Original Control Principle The original control principle block diagram is shown in Figure 1. The handheld controller provides a 4-20mA signal to the preamplifier. The preamplifier circuit is a DC output internal feedback push-pull circuit. After the given signal is compared with the 4-20mA signal fed back from the electric actuator position transmitter, the electronic switch and trigger control the electric actuator servo motor to rotate forward or backward. At the same time, the 4-20mA signal fed back from the electric actuator position transmitter also changes accordingly and sends the feedback signal back to the preamplifier for comparison until the given and feedback signals are balanced. Since the feedback signal is connected in series with the handheld controller display and the servo preamplifier, the change process of the feedback signal is displayed on the handheld controller while the feedback is compared with the given signal. Figure 1: [align=center] Figure 1 Servo Amplifier Control Principle[/align] The "zero adjustment" potentiometer and "stabilization" potentiometer in the servo preamplifier are crucial for the electric actuator to generate oscillations and are also prone to failure. The thyristor, diode, current-limiting inductor, and single-cell transistor in the servo electronic switch and trigger circuit are also prone to failure. Utilizing Servo-Free Technology to Control Actuators During the technical upgrade of Kiln No. 3, which began in November 1999, a DCS system was established for the Kiln No. 3 system. Simultaneously, based on our long-term experience in using actuators and servo-free retrofitting, servo-free control retrofitting was carried out on five actuators in the No. 3 coal mill, including the inlet valve of the bag filter exhaust fan and the cold air valve, with RC arc extinguishing and RV varistors. Using a ladder diagram program (Figure 2), the comparison and amplification functions of the servo amplifier were simulated for the 220V and 380V electric actuators. The drive control of the electric actuators was implemented using a digital output module, and the process valve position feedback was collected using an analog input module. Image 2: Figure 2 Ladder Diagram Program for Servo-Free Control. In the ladder diagram program, the value of 400101 is given by the operator, and after division and multiplication, it is rounded down and assigned to 400135; similarly, 300019 is the feedback value, and after division and multiplication, it is rounded down and assigned to 400140. After comparing 400135 and 400140, the corresponding 000033 coil is driven, causing the intermediate relay K1 in the hard circuit (see Figure 3) to close, the AC contactor KM1 to operate, and the motor to rotate forward; conversely, the 000034 coil is driven, causing the intermediate relay K2 in the hard circuit to close, the AC contactor KM2 to operate, and the motor to rotate in reverse, thus realizing the servo-free forward or reverse rotation of the servo motor of the field electric actuator. Image 3: [align=center][align=center]Figure 3 Electric Actuator Control Principle[/align][/align] DCS: Control System; ESR: Actuator Position Transmitter; Q: Low-voltage Circuit Breaker; Q1, Q2: Maintenance and Repair Switches; RC: Arc Extinguishing Device; RV: Varistor. In the ladder diagram program, "#100" is the rounded value. "100" actually represents the accuracy of the valve opening, i.e., the dead zone control range. In high-precision control, "#50" or smaller can be used, which is equivalent to the "stabilizing" potentiometer in the servo preamplifier. 000581 is the reset coil in the program. "#40" means that the drive output is held for 40 seconds before automatically disconnecting. After modification, the hard circuit can simultaneously handle 220V and 380V. Select 220V or 380V actuators. Due to centralized control, each electric actuator requires two intermediate relays and one local control box. If it can be negotiated with the manufacturer to install the contactor, RC arc extinguishing device, and varistor inside the electric actuator, installation and operation will be easier. Our company has already made the above modifications to the Siemens actuators on the production line.