Electron beam multi-pole shoe coil deflection system control method based on FOC algorithm

Abstract

The invention provides an electron beam multi-pole shoe coil deflection system control method based on an FOC algorithm. An electron beam multi-pole shoe deflection coil and a stator coil of a motor have similar structures, and high-precision control can be realized by using an FOC (Field-Oriented Control) algorithm in the field of the motor, so that the technology is introduced into an electron beam deflection control system, Park transformation special for an electron beam is designed, PI parameters are dynamically adjusted, and the control precision is improved. And a new idea is provided for an electron beam deflection control system. The whole FOC control process is detected through a back electromotive force observer, the position of an electron falling point is measured, and therefore closed-loop control is achieved. The measured position information of the drop point is compared with expected position information input by a user, and the FOC high-frequency scanning driving circuit can make corresponding adjustment according to the obtained difference error position information until the error position information is eliminated. The technology can be widely applied to the derivative fields of the electron beam industry, such as electron beam lithography, scanning electron microscope scanning, electron beam additive manufacturing (EBM) and the like, and has a good application prospect.

Description

Technical Field

[0001] The present invention relates to the fields of motors and electron beams, and particularly to the innovation of an electron beam deflection coil control system, using the FOC (Field-Oriented Control) control algorithm commonly used in the three-phase motor field to control the electron beam multi-pole shoe coil deflection system. Background Art

[0002] Electron beam welding technology is a fusion welding process that uses a high-speed focused electron beam to weld metal workpieces, playing an increasingly important role in fields such as aerospace, marine vessels, and automotive manufacturing. With the advancement of the industrialization process, metal manufacturing parts in the processing are becoming increasingly complex, and the requirements for their performance and processing accuracy are getting higher and higher. Traditional processing technologies are difficult to adapt to the development trend of high-precision manufacturing technologies, and many emerging technologies have emerged. Among them, electron beam processing technology is a processing technology with significant advantages. Electron beam processing technology realizes processing by emitting a high-energy electron beam from an electron gun and bombarding the surface of the workpiece. The power supply of an electron beam device usually reaches thousands of volts, and the power supply is respectively connected to the cathode and anode inside the electron gun. Due to the huge voltage difference between the two poles, electrons generated from the tungsten filament will rush into the magnetic field with extremely high kinetic energy under the action of a large electric field, passing through the centering coil, deflection coil, focusing coil, and astigmatism correction coil, and finally hitting the surface of the workpiece to be processed.

[0003] Electron beam processing is carried out in a vacuum environment. It is very difficult to observe the processing quality or even the processing landing point at close range because the processing of current electron beam devices needs to be in a vacuum environment, and processing personnel can only difficultly see the inside processing situation through a window. And the welds that need to be processed by the electron beam usually have a large depth-to-width ratio, so there are high requirements for the accuracy of the electron beam landing position, and the landing position of the electron beam is controlled by a deflection system. The electron beam deflection control system is a device used to precisely control the movement direction of the electron beam, widely used in fields such as electron microscopes (SEM / TEM), electron beam lithography, electron beam welding, particle accelerators, and medical radiotherapy equipment. Its core function is to change the trajectory of the electron beam through an electromagnetic field or an electrostatic field to achieve high-speed and high-precision deflection positioning.

[0004] The electron beam deflection control system is an important link that the electron beam industry urgently needs to improve.

[0005] First, the traditional multi-pole shoe coil control system has great limitations. The deflection control of the electron beam by the deflection coil is mainly achieved by applying different deflection currents to the deflection coil. This method belongs to open-loop control. When the supplied current is unstable and fluctuates, the electron beam will become unstable and swing continuously, unable to be well fixed at a certain position, and there is no feedback to form a good closed-loop system. Even if a simple closed-loop such as PID control can be achieved, the PID parameters of the current deflection coil control system are fixed, and the electron beam will show hysteresis or overshoot, making it difficult to meet the deflection requirements of high frequency and high precision, and difficult to adapt to the non-linear magnetic field changes, resulting in electron beam positioning delay or jitter.

