1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

instantaneous vehicle speed

Discussion in 'Automotive Electronics' started by elecLear78, Nov 24, 2015.

  1. elecLear78

    elecLear78 Member

    Joined:
    Jul 16, 2013
    Messages:
    227
    Likes:
    1
    Location:
    india
    I am confused with instantaneous vehicle speed and average vehicle speed. Instantaneous vehicle speed gives a speed at a particular instant of time. Now my doubt is what is this instant of time? In the sense if a speed sensor is connected to the vehicle to know the speed you have to read number of pulses say for 1 sec to determine the speed. So, if you say instant is it at the beginning or at the end of one second. And the pulses should be read for how much duration is it 1sec or 1 m sec?
     
  2. Misterbenn

    Misterbenn Active Member

    Joined:
    May 30, 2005
    Messages:
    510
    Likes:
    28
    Location:
    Birmingham, UK
    The speed measured in your example would be the average over 1s, for example the car could be going 100mph for 0.1s and then 0mph for the remaining 0.9s but your sensor would give you an average speed of 10mph. However this clearly isn't physically possible the inertia of the car is too great to change that quickly and i imagine most sensors probably measure much quicker.

    Als0 I would imagine most sensors would count the time it takes to receive X number of pulses so the quicker you are going the smaller your 'instant'

    Again i suspect you would count a number of pulses and then use the time recorded, but i might be wrong.
    If you were counting pulses over an amount of time then your resolution would depend very much on the sample time. for example if you only produce one pulse per time sample per 10mph then at 5mph you would measure 0mph followed by 10mph followed by 0mph and you would have to average this to get 5mph.
    But if you have a 10X longer sample time to get 10 pulses per time sample per 10mph, then at 5mph you would get 5 pulses every time and you would measure 5mph.
    So in answer to your question it depends on the pulses/revolution that your sensor puts out and the resolution you want to achieve.
     
  3. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,139
    Likes:
    566
    Location:
    AZ 86334
    Imagine a magnet glued to the drive shaft with a hall-effect sensor that makes two pulses per revolution of the drive shaft. The rear end ratio is 3:1, so you get 6 pulses per revolution of the rear wheel (this is typical of most cars).

    Now imagine measuring the period between two successive pulses, and doing the math to compute vehicle speed from that...

    You can update the computation six times per wheel rotation, or report the speed based on the average of up to hundreds of pulses...
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

    Joined:
    Apr 17, 2007
    Messages:
    7,373
    Likes:
    974
    Location:
    Loveland, CO USA
    ONLINE

    If you really really want instantaneous vehicle speed (and you don't) then you can count the TIME between pulses. In the time for the wheel to turn one turn, or one fraction of a turn, you can know the speed. BUT the number will change too much. So most meter will show the average over 1 second. (maybe 3 seconds) Some meter will take a weighted average over 10 or 100 samples to get a good clean number.

    If you have a digital readout and the number is changing 100 time every second, it is totally unreadable.

    The average speed is usually for the entire trip. Or average from "reset".
     
  6. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,970
    Likes:
    1,099
    Well if you had s(t) or distance as a function of time.

    Numerically differentiating it, v = ds/dt

    In a vehicle, you have a sensor that will pulse every so much distance traveled and you need mph and you need what kind of resolution and what kind of update rate? So, pick a period that works. If it ends up being every second, so be it.
     
  7. elecLear78

    elecLear78 Member

    Joined:
    Jul 16, 2013
    Messages:
    227
    Likes:
    1
    Location:
    india
    So there is nothing like instantaneous it is all average.
     
  8. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,970
    Likes:
    1,099
    For the most part. remember the definition of delta s/delta t in the limit as delta t approaches 0 is your instantaneous speed.

    As long as your speed agrees with the radar gun, laser etc, your fine, right? As long as you didn't put over sized tires on.

    Average is basically reserved for total distance / total time for say a trip.
     
