We consider level control a fundamental control technique. Level controls are used in all types of applications:
There are several types of level control.
In simple level control, such as a well that acts as a buffer between processes, there is a high level and a low level. When the liquid reaches the high level, you pump the level down until the liquid reaches the low level at which time you stop pumping out of the tank or vessel.
The opposite could also be true. For example, if you have a tank or vessel that acts as a local buffer, then when the liquid goes down to the low level, you might start a pump or open a valve to refill the tank or vessel until the high level turns on.
Although this type of control is very easy and simple to implement, it is not able to maintain a setpoint level in the tank or vessel.
In proportional control typically the tank level is an analog signal that accurately indicates the current level of the tank or vessel. The controller is typically a PID controller that controls an analog output based on the current level and the setpoint. For example, if you want a tank or vessel to be 60% full, maintain a level of 12.5 feet, or 265 gallons, you would get an accurate level sensor, scale the level sensor output to your units and then feed that level to the PID controller. The PID controller would then control a valve or pump to maintain the level. You can often maintain very tight levels using proportional control.
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