Brushless direct current motor driven high-precision optical switch device for cold atomic clock and control method

Abstract

The invention discloses a brushless direct current motor driven high-precision optical switch device for a cold atomic clock and a control method, the device comprises an FPGA control panel, a three-phase brushless direct current motor, a light blocking blade, a driving circuit and a photoelectric detection circuit, the light blocking blade is fixed on a motor rotating shaft, and the blade is provided with a 15-degree light hole and a symmetrically excavated mass balance structure. It is ensured that the laser on-off time accounts for 17% and the rotational inertia is uniform; the FPGA starts the motor through six-step commutation, estimates an electrical angle in real time in combination with Hall signal interpolation, is switched to a sine wave control mode to generate a seven-segment SVPWM waveform, and dynamically adjusts PID parameters to realize that the periodic fluctuation of the motor is less than + / -0.25 ms. The photoelectric detection circuit feeds back the rotating speed to form closed-loop control, and the drive circuit is integrated with a dead-zone compensation module to reduce harmonic interference. The extinction ratio is superior to 40dB, the frequency accuracy and long-term stability of the cold atomic clock are improved, and the cold atomic clock is suitable for high-precision sequential control scenes such as laser cooling and atomic detection.

Description

Technical Field

[0001] This invention relates to optical switch control technology used in cold atomic clock experiments, specifically a brushless DC motor-driven, high-precision optical switch device and control method for cold atomic clocks. By using a brushless DC motor to drive light-blocking blades, combined with a precise FPGA-based control algorithm and a photoelectric feedback system, this device achieves rapid, periodic switching of the laser, meeting the stringent timing accuracy, extinction ratio, and stability requirements of cold atomic clocks. Background Art

[0002] In cold atomic clock experiments, the periodic on-off switching of lasers is a key step in the preparation and detection of cold atoms. Precisely controlling the on-off timing of cooling and repumping light using optical switches can prevent stray light from interfering with atomic energy levels. However, existing technologies have the following problems:

[0003] Cooling and repumping light generate a significant amount of stray light, which is affected primarily by two factors. First, during the free evolution phase, repumping light can pump atoms from F=3 to F=4, while residual stray light from the cooling light can potentially pump atoms from F=4 to F=3. This can affect the contrast of the Ramsey fringes, and the impact becomes increasingly pronounced with increasing evolution time. Second, stray light can also cause optical frequency shifts in atomic energy levels, thereby affecting the frequency accuracy of the atomic clock. Since stray light itself is not stable, its perturbations can also destabilize the optical frequency shifts, which in turn affect the stability of the atomic clock frequency. Therefore, reducing stray light is crucial for improving both the accuracy and stability of atomic clocks.

[0004] In order to reduce the impact of stray light, it is necessary to select an efficient and stable optical switch. However, the existing technology has the following problems:

[0005] Although the response time of the acousto-optic modulator is very fast when used as an optical switch, its extinction ratio is not ideal, and some stray light still leaks, affecting the accuracy and stability of the atomic clock.

[0006] Commercial mechanical switches are not only bulky, but also lack stability and reliability, making them unsuitable for use in atomic clock systems.

[0007] Although stepper motors are easy to control, they have a limited lifespan. They are also prone to generating torque pulsation during commutation, which causes speed fluctuations and makes it difficult to achieve timing control accuracy better than milliseconds.

[0008] To address these issues, existing technologies have attempted to use DC motors to drive light-blocking blades to improve extinction ratio and reliability. However, the control accuracy of DC motors is limited by inherent flaws in traditional control methods (such as six-step commutation). These include torque fluctuations and harmonic interference during commutation, which make speed fluctuations difficult to suppress. This makes it impossible to meet the optical switch cycle stability requirements (better than 0.25ms) required by cold atomic clocks. Summary of the Invention

[0009] To address the challenges of existing technologies, this paper proposes a high-precision optical switching device and method for cold atomic clocks. This device aims to address the issues of insufficient extinction ratio, low control accuracy, and poor stability inherent in existing optical switching technologies. The core innovation of this invention lies in combining the high reliability of a brushless DC motor, a precise FPGA control algorithm, and an optimized light-blocking blade structure to achieve high stability in the laser on-off cycle and minimize stray light leakage.

