, respectively connect the two points of each group, fit two straight lines, and calculate the slope of the two straight lines; (4) Finally, correct the two straight lines obtained in step (3), and determine the rotation angle corresponding to the maximum magnetic induction intensity according to the characteristic that the angle corresponding to the intersection point of the two straight lines is consistent with the maximum magnetic induction intensity rotation angle; The collected data in step (1) is rotated and scanned by the magnetic field source with a step size of 45°, and the scanning range is 180°; Every time the magnetic field source rotates 45°, the three-axis magnetic sensor collects the magnetic induction intensity signals in three orthogonal directions of its spatial position, and the signal is stored after analog-to-digital conversion; when the magnetic field source completes a plane scan, it can be collected Four magnetic induction data, denoted as: Ba, Bb, Bc, Bd; The data grouping described in step (2) is to divide them into two groups located on both sides of the maximum magnetic induction intensity according to the size relationship of the four collected data Ba, Bb, Bc, and Bd: The straight line fitting and slope calculation described in step (3) is to connect the 2 points of each group after step (2) into a straight line; Located on the side of the maximum magnetic induction intensity, point Located on the other side of the maximum magnetic induction intensity, and according to with Calculate the slopes of the two straight lines separately; The determination of the rotation angle corresponding to the maximum value of the magnetic induction described in step (4) is to calculate the mean value of the absolute values of the slopes of the two straight lines on the basis of step (3) , and then correct the two straight lines obtained in step (3) according to one of the four methods in the table below: The angle corresponding to the intersection of the two straight lines after correction is consistent with the rotation angle of the maximum magnetic induction intensity. Indicates the rotation angle corresponding to the maximum magnetic induction intensity calculated according to the first method, , then there are: . 2. An electromagnetic tracking system based on the rotating magnetic field fast tracking method according to claim 1, characterized in that: comprise a magnetic field source unit, a three-axis magnetic sensor unit and a magnetic field source unit composed of two relatively fixed and known magnetic field sources The control processing unit has three parts; among them, the magnetic field source unit is composed of a coil wound on a magnetic rod, and the magnetic rod is fixed on a pan head that can rotate in two planes, horizontal and vertical; the three-axis magnetic sensor unit includes a three-axis magnetic sensor , signal conditioning circuit and analog-to-digital conversion circuit; the control processing unit includes: data sampling and storage unit, sampling control unit, magnetic field source rotation/excitation control unit, data grouping module, slope calculation module, correction module, corresponding to the maximum value of magnetic induction The rotation angle determination module of the magnetic bar; the rotation of the pan/tilt is controlled by the magnetic field source rotation/excitation control unit to realize the arbitrary orientation of the magnetic bar in space; in addition, the magnetic field source rotation/excitation control unit also provides suitable excitation current for the coil; three The axial magnetic sensor perceives the magnetic induction intensity in three orthogonal directions of the spatial position of the sensor; the signal conditioning circuit filters and amplifies the signal sensed by the three-axis magnetic sensor, and the analog-to-digital conversion circuit adjusts the signal under the control of the sampling control unit The analog signal output by the circuit is converted into a digital signal and sent to the control processing unit for further processing; the control processing unit sends the data output by the sensor to the data sampling and storage unit for storage, and after the scanning of the control magnetic field source is completed, through the data packet The module groups the data, calculates the slope through the slope calculation module, and performs correction processing through the correction module. Finally, according to the angle corresponding to the intersection point of the two straight lines, the determination module obtains the maximum magnetic induction intensity from the rotation angle of the magnetic rod corresponding to the maximum magnetic induction intensity. The magnet rotation angle corresponding to the value.