Support rod actuator, six-degree-of-freedom secondary mirror adjusting mechanism and control system and method thereof

Abstract

The invention discloses a strut actuator, a six-degree-of-freedom secondary mirror adjusting mechanism and a control system and method thereof. The support rod actuator adopts a linear variable differential transformer (LVDT) as a sensor and comprises a primary coil, two secondary coils and an iron core. When sine wave signals are applied to the two ends of the primary coil, along with movement of the iron core, the voltage difference value induced by the two secondary coils and the displacement amount of the iron core are in a linear relation, and the linear relation is not sensitive to space irradiation. The two ends of the supporting rod actuator are connected with the upper platform and the lower platform through hooke joints, and the supporting rod actuator composed of a gear motor driving assembly and an LVDT micro-displacement sensor is restrained by a flexible rhombic support to form the micro-deformation equal-proportion amplification effect in the horizontal direction and the vertical direction. The configuration of the six support rods can effectively realize the adjustment of the six-degree-of-freedom pose of the secondary mirror, and a closed-loop six-degree-of-freedom pose control method is adopted to avoid the problem of inaccurate pose adjustment caused by the motion deviation of the support rods due to gaps and return errors brought by the motion of the stepping motor.

Description

Technical Field

[0001] The invention relates to the technical field of a main optical system of a space camera, and in particular to a support rod actuator, a six-degree-of-freedom secondary mirror adjustment mechanism, and a control system and method thereof. Background Art

[0002] The main optical system of a space camera generally adopts a three-mirror anastigmatism design, including a primary mirror, a secondary mirror, and a tertiary mirror. The optical system structure is as follows: Figure 1 As shown in the figure, the relative posture change of the primary reflector and the secondary reflector has a significant impact on the imaging quality of the space target. During the ground assembly and adjustment of the camera, due to the installation error of the reflector during the assembly process, the elastic deformation of the structure, the active gap when the reflector is fixed, the rigid position error of each optical element in the optical system is caused, and during the on-orbit use, the mechanical environment during satellite launch, the change of ambient temperature after the camera enters orbit, and the stress release and deformation of the optical and mechanical parts, the relative slight posture changes between the optical elements including the primary mirror, the secondary mirror and the three mirrors include translational motion along the three axes and rotation around the three axes. The posture change range is in the order of hundreds of microns and hundreds of arc seconds. This posture change will cause the MTF of the camera to decrease and affect the imaging quality of the camera. Therefore, the secondary mirror adjustment device with the on-orbit micro-posture active adjustment function can adjust the changes in the relative posture misalignment of the primary-secondary mirror of the camera optical system in the main optical system, which is of great significance to optimize the on-orbit imaging quality of the camera. Therefore, the micro-posture adjustment system of the secondary mirror parallel mechanism of the space remote sensing camera is mainly used to adjust the posture of the secondary mirror to ensure the imaging quality.

[0003] Due to the high rigidity requirements of the secondary mirror structure and the secondary mirror adjustment device in the launch mechanics of the launch vehicle, there are high requirements for micro-nano drive and micro-displacement measurement technology and its aerospace engineering. In the existing technology, the measurement sensor of the secondary mirror adjustment device adopts a capacitive sensor or a straight grating encoder. The capacitive sensor has a time drift characteristic and needs to be calibrated for long-term space applications, which cannot meet the requirements of space applications. The photoelectric-to-photoelectric conversion accuracy of the straight grating sensor is affected by the space radiation environment and cannot meet the requirements of high-orbit satellite applications. Summary of the invention

[0004] The present invention provides a support rod actuator, a six-degree-of-freedom secondary mirror adjustment mechanism and a control system and method thereof, with the aim of effectively realizing the adjustment of the six-degree-of-freedom posture of the secondary mirror, and adopting a closed-loop six-degree-of-freedom posture control method to avoid the problem of inaccurate posture adjustment caused by the gap caused by the movement of the stepper motor and the support rod movement deviation caused by the return error.

[0005] In a first aspect, a strut actuator is provided, comprising: a flexible diamond bracket, an LVDT stator, an LVDT mover, an LVDT mover bracket, an LVDT stator bracket, a reduction drive assembly, and a differential screw assembly;

[0006] The flexible diamond-shaped bracket includes a first end and a second end opposite to each other in the axial direction, and also includes a third end and a fourth end opposite to each other in the radial direction; the reduction drive assembly is fixedly installed between the first end and the second end of the flexible diamond-shaped bracket through a differential screw assembly; the LVDT stator bracket surrounds the outer periphery of the reduction drive assembly so as not to affect the rotation of the reduction drive assembly; one end of the LVDT stator bracket is fixed to the third end of the flexible diamond-shaped bracket, and the other end extends to the vicinity of the fourth end of the flexible diamond-shaped bracket and is installed with the LVDT stator; the LVDT mover bracket is installed at the fourth end of the flexible diamond-shaped bracket and is used to set the LVDT mover, and the LVDT mover cooperates with the LVDT stator;

[0007] The reduction drive assembly outputs rotational motion, driving the lead screw in the differential screw assembly to rotate, thereby causing the distance between the first end and the second end of the flexible diamond bracket to change; thereby changing the distance between the third end and the fourth end of the flexible diamond bracket; a displacement is generated between the mover and the stator of the LVDT, which is used to measure the change in the distance between the third end and the fourth end of the flexible diamond bracket.