[0010] The technical solutions of the present invention are as follows:

[0011] A high-precision optical switch device includes: an FPGA control board, a DC motor drive circuit, a brushless DC motor, a light-blocking blade, and a photoelectric detection circuit; wherein:

[0012] The FPGA control board is used to communicate with the host computer and output pulse width modulation signals; the DC motor drive circuit is used to convert the DC signal output by the FPGA into an AC signal to drive the brushless DC motor; the brushless DC motor is used to drive the blades to rotate periodically; the light-blocking blades are used to act as optical switches, periodically turning the laser on and off; the photoelectric detection circuit is used to measure the period of the brushless motor for speed PID control.

[0013] Furthermore: the DC motor drive circuit is controlled by FPGA, can output and measure the three-phase current of the DC motor, and can receive the Hall position signal output by the DC motor to control the DC motor.

[0014] Furthermore, the brushless motor is a three-phase brushless DC motor, a Hall sensor is installed on the motor, and the rotating shaft and the blade with the small hole are glued with 3M glue.

[0015] Furthermore: the light-blocking blade is dug out with a small hole at a certain angle so that the ratio of the time the laser passes through the blade and the motor cycle meets the timing of the atomic clock. At the same time, a certain volume is dug out from the outer periphery of the other side of the blade so that the mass of the blade is uniform when it rotates.

[0016] Furthermore: the photoelectric detection circuit board converts the light intensity signal into a voltage signal, and calculates the period of the brushless motor according to the period of the voltage signal.

[0017] The control method of the present invention comprises the steps of:

[0018] 1) The host computer sets the target speed, PID parameters, and PWM frequency of the brushless motor;

[0019] 2) When the motor starts, a six-step commutation method is used to control the motor. The direction of the brushless motor current changes every 60 degrees based on the position information from the Hall sensor. The motor speed is measured based on the voltage signal from the photoelectric detection circuit, and the duty cycle of the PWM output is then controlled by PID control.

[0020] 3) When the motor approaches the target speed, a sine wave is used to control the motor. The Hall effect signal and the measured motor period are used to interpolate and estimate the motor's electrical angle. The required space vector and duty cycle are calculated. The SVPWM module is then built to output PWM to ensure the motor rotates smoothly at the target speed.

[0021] 4) When the motor speed decreases or increases significantly due to environmental reasons, if it still cannot stabilize after 1 second, it will re-enter the six-step commutation mode to approach the target speed;

[0022] 5) When the motor stops for a long time or rotates at maximum speed, the FPGA no longer inputs PWM signals to the motor driver, causing the motor to stop running and troubleshoot the corresponding fault.

[0023] The brushless motor control system of the present invention includes:

[0024] The speed calculation module is used to process the voltage signal from the photoelectric detection circuit and measure the motor cycle and speed;

[0025] Position calculation module, used to calculate the current electrical angle of the motor;

[0026] The PID module is used to compare the current speed with the target speed, use PID control to output the PID signal, and determine the duty cycle of the PWM and SVPWM modules;

[0027] PWM module, used to output PWM wave in six-step commutation mode;

[0028] SVPWM module, used to output 7-segment SVPWM wave in sine wave control mode;

[0029] Communication module, used to realize communication between FPGA and host computer;

[0030] The main control module is used to determine the control mode of the motor and whether it is stopped.

[0031] Furthermore, the high-precision motor control method obtains the current electrical angle of the motor by adding the angle corresponding to the Hall signal jump to the current angular velocity and multiplying the time after the Hall signal jump, and recalibrates each time the Hall signal jumps.

[0032] Furthermore: the high-precision motor control method performs dead time compensation according to the polarity of the three-phase current.

[0033] A cold atomic clock system integrates the above-mentioned high-precision optical switch device to control the periodic on and off of cooling light and repumping light. The extinction ratio is better than 40dB and the periodic fluctuation is less than ±0.25ms.

