How to handle stream of inputs and generate output based on input combination in UML State machine diagram

Question

Following is a safety controller with input and output Condition given below for designing a state machine: Here SignalOk, SignalWeak and SignalLost are measurements signal quality of steering angle. SteeringAngle signal itself contains the original steering data. In case of 3 consecutive SignalOk, system controller will output ValidSignal with the steering angle data. In other cases, signal will be considered as CorrputSignal. I am using UML 2 state charts(Harel charts). This is so far what I have done:

N.B.:Parallel states and broadcasting is not supported yet, but nested states are supported. I don't know how to model this stream of inputs in state machine, any kind of help will be appreciated.

Answer 1

First I would recommend renaming the states, so that they don't resemble actions. I suggest to name them First Ok received, Second Ok received and Ok confirmed.

Since the SteeringAngle shall be ignored the first two times, the only transition triggered by it should be an internal transition in Ok confirmed. This transition will also invoke ValidSignal.

Nothing is specified about the order of SteeringAngle and SignalOk. Therefore, SteeringAngle should be deferred in Second Ok received. This way, even it it comes first, it will stay in the event pool.

Any reception of SignalWeak or SignalLost should return to Ready. You could do this with a local transition of Operational to Ready.

One additional recommendation: Define an Initial state in Operational and target the SystemOk transition to Operational. The effect is the same, but it results in a better separation of the two top level states.