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Thread: Wheel Speed Sensor Setup (for TCS)

  1. #1
    We're setting up a traction control system on our car, and are trying to determine the best way in supplying the front wheel speeds to the ECU. There is a speed sensor on each wheel supplying pulse outputs, and our ECU is a Dragonfire, which is a first off prototype, where the traction control function has not been used before.

    Our problem is that we're not sure whether to supply the ECU with the average front wheel speed, or the fastest, or slowest, or what? Whcih option have people found works best? How do commercial TCS do this? We're using a spool at the back, so rear wheel speed pickup is not an issue.

    Thanks in advance.

  2. #2
    I would use fastest front wheel speed. Your front wheels aren't being driven, i'm assuming, so the fastest front wheel speed should show actually vehicle velocity.
    Erich Ohlde
    Jayhawk Motorsports
    FSAE 04 - 09

    All electrical components and wiring harnesses depend on proper circuit functioning, which is the transmission of charged ions by retention of the visible spectral manifestation known as "smoke". Smo

  3. #3
    There is a speed sensor on each wheel supplying pulse outputs, and our ECU is a Dragonfire, which is a first off prototype, where the traction control function has not been used before.

    National semiconductor makes a frequency to voltage converter that will accept the wheel speed frequency input and output a voltage proportional to that. At the back of the LM2917 datasheet, there are three circuits that describe how to do the three functions you described, if you already haven't figure that out. I have not implemented a TCS, but I would venture to guess that the critical speed would be the high speed wheel. If you are in a turn, and the weight transfer puts more load on the outside tire, which is turning faster, that it would be more important than the slower and unloaded wheel. However, the unloaded wheel is still providing grip, so the average may be better. I dunno? If you wanted to try all three, slowest, fastest, and average, you could use a three position switch/analog mux to select the function you want to test.

    DataSheet Link: LM2917
    UNM FSAE 2003 to 2005

  4. #4
    Axle speed is what you really want, you can do it with front wheel speeds, a steering pot, and a math channel.

    In lieu of that, I'd use fastest, as the inside will be more likely to lock.
    mmmm..... Garlic.

  5. #5
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    I am curious to know how many teams have accelerometers ("G-sensors") in their electronics packages? If so, does anybody use these for their traction control. Ie. if the engine/rear wheels are turning faster than the accelerometer says the car should be going, then cut power. An accelerometer built into the ECU box might be easier to do and more reliable than wires going to sensors on the front wheels. It might also be more accurate - a driver resting his foot on the brake pedal might lock the inside wheel (when it is off the ground during corner exit) so that average front wheel speed measured is only half of true speed.

    Accelerometers can also be used to measure yaw rate for "Stability Control". Six accelerometers, suitably positioned, can track all motions of the car.

    Z

  6. #6
    Response time and accuracy (of measuring car speed) make accelerometers a very poor choice for traction control. You need much better resolution than that! If you have both, compare the two-night and day. They do make good means of fine tuning your wheelspeed based algorithms, as you can change gains or targets based on lat or long accel.

    You can write some real simple codes to deal with one wheel lockup and the like.

    tooth-triggered speed sensors are some of the most reliable out there. They are also super cheap. They don't drift, they don't offset. If they break, they generally stop working- they don't give erroneous signals. Brake heat is a minor issue on these cars.
    mmmm..... Garlic.

  7. #7
    However if you are really against wheelspeeds I'd say an optical sensor would not be a bad choice. Although I'd still like to know what my front tires are doing.
    mmmm..... Garlic.

  8. #8
    what happens if an optical sensor gets mud on
    it?
    UNM FSAE 2003 to 2005

  9. #9
    Not sure if I was clear on my previous post, I meant an optical sensor on the chassis, pointed at the ground, to measure overall vehicle speed.

    As for 'mud', well I can't imagine how you'd do that. There are optical sensors that handle rain and dirt just fine. If you are getting into mud you have more problems than traction control.
    Common sense and good mounting, shielding, etc will prevent any issues there... I'm sure teams have run them, as well as IR sensors that also can't be blocked, with no ill effects.

    Of course like I said I think wheelspeeds are still more robust and accurate than optical, just that optical would be better than using an accel.
    mmmm..... Garlic.

  10. #10
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    Garlic,

    My thinking with the accelerometers was that they could be built-in with the ECU, so no connectors, wiring runs with dozens of cable-ties, holes-in-chassis, etc. Also the acc. doesn't have to "see" anything (except "inertial space") so no problems with mud-in-the-eye, or rocks-in-the-teeth . (I'm thinking about their application off-road - where mud and rocks are standard.)

    Do you know, or can you point to, the response time/(in)accuracy/cost of accelerometers? Also, what sort of signal do they put out?

    Z

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