1. Open-loop control is a control system where the output is not fed back to the input. The controller applies a fixed input signal to the system, regardless of the output. An example of an open-loop control system is a washing machine. The washing machine has a timer that applies a fixed sequence of wash, rinse, and spin cycles, regardless of the condition of the clothes inside the machine.
2. Closed-loop control is a control system where the output is fed back to the input. The controller adjusts the input signal based on the output signal to maintain the desired output. An example of a closed-loop control system is a thermostat. The thermostat senses the temperature in the room and adjusts the heating or cooling system to maintain the desired temperature.
3. The span of a control loop is the range of values that the controller can use to adjust the input signal. The spilled range is the range of values that the controller cannot use to adjust the input signal. For example, if a temperature controller has a range of 0-100 degrees Celsius and a deadband of 5 degrees Celsius, then the spilled range is 0-5 degrees Celsius and 95-100 degrees Celsius.
4. A cascade control loop is a control system where the output of one controller is used as the setpoint for another controller. An example of a cascade control loop is a temperature control system for a chemical reactor. The first controller adjusts the heating or cooling system to maintain the desired temperature of the reactor. The second controller adjusts the flow rate of the reactants to maintain the desired temperature of the reaction.
5. A ratio controller is a control system that maintains a fixed ratio between two inputs. An example of a ratio controller is a fuel-air ratio controller for a combustion system. The controller adjusts the fuel flow rate to maintain a fixed ratio between the fuel flow rate and the air flow rate.
6. Feedback control is a control system where the output is fed back to the input to adjust the input signal. Feedforward control is a control system where the output is used to adjust the input signal before it enters the system. An example of a feedback and feedforward control system is a cruise control system for a car. The feedback control adjusts the throttle to maintain the desired speed. The feedforward control adjusts the throttle to compensate for changes in the slope of the road.
7. A safety instrumented system (SIS) is a control system that is designed to prevent or mitigate hazardous events. An example of an SIS is a safety shutdown system for a chemical plant