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EphraimB

Speedometers for use in the Subway

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I want to buy a speedometer or get a speedometer app on my android phone to see how fast my, for example, (A) train is going. Unfortunately, I only see GPS speedometers on the Google Play Store and only see GPS speedometers on Amazon. Is there any way I can buy a speedometer that doesn't use a GPS that I can crazy glue to my phone case?

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Do you know how speedometers or speed measuring devices work?

You should read up on them, then think about your question.

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22 minutes ago, EphraimB said:

I want to buy a speedometer or get a speedometer app on my android phone to see how fast my, for example, (A) train is going. Unfortunately, I only see GPS speedometers on the Google Play Store and only see GPS speedometers on Amazon. Is there any way I can buy a speedometer that doesn't use a GPS that I can crazy glue to my phone case?

I want you to not keep starting new threads, and I want you to take Deucey’s advice. 

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5 hours ago, EphraimB said:

I want to buy a speedometer or get a speedometer app on my android phone to see how fast my, for example, (A) train is going. Unfortunately, I only see GPS speedometers on the Google Play Store and only see GPS speedometers on Amazon. Is there any way I can buy a speedometer that doesn't use a GPS that I can crazy glue to my phone case?

Imagine yourself in a box. Every time the train moves, you would feel the force of acceleration. You have a measuring device that tells you how the force of acceleration. Then, to calculate the speed (because you are inside a box and cannot obtain information on the outside world via means like GPS, you must use the equations for acceleration to calculate the current speed. Of course, you must also take into account varying rates of acceleration.

One problem you will run into soon enough is when the train stops. You’ll notice that the calculations (after accounting for all the accelerating and decelerating forces) leave an error. Maybe your math says that the train is still moving at 1 meters per second, but you do not feel any bumps or rattling that is usually indicative of a moving train. This problem stems from the fact that no matter how precise your measurements, you will have quantization errors. A precise measurement may have 40 digits to the right of the decimal. Most computers only do up to 16 digits. And high school students in physics labs use 2 digits when doing math by hand. This math must also be done frequently (thousands of times per second), because—if say—you sampled every minute, the train may have accelerated and decelerated already, meaning you miss the window of opportunity to capture that data.

Even after all that, the cumulative error from repeatedly adding and multiplying the data will give you a very inaccurate measurement. Only attaching a meter to the train’s wheels and counting the rotations will give you some reliable number since it is simply a count of rotations multiplied by wheel circumference divided by the amount of time elapsed.

Hence, even speedometer apps on a mobile device will opt to use the GPS rather than the phone’s accelerometer.

3 hours ago, LGA Link N train said:

My oh my. If you keep making new threads this forum will explode!!!!! 

So please, keep some of these ideas in the random thoughts thread. 😡😡

You don’t seem to understand the criteria for making a new thread. This one is fairly specific and not related to anything else already posted.

Edited by CenSin
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Not a lot in the way of off-the-shelf tech is gonna give this to you. I wouldn't recommend it, but, You might have some luck pointing a baseball radar gun out the side at the tunnel wall. You'll have to do some math to correct for something called "cosine error" that's a favorite defense of people who get speeding tickets. I'm not sure your fellow passengers or law enforcement officers will understand or appreciate what you're doing, and might have some pointed questions you'd wish to avoid. There are also better ways to spend $200

In terms of new solutions?

Accelerometer data combined with the known-distance between stations should be able to give you pretty reliable speed indications, albeit not realtime. You'd have to write an app that took the time between the stations and the distance between the stations into account to derive an average speed, and then factor in the acceleration curve to derive the speed over that run. Janky, but the known distance would provide a check for the acceleration based speed detection. (fun fact: This is how submarines navigate underwater. their accelerometers are far more precise than the ones in your smartphone, and there's still a lot of drift)

Another idea, pretty wacky. Tunnel lighting is at fairly regular intervals. In theory, if the interval were known, you could use the illumination from the lights, parsed by the camera, as a form of rudimentary pulse-code, and from this, derive the speed. You'd need a window seat. Add in accelerometer data and the two means can correct for each others errors. 

 

So - Get coding!

Alternatively just look into the rearward-facing cab in the last car of the A. Speedo will probably have a reasonably accurate readout. 

 

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