[0006] Second, the magnetic field coupling between multi-pole shoe coils will cause mutual interference during dynamic regulation, reducing the deflection accuracy. This phenomenon is relatively common in scanning electron microscopes and electron beam lithography machines. Therefore, the problem of magnetic field coupling interference of multi-pole shoe coils is also a difficult problem faced by the electron beam multi-pole shoe coil deflection system. The coupling may lead to uneven focusing force, causing phase difference and resulting in electron beam distortion. Eddy current coupling will reduce the response speed of magnetic field regulation, leading to dynamic response delay. At the same time, the coupling may introduce the risk of oscillation in the feedback loop, and the stability problem should not be ignored.

[0007] Third, the electron beam deflection coil is usually wound by many turns of coils, so there will originally be a certain inductance in the coil. And the inductance is highly sensitive to current, which is reflected in its hindrance to the change of current. High inductance leads to a long current establishment time, limits the bandwidth of the deflection system, and makes it difficult to track high-frequency scanning signals.

[0008] Fourth, the deflection coil usually has two sets of deflection coils in the X and Y directions for coordinated adjustment. However, the existing system lacks the ability to decouple the coupled field, and the scanning trajectory is prone to distortions such as diamond distortion, and the phenomenon of out-of-sync multi-axis control occurs. Fifth, the coil resistance changes with temperature, resulting in the drift of the current-magnetic field relationship. Affected by the temperature drift, frequent calibration is required, which is very cumbersome and involves a large amount of work, and is not suitable for the current production requirements.

[0009] Fifth, the coil resistance changes with temperature, resulting in the drift of the current-magnetic field relationship. Affected by the temperature drift, frequent calibration is required, which is very cumbersome and involves a large amount of work, and is not suitable for the current production requirements.

[0010] Regarding the above pain points, there is currently no effective control solution for the electron beam multi-pole shoe coil deflection system. Referring to other fields, corresponding solutions have been proposed. For example, the FOC (Field-Oriented Control) magnetic field orientation control technology in the motor field. The stator structure of the motor is very similar to the multi-pole shoe deflection coil structure in our electron beam industry, and this algorithm has successfully achieved high-precision and fast control of the motor magnetic field. The present invention proposes and adopts a method for the electron beam multi-pole shoe coil deflection system, filling the current gap in the electron beam industry and realizing the cross-border integration between the electron beam field and the motor field. Summary of the Invention

[0011] The present invention provides a control method for an electron beam multi-pole shoe coil deflection system based on the FOC (Field-Oriented Control) algorithm. This technology can transplant the FOC magnetic field orientation control technology in the motor field to the electron beam multi-pole shoe coil deflection system, making the electron beam multi-pole shoe coil deflection system have the characteristics of closed-loop, controllable, high-frequency, and high-precision. This technology can be extended to fields such as electron beam additive manufacturing and lithography. To achieve the above object, the present invention adopts the following technical solutions:

[0012] First step, design the multi-pole shoe coil structure with reference to the motor stator structure. Since FOC control is three-phase power control, the coil leads should also be three wires. And the subdivision degree of only a three-pole magnetic field is very low, so it is preferably designed as a multiple of three and greater than or equal to six, and the wiring of the coil is completed with reference to the star connection method or the delta connection method.

[0013] Second step, connect the three-phase wires of the coil to the FOC drive board. This drive board is composed of three half-bridge circuits. Each time, only the upper or lower MOS transistor of the same half-bridge circuit is turned on, and the on-off of the coil is completed by frequently switching the corresponding MOS transistors, thereby realizing the circuit drive of the coil.

[0014] Third step, write the FOC drive program. The core of the program is the Clarke transformation and the Park inverse transformation. These two transformations complete the transformation from the three-phase coordinate system to the two-phase coordinate system and the transformation of the two-phase coordinate system to coincide with the rotor coordinate system. The FOC program is a decoupling process, and the goal is to convert the complex three-phase problem into a simple two-phase problem for analysis. On this basis, PID is added to have a better stabilizing effect on the output rotor speed and angle.