[0008] In combination with the first aspect, in certain implementations of the first aspect, the LVDT stator includes a primary coil and two secondary coils, and the LVDT mover includes an iron core; when a sinusoidal wave signal is applied to both ends of the primary coil, as the iron core moves, the voltage difference induced by the two secondary coils is linearly related to the displacement of the iron core.

[0009] In a second aspect, a six-degree-of-freedom secondary mirror adjustment mechanism is provided, comprising a moving platform, a static platform, a Hooke's hinge, and six support rod actuators as described in any one of the implementations of the first aspect above; the third end and the fourth end of the flexible diamond-shaped bracket of the six support rod actuators are respectively fixed to the static platform and the moving platform through the Hooke's hinge to form a six-degree-of-freedom parallel mechanism; wherein,

[0010] A static coordinate system O-XYZ is established at the center of the circle where the lower hinges of the six Hooke's hinges of the static platform are located, with the origin of the coordinate system being O; a dynamic coordinate system P-xyz is established at the center of the circle where the upper hinges of the six Hooke's hinges of the dynamic platform are located, with the origin of the coordinate system being P; the coordinate vector of the rotation center of the upper hinge in the dynamic coordinate system is A i (A1, A2, A3, A4, A5, A6), the coordinate of the rotation center of the lower hinge in the static coordinate system is marked as B i (B1, B2, B3, B4, B5, B6); the direction of the line connecting the third and fourth ends of the flexible diamond bracket of the i-th support actuator corresponds to A i -B i The connection direction of

[0011] Each strut actuator outputs displacement independently. The six strut actuators output different displacements to achieve different postures of the moving platform. The posture of the motion platform of the six-degree-of-freedom parallel mechanism is expressed by the generalized coordinate vector q, where q = [x, y, z, α, β, γ] T , expressed as a translation of three axes and a rotation around three axes.

[0012] In a third aspect, a control system for a six-degree-of-freedom secondary mirror adjustment mechanism as described in any one of the implementations of the second aspect is provided, comprising:

[0013] The wavefront phase difference calculation model uses multi-frame phase difference inversion based on focal plane imaging to describe the phase difference of Zernike polynomial coefficients;

[0014] The six-DOF secondary mirror adjustment mechanism measurement module reads the resolved code values ​​of the LVDT sensors in the six strut drive assemblies by edge by edge, and performs linear interpolation with the calibrated code value-displacement data of each strut to obtain the current displacement information of each strut;

[0015] The six-degree-of-freedom secondary mirror adjustment mechanism control module receives the secondary mirror adjustment posture x, y, z, α, β, γ instructions output by the wavefront difference solution module, calculates the expected length of each support rod according to the inverse solution formula of the six-degree-of-freedom platform kinematics, and calculates the displacement of each support rod that needs to be adjusted according to the LVDT displacement measurement information of each support rod output by the six-degree-of-freedom secondary mirror adjustment mechanism measurement module. It converts the displacement into the step amount and direction of the motor of each support rod actuator assembly according to the step amount of the stepper motor, and coordinates the movement time of each support rod in place to realize the movement of each support rod.

[0016] In conjunction with the third aspect, in certain implementations of the third aspect, the six-degree-of-freedom secondary mirror adjustment mechanism measurement module includes an excitation signal generation module, an induction signal conditioning module, and an induction signal solving module;

[0017] The excitation signal generation module outputs a sine wave excitation signal with adjustable amplitude and frequency to meet the LVDT input signal requirements;

[0018] The induction signal conditioning module receives the Va and Vb signals induced by the A-phase winding and the B-phase winding of the LVDT, conditions them, and adjusts the amplification factors a and b of the two operational amplifiers in the first-stage amplification circuit separately to meet the requirement that the effective value of the sum of the Va+Vb of the signal is conditioned to 2Vrms, and at the same time, the maximum effective value of the difference between the Va-Vb of the signal is conditioned to 2Vrms; the signal resolution module uses the input sine and cosine signals and an integrated circuit based on the amplitude detection resolution principle to complete the output of the AC signal to 16-bit parallel binary data, and reads the data through the FPGA control timing.

[0019] In a fourth aspect, a control method for a six-degree-of-freedom secondary mirror adjustment mechanism as described in any one of the implementations of the second aspect is provided, comprising:

[0020] Through the phase difference inversion of multiple frames based on focal plane imaging, the Zernike polynomial coefficients are obtained to obtain the desired adjustment posture x1, y1, z1, α1, β1, γ1 of the six-degree-of-freedom secondary mirror mechanism;

[0021] Read the digital value of the LVDT sensor of each support rod and find the code value-displacement mapping table of each support rod calibration to calculate the displacement Δl of each support rod relative to the initial origin i , i = 1, 2, 3, 4, 5, 6, and calculate the current position x0, y0, z, α0, β0, γ0 through the kinematic solution formula of the six-degree-of-freedom platform;

[0022] According to the secondary mirror, adjust the posture x1, y1, z1, α1, β1, γ1 command content, and calculate the expected length L of each support rod through the kinematic inverse formula of the six-degree-of-freedom platform according to the posture command i ;