  9. misterT

    misterT Well-Known Member Most Helpful Member

    Joined:
    Apr 19, 2010
    Messages:
    2,697
    Likes:
    368
    Location:
    Finland
    If you accelerate from 0 to 100, then at some point (instantaneous moment) your speed must be exactly 50. Very difficult to measure that exact moment, but it does exist.

     
    • Like Like x 1
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    Pretty well - the measurement is 'distance PER time period' (so miles PER hour, or kilometers PER hour, or metres PER second) - so there's not really any instantaneous reading possibly (as a time period of zero gives infinite speed).
     
  11. misterT

    misterT Well-Known Member Most Helpful Member

    Joined:
    Apr 19, 2010
    Messages:
    2,697
    Likes:
    368
    Location:
    Finland
    But you will also travel 0 meters. 0/0?

    Of course real world measurements are always averages over some time period. But you can't say that there is no thing like instantaneous speed.
     
  12. JimB

    JimB Super Moderator Most Helpful Member

    Joined:
    Sep 11, 2004
    Messages:
    6,392
    Likes:
    593
    Location:
    Peterhead, Scotland
    ONLINE
    Going back to the original question, is this a real world question, or is it some abstract theoretical question totally divorced from practical reality.

    As for time period of zero, that all depends on how familiar you are with the niceties of differential calculus...

    v = ds/dt, as t tends to zero, you get the instantaneous velocity

    JimB
     
  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    Why not? :D

    The actual units used don't even allow for instantaneous values, as it's distance travelled divided by time taken.
     
  14. elecLear78

    elecLear78 Member

    Joined:
    Jul 16, 2013
    Messages:
    227
    Likes:
    1
    Location:
    india
    Can someone please provide me a link to a good video on vehicle speed sensor or explain the principle of its operation, types, its output, location. I am confused how it is generating a pulse output.
     
  15. crutschow

    crutschow Well-Known Member Most Helpful Member

    Joined:
    Mar 14, 2008
    Messages:
    10,623
    Likes:
    479
    Location:
    L.A., USA Zulu -8
    In practice, "instantaneous" is a short enough time period that the vehicle speed changes much less than the resolution of the measurement during the measurement period.

    Speed pulses can be generated by having one or more magnets on the vehicle drive shaft which generate a voltage pulse in an adjacent coil for each time the shaft revolution causes the magnet to pass close to the coil.
     
  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,970
    Likes:
    1,099
    Don't forget that each wheel rotates at a slightly different speed, so what's that?

    Tire sizes are "slightly different", cornering etc. Traction on/off. All sorts of things.

    How are you going to measure? counts / femtosecond? It's still average.
     
  17. crutschow

    crutschow Well-Known Member Most Helpful Member

    Joined:
    Mar 14, 2008
    Messages:
    10,623
    Likes:
    479
    Location:
    L.A., USA Zulu -8
    The best you can do is measure the pulse period, which gives the average for that period of time.
     
  18. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,139
    Likes:
    566
    Location:
    AZ 86334
    I tried to in post #3! Read it again...
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

    Joined:
    Oct 30, 2010
    Messages:
    9,970
    Likes:
    1,099
    In an old car, there was a cable from the transmission to the speedometer. You have the circumference of the wheels and a fixed gear ratio, or at least the transmission knew. So, the very simplest form was a magnetic reed switch. A magnet on some diameter of the cable closing as some increment of distance is traveled. Lets say for argument 1 foot.
     
  20. elecLear78

    elecLear78 Member

    Joined:
    Jul 16, 2013
    Messages:
    227
    Likes:
    1
    Location:
    india
    I am assuming that for a hall affect if it passes through a magnet on the wheel then a logic high is generated and when there is none a logic low . So my doubt is magnet will be of finite width so how does the pulse exactly look like this.

    upload_2015-11-27_20-54-23.png

    But i heard the pulse will be something like this.
    upload_2015-11-27_20-56-30.png

    How is this achieved?
     
  21. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    Easily by passing the pulses through a flip/flop (which halves the frequency) - but there's no need (or point) in a symmetrical pulses, just use the one direct from the sensor.
     
    Last edited: Nov 27, 2015

Share This Page