Abstract: A motion-type PLC was successfully applied to the automatic tuning system of the high-frequency D circuit in an accelerator, achieving automatic system tuning. This met the requirement of stable system operation under strong electromagnetic interference. Based on the output of the phase detector, the two output pulses of the PLC were programmed using appropriate programming software to control the frequency of the output pulses. Combined with a three-phase hybrid stepper driver, stepper motor, and mechanical transmission system, the stepper motor was controlled to achieve precise tuning of the D circuit. Using a PLC reduces the difficulty of system debugging and wiring, shortens the development cycle, and provides flexibility for system debugging and maintenance. Keywords: D circuit, automatic tuning, motion-type PLC, accelerator I. Overview: The automatic fine-tuning of the D circuit frequency is a device specifically designed for the high-frequency D circuit of an accelerator to achieve automatic tuning. The D circuit consists of a high-frequency cavity, which is an important component of the accelerator. During accelerator operation, a high-frequency voltage is applied to this D circuit to accelerate particles. A schematic diagram of the D circuit is shown in Figure 1. Tuning of the D circuit is achieved by changing the position of the shorting plate (coarse tuning), and fine tuning is achieved by changing the distance between the fine-tuning capacitor and the D-type box. During normal operation, as long as the frequency of the high-frequency amplifier is fixed, the position of the shorting plate is also fixed. However, due to various factors (including thermal deformation, mechanical vibration, etc.), the parameters of the D circuit are unstable. Therefore, the D circuit cannot maintain a resonant state, and the Q value of the D circuit is very high (up to 8800 at 5.5MHz), so the D voltage will be unstable. Therefore, a frequency fine-tuning system is needed to achieve automatic tuning of the D circuit to achieve frequency stability, thereby improving beam quality and meeting the experimental requirements for beam current. The specific parameter requirement is: frequency stability of 1×10⁻⁶ (with automatic and manual modes). With the continuous advancement of science and technology, the original frequency tuning system with separate components is aging and technologically outdated, and can no longer meet the requirements of physical experiments. Based on the extensive experience accumulated from the original system, we designed a new automatic fine-tuning system. This system consists of four parts: a 360-degree electronic phase shifter, a phase detector, a motion-type PLC, a stepper motor driver, a stepper motor, and mechanical parts. [align=center]Figure 1[/align] II. System Composition: 1. 360° Electronic Vector Synthesis Phase Shifter (See Figure 1) [align=center]Figure 2. Block Diagram of Vector Synthesis Phase Shifter Principle[/align] When UA remains constant, UC also remains constant. Therefore, the condition for phase shift without amplitude change is that the multiplication factors K1 and K2 are determined by the control voltage. In this way, w and U maintain an approximately linear relationship. Phase shift is achieved by adjusting K1 and K2. 2. Phase Detector The function of the phase detector is to convert the phase difference into voltage, i.e., the conversion of Dw to V. As can be seen from Figure 2, the two original signals U1 and U2 input to the circuit come from the high-frequency transmitter and the D circuit, respectively. The former uses capacitive coupling, while the latter uses inductive coupling. The high-frequency transmitter and the D circuit are also capacitively coupled. Therefore, when the D circuit resonates, the phase difference between U1 and U2, Dw = p/2. When detuned, Dw < p/2 or Dw > p/2. This is the basic point for judging the resonance of the D circuit. Because the U2 line is too long, it will cause phase shift during transmission. Therefore, we connect an electronic phase shifter in series at the input for phase compensation. Based on this relationship, we selected the XR-2208M multiplier as the phase detector. This multiplier has an output frequency of up to 100MHz, forming an 1808 phase detector. The phase difference between the two input signals and the output DC voltage have the following relationship: Dw — the phase difference between the two input signals; Kd — the conversion gain of the phase detector. When the input signal is 50mV.rms, Kd ≈ 2V/radian. This is independent of the signal amplitude. This phase detector has three output levels. When Dw = p/2, VDw = 0 (resonance). When Dw < p/2, VDw > 0 (detuned). When Dw > p/2, VDw < 0 (detuned). 