A control circuit is commonly referred to as a “loop.” A control loop may be classified as either “open” or “closed” depending upon whether the control adjustments are manual settings (open loop) or automatically determined by some type of feedback controller (closed-loop).
Open loop (Fig. 4-18a) — In an open-loop control system, an operator makes a manual adjustment to a device (valve) which controls the flow of a manipulated variable (steam) to attempt to achieve some set-point (desired temperature) value of a controlled variable (hot water). However, this adjustment is only valid for the conditions under which the operator made the adjustment. Any disturbance such as a change in inlet water temperature, steam temperature, heat loss to the surroundings, or throughput will cause the outlet temperature to change.
Closed loop (Fig. 4-18b) — If appropriate measuring and controlling elements are added to the system, the loop is closed by the inclusion of an automatic feedback controller. The controller detects any difference between the set-point and measurement signals (error signal) and produces an output signal to drive the valve in the proper direction to adjust the heat input to cause the measurement to reach the set-point value.
Feedback control (Fig. 4-18c) — The basic components of a feedback control loop are shown in block diagram form in the figure. The “comparator” actually represents the entire controller and any associated signal converters. The “control element” is the valve, the “feedback element” is the transmitter, and the “process” is the mixing of the steam and cold water inside the water heater.
Feedforward control (Fig. 4-18d) — Feed forward control (often called “Predictive Control”) is actually a form of open-loop control. An input variable (cold water temperature) is monitored and the manipulated variable (steam flow) is adjusted accordingly to compensate for changes in the input variable. Feedforward control is almost always used in conjunction with feedback control to overcome the effects of some expected disturbance.