[0023] According to the expected length L of each support rod i and the current displacement of each support rod Δl i , the expected elongation or shortening of each support rod is obtained by calculation i =L i -D i +Δl i (i=1,2,3,4,5,6),D i is the standard length of each support rod;

[0024] The step amount N of each support rod is calculated by dividing the expected extension or shortening of each support rod by the minimum step distance. i =Δd i / s i ,s i The minimum step length of the motor is set, and the driving step frequency of each support rod is optimized to achieve that each support rod reaches the desired displacement at the same time;

[0025] According to the stepping amount, stepping frequency and movement direction of each support rod, a driving pulse width modulation signal of each support rod is generated and sent to the power driving module to drive the stepping motor to move;

[0026] Determine whether the difference between the current posture and the expected posture meets the threshold condition;

[0027] If the condition is satisfied, the movement of each support rod stops; if the condition is not satisfied, each support rod continues to move, and the above steps are repeated until the threshold condition is satisfied.

[0028] In combination with the fourth aspect, in some implementations of the fourth aspect, the control method further includes ground calibration of a six-degree-of-freedom secondary mirror adjustment platform, including:

[0029] Initialize system variables, including platform structure parameters and workspace;

[0030] Average step length of a single rod i The calibration is carried out by using a laser interferometer and self-collimation technology to collect the measured displacement corresponding to a number of steps of the support rod driver movement for calibration;

[0031] The single-rod driver LVDT measurement and calibration is to drive the single rod to move from the extreme stroke on one side to the extreme stroke on the other side at a fixed periodic interval, a fixed number of steps, and a fixed frequency. During the movement, the LVDT solution digital quantity is read through the LVDT measurement module, and the elongation of the measured rod is collected by the laser interferometer. N sampling points are selected according to the range of motion of the rod to obtain a two-dimensional array consisting of N LVDT code values ​​and N rod elongations. This array is the rod motion calibration code value-displacement mapping data Di[N,2], (i=1,2,…,6).

[0032] In conjunction with the fourth aspect, in some implementations of the fourth aspect, the control method includes:

[0033] Read the calculated digital quantity dn of the current LVDT sensor of each support rod, and find the code value-displacement mapping table Di[N,2] of each support rod calibration, and use the linear interpolation method to fit the displacement Δl of each support rod relative to the initial origin i (i=1,2,3,4,5,6), when dn∈(Di[n,1],Di[n+1,1]), where n∈(1,N-1), then Δl i =(Di[n+1,2]-Di[n,2]) / (Di[n+1,1]-Di[n,1])*(dn-Di[n,1])+Di[n,2]; and calculate the current position x0, y0, z, α0, β0, γ0 through the kinematic solution formula of the six-degree-of-freedom platform.

[0034] In conjunction with the fourth aspect, in certain implementations of the fourth aspect, the support rod LVDT micro displacement Δl i Measurements include:

[0035] An excitation signal generator is used to generate a sinusoidal excitation signal with power driving capability and output it to the excitation end of the LVDT sensor;

[0036] The signal conditioning module receives the induced voltage signals Va and Vb from the sensing end of the LVDT sensor, obtains the Va+Vb signal and the Va-Vb signal through calculation and conditioning, ensures that the maximum effective values ​​of the Va+Vb and Va-Vb signals meet the solution requirements, and applies a bandpass filter to filter the Va+Vb and Va-Vb signals;

[0037] The shaft angle calculation module is applied, and the excitation reference signal, Va+Vb and Va-Vb signals are used as inputs to obtain 16-bit binary data through amplitude-discrimination shaft angle calculation.

[0038] FPGA is used to read and process the shaft angle solution data.

[0039] In conjunction with the fourth aspect, in certain implementations of the fourth aspect, the parameters of the rotation matrix R are obtained according to the posture adjustment information, and the kinematic inverse solution of the six-degree-of-freedom platform is used to calculate:

[0040] ‖L i ‖=‖RA i +PB i ‖

[0041] Where R is the rotation transformation matrix of the motion coordinate system; A i B is the coordinate vector of the center point of the hinge of the moving platform in the motion coordinate system; i is the coordinate vector of the center point of the static platform hinge in the static coordinate system; P is the offset vector of the origin of the moving coordinate system relative to the origin of the static coordinate system; L i is the length of the support rod; after the above calculation, the expected length L of each support rod is obtained i (i=1,2,3,4,5,6).

[0042] Compared with the prior art, the solution provided by the present invention includes at least the following beneficial technical effects:

[0043] This patent proposes a six-degree-of-freedom parallel mechanism micro-position adjustment system and method driven by a reduction motor and measured by an LVDT micro-displacement sensor. The two ends of a single support rod are connected to the upper and lower platforms through a Hooke's joint. The support rod actuator composed of a reduction motor drive assembly and an LVDT micro-displacement sensor forms a proportional amplification effect of micro-deformation in the horizontal and vertical directions through a flexible diamond bracket constraint. The configuration of the six support rods can effectively adjust the six-degree-of-freedom position of the secondary mirror, and a closed-loop six-degree-of-freedom position control method is used to avoid the problem of inaccurate position adjustment caused by the gap caused by the stepper motor movement and the deviation of the support rod movement caused by the return error. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 Schematic diagram of the optical system structure.

