Re: [Jsbsim-devel] Matlab-JSBSim Interface

[Philippose R. <sa...@ro...>]

Hello Guys,
   Thanks a lot Wysong, Tim and Jon for the
stimulating discussion, the ideas and the suggestions
regarding the Matlab Interface!

   First...Wysong...well...my intention right from the
beginning..was to try and come up with a
"Co-simulation" interface between JSBSim and Simulink,
because my base intention (really long term :-)!) is
to work on a UAV based on a model helicopter. For
this, I wanted to run the control systems design on
Simulink and use JSBSim for the Flight dynamics part
of it.

   Tim, interesting to have a GaTech Aero dude in the
group :-) I must confess...this whole fascination with
UAVs and helicopters goes back atleast 7 years when I
first saw the UAV Competition website (in which GaTech
seems to always lead :-) With the Yamaha helicopter
(GTar?)) Its only now that I have gotten the
opportunity, the resources...and JSBSim to actually do
something tangible. (Well...got rejected for a PhD
from GaTech last year :-)!)

   So coming back....I dont know if any of you have
used a software called "Simpack"...its used mainly for
Railway simulation (My Master thesis was about Robust
ABS brake control systems for trains using QFT for
frequency domain and LQR for time domain)...Simpack
has a Co-Simulation interface which runs over
sockets...its basically an S-Function which takes as
input the different inputs you want to send to your
model...and gives as output the output variable from
the model. This is what I am aiming at reaching.

  As for using an "openloop" assumption that the two
will be automatically synchronised by using Blocking
mode will lead to trouble...the reasons I see for this
are:

   1. You cannot perfectly synchronise the start of
the simulink simulation and the JSBSim
Simulation...JSBSim needs some time before it starts
putting out values. And since you need to drive the
JSBSim simulation via Simulink...Simulink will
start-up faster.

   2. 120Hz is 0.0083333333.....seconds...you will end
up with a drift due to differences in sample steps
unless you can specify the time step upto exactly the
sae number of decimal points as in JSBSim.

   3. You cant really run a normal simulink model in
real-time...unless you use Realtime workshop to
generate code and compile
it....etc...etc....basically...if you choose a fixed
time step simulation mode and press "play"...Simulink
does not simulate in real-time. On the other
hand...JSBSim I assume simulates in real time when the
"--realtime" switch is provided. So you will end up
with a confused simulation system :-)!

   Becuase of all this, and because JSBSim is more
felxible than Simulink, I thought we could probably
drive the JSBSim clock from Simulink by sending the
current simulation time along with the data sent out
from Simulink...and then JSBSim simulates the current
time step...and gives back the results.

   As for a union...thats something I hadnt thought of
:-) So...shall try to look into that option
too...thanks Wysong :-)! Only problem...in the windows
socket "recv" command...the incoming data comes in as
a character array pointer...and in the "send"
command...the outgoing data is also a "char
*"...doesnt that mean that you are restricted to send
on character arrays? As in...for the network
layer...there is only one datatype...the "char".

   And Jon...I had mentioned that I wanted to ask you
something specific about the sockets....shall put that
into a second mail..I think this is already too long
:-)!

   Driving went quite well today :-)! Just...the
Germans seem to be a little more agressive than me :-)
Places where I would feel most comfortable at around
65km/h..I had to hold the steering wheel tight and go
to over 85 :-)!! 

   Enjoy the rest of the day :-)!

   And...keep the ideas flowing :-)!

Regards,

Philippose


  

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