As a hobby I build radio-controlled scale (1:14) models of semi trucks. To go along with them I would like to build a working scale model of a "radar trailer". Here's a link to one if you don't know what I'm talking about: http://www.ru2systems.com/products/trailers/Fast650.php
Before I invest in parts I thought I should ask for some expert advice to see if this idea is even possible and the best strategy on building it, etc. I put radar in quotes as I don't plan to use radar but some other sensor and hopefully at least mimic the radar trailer's operation.
It would be battery operated, of course arduino controlled and I would like to have a 2 digit 7-segment display to display the scale speed after measuring and some math. I'm thinking I could use one of sensors that can measure distance, then take a few readings over time figure out how much the truck has traveled in that time. Adjust for scale and then display the speed. Just so you know the trucks we drive aren't incredibly fast like the RC race cars. We drive at a slow pace through a scale "town" at our rally/meets.
So for my questions:
Is this idea even possible?
Which sensor would be best? I was thinking of either of these two. Feel free to suggest a better one, if there is one.
IR Distance sensor
Maxbotix Ultrasonic Rangefinder - LV-EZ1 - EZ-1
Do I need to account for the angle of the sensor and the on coming truck?
I know I'm probably missing important details but any suggestions/tips/hints?
Thanks in advance.
Brian
Disclaimer:
I have no affiliation with adafruit industries or ru2systems. I just happened to find the information I wanted at their sites.
Do I need to account for the angle of the sensor and the on coming truck?
Technically, yes. Practically, I doubt it. If the angle is small, the difference is so small as to not matter. As the truck gets close, the angle may become large enough to cause problems but the solution is simple: stop updating the display or average the last N readings.
I have seen a lot of examples of distance measurement but none where they found the speed of a moving object
Speed is just rate of change of distance. It's simple calculation.
Your problem may come from the rate that you can take measurements.
Speed guns work continuously, calculating speed from the Doppler frequency of the echo.
However ultrasonic rangefinders make discrete round-trip time measurements, limited by the speed of sound and the sensitivity of the receiver - you may only manage 15 to 20 measurements a second.
At scale ranges, this may equate to very few measurements.
In my past life I was a Traffic Safety radar engineer. If you google "Doppler Effect" you can find a wealth of information. For example, if your radar trailer operates in X Band then you will get a shift of 31.389 Hz per mile per hour. If its in K Band, it would be 72.023 Hz per mile per hour. In your application, take the absolute value of the doppler shift and then it doesn't matter if it is coming towards you, or away from you, it will give the same speed. Yes, the angle matters. It's called the cosine error. The farther you are from straight in line with the object, the slower the measured speed will be.
However, as you a building a scale model without a true radar, simply measure the "scaled distance" traveled over the "true time" and you will get the same result.