How does a PLC control the opening degree of a control valve?

PID control involves comparing the set value (cascade setpoint) with the actual valve position, and then using the PID proportional, integral, and derivative terms to determine the OP opening degree of the control valve. There is a linear relationship between the OP value and the opening degree, ranging from 0% to 100%.

Q: I want to know how the output value from PID control determines the opening degree of the valve A: The output value of PID control is AO, for example 4~20mA; the magnitude of this current value corresponds to the opening degree of the valve. Q: Does the PID module need to know the current opening degree of the valve first? (How does it find that out?) A: It needs to know it, as it is used as a feedback signal. The current valve opening can be transmitted to the PLC via the valve position signal (which is required when selecting the valve model). Q: If the control valve is connected to AO, is it still necessary to connect it to AI as well? What determines the opening degree of the valve exactly? If AI needs to be connected, then the valve position signal must be connected to AI. You can just search for the PID algorithm online.

The PID module in a PLC only requires the input of the variables that need to be controlled; the output is used for controlling the valve, while the actual valve opening degree is not necessary.

The PID controller does not know the actual valve opening degree; it only receives the feedback value of the control variable corresponding to that opening degree, compares it with the set value to perform PID calculations, and then uses those results to control the valve opening degree.

OP, simply put, it’s a programming issue – adding a control loop. The valve position signal can be output as a 4-20mA analog signal through a valve position transmitter or a positioner with feedback functionality, or it can be output as a digital signal through a valve position switch.

How does the output value from PID control determine the opening degree of the valve? (It is determined by comparing the temperature setpoint with the actual temperature; the PLC performs PID calculations and generates a 4-20 mA DC signal to control the valve’s opening degree.) The PLC software itself includes a PID module. Does this PID module need to know the current opening degree of the valve? (It isn’t necessary to know this value when controlling the valve.) If the control valve is connected to an AO signal, is it still necessary to connect it to an AI signal as well? (The control valve only needs to receive AO signals; it doesn’t require AI signals.) What exactly determines the valve’s opening degree? (It is controlled by the PLC’s output AO signal.)

1. Since the original poster already knows how to convert the temperature input into actual temperature values, the AO output follows the opposite process. Leaving PID aside, writing a value within a normal range (such as 0-27648 for Siemens S7-300) to the output register address corresponds to a certain mA value. 2. The PID module does not need to know the actual opening degree of the valve; it only knows the desired opening degree of the valve (that is, the PID’s output, as the PID certainly knows its own output). When there are no faults in the circuit, the “actual opening degree” and the “desired opening degree” of the valve are “the same” ； If there is a discrepancy, then call an instrument technician for maintenance.

PID control can be understood in this way: the regulator controls the magnitude of the output signal (that is, the signal sent to the control valve) based on the difference between the set value and the actual measured value. If there is no deviation, the output remains unchanged (the control valve maintains its current opening degree); if there is a deviation, the output will increase or decrease (corresponding to an increased or decreased opening degree of the control valve) until the two values match. So the output is only an incremental change; there is no need to know the current opening degree.

Reply to 9# Da Meng Xian Jue: Thank you all for your help. Quoting the answer from floor 9, PID control can be understood in this way: the regulator controls the magnitude of the output signal (that is, the signal sent to the control valve) based on the difference between the set value and the actual measured value. If there is no deviation, the output remains unchanged (the control valve maintains its current opening degree); if there is a deviation, the output will increase or decrease (corresponding to an increased or decreased opening degree of the control valve) until the two values match. So the output is only an incremental change; there is no need to know the current opening degree. Excuse me, regarding the 9th floor: I’m still not quite clear. Isn’t the output signal a 4-20mA current signal? And isn’t a 4-20mA signal related to the valve’s opening degree (I’m not sure how the increment is determined)? If the PLC doesn’t know the current opening degree of the valve, how can it determine an increment? For example, if the current valve opening degree is 50% and the PID module determines that the valve needs to be opened further, what value should the mA signal be if the PLC doesn’t know the current opening degree? Also, doesn’t the screen of the general control station need to display the valve opening degree?

Reply to 10# wyj806: Of course, the PLC knows the current opening degree of the valve; the value of the current output corresponds to the valve’s opening degree! Under normal circumstances, these two values are identical. Only for some very important valves is feedback on the opening degree used, so that the operator can see the actual opening degree of the valve!