Identifying the correct system and actors for use case diagram

Question

I am having trouble correctly identifying the actor and system for a use case diagram.

I am developing sensors that interact with an already existing system, specifically four different sensors. I have tried using the sensors as secondary actors, which would usually make sense but this presents another issue. Actors are considered external and the sensor hardware cannot be described in a block definition diagram using SysML since it is not a part of the system, as far as I know. Am I overthinking?

Instead I was wondering if I should use the sensors as a system, and use the already existing system as a secondary actor? This is for a school project so I have to make sure I document my work the correct way.

Thanks

Answer 1

Yes by definition an actor is external to the system they react with, then all depends on what the system is, this is a matter of perspective

I was wondering if I should use the sensors as a system, and use the already existing system as a secondary actor?

exactly, when the system you describe is your sensors the already existing ones are out of it and they can be represented as actor, and even as primary actor if they activate your use cases.

If you describe the already existing sensors the roles are reversed and your sensors becomes actor

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[edit from your remark]

If the overall system includes the already existing system and your sensors, the already existing system becomes a sub system of the overall system, and your sensors is also a sub system of the overall system.

When you describe your sub system, the sensors from the already existing sub system can be still actor etc, nothing change. A sub system is a system.