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The control system performance can be improved by combining the feedback (or closed-loop) control of a PID controller with feed-forward (or open-loop) control. Knowledge about the system (such as the desired acceleration and inertia) can be fed forward and combined with the PID output to improve the overall system performance.
A setpoint can be any physical quantity or parameter that a control system seeks to regulate, such as temperature, pressure, flow rate, position, speed, or any other measurable attribute. In the context of PID controller , the setpoint represents the reference or goal for the controlled process variable.
In a chemical process, independent variables that can be adjusted by the controller are often either the setpoints of regulatory PID controllers (pressure, flow, temperature, etc.) or the final control element (valves, dampers, etc.).
A proportional controller schematic is shown here. The set point is transmitted through the flapper plate via the fulcrum to close the orifice and increase the chamber pressure. The feedback bellows resists and the output signal goes to the control valve which opens with increasing actuator pressure.
In open-loop control, the control action from the controller is independent of the "process output" (or "controlled process variable"). A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building.
During the Industrial Revolution in the 18th century, there was a growing need for precise control over boiler pressure in steam engines. In the 1930s, pneumatic and electronic controllers, such as PID (Proportional-Integral-Derivative) controllers, were breakthrough innovations that laid the groundwork for modern control theory.
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