i was under the impression the monitoring would only take place during certain hours, and the locations would change regularly (im thinking like youre a parking lot money collector or something?). im getting the hint now that this is 24/7 permanent installations?
The traffic occurs differently in different locations. Some places, there's a surge of traffic in the morning, then it's quiet until either lunch, or until quitting time.
Others, it's continuous throughout the day, such as at a multi-level parking garage for a Home Depot or a mall or something like that, so the system must be running continuously. So yes, this system needs to be designed to be running 24/7.
Do you have any more additional requirement throwing out here? I didn't know money is so tight that a mere $800 sensor would be a problem, let alone 4 of these $800 to cover 2 dimensional measurement of the object.
There's the usual stuff though - our own costs, and then we also want to make a profit when selling the equipment.
Some installations don't want to throw much money at this sort of thing.
Also how much space do you have to install these sensor? I think if you have flexibility a simple 2 vertical sensor far enough to catch both the front and the rear of the smallest vehicle of interest (something you already have), and 2 horizontal sensor far enough for the same to measure both the width and height of an object (to exclude non interesting object like a shopping cart, animal, human, etc), should be able to get you started quickly.
Some parking garages have
low ceilings. I've seen as low as 5'10". And other facilities have ceilings that are 14' high.
IMO the key is still trying NOT to rely on reflectivity of any kind (i.e. have a sender on one end and a receiver on the other end) at least on one pair of the sensors, if you must do an overhead light curtain that is relying on reflectivity, at least do one break the beam type to verify any reading to eliminate false positive. From my experience you just cannot trust reflectivity if you are expecting random shape object (cars) and environmental condition (water, ice, etc).
This is regardless of whether you use radar, ultra sound, laser, light bulb, infra red, and what not.
Some places can't use a receiver on the other end, if it's being fired sideways. Example....and without a top-notch MS-Paint diagram, this might be tough to describe. I'll try to add something tomorrow.
In some garages, one component could be mounted to a post by lanes of travel. Two problems then: 1) There are likely to be two lanes of travel present. 2) On the opposite side of the lanes of travel are more parking spaces, so there isn't anywhere to mount a receiver module where it would consistently get a good view.
And of course, ground-mounting an electronic device would be a problem.
(Dirt, crushing weight, etc.)
Radar: There are sensors available that can be set to ignore anything beyond a certain set distance. So I could flip a switch on it, and it will ignore anything more than 4 meters from the sensor. But, this sort of sensor is good for motion, but many of them have problems with continuous presence detection of an object that stops in the field of vision.
But yes, you're dead on about the reflectivity issue. It's just so unpredictable that it can result in either missing the targets, or else false trips.
Vision sensors are the reason I haven't posted much in a while. I can tell you some of what I have learned in the past couple months. Machine vision is a really interesting field and open to a lot of methods to solve the problems, funding is also very very good in this area, hint: DARPA.
If you want to do human vision level of sensors you need to read some books on how the eye brain interaction works, the brain takes shortcuts that you can use when designing machine vision.
FPGA. There is no way around it. The information has to be processed in real time and FPGA are the easiest way to do it. You would need a 100 core cpu to do vision as well as 1 FPGA and a 1 core cpu. It isn't speed that is the issue, it is that the information from many different paths has to be present before the cpu can even begin to decode the information and FPGA excel at joining multiple sources into one. If you try to shift in information pixel by pixel for the cpu to analyze it becomes very hard to scale further because you soon reach processing limits that the FPGA can solve easily by 'chewing up the bits' so the cpu can better process the data.
If you can , get a copy of this book, sort of the bible of image processing.
http://www.amazon.com/Image-Processi...ssing+handbook
FPGA: I have heard of them, and I know that they're quite flexible and capable. That's about as far as it's gone though. We looked at using them for embedded applications at work, but they're massive overkill for most of what we do. For the most part, a 28-pin PIC chip gets the job done. You know, super-high-tech stuff.
Thank you for the link though. Maybe some day it will indeed come in handy.
You totally can do it with a DSP or other high performance processor. The algorithm design and tuning is the key. The implementation of the hardware and software, not as much. It is like saying you need to build a suspension bridge before you evaluate all the options (i.e. non suspension bridge, tunnel, ferries, or a road around the crossing) available.
Also IMO detecting whether there is a vehicle presence is a job that can be done much simpler than needing optical cameras and image processing. We are not trying to scan for criminal or terrorist here, we are just counting cars.
Yeah, you'd think so.
We just haven't been able to find the ideal/perfect solution yet.
Thus far, doing it with the infrared light curtains has given pretty good results in most applications. But some of the really busy garages, with lots of pedestrian traffic especially, have proven to be challenging, particularly when the owners want 100% counting accuracy. The other issue is when people don't drive underneath the sensors - so lane delineation is needed to help funnel traffic to where it can be counted. In some garages, lane delineators, such as flexible posts, can get destroyed in a matter of weeks, which adds maintenance overhead.
I think the only option for 100% accuracy is single-space-detection, with a sensor in every parking location. The goal here is to deliver excellent accuracy without the need to undertake that kind of installation, and to also cut down on maintenance requirements.
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