[0045] Figure 2Schematic diagram of the strut actuator structure.

[0046] Figure 3 Schematic diagram of the six-degree-of-freedom secondary mirror adjustment mechanism.

[0047] Figure 4 Schematic diagram of the coordinate system of the six-degree-of-freedom parallel mechanism.

[0048] Figure 5 The schematic diagram is a structural diagram of a six-degree-of-freedom micro-position adjustment control system for a secondary mirror of a space remote sensing camera.

[0049] Figure 6 This is the flow chart of the control method for six-degree micro-posture adjustment of the secondary mirror mechanism.

[0050] Figure 7 This is the structural block diagram of the support rod micro-displacement signal detection circuit. DETAILED DESCRIPTION

[0051] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0052] Figure 2 Schematic diagram of a support rod actuator provided by the present invention. Figure 2 As shown, it includes a flexible diamond bracket 1, an LVDT stator 2, an LVDT mover 3, an LVDT mover bracket 4, an LVDT stator bracket 5, a reduction drive assembly 6, and a differential screw assembly 7. The flexible diamond bracket 1 includes a first end and a second end opposite to each other in the axial direction, and also includes a third end and a fourth end opposite to each other in the radial direction.

[0053] The reduction drive assembly 6 is fixedly mounted between the first and second ends of the flexible diamond bracket 1 through a differential screw assembly 7. The LVDT stator bracket 5 surrounds the outer circumference of the reduction drive assembly 6 so as not to affect the rotation of the reduction drive assembly 6. One end of the LVDT stator bracket 5 is fixed to the third end of the flexible diamond bracket 1, and the other end extends to the vicinity of the fourth end of the flexible diamond bracket 1. The LVDT mover bracket 4 is mounted on the fourth end of the flexible diamond bracket 1 and is used to set the LVDT mover 3. The LVDT stator 2 is mounted on the LVDT stator bracket 5 and cooperates with the LVDT mover 3.

[0054] The reduction drive assembly 6 outputs high-precision rotational motion, driving the screw in the differential screw assembly 7 to rotate, thereby driving the distance between the first end and the second end of the flexible diamond bracket 1 to change. This can change the distance between the third end and the fourth end of the flexible diamond bracket 1. The displacement between the mover and the stator of the LVDT can measure the change in the distance between the third end and the fourth end of the flexible diamond bracket 1, thereby characterizing the change in the distance between the first end and the second end of the flexible diamond bracket 1.

[0055] The linear variable differential transformer (LVDT) used in the present invention is an electromagnetic induction displacement detection sensor, which consists of a primary coil, two secondary coils, an iron core, a coil frame and a shell. When a sinusoidal wave signal is applied to both ends of the primary coil, as the iron core moves, the voltage difference induced by the two secondary coils is linearly related to the displacement of the iron core, and it is insensitive to space radiation.

[0056] like Figure 3 As shown, the present invention also provides a six-degree-of-freedom secondary mirror adjustment mechanism. The six-degree-of-freedom secondary mirror adjustment mechanism may include a moving platform 11, a static platform 13, a Hooke's hinge 14, and six Figure 2 The third and fourth ends of the flexible diamond-shaped brackets 1 of the six support rod actuators 12 are fixed to the static platform 13 and the dynamic platform 11 respectively through the Hooke's hinge 14 to form a Stewart configuration 6-UCU (U-Hooke's hinge pair, C-cylindrical pair) six-degree-of-freedom parallel mechanism, as shown in FIG. Figure 4 The support rod actuator 12 assembly with flexible support constraints used in the present invention is connected to the upper and lower platforms through the Hooke's hinge 14, which can simultaneously meet the requirements of high rigidity, multiple degrees of freedom and micro deformation.

[0057] A static coordinate system O-XYZ is established at the center of the circle where the lower hinges of the six Hooke's hinges 14 of the static platform 13 are located, with the origin of the coordinate system being O; a dynamic coordinate system P-xyz is established at the center of the circle where the upper hinges of the six Hooke's hinges 14 of the dynamic platform 11 are located, with the origin of the coordinate system being P. The coordinate vector of the rotation center of the upper hinge in the dynamic coordinate system is A i (A1, A2, A3, A4, A5, A6), the coordinate of the rotation center of the lower hinge in the static coordinate system is marked as B i (B1, B2, B3, B4, B5, B6). The direction of the line connecting the third end and the fourth end of the flexible diamond-shaped bracket 1 of the i-th support rod actuator 12 corresponds to A i -B i The connection direction.

[0058] After the coordinate system is determined, the position and posture of the motion platform 11 of the six-degree-of-freedom parallel mechanism is expressed by a generalized coordinate vector q, where q = [x, y, z, α, β, γ] T , which is expressed as translation of three axes and rotation around three axes. These six parameters determine the spatial position of the moving platform 11. Each strut actuator 12 can output displacement individually. The six strut actuators 12 output different displacements to achieve different positions of the moving platform 11.

