Method for eliminating abnormal angular drift of frame axis of two-axis inertial platform

Abstract

The invention relates to a method for eliminating abnormal angular drift of a frame shaft of a two-shaft inertial platform, and the method comprises the steps: enabling an input quantity to be converted into an angular position from an angular speed through optimizing and stabilizing an integrator sequence in a loop control link; the phenomenon that when the carrier has co-frequency out-of-phase interference, additional constant is introduced due to trigonometric function product and difference arithmetical operation in the decoupling link, and consequently additional constant angle drift exists in a frame axis is avoided. Besides, in combination with the control principle of a stable loop and an angle tracking loop of the two-axis inertial platform, a stage body angular velocity accumulated quantity clearing link is added when the stable loop is switched from the angle tracking loop to the angle tracking loop, the stage body angular velocity accumulated quantity generated in the angle tracking loop control process is cleared, and on the premise that the control precision is not affected, the control precision is improved. The main purpose is to eliminate the abnormal angular drift introduced to a frame shaft stabilizing loop by the angular velocity accumulated quantity of the two-shaft stabilizing platform in the freedom-degree-free direction along with the rotation of a platform body shaft in an angular position decoupling link.

Description

Technical Field

[0001] The invention relates to a method for eliminating abnormal angular drift of a two-axis inertial platform frame axis, and belongs to the technical field of servo loop control of a two-axis inertial platform system. Background Art

[0002] Traditional inertial platforms are generally three-axis or four-axis inertial platforms. Even if the same-frequency or difference-frequency angular motion occurs and the control mode is switched, the platform is stable in the inertial space, and the angular velocity or angle in the three directions of the platform is close to 0, which will not cause large angular drift.

[0003] When the carrier undergoes angular motion at the same frequency but out of phase, if the angular velocity of the gyroscope is first decoupled and then the angular velocity of the frame axis after decoupling is integrated, additional constant angular drift will occur in the frame axis due to the introduction of additional constants by the product and difference operations of trigonometric functions in the decoupling link. The calculation process of the additional constant angular drift is as follows:

[0004] First, the angular velocity of the platform is expressed as

[0005]

[0006] Among them, A x , A y , A z is the amplitude of the angular velocity of the platform; ω is the frequency of the angular velocity of the platform; γ, α, β are the initial phases of the angular velocity of the platform in the X, Y, and Z directions respectively; t is time.

[0007] If the angular velocity decoupling of the gyroscope-sensitive platform body in the X direction and the platform body in the Z direction is performed first, the angular velocity ω of the frame axis direction after decoupling is obtained. z (t) is expressed as follows:

[0008]

[0009] in, Approximately -k1cos(2ωt-α), Approximately -k2sin(ωt-α), k1 and k2 are the amplitudes of the composite function. z (t) The calculation results are as follows:

[0010] ω z (t)=k1[cos(ωt)cos(ωt-α)sin(ωt-γ)-sin(ωt)sin(ωt-α)sin(ωt-γ)]+k2[cos(β-α)+sin2ωtsin(β+α)-cos2ωtcos(β-α)]

[0011] By ω z(t) From the calculation results, it can be seen that k2cos(β-α) is a constant. This constant term is caused by the fact that the same-frequency angular velocity signal is generated by the product and difference operations of trigonometric functions in the decoupling link, which leads to the introduction of additional constant angular drift of the frame axis.

[0012] The existing two-axis inertial platform ignores the same-frequency and different-phase dynamic interference, which produces a constant angular drift due to the product and difference operations of trigonometric functions in the decoupling link. It is believed that the probability of the same-frequency and different-phase interference occurring in actual flight is extremely low.

