Vehicle loop detector, suggestions for best practices - Page 1
Forum section: electronics / projects

author: rvalente
author_rank: Frequent Contributor
timestamp: November 18, 2016, 11:54:00 am
content: Mates,I need to design a loop detector, very similar to the models used in traffic for vehicles but this will be used for steel wire rolls detection (steel factory).The sensor dimensions are about 1000x1000mmI googled some stuff and they look pretty much the same:The coil is isolated by 1:1 trafo, which is part of a colpitts oscillator, the this freq is read by a frequency to voltage converter or some kind of micro for the freq. Is this the best design?How the loop is made? Which wire? Insulation? Twisted pair?Can someone please share some professional schematics?Greetings, Renan

author: ealex
author_rank: Frequent Contributor
timestamp: November 18, 2016, 04:40:12 pm
content: helloI've worked with vehicle loop detectors in the past.the loop was installed in the following way:- cut a rectangle in the asphalt, 2x1 meters for the loop and a channel from the loop to the equipment rack for the cable- install 3 loops of wire in the rectangle - as you would make an inductor- twist the cables from the loop to the detectorwe used thin insulated copper wire, normal PVC insulation, nothing fancy.I've looked in the detectors as well : as you said, they were based on an oscillator and a micro that measured the frequency and based on rate of change and some other configurable parameters triggered a relay

author: SeanB
author_rank: Super Contributor
timestamp: November 18, 2016, 05:00:36 pm
content: Just grab an off the shelf vehicle sensor loop unit, it will be cheaper than building it yourself, and just use the design specs of the unit to make your sensor. If you want to detect the ( I will guess 5 tons) steel rolls you just bury the sensor loop ( 1.5mm cable, like above the correct number of turns for the area and kept away from any large ferrous items, and in a slot cut in the concrete floor, sealed with some bitumen after laying the cable, and twisted wire to the controller) and plug it in, power it up and select the type of sensing you want, either tracking ( which will eventually disregard a roll in the loop if it is left there) or using a threshold, which will detect the rolls easily as they are such a large mass change.However, if you want to detect a small roll, like a 500g oner, then you are stuck with inductive sensor units with a short range, that are placed in a threaded hole the part will either move past or over. Housing needs to be a non ferrous material.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 18, 2016, 05:35:17 pm
content: Hello Guys,This is the kind of steel the guys at the steel factory wanna detect, the presence signal will them trigger the internal logistics to keep the machines well fed.This big rolls are about 0.5T each.Here in Brazil this kind of technology is only used in speed trap cameras and some gate barriers, there is almost no use in traffic controllers (yeah, stupid) so its kind of very very rare technology for some cloning.That's why I asked for some schematics.This guy wants to install around 50 units of this device, so its worthwhile for us to build these...

author: SeanB
author_rank: Super Contributor
timestamp: November 19, 2016, 06:13:19 am
content: Can you build it cheaper than this?http://www.ebay.com/itm/AC-220V-Traffic-Inductive-Loop-Vehicle-Detector-Signal-Control-Sensor-Tool-/142180933711?hash=item211aa4e04f:g:CLIAAOSwcUBYLDiX#shpCntIdDeliver to Brazil, and will do the job, plus plug in so if you need to replace it it is simple to do.http://nortechdetection.com.au/wp-content/uploads/2014/11/PD132-134-enhanced-sf.pdfManual for similar one....

author: rvalente
author_rank: Frequent Contributor
timestamp: November 19, 2016, 02:36:15 pm
content: Hi,No, this value is pretty good deal, I'll buy or two for some tests.After reading the manual I got worried about cross talking.With all the sensors loop that close I'm pretty sure there will be cross talk, but differently from traffic I do not a very fast response so I could scan the steel rolls one-by-one and avoid it.What do you think?

author: Gyro
author_rank: Super Contributor
timestamp: November 19, 2016, 06:48:18 pm
content: One company I worked for also used to manufacture traffic controllers, loop detectors etc.One thing to remember is that typical vehicle loop detectors don't have continuous presence detection (they don't expect a vehicle to stay there for ever). They must be able to cope with someone dumping a load of scaffolding on the pavement next to the loop, parked vehicles near them, environmental factors etc. After a (normally programmable) time, the detector will reset and treat the new coil inductance as the default state. I'm not sure if this will be an issue for you. The other thing is that the loop dimensions are critical to the object size, for vehicle detectors the inductance typically goes down as the steel vehicle sits over and de-tunes it (the opposite of what you might expect). I'm not sure how this might relate to coils of steel wire in very close proximity rather than vehicles elevated on wheels. In vehicle loops, crosstalk isn't usually a problem (for instance several loop terminations might run back to the controller together on a road junction to save on slot cutting (not ideal, but I've seen it done).I think, yes, you need to do some tests for your particular situation. I'm not sure how practical loops secured with duct tape might be for a temporary testing and optimizing.

author: Marco
author_rank: Super Contributor
timestamp: November 19, 2016, 07:18:01 pm
content: Quote from: rvalente on November 19, 2016, 02:36:15 pmSure, could work. Just add a SSR in every loop so you can interrupt it when you don't need it.Looking at the user manual, this detector does have a permanent presence mode. Which I assume compares the current inductance to that of the last reset.PS. these seem to be Chinese clones of the Nortech Detection devices (ebay has them for other supply voltages too, with some Chinese on the labels).

