Читать книгу Automation of Water Resource Recovery Facilities - Water Environment Federation - Страница 208

Processes without a direct feedback of the process variable are called open-loop systems. Those with a direct feedback control loop are called closed-loop systems. An example of an open loop would be driving a car (without cruise control). The control action is depressing the gas pedal. If the pedal remains in the same position, the car will speed up and slow as the car goes over hills. Using cruise control is an example of closed-loop control. The cruise control adjusts the amount of gas feed to maintain a constant speed for the car. While going uphill, cruise control will increase the gas feed rate, while, going downhill, it will decrease the gas rate.

Closed-loop control is used when there is a relationship between the control action and the process variable and the process variable can be easily measured. Closed-loop control is common in wastewater treatment control. An example includes dissolved oxygen control in an aerobic activated sludge system, which was mentioned previously. However, there are instances when open-loop control is needed and can be applied effectively. An example is “flow pacing”, or adjusting chemical flow proportionally to plant flow. This keeps the dosage rate of the chemical constant, but does not control the chemical feed based on a measurement of the desired effect of the chemical.