[0013] The two-axis inertial platform is different from the three-axis inertial stabilization platform in that it lacks one control degree of freedom. When the servo loop works in the stable loop control state, the angle of the table body in the Y direction is 0, and the angle of the table body in the X direction and the angle of the table body in the Z direction are decomposed into the frame axis direction after the angular position signal is decoupled. The angle of the frame axis direction is 0, but the angle of the table body in the X direction and the Z direction is not completely 0 due to the lack of one control degree of freedom. Therefore, when the servo loop works in the angle tracking loop control state, the table body in the X direction and the Z direction have accumulated a large angle value. When the servo loop switches from the angle tracking loop control state to the stable loop control state, the angle of the table body in the X direction and the Z direction that cannot be overcome by the actuator due to the lack of one degree of freedom will be introduced into the frame axis stabilization loop through the angular position decoupling link within the control cycle of the switching control state, and part of the angle of the table body in the X direction and the Z direction cannot be controlled to 0 by the servo loop. Therefore, the frame axis stabilization loop will introduce abnormal angle drift within the control cycle at the time of control state switching. Summary of the invention

[0014] The technical problem solved by the present invention is to overcome the deficiencies of the prior art and provide a method for eliminating abnormal angular drift of a two-axis inertial platform frame axis, thereby eliminating abnormal angular drift introduced by co-frequency and heterogeneous dynamic interference and control state switching.

[0015] The solution to the technical problem of the present invention is: a method for eliminating abnormal angular drift of a two-axis inertial platform frame axis, the method comprising a platform axis stabilization loop control process:

[0016] The angular velocity of the table in the Y direction is sensed by the fiber optic gyroscope and is processed successively by the integral link, the proportional integral link, the lead correction link, and the lag correction link to obtain the table axis control signal. After power amplification, the table axis control signal is output as a control torque signal to the torque motor, forming a closed-loop control of the table axis stability loop.

[0017] Preferably, the above method for eliminating abnormal angular drift of the frame axis of the two-axis inertial platform also includes a frame axis stabilization loop control process:

[0018] The fiber optic gyroscope senses the angular velocity of the table in the X direction and the Z direction of the table. The angular velocity of the table in the X direction and the Z direction of the table are respectively obtained through the integration link to obtain the angle of the table in the X direction and the angle of the table in the Z direction. The angle of the table in the X direction and the angle of the table in the Z direction are decomposed into the frame axis direction after the angular position signal is decoupled. The angle of the frame axis direction is processed by the proportional integral link, the lead correction link, and the lag correction link to obtain the frame axis control signal. The frame axis control signal is output after power amplification to the torque motor to form a closed-loop control of the frame axis stability loop.

[0019] Preferably, the integral link in the platform axis stabilization loop is discretized by bilinear transformation and expressed as follows:

[0020]

[0021] in, is the angular velocity of the stage in the Y direction; is the angle after integrating the angular velocity in the Y direction of the platform, θ yp [0] is the accumulated value of the angular velocity in the Y direction of the platform in the previous control cycle.

[0022] Preferably, within the 1ms control cycle when the stage axis angle tracking loop control state switches to the stage axis stabilization loop control state, the accumulated value of the stage Y direction angular velocity in the previous control cycle is Ensure that the initial value of the input of the table axis stability loop is 0. It is worth noting that the clearing step is no longer executed after the control state is switched, and the angular velocity accumulated value clearing step is only executed within the 1ms period of the control state switching.

[0023] Preferably, the integral link in the frame axis stabilization loop is discretized by bilinear transformation and expressed as follows:

[0024]

[0025] in, are the angular velocities of the stage in the X and Z directions respectively; are the angles after integrating the angular velocities in the X and Z directions of the platform, They are respectively the accumulated values ​​of the angular velocity in the X and Z directions of the platform in the previous control cycle.

[0026] Preferably, within the 1ms control cycle when the frame axis angle tracking loop control state switches to the frame axis stabilization loop control state, the accumulated value of the angular velocity in the X direction of the platform in the previous control cycle is Let the accumulated value of the angular velocity in the Z direction of the platform in the previous control cycle be Ensure that the initial value of the frame axis stability loop input is 0. It is worth noting that the clearing step is no longer executed after the control state is switched, and the angular velocity accumulated value clearing step is only executed within the 1ms period of the control state switching.

[0027] Preferably, the angular position signal decoupling formula is as follows:

[0028]

[0029] in, is the angle of the frame axis; They are the angle of the stage in the X direction and the angle of the stage in the Z axis direction, is the rotation angle of the table axis.

[0030] The accumulated angular velocity value is cleared only within the 1ms period of control state switching. The angular velocity accumulation in the direction without degrees of freedom of the two-axis stable platform can be eliminated in the angular position decoupling link along the platform body axis. The rotation introduces abnormal angular drift into the frame axis stabilization loop.

