I need a formula to code my microprocessor to tell me how fast my model train is going in MPH

Question

Thank you for helping me with this. I am not very good with long algebra problems. It must be my "ADD". I am a 63 year old experimenter and I experiment with just about everything; engineering, electronics, software and mathematics.

I am building a speedometer for large (1/8th) scale locomotives. I am using a Basic Stamp microprocessor which is capable of counting the teeth on one of the drive sprockets during a number of "periods". Each period is 400 us (microseconds).

The code function used is called COUNT. If I tell the system to COUNT 320 it gives me back the number of teeth that it COUNTs during 320 X 400 us. BTW, I have empirically done this and 320 is fairly close to correct. The result needs to be given in actual "full scale" MPH *10 enabling me to read tenths of a MPH (ie: 75 = 7.5 MPH). Remember I want to display actual MPH, not scale MPH.

Here is the other info needed:

The locomotive wheel is 15.71" in circumference and has a sprocket with 72 teeth so that for each 15.71" traveled, my gear tooth sensor gets 72 "ticks". In actuality, I have a 5" diameter wheel and a 2:1 gear box with a 36 tooth sprocket. There is a geartooth sensor on the sprocket that COUNTs teeth as they go by and gives them to my microprocessor.

Can you help me create a formula I can use for doing this? It would be greatly appreciated. I have rather complicated code running my Locomotives and having a accurate speedometer would be wonderful.

Thanks for helping me with my project.

Greg

Answer 1

Here is what you might try:

 

Measure the speed of the train over a measured length of track. That is, take a stop watch and time how long it takes to cover that track. Let's say the track is 4 feet long and it takes 4 seconds to cover that track. Therefore, the train's speed is 4ft/4sec or 1ft/sec. You can convert this to MPH by multiplying that value by 60*60/5280. That ratio is .6868. Therefore, the train is going .69 MPH if what you want is actual speed of the vehicle. If it is a 1/8th model, then the model speed would be 8 times the real speed or 5.494 Model MPH.

 

The point is, once you measure the actual speed, then you can work backwards to determine what your programming values should be.

 

Good luck.