CN103424111A - Method for reducing square two-frequency laser gyroscope magnetic sensitivity - Google Patents

Abstract

The invention discloses a method for reducing square two-frequency laser gyroscope magnetic sensitivity. The method includes the steps that a, at least two adjacent light path long control mirrors are installed in four reflectors on a square two-frequency laser gyroscope; b, an external magnetic field is initiatively applied to the laser gyroscope in a normal work state, and gyroscope difference frequency change caused by application of the external magnetic field is detected; c, angles of pitch of the two light path long control mirrors are controlled so as to adjust coplane degrees of a resonance light path of a gyroscope resonance cavity, the gyroscope difference frequency change caused by the external magnetic field is made to be the smallest and the state is kept finally. Compared with the prior art, the method is characterized in that path length control mirrors commonly used in the square two-frequency laser gyroscope in the prior art are replaced by light path long control mirrors, the angle control function of the control mirrors is utilized, gyroscope difference frequency signals serve as detection objects directly, non-coplanar angles of the resonance light path are reduced through adjustment, and therefore the purpose of reducing the square two-frequency laser gyroscope magnetic sensitivity can be achieved.

Description

Method of Reducing Susceptibility of Square Two-frequency Laser Gyroscope

technical field

The invention relates to an optical angular velocity sensing technology, in particular to a method for reducing the magnetic sensitivity of a square two-frequency laser gyroscope.

Background technique

The laser gyroscope is an angular velocity sensor based on the Sagnac effect. It reflects the magnitude of the input angular velocity through the resonance frequency difference of a pair of bidirectional traveling waves running in the ring resonator. One of them is a quadrilateral formed by four mirrors. A two-frequency laser gyro with a ring resonant optical path (as shown in Figure 3), its structure usually includes an octagonal gyro block a, on which are processed 4 patch surfaces supporting mirrors and used for light transmission and filling gain The capillary holes of the medium, two plane reflectors b, c and two spherical reflectors d, e optical glue form a ring resonant cavity on the four patch surfaces of the gyroscope block a; among them, the spherical mirrors d, e are also used as process The long control mirror, whose mirror surface can be translated back and forth under the control of its control unit, in the figure f, g, h, i are the four sides of the ring resonant optical path; ideally, two of the four patch surfaces are opposite The surface of the patch should be parallel to each other (actually important is the mirror pasted on it), so that the quadrilateral resonant optical path formed by the four mirrors is strictly coplanar. However, due to the inevitable processing error of the patch surface and the cavity adjustment error (through the spherical plate), it is difficult to make the four sides of the final resonant optical path strictly coplanar. The angle α is called its non-coplanar angle, and this non-coplanarity will increase the magnetic sensitivity of the laser gyroscope, reducing its accuracy and deteriorating its performance.

At present, due to the lack of effective means to monitor whether the resonant optical paths are coplanar during cavity tuning, the coplanarity of the optical paths is mainly guaranteed by machining accuracy. Although there are also methods for monitoring the coplanarity of the resonant optical path during cavity tuning, for example, Robert H. Moore et al. have proposed that two beams of polarized light are bidirectionally input into the ring resonant cavity, and then by detecting and comparing the forward and counterclockwise resonant light The ellipticity of the output light is used to monitor the coplanarity of the resonant optical path (United States patent 4850708, 1989). However, due to the influence of many factors, this method is difficult to be free of errors. More importantly, these methods are all aimed at passive cavities. Consistent, and there is nothing to do about the cavity deformation caused by external factors in the future.

For the ring resonator shown in Figure 3, ideally, the relationship between the frequency difference of the clockwise and counterclockwise lasers operating in the gyro resonator and the input speed is

                                       （1）

(1)

是顺、逆时针运行的两束激光的频率差；

是激光环路所围的面积；

是环路光程；

是激光波长；为沿激光陀螺敏感轴方向输入的角速度；

称之为比例因子。 in,

is the frequency difference between two laser beams running clockwise and counterclockwise;

is the area surrounded by the laser ring;

is the optical path of the loop;

is the laser wavelength; is the angular velocity input along the sensitive axis of the laser gyro;

Call it the scaling factor.

