Precise Water Delivery System for Coffee Machines

Xiang Zheng Tu

 

The above figure shows a precise water delivery system for coffee machines, which is provided by POSIFA Microsystems Company. The system comprises a cold water inlet, a water filter, a solenoid valve, a thermal flow sensor module, a one-way valve and a water heater. The cold water may be supplied by running water or a water container in which water flow is driven by water self weight. The cold water enters the inlet and then successively passes through the solenoid valve, the thermal flow sensor module, the one-way valve. Finally the cold water is heated in the water heater and the hot water is ready to flow into a spray head, and onto a ground coffee, which is contained in a brew basket mounted below the spray head.

On or off of the cold water flow is realized by the solenoid valve. The valve is controlled by an electric current through a solenoid. If the valve is open when the solenoid is not energized, then the valve is termed normally open (N.O.). Similarly, if the valve is closed when the solenoid is not energized, then the valve is termed normally closed. The solenoid is controlled by a switched circuit. A digital output is connected to the base of a transistor which controls the current to a normally open relay. When the relay coil is energized, it closes the contacts, which allows current from the DC supply to flow through the solenoid. When the solenoid coil is energized, the valve opens, allowing cold water to flow through the valve.

The digital output is send by the thermal flow sensor module. The thermal flow senor module measures the flow rate of the cold water and calculates an amount of cold water flowing through in a certain time by integration, which should equal to a desired amount of cups of finished coffee to be made. When the corresponding digital output matches the number of cups, the control circuit is open to the solenoid valve thus closing the valve and stopping the flow of cold water. As seen in the above figure, there is another similar circuit for controlling the water heater. Likewise, at the same time, another digital output is sent to the contactor of the circuit for shutting down the power supply to the water heater.

The thermal flow sensors of the module rely on the ability of fluid flows to affect thermal phenomenon by way of heat transfer that, in turn, is transduced into a varying electrical signal capturing the sensor response to flow change. The sensors are thermally isolated so only heat transfer due to flow can occur. Other heat transfer pathways such as through substrate or electrical leads result in thermal losses that degrade sensor performance and are minimized in the device design. The thermal flow sensors measure mass flow rate and response is independent upon a constant fluid temperature.