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Sensors and speeds - Need help with Math

Thread starter #1
Hi, I'm no math guru but I would like to know this math puzzle.

Situation: A LASER on one side and a LASER sensor on the other side. If a car passes and breaks the LASER beam how long will it block the beam if lets say the car is 2.5meters long and running at around 200Mph.

Thank you!
 

ronsimpson

Well-Known Member
Most Helpful Member
#2
2.5meters long and running at around 200Mph
first you have mixed number systems.
Miles per hour and meters. You need to convert meters to feet OR convert Mph to kmph.
Which system do you normally use?
-----------------edited------------
Mph is this meters per hour OR miles per hour?
 

ronsimpson

Well-Known Member
Most Helpful Member
#4
I will make a change so I can do the math in my head.
Change car from 2.5m to 2m long.

2m/200m/hr=0.01hr

Another way to think about it: If the car is 2m long and going 200m/h then there is time to drive 100 cars past in one hour. So the time is 1/100 hour.

2.5/200=.0125hr.

0.0125 X 60 = .75 minutes

.75 X 60 = 45 seconds.

Please test my work.
 
Thread starter #6
I will make a change so I can do the math in my head.
Another way to think about it: If the car is 2m long and going 200m/h then there is time to drive 100 cars past in one hour. So the time is 1/100 hour.
You lost me there! Haha

Don't worry I'll digest and test. Thank you so much ronsimpson!
 
Thread starter #9
Hello Ratchit, thanks for helping. 0.028 sec sounds realistic enough. I was worried that a car too fast won't get detected, well we have none here that could go 200miph. Sensor threshold is around 0.0005 sec average.
 

Ratchit

Well-Known Member
#11
Where did you get 200 miles/hr?
Very confusing?
I looked up Mph as stated in post #1 by the OP in http://www.acronymfinder.com/MPH.html , and found it means miles/hour.

Post #1= 200Mph
Post #3=300kmph. I could swear it said 300mph. "metric"

If I misread post #3 then 1/1000 of the time I said.

(2.5m/300,000M/hr) X (3600sec/hr) = 0.03 sec
No matter what speed values and units the OP said or meant, the method I showed uses units cancellation to give traceable calculation results no matter how the distance or speed are described.

Ratch
 

spec

Well-Known Member
Most Helpful Member
#12
... I was worried that a car too fast won't get detected, well we have none here that could go 200mph. Sensor threshold is around 0.0005 sec average.
Hy BBM,

In case you are interested for future reference, here are the important characteristics of the optical sensor that are relevant to your drag racing application.
(1) On sensor sensitivity. This is the minimum amount of light that must fall on the detector to trigger it.
(2) Off sensitivity. This is the maximum amount of light that will de-trigger the detector.
(3) On response time. This is the length of time between the light first falling on the detector and the detector triggering
(4) Off response time. This is the length of time between the light stopping falling on the detector and the detector de-triggering.

In terms of drag racing, the two critical parameters are, (2) and (4) above. (2) to ensure the detector actually de-triggers and (4) to ensure time accuracy.

I wonder what sort of overall time accuracy you need?

spec
 
Last edited:

ronsimpson

Well-Known Member
Most Helpful Member
#13
Years ago we made a drag race car timer. (street racing mostly) How to tell then the car starts/ends a run?
I used a Mercury tilt switch. When the car lunches forward the timer starts, and when the gas is released (stop acceleration) the timer stops. Not real accurate but good enough for street racing.
Years ago=back when Mercury did not kill people and you could get tilt switches.
 

MrAl

Well-Known Member
Most Helpful Member
#14
Hi,

Regardless what you get in time, i would say that a pin diode would catch both the front and back of the car. They are pretty fast.

If you work in ALL the same units the calculation is much much easier. For example, at 300kmph and a car 3m long, we have:
300k=300000m first convert to same units as the car, then:
3/300000=1/100000 hr

then multiply by 3600 to get into seconds because there are 3600 seconds in one hour:
time between rise and fall of sensor = 3600/100000 = 0.036 seconds

See how easy that can be :)

One thing to remember is that if you are doing multiple cars and they are all different, you'll have to know the length of each car where the length is the length that the sensor sees which may be from bumper to bumper or from the front of the hood to the back of the tail lights or similar.
 