However, due to the existence of many error factors, the frequency difference between clockwise and counterclockwise lasers in the gyro resonator is not strictly in accordance with the expression output result of formula (1), but contains many error terms, the frequency difference caused by the magnetic field Change is one of them. Without considering other factors, this difference frequency can be expressed as

                                        （2）

(2)

为磁场所导致的频差的变化量。对于上面所描述的方形陀螺，由于谐振光路的轻微非共面，其顺、逆时针谐振光的频率差受磁场的影响可表示为 in

is the change in frequency difference caused by the magnetic field. For the square gyro described above, due to the slight non-coplanarity of the resonant optical path, the frequency difference between the clockwise and counterclockwise resonant light is affected by the magnetic field and can be expressed as

                                     （3）

(3)

Where α is the non-coplanar angle of the resonant optical path, B is the magnetic field strength, and k is a coefficient related to the gain medium, the optical characteristics of the mirror, etc. At this time, the sensitivity of the laser gyroscope to the magnetic field can be expressed as the magnetic sensitivity K _B

                                      （4）

(4)

Obviously, a direct and effective method to reduce the magnetic sensitivity K _B of the gyroscope is to reduce the non-coplanar angle α of the resonant optical path.

Contents of the invention

The object of the present invention is to provide a reduced square laser gyroscope that can take the magnetic sensitivity of the square two-frequency laser gyro as the monitoring object, monitor it in real time during the working process, and correct the coplanarity deviation of the resonant optical path caused by the deformation of the cavity at any time. Laser Gyro Magnetic Sensitivity Method.

The method for reducing the susceptibility of a square two-frequency laser gyro provided by the invention comprises the following steps:

a. Install at least two adjacent optical path length control mirrors among the four mirrors on the square two-frequency laser gyroscope;

b. Actively apply an external magnetic field to the laser gyroscope in normal working state, and detect the resulting gyroscope difference frequency change;

c. Control the pitch angle of the two optical path length control mirrors to adjust the coplanarity of the resonant optical path of the gyro resonator cavity; minimize the change in the difference frequency of the gyro caused by the external magnetic field, and finally maintain this state.

The pitch angle of the optical path length control mirror adopts a closed-loop measurement and control method or an open-loop measurement and control method to adjust the angle. The closed-loop measurement and control method is to monitor its magnetic sensitivity at any time when the square two-frequency laser gyro is in a normal working state. Dynamic control always minimizes the magnetic sensitivity of the gyroscope. The open-loop measurement and control method is to measure a group of gyroscopes with the smallest magnetic sensitivity of the square two-frequency laser gyroscope before leaving the factory. The control voltage on the pitch angle of the long control mirror is used as the control parameter. , the optical path length control mirror of the gyro will always control the optical path of the gyro with this set of control parameters.

In the closed-loop measurement and control mode, an external magnetic field needs to be applied all the time.

In the open-loop measurement and control mode, the gyro does not need to apply an external magnetic field when it is in use.

The external magnetic field adopts an alternating magnetic field, and the alternating magnetic field is generated by a coil, and the coil is installed on the gyro cavity in the gyroscope or outside the gyro cavity in the gyroscope.

When the control parameters are measured before leaving the factory, the external magnetic field adopts an alternating magnetic field, and the alternating magnetic field is generated by a coil, and the coil for generating the magnetic field is installed outside the gyroscope cavity.

When the control parameters are measured before leaving the factory, the external magnetic field can adopt a constant magnetic field when the square two-frequency laser gyroscope is in a fixed rotation state. The constant magnetic field is generated by a coil or a permanent magnet. The coil or permanent magnet that generates the magnetic field The magnet is installed outside the gyroscope cavity.

In the step c, controlling the pitch angles of the two optical path length control mirrors adopts the small jitter extremum method.