[0059] The secondary mirror six-degree-of-freedom micro-position adjustment control system mainly includes three aspects: secondary mirror adjustment position solution, secondary mirror adjustment position measurement and secondary mirror six-degree-of-freedom adjustment control. Figure 5As shown. ① The wavefront phase difference solution module, through the phase difference inversion of multiple frames based on focal plane imaging, performs the phase difference description of the Zernike polynomial coefficients; ② The six-degree-of-freedom secondary mirror adjustment mechanism measurement module, reads the solution code value of the LVDT sensor in the six support rod driver components by dividing the edges, and performs linear interpolation with the code value-displacement data calibrated by each support rod to obtain the current displacement information of each support rod (including direction and size); ③ The six-degree-of-freedom secondary mirror adjustment mechanism control module, receives the secondary mirror adjustment posture x, y, z, α, β, γ instruction content output by the wavefront phase difference solution module, calculates the expected length of each support rod according to the inverse solution formula of the six-degree-of-freedom platform kinematics, and calculates the displacement amount (including direction and size) that each support rod needs to adjust according to the LVDT displacement measurement information of each support rod output by the six-degree-of-freedom secondary mirror adjustment mechanism measurement module, and converts it into the step amount and direction of the motor of each support rod actuator 12 component according to the step amount of the stepping motor, and coordinates the movement time of each support rod in place to achieve high-precision movement of each support rod.

[0060] The workflow of the secondary mirror's six-degree-of-freedom micro-position adjustment control system is divided into the following steps.

[0061] 1) The system is powered on and the program is loaded and initialized.

[0062] 2) The wavefront aberration solution module obtains the Zernike polynomial coefficients through multi-frame phase difference inversion based on focal plane imaging, and obtains the wavefront aberration correction module parameters, that is, the expected adjustment posture x1, y1, z1, α1, β1, and γ1 of the six-degree-of-freedom secondary mirror mechanism.

[0063] 3) The six-degree-of-freedom secondary mirror adjustment mechanism measurement module reads the current solution digital quantity of each support rod LVDT sensor and searches the code value-displacement mapping table of each support rod calibration to calculate the displacement Δl of each support rod relative to the initial origin i (i=1,2,3,4,5,6), and calculate the current position x0, y0, z, α0, β0, γ0 through the kinematic formula of the six-degree-of-freedom platform.

[0064] 4) The six-degree-of-freedom secondary mirror adjustment mechanism control module receives the secondary mirror adjustment posture x1, y1, z1, α1, β1, γ1 command content, and calculates the expected length L of each support rod according to the posture command through the kinematic inverse solution formula of the six-degree-of-freedom platform i (i=1,2,3,4,5,6).

[0065] 5) The control module determines the desired length L of each support rod. i and the current displacement of each support rod Δl i , the expected elongation or shortening of each support rod is obtained by calculation i =L i -D i +Δl i(i=1,2,3,4,5,6),D i The standard length of each support rod.

[0066] 6) The control module calculates the step amount N of each support rod based on the calculated expected extension or shortening amount of each support rod divided by the minimum step distance. i =Δd i / s i ,s i The minimum step size of the motor is set, and the driving step frequency of each support rod is optimized to achieve that each support rod reaches the desired displacement at the same time.

[0067] 7) The control module generates a driving pulse width modulation signal for each support rod according to the step amount, step frequency and movement direction of each support rod and sends it to the power drive module to drive the stepper motor to move.

[0068] 8) Determine whether the difference between the current posture and the expected posture meets the threshold condition.

[0069] 9) If the conditions are met, the movement of each support rod stops.

[0070] 10) If it is not satisfied, the supporting rods continue to move and step 3) is repeated until it is satisfied.

[0071] The present invention is based on a high-precision micro-displacement detection method of an LVDT measuring sensor, a LVDT data calibration data and a calibration data fitting method, and a closed-loop posture adjustment control method, which further improves the measurement accuracy of the length of each support rod of the secondary mirror adjustment mechanism, thereby achieving high posture measurement accuracy and posture adjustment accuracy of the platform.

[0072] The structure of the six-degree-of-freedom secondary mirror adjustment mechanism measurement module is as follows: Figure 6 As shown, it is used to calculate the displacement Δl of each support rod relative to the initial origin. i The main components include an excitation signal generation module, an induction signal conditioning module, and an induction signal resolution module. The excitation signal generation module can output a sinusoidal excitation signal with adjustable amplitude and frequency to meet the LVDT input signal requirements. The induction signal conditioning module receives the induced Va and Vb signals of the A-phase winding and the B-phase winding of the LVDT, and performs signal conditioning on them. It adjusts the amplification factors a and b of the two operational amplifiers in the first-stage amplifier circuit separately to meet the signal "sum value X" (Va+Vb) effective value conditioning to 2Vrms, and at the same time adjusts the maximum effective value of the signal "difference Y" (Va-Vb) to 2Vrms. The signal resolution module uses the input sine and cosine signals and an integrated circuit based on the amplitude detection resolution principle to complete the output of the AC signal to 16-bit parallel binary data, and reads the data through the FPGA control timing.

[0073] The workflow of the secondary mirror's six-degree-of-freedom micro-position adjustment control system is divided into the following steps.

[0074] (1) Ground calibration of the six-degree-of-freedom secondary mirror adjustment platform

[0075] Step 1, initialize system variables (platform structure parameters, workspace).

[0076] Step 2: Average step length of a single rod s i The calibration is carried out by using a high-precision laser interferometer and self-collimation technology to collect the measured displacement corresponding to a certain number of steps of the support rod driver for calibration.