[0031] The beneficial effects of the present invention compared with the prior art are:

[0032] (1) The present invention eliminates the additional constant angle drift of the frame axis caused by the product and difference mathematical operations of trigonometric functions in the decoupling link of the carrier's same-frequency and out-of-phase angular motion by optimizing the servo loop control structure of the two-axis inertial platform;

[0033] (2) The present invention adds a step of clearing the integral values ​​of the three directions of the platform angle in the stabilization loop control link during the control cycle of switching the two-axis inertial platform angle tracking loop to the stabilization loop control state, thereby eliminating the abnormal angular drift introduced into the frame axis stabilization loop by the accumulated angular velocity in the direction without degrees of freedom of the two-axis stabilization platform in the angular position decoupling link as the platform axis rotates. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a structural diagram of the stabilization loop of the two-axis inertial platform of the present invention;

[0035] Figure 2 It is the frame axis drift result under the structure of first decoupling and then integrating the angular velocity of the platform;

[0036] Figure 3 It is the frame axis drift result under the structure of integrating the angular velocity of the platform first and then decoupling;

[0037] Figure 4 To avoid abnormal angle drift introduced when switching control states without clearing the angle accumulation value in the integral link;

[0038] Figure 5To eliminate the abnormal angle drift introduced when the control state is switched under the angle accumulation value in the integral link. DETAILED DESCRIPTION

[0039] The present invention will be further described below in conjunction with the embodiments.

[0040] There are two reasons for the abnormal angular drift of the two-axis inertial platform. One is that when there is angular motion of the same frequency and out of phase in the direction of the platform axis and the frame axis, the additional constant angular drift of the frame axis is caused by the product and difference mathematical operation of trigonometric functions in the angular position decoupling link. The other is that the accumulated angular velocity in the direction without degrees of freedom during the control period of switching the control state of the stabilization loop from the control state of the angle tracking loop introduces abnormal angular drift to the stabilization loop of the frame axis in the angular position decoupling link as the platform axis rotates.

[0041] The present invention takes into account the co-frequency and heterogeneous dynamic interference environment of the carrier and the control state switching function in dynamic flight, and eliminates the additional constant angular drift of the frame axis caused by the product and difference mathematical operations of trigonometric functions in the decoupling link of the co-frequency and heterogeneous angular motion of the carrier in the decoupling link by adjusting and optimizing the control sequence structure of the integral link and other control links in the stabilization loop; by adding a two-axis inertial platform angle tracking loop to the three-direction angular integral value clearing link of the stabilization loop control link in the stabilization loop control state switching cycle, the angular velocity accumulation in the direction without degrees of freedom of the two-axis stabilization platform is eliminated in the angular position decoupling link along with the rotation of the platform axis to introduce abnormal angular drift into the stabilization loop of the frame axis

[0042] The two-axis inertial platform stabilization circuit includes two circuits: the platform axis and the frame axis. When there is an interference torque M at the platform axis end f When the framework system Converted into angular velocity, fiber optic gyroscope G FOG (s) is sensitive to angular velocity and outputs the corresponding digital signal to the servo control link, which includes an angular position decoupler Controller G ci (s), and finally outputs the control torque signal to the torque motor G after power amplification. m (s), the torque motor generates the corresponding feedback torque M D To offset the disturbance torque M f , so that the platform's body axis and frame axis are stabilized in the inertial space, ensuring the normal operation of the platform's stabilization circuit.

[0043] When there are angular motions of the same frequency and different phases in the three directions of the platform axis, if the angular velocity output by the gyroscope is directly decoupled Decoupling will cause The product and difference of the trigonometric functions produces an additional constant angular drift k2cos(β-α). Among them, k2 is related to the amplitude and angular frequency of the carrier's angular motion, and α and β are the initial phases of the angular velocity in the Y and Z directions of the platform, respectively.