[0034] Compared with the prior art, the technical effects of the present invention are:

[0035] 1) Sinusoidal control reduces torque ripple during commutation, making the motor more stable. Seven-segment SVPWM effectively reduces PWM harmonics, ultimately achieving a motor rotation period of 87.5ms with fluctuations of less than 0.25ms. This invention offers easy control, excellent stability, and high reliability, making it of great significance in cold atomic clock experiments.

[0036] 2) Combining six-step commutation starting with sinusoidal wave smoothing control, it takes into account both fast response and high precision, solving the torque pulsation problem in the traditional single control mode.

[0037] 3) The coordinated design of the light-transmitting aperture and mass-balanced cutouts ensures timing accuracy while reducing mechanical vibration and avoiding stray light interference. The mass-balanced design of the light-blocking blades and the real-time feedback control of the FPGA ensure the long-term stability and reliability of the system.

[0038] 4) Seven-segment modulation combined with dead-zone compensation reduces harmonic components to below 5%, reducing motor heating and electromagnetic interference.

[0039] 5) FPGA-based real-time monitoring and mode switching mechanism ensures automatic system protection in abnormal situations and improves reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of an embodiment of a high-precision optical switch device for a cold atomic clock according to the present invention.

[0041] Figure 2 This is a flow chart of the motor control method of the high-precision optical switch device of the cold atomic clock according to the present invention.

[0042] Figure 3 This is a graph showing the change in motor speed along with the motor rotation period achieved by the present invention. DETAILED DESCRIPTION

[0043] The specific implementation methods of the present invention are described in detail below with reference to the embodiments to ensure that those skilled in the art can implement the present invention according to the description.

[0044] See also Figure 1 , Figure 1 Schematic diagram of an embodiment of a high-precision optical switch device for a cold atomic clock according to the present invention. As shown in the figure, a high-precision optical switch device for a cold atomic clock includes:

[0045] FPGA control board. This embodiment uses the XILINX Artix-7 series FPGA (model XC7A35T) as the main control chip, which integrates the following functional modules:

[0046] The speed calculation module is used to process the voltage signal from the photoelectric detection circuit and convert it into a digital signal. A voltage higher than 0.2V is a high level, and a voltage lower than 0.2V is a low level. After the digital signal is processed by a 1us delay filter, the time interval between the two rising edges is the motor cycle, and the motor speed is calculated from the cycle.

[0047] The position calculation module is used to calculate the current electrical angle of the motor. The current electrical angle of the motor is obtained by adding the angle corresponding to the Hall signal jump to the current motor speed and multiplying it by the time after the Hall signal jump. The motor is recalibrated every time the Hall signal jumps.

[0048] The PID module is used to compare the current speed with the target speed, use PID control to output the PID signal, and determine the duty cycle of the PWM and SVPWM modules;

[0049] The PWM module is used to output PWM waves in the six-step commutation mode. The duty cycle is determined by the PID signal, and the output is determined by the Hall signal.

[0050] The SVPWM module is used to output a 7-segment SVPWM wave in the sinusoidal wave control mode. In this case, the motor drive adopts a three-three conduction mode. Each phase has one bridge arm turned on. The upper bridge arm conduction can be regarded as 1, and the lower bridge arm conduction can be regarded as 0. At this time, there are 6 space voltage vectors (001, 010, 011, 100, 101, 110) and two zero vectors (000 and 111). These 6 space voltage vectors can be used as base vectors. By appropriately inserting the zero vector and reducing the number of MOS tube switches, a 7-segment SVPWM can be obtained. The dead time is inserted according to the positive and negative phase current vectors. The duty cycle of the voltage vector is determined by the PID signal and the electrical angle of the motor.

[0051] The communication module is used to realize the communication between FPGA and host computer, transmit PID parameters, target speed, PWM frequency to FPGA, and transmit motor speed to host computer;

[0052] The main control module is used to determine the control mode of the motor and whether it has stopped. When the motor starts to rotate, it is in six-step commutation mode. When the motor speed is less than 50 rpm from the target speed, it changes to sinusoidal wave control mode. In sinusoidal wave control mode, when the motor speed is greater than 50 rpm from the target speed and maintains for 1 second, the motor changes to six-step commutation mode. When the motor speed reaches 0 or the maximum speed after 1 minute, the module stops outputting PWM signals and the motor stops.

