Single ended breakbeam

Thread Starter

jbord39

Joined Mar 31, 2010
41
Where did you get that information? Baseball speed guns work just like police radar and the ones I'm familiar with must be used at or near home plate so that the ball is coming directly at the gun to detect the speed accurately.
I was just under that impression. Thanks for the clarification.
 

John P

Joined Oct 14, 2008
2,025
That was my initial idea as well. The problem is that this would require some sort of computer vision software. To detect the edge in an environment with moving clouds and ambient lighting changes could be difficult. That is why I was hoping for a sensor-based approach. What implementation would you use to detect the edges?
I'm assuming the simplest form of computer vision. Let's say the camera line sees a strip of basically the same light level, which is reasonable for a patch of road, before a car comes along. Then for a certain time there's a step up or a step down in brightness in that line, traveling left to right or right to left, and that's the front end of a car. The time it takes to cross the camera's view, or some segment of the view, (given the length of highway that the scan line covers, which is known) corresponds to the object's speed. To detect the object can you just use a comparator, with the other input being a low-pass filtered level derived from the brightness of the empty road in scans where there was no vehicle?

You could probably figure out some sort of "sanity check" to try to eliminate false readings--an object must have a certain time on the sensor versus its calculated speed for instance. That would eliminate the edge of a cloud, which might have the right travel speed, but which would be on the sensor too long to be real. In fact after the cloud had been in place a while, the low-pass filter would respond to it and consider the clouded road to be the ambient level for future vehicles to be compared to.
 

Thread Starter

jbord39

Joined Mar 31, 2010
41
Does anyone have any recommendations for using a PIR? Two such sensors set up with a known distance could easily calculate speed. The problem I am having is all the PIR sensors I can find are made for activating alarms, and have a set minimum response time.

Thanks,

John
 

KMoffett

Joined Dec 19, 2007
2,918
Pyroelectric sensors work on detecting heat change. The primary element is not a fast response device. So, you will not ever get a fast change in output.

Ken
 

Thread Starter

jbord39

Joined Mar 31, 2010
41
Pyroelectric sensors work on detecting heat change. The primary element is not a fast response device. So, you will not ever get a fast change in output.

Ken
Thanks for the information. So if I want to attempt to implement this using sensors rather than a simplified computer vision, what routes should be considered?

I am thinking either two pulsed laser/detector or pulsed led/detectors spaced say 20 feet apart pointed perpendicular to the road, aimed at the wheels. When a car passes, the wheels should reflect some signal back at the receiver.

The problem is making sure that both the receiver is sensitive enough and that the beam width is small enough. If the beam width is very wide than the margin of error would be wide, but also easier for the receiver to pick up. With a very narrow beam (laser) the chance of the receiver completely missing the reflected beam seems high.

Is there any way (or maybe particular frequency range) that would keep the beam tight until it hit the car, and upon reflection spread out wide?

Thanks,

John
 

KMoffett

Joined Dec 19, 2007
2,918
Wheels are not a very good reflector. Many are very absorbent (i.e. black), some have highly reflective hub caps, some have domed hubcaps, some have faceted hubcaps. most vehicles have two wheels per side, others have many more. A retro-reflector on the opposite side of the lane still seems to simplest and most reliable.

Ken
 

Thread Starter

jbord39

Joined Mar 31, 2010
41
Wheels are not a very good reflector. Many are very absorbent (i.e. black), some have highly reflective hub caps, some have domed hubcaps, some have faceted hubcaps. most vehicles have two wheels per side, others have many more. A retro-reflector on the opposite side of the lane still seems to simplest and most reliable.

Ken
Thanks for the information. I agree that it is for sure the simplest way to handle the problem, but I am not so sure about reliability. The mechanism relies on an operator setting up the reflector correctly and the reflector not getting even slightly moved by wind or a pebble for up to 12 hours. The idea behind keeping it as a single device is to minimize the setup time and the errors that can be introduced by even having to set something up.

I know that inductive loops are commonly used (around and underneath the road) to detect moving cars. I cannot do this, but does anyone have an idea whether a loop with many turns near the road would have an induced voltage above the noise floor? The loop size would have to be at a max 1m^2.
 
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