Flag (oscillator)

Flag

The flag oscillates when connected to an air flow. It fills up with air, but as the air reaches the end of the flag, the flag opens, thus releasing all the pressure, and the flag drops again, stopping the airflow.

The frequency at which the flag oscillates is determined by the flow rate. The oscillation frequency can be reduced by connecting a capacitor in line with the flag.

The flag can be used to:

  • Make actuators oscillate by placing them in line with the flag.
  • Observing the approximate flow rate at a point in the circuit, by observing the frequency at which the flag oscillates. As such it can function as a basic flow sensor.

How does it work?

The flag is initially closed because the majority of the fabric sleeve hangs down, creating a fold in the fabric tube that prevents air from flowing out. The fabric after the fold, which is hanging down, has a certain mass, and is basically pulling down on the fabric tube. As more air flows into the flag, the fold moves up, and there will be less fabric after the fold hanging down, so that mas decreases. It then becomes progressively easier for the tube to inflate.

When the tube is fully inflated, there is no fold anymore and the flag opens up at the end. When the flag is open, all the air can rush out and the pressure drops sharply. This in turn causes the fabric of the flag to fall and create a fold, thus closing the flag. The whole process then starts over.

Symbol

Schematic symbol for the flag

Note: The atmospheric pressure around us is considered ‘normal’ pressure. This is the ‘ground’ pressure that all other pressures in the circuit are measured against. The pump can create up to 1 bar of pressure compared to the ‘ground’ pressure of the atmosphere around us.
Because the flag is releasing air to the atmosphere around us, we can say that one end of the flag is connected to ground. That is why there’s a ground-symbol (‘GND’) attached to the flag symbol in the schematic.