Simple DCS loop control problem

The DCS receives the data signals sent back by the flow meter, compares them with the set flow rate or the desired flow rate, generates a control signal for the control valve, and sends it back to the field to adjust the opening degree of the valve. I’m a beginner too.

It’s hard to quantify this; if a fixed ratio is established, then it is also required that the valves have equal percentages and fixed CV values. It is the PV signal of flow rate that enters the DCS; it involves single-loop control, where a comparison is made with the set SV value, and then, through a PID controller, an output value for the valve opening degree is determined. During this process, the valve opening degree keeps changing until it eventually stabilizes.

Taking the flow controller tag FIC101 as an example, if the range is 0-10 t/h. The current flow rate is 5 t/h, and the operator set it to automatic at 8 t/h. The starting point of the entire control chain should be the flow measurement transducer located at the site. Taking an orifice plate as an example, the orifice plate generates a pressure difference that is directly proportional to the flow rate; this pressure difference is transmitted through pressure conduits to the pressure sensing diaphragm of the transducer. The pressure-sensitive element then converts changes in this pressure difference into electrical signals in the form of 4-20mA current variations. In this example, 5 t/h represents 50% of the total range of the measured value, which corresponds to 12 mA. On the premise of using a fixed channel for connecting the cable to the AI card of the cabinet controller module, the 12mA current signal is converted into a digital signal within the range that the controller can accept, at 50%. In the controller of the cabinet control unit, there is a FIC101 module that implements the control strategy based on the instrument control design. The input signals to this module come from the fixed-channel conversion signals of the AI card mentioned earlier; these signals are then converted using the preset range of 0–10 t/h to obtain the measured value PV, which in this case is 5 t/h. This value is sent to the PID function block, which incorporates functions such as PID calculations, parameter settings, various coefficients, and alarm mechanisms. The PID function block compares the set value SP entered by the operator at the control station, namely 8 t/h, with the measured value PV of 5 t/h to determine the deviation. This deviation is used as an input in the PID calculation formula (you can look it up on Baidu if you can’t see it here) to calculate the final output value: the OP of the controller. For example, it may be calculated that the OP should be set at 70% at the next moment. The OP value is converted into a range (for valves, the OP value and the range are both typically 0-100%); this value is then sent to one of the channels on the AO card of the controller unit. On the AO card, the range is further converted into a 4-20mA current in the circuit associated with that channel, specifically 15.2mA. The circuit of that channel on the AO card is connected to the valve positioner cable of the on-site control valve. In the valve positioner, the 15.2mA electrical circuit current is converted electrically to regulate the supply pressure of the pneumatic control valve from 0 to around 4 kilograms; by applying this pressure and comparing it with the valve opening degree, the control valve is adjusted to 70% open. Under the combined influence of the valve characteristics and the flow conditions in the pipeline (pressure difference, resistance), the valve opening degree determines the flow rate; as the opening degree increases, the flow rate increases as well. The increased flow rate is detected by the flow orifice plate and transmitter, and the same process is repeated to enable the PID calculation formula to compute the deviation for the next cycle. This repeats over and over again within each DCS scanning cycle. I hope I can answer your question.

It should be quantified enough now; add a premium