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Thread: Simulate what you can test and test what you can simulate

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  1. #1
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    Simulate what you can test and test what you can simulate

    To reinforce the notion of using simulation pre-construction, I stumbled into this paper from a few years back and though some members might want to take a stab at doing some get-real simulation engineering with more than the usual amount of chassis detail. "You mean the tires are not the only compliant parts in a car ? OMG"

    A good project for a novice Excel or Matlab wannabe. Add a bit more complexity and some TTC tire data facts with a better tire model and you might just be ready to build a car good right off the trailer and ready to test.

    https://cecas.clemson.edu/ayalew/Pap...cles/641_1.pdf

  2. #2
    Bill, I thought you might enjoy the below paper as well. Would be relatively easy to get tyre relaxation data from TTC, get Fy and Mz compliance effects in (that data is harder to come by for FSAE teams as it's not on the internet) and do a chirp steer test. Compare the TFs with what you may measure experimentally.

    Okay then, what does a "good" TF look like for yaw, normal accel gains and understeer.

    https://cecas.clemson.edu/ayalew/Pap...ance/755_1.pdf

  3. #3
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    Simulation and Testing

    Thank you for that reference. I'll bite offand chew a few comments:

    1) There is relaxation data available on the TTC, AND an example of it processed posted by me, including it's use in simulations where speed adjustments are made.

    2) The only real compliances applicable to a FSAE car are probably steer and camber related, more so front effects (front steering).

    3) The TTC relaxation tests are representative of the steered (By a steering mechanism). This is the Control Input. The unsteered tires (as is likely rears) don't have the same responses as they are plunged into a turn because of the vehicle's yaw and sideslip response and not steered by the driver.

    4) Note that the steered tires Mz response is the most interesting because of it's effect on compliance AND on the rigid body yawrate transient. A feeling driver ought to e able to sense this torque in the steering wheel if the steering mechanism is good and the drivers hands and fingers aren't welded to the rim.

    5) Experimental methods CAN be used to ascertain transient tire characteristics in the long range view. Given a chirp or square wave pulsed
    steer input to the car AND a Cornering Compliance based set of transfer functions (Cornering Compliances as in there's more than just some
    tire compliance in the vehicle), a Optimizer can be coaxed into producing the cornering compliances that mimic the road test results
    (Yaw Velocity and Sideslip gains and steering sensitivity (Ay by SWA). However, if you don't include a tire relaxation term, the solution
    will not converge. This is because there will be a phase error that can not be closed. An extra velocity term is necessary for both gain and phase
    characteristics to be matched. This will be the lumped tire relaxation effect.

    6) Understeer is a derivative, not a difference variable. I have seen more than one 'vehicle dynamicist' use difference metrics and then get bamboozled by the appearance of real road test results which are not increasing or decreasing functions, but curl back on themselves. This would really mean a severely oversteering vehicle yet it would be published as an understeering condition.
    Limited slip differentials come to mind as players in this nomenclature game.

    I can post a few transfer function views of what the bob & wow FSAE cars might be (best of the best and worst of the worst) if anybody is interested.

  4. #4
    I am aware of a number of teams looking into the relaxation data based on your TTC posts. How far they are towards instituting it into a viable simulation and specification tool, I am unsure.

    Hadn't thought much about the rear transient development too much, to be honest. Will need to give that some cycles in my thought process at some stage. Agreed, that their definition of US is not particularly satisfactory.

    I know there is at least one soul interested in your BOB and WOW approximations are for FSAE vehicles.

  5. #5
    I assume, if I had good confidence in my design-time parameters, the difference between the steady-state simulation and the test result (if I had any) could be attributed to some sort of compliance. How would I pick my understeer target in the first place?

  6. #6
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    Chassis design specs.

    Quote Originally Posted by turtle View Post
    I assume, if I had good confidence in my design-time parameters, the difference between the steady-state simulation and the test result (if I had any) could be attributed to some sort of compliance. How would I pick my understeer target in the first place?
    That's a very good question. Since FSAE cars don't run at break-neck speeds, one of the two principle advantages of an understeering car is the attenuation of steering gain with increasing speed. (it's speed squared actually). So, you would want to know what the min and max speeds you would be maneuvering with so the drivers ability to control the car with a 'comfortable' steering wheel angle range is established.
    Obviously the steering mechanism ratio factors into this, with an additional constraint of the max tolerable steering wheel rim force. BTW, my own sims of this type of car indicate that just weight/tires, speed, and wheelbase show them to be kinda lazy, (High cornering level/ability, but disappointing transient responses, even with 50/50 weight distribution.

