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Inductive Loop Relay Problem

  1. Oct 18, 2011 #1
    Hi all

    Well I tried googling this but could not find a satisfactory answer , so here I am seeking help from all you experts :biggrin:

    So my question is regarding inductive loop sensors used on the road to give presence, vehicle classification etc information. These inductive loops are connected to a vehicle detector ( Relay - pulse or presence output). This relay ( all kind/ not specific to a brand) produce a digital output - it gives 1 when the vehicle is over the inductive loop and 0 for the rest of the time. This data is collected for lets say 30 sec and then transferred on to higher level to be used with SCATS or SCOOTS.

    OK so what ive found is the digital output sometimes get stuck on/off ( regardless of vehicle going over loop or not), or the output is broken, also problems such as chattering and hanging on/off occurs. I cant seem to get to the core of these problems. Why do they occur? The inductive loop does its job by providing the magnetic profile of the vehicle as it goes over loop , then why is relay incapable of detecting it?

    Any suggestions/ ideas are welcome :shy:

  2. jcsd
  3. Oct 18, 2011 #2


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    These loops in the roadways are very prone to interference so good design and good shielding are needed to make them work properly. They are used routinely to operate traffic lights and seem to work OK with good design.

    Yours sounds like it is just faulty, though.

    If it used to be OK, but is now faulty, look for corroded lead shielding, dry solder joints, resistors that have gone high in resistance.

    You may be able to bring the box to a workshop and establish whether you can duplicate the faults there without the road loop. You will need a schematic diagram or a working model of the same device.

    Logic gates and microprocessors are very sensitive to interference on input pins that have been left unconnected. So, a pin that slowly builds up a voltage can have a dramatic effect on a circuit. This is mainly a design problem and you may be able to get help from the maker of the equipment.
  4. Oct 18, 2011 #3
    Thanks vk6kro. I get your point. But what I am wondering about is- is it the way we interpret the magnetic profile of a vehicle over loop that causes the error / or the electronics in the relay?

    I have found that loops undercount and overcount a lot leading to errors- hanging on ( relay output 1 even when no vehicle is on the loop) ?? Where is this error generated?
  5. Oct 18, 2011 #4

    jim hardy

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    things that work great in the lab often fail when subjected to the real environment.

    take some failed ones apart and see what's broke - one observation is worth a thousand expert opinions.

    Then look where they're installed. rebar in concrete will affect nearby induction loops and will have to be calibrated out.

    is there a link to the tech manual for these things?
  6. Oct 18, 2011 #5


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    They probably work like an upside down metal detector, so the output would vary with cars passing over the loop, but probably this variation would only be quite small for motor bikes and small cars because of the distance to the car from the loop.

    So, the electronics would have voltage comparators in it which decide the difference between "no car" and "car".
    This is where design makes all the difference because noise on the input signal can make the sensor decide there is a car when there isn't one.

    It is important to stay focussed, though. If the thing worked before, there is no need to redesign it. Something has gone faulty and you just need to find it.

    If cars pass over the loop very close together, then they can easily count as one car. There probably isn't much you can do about this.
  7. Oct 19, 2011 #6
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