[0053] This embodiment utilizes a three-phase brushless DC micromotor (model EC45-3650) with three built-in Hall sensors (spaced 120° apart) for real-time rotor position feedback. The drive circuit utilizes the MP6540 chip to construct a three-phase inverter bridge. The input voltage is 12V DC, and the output is a three-phase AC signal to drive the motor. The circuit also incorporates a dead-time compensation module (2μs delay) to prevent direct current flow between the upper and lower bridge arms.

[0054] Light-blocking blades are made of lightweight aluminum alloy (density 2.7g / cm 3 ), 0.5mm thick and 30mm in diameter. 15° fan-shaped holes are opened along the circumference of the blade to ensure that the laser transmission time accounts for 17% (about 14.9ms) of the motor cycle (87.5ms), which accurately matches the timing requirements of the cold atomic clock. Two 2mm holes are symmetrically dug out on the periphery of the blade. 3 The rectangular area makes the moment of inertia evenly distributed and reduces the vibration amplitude to within ±5μm.

[0055] The photoelectric detection circuit in this embodiment uses an OPT101 photodiode to detect changes in the intensity of light reflected from the blades. The output current signal is converted into a 0-5V voltage signal by an LM358 operational amplifier. The FPGA collects the voltage signal through an ADC module (sampling rate 1MHz) and extracts the motor cycle based on a threshold value (high level > 0.2V, low level < 0.2V).

[0056] The FPGA control board is used to communicate with the host computer and output pulse-width modulation signals; the DC motor drive circuit is used to convert the DC signal output by the FPGA into an AC signal to drive the brushless DC motor; the brushless DC motor is used to drive the blades to rotate periodically; the light-blocking blades are used to act as optical switches, periodically turning the laser on and off; the photoelectric detection circuit is used to measure the period of the brushless motor for speed PID control.

[0057] The photoelectric detection circuit board uses a photodiode to convert the light intensity signal into a current signal, and then uses an operational amplifier to convert it into a voltage signal. The period of the brushless motor is calculated based on the period of the voltage signal.

[0058] Figure 2This is a flow chart of the motor control method for the high-precision optical switch device of the cold atomic clock of the present invention. It describes the logical flow of six-step commutation, sinusoidal wave control and fault handling in steps. The specific steps are as follows:

[0059] S1. Parameter settings:

[0060] The host computer sets the target speed, PID control parameters and PWM frequency of the brushless motor.

[0061] S2. Six-step commutation startup phase

[0062] During motor initialization, the FPGA changes the direction of the brushless motor's current every 60 degrees based on the position information from the Hall sensor. The motor's speed is measured based on the voltage signal from the photoelectric detection circuit, and the PID control output PWM duty cycle is then used.

[0063] S3. Sine wave control stage

[0064] When the motor approaches the target speed, it switches to sine wave control mode and the FPGA performs the following operations:

[0065] -Using the Hall signal and the motor cycle, the electrical angle is estimated in real time through interpolation;

[0066] -Generate three-phase sinusoidal voltage reference value according to the electrical angle, and convert it into U in the rotating coordinate system through Clarke transformation and Park transformation d , U q , and then combined with the SVPWM algorithm to generate a seven-segment PWM waveform;

[0067] - According to the polarity (positive / negative) of the three-phase current, a 2μs dead time is inserted into the PWM signal to avoid inverter bridge short circuit;

[0068] S4. Speed fluctuation response:

[0069] The motor speed is monitored. If the speed deviation is greater than 50 rpm and lasts for 1 second due to environmental interference, the FPGA main control module automatically switches back to the six-step commutation mode and readjusts the speed.

[0070] If the motor stops (speed = 0) or reaches the maximum speed (3000 rpm) for more than 1 minute, the FPGA will no longer input PWM signals to the motor driver, causing the motor to stop running. The host computer will also trigger the "fault code E01" alarm to troubleshoot the corresponding fault.

[0071] Figure 3 This is the brushless motor period fluctuation curve. It can be seen that the brushless motor's upper and lower period fluctuations are better than 250us.