A specific method of the small jitter extreme value method includes the following steps:

(1) Detect the change in the difference frequency of the gyroscope caused by the external magnetic field, increase or decrease the voltage on the pitch angle control unit of the angle control part of the square two-frequency laser gyroscope on an optical path length control mirror with a small step, and At the same time, increase or decrease the control voltage applied to the pitch direction of another optical path length control mirror, so that the light intensity is at a maximum value;

(2) Detect the gyro difference frequency change caused by the magnetic field again. If the change decreases, continue to increase or decrease the control voltage on each optical path length control mirror in small steps; if the change increases, Then the small steps in the opposite direction increase or decrease the control voltage on each optical path length control mirror;

(3) Repeat the above process until the magnetic susceptibility of the gyro increases no matter whether the voltage is increased or decreased in a small step, and the gyro is in the lowest state of magnetic susceptibility at this time, and the relevant control voltages at this time are The optimal control parameters required.

Compared with the prior art, the present invention uses the optical path length control mirror to replace the path length control mirror commonly used in the current square two-frequency laser gyroscope, and uses the angle adjustment function of the control mirror to directly take the gyro difference frequency signal as the detection object, By adjusting and reducing the non-coplanar angle of the resonant optical path, the magnetic susceptibility of the square two-frequency laser gyro can be reduced.

Description of drawings

Figure 1 is a schematic diagram of the structure of the square two-frequency laser gyroscope of the present invention.

Fig. 2 is the second schematic diagram of the structure of the square two-frequency laser gyroscope of the present invention.

Fig. 3 is a schematic structural diagram of the prior art.

Detailed ways

The method for reducing the susceptibility of square two-frequency laser gyroscopes of the present invention comprises the following steps:

a. Install at least two adjacent optical path length control mirrors among the four mirrors on the square two-frequency laser gyroscope;

b. Actively apply an external magnetic field to the laser gyroscope in normal working state, and detect the resulting gyroscope difference frequency change;

c. Control the pitch angle of the two optical path length control mirrors to adjust the coplanarity of the resonant optical path of the gyro resonator cavity; minimize the change in the difference frequency of the gyro caused by the external magnetic field, and finally maintain this state.

Wherein, the pitch angle of the optical path length control mirror is adjusted by a closed-loop measurement and control method or an open-loop measurement and control method. The closed-loop measurement and control method is to dynamically control its magnetic sensitivity at any time when the square two-frequency laser gyroscope is in a normal working state. Always minimize the magnetic sensitivity of the gyro. The open-loop measurement and control method is to measure the minimum magnetic sensitivity of a set of gyro before the square two-frequency laser gyro leaves the factory. The control voltage on the pitch angle of the long control mirror is used as the control parameter. The long path control mirror has been controlling the gyro optical path with this group of control parameters.

In the closed-loop measurement and control mode, an external magnetic field is always applied. The external magnetic field adopts an alternating magnetic field. The alternating magnetic field is generated by a coil, and the coil is installed on the gyro cavity in the gyroscope or outside the gyro cavity in the gyroscope.

In the open-loop measurement and control mode, the gyro does not need to apply an external magnetic field when it is in use.

When the control parameters are measured before leaving the factory, they can be measured and controlled by a method similar to the closed-loop measurement and control method, that is, the external magnetic field adopts an alternating magnetic field, and the alternating magnetic field is generated by a coil, but the coil that generates the magnetic field should be installed in the gyro cavity in vitro.

In addition, before leaving the factory, the square two-frequency laser gyro can be placed in a fixed rotation state to obtain a set of control parameters. At this time, the external magnetic field can use a constant magnetic field. The constant magnetic field is generated by a coil or a permanent magnet, and the coil or permanent magnet that generates the magnetic field The magnet is installed outside the gyroscope cavity.

Since the laser gyro in the open-loop measurement and control mode does not need to apply an external magnetic field, the coils or permanent magnets required for the detection of the magnetic field before leaving the factory are not required on the delivered laser gyro.