[0015] Step 4: Use the coil back electromotive force observer to obtain the electron beam landing position. The movement of the electron beam in the magnetic field is analogized to the rotor of an electric motor, where the rotor position is equivalent to the electron beam current offset. The back electromotive force observer is used to measure the voltage disturbance of the deflection coil caused by the beam movement. With this step, the closed-loop control of FOC can be achieved. The collected position information is compared with the position information set by the user. Once a deviation occurs, adjustment can be made immediately, with rapid response, and finally adjusted to the appropriate position.

[0016] Step 5: The user inputs the target position of the electron beam, and repeat Step 3 and Step 4 until the electron beam deflection is finally completed.

[0017] The control method of the electron beam multi-pole shoe coil deflection system based on the FOC algorithm works through the above modules and steps. This invention can effectively solve the problems faced by the current electron beam deflection control system industry. The beneficial effects of this invention are as follows:

[0018] 1. Compared with the deflection control systems used in the current electron beam processing process, the FOC control algorithm has the advantages of high-precision control, fast dynamic response, strong anti-interference ability, system optimization and cost reduction in the electron beam industry. It shows better applicability and good sensitivity in the complex environment of the electron beam working area. In addition, by automatically adjusting the excitation component according to the electron beam energy, energy efficiency optimization can be achieved, copper loss of the coil can be reduced, and thus the service life of the system can be extended.

[0019] 2. Effectively solve the problem of magnetic field interference between multi-pole shoe coils. In the electron beam deflection system, the magnetic fields of adjacent pole shoes will be superimposed on each other, resulting in non-linear distortion, beam current positioning error and decline in focusing ability. By directly injecting a compensation term into the d-q axis control command, applying a test current pole by pole, measuring the magnetic field response of other pole shoes, and constructing a coupling matrix; then superimposing the real-time calculated compensation amount on the d-q axis command, the problem is solved in the program.

[0020] 3. Design an electron beam PI parameter dynamic tuning method. The traditional control system uses fixed PI parameters for adjustment, which cannot well adapt to the actual situation of electron beam processing, resulting in an unstable system. The PI parameter dynamic tuning method of this invention is characterized in that it adjusts K according to the square root relationship of the acceleration voltage p , and corrects the temperature coefficient τ with temperature elec , and realizes smooth transition through the tanh() function to avoid oscillation caused by parameter mutation.

[0021] 4. The sensorless method is used to detect the position of the electron beam. The current and voltage of the coil are the direct control objects, so it is much simpler to detect them than to monitor the landing position of the electron beam. The real-time current and voltage of the coil are collected, and based on the basic characteristics of the coil, such as its inner diameter, the number of pole shoes, and the number of turns of the coil, a mathematical model between the back electromotive force and the coil voltage and current is established. Using the established mathematical model, the back electromotive force is obtained through numerical calculation, and then the landing position of the current electron beam is deduced.

[0022] 5. Since the electron beam has no physical rotor, an "equivalent position" needs to be reconstructed through the back electromotive force. The present invention adopts a closed-loop control architecture, aiming to feedback the electron beam landing position data in real time through real-time monitoring, analysis, and adjustment, and adjust the current phase or amplitude through the FOC algorithm to offset the eddy current lag effect. This architecture integrates three functions of monitoring, analysis, and execution, runs through the entire deflection control process, and forms a closed loop to realize the electron beam deflection control system. Brief Description of the Drawings

[0023] Figure 1 It is a schematic diagram of a control method for an electron beam multi-pole shoe coil deflection system based on the FOC algorithm of the present invention;

[0024] Reference numerals in the figure: 1. Electron gun; 2. Multi-pole shoe deflection coil; 3. Electron beam landing position; 4. FOC high-frequency scanning drive circuit; 5. FOC algorithm Specific Embodiment

[0025] Now, the present invention will be further described in detail with reference to the accompanying drawings. The above drawings are all simplified schematic diagrams, only illustrating the basic structure of the present invention in a schematic way, so they only show the components related to the present invention.

[0026] The present invention provides a design idea for a control method of an electron beam multi-pole shoe coil deflection system based on the FOC algorithm, including an electron gun 1, a multi-pole shoe deflection coil 2, an electron beam landing position 3, an FOC high-frequency scanning drive circuit 4, and an FOC algorithm 5.

