Pneumatically controlled valve unit, valve system, and method of operating a valve unit

A pneumatically controlled valve unit having a main valve for controlling a fluid flow of a fluid to be dosed, the valve unit comprising:

• a piston shiftably mounted in a valve chamber, operating against a return device, and connected to a valve closing body of the main valve to open and close the main valve,
• wherein the piston delimits a pressure chamber within the valve chamber such that the piston can be shifted against the return device by pressurizing the pressure chamber,
• wherein a position sensor is provided which is set up to detect the position of the piston in the valve chamber,
• wherein one respective pressure line leads from the pressure chamber to a pressure fluid port and a ventilation port, and an aeration valve is arranged in one pressure line and a ventilation valve is arranged in the further pressure line,
• wherein a control unit is provided which is set up to switch the aeration valve and the ventilation valve on the basis of the position of the piston detected by the position sensor to regulate a pressure in the pressure chamber for shifting the piston,
• wherein the control unit is set up to close the aeration valve or the ventilation valve as soon as the piston is in the open position or in the closed position, and
• such that the pressure in the pressure chamber does not rise up to the system pressure of a connected compressed-air network upon opening of the main valve and the pressure in the pressure chamber does not drop to ambient pressure when closing the main valve.