Controlling the pitch angle of two optical path length control mirrors in step c is to adopt the small jitter extreme value method, and a kind of specific method of this small jitter extreme value method comprises the following steps:

(1) Detect the change in the difference frequency of the gyroscope caused by the external magnetic field, increase or decrease the voltage on the pitch angle control unit of the angle control part of the square two-frequency laser gyroscope on an optical path length control mirror with a small step, and At the same time, increase or decrease the control voltage applied to the pitch direction of another optical path length control mirror, so that the light intensity is at a maximum value;

(2) Detect the gyro difference frequency change caused by the magnetic field again. If the change decreases, continue to increase or decrease the control voltage on each optical path length control mirror in small steps; if the change increases, Then the small steps in the opposite direction increase or decrease the control voltage on each optical path length control mirror;

(3) Repeat the above process until the magnetic susceptibility of the gyro increases no matter whether the voltage is increased or decreased in a small step, and the gyro is in the lowest state of magnetic susceptibility at this time, and the relevant control voltages at this time are The optimal control parameters required.

Specific embodiment one of the present invention:

Fig. 1 is a schematic diagram of a square two-frequency laser gyroscope with a coil installed on the gyroscope cavity proposed by the present invention, which includes two plane mirrors 1, 2 and two optical path length control mirrors 3, 4 and is controlled by the four The mirror constitutes a ring resonant cavity, the winding holes 5 and 6 processed on the side of the gain area of the ring resonator, the coils 7 and 8 straddling the gain area, the gyro difference frequency signal output part 9, and the gyro light intensity signal output part 10, wherein the optical path length control mirrors 3 and 4 can not only translate back and forth under the control of the path length control unit, but also can produce deflection under the drive of the angle control unit, that is, they can not only translate back and forth to control the optical path length, but also can The angular deflection controls the direction of the light path.

When the external magnetic field is in the unknown random rotation state of the square two-frequency laser gyroscope shown in Figure 1, when using the alternating magnetic field mode, first apply an alternating current to the coils 7, 8

                                   （5）

(5)

This current will generate an AC magnetic field of the same frequency as

                                        （6）

(6)

From formulas (2) and (3), it can be seen that under this external magnetic field, the gyro difference frequency is

                                     （7）

(7)

中分辨出以频率ω变化的

大小。注意：在公式（5）、（6）、（7）中主要关注的电流、磁场、差频的变化频率，因此没有考虑它们的相位关系；另外频率ω需要综合考虑陀螺的抖动频率和信号采样频率适当选取。 Obviously, we can easily obtain from

In the resolution of the change with frequency ω

size. Note: In the formulas (5), (6), and (7), the frequency of change of the current, magnetic field, and difference frequency is mainly concerned, so their phase relationship is not considered; in addition, the frequency ω needs to comprehensively consider the jitter frequency of the gyro and signal sampling The frequency is properly selected.

的情况下，便可通过光路程长控制镜3、4的俯仰方向调整谐振光路的共面性，实现在工作状态中随时对陀螺的磁敏感性进行动态控制，其中控制光路程长控制镜俯仰角的小抖动极值法，其具体包括如下步骤： Subsequently, the closed-loop measurement and control method is used to continuously obtain the gyroscope's

In the case of , the coplanarity of the resonant optical path can be adjusted by controlling the pitch direction of the mirrors 3 and 4 through the long optical path, so as to realize the dynamic control of the magnetic sensitivity of the gyroscope at any time in the working state, wherein the long optical path is controlled to control the pitch of the mirror The small jitter extremum method of the angle, which specifically includes the following steps:

的变化量，微小步幅增加或减小在方形二频激光陀螺的光路程长控制镜3上角度控制部件的俯仰角控制单元上的电压，并同时增加或减小加在光路程长控制镜4上俯仰方向的控制电压，使光强处于极大值； (1) Detect the gyro difference frequency caused by the external magnetic field

The amount of change, small steps increase or decrease the voltage on the pitch angle control unit of the angle control part on the optical path length control mirror 3 of the square two-frequency laser gyroscope, and increase or decrease the voltage applied to the optical path length control mirror at the same time 4. The control voltage in the pitch direction makes the light intensity at the maximum value;