[0027] First, an electron beam current is emitted by the electron gun 1. When the electron beam passes through the multi-pole shoe deflection coil 2, since a current is passed through, the multi-pole shoe deflection coil 2 will generate a uniform deflection magnetic field in this area, and this magnetic field will deflect the electron beam, just like the direction of the red electron beam changes as shown in the figure. At this time, we need to know the electron beam landing position 3.

[0028] In order to accurately control the landing position 3, the present invention introduces the FOC algorithm 5 and modifies it specifically for adaptation to the electron beam. The process of this algorithm is as follows: First, the three-phase current of the electron beam is sampled to obtain I a 、I b 、I c, and then I a , I b , I c are transformed by Clark transformation to obtain I α , I β , and then I α , I β are transformed to the d-axis and q-axis through the Park transformation dedicated to the electron beam, where the d-axis is used for beam current focusing and the q-axis is used for beam current deflection. Since the electron beam has no physical rotor, this step actually transfers the α, β coordinate system by an angle corresponding to the electron beam deflection angle θ, thereby constructing the d, q coordinate system.

[0029] At this time, the real-time measurement values of I d , I q are obtained. By comparing them with the user-set values Id_ref and Iq_ref, a deviation value can be obtained. At this time, through an algorithm for dynamically adjusting the PI parameters of the two acceleration voltages, the adjusted U q , U d are obtained respectively. At this time, the obtained data U q , U d are synthesized into a voltage space and input into the SVPWM module for modulation, and the state encoding values of the three half-bridges at this moment are output.

[0030] The FOC high-frequency scanning drive circuit 4 quickly responds to the state encoding values of the three half-bridges and follows them well, and the output value will control the MOS tube switches of the three-phase inverter, thereby driving the multi-pole shoe coil to deflect the electron beam.

[0031] The back electromotive force observer is involved throughout the FOC. Its core functions include providing the electron beam position in real time, improving the response speed of the current loop, and fault detection. First, in the current sampling and Clarke transformation stage, the observer real-time collects the phase current and terminal voltage of the multi-pole shoe coil, and separates the back electromotive force component e emf through the voltage equation; in the Park transformation and magnetic field decoupling stage, the observer provides the real-time deflection angle θ of the electron beam for the rotation matrix of the Park transformation; in the current loop control stage, the back electromotive force output by the observer is used for feedforward compensation to cancel the coil induced voltage disturbance, thereby improving the current loop bandwidth.

[0032] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A control method for an electron beam multi-pole shoe coil deflection system based on the FOC (Field-Oriented Control) algorithm, characterized in that, Including: Including an FOC high-frequency scanning drive circuit, an electron gun, a multi-pole shoe deflection coil, and an FOC algorithm. Among them, when the user inputs the target position, the single-chip microcomputer on the FOC drive board will perform data processing, calculate the coils that should be turned on and off in this cycle, and achieve different input currents by setting the duty cycle. The single-chip microcomputer transmits the calculated data to the FOC high-frequency scanning drive circuit to control the rapid on and off of the MOS transistors in the half-bridge circuit on the control circuit, and the corresponding current will be passed through the deflection coil, so that the electron beam deflects and reaches the target position. Through the coil back electromotive force observer, the current and voltage of the coil are monitored, and the back electromotive force signal corresponding to the electron beam current position is deduced based on the mathematical model. This signal is compared with the input target position information. When a deviation occurs, it will be adjusted immediately until the deviation is eliminated.

2. The control method of an electron beam multi-pole shoe coil deflection system based on the FOC algorithm according to claim 1, characterized in that The FOC high-frequency scanning drive circuit mainly consists of a half-bridge drive circuit, a current sampling circuit, an STM32 main control system, and a necessary power supply circuit. Among them, the half-bridge drive circuit is also called a three-phase inverter bridge, which is composed of 6 MOS transistors. The MOS transistor is a power switch. By quickly adjusting the on and off state of the MOS transistor, the corresponding current will flow in and out of the corresponding coil, generating different deflection magnetic fields. STM32 has a high-precision ADC, and the multi-pole shoe FOC control cycle can be compressed to within 5 microseconds.