[0044] like Figure 1 As shown, in order to suppress the value of the additional constant angular drift k2cos(β-α), the present invention establishes the transfer function of each link in the two-axis inertial platform stabilization loop; designs a stabilization loop controller according to the index requirements, wherein the controller includes a proportional amplification link, a pure integral link, a proportional integral link, a lead correction link, a lag correction link, etc.; optimizes and adjusts the control sequence of each link in the stabilization loop, and converts the integral link Pre-coupler Previously, the integrated angular velocity of the platform becomes an angle value within a certain period of time, and when the trigonometric function operation in the decoupling link is performed, no additional constant will be generated due to the product and difference relationship between the angular velocities of the same frequency.

[0045] The control process of the platform axis stability loop is as follows:

[0046] When the table axis is subjected to the disturbance torque M f When the angular velocity of the table axis is 0, it is converted into the angular velocity in the Y direction of the table body through the frame system, and the angular velocity in the Y direction of the table body axis is sensed by the fiber optic gyroscope. After being processed by the integral link and the controller (proportional integral link, lead correction link, and lag correction link), the table body axis control signal is obtained. The table body axis control signal is output as a control torque signal to the torque motor after power amplification, forming a closed-loop control of the body axis stability loop.

[0047] The frame axis stabilization loop control process is as follows:

[0048] When the frame axis is subjected to the disturbance torque M f When the angle is 0, it is converted into the angular velocity of the table body in the X direction and the Z axis direction through the frame system, and the angular velocity of the table body in the X direction and the Z axis direction is sensed by the fiber optic gyroscope. The angular velocity of the table body in the X direction and the Z axis direction is respectively obtained through the integration link to obtain the angle of the table body in the X direction and the angle of the table body in the Z axis direction. The angle of the table body in the X direction and the angle of the table body in the Z direction are decomposed into the frame axis direction after the angular position signal is decoupled. The angle of the frame axis direction is processed by the proportional integral link, the lead correction link, and the lag correction link to obtain the frame axis control signal. The frame axis control signal is output as a control torque signal to the torque motor after power amplification.

[0049] The open-loop transfer function of the platform axis and frame axis stability loop system is:

[0050]

[0051] J is the inverse matrix of the frame axis moment of inertia;

[0052] Fiber Optic Gyroscope Transfer Function

[0053]

[0054] Among them, B w is the frequency bandwidth.

[0055] Torque motor transfer function:

[0056]

[0057] Among them, K g is the moment coefficient, T g is the torque motor time constant, L m is the motor inductance, R m Motor winding resistance;

[0058] G c (s) is a stable loop controller, including an integral link, a proportional integral link, a lead correction link, and a lag correction link.

[0059] In a specific embodiment of the present invention, the PID parameters are adjusted using a robust response time method, and the transfer function of the proportional-integral link is: The transfer function of the lead correction link is: The transfer function of the lag correction link is:

[0060] The transfer function of the stability loop controller of the platform axis and frame axis is:

[0061]

[0062] K ci represents the gain coefficient of the stability loop controller. i=y represents the gain of the platform axis stability loop controller, and i=z represents the gain of the frame axis stability loop controller.

[0063] Transformation matrix from the table coordinate system to the frame coordinate system

[0064]

[0065] In order to avoid the introduction of angle product values ​​when the control state is switched, the present invention adds a link for clearing the angle integral values ​​of the three directions of the platform body in the stabilization loop control link within the control state switching period of the two-axis inertial platform angle tracking loop to eliminate the abnormal angular drift introduced into the frame axis stabilization loop by the accumulated angular velocity in the direction without degrees of freedom of the two-axis stable platform in the angular position decoupling link as the platform axis rotates.

[0066] The integral link in the platform axis stabilization loop is expressed as follows after being discretized by bilinear transformation:

[0067]

[0068] in, is the angular velocity of the stage in the Y direction; is the angle after integrating the angular velocity in the Y direction of the platform, is the accumulated value of the angular velocity of the stage in the Y direction in the previous control cycle. In a specific embodiment of the embedded digital control, the control cycle is 1 ms.

[0069] In the 1ms control cycle when the table axis angle tracking loop control state switches to the table axis stabilization loop control state, let the accumulated value of the table Y direction angular velocity in the previous control cycle

[0070] The integral link in the frame axis stabilization loop is expressed as follows after being discretized by bilinear transformation:

[0071]

[0072] in, are the angular velocities of the stage in the X and Z directions respectively; are the angles after integrating the angular velocities in the X and Z directions of the platform, They are respectively the accumulated values ​​of the angular velocity in the X and Z directions of the platform in the previous control cycle.