(2) Detect the gyro difference frequency change caused by the magnetic field again. If the change decreases, continue to increase or decrease the control voltage on each optical path length control mirror in small steps; if the change increases, Then the small steps in the opposite direction increase or decrease the control voltage on each optical path length control mirror;

(3) Repeat the above process until the magnetic susceptibility of the gyro increases no matter whether the voltage is increased or decreased in a small step, and the gyro is in the lowest state of magnetic susceptibility at this time, and the relevant control voltages at this time are The optimal control parameters required.

Specific embodiment two of the present invention:

Fig. 2 is a schematic diagram of a square two-frequency laser gyroscope with a coil installed outside the gyroscope cavity proposed by the present invention. A ring resonant cavity is formed, a gyro difference frequency signal output part 15, a gyro light intensity signal output part 16, and coils 17 and 18 placed outside the gyro cavity for generating a magnetic field.

中分辨出以频率ω变化的

大小，采用闭环测控方式，在不断的获得陀螺的

的情况下，便可通过光路程长控制镜13、14的俯仰方向调整谐振光路的共面性，实现在工作状态中随时对陀螺的磁敏感性进行动态控制，其中控制光路程长控制镜俯仰角的小抖动极值法，其具体包括如下步骤： The coils 17, 18 are installed together with the gyro cavity in the gyroscope. When using the alternating magnetic field method in the first embodiment, an alternating current is first applied to the coils 17, 18, and then the coils 17, 18 are subjected to technical means such as correlation detection or spectrum analysis.

In the resolution of the change with frequency ω

Size, using closed-loop measurement and control method, in the continuous acquisition of gyroscope

In the case of gyro, the coplanarity of the resonant optical path can be adjusted by controlling the pitch direction of the mirrors 13 and 14 through the long optical path, so that the magnetic sensitivity of the gyroscope can be dynamically controlled at any time in the working state, wherein the long optical path is controlled to control the pitch of the mirror The small jitter extremum method of the angle, which specifically includes the following steps:

的变化量，微小步幅增加或减小在方形二频激光陀螺的光路程长控制镜13上角度控制部件的俯仰角控制单元上的电压，并同时增加或减小加在光路程长控制镜14上俯仰方向的控制电压，使光强处于极大值； (1) Detect the gyro difference frequency caused by the external magnetic field

The amount of change, the small step increases or decreases the voltage on the pitch angle control unit of the angle control part on the optical path length control mirror 13 of the square two-frequency laser gyroscope, and increases or decreases the voltage applied to the optical path length control mirror at the same time 14, the control voltage in the pitch direction makes the light intensity at the maximum value;

(2) Detect the gyro difference frequency change caused by the magnetic field again. If the change decreases, continue to increase or decrease the control voltage on each optical path length control mirror in small steps; if the change increases, Then the small steps in the opposite direction increase or decrease the control voltage on each optical path length control mirror;

(3) Repeat the above process until the magnetic susceptibility of the gyro increases no matter whether the voltage is increased or decreased in a small step, and the gyro is in the lowest state of magnetic susceptibility at this time, and the relevant control voltages at this time are The optimal control parameters required.

Specific embodiment three of the present invention:

In the test room of the square two-frequency laser gyro before leaving the factory, the square two-frequency laser gyro with the coils shown in Figure 2 installed outside the gyro cavity was used, and the method and method adopted in the second embodiment were used to obtain a set of optical path length control Optimum control parameters of the control voltage in the pitch direction on the mirrors 13 and 14; then, only the gyroscope block is installed in the gyroscope, and the open-loop measurement and control method is adopted, and the minimum optical path length control of the gyroscope magnetic sensitivity is measured by the above method. A group of control voltage parameters of the pitch angle of the mirror is used as the normal operating parameters of the gyroscope in this embodiment, and the optical path length control mirror of the gyroscope has been controlling the optical path of the gyroscope with this group of parameters.

