Does a valve positioner only receive control signals? Feedback opening degree?

The DCS sends a 4-20mA signal to the valve positioner; the opening degree of the valve, as feedback, is not provided by the valve positioner. The feedback rod on the control valve can only display the valve’s opening position locally, and it cannot send a signal back to the DCS. Only when a feedback switch is installed can it send a two-digit digital signal back to the DCS, allowing the DCS to determine whether the valve is open or closed. If the feedback switch malfunctions, the valve opening degree displayed in the DCS will be incorrect. If the DCS outputs 0 or 100%, and the feedback switch indicates open or closed, then the feedback is correct. If 50% is sent, one needs to check on-site whether the feedback rod is at halfway along its travel path; if it is, then the valve’s opening degree is correct. Therefore, during the process of opening or closing the valve, there is no feedback display – a display appears only when the valve has reached its fully open or fully closed position. To know the exact percentage of opening during the opening process, one must check the position of the feedback rod on-site; the same principle applies when adjusting the valve on-site.

Some models of intelligent electrical positioners come with a feedback signal; this feedback is two-wire based, just like in pressure transmitters, in the 4-20mA format. For specific parameters, check the respective transmitters

I’ve never seen a control valve with a feedback valve position; that feedback rod only extends up to the actuator.

Does the valve positioner only receive control signals from the DCS? Yes, does it feedback the opening degree to DCS? No, the control valve feeds back its opening degree to the DCS; is the opening degree fed back through that feedback rod? No. So if the valve is controlled on the DCS to be opened to a certain position, but it doesn’t reach that position, will there be an alarm? No, it’s just a mismatch between output and feedback; as long as the valve opening is not at its maximum or minimum, it does not affect the control performance. At this time, is the opening value displayed on the DCS the one you set, or the actual opening value at the site? The feedback is the actual opening degree of the valve. In other words, how can we confirm that the DCS’s control over it is effective? Check whether the control parameters meet the control requirements.

　　The control valve can send to the control room, via attachments, a continuous signal of the valve opening degree ranging from 0 to 100%; switch signals indicating whether the valve opening degree is greater than or less than a certain value for 1 to 4 settings; and video footage showing the overall status of the valve. 　　But, but you must choose to use the appropriate attachment, which can be a valve positioner with the relevant functions, or an independent valve position signal transmission attachment. 　　The valve position displayed by DCS can be the output signal of the control unit ； It is also possible to display the actual valve position on-site, provided that the appropriate control valve accessories are available. 　　Due to the operation of the control valve, this is usually reflected in the process (controlled) parameters; therefore, in most cases (unless there is a specific requirement), it is not necessary to display the valve position in the control room. The valve position shown on the DCS is the output signal of the control unit, rather than the actual valve position. Because theoretically, the output signal of the control unit should be consistent with the actual valve position. 　　If it is necessary to check the valve position at any time, one or more methods can be selected for verification.