[0073] In the 1ms control cycle when the frame axis angle tracking loop control state switches to the frame axis stabilization loop control state, let the accumulated value of the angular velocity in the X direction of the platform in the previous control cycle Let the accumulated value of the angular velocity in the Z direction of the platform in the previous control cycle be

[0074] After switching to the stable loop, the integrated value is cleared and the stable loop begins to isolate the angular motion of the external carrier after the switching moment.

[0075] The formula for decoupling the angular position signal is as follows:

[0076]

[0077] in, is the angle of the frame axis; They are the angle of the stage in the X direction and the angle of the stage in the Z axis direction, is the rotation angle of the table axis.

[0078] Figure 2The drift result of the frame axis under the structure of first decoupling and then integrating the angular velocity of the platform. The platform is placed on a three-degree-of-freedom swing turntable, which is divided into the inner frame axis, the middle frame axis and the outer frame axis. The platform works in a stable loop state. The stable loop control structure adopts a control structure of first decoupling the angular velocity and then integrating the angular velocity. The turntable swings at 1Hz and 6° in three directions at the same time. Figure 2 It can be seen that the angular velocity of the same frequency angular motion is decoupled by the decoupler The mathematical operation of the product and difference of the trigonometric functions generates an additional constant angular drift of the frame axis, which is 1° / s. This drift will cause a constant additional angular velocity to be added to the angular velocity in the direction of the frame axis. This constant additional angular velocity will affect the platform's tracking of the specified coordinate system and the completion of high-precision dual-axis rotation modulation control.

[0079] Figure 3 The drift result of the frame axis under the structure of integrating the angular velocity first and then decoupling the platform. The platform is placed on a three-degree-of-freedom swing turntable, which is divided into the inner frame axis, the middle frame axis and the outer frame axis. The platform works in a stable loop state, and the servo control structure adopts a control structure of integrating the angular velocity first and then decoupling. The turntable swings at 1Hz and 6° in three directions simultaneously. Figure 3 It can be seen that there is no additional angular drift in the angle of the frame axis direction. Because the angular velocity of the platform after integration becomes the angle value within a certain period of time, no additional constant will be generated due to the product and difference relationship between the angular velocities of the same frequency during the trigonometric function calculation in the decoupling link.

[0080] Figure 4 In order to avoid the abnormal angular drift introduced when the control state is switched without clearing the angle accumulation value in the integral link, the platform is placed on a marble table, and the two axes of the platform are placed at the initial position of 0°, with the frame axis facing east and the ground speed in the east close to 0° / s. In the static base state, the stabilization loop adopts a control method that does not clear the angle accumulation value of the integral link when the control state is switched. The control loop is first run in the angle tracking loop state for 100s, and then the platform body axis rotates at an angular velocity of 10° / s while switching to the stabilization loop control state. Figure 4 This is the angular velocity drift curve of the frame axis after the control state is switched, and the slope of the curve after switching is -0.86° / s. In theory, the rotation of the table axis will not introduce additional angular drift to the frame axis, but because the two-axis platform lacks a degree of freedom, the method of integrating first and then decoupling will introduce the angle value of the missing degree of freedom before the switch due to the rotation of the table axis at the moment of control state switching.

[0081] Figure 5In order to clear the abnormal angular drift introduced when the control state is switched under the angle accumulation value in the integral link. The platform is placed on a marble table, and the two axes of the platform are placed at the initial position of 0°, with the ring frame axis facing east and the ground speed in the east close to 0° / s. In the static base state, the stabilization loop adopts the control method of clearing the angle accumulation of the integral link when the control state is switched. The control loop is first run in the angle tracking loop state for 100s, and then the platform body axis rotates at an angular velocity of 10° / s while switching to the stabilization loop control state. Figure 5 This is the angular velocity drift curve of the frame axis after the control state is switched. The slope of the curve after switching is -0.076° / s. Clear the integral link angle accumulation θ xp [0], θ zp [0], the frame axis angular velocity drift is compared to Figure 4 (Uncleared case), the frame axis angle drift slope is reduced by about 11 times.

