SparkFun Forums

[fkatzenb]

I kind of expected the answers you just gave, which is appeciated. I knew my ideas, were half complete as they are based on just watching some of my coworks and fellow engineers do structures and fatique of our flight test aircraft. I do have access to matlab.

For my case, we are purely looking at shock designs (custom in development ones... side project for as I am an electrical guy with tons of aircraft testing experience and a huge love for all things mechanical from racing to engine building which I do.)

I had thought of linear transducers for this instead. I found 5" travel plunger style pots which when mounted near the pivit point of the control arms would provide enough resolution (with the known suspension geometry), we could sample them all at the same time and then just use time to determine suspension accel and deaccels for determining shock survivability. However the part that really matters is while we could determine shock travel and stuff, we can't differentiate from bumps to surface contact to determine if we are faster and hold more lateral grip thru a turn of uneven surfaces. Yes we have start stop times, however its not like a road track where you are more consistant in driving because a lap may only take 1-2 mintues and the surface doesnt change.

Here the variables are huge, so we have to look at things differently.


Frank

[ohararp]

Frank, I did mostly turbine engine related vibe issues. I forgot to mention that for traction you may want to place (encoders-ABS tone rings would work) on each wheel and at the drive shaft for traction analysis. Looking a wheel slip relative to the driveshaft could help isolate the problem there.

As far as testing goes its all about the test plan. You may want to look at a design of experiments type approach for a deterministic view of what's going on. Overall though i'd recommend not just random sampling out in the desert but prescribed tests (quarter mile, single bump, whoops, g-pad) and draw from that.

Hope that helps. Sounds like everything you have been through before.

[fkatzenb]

Did you see the filter thread? Does that make things more feasible? http://www.sparkfun.com/cgi-bin/phpbb/v ... php?t=4273


Yep, been there done that, but just not on cars. I know they wanted to go to a test site with the speed bumps, offset bumps, so that is no problem whether using wheel encoders with lvdts OR an accelerameter... things are alot more controlled, etc. So really it sounds like we shouldnt bother trying to datalog their Baja run as there is no comparison data... just g impact forces for failure point study. I wish I could just swip one of the aircraft's EGI/IMU units for use.

On a second note, the guy who has set me up with helping these shock guys get data wants to do something similar. He has a gyro we are going to start test flights on with a never before used motor. It is a pretty sweet engine setup and has an awesome torque curve. Anyway, we need to establish baseline accerations at various density altitudes and fuel loads. In of itself is easy, however gathering the data we were attempting to use accelerometers for as that is the only thing we can do. Because secondly is he wants to try different prop configurations and do the same testing to look for any improvements in acceleration and cruise conditions. The engine logging stuff is already there and it even records baro alt and OAT. All we would have to do then is no weight with and without fuel and record fuel flow or fuel quantities to know our weight at all times. What would you recommend?



Frank

[ohararp]

Frank I always recommend keeping the raw voltages (or counts if you are using a coltage reference) and then post processing them in matlab. By doing this you can always recover your raw data and try a different filter. Generally software filters are one way (like a diode) and don't allow you to recreate the test. Trust me the gov't has wasted alot of money when some "smart" test engineer hasn't done this.

If you are looking at accel rates couldn't you just use a pitot tube and monitor velocity this way? I have seen some "drag strip" performance evaluators that use rapid gps updating to determine stuff. I think they were from a German company. They might be 10 hz output types. They were a little pricey at $1000. This would probably work in the air as well as on the ground.

[fkatzenb]

ya i thought of pitot... undecided on that. For the gyro, I was thinking on my drive to work, that really I just need a high rate GPS and a vertical accel. For that we are just looking for level accels, but since its a gyro we have that moment about the disk since the thrust line is lower then the plain of lift, so we will start to climb some.

As for the vehicle using the filters with the accel, good point on using a soft filter by feeding the data into a Matlab process so we always have the raw data and can make changes, etc.

Even if we end up not doing any of this, I was thinking about purchasing it anyway, and just having some fun working with filters and such.


Frank

PS> Thanks for the conversation!

[NateW]

You mentioned testing with the engine running - My guess is that the signals you're looking for are much lower-frequency than the noise from the engine, so it seems to me that it would be useful to experiment with a few different mounting schemes to see what you can do to soak up vibrations. (In other words, implement low-pass filters in hardware.) RC helicopters use gyros and while engine vibration does impact performance, a little bit of foam helps quite a bit. The soft foam used for sealing gaps around windows worked well.