Specific embodiment four of the present invention:

减小，当

减小到0时，光路程长控制镜13、14的俯仰方向的控制电压即为最佳控制参数。随后，只将陀螺块体安装在陀螺仪内，采用开环测控方式，利用上述方式测得激光陀螺磁敏感度最小时光路程长控制镜的一组控制电压的最佳控制参数，并将其作为该实施例中陀螺的正常工作参数，陀螺的光路程长控制镜就一直以这组参数控制陀螺光路。 In the test room of the square two-frequency laser gyro before leaving the factory, the square two-frequency laser gyro with the coil shown in Figure 2 installed outside the gyro cavity is used to power up the square two-frequency laser gyro by means of a constant magnetic field and wait for it to be normal and stable. After working, keep the gyro stationary or rotate at a constant speed, first detect the gyro difference frequency without the influence of the magnetic field in the absence of an external magnetic field, and then pass a DC current to the coil to generate a constant external magnetic field, and then detect the magnetic field effect at this time The gyro difference frequency, the difference between the two is the change in the gyro difference frequency caused by the magnetic field . Then adjust the coplanarity of the resonant optical path by the pitch direction of the optical path length control mirror 13,14 to make

decrease when

When it is reduced to 0, the control voltage in the pitch direction of the optical path length control mirrors 13 and 14 is the optimal control parameter. Then, only the gyroscope block is installed in the gyroscope, and the open-loop measurement and control method is used to measure the optimal control parameters of a set of control voltages of the laser gyroscope with the minimum magnetic sensitivity of the laser gyroscope and the optical path length control mirror, and use it as In this embodiment, the normal working parameters of the gyroscope and the optical path length control mirror of the gyroscope always control the optical path of the gyroscope with these parameters.

保持不变，由公式（2）可知，此时

若有变化，则都是因

有变所导致的，因此，此时即便所加磁场为恒磁场，只要对比一下加磁场和不加磁场时

，就能很容易的获得

，从而使检测过程更加简化。此外，既然此时采用恒磁场，也就可以用永磁铁来代替线圈提供磁场。 In a specific embodiment of the present invention, the coil that generates the magnetic field is installed on the gyro cavity (as shown in Figure 1) and is compact in structure, and is suitable for closed-loop control mode; the coil that generates the magnetic field is installed outside the gyro cavity (as shown in Figure 2 Shown) Although it is not installed on the gyro cavity (as shown in Figure 1), it has the following advantages: 1. It does not destroy the original structure of the gyro cavity; 2. There is more space to use more turns. More coils can increase the magnetic field and improve the detection sensitivity; 3. Great flexibility; 4. It can be used in both closed-loop control mode and open-loop control mode. Or keep the detection under the state of uniform rotation, since the input angular velocity Ω of the gyro is constant at this time, the difference frequency of the gyro generated by the rotation during the whole detection process

remains unchanged, it can be seen from formula (2), at this time

Any changes are due to

Therefore, even if the applied magnetic field is a constant magnetic field at this time, just compare the applied magnetic field with that without

, it is easy to obtain

, which simplifies the detection process. In addition, since a constant magnetic field is used at this time, a permanent magnet can be used instead of a coil to provide a magnetic field.

In order to quickly and accurately find this optimal control parameter, it is best to use a slightly larger step to initially determine the control parameters, and then use a smaller step to accurately obtain the control parameters around this group of parameters.

When using the constant magnetic field method, in order to avoid the influence of other external magnetic fields such as the earth's magnetic field, the above-mentioned detection should be carried out in a magnetic shielding environment, otherwise, the above-mentioned process should be repeated many times, that is, the above-mentioned inspections in the case of coil power-off and coil power-on are repeated. Prosecution process to eliminate the influence of other magnetic fields.

In fact, cavity deformation is usually a relatively slow process, so even if the closed-loop control method is adopted, it does not have to be in the state of measurement and control all the time, but can be measured and controlled at regular intervals according to needs, and the rest of the time is controlled according to the control parameters obtained last time open-loop control state.