[0077] Step 3, single-rod driver LVDT measurement and calibration, drive the single rod to move from one side's limit stroke to the other side's limit stroke at a fixed periodic interval, fixed number of steps, and fixed frequency. During the movement, the LVDT solution digital quantity is read through the LVDT measurement module, and the elongation of the measured rod is collected by the laser interferometer. N sampling points are selected according to the range of motion of the rod, and a two-dimensional array consisting of N LVDT code values ​​and N rod elongations can be obtained. This array is the rod motion calibration code value-displacement mapping data Di[N,2], (i=1,2,…,6).

[0078] (2) Six-DOF secondary mirror adjustment control method

[0079] Step 1, initialize system variables (platform structure parameters, LVDT calibration data, step calibration data).

[0080] Step 2: Perform wavefront phase difference calculation to obtain the desired adjusted posture information x1, y1, z1, α1, β1, and γ1.

[0081] Step 3: Read the calculated digital quantity dn of the current LVDT sensor of each support rod, and find the code value-displacement mapping table Di[N,2] of each support rod calibration, and use the linear interpolation method to fit the displacement Δl of each support rod relative to the initial origin. i (i=1,2,3,4,5,6), when dn∈(Di[n,1],Di[n+1,1]), where n∈(1,N-1), then Δl i =(Di[n+1,2]-Di[n,2]) / (Di[n+1,1]-Di[n,1])*(dn-Di[n,1])+Di[n,2]; and calculate the current position x0, y0, z, α0, β0, γ0 through the kinematic solution formula of the six-degree-of-freedom platform.

[0082] Support rod LVDT micro displacement Δl i The measurements are as follows.

[0083] Step 3.1, using an excitation signal generator to generate a sinusoidal excitation signal with a certain frequency and amplitude and power driving capability, outputting it to the excitation end of the LVDT sensor.

[0084] Step 3.2, the signal conditioning module receives the induced voltage signals Va and Vb from the sensing end of the LVDT sensor, obtains the Va+Vb signal and the Va-Vb signal through calculation and conditioning, ensures that the maximum effective values ​​of the Va+Vb and Va-Vb signals meet the solution requirements, and applies a bandpass filter to filter the Va+Vb and Va-Vb signals.

[0085] Step 3.3, apply the shaft angle solution module, take the excitation reference signal, Va+Vb and Va-Vb signals as input, and obtain 16-bit binary data through amplitude detection shaft angle solution.

[0086] In step 3.4, FPGA is used to read and process the shaft angle solution data.

[0087] Step 4: Based on the posture adjustment information, obtain the parameters of the rotation matrix R, and use the inverse kinematic solution of the six-degree-of-freedom platform to calculate as follows:

[0088] ‖L i ‖=‖RA i +PB i ‖

[0089] Where R is the rotation transformation matrix of the motion coordinate system; A i B is the coordinate vector of the hinge center point of the moving platform 11 in the motion coordinate system; i is the coordinate vector of the hinge center point of the static platform 13 in the static coordinate system; P is the offset vector of the origin of the moving coordinate system relative to the origin of the static coordinate system; L i is the length of the support rod. After the above calculation, the expected length L of each support rod is obtained. i (i=1,2,3,4,5,6).

[0090] Step 5: The control module determines the desired length L of each support rod. i and the current displacement of each support rod Δl i , the expected elongation or shortening of each support rod is obtained by calculation i =L i -D i +Δl i (i=1,2,3,4,5,6), D i The standard length of each support rod.

[0091] Step 6: The control module calculates the step amount N of each support rod based on the calculated expected extension or shortening amount of each support rod divided by the minimum step distance. i =Δd i / si ,s i The minimum step size of the motor is set, and the driving step frequency of each support rod is optimized to achieve that each support rod reaches the desired displacement at the same time.

[0092] Step 7: The control module generates a driving pulse width modulation signal for each support rod according to the stepping amount, stepping frequency and movement direction of each support rod and sends it to the power driving module to drive the stepping motor to move.

[0093] Step 8, determine whether the difference between the current posture x0, y0, z, α0, β0, γ0 and the expected posture x1, y1, z1, α1, β1, γ1 meets the threshold condition.

[0094] Step 9: If the condition is satisfied, the movement of each support rod stops; if not, each support rod continues to move, and step 3) is repeated until the condition is satisfied.

[0095] This patent proposes a six-degree-of-freedom parallel mechanism micro-position adjustment system and method driven by a reduction motor and measured by an LVDT micro-displacement sensor. The two ends of a single support rod are connected to the upper and lower platforms through Hooke joints. The support rod actuator composed of a reduction motor drive assembly and an LVDT micro-displacement sensor forms a proportional amplification effect of micro-deformation in the horizontal and vertical directions through a flexible diamond bracket constraint. The configuration of the six support rods can effectively adjust the six-degree-of-freedom position of the secondary mirror, and a closed-loop six-degree-of-freedom position control method is used to avoid the problem of inaccurate position adjustment caused by the gap caused by the stepper motor movement and the deviation of the support rod movement caused by the return error.

[0096] Although the present invention is 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 without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be based on the scope defined by the claims of the present invention.

