2293Open Loop Control

An open-loop control system is characterized as one whose output has no impact on the measured variable, and so any form of process feedback is impossible.

Examples of Open Loop Control:

• Turning off the building heating boilers in April and leaving them off until September. A common control strategy, this follows the common sense notion that boilers should not be needed in summer, but does not address spring and fall vary well and can lead to both discomfort and energy loss when weather is unseasonably cold or warm, usually near the calendar cutoff dates. This is open-loop because the outside air temperature is unaffected by whether or not the boiler is on. This is also open-loop to the building itself, because call for heat feedback will not be heard.

Figure 22.10 Closed Loop Control

Automatic reset of hot water temperature from outside air. A common control strategy, this provides general compensation based on the common sense notion that 'the colder it gets outside the more heat we'll need,' however is open-loop since the outside air temperature is unaffected by water temperature.

Starting the building HVAC system one hour before occupancy. A common strategy, this follows the common sense notion that the building will need some time to warm up (or cool down) after being off all night, or all weekend. It provides general compensation for the thermal lag in the building mass, but is open-loop because variations in actual time required in different seasons is not considered. For example it may take (4) hours after a long and cold weekend but only a half-hour after a single night in spring, but the controller is blind to these facts. Thermostat in room 1 controls the hot water control valve serving room 2. In this example, closed loop feedback control is changed into open loop control due to a design or installation error.

Figure 22.11 Open Loop Control
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