author: rvalente
author_rank: Frequent Contributor
timestamp: November 19, 2016, 08:25:24 pm
content: Quote from: Gyro on November 19, 2016, 06:48:18 pmThe inductance goes down with the metallic mass over the coil? I was think the opposite, the inductance would go up, like a air coil vs coil over iron.As the steel wire rolls would may stay there for a long time I do not think the re-tune technique would be a good idea, maybe calibrate once and call it a day.These rolls are load and unload by fork-lift, so its far from gentle, I don't think the silver tape would stay for more then few operations eheheh, but I could talk to my customer and we could cut the floor for 2..3 coils.Quote from: Marco on November 19, 2016, 07:18:01 pmSince there is the permanent presence mode, I'll start with this guy, If not satisfactory I could develop our own.The good idea to develop our own is to add some extra features, like 4 channels and a simple modbus network, since it be in automated factory.Do you by chance can share any schematics?

author: Gyro
author_rank: Super Contributor
timestamp: November 19, 2016, 08:45:03 pm
content: It's amazing how quickly all traces of a company you used to work for (Microsense) can dissapear from the web (we used to be a major supplier of controllers and street furniture in the UK). So much for the internet being an all encompasing knowledgebase.Using it as a search term did pull up a couple of loop installation manuals which could be quite helpful to you:http://www.siemens.co.uk/traffic/pool/downloads/handbooks/road_signals/667_he_20663_000.pdfhttp://buckholztraffic.com/ARTICLES/ARTICLE_4_Required_Inductance.pdfhttp://www.marshproducts.com/pdf/Inductive%20Loop%20Write%20up.pdfNotice that the loop frequency goes up on presence (inductance goes down). As I say, this might be different with wire coils on the surface - I suppose worst case bare wire coils like that might act as completely shorted turns, severely damping the loop (though only if horizontal - a possible error situation?).

author: rvalente
author_rank: Frequent Contributor
timestamp: November 19, 2016, 09:17:16 pm
content: Quote from: Gyro on November 19, 2016, 08:45:03 pmGreat source of information, thanks a lot!What do you think of this design:http://www.elektronika.ba/843/inductive-loop-vehicle-detector-v2-1/

author: Gyro
author_rank: Super Contributor
timestamp: November 19, 2016, 09:39:20 pm
content: Really hard to say, but it looks a reasonable design. It supports the normal NEMA spec loop inductance range and has transformer isolation of the loop. It also has selectable and infinite presence time.I'm not sure what the environmental conditions are like at your site (temperature). I remember we used to use combinations of capacitors (polyester and polycarbonate) to temperature compensate the oscillator but that was for installation in street cabinets, exposed to freezing temperatures, hot sun etc. Stability becomes more important as presence time is increased (especially to infinity!).I think the performance of this detector probably ultimately comes down to the quality of the software design.

author: Marco
author_rank: Super Contributor
timestamp: November 20, 2016, 12:41:45 am
content: In software you don't really need a capacitor at all, simply excite it with a square wave through a resistor and do vector analysis.

author: Gyro
author_rank: Super Contributor
timestamp: November 20, 2016, 10:52:56 am
content: QuoteYes you could, but industry practice shows that road loops work much better in a resonant circuit. The inductance changes are surprisingly subtle (for reliable detection anyway),. With a resonant circuit you need to pump far less energy into the loop and given that it is completely unshielded, for EMC approval you want as little excitation as possible and definitely no square waves. Over the years, frequency (or more properly tuning) shift has been shown to be most effective method.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 20, 2016, 02:38:23 pm
content: Talking about Brazil in a industrial environment the temperatures would never go below 15°C, so to qualify from 10 to 60°C is enough.I'd go with the resonant design, the capacitor does the job and the micro just has to measure some freq deviation, easy enough.Also there is the positive point of less energy in the coil, less irradiated noise, which I believe would improve the cross talk to the neighbor coil.Well guys, I much appreciate by the help, I'd reply when get some updates.If you happen to have any schematics to share, please do.

author: ealex
author_rank: Frequent Contributor
timestamp: November 20, 2016, 05:02:05 pm
content: a note on making a temporary test loop (i've done this and they lasted ~ 1 week in normal traffic ) :- cut strips of hard insulating foam, the thin type used under laminated flooring- lay them in the form on the loop- lay the wires on the foam taking care not to overlap them- on the end-point, where you'll have to overlap, add 1-2 pieces of foam under the overlapping wire- place another layer of foam strips above the loop- do the same thing for the output wires if they will be in the waythe loop was glued to the asphalt using some special road marking tape - felt like a heavy / thick cloth, maybe glass fiber based.we used a solvent based glue : dry and warm the road surface with a gas flame, apply the glue, leave it until most of the solvent evaporates (it no longer felt sticky ), apply the loop and then the tape and press it really well.the foam got compressed quickly - the entire thing got quite flat and the traffic was not disturbed.i don't know where you'll find that tape, maybe you can find some equivalent locally.the entire thing took a lot of manual labor and we had to block the lane for several hours, so it might not be fit for several tests.

author: C
author_rank: Super Contributor
timestamp: November 20, 2016, 05:32:52 pm
content: If your going to build something, might want to think of system as more of a network analyzer. Using this idea, cross talk could become additional input information an not a problem. When you get to resonant frequency you have low input to coil with high output.Looks like you have a large amount of time to use, so you could do a step through each loop doing a scan. A combination of quick scan to verify same resonant frequency which fails to a wider scan to find new frequency.The extra information would also assist in setting levels for detection.For your initial testing, you might want to put your loops in bottom side of plywood sheets.