[0072] Through the above-mentioned specific implementation methods, the present invention has demonstrated significant creativity in hardware design, control algorithm and system integration, can effectively solve the accuracy and stability problems of optical switches in cold atomic clock experiments, and has clear industrial application value.

Claims

1. A brushless DC motor driven high-precision optical switch device for a cold atomic clock, characterized in that: include: FPGA control board, used to communicate with the host computer and generate pulse width modulation (PWM) signals, integrating speed calculation module, PID control module and space vector pulse width modulation (SVPWM) module; A three-phase brushless DC motor with a built-in Hall sensor has a light-blocking blade fixed to its shaft. The light-blocking blade has a fan-shaped light-transmitting hole with an angle of 15°±0.5° and is symmetrically hollowed out on the periphery to evenly distribute the moment of inertia. It acts as an optical switch, periodically turning the laser on and off. The drive circuit is used to convert the PWM signal output by the FPGA into an AC signal to drive the motor, and integrates a dead time compensation module to prevent the bridge arm from shooting through; The photoelectric detection circuit, including a photodiode and an operational amplifier, is used to convert the light signal reflected by the blade into a voltage signal, extract the motor cycle through digital filtering, and feed it back to the FPGA to form a closed-loop control.

2. The high-precision optical switch device according to claim 1, characterized in that: The driving circuit is controlled by FPGA, can output and measure the three-phase current of the DC motor, and can receive the Hall position signal output by the DC motor to control the DC motor.

3. The high-precision optical switch device according to claim 1, characterized in that: The light-blocking blade is dug out with a small hole at a certain angle so that the ratio of the time the laser passes through the blade and the motor cycle meets the timing of the atomic clock. At the same time, a certain volume is dug out from the outer periphery of the other side of the blade so that the mass of the blade is uniform when it rotates.

4. A control method for the high-precision optical switch device according to any one of claims 1 to 3, characterized in that: The following steps are involved: 1) Set the target speed, PID parameters and PWM frequency of the three-phase brushless DC motor through the host computer; 2) When the motor starts, a six-step commutation method is used to control the motor. The direction of the brushless motor current changes every 60 degrees based on the position information from the Hall sensor. The motor speed is measured based on the voltage signal from the photoelectric detection circuit, and the PID control output PWM duty cycle is then used. 3) When the motor approaches the target speed, a sine wave is used to control the motor. The Hall effect signal and the measured motor period are used to interpolate and estimate the motor's electrical angle. The required space vector and duty cycle are calculated. The SVPWM module is then built to output PWM to ensure the motor rotates smoothly at the target speed. 4) When the speed error is less than ±50 rpm, it switches to sinusoidal control mode and generates a seven-segment SVPWM waveform based on electrical angle interpolation. If the speed deviation exceeds ±50 rpm and persists for 1 second, it switches back to six-step commutation mode. 5) When the motor stops or reaches the maximum speed (3000 rpm) for more than 1 minute, the PWM output is terminated and a fault alarm is triggered.

5. The high-precision motor control method according to claim 4, characterized in that: The brushless motor control system includes: The speed calculation module is used to process the voltage signal from the photoelectric detection circuit and measure the motor cycle and speed; Position calculation module, used to calculate the current electrical angle of the motor; The PID module is used to compare the current speed with the target speed, use PID control to output the PID signal, and determine the duty cycle of the PWM and SVPWM modules; PWM module, used to output PWM wave in six-step commutation mode; SVPWM module, used to output 7-segment SVPWM wave in sine wave control mode; Communication module, used to realize communication between FPGA and host computer; The main control module is used to determine the control mode of the motor and whether it is stopped.

6. The high-precision motor control method according to claim 4, characterized in that: The current electrical angle of the motor is obtained by adding the angle corresponding to the Hall signal jump to the current angular velocity and multiplying it by the time after the Hall signal jump. The motor is recalibrated every time the Hall signal jumps.

7. A cold atomic clock system, characterized in that: The high-precision optical switch device according to any one of claims 1 to 3 is integrated to control the periodic on-off of cooling light and repumping light, with an extinction ratio better than 40 dB and a period fluctuation less than ±0.25 ms.

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