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Valve positioners, classified by structure into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners, are essential accessories for control valves. They are typically used in conjunction with pneumatic control valves; they receive the output signal from the controller and use this signal to control the pneumatic control valve. When the control valve moves, the displacement of the valve stem is fed back to the valve positioner through mechanical mechanisms, and the valve’s position is transmitted to the higher-level system via electrical signals. Based on their structural design and working principles, valve positioners can be divided into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners. Valve positioners can increase the output capacity of control valves, reduce delays in the transmission of control signals, speed up the movement of the valve stem, improve the linearity of the valve, overcome frictional forces on the valve stem and eliminate the effects of unbalanced forces, thereby ensuring the accurate positioning of the control valve. There are some common issues that arise when valve positioners are in use. Today, we have compiled a list of these issues to help everyone better understand them during operation. 1. The valve positioner cannot self-tune: This message appears on the screen during self-tuning, indicating that the electrical angle is not within the specified range. Care must be taken during installation, as shown in the circled area in the figure below – the angle must not exceed the indicated range. 2. No display on the LCD: Use a multimeter to measure the DC voltage at terminals 11 and 12 while it is under load, and check whether the positive and negative terminals are connected correctly. If there are no issues, verify that the voltage at terminals 11 and 12 lies within the range of 8.2–8.7 V; if the voltage is within this range, it is usually a problem with the motherboard ； If the voltage is not within this range, the fault lies in the DCS interface or the transmission wires. 3. Actuator oscillation: Check for air leaks at the inlet fittings of cylinders 0UT1 and 0UT2 ； Set the operating mode to 1.3 manual mode, and check whether one of the ports of 0UT1 and 0UT2 is leaking air continuously. If there is continuous leakage, it may be due to a slight blockage inside the valve actuator body. If the positioner has no air leaks, but the air output from one of its ends remains unchanged during normal and accelerated operation modes when it is operated manually, this can also cause oscillation. In such cases, it is necessary to reset the positioner to its factory settings using option P11 in the menu, and then perform re-tuning. 4. The cylinder does not respond to signals: Set the operation mode to manual, and use the increase and decrease buttons to check whether OUT1 and OUT2 generate any output. If the two ports have alternating outputs, the problem lies with the cylinder or the load ； If there is output from only one port, or from both ports simultaneously, or if output occurs continuously from one port, it indicates that there are impurities stuck inside the components of the valve positioner, and the positioner needs to be replaced. 5. Abnormal position feedback signal: Use a multimeter to measure the DC voltage at terminals 31 and 32 under load; it should be between 17-23 V. If the voltage is within this range, it is usually a problem with the feedback circuit board ； If the voltage is not within this range, the fault lies in the DCS interface or the transmission wires.