[Rtai] Small control system proyect.

[Paolo Mantegazza]

Paolo Mantegazza wrote:
> Robert Perez wrote:
>> Apart from shm functions I believe that no more lib functions are 
>> called (but perhaps I will need some later).
>>
>> If I understood well the docs of RTAI I must not call linux functions 
>> from control programs to maintain real time.
>>
>>  
>>
>> The idea I have is: master program set a flag in shm of control_prog1 
>> to start an execution cycle and then master goes to sleep for 20ms.
>>
>> control_prog1 awakes, executes a cycle then awakes the master to tell 
>> it that it has finished and then goes to sleep.
>>
>> Master awakes, takes out the permissor from control_prog1 and gives it 
>> to control_prog2 and wakes up that process and then goes to sleep again.
>>
>> That is the main loop of the processes involved unles a control_prog 
>> don't finish the cycle in 20ms, then the master awakes and send the 
>> overruned
>> control_prog to sleep even if it didn't finish the cycle.
>>
>> When the turn comes again for that cprog the master wakes it up and 
>> the cprog continues from the same place it was inside the cycle.
>>
>>  
>>
>> I think that to wake up the master module I could use some semaphore 
>> or futex (I'm reading docs on how to use them), but I don't
>>
>> know if I can from the master, wake another RT_TASK.
>>
>> I have seen that we could use names for RT processes and then access 
>> them by name.
>>
>> Do you think this is a good way ?
>>
> 
> All of what you want to do can be done in strict real timer under RTAI. 
> I see some kind of sequential states driven execution. For it I'd use 
> just intertask messages. Since the master wait for an answer from 
> control progs the latter and the master could use rt_receive, the master 
> does it with a timeout. When the ctask finishes it rt_send the master 
> and rt_receive for the next cycle. An overrun condition can than be 
> handled with RTAI real time signals or simply using rt_task_suspend. In 
> the latter case when it is time to resume the overruner again the master 
> uses rt_task_resume and goes to rt_receive with timout again, and so on 
> and so on. Notice there is no need of shm and it can be done between 
> processes and threads alike.
> 
> Paolo.
> 

BTW, I forgot to mention that all of what above can be done both locally 
and remotely, UDP at the moment. Since I saw at your links that you are 
talking about distributed controllers such a feature might turn out to 
be quite useful. The same code can work in whatever way and needs just a 
configuration launch scripts, while with shm you are bound to work locally.

Paolo

>>
>> Thanks in advance.
>>
>>  
>>
>>  
>>> Date: Fri, 8 May 2009 08:58:57 +0200
>>> From: mantegazza at aero.polimi.it
>>> To: robertperez67 at hotmail.com
>>> CC: rtai at rtai.org
>>> Subject: Re: [Rtai] Small control system proyect.
>>>
>>> Robert Perez wrote:
>>>> Hi all.
>>>>
>>>>
>>>>
>>>> I want to ask if it's possible to use linux + RTAI for the following 
>>>> scenario:
>>>>
>>>>
>>>>
>>>> I want one (master) process to allow other (child) processes to run 
>>>> in an ordered fashion (by writing an enable bit in shm)
>>>>
>>>> In addition this (master) process should be able to put a child 
>>>> running process to sleep if a
>>>> certain condition is true and then awake the next one and so on.
>>>>
>>>>
>>>>
>>>> Of course all of these should be RT processes.
>>>>
>>>> This is for a hobby control system proyect I'm planning.
>>>>
>>>> Also the child processes should be able to talk through shm.
>>>>
>>>>
>>>>
>>>> I got it working part of it in linux but I will need real time to 
>>>> make it a serious project.
>>>>
>>>> I am considering moving to RTAI but I want to know if its possible 
>>>> before reading lots of documentation.
>>>>
>>>> BTW it is coded in C++ and I don't know if C++ is supported in RTAI.
>>>>
>>>>
>>> A few things you can see here: 
>>> http://www.aero.polimi.it/mbdyn/mbdyn-rt/index.html, is in C++. It is 
>>> just a matter of initializing and staticalize everything in advance 
>>> or through a server process.
>>>
>>>> Could somebody please tell me what functions should I use, or an 
>>>> idea on how to implement it ?
>>>>
>>> SHM a part what function calls do you use to have it work with Linux?
>>>
>>> Paolo
>>>
>>>>
>>>> Thank you all and keep the good work!.
>>>>
>>>>
>>>>
>>>> Robert Perez.
>>>>
>>>> PD: for those interested, the humble proyect is here 
>>>> http://sourceforge.net/projects/freedcs/ and website 
>>>> http://freedcs.sourceforge.net/
>>>>
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