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Thread: Testing, Testing 1,2 ...

  1. #41
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    Steer under or steer over ?

    A good question about the UG you probably ought to determine for the sake of beating up those design judges who are way out of their element.

    Just the facts, Ma'am:

    1) Constant speed procedure. Takes a LOT of real estate to get the job done. This includes getting it up to speed, collect some data and then hitting the binders without smoking the tires. The test can produce a complete profile of a vehicles capability: nonlinear steady state gains and transient response characteristics of lateral acceleration yaw velocity and roll. You would normally want to a assess steer removal responses, too, because getting into a corner ain't the same cigar as asking the tires to straighten you out. Transient responses can get interesting, too. For all the hype about rice cars, these ugly machines tend to produce traits that even Grandma would object to if her Honda acted this way. Either your simulation matches the results or it doesn't. You can't hide behind a "test Variation" clause because these cars ought to be super-duper, but many of them obviously aren't. (just look at some recent pictures posted on Faceplant). Testing at and airport using crossed runways is a Best Practises move. Plus you can run at any speed the car will go and maintain the speed during the cornering segment. Some people cheat by going in hot and scrubbing bubbles to get a constant average speed.

    2) Constant steer procedure: An easy one as long as the steering is nicely designed and assembled. Otherwise you will have some (usually) puzzling steer position effects. No transient metrics, just steady state gains. As some may recall, I ran this procedure on my Bass boat on Woodland Lake a few years ago to look at propeller blade effects. Still a good test to validate a nonlinear computer simulation. Yes there are/can be speed (aero, tire, tractive force) effects, but the results need to pass a laugh test. All you would really need is yawrate and speed to get you a plot of UG vs. Ay out to the "limit".

    3) Constant Radius Test: Known amount of real estate makes it very practical. Requires a high bandwidth driver 'cause its a closed loop test. (that means you need at least a learners permit to get to the test track, much less operate on it. One benefit of this procedure is the determination of the 'Tangent Speed'. I.e. the speed where body sideslip is ZERO. You don't need an expensive sideslip transducer to get a value of ZERO at some speed around the circle (where the vehicles axis is tangent to the test circle). The tangent speed is a great contest and tuning metric. Changes to the front or rear of the car that raises the tangent speed are 'good'. Things that lower it are 'bad'. There are two ways to run the test. One is to slowly increase speed while staying on track. This implies throttle and differential and aero blending but that's the way it goes. The other method uses segments of constant speed runs. Run it in both directions, please. Your simulation will naturally include all these complex interactions for which you will have no data to characterise them.

    4) The constant confusion test is a last resort used in the Design Event where you wave your hands and paw through you many notebooks and logs to explain why a naturally occurring oversteering yet stable vehicle is horribly understeering during the autocross. Say it ain't so even though you memorized RCVD in 3 languages.

    Don't forget about a Frequency Response test ! When you pipe your iPhone into a Bose, you don't compare the sound at one frequency or loudness level, do you ? All the steady state values and transient response stuff (bandwidth, get it ?) are right there in front of you with a few hand typed Matlab statements. AND, the evil effects of nonlinearities (tires and steering) are way overstated and hyped by the non-believers. (It's Fake Vehicle Dynamics theory).

    All these tests are intended to confirm that you have accounted for all the tire and chassis structural modes in the vehicle that you said you have. And what you said you have I'm sure you feel is the bestest, happiest, warmest, amazing, wonderful, awesome, and incredible university-speak that money can buy. But once you produce a test result with a measured UG, the lawyers will be all over you. Just make sure you have the sim that goes along with it.

    Make it so...

  2. #42
    Lets get excited about frequency response.

    So you've got your UG test, but now with the constant speed test we have transient data? Who cares about what the transient test procedure is, as long as there is enough frequency content - time domain, frequency domain, (z-domain? o_O).

    No matter how the system gets identified you hope you get correlation in both time and frequency domains. Simulating and parameter estimation in time domain is conceptually closer to watching the real thing. Frequency domain is ???. You have a preference?

  3. #43
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    The 2 processes are essentially interchangeable. Each one promotes a set of constitutive describing functions based on gain, damping and natural frequency. What makes FSAE cars so special is their VERY low understeer (theoretically) and thus corresponding heavily damped (i.e. sluggish) responses. You can see this is recent videos posted. Actually, the mention and consideration of "damping" (as in 'yaw damping') is almost ridiculous in a FSAE car context because the tires and architecture of them eliminates the traits associated with such metrics. The fact that some cars may have a 'yaw damping' trait means that the car is hurting for some engineering.

    All this boils down to real estate (how much room have you got ?) and what kind of theory do you subscribe to. And if you have the background in solutions to differential equations, a time based formulation of a vehicle's step steer transient response contains 2 gain terms, a damping term, a frequency term and a phase term.

    Note that the interchangability of Time and Frequency domains is demonstrated by the chirp_test and VHSIM simulations posted on my Google drive.

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