    The second contribution of understeer is to increase the car's natural frequencies. Thus they become more responsive. You would want the response times / bandwidth to be within the driver's own range of perception so that they don't have to wait for a response to develop and thus be tempted to anticipate a sluggish vehicle.

    Since the tires on these cars are so large compared to their design static and dynamic axle loads, getting even a neutral steer car is difficult to obtain unless you do it the smart way instead of the easy way. And, it's not likely that the open loop oversteering car is unmanageable, you just have to control it all the time. Understeer costs you max lateral capability because you don't get to use the max force of both axles.

    If I were running a team, I would build a car, validate it with simple measurements, (constant radius and frequency response tests), then run a matrix of tire properties (size and pressure) and weight distribution and ask several drivers to evaluate the conditions. One who can tell the difference between all conditions is still in the running. The rest wash out,
    Then run lap times on a course which challenges the car and the driver. The correlation is pretty much established as to what your steering and steering torque gains are as well as the response times you can afford.

    The learnings from this are tremendous. That's why you build simple, test as often as possible and train your driver(s). Then you will have good cars on the hauler when you ship them and can enjoy the scenery at the track instead of the taste of grease, skinned knuckles and cold hot dogs.

  7. #7
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    And the answer is: SYNTHESIS

    Quote Originally Posted by turtle View Post
    I assume, if I had good confidence in my design-time parameters, the difference between the steady-state simulation and the test result (if I had any) could be attributed to some sort of compliance. How would I pick my understeer target in the first place?
    You would not pick the understeer as a target for a FSAE car. That would be a constraint, goal or target if you were designing a vehicle with large payload changes or anticipating forseeable misuse by owners replacing tires with aftermarket skins, etc.
    In this case, you would want to pick front and rear cornering compliances to deliver an optimum transient response suitable, acceptable and comfortable to your driver. That means you would choose the system dynamics of your car to work within their skills, reflexes and reactions.

    Here is a simple textbook kind of example of the technique.

    Lets say you have some sort of foundation of your car's architecture: Total weight, weight distribution, and speed range to be encountered.

    Plop them into a synthesis tool which can map out transient responses from hypothetical top tier chassis + tire subsystem specs:
    VHSYNTH.jpg

    Run a play which produces an array of response time vs. front and rear cornering compliance. Lateral acceleration response time is clearly the specific trait correlated to pleaseability. A crude display of this is also shown.
    DFDRTAY.JPG

    Now you need the response time target for your driver and a list of possibilities for one end of the car as far as the tires and K&C recipe:

    Based on cost, availability, performance and tuneability, let's say your team could produce a front cornering compliance (DF) of 3.00 deg/g. Then your requirement for a rear cornering compliance (DR) would be 2.47 deg/g on a 0.30 sec response time requirement, leaving you with an underteer of 0.53 deg/g.

    If your front tires and chassis can produce a DF of 2.00 deg/g (more likely), then your DR could be 2.017 deg/g and a slightly oversteering car. Does this all start to sink in ?
    This changes a bit if your driver has better reactions (let's say .24 response time more likely for Fangio) then your DF of 2.00 deg/g would need a DR of 1.79 deg/g to queue with such a driver.

    Keep in mind there are a few other issues here but the process is the same. Here I chose DF as a starting reference. Most of the time it would be DR, but you get the picture. And there is consideration for what happens as your g levels rise. FSAE car tires are so large, that their properties really don't change that much
    over the range of useage as I see. Wild guesses as to great nonlinearity are from wild imaginations. That's why the TTC is necessary.

    The next drill down would be to evaluate all the tire data to find tire properties that can deliver your DF when you add some sort of steering system. That's not always easy because of tire availability, steering mechanism limitations and packaging, etc. Finally, given the gain of the car in the speed range you are working with, you choose a steering ratio that minimizes hand movement and broken arms and shoulders from excessive steering torque. (did someone mention power steering for low speed maneuverability ???)

    Let me also remind you that as you reduce the steer ratio, reactions loads go up big time, so steering compliances get jacked up. It's very possible (and common) for a 'quicker' gear to produce a 'slower' car because the increased compliance slows the car's gain down faster than the quicker gear speeds it up. Sheet Happens !

