web developer & system programmer

coder . cl

ramblings and thoughts on programming...

hierarchical state machines

published: 29-10-2012 / updated: 29-10-2012
posted in: c, c++, development, programming, projects, proview, tips
by Daniel Molina Wegener

I need to work with a hierarchical state machine. The main problem that I am currently facing is how to pass messages or trigger events on each event transition. Since I am using C Plus Plus, I will use class methods. In this case, I will use a hardware device class method. On each state transition, an event will be thrown, and the given event should trigger an action — in this case a class method — that will complete the state transition. The advantage of the hierarchical state machine is the fact that you can manage composite hardware devices without problems, because each sub-device can be handled individually with its own state, but increases the complexity of the program.

On this case, I will iterate over the list of units on the scheduler — where each unit is a composite device — it will assign a new work state on each device, calling the run method on the device object. The run method will launch the child state machine of the required action, but the parent state machine only has few states storing the device status, so the processing scheduler knows which event should be triggered on each scan cycle. The required state transitions on the parent state machine, which are informational rather than hardware related, will be useful handling nested event handling.

Each cycle will determine whether read or write actions should be triggered, even if they are manual or automatic operations on each device, and only then the program will be able to call read and write once they are required. There are some actions not using the communication channel, so I can avoid calling read and write, enhancing the application performance a little, but still I will need to check the device state to check for some automatic reads, required to display the device state and ModBus holding registers.

You can find several models of hierarchical state machines, but I prefer to write one using events bound to class methods, where each class method drives an action on the device. This approach is simpler than others using signals and similar event handling methods, but I need to keep the source code as simple as possible, because increasing the complexity should increase the development time. Finally, the unit class will be only a gateway to the each hardware device, where each state will be triggered by the scheduler. Before this model, I was using a priority queue using messages, very similar to message queues, but is required on this system, to have direct access to the device state, and that message queue is not so synchronous as it is required, where a direct feedback from the device should be placed on each state transition.

Now I have finished the scheduler and the state machine processor, I just need to complete the integration between each component and I will deliver the new device management application. Also, the HMI has changed a little, and the new model allows some direct actions like resetting the device and its state.

No coments yet.

post a comment

XHTML: You can use these tags: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>