Thread starter #15
Hi everyone, I know this is an old thread but I just wanted to give closure to it.

The purpose of the calculation I was trying to do is to find the speed of the car in the finish line. So what I was planning is to calculate the difference as the front tires hit the laser beam and the as the rear tires hit the laser beam again. But it was ditched because it wouldn't make sense. The final version I did is placed a sensor 66ft before the finish line and calculate the time difference between the two sensors (in actual pro racing this is called Trap Speed). And it is working awesomely.

As per the worries about sensors response/delay, I followed Compulink's way of calibrating their timer. Compulink is the undisputed official timing system in all of america's NHRA race tracks.

Since sensors and beams have delays, even Compulink's have delays (despite being super high-end), they calibrate it using the beams and sensors on both lanes. Hardware specs are the same with the same delays and response time. So what they do is tie-up the signal input ports of the sensors for both lanes and run a car on only one lane. You should get an equal time for both lanes upto 10,000th of a sec (threshold is 0.0003sec difference). Then repeat the process by running another car on the other lane, result should be the same. This way you can accurately say that if both cars hit the finish line on exactly the same time, you'll get a tie.

Now, if the starting line sensors have exactly the same respone time on the finish line sensors, then the response delay is cancelled and you'll get the accurate time. And since accuracy is upto 10,000th of a sec, and you only need upto the hundredths of a sec, its pretty much accurate (races are only timed upto the hundredth, and will only use the thousandth incase of a tie). Note that top fuel times are upto 1,000th of a sec and 10,000th incase of a tie. And a tie is so rare that it only happened here (well not really a tie):


So in the end, our timer works well and see it in action here:


Please no hate on the video, our cars run only upto 250kph (barely). We are no top fuel.

Thank you all for the love and time.

Biboy
 
Last edited:

MrAl

Well-Known Member
Most Helpful Member
#16
Hi,

Havent gone to the drag races in years now, used to go when i was younger with some friends.
Not even sure if the race track is there anymore :)
Just checked, yeah it is still there in Englishtown, NJ.
 

ci139

Active Member
#17
Hello Ratchit, thanks for helping. 0.028 sec sounds realistic enough. I was worried that a car too fast won't get detected, well we have none here that could go 200miph. Sensor threshold is around 0.0005 sec average.
you can always set up double or more at a fixed distances and read all break beam times
interleave Δx = 1m (a meter) average speed at that meter is v = Δx/Δt = -1m/(t."1-st break beam" "minus" time."second break beam")
200mph = [in=.0254,feet = 12 in = .3048m] = should be 321.87 km/h (not what they dimension it) = [so using us survey mile = 5280 ?survey?ft!!! = [i'm using my feet] = 1609.344m] = 200mph·1.609344km/(us survey mile derived from ci139's feet) = 321.8688km/h = v*10³/3600s = 89.408m/s
. . .
so to pass 1m takes in average Δt=Δx/v=1m/89.408m/s=0.011184681 s = 11184.68146 µs (microseconds) to get that @ 6 decimal precision the clock must be quanted at least to Δt/(10^7)=1.118468146ns (nanoseconds) e.g apx. a GHz clock if you average at 10m run you pass with 100MHz accuracy
(further on loosening ranges . . .) if you don't need 6 decimals but the 5 will do the 10MHz clock will do (there will be random error extra to that other than mathematical)
 

gary350

Well-Known Member
#18
Hi, I'm no math guru but I would like to know this math puzzle.

Situation: A LASER on one side and a LASER sensor on the other side. If a car passes and breaks the LASER beam how long will it block the beam if lets say the car is 2.5meters long and running at around 200Mph.

Thank you!
At 60 mph a car will travel 1 mile in 1 minutes.

200mph/60=3.33333333 times faster.

5280/3.3333333333= 1584 ft per minutes

1584/60= 26.4 ft per second = 316.8 inches

1 meter = 39.370 inches

2.5 meters= 98.425 inches

316.8 / 98.425 = 3.218 seconds is the answer.
 

JimB

Super Moderator
Most Helpful Member
#20
Is there a reason why this 6th grade math problem suddenly became relevant again?
I had wondered that myself, however looking at the latest calculation where a car which is 2.5 metres long and moving at 200mph, takes 3.2 seconds to pass a point, the answer has to be...

...time dilation.

There seems to be some Relativity thing going on.

JimB
 

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