Claims

1. A support rod actuator, characterized in that: include: Flexible diamond bracket (1), LVDT stator (2), LVDT mover (3), LVDT mover bracket (4), LVDT stator bracket (5), reduction drive assembly (6), differential screw assembly (7); The flexible diamond-shaped bracket (1) comprises a first end and a second end opposite to each other in the axial direction, and also comprises a third end and a fourth end opposite to each other in the radial direction; the reduction drive assembly (6) is fixedly mounted between the first end and the second end of the flexible diamond-shaped bracket (1) through a differential screw assembly (7); the LVDT stator bracket (5) surrounds the outer periphery of the reduction drive assembly (6) so as not to affect the rotation of the reduction drive assembly (6); one end of the LVDT stator bracket (5) is fixed to the third end of the flexible diamond-shaped bracket (1), and the other end extends to the vicinity of the fourth end of the flexible diamond-shaped bracket (1) and is mounted with the LVDT stator (2); the LVDT mover bracket (4) is mounted on the fourth end of the flexible diamond-shaped bracket (1), and is used to set the LVDT mover (3), and the LVDT mover (3) cooperates with the LVDT stator (2); The reduction drive assembly (6) outputs a rotational motion, driving the screw in the differential screw assembly (7) to rotate, thereby causing the distance between the first end and the second end of the flexible diamond-shaped bracket (1) to change; thereby changing the distance between the third end and the fourth end of the flexible diamond-shaped bracket (1); and generating a displacement between the mover and the stator of the LVDT, which is used to measure the change in the distance between the third end and the fourth end of the flexible diamond-shaped bracket (1).

2. The strut actuator according to claim 1, characterized in that: The LVDT stator (2) comprises a primary coil and two secondary coils, and the LVDT mover (3) comprises an iron core; when a sine wave signal is applied to both ends of the primary coil, as the iron core moves, the voltage difference induced by the two secondary coils is linearly related to the displacement of the iron core.

3. A six-degree-of-freedom secondary mirror adjustment mechanism, characterized in that: The invention comprises a moving platform (11), a stationary platform (13), a Hooke's hinge (14), and six strut actuators (12) as claimed in claim 1 or 2; the third end and the fourth end of the flexible diamond bracket (1) of the six strut actuators (12) are respectively fixed to the stationary platform (13) and the moving platform (11) through the Hooke's hinge (14) to form a six-degree-of-freedom parallel mechanism; wherein, A static coordinate system O-XYZ is established at the center of the circle where the lower hinges of the six Hooke's hinges (14) of the static platform (13) are located, and the coordinate origin is O; a dynamic coordinate system P-xyz is established at the center of the circle where the upper hinges of the six Hooke's hinges (14) of the dynamic platform (11) are located, and the coordinate origin is P; the coordinate vector of the rotation center of the upper hinge in the dynamic coordinate system is A i (A1, A2, A3, A4, A5, A6), the coordinate of the rotation center of the lower hinge in the static coordinate system is marked as B i (B1, B2, B3, B4, B5, B6); the direction of the line connecting the third end and the fourth end of the i-th support rod actuator (12) and the flexible diamond support (1) corresponds to A i -B i The connection direction of Each support rod actuator (12) outputs displacement independently, and the six support rod actuators (12) output different displacements to realize different positions and postures of the motion platform (11). The position and posture of the motion platform (11) of the six-degree-of-freedom parallel mechanism is represented by a generalized coordinate vector q, where q = [x, y, z, α, β, γ] T , expressed as a translation of three axes and a rotation around three axes.

4. A control system for a six-degree-of-freedom secondary mirror adjustment mechanism as claimed in claim 3, characterized in that: include: The wavefront phase difference calculation model uses multi-frame phase difference inversion based on focal plane imaging to describe the phase difference of Zernike polynomial coefficients; The six-DOF secondary mirror adjustment mechanism measurement module reads the resolved code values ​​of the LVDT sensors in the six strut drive assemblies by edge by edge, and performs linear interpolation with the calibrated code value-displacement data of each strut to obtain the current displacement information of each strut; The six-degree-of-freedom secondary mirror adjustment mechanism control module receives the secondary mirror adjustment posture x, y, z, α, β, γ instruction content output by the wavefront difference solving module, calculates the expected length of each support rod according to the inverse solution formula of the six-degree-of-freedom platform kinematics, and calculates the displacement amount required to be adjusted for each support rod according to the LVDT displacement measurement information of each support rod output by the six-degree-of-freedom secondary mirror adjustment mechanism measurement module, converts the displacement amount and direction of each support rod actuator (12) component motor according to the step amount of the stepping motor, and coordinates the movement time of each support rod in place to realize the movement of each support rod.

5. The control system according to claim 4, characterized in that: The six-degree-of-freedom secondary mirror adjustment mechanism measurement module includes an excitation signal generation module, an induction signal conditioning module and an induction signal solving module; The excitation signal generation module outputs a sine wave excitation signal with adjustable amplitude and frequency to meet the LVDT input signal requirements; The induction signal conditioning module receives the Va and Vb signals induced by the A-phase winding and the B-phase winding of the LVDT, conditions them, and adjusts the amplification factors a and b of the two operational amplifiers in the first-stage amplification circuit separately to meet the requirement that the effective value of the sum of the Va+Vb of the signal is conditioned to 2Vrms, and at the same time, the maximum effective value of the difference between the Va-Vb of the signal is conditioned to 2Vrms; the signal resolution module uses the input sine and cosine signals and an integrated circuit based on the amplitude detection resolution principle to complete the output of the AC signal to 16-bit parallel binary data, and reads the data through the FPGA control timing.