3. A control method for an electron beam multi-pole shoe coil deflection system based on the FOC algorithm according to claim 1, characterized in that, The electron gun is the control terminal of electron beam processing, and all final processing by the FOC algorithm requires the electron gun to achieve. An electron gun is a device that generates, accelerates, and focuses an electron beam, and is widely used in fields such as cathode ray tubes, electron microscopes, electron beam welding, particle accelerators, and X-ray tubes. Its core function is to convert electrical energy into a high-speed electron beam and control its direction, intensity, and focusing. The electron gun consists of a cathode, a control electrode, an anode, a focusing system, and a deflection system, etc.

4. The control method of an electron beam multi-pole shoe coil deflection system based on the FOC algorithm according to claim 1, characterized in that, The multi-pole shoe deflection coil is an electromagnetic device used to precisely control the deflection path of a charged particle beam. Its core function is to generate a complex magnetic field distribution through multiple independently controlled electromagnetic coils to achieve high-precision and low-distortion beam deflection and focusing. To adapt to the FOC algorithm, the number of its poles needs to be a multiple of three. In order to obtain a higher-precision magnetic field, it is at least designed as a multi-pole shoe deflection coil with six poles or more.

5. A control method for an electron beam multi-pole shoe coil deflection system based on the FOC algorithm according to claim 1, characterized in that, For the FOC algorithm, the first step is the decoupling of magnetic field vectors. A special Park transformation for the electron beam is designed for the present invention. The traditional Park transformation is used for the torque and excitation components in motors. However, there are multi-pole shoe coils in the present invention, so there is a multi-pole magnetic field. The independent control of the multi-pole magnetic field is achieved through the Clarke / Park transformation. The special Park transformation for the electron beam decomposes the multi-pole shoe magnetic field into a vector decoupling process on the d-q axes. The d-axis component is used for beam focusing to control the beam spot size, while the q-axis component is used for beam deflection to control the trajectory angle. After this step, the magnetic field distribution on the d-q axes is mapped to the current distribution of the actual coils. However, there is a problem of cross interference in the multi-pole shoe magnetic field. Therefore, a dynamic coupling compensation technique is also required in this step. This technique includes a feedforward decoupling algorithm and adaptive decoupling. A magnetic field coupling matrix between the pole shoes is constructed, and a feedforward compensation term is injected into the d-q axis commands. An adaptive algorithm is used to update the coupling coefficient online.

6. The control method of an electron beam multi-pole shoe coil deflection system based on the FOC algorithm according to claim 1, characterized in that Regarding the FOC algorithm, the second one is a PI parameter tuning algorithm dynamically adjusted according to the accelerating voltage. The electron beam is easily affected by the accelerating voltage. When the accelerating voltage is below 10 keV, it belongs to a low-energy beam current. At this time, the electron beam is easily disturbed by the magnetic field and keeps swinging. When the accelerating current is around 100 keV, it belongs to a high-energy beam current. At this time, the inertia of the electron beam current is large. Although it can respond quickly when the current changes, it is prone to overshoot due to the large inertia. Therefore, dynamic PI is needed to adjust, which is a method for dynamically tuning the PI parameters of the electron beam. The energy range of the electron beam is judged according to the accelerating voltage, and the proportional gain K p 、K i is adjusted according to the square root relationship to achieve dynamic PI adaptive adjustment.

7. A control method for an electron beam multi-pole shoe coil deflection system based on the FOC algorithm according to claim 1, characterized in that, For the FOC algorithm, the third step is the sensorless beam position detection based on the back electromotive force (back-EMF) observer. The back-EMF observer is involved throughout the FOC process. Its core functions include providing the electron beam position in real time, improving the response speed of the current loop, and fault detection. By monitoring the coil current and voltage signals in real time, the back-EMF is calculated based on a mathematical model. The back-EMF is the induced electromotive force generated when the electron beam magnetic field moves relative to the deflection coil. Its amplitude and frequency are directly related to the electron beam position and speed. After integrating the back-EMF, it is input into the pre-stored multi-pole shoe magnetic field distribution matrix to calculate the beam offset coordinates. Therefore, by accurately observing the back-EMF, the sensorless estimation of the electron beam position and speed can be achieved. Since the electron beam has no physical rotor, the "equivalent position" needs to be reconstructed through the back-EMF.

Images