Tuesday, June 09, 2015

Simulation

One of the failings of silver ball was that I never got an accurate hardware in the loop simulator running.
On this project I'm not going to make the same error. 

After evaluating a number of possible solutions I've decided to use JSBSim  and I've secured the assistance of the 
Jon Berndt,  Development coordinator and chief architect for the JSBSIM, in getting that done. (I've made an arrangement where he will help me get started for a consulting fee, his time permitting)

One of my questions to him was:

>A details question.... I realize that JSBSim was built to be a flight simulator.
>I'm trying to repurpose it as a rocket simulator....

>Internally you seem to use ECEF coordinates.... I have not dug deep enough to figure out 
>if you model the inertial effects of a rotating earth...
>will one see a performance difference between an equatorial launch and aa polar launch?

>If I have a vehicle attached to the surface of the earth will it show the rotation rates of 0, or will it show the 
>the rotations rates one would see in an Earth centered inertial frame?



His answer greatly increased my confidence that I made the correct selection...
And, yes, JSBSim had a full overhaul of its EOM many years ago for full and complete EOM. It was the only non-NASA sim used in a large check case development effort that is published here: http://nescacademy.nasa.gov/flightsim/

The EOM integration takes place in ECI, full rotation effects are factored in and – yes – you would see the benefit from an equatorial launch compared to a higher-latitude launch. It is a full, vehicle simulator that can model anything from a rocket to a ball to a blimp to a sub/supersonic aircraft.

The vehicle sitting on the launch pad should show no Earth relative motion, but should show a translation and rotation relative to inertial. This was one aspect that was compared across NASA sims and JSBSim in the check case effort, and the comparison effort investigated tiny differences to make sure we were all on the same page and using the same Earth rotation rate. At this stage, I am very confident in JSBSim. That’s one nice thing about open source and broad use: the more people use it in different ways, the better the development effort goes, and the more robust the code becomes.


The simulator in general is designed to be data driven via external XML files. So one can model things like autopilot loops etc...I really want this to be a hardware in loop simulator so one of the modifications I'm asking Jon to help me put my c/c++ code into  the control loop. Eventually this will entail me taking the current state, simulating the IMU and GPS outputs feeding it into flight avionics and then taking the response from the flight avionics and giving it back to the sim to have it operate on the commanded changes.


19 comments:

Anonymous said...

Does JSBSim model Earth as a sphere or as an ellipsoid? The latter could, e.g., allow to see the turning of the orbital plane and better models distances from Earth center.

jccooper said...

I've seen JSBSim mentioned enough on aRocket that this post finally got me to look it up. I read through their (unfinished but) excellent manual and poked at some of the code, and it looks like a good choice. They seem already to have put some thought into rockets, and they mention on the website HITL been done before.

Their architecture is simple enough that once the vehicle itself is set up in their XML, I think it ought not be too hard to integrate your own controller, and/or create a software workalike (which is probably the first step.) Just a matter of creating your own input/output wrappers around the main simulation loop, as they suggest on p80. (It seems you could also communicate via a socket, but that seems unnecessary.)

Do you have a vehicle description already? I'm kind of inclined to try it out. If not, maybe I'll try out the Space Shuttle model they have. (The manual mentions Little Joe, which would be better, but I don't see it included.)

Paul Breed said...

One of the deliverable from Jon is a set of XML files for my conceptual otrag.
When that is done I'll make it available here.

Jon Berndt said...

In answer to anonymous, there are two gravity models. One is a simple inverse square law model, and the other is a simple spherical harmonic model (it's the one described in Stevens and Lewis aircraft simulation and control book). Geodetic position is determined as well as geocentric and ECEF. To jccooper, the manual is a bit outdated, and some of the models in the JSBSim distribution are works in progress (and have been for ten or fifteen years!). The most complete models are probably ones distributed with FlightGear. JSBSim is one of the flight models incorporated into FlightGear (and is the flight model for OpenEaagles and Outerra,as well). For a lot of the models in the JSBSim distribution there was not enough good information to complete those models. I seem to recall that the full shuttle aero model, for instance, is in a couple of books that are each 6 inches thick. The C172 model and the 737 model are pretty well developed. The J246 model is as well, but only for first stage flight and part of second stage. Some of these models were only meant to prove certain concepts in the sim as they were developed, but some are used for academic purposes. Also, some of the most complete models are held by industry and are not releasable - that's one reason we separated code from model. To get some feeling for how JSBSim is being used you could also go to the AIAA web site and search their papers for JSBSim. At last count there were about 40 papers. It is finding a lot of use in the UAV field in recent years.

I also wanted to mention that while I did write a lot of the initial code and set up the initial architecture, there have been a handful of very solid engineers that have made major contributions to JSBSim as members of the development team, and the open source paradigm that JSBSim was developed in also was instrumental in getting bug reports and feature requests. I feel that it's a pretty solid piece of work, now, thanks to the collaborative development environment.

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