3. Programmable Logic Controller (PLC): We selected a Panasonic Electric Works PLC for motion control, featuring two 10kHz pulse outputs; with two output channels, each channel has a maximum frequency of 5kHz. It also has two A/D converters and one D/A converter. 4. German Baigella three-phase hybrid stepper motor and drive system driver WD-007: This driver operates on AC servo principles, with an input voltage of 220VAC, a control pulse signal voltage of 5VDC, and an output of 3x325VAC. It features overheat, overcurrent, undervoltage, and overvoltage protection. The number of steps per revolution can be set to 500, 1000, 5000, or 10000 steps/revolution according to user requirements. The stepper motor is VRDM-3910, with a maximum torque of 4Nm. 5. The coarse adjustment of the mechanical transmission part adopts worm gear and spiral shaft transmission. Its advantage is that it can be self-locking, but the transmission efficiency is low and the power loss is large. Fine adjustment is achieved by bevel gear transmission. The worm gear pitch is 2mm and the bevel gear pitch is 0.5mm. The minimum pulse equivalent can reach 0.004mm/pulse, which can meet the tuning requirements. III. Implementation method and programming: We achieve the tuning of the D circuit by programming the PLC output pulse. The automatic tuning process is to send the output of the phase detector to the A/D converter. Based on the A/D value, the program controls the pulse output frequency and direction. Combined with the three-phase hybrid stepper driver, stepper motor and mechanical transmission system mentioned above, the position movement of the capacitor board is completed, thereby realizing the precise tuning of the D circuit. (As shown in Figure 3) [align=center] As shown in Figure 3[/align] The PLC pulse output instruction F168 (SPD1) can be used to realize trapezoidal control. According to the specified initial speed, maximum speed, acceleration/deceleration time and target value, it can automatically output pulses. The instruction F169 (PLS) can perform a JOG (jog) operation and output pulses from a specified channel when the execution condition (trigger) is ON. Utilizing incremental, absolute, and origin-return control modes, along with system registers, for output pulse programming eliminates the need for travel limit switches, reduces system wiring, and improves system flexibility. The specific programming method is as follows: Using PLC instruction F168 (SPD1), ladder control is automatically executed according to a given parameter table. In the above program, the pulses output through output terminal Y0 have an initial speed of 500Hz, a maximum speed of 5000Hz, an acceleration/deceleration time of 200ms, and a total movement of 10000 pulses. At this time, the high-speed counter values ​​(DT9044 and DT9045) will increase with the number of pulses. The pulse output instruction (F169) outputs pulses from the specified channel and performs JOG (jog) operation when the execution condition (trigger) is ON. When X2 is ON, Y0 outputs pulses with a frequency of 300Hz and a duty cycle of 10%. At this time, the direction output Y2 is OFF, and the passing value count of the high-speed counter CH0 (DT9044 and DT9045) increases. When X6 is ON, Y1 emits a pulse with a frequency of 700Hz and a duty cycle of 10%. At this time, the direction output Y3 is OFF, and the passing value count of the high-speed counter CH1 (DT9048 and DT9049) decreases. IV. Conclusion: 1. This automatic tuning system uses a PLC as the control device, and uses software programming to complete the logical relationship processing of each signal and pulse output programming, simplifying the design of the hardware circuit and improving the flexibility of the entire system. 2. The PLC is an electronic device designed specifically for industrial control, using integrated circuits as basic components. Multi-level anti-interference and carefully selected component measures are adopted in the design and manufacturing process, and the internal processing does not rely on contacts. It is suitable for the strong electromagnetic operating environment of the accelerator high-frequency system, fundamentally ensuring the stability and reliability of the control system. It solves the requirement for stable operation of control equipment in a strong electric and magnetic environment. Currently, the installation and commissioning of this system has been completed and it is in use. The PLC-based automatic frequency tuning system for the high-frequency D circuit of the accelerator operates stably, achieving the designed frequency stability target of 1*10⁻⁶. References: 1. [M] NAISPLC "FP Series Programming Manual", Matsushita Electric Works, Ltd. 2. [M] Gao Zhongyu et al., "Mechatronics System Design", Machinery Industry Press, April 1997. 3. [M] Zhang Suwen et al., "High-Frequency Electronic Circuits", Higher Education Press, April 1993.