  8. #8
    It seems to be a ghost town here, but there is a lot of useful information to digest. I will just go right ahead and ask more questions even if I don't fully understand everything just yet. Maybe some homework is in order

    Quote Originally Posted by DougMilliken
    You might take a look through SAE 205A "Motions of Skidding Automobiles", Radt & Milliken. 1960. If you can't get the paper from your library or SAE, let me know.
    Looks like the library archive goes to 1967 on microfiche (!!). The paper preview starts with a presentation of the model though I would be interested in what is presented after the model.

    Quote Originally Posted by MCoach
    While running these tests, if you place an O-ring on your damper shaft and push it up to the face of your body, you can that to find your max damper travel during each event.
    This is a very clever trick that I've got to try now.

    Quote Originally Posted by BillCobb View Post
    You would not pick the understeer as a target for a FSAE car. That would be a constraint, goal or target if you were designing a vehicle with large payload changes or anticipating forseeable misuse by owners replacing tires with aftermarket skins, etc.
    In this case, you would want to pick front and rear cornering compliances to deliver an optimum transient response suitable, acceptable and comfortable to your driver. That means you would choose the system dynamics of your car to work within their skills, reflexes and reactions.
    Wow! Lots of information to break down here. I understand the motivation of trying to design an FSAE for 'optimum' transient response. What I do not understand is how you design for variation in your vehicle model like in the road vehicle case you mentioned. Is there any reason to consider something like payload variation (heavier driver??) for an FSAE? And if so, how do you 'design' for these variations?

    Returning to the theme of simulation/testing, let's go back to the scenario of being broke without a fancy data acquisition system. Synthesis is one part of the puzzle, but what about characterization? Is there a simple way to invert this process from some sort or road test information to derive how far off the car was from it's design targets? I know it's discussed a bit in the other threads you have but I might as well bring it up.

    Another piece of the puzzle is finding the right driver. As much as I would like to think that I am a karting champion, the reality is that many of us are not the greatest driver. How can the transient response specs be selected early in the process such that the vehicle design accommodates the wide range of amateur drivers that I have to work with?

  9. #9
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    Ghost Buster

    It's a ghost town because so few members know what 'simulation' actually is. They confuse simulation with 'tools' . In that case, my best simulations here on the Farm are my Milwaukee battery impact driver and a 1/2" (13mm) box wrench. The catch all is a pair (2) of vice grips and a large flat head screw driver for mounting tires and fixing flats. These are my highly correlated simulations. I get good lap times around the fields with good tires.

    In fact, this FORUM is a TOOL, thus, I'd guess its a simulation !

    Meanwhile, 'Design Variation' usually involves 'Build Variation' because of all the slotted holes teams think they need. Instead, a well done suspension parameter modeling TOOL which include not only geometry but compliant bodies is used to produce a variation analysis. You fire thousands of random misbuilds at the 'tool' (Monte Carlo Analysis) and recover the SDF's (Suspension Design Factors). That include ride/Roll steer, compliance steer and camber, roll centers (force method), asymmetries, etc.
    This is then analyzed in bulk to identify the points and member elements with the highest participation is misbuild conditions. THOSE condition get the most emphasis during construction and part selection. You ought then to be able to 'net build'. You don't need all those drill holes that show only that you have no idea what you are doing, just wandering around, lost in the uncertainty forest. That's Monte Python Analysis. An alternative to the Monte Carlo technique is the BOB and WOW process. Best of the Best and Worst of the Worst. Build boxes around values for your key design parameters and evaluate your System Model with the 4 values in each position.

    For testing, search elsewhere in the forum for a "Characteristic Speed Test" procedure I submitted. Knowledgeable readers may recall that I measured the understeer of my speed boat. It's understeering with a 3 blade propeller, neutral with a 4 blade and oversteering with a 5 bladed Mercury Hi-Five ported prop.

    The right driver ? Eliminate the wrong driver(s). Its a show-me contest usually involving several cars: good, bad and ugly. A good driver produces the best times, saves the car parts (and tires), is smooth (smoove), and can tell you why. SCCA Autocross might be a good starting qualification, good technical background, a good balance of left and right brain functions (need the haptic elements). has had early childhood exposure to more than 0.1 g lateral force, and has not had severe head injuries from crashing their hot air balloon.

    That is all.

  10. #10
    I guess it comes down picking the combination of front and rear cornering compliances I can afford to have then.

    Speaking of simple measurements - if I had nothing in terms of instrumentation, what would be the single most valuable test I could do with parts off the shelf? EDIT: Bonus - and what would be the simplest simulation I could use to correlate/validate/study the test results with?

    Curious what an unstable driver could do with an unstable car, assuming it would create a closed-loop stable system.
    Last edited by turtle; 05-14-2017 at 12:48 AM.

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