What do open-loop and closed-loop mean in instrument control?
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During the installation of the pump’s frequency converter, the designer asked whether the control should be open-loop or closed-loop. What does this mean?
Open loop means that the effect of control is not monitored, while closed loop involves monitoring the effect of control and adjusting it based on the deviation between the actual value and the set value
In an open-loop system, the regulation during operation is based on manually set values, whereas in a closed-loop system, it is an automatic control system composed of instruments or transmitters and a control system
Simply put, open-loop control means that the variable frequency control of the pump does not require feedback or real-time adjustments to the pump’s speed. A closed loop system means automatically adjusting the pump speed based on feedback from the actual operating conditions.
In open-loop mode, only the frequency of the pump motor’s output is controlled in one direction; there is no monitoring of the pressure and flow rate of the medium at the pump outlet, or even if such measurements are available, they do not affect the variable-frequency output； In a closed-loop system, the pressure and flow rate of the medium at the pump outlet affect the output of my variable frequency drive. Asking whether your frequency converter is open-loop or closed-loop should refer to the frequency converter itself. Some frequency converters come equipped with built-in closed-loop control functions, including a closed-loop input switch through which pressure or flow rate (resistance or current signals) can be supplied. Once in closed-loop operation, the frequency converter will automatically control and adjust itself based on the inputted pressure or flow rate values compared to the set values. In the inverter, the PI parameters can also be set and modified. In DCS or PLC control systems, closed-loop control for frequency conversion is generally not used; the automatic control function is carried out by the DCS or PLC system, with the frequency converter merely receiving signals from the control system. In this way, although the outlet parameters of the pump are controlled, it remains an open-loop system for the frequency converter.
In an open-loop system, there is no feedback, and thus no control loop can be formed.
Open-loop control refers to a situation where the controller outputs directly, with no input signal and thus no feedback, resulting in no limitation on control errors. Closed-loop control refers to a situation where the output signal controls the target, with a feedback signal being compared to the input; in other words, there is feedback, resulting in small control errors.
In an open-loop system, the regulation during operation is based on manually set values; essentially, the operator must intervene manually. In a closed-loop system, an automatic control system is formed by sensing instruments (such as thermometers, pressure transmitters, flow meters, etc.) and actuators (such as control valves, pumps, frequency converters, etc.). The operator only needs to set the parameters once (such as process parameters like pressure, flow rate, temperature, etc.), after which the system carries out the regulation itself. If a control system includes sensing instruments for monitoring various parameters, it must be a closed-loop system