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Building a state machine

In this example we build a finite state machine that controls a temperature sensor: the sensor initialises, measures, and can be halted and resumed.

The state machine below follows the pattern described in the OPC UA reference manual.

State Machine

Declaring the state machine​

A FiniteStateMachineType is defined by two things, and both are required: the states it can be in, and the transitions between them. Exactly one state carries isInitialState: true.

Temperature control state machine
stateMachines:
  - browseName: TemperatureControlStateMachineType
    subtypeOf: ua:FiniteStateMachineType
    description: |
      Controls a temperature sensor: it initialises, measures, and can be halted
      and resumed.
    # Promoting an inherited optional makes it mandatory on every instance, so a
    # client can always read the current state by name.
    promotedToMandatory:
      - ua:CurrentState.ua:Name
      - ua:LastTransition.ua:TransitionTime
    states:
      - name: Initializing
        isInitialState: true
        value: 0
        description: |
          **Initializing** — the sensor is powering up and self-checking.
      - name: Measuring
        value: 1
        description: |
          **Measuring** — the sensor is acquiring temperature values.
      - name: Halted
        value: 2
        description: |
          **Halted** — acquisition has been stopped and can be resumed.
    transitions:
      - from: Initializing
        to: Measuring
        value: 0
      - from: Measuring
        to: Halted
        value: 1
      - from: Halted
        to: Measuring
        value: 2

Each transition names the state it comes from and the state it goes to. The value is the transition's number in the address space — clients use it to tell transitions apart.

Promote what clients need to read

ua:CurrentState and ua:LastTransition are inherited as optionals. Listing them under promotedToMandatory makes them mandatory on every instance, so a client can always read the current state by name without checking whether the node exists.

Using it on an object type​

A state machine becomes useful once an object type owns one. Declare it as a component whose typeDefinition is the state machine type:

A sensor driven by the state machine
objectTypes:
  - browseName: TemperatureSensorType
    subtypeOf: di:ComponentType
    description: A temperature sensor driven by a state machine.
    components:
      - browseName: ControlState
        typeDefinition: TemperatureControlStateMachineType
        description: The state machine controlling this sensor.

instances:
  - browseName: MyTemperatureSensor
    typeDefinition: TemperatureSensorType
    organizedBy: /ua:Objects

Generating this model produces MyTemperatureSensor with a ControlState component, its three states, and the three transitions between them.

Going further​