Control System Types
• Closed loop (feedback) control
• Open loop (feedforward) control
Does sensor measure controlled variable?
If yes the control system is Closed loop, if no the system is Open loop.
In the closed system, controller responds to error in controlled variable. Previous example of the steam heating coil is a closed loop. In general, HVAC control systems are primarily closed loops.
In the open loop system, there is an indirect link between controller and controlled variable. The system action is based on external variable. The relationship between external variable and controlled variable is assumed. An example of open loop is electric blanket.