Definitions of valve commands and feedback (AI and AO), identification of fault points, and troubleshooting

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In control systems, the AI (analog input) and AO (analog output) types associated with the “commands” and “feedback” for valves, along with the logic for fault detection, are as follows: I. Definitions of AI/AO for valve commands and feedback 1. Valve command (control signal) – AO (analog output): A 4-20mA signal generated by the DCS/PLC, used to control the opening degree of the valve (e.g., 4mA corresponds to fully closed, while 20mA corresponds to fully open). Nature: AO is the output signal of “control system → valve positioner” and belongs to the category of “active control signals”. 2. Valve feedback (valve position signal) — AI (analog input): Defined as the 4-20mA signal output by the valve positioner, which indicates the actual opening degree of the valve (e.g., 4mA corresponds to fully closed, 20mA corresponds to fully open); this signal is sent back to the DCS/PLC. Nature: AI is the input signal for “valve positioner → control system”, and it belongs to the category of “passive feedback signals”. II. Identification and Handling of Common Faults 1. Large discrepancy between command and feedback (e.g., command at 50%, feedback at 10%). Possible causes: faulty actuator, or the actuator receiving the command but failing to operate the valve properly (e.g., blocked air supply, faulty servo motor) ； Valve sticking: Stuck valve stem or scaled valve core, preventing the valve from operating as instructed ； Feedback signal failure: Broken feedback circuit, damaged locator feedback module (distorted feedback signal). Judgment method: Use a multimeter to measure the AO command value output by the DCS (to check whether it matches the set value, e.g., 50% should correspond to 12mA) ； Measure the AI feedback value output by the locator (to determine whether it matches the actual valve position; for example, an actual opening of 10% should correspond to 6.4 mA) ； Manually operate the valve (using the actuator’s manual knob) and observe whether the feedback changes as the valve position changes: If the manual operation yields normal feedback, then there is a fault in the part of the actuator that receives commands ； If the manual operation feedback is still abnormal → the valve is stuck or there is a fault in the feedback circuit. Treatment: Remove sticking in the valve (disassemble and clean the valve stem and valve core) ； Repair the locator (clear the air path, replace servo components) ； Repair the feedback circuit (reconnect broken wires and replace damaged cables). 2. The command is normal, but the feedback remains unchanged (the feedback stays at 0% or 100%). Possible causes: a fault in the feedback circuit, a broken AI signal (resulting in a feedback value of 0 mA, corresponding to 0%), or a short circuit (resulting in a low feedback current, such as near 0%) ； Damage to the locator feedback module: issues such as problems with potentiometers or sensors prevent the generation of feedback signals ； Mechanical failure of the feedback component: such as the separation of the connection mechanism between the actuator and the valve stem (no change in feedback as the valve position changes). Judgment method: Measure the output terminal of the positioner’s feedback signal (to check for 4-20mA variation); no variation → fault in the positioner or mechanical connection ； Test the input terminal of the DCS AI card (to check if it matches the output of the locator): If they do not match, there is a wiring fault. Treatment: Repair the feedback circuit (tighten connections, replace cables) ； Replace the positioner feedback module (such as potentiometer, Hall sensor) ； Reconnect the feedback mechanism (tighten the linkage and coupling). 3. The command produces no output (the output is fixed at 0%), but the feedback is normal. Possible causes: Fault or damage to the DCS AO card, preventing it from generating a 4-20mA signal ； Instruction circuit fault: The AO circuit is broken or short-circuited, preventing signals from being transmitted to the locator ； Incorrect DCS parameter setting: For example, the circuit is forced to \"Manual 0%\" or there is a configuration error. Judgment method: Check the DCS configuration (whether it is mandatory and whether the range is correct) ； Check the DCS AO output (for 4-20mA signal): No output → Card failure ； Check the locator command input terminal (whether it matches the DCS output): Mismatch → Circuit fault. Processing: Reset DCS forced status, correct configuration ； Replace the AO card or repair the wiring (repair broken wires and eliminate short circuits). 4. Neither commands nor feedback are present (no signal at all). Possible causes: power supply failure, interruption in the power supply to the locator (24VDC); both commands and feedback rely on power supply ； Public line failure: For example, if the power and signal cables for the locator share the same cable, the entire cable is broken. Judgment method: Measure the voltage at the power terminal of the locator (whether there is 24VDC): No voltage → Power supply failure ； Check the power circuit (fuses, circuit breakers, wiring). Treatment: Restore power supply (replace fuse, close circuit breaker, secure power connections) ； Repair public lines (replace broken cables). Summary of instructions (AO): The control signals output by the control system; faults are often related to DCS cards and output circuits ； Feedback (AI): The signal reflecting the actual opening degree of the valve; failures are often related to the actuator, feedback circuitry, and the mechanical parts of the valve. For diagnosis, it is necessary to conduct checks step by step \"from the signal source to the terminal\" (check the DCS output → the actuator input → the actuator output → the DCS input), and combine this with manual operations to quickly locate the fault point.