Post by thread:help wanted on detection loop

At 08:59 PM 7/21/99 +0200, you wrote: >hello, > >I'm working on a project to operate my driveway gate by using satellitedish >motors. for the mechanical part it went ok, but for the electronical part i >have some difficulties. I want to operate my gate by driving over an >detection loop in my driveway, the kind they use by traffic lights. maybee >one of you can give me a hint of how these detection loops are working. > >Please advise, >Regards, >Henk Smit. > > The standard technique is to use a multi-turn loop buried in the roadway and use the inductance as one component of a tuned circuit. I vehicle passing over it changes the inductance and hence the resonant frequency. The problems are the standard ones: detecting a small change in frequency, stability of the circuit under changes of temperature, voltage, etc. I have seen simple units advertised for a few hundred dollars, but it should not be too hard to design one using a PIC to run things. Typical frequencies seem to be 40-100khz. One could also put the inductor into a bridge circuit and measure the actual inductance. One of the drawbacks is the need to dig up the drive to bury the sensor -- a real pain if the drive is already paved. There are a number of other techniques in use also. There are some magnetic sensors that can be used to detect the presence of an automobile directly. Optical methods are also used (beam across the road, etc) but are subject to being tripped by humans, animals, rain. Infrared detectors (for the heat of the engine) are also possible, but this wouldn't detect a stopped car after it got cold. Pressure sensors in the drive are often used in traffic counters, and this is very simple, but not suitable for very high traffic areas (wears out, etc). I have also seen an ultrasonic sensor employed for detecting vehicles, but is also subject to certain false alarms, blah, blah, ... I suppose there are probably another dozen on so physical phenomenon that could also be used as well. Whichever technique is used there are quite a few engineering considerations if a solid reliable system is required. Bob McClure

> > Optical methods are also used (beam across the road, etc) but are subject > to being tripped by humans, animals, rain. > I was thinking about this also. I thought of two ways to make the system more reliable, but would not necessarily work in all situations. First, use a software timer to detect how long the beam is broken. Short breaks would indicate humans or animals. Second, another beam placed ten - fifteen feet further down the drive than the first could be used to tell if someone was standing in front of the first beam. In other words, first beam is broken, set a timer, if timer expires and beam is still broken, it's probably a vehicle but it could just be obstructed. Now watch the second beam to see if it gets broken ... Eric

H. Smit wrote: > > hello, > > I'm working on a project to operate my driveway gate by using satellitedish > motors. for the mechanical part it went ok, but for the electronical part i > have some difficulties. I want to operate my gate by driving over an > detection loop in my driveway, the kind they use by traffic lights. maybee > one of you can give me a hint of how these detection loops are working. > > Please advise, > Regards, > Henk Smit. They detect the change in inductance of a coil caused by a nearby metal object. Usually they're set up in a figure 8 pattern so nearby mag fields don't affect them. Only having a metal object over one half of the figure 8, but not the other. -- Anniepoo Need loco motors? http://www.idiom.com/~anniepoo/depot/motors.html

I've thought about this somewhat, you don't necessarily want to let the neighborhood burglars open your gate automatically; For situations where you want only "Authorized" vehicles to open the gate, you could have a coil under the vehicle & inductively (transformer) couple between the two - use some kind of Challenge/Reply protocol (Could involve a rolling code or a KeeLoq onboard the vehicle, rather obvious use for a KeeLoq.) If you want authentication, you need to SET the frequencies used by each, and use the coils as a transformer, not just detect an impedance change. Also, if you wind the coil carefully, you could use a concrete saw & install the coil without digging the entire driveway up (though it IS probably easier to wind the coil in a PVC pipe, bury the pipe, and prevent any possible expansion/contraction from breaking your wires) - The system I worked on which had such a coil used heavy wire (12 gauge IIRC) for the buried loop, IIRC they just poured concrete directly over the wires (Wires were taped together to the rebar stakes used to set the coil form, IIRC.) That was 6 years ago, and I wasn't involved in that side of the project (I was doing the Windows comm/control code on that.) Mark {Quote hidden}> From: pic microcontroller discussion list [spam_OUTPICLISTTakeThisOuTMITVMA.MIT.EDU] On > Behalf Of H. Smit > Sent: Wednesday, July 21, 1999 2:00 PM > To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU > Subject: help wanted on detection loop > > hello, > > I'm working on a project to operate my driveway gate by using satellitedish > motors. for the mechanical part it went ok, but for the electronical part i > have some difficulties. I want to operate my gate by driving over an > detection loop in my driveway, the kind they use by traffic lights. maybee > one of you can give me a hint of how these detection loops are working. > > Please advise, > Regards, > Henk Smit.