Claims (9)

1. A method for reducing the susceptibility of square two-frequency laser gyro, comprising the steps: a. Install at least two adjacent optical path length control mirrors among the four mirrors on the square two-frequency laser gyroscope; b. Actively apply an external magnetic field to the laser gyroscope in normal working state, and detect the resulting gyroscope difference frequency change; c. Control the pitch angle of the two optical path length control mirrors to adjust the coplanarity of the resonant optical path of the gyro resonator cavity; minimize the change in the difference frequency of the gyro caused by the external magnetic field, and finally maintain this state. 2. the method for reducing square two-frequency laser gyro susceptibility according to claim 1, is characterized in that: the pitch angle of described optical path length control mirror adopts closed-loop measurement and control mode or open-loop measurement and control mode to carry out angle adjustment, and described The closed-loop measurement and control method is to dynamically control the magnetic sensitivity of the gyroscope at any time when the square two-frequency laser gyroscope is in a normal working state, so as to always minimize the magnetic sensitivity of the gyroscope. Before the gyro leaves the factory, a set of control voltages on the pitch angle of the gyro magnetically sensitive minimum light path length control mirror are measured as control parameters. After that, the gyro optical path length control mirror will always use this set of control parameters to control the gyro optical path. 3. The method for reducing the magnetic susceptibility of a square two-frequency laser gyro according to claim 2, characterized in that: in the closed-loop measurement and control mode, an external magnetic field needs to be applied all the time. 4. The method for reducing the magnetic sensitivity of a square two-frequency laser gyro according to claim 2, characterized in that: in the open-loop measurement and control mode, the gyro does not need to apply an external magnetic field when it is in use. 5. The method for reducing the susceptibility of square two-frequency laser gyro according to claim 3, characterized in that: the external magnetic field adopts an alternating magnetic field, and this alternating magnetic field is produced by a coil, and the coil is installed in the gyroscope On the gyro cavity of the gyroscope or outside the gyro cavity in the gyroscope. 6. the method for reducing square two-frequency laser gyro susceptibility according to claim 2, is characterized in that: when measuring described control parameter before leaving the factory, described external magnetic field adopts alternating magnetic field, and this alternating magnetic field is by The coil generates the magnetic field, and the coil for generating the magnetic field is installed outside the gyroscope cavity. 7. the method for reducing the susceptibility of square two-frequency laser gyroscope according to claim 2, is characterized in that: when measuring described control parameter before dispatching from the factory, described external magnetic field is in the fixed rotation of square two-frequency laser gyro In this state, a constant magnetic field can be used, and the constant magnetic field is generated by a coil or a permanent magnet, and the coil or permanent magnet that generates the magnetic field is installed outside the gyroscope cavity. 8. The method for reducing the susceptibility of square two-frequency laser gyro according to claim 1, characterized in that: controlling the pitch angles of the two optical path length control mirrors in the step c is to adopt the small jitter extremum method. 9. the method for reducing the susceptibility of square two-frequency laser gyro according to claim 8, is characterized in that: a kind of specific method of described small jitter extremum method comprises the steps: (1) Detect the change in the difference frequency of the gyroscope caused by the external magnetic field, increase or decrease the voltage on the pitch angle control unit of the angle control part of the square two-frequency laser gyroscope on an optical path length control mirror with a small step, and At the same time, increase or decrease the control voltage applied to the pitch direction of another optical path length control mirror, so that the light intensity is at a maximum value; (2) Detect the gyro difference frequency change caused by the magnetic field again. If the change decreases, continue to increase or decrease the control voltage on each optical path length control mirror in small steps; if the change increases, Then the small steps in the opposite direction increase or decrease the control voltage on each optical path length control mirror; (3) Repeat the above process until the magnetic susceptibility of the gyro increases no matter whether the voltage is increased or decreased in a small step, and the gyro is in the lowest state of magnetic susceptibility at this time, and the relevant control voltages at this time are The optimal control parameters required.

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