6. A control method for the six-degree-of-freedom secondary mirror adjustment mechanism as claimed in claim 3, characterized in that: include: Through the phase difference inversion of multiple frames based on focal plane imaging, the Zernike polynomial coefficients are obtained to obtain the desired adjustment posture x1, y1, z1, α1, β1, γ1 of the six-degree-of-freedom secondary mirror mechanism; Read the digital value of the LVDT sensor of each support rod and find the code value-displacement mapping table of each support rod calibration to calculate the displacement Δl of each support rod relative to the initial origin i , i = 1, 2, 3, 4, 5, 6, and calculate the current position x0, y0, z, α0, β0, γ0 through the kinematic solution formula of the six-degree-of-freedom platform; According to the secondary mirror, adjust the posture x1, y1, z1, α1, β1, γ1 command content, and calculate the expected length L of each support rod through the kinematic inverse formula of the six-degree-of-freedom platform according to the posture command i ; According to the expected length L of each support rod i and the current displacement of each support rod Δl i , the expected elongation or shortening of each support rod is obtained by calculation i =L i -D i +Δl i ,D i is the standard length of each support rod; The step amount N of each support rod is calculated by dividing the expected extension or shortening of each support rod by the minimum step distance. i =Δd i / s i ,s i The minimum step length of the motor is set, and the driving step frequency of each support rod is optimized to achieve that each support rod reaches the desired displacement at the same time; According to the stepping amount, stepping frequency and movement direction of each support rod, a driving pulse width modulation signal of each support rod is generated and sent to the power driving module to drive the stepping motor to move; Determine whether the difference between the current posture and the expected posture meets the threshold condition; If the condition is satisfied, the movement of each support rod stops; if the condition is not satisfied, each support rod continues to move, and the above steps are repeated until the threshold condition is satisfied.

7. The control method according to claim 6, characterized in that: The control method further includes ground calibration of a six-degree-of-freedom secondary mirror adjustment platform, including: Initialize system variables, including platform structure parameters and workspace; Average step length of a single rod i The calibration is carried out by using a laser interferometer and self-collimation technology to collect the measured displacement corresponding to a number of steps of the support rod driver movement for calibration; The single-rod driver LVDT measurement and calibration is to drive the single rod to move from the extreme stroke on one side to the extreme stroke on the other side at a fixed periodic interval, a fixed number of steps, and a fixed frequency. During the movement, the LVDT solution digital quantity is read through the LVDT measurement module, and the elongation of the measured rod is collected by the laser interferometer. N sampling points are selected according to the range of motion of the rod to obtain a two-dimensional array consisting of N LVDT code values ​​and N rod elongations. This array is the rod motion calibration code value-displacement mapping data Di[N,2], i=1,2,…,6.

8. The control method according to claim 7, characterized in that: The control method comprises: Read the calculated digital quantity dn of the current LVDT sensor of each support rod, and find the code value-displacement mapping table Di[N,2] of each support rod calibration, and use the linear interpolation method to fit the displacement Δl of each support rod relative to the initial origin i , when dn∈(Di[n,1],Di[n+1,1]), where n∈(1,N-1), then Δl i =(Di[n+1,2]-Di[n,2]) / (Di[n+1,1]-Di[n,1])*(dn-Di[n,1])+Di[n,2]; and calculate the current position x0, y0, z, α0, β0, γ0 through the kinematic solution formula of the six-degree-of-freedom platform.

9. The control method according to claim 6, characterized in that: Support rod LVDT micro displacement Δl i Measurements include: An excitation signal generator is used to generate a sinusoidal excitation signal with power driving capability and output it to the excitation end of the LVDT sensor; The signal conditioning module receives the induced voltage signals Va and Vb from the sensing end of the LVDT sensor, obtains the Va+Vb signal and the Va-Vb signal through calculation and conditioning, ensures that the maximum effective values ​​of the Va+Vb and Va-Vb signals meet the solution requirements, and applies a bandpass filter to filter the Va+Vb and Va-Vb signals; The shaft angle calculation module is applied, and the excitation reference signal, Va+Vb and Va-Vb signals are used as inputs to obtain 16-bit binary data through amplitude-discrimination shaft angle calculation. FPGA is used to read and process the shaft angle solution data.

10. The control method according to claim 6, characterized in that: According to the posture adjustment information, the parameters of the rotation matrix R are obtained, and the inverse kinematic solution of the six-degree-of-freedom platform is used for calculation: ‖L i ‖=‖RA i +PB i ‖ Where R is the rotation transformation matrix of the motion coordinate system; A i B is the coordinate vector of the hinge center point of the moving platform (11) in the motion coordinate system; i is the coordinate vector of the hinge center point of the static platform (13) in the static coordinate system; P is the offset vector of the origin of the moving coordinate system relative to the origin of the static coordinate system; L i is the length of the support rod; after the above calculation, the expected length L of each support rod is obtained i .

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