Mass sensors at traffic lights - Page 1
Forum section: electronics / beginners

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 07:07:29 am
content: At traffic lights there are pads that contain mass sensors so that the signal knows if there are/aren't vehicles waiting. I know motorcycles sometimes don't have enough mass to trigger them. I'm curious about why the technology that's used isn't sensitive enough for motorcycles. Is the problem that difficult or is it just a case of poor choice for the implementation? I think that it's just a loop of fine wire embedded into the asphalt/concrete and then sealant is applied that covers the wire and the groove for the wire. I guess they measure the inductance??

author: VK5RC
author_rank: Unknown
timestamp: April 14, 2014, 07:15:30 am
content: I think they are 2 loops joined along the long axis (of the road) if that makes sense, I ride a push bike and to get detected you need to have a metal rim, carbon fibre never works, aluminium does however, but you need to be exactly where the two rectangular loops touch in the middle of the lane.I too would be interested to know how they work.

author: kizzap
author_rank: Unknown
timestamp: April 14, 2014, 07:41:45 am
content: Quote from: VK5RC on April 14, 2014, 07:15:30 amPretty much this.They are generally an inductive loop, placed over the ground where a car will generally wait. As a car drives over the loop, the inductance of the coil that the controller sees changes, telling it there is a car there. The controller then decides what and when it is going to do with the information it has.As an aside, I believe some traffic lights have secondary coils placed further into the intersection in the lanes, which I believe are either used to trigger red light cameras, or to tell the controller that there are vehicles still in the intersection.

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 14, 2014, 09:46:39 am
content: I'm starting work on a project that uses these loops. The loop in the tarmac will have a certain inductance, when a metal object enters the area the inductance changes. If you make the inductive loop part of a colpits oscillator the frequency of the oscillator will change according to the presence of the metal mass. Feed that though a mosfet/opamp and you have a signal that you can read with a micro.Now the difficult part is that the loop inductance will slowly vary naturally with temperature and even atmospheric pressure. You need algorithms to filter out the slow changes but detect the large changes.For bicycles you can do the same thing but the loop needs to be smaller and two loops can be used to increase sensitivity.

author: VK5RC
author_rank: Unknown
timestamp: April 14, 2014, 10:57:47 am
content: Interesting, the aluminium I think lowers reactance, so the software must be pretty good to pick both increases and decreases in reactance (oscillating frequency) and not get triggered by rain downpours (with a little salt on the road for good measure).

author: jeremy
author_rank: Super Contributor
timestamp: April 14, 2014, 11:21:04 am
content: A friend of mine who worked on these said that they can be made with a few loops to determine the speed of cars entering the highway

author: kayvee
author_rank: Regular Contributor
timestamp: April 14, 2014, 11:30:00 am
content: Presence, passage, direction,speed, vehicle classification.Loop size, shape, position, configuration is the key.

author: deth502
author_rank: Frequent Contributor
timestamp: April 14, 2014, 02:23:39 pm
content: yes, they do (did) work as inductive loops, as stated, but, in the u.s. at least, these are all being phased out and replaced with video detection.

author: StevenB
author_rank: Contributor
timestamp: April 14, 2014, 02:29:47 pm
content: QuoteWhich in my experience sucks when it is foggy.

author: kolbep
author_rank: Frequent Contributor
timestamp: April 14, 2014, 02:37:46 pm
content: Here is a Loop Detector that we install at complexes /businesses to open their gates when a vehicle wants to exit.http://www.et.co.za/loop_detector.htmlYou have a few loops of stranded wire buried under the tar, and connected to this detector.Power up the detector, and it detects which is the best frequency based on the loop.Then when a vehicle (or spade, or piece of scrapmetal) goes over the loop, the inductance changes, shifting the resonant frequency, and the gate is opened.You can also put an external trigger in to the controller, so that you have to push a button, which then activates the detector, so that you can basically Authorise the gate to open when the car reaches loop). This will prevent a shopper from just driving your car out. They first have to 'ask' you to activate the loop detector, so that they can take your car. Of course if this happens, they will either take you with them, or use your remote, or use your car as a heavy duty gate opener.You can see I am South African.....

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 14, 2014, 03:03:09 pm
content: Has anyone used magnetometers in place of inductive loops? Looks to me like there are some advantages, mainly that they should be more robust and easier serviced.I've found some honeywell IC magnetometers but I was wondering if there were any low power off the shelf solutions available.

author: senso
author_rank: Frequent Contributor
timestamp: April 14, 2014, 03:59:27 pm
content: How can a chip that needs a little pcb be more reliable than a simple piece of insulated cooper(or aluminium) cable?

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 14, 2014, 04:36:40 pm
content: ^When the cable is inserted into a 5mm gap in tarmac and repeatedly driven over.In fairness they are fairly reliable if left alone, trouble is that there is roadworks and resurfacing. Also if a fault develops it will likely be intermittent at first, meaning you may have to dig up the old cable to find out, that's some serious digging and hassle.A magnetometer can sit by the roadside or buried in a 100mm x 100mm box, much easier for installation and service.

author: SeanB
author_rank: Super Contributor
timestamp: April 14, 2014, 05:46:24 pm
content: Most of the loop detectors here sit in a slit in the concrete/tar about 25mm below the surface, and generally are made from 1.5mm stranded wire, generally used for wiring houses. This is then filled in with tar or a polyurethane sealer to protect it from pretty much everything. The ends are crimped to the cable to the detector if it is a distance away or just go to it. This pretty much survives anything aside from a road resurfacing or the roadway disintegrating around it. Even the cable thieves do not try to take them out, way too much work for the small amount of copper in the loop.They are pretty good at detecting cars, bikes, trucks and buses, and are a lot better than the old system which used a pneumatic tube across the road that would detect cars and trucks but was unreliable with motorcycles and bicycles.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 05:49:10 pm
content: Quote from: deth502 on April 14, 2014, 02:23:39 pmI doubt this very much. Some pointers to back this up would be interesting. Far too expensive and too many moving parts I would guess.

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2014, 05:51:24 pm
content: Quote from: deth502 on April 14, 2014, 02:23:39 pmFor technical reasons or for surveillance reasons?PS. nevermind, if I take the engineer's word for itin this article(which I do) they don't all currently have the ability to transmit video. Of course once they become the standard making them multi-purpose is going to become very tempting.

author: deth502
author_rank: Frequent Contributor
timestamp: April 14, 2014, 06:00:30 pm
content: Quote from: GiskardReventlov on April 14, 2014, 05:49:10 pmyou can doubt it all day long, it wont change the facts. there are ZERO moving parts in either an inductive loop, or a video detection system.

author: deth502
author_rank: Frequent Contributor
timestamp: April 14, 2014, 06:04:56 pm
content: Quote from: Marco on April 14, 2014, 05:51:24 pmwho knows in the future, but yes, as youve seen they dont actually transmit anything. as to weather or not they are actually recording anything, that is a possibility, although unlikely.one advantage, for now, is improved traffic safety. the sheepish masses dont know any better, when the new detection cameras are installed, everyone thinks that they are there to photo and ticket ppl who run the red light, which they arent, but it puts manners on the drivers.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 06:10:17 pm
content: Quote from: deth502 on April 14, 2014, 06:00:30 pmHa! My doubts were for your assertion that "but, in the u.s. at least, these are all being phased out and replaced with video detection. "What equipment does the video detection system use? How does it work? What about fog, snow, rain, etc.? Is it a completely autonomous system?

author: deth502
author_rank: Frequent Contributor
timestamp: April 14, 2014, 06:18:32 pm
content: there are many different manufacturers with different systems. the systems are autonomous. not a lot of fog in my area, but we havent had any issues with rain or snow in the last 5 or so years since our first intersection was retrofitted. at this point, we only have 1 inductive loop signal left. there are no immediate plans to replace it, but you can rest assured, the next time any major roadwork is done through that intersection, it will be switched out. this is not our decision. as a local govt, we are responsible to install and maintain the signals, but the state d.o.t. controls where they are and how they operate.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 06:25:47 pm
content: Quote from: SeanB on April 14, 2014, 05:46:24 pmThe same implementation is used in u.s. too. Sometimes it's gray and variations of that. Sometimes black.QuoteI've never seen that here for traffic signals, what vintage are you talking here?They do use these to do traffic counts from time to time and gather bulk speed data.

author: SeanB
author_rank: Super Contributor
timestamp: April 14, 2014, 06:34:16 pm
content: So you replace a reliable simple low cost low power sensor with a new one that is both complex, high power use and likely to give problems down the line with failing electronic systems, along with not being reliable in heavy rain, snow and with direct sunlight hitting the camera lens.I smell pork pies there.the pneumatic systems were installed in the 1970's and were replaced with loop detectors pretty much within the decade as they became more reliable. To do speed timing here they use a piezo cable that generates a high voltage pulse when a wheel hits it, and place 2 lines 1m apart with a camera to get the happy snap that you get in the mail. Newer ones use LIDAR or RADAR and are generally easier to set up. I wave at them as I go by, and blow the horn to wake them up as well. The line units are mostly used in smaller municipalities as they got them cheap as bigger metros upgraded, and although these use film they are still profitable to operate. New ones use digital cameras and record to flash media, they also can do double duty as a red light violation camera if placed in the permanent housing at the intersection.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 06:37:06 pm
content: Quote from: VK5RC on April 14, 2014, 07:15:30 amIt'd be great to see a picture of it if you can.I'll try to get some pictures here, the loops are revealed by the groove and the sealant they use to fill the groove.They are probably 2m x 2m or so and then the single groove goes off to the side of the rode to the signal switch.In large metro areas, like San Diego for example, they also have signals connected to satellites, that technology has been in use since at least the 80's. It allows for squeezing more traffic through and keeping vehicles moving. Some metros use something similar but do not use satellites, instead they use landlines and humans make the call to extend a green light or red light to improve flow. Vegas does this.

author: SeanB
author_rank: Super Contributor
timestamp: April 14, 2014, 06:53:59 pm
content: Here since the 1980's they connected most of the signals to a central control room with an upgrade to a computerised control for each intersection. This replaced the old Automotor controls, of which a few are still left running small intersections. That way they can remotely program light duration and have traffic pattern control.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 06:57:02 pm
content: Quote from: SeanB on April 14, 2014, 06:34:16 pmYou're talking about the video? Yes too $$$.QuoteThe piezo must be a much simpler solution than a pneumatic. Interesting how these techs evolve toward simple.The red light cameras are being phased out in So. Cal. Expensive infrastructure, very shady deals with the vendors, then the municipalities were left holding the bag when the gear started to fail. Vendors are long gone under a new name, etc. They require hands-on, they piss people off. The smart traffic people are doing smart things. For example increasing the time for a yellow or increasing the time be when the light turns red in one direction and green in the other.The vendors were taking a cut of the red light ticket revenue. And it was big, big $$$. So corruption was rampant. I heard cases of cars getting ticketed for running a red light in LA but the person lived in SF and never was in LA.

author: SeanB
author_rank: Super Contributor
timestamp: April 14, 2014, 07:04:46 pm
content: Sounds like the vendors here........

author: deth502
author_rank: Frequent Contributor
timestamp: April 14, 2014, 10:20:13 pm
content: Quote from: SeanB on April 14, 2014, 06:53:59 pmall of ours are independent, due to the distances that they are apart.there is no "control room", but the spec is (state law in my state, ymmv) that if the signals are less than x amount apart, they must be connected to a common controller to synchronize them for better traffic flow. still all computer controlled and autonomous though.actual power consumption differences between both methods, i cant say, i dont pay the electric bills, and im not privy to that financial information, but under the guise of reduced power consumption, we were mandated to retrofit all of our incandescent bulb signals to led's a few years back.

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 14, 2014, 11:48:59 pm
content: Quote from: bookaboo on April 14, 2014, 09:46:39 amSounds like you'd need to calibrate the device for each intersection.Most intersections don't trigger for a bicycle and even motorcycles have problems too. I think this answers my question about implementation. If they don't add small loops then smaller vehicles don't trigger. This presents adilemma because smaller vehicles can get stuck at red lights that don't "see" them.

author: StevenB
author_rank: Contributor
timestamp: April 16, 2014, 04:21:09 pm
content: Quote from: SeanB on April 14, 2014, 06:34:16 pmI don't understand why the simple inductive system is being replace by cameras either. I assume it is easier to maintain and adjust? I have definetly seen issues during fog (have to sit throught each cycle timing out instead of detecting no cars). I imagine this is a calibration issue. It's fustrating when you are late for work!

author: Yago
author_rank: Frequent Contributor
timestamp: April 16, 2014, 05:27:37 pm
content: In UK it looks like there are cameras appearing at the top of traffic light junctions and crossings.Might be that our roads are so knackered that any loops etc are just being pulled apart as the road fails. I don't think that what we have qualifies as just pot holes now!

author: GiskardReventlov
author_rank: Frequent Contributor
timestamp: April 17, 2014, 05:01:20 pm
content: Quote from: Yago on April 16, 2014, 05:27:37 pmSome places put up fake cameras. If they can't afford to fix pot holes they either use fake cameras or they spent all the money on real cameras and can't fix the pot holes.I heard that vandals have used lasers on cameras to burn out the image sensor. Not sure if that's true but I think it's possible. But preventable I'd think.

author: SeanB
author_rank: Super Contributor
timestamp: April 17, 2014, 06:56:15 pm
content: Here they just shoot them or necklace them. You need a guard for the camera or it will be towed till it falls apart. I see the local metro are trialling a vehicle sensor, but this one is a simple microwave vehicle sensor rather than a vision system. They are also at the same trial intersection trialling some countdown times LED lamp units, which do look quite nice with the animated pedestrian in green with the orange countdown above it.