Qt Quick Waterpump Example

Interacting with an OPC UA server to build a QML-based HMI for a simple machine.

Qt Quick Waterpump shows how to use Qt OPC UA QML API to interact with an OPC UA server to build a QML based HMI for a simple machine.

The Simulation

The OPC UA server included in this example runs a simulation of a machine containing two tanks, a water pump and a valve. Water can be pumped from the first tank into the second tank and then be flushed from the second tank by opening the valve. Both operations have a user-configurable setpoint which controls how much water is pumped to or flushed from the second tank.

The following nodes exist on the server:

ns=2;s=MachineThe folder containing the method and variable nodes for the machine
ns=2;s=Machine.StateThe state of the machine
ns=2;s=Machine.Tank1.PercentFilledThe current fill status of the first tank
ns=2;s=Machine.Tank2.PercentFilledThe current fill status of the second tank
ns=2;s=Machine.Tank2.TargetPercentThe setpoint for pumping and flushing
ns=2;s=Machine.Tank2.ValveStateThe state of the valve of the second tank
ns=2;s=Machine.DesignationA human readable designation of the machine for display purposes
ns=2;s=Machine.StartCall this method to start the pump
ns=2;s=Machine.StopCall this method to stop the pump
ns=2;s=Machine.FlushTank2Call this method to flush tank 2


This example uses QML items to read and write values as well as method calls. All operations are asynchronous and handled by QML in the background.

A QML item represents the machine and its components. This item does not have any graphical interface and only provides access to the properties of the machine. It has two tanks which are instances of a separate Tank component.


The machine item exposes all properties and subitems as properties.

    readonly property alias tank1: tank1
    readonly property alias tank2: tank2
    readonly property alias state: machineState.value
    readonly property alias tank2PercentFilled: tank2.percentFilled
    readonly property alias tank2valveState: tank2.valveState
    readonly property alias designation: designation.value
    property alias resetMethod: resetMethod
    property alias startMethod: startMethod
    property alias stopMethod: stopMethod
    property alias flushMethod: flushMethod
Asynchronous Handlers

The asynchronous parts of the API are handled by QML automatically because signals are emitted for every property change by default.

For example, the button to flush the second tank is enabled only if the backend is connected to the server, the machine is idle, and the tank level is above the setpoint. On click, the flushTank2() method is called on the server.

    Tank2Unit {
        flushButtonEnabled: connection.connected && machine.state === Machine.MachineState.Idle && machine.tank2.percentFilled > machine.tank2.targetPercent
        percentFilled: machine.tank2.percentFilled
        valveState: machine.tank2valveState


The server is started automatically by the HMI application. After connecting to the server by clicking the Connect button, drag the slider to set a setpoint, then click Start to start pumping water from the first tank to the second tank. After setting a setpoint lower than the current value of the second tank, a click on Flush opens the valve.

If there is no water left, click Reset simulation to refill the first tank.


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