[0082] It can be seen from the above that, under the premise of not affecting the control accuracy, the present invention enables the two-axis inertial platform to still exert its control advantage of multi-mode switching in a maneuvering flight environment, while eliminating the abnormal angular drift of the frame axis.

[0083] In summary, the present invention optimizes the order of integrators in the stabilization loop control link so that the input is converted from angular velocity to angular position. By combining the control principles of the stabilization loop and the angle tracking loop of the two-axis inertial platform, a link for clearing the accumulated angular velocity of the platform at the time of switching from the angle tracking loop to the stabilization loop is added, thereby achieving the purpose of eliminating abnormal angular drift of the frame axis.

[0084] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for eliminating abnormal angular drift of a two-axis inertial platform frame axis, characterized in that Including the platform axis stability loop control process: The angular velocity of the table in the Y direction is sensed by the fiber optic gyroscope and is processed successively by the integral link, the proportional integral link, the lead correction link, and the lag correction link to obtain the table axis control signal. After power amplification, the table axis control signal is output as a control torque signal to the torque motor, forming a closed-loop control of the table axis stability loop.

2. A method for eliminating abnormal angular drift of a two-axis inertial platform frame axis according to claim 1, characterized in that: Also included is the frame axis stabilization loop control process: The fiber optic gyroscope senses the angular velocity of the table in the X direction and the Z direction of the table. The angular velocity of the table in the X direction and the Z direction of the table are respectively obtained through the integration link to obtain the angle of the table in the X direction and the angle of the table in the Z direction. The angle of the table in the X direction and the angle of the table in the Z direction are decomposed into the frame axis direction after the angular position signal is decoupled. The angle of the frame axis direction is processed by the proportional integral link, the lead correction link, and the lag correction link to obtain the frame axis control signal. The frame axis control signal is output after power amplification to the torque motor to form a closed-loop control of the frame axis stability loop.

3. The two-axis inertial platform system stabilization loop control process according to claim 1, characterized in that: The integral link is expressed as follows after being discretized by bilinear transformation: in, is the angular velocity of the stage in the Y direction; is the angle after integrating the angular velocity in the Y direction of the platform, It is the accumulated value of the angular velocity in the Y direction of the platform in the previous control cycle.

4. The two-axis inertial platform system stabilization loop control process according to claim 3, characterized in that: In the 1ms control cycle when the stage axis angle tracking loop control state switches to the stage axis stabilization loop control state, let the accumulated value of the stage Y direction angular velocity in the previous control cycle Ensure that the initial value of the input of the table axis stability loop is 0. It is worth noting that the clearing step is no longer executed after the control state is switched, and the angular velocity accumulated value clearing step is only executed within the 1ms period of the control state switching.

5. The two-axis inertial platform system stabilization loop control process according to claim 2, characterized in that: The integral link is expressed as follows after being discretized by bilinear transformation: in, are the angular velocities of the stage in the X and Z directions respectively; are the angles after integrating the angular velocities in the X and Z directions of the platform, They are respectively the accumulated values ​​of the angular velocity in the X and Z directions of the platform in the previous control cycle.

6. The two-axis inertial platform system stabilization loop control process according to claim 5, characterized in that: In the 1ms control cycle when the frame axis angle tracking loop control state switches to the frame axis stabilization loop control state, let the accumulated value of the angular velocity in the X direction of the platform in the previous control cycle Let the accumulated value of the angular velocity in the Z direction of the platform in the previous control cycle be Ensure that the initial value of the frame axis stability loop input is 0. It is worth noting that the clearing step is no longer executed after the control state is switched, and the angular velocity accumulated value clearing step is only executed within the 1ms period of the control state switching.

7. The two-axis inertial platform system frame axis stabilization loop control process according to claim 2, characterized in that: The angular position signal decoupling formula is as follows: Among them, θ zA [1] is the angle of the frame axis; They are the angle of the stage in the X direction and the angle of the stage in the Z axis direction, is the rotation angle of the table axis. The accumulated angular velocity value is cleared only within the 1ms period of control state switching. The angular velocity accumulation in the direction without degrees of freedom of the two-axis stable platform can be eliminated in the angular position decoupling link along the platform body axis. The rotation introduces abnormal angular drift into the frame axis stabilization loop.

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