# instantaneous vehicle speed

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

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1. ### elecLear78Member

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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. ### MisterbennActive Member

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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. ### MikeMlWell-Known MemberMost Helpful Member

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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...

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5. ### ronsimpsonWell-Known MemberMost Helpful Member

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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. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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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. ### elecLear78Member

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So there is nothing like instantaneous it is all average.

8. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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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. ### misterTWell-Known MemberMost Helpful Member

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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.

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10. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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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. ### misterTWell-Known MemberMost Helpful Member

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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. ### JimBSuper ModeratorMost Helpful Member

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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 GoodwinSuper ModeratorMost Helpful Member

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Why not?

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

14. ### elecLear78Member

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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. ### crutschowWell-Known MemberMost Helpful Member

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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. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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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. ### crutschowWell-Known MemberMost Helpful Member

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The best you can do is measure the pulse period, which gives the average for that period of time.

18. ### MikeMlWell-Known MemberMost Helpful Member

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I tried to in post #3! Read it again...

19. ### KeepItSimpleStupidWell-Known MemberMost Helpful Member

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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. ### elecLear78Member

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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.

But i heard the pulse will be something like this.

How is this achieved?

21. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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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