Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sensor in car to calculate weight distribution?

  1. Jan 14, 2013 #1

    I’m currently working on a project in which I need to know the weight distribution in the four tires of a car at all times. I was thinking to install strain gauges somewhere in the chassis, or displacement sensors by the suspension. However I don’t feel these methods will give me accurate results. The other option was to install an accelerometer at the center of mass of the vehicle that would tell me how much it inclines when braking and cornering.
    Please tell me your thoughts on this. Any help is good!

    Thanks in advance

  2. jcsd
  3. Jan 15, 2013 #2

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    i tried that about 20 years ago. Used a long stroke by directional cylinder and a liquid filled pressure gage. Mounted it on the right front of the race car and placed a huge cam corder ( that's all we had back then) so it could record the pressure and where it was relative to the race track...worked half way good but this was before we knew about roll centers , Center of Gravity, mass centroid, polar moments and all the stuff that we should have looked at first before trying to bandaie an ill handling car...
    today i think you can hang some cheap displacement sensors on t he suspension and plumb the data into some type of dat collector and really skull out what's happening..

    anyway..welcome and good luck
  4. Jan 16, 2013 #3
    Are you talking about measuring it directly, or being able to calculate it from other sensors?

    Anything to do with the motion of the body will be horribly inaccurate. You'd need to know far too much design detail about the car to make it work.

    Modern ABS 'knows weight distribution' when the brakes are applied by measuring wheel slip against applied braking force. Not sure how you would do it at all times though...
  5. Jan 17, 2013 #4
    Ranger Mike and Kozy. Thank you for your answers.

    I was thinking that installing a 6 axis accelerometer in the center of mass of the car might not give accurate results. The other option was to install three sensors that would give me the velocity, steering and braking force:
    -Velocity: Pitot tube
    -Steering: Turning sensor in the steering column (+account for power steering)
    -Braking force: Potentiometer installed in the axis of rotation of the brake pedal and map such analog signal to a set of empirical data for braking force.

    The other option was to use the information from the engines traction control and knowing the coefficient of friction with the ground use that information to calculate the force on each tire. (This is similar to what Kozy is saying). However you will only get information when there is slippage.

    If you have any other ideas please let me know! Thanks again
  6. Jan 18, 2013 #5
    Something about your velocity/ steering/ braking force seems like it would be horribly inaccurate. A pitot tube would be affected by wind speed (maybe better to get velocity off the wheels....but even that is inaccurate). Steering and braking force...those are all sensing the inputs. Braking force can and will change depending on conditions, such as wear, weather, and especially heat. Steering just the same....it will vary depending on many conditions.

    I would guess accelerometers and/ or a gps would be better suited for calculating speed/ braking/ acceleration. But even then you would have to accurately know many car parameters, which would be subject to change. Everything would be thrown off depending on how much fuel you have or what you had for lunch.

    The most accurate way would probably be to somehow place force sensors between each shock and mount. I don't know how difficult that would be though.

    Or maybe figure out the spring constants at each corner, and install a sensor to measure the spring compressions?
  7. Jan 18, 2013 #6
    Lsos first of all thanks for your input.

    The results don’t have to be extremely accurate. Remember that we are going to use them to drive a dynamic spoiler that can only move within a certain range. What we are trying to do is use the three parameters described before (velocity, braking force and steering angle) to calculate a variable X. This variable can be either positive or negative, depending on whether the driver is steering to the right or left, and it will be mapped to an array of data that will represent the displacement of the actuators. (from 0” to 12” or 18”). Also the actuators will update their positions around 10 times per second.

    The velocity we have decided to obtain from the ECU, the braking force will be calculated placing a rotating potentiometer by the braking pedal, and the steering will be calculated by either a rotating potentiometer around the steering column or an accelerometer on the mass center of the car.
    Thanks again to everyone for their inputs and let me know what you think of this update.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook