Technology Insight - Inductive Loops

Technology Insight - Inductive Loops

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Hey everyone! In this Technology Insight episode of   ITS Now, we are looking at Inductive Loop vehicle  detection, which is probably the most important   individual technology used in ITS applications,  because it is used so widely in applications as   diverse as traffic signals, incident detection,  vehicle classification, presence detection etc.  I think it is probably worth stating that these  are inductive rather than induction loops,   which they are sometimes erroneously referred  to as. The later are actually used to wirelessly   transfer energy, so within the ITS sphere, these  are starting to be used for wirelessly charging   electric vehicles, but no you will not be  able to charge your shiny new EV off the   detection loops at a set of traffic signals! Anyway, I’m Alistair, and this, is ITS Now. Inductive loops have been utilised extensively  since the 1960’s across a wide range of uses,   including traffic signals, vehicle count sites,  access control and incident detection. Although   newer technologies have been introduced over the  years, loops are still considered to be highly   reliable, and are the standard against which  other methods of detection are compared with. Many will be familiar with the outline of vehicle  detection loops in our roads, a thin black line in   the tarmac. Loops come in a variety of shapes and  sizes depending on their use and the standards of  

the country being used in, but include rectangles,  chevrons, circles, diamonds and even butterflies! In the UK, the requirements for inductive  loops is set out in the Manual of Contract   Documents for Highway Works, or MCHW for short,  Highway Construction Details ‘G’ Series drawings.   The set of 32 drawings provide details of the  various construction elements required for   different inductive loop detection applications.  Further technical guidance and links to other   documents is given in MCH1540, the specification  for the installation of detector loops.   Also, the Federal Highway Administration  Traffic Detector Handbook covers this. To install a loop, a slot is cut in the required  shape and dimensions for the desired detection   functionality. A connecting slot (cut back) to  the edge of the carriageway is also required   to allow the loop to be connected. It is essential to ensure that the  

slots are clear of debris before the cable  is installed and there mustn’t be any water   in the slot when they are backfilled, this is  normally done using a handheld leaf blower.  A specialist insulated flexible wire (Hypalon)  is then wound around the ‘loop’ slot 3 or 4   times (dependent upon the size, required  sensitivity and function of the loop)   then the ends (loop tails) are typically wired  back through the cut back to a jointing chamber   situated in the verge or footway. It is important  to ensure that the loop tails are twisted,   so that the tails do not detect vehicles or  other metallic objects passing in their vicinity. 

The slot is then backfilled  to protect it from damage,   often part filled with an epoxy resin, then  finished with a bitumous hot-pour sealant on top.  Although inductive loops provide a reliable  means of accurately detecting vehicles,   their design and installation needs to be  carried out carefully to ensure that the   equipment provides a reliable level of service. An  assessment of the carriageway condition should be   carried out to ensure that it is good enough for  loops to be installed in to. Evidence of issues  

such as ‘tram lining’ (where the road surface has  been compacted by the wheel tracks of vehicles),   differential subsidence, heave or cracking and  breakup of the wearing course may result in a   reassessment of the method of detection to be  used or for the carriageway to be re-surfaced.  It is worth noting that in some locations,  including the US, some loops are pre-formed   and installed in the roadbase, before the wearing  course, the top surface of tarmac, is rolled out   over them. Stick down loops are also sometimes  used in situations like concrete multi-story   car park structures, where it isn’t possible to  slot cut the loops into the carriageway surface.   However, these wouldn’t stand up to  enough wear for highway applications. The wire in the pit is jointed to a feeder cable,  this was traditionally done using a chemical or   heat-shrink joint, but increasingly this is now  done using a lightweight plastic bottle joint.  The feeder cable is fed through the duct  system to the TSC or equipment cabinet   where it is connected to a detector pack. The  feeder cables contain twisted pairs of cables,  

usually in an earthed steel wire armoured  sheath to provide both mechanical protection   and to minimise electrical interference. Once switched on, the loop coil emits an   inductive field, which is disrupted when a metal  object passes through it, such as a car or lorry.   The detector pack processes information regarding  the changes in the signal it receives and when an   anomaly is detected, an output is generated  to indicate the presence of a vehicle.   (The process involved is essentially similar to  that used by a handheld metal detector, but in   this case the sensor is buried in the ground and  is affected by metal objects passing overhead).   Although this technology has had  widespread use since the 1960’s,   it is still seen as the baseline that other  detection technologies are measured against.

To show you how this works, I have  set up an example to demonstrate an   example of a single loop. So, what we've got here   is this loop of wire going round. I've set them  around four cones, so it's reasonably taught, and   this is a three flex ordinary electrical wire, so  it isn't what you'd use in an actual installation,   that would use something called Hypalon wire,  which is a quite a thin, black, flexible wire,   which would actually be laid into the slot. Most  loops use three turns in them. So that's the the  

loop itself and then here you see the loop tail,  this black and white cable which is twisted,   and this would normally go back into a joint  in the verge, in the chamber in the verge   and then take them back using a loop feeder  cable, back into the controller cabinet or into a   roadside cabinet of some sort, with a detector  pack in. So, here we've got two four channel   detector packs, we're using this one and  you can see the lights on here, which have   been switched on so that you can see them. So,  we're only using one channel at the moment, so   this loop is actually coming into channel one  here, and we'll see that going in a minute.   These are standard types of formats that are used  for detector cards, so the interface on the back   then going to a traffic signal controller or a  count site, or whatever, those bit patterns are   all defined, so you can actually use different  brands of detector packs, without any hassle.   So, here I've just got this on a power supply,  but normally, as I say, this would all be within   a roadside cabinet. So, anyway, we've got the  loop tail, so what I thought I'd do here, I've got   an electromagnetic detector, it seems to  have just faulted! Here we go, right, so here   you can see how those lights are   continuously monitoring, so that shows that  there's an electromagnetic field here that its   detecting. If we go and do the same thing with the  loop tail now, as you can see that doesn't happen,  

and that's because of the twist on here, so  that's why it's so important that the the tails   from the very point where at the corner of the  loop, is that they are twisted all the way back   in the cutback slots, all the way into the  chamber, to where they're actually jointed,   because otherwise, they'll pick up either vehicles  going over the cutback or just random noise, which   means that the whole installation would be no  good basically, it would be unreliable, it would   be double counting, adding extra vehicles for  other lanes etc. Anyway, so how does this work,   so it's basically like a metal detector, only  instead of looking for metal in the ground,   we're looking for a car basically, going  over the top of this, so what I'll do   is see that LED coming on, as I push this through  this field, and this extra light comes on,   you can probably hear it as well, clicking  on and off. And that's all there is to it,   this is more or less the the size of an  ordinary Chevron Loop. I've just set it out   square because it was easier here, but obviously  the shapes differ, you can get round ones and   different sizes and shapes etc, but that's  the basic way in which these things work. Next, we’re going to look at an installation of  motorway Incident detection loops to show you   how these are typically installed. You will see  that this consists of two loops in each lane,   with cut-backs to the verge to  allow these to be terminated. 

The installation of an inductive  loop will normally require the   lanes either side of where a loop  is being installed to be closed.   This is because the footprint of a loop  stretches across most of the effected   lane and the slot cutting machines are also  quite wide as well, so additional clearance   is required to protect the installation  operative whilst undertaking these tasks.  On multi lane installations like the  example we are going to show you,   the cut-backs from the outer lanes also cross  the inner lanes to reach the termination point   in a chamber in the verge, so it isn’t really  practical or safe to try to maintain a live lane,   so this example was done under a  full closure. It can therefore be   difficult to get an opportunity to  replace loops, so regardless of use,   these are often done at night when traffic volumes  are lighter but adding considerably to the cost.

As we saw in the video, testing of the loops is  essential. The generic test requirements for loops   is given in drawing G6, although many highway  authorities have their own versions of this.   Tests are normally undertaken at the  end of the loop tails before they are   terminated to the feeder cable, then once  this is done they are also tested at the   far end of the feeder cable in the equipment  cabinet where the detector cards are housed.  The electrical tests include measuring  the resistance and inductance of the   loop then loop and feeder cable, followed by  a series of resistance measurements to earth.   The length of the loop tails and feeder  cables are also measured to ensure that   the results of the electrical testing is accurate.  Measurements of the distance between the leading  and trailing edge of the loop, and if installed as   a pair of loops in a lane, the distance between  the two loops may also be required to ensure   that the system will interpret parameters such  as vehicle speed and vehicle length correctly. 

The actual location of the loop  may also need to be measured,   MOVA loops for example, it is important to  record the distance to the traffic signal   stop line because the dataset needs to know what  this to accurately time the signal phases. The   From Stop Line distance (referred to as FSL for  short) is measured from the nearside leading-edge   corner of the loop, not the distance between  the nearest edge of the loop to the stop line. Once installed, terminated and operating, the  setup of the detector card needs to reflect the   requirements of the particular use. Different  manufacturers have varying means to set these,   but typically on four channel cards, each channel  is set to minimise cross talk between adjacent   channels, and the sensitivity of each input is  adjusted to minimise false triggers, whilst also   allowing more difficult to detect vehicles, such  as bicycles, to be detected reliably if needed. It is also essential to ensure that the loops  are terminated into the correct channel inputs   of the detector cards. If there is a road or lane  closure, it might be possible to drive a vehicle   over each loop in turn to verify this, but if  the site is open to traffic, it is much harder   to achieve accurately. In this situation,  it is necessary to have a ‘spotter’ located  

adjacent to the loop being verified, and  using a handsfree mobile phone or radio,   tell the engineer at the cabinet when  vehicles are actually passing over the loop.  The engineer will then need to ensure that the  operation they are seeing on the detector card   panel matches the information the spotter  is calling out to them. If it doesn’t,   they will need to investigate what is going on. Please remember to click on the Like and  Subscribe, to keep updated on ITS Now! At the corners of loops, a small  intersecting slot, called a crosscut,   is often used to reduce the strain on the loop  cable by removing the sharp internal corner that   could possibly damage the Hypalon cable. It is  important that the crosscut is accurately located   at the junction point of the two side cuts and  doesn’t leave a small area of tarmac between them,   these are referred to as  ‘floaters’ because over time there   is a tendency for these to break away and cause a  pothole to form at the corner point of the loop.

One of the weakest parts of a loop installation  is the transition point, where the loop tails   go from the carriageway surface to the verge  or footway. It has been common in the past to   take the loop tails back to the jointing pit  by cutting them directly through the kerb or   into the verge, but this can leave the tails  vulnerable to damage and weaken the kerbing.   The design should therefore incorporate a  reliable means of making the transition from   the carriageway into the verge or footway,  such as using a carriageway loop tail box.  These are used by cutting the loop tail  directly into a ductile iron box set into   the carriageway surface. The tails are  then routed through a flexible conduit  

in the bottom of the box, which  connects to a pit in the verge,   by running under the kerb. The tails are then  jointed in the pit to the loop feeder cable. Whilst loops are being slot-cut, it is essential  to ensure that the slots are cut to the required   depth, contractors may try to cut these too  shallow, in order to minimise the time required.   A number of issues can occur if  the slots are not deep enough   that would lead to premature failure: • the backfill is more likely to come   away, exposing the loop cable, which will  result in the cable being worn and broken  • the loops will be destroyed by  ‘planing out’ during resurfacing works During the life of a loop installation, damage  to the road surface can result in the loop not   working. Damage tends to be caused by two  issues, the first are utility companies,  

who are notorious for trenching through  loops, whilst carrying out works.   The second is caused by the  deterioration of the carriageway,   breakup of the surface will result in the loops  becoming exposed and damaged, and tram lining   (differential compaction) can result in the loop  wire becoming stretched and subsequently breaking.  Installed properly, inductive  loops will provide a reliable,   long lasting and effective means of detecting  vehicular traffic. They don’t suffer from the  

obscuration or climatic issues that effect most  forms of verge mounted above-ground detection. I think this episode shows the huge  array of information that relates to   an individual topic area such as  indictive loops, in reality there   is probably enough content to make a  whole series on this subject alone.   If you did find this useful, have a look at our  books in the publication page of  Thank you to Crown Highways for their assistance  with making this episode, if you’d like to   be involved in a future Technology Insight  episode, please feel free to get in contact.  Could I ask you to please subscribe to  our channel, this really will help us   to bring you further videos about ITS,  thank you to everyone who has, so far. 

Thanks for watching, see you next time.

2022-10-03 11:08

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