Buried sensor for 'People' detection?

[Electroguard]

Hi,

Does anyone know of a practical 'people' detector that uses buried cables to detect when people walk across, eg: for perimeter detection around a garden etc.

I know it must be technically feasible because some expensive commercial systems do exist, but I don't know what sensing principle they are using or what they are actually sensing.

I was wondering if a pair of cables buried a couple of feet apart might perhaps be able to register a capacitance disturbance, but I don't think thats very likely given the area of the detector in ratio to the small and fleeting disturbance of a person stepping across.

However, I'm old enough to remember AM transistor radios with telescopic aerials, and making 'music' from a theramin, so perhaps it may be possible to read a difference in the RF coupling of an underground transmitter wire 'aerial' adjacent to a receiver wire detector.

Whatever sensing principle is used, I'm expecting to need to do an Arduino controller to monitor and compensate for drift of such a big sensor.

Has anyone been down a 'buried cable' sensor path before?

Thanks in advance for any kind suggestions.

 

[nsaspook]

Exactly, it's hard to do with a single sensor for a large area as the S/N ratio from capacitive proximity would be too small with lots of false positives if you crank-up the gain to make it work.

This is more about tracking systems in general but the sensor requirements for networks are much the same.
https://thiagot.com/papers/teixeira_techrep10_survey_of_human_sensing.pdf

 

[dr pepper]

I've fiddled with this back before microcontrollers were popular, as mentioned noise is a problem, if you make something that will pick up a person 2m away, even 1m away it'll probably pick up a car many times that distance, and rfi from phones etc.

Capacitance is one way of doing it, include the sense wire in a rf oscillator, and if the freq changes because someone has come close trigger an alarm.

A theramin beats 2 rf frequencies togther, one frequency is a fixed oscillator and the other is a variable freq oscillator, the 'wand' being part of the inductance/capacitance tank, the diffrence of the 2 freqs is the output.

 

[AnalogKid]

That report looks very interesting. Haven't read it yet, but my first thought is this: If you do the theremin approach with one oscillator slaved to another with a slow servo loop, the system would auto track changes in the environment without generating an alarm. However, something moving faster than the control loop bandwidth, like a walking person, would create a literal "disturbance in the force." This is similar to the operation of a traffic light sensor.

ak

 

[nsaspook]

This is similar to the operation of a traffic light sensor.

ak

Even those types of systems (inductive loop) have detection problems as anyone stopped at a light on a motorcycle knows.
https://www.fhwa.dot.gov/publications/research/operations/its/06108/02.cfm

LOOP SYSTEM SENSITIVITY
Loop system sensitivity is defined as the smallest change of inductance at the electronics unit terminals that will cause the controller to actuate. This sensitivity must be equal to or greater than the threshold for the electronics unit. Many states specify that the electronics unit must respond to a 0.02 percent change in inductance. NEMA Standards (see Section 15.3.2 of Appendix J), recognizing the differences in electronics unit design (

L/L or

L), specify the sensitivity threshold for three classifications of test vehicles when they are centered in a single 6- x 6-ft (1.8- x 1.8-m) three-turn loop with 100 ft (30.5 m) of lead-in cable. The vehicle classes are:

• Class 1: 0.13 percent (
  
  L/L) or 0.12
  
  H (
  
  L) inductance change (small motorcycle).
• Class 2: 0.32 percent (
  
  L/L) or 0.3
  
  H (
  
  L) inductance change (large motorcycle).
• Class 3: 3.2 percent (
  
  L/L) or 3.0
  
  H (
  
  L) inductance change (automobile).

 

[AnalogKid]

If it were easy, anyone... Been a looong time since I messed with a theremin, but the delta-L caused by waving your hand near a little whip antenna has got to be tiny, yet a 500 Hz change in an oscillator's frequency is very easy to detect, so at the concept level it seems like a good place to start. I suspect a big problem will be reduced sensitivity caused by burying the sensor, but I'm just winging it here.

ak

 

[JimW]

Keep in mind that a Theremin uses the ground as a reference point for the variable oscillator. If the pitch antenna is anywhere near ground (or a desk, car, elephant) then you have a problem. So I think having a cable anywhere on or under the ground is an issue. Could you build a non-standard variable oscillator that uses the interference in the electrical field between two cables (as mentioned by the O.P.)? I would say maybe, but still unlikely if both are on or under the ground. The electric field is going to be contained by the ground and have only the smallest impact by anything moving above it.

I will freely admit that this is not my field, but I have built several theremins and have had to work through some debug issues on them based on their installation.

-Jim

 

[dr pepper]

There are commercial systems that do this, so it can be done.
Maybe they use sub mhz frequencies.
And probably a load of digital junk including dsp's.

 

[AnalogKid]

Now, be polite. that digital "junk" is just analog stuff with disabilities.

ak

 

[dr pepper]

Oops, maybe I should have been a little more PC.

 

[Tony Stewart]

The body has no metallic conductors to resonate with H field magnetics like a vehicle
The dielectric is the key quality of the human body by remote sensing except proximity is challenged by having access only from one side giving very poor sensitivity.

One could have plurality of optical transponders from the ground but this might be a maintenance nightmare.

From my past antenna testing, I discovered that VHF works very well with a dipole and a directional coupler to measure reflected signals up to 5 wavelengths or so using a hot carrier diode to generate a small DC voltage from the return signal generated from the proximity of any person.

This could be a 1/2 wave dipole or a loop antenna situated around a patio stone, such that anyone stepping near the stone can be easily detected.

3dB splitter could be used in place of a stripline directional coupler and Schottky diode used in place of hot carrier diode with signal conditioning required to amplify the rectified reflected carrier, with DC gain , very low HPF , ac gain and comparator to detect threshold of motion, then one shot to extend duration of motion with amplified DC for another threshold comparison. The AC threshold is lower than the DC threshold.

 

[Electroguard]

Thanks for your responses guys. Not to worry, it was more out of interest than any critical need. I'm a bit relieved in a way, because the property used to be a lorry yard and the ground is mostly compacted hardcore. I rent out 4 mobilehomes here, and from bad past experiences I need to know when people are straying where they shouldn't.

I've got a couple of PTZ CCTV cameras, but even though one of them is auto-tracking, they can't see anything going on outside their field of view unless something controls them to move, so I've done an Arduino controller which can send PTZ IR control sequences according to triggered inputs and thus turn the cameras to face the action (and speak voice announcements for off-limit areas), hence the need for more effective longer range sensors than the current beam-break and standard 12m PIRs that I have at present.

If anyone had known of any underground sensor system that already worked then I would have enjoyed taking things further, but it's beyond me to try to develop something like that from scratch, so I'll be reverting to plan B and ordering three long range (40m) PIRs.

Thanks again for your replies.

 

[Tony Stewart]

if there weren't any moving foliage, motion sensing VGA camera software from Logitech worked well for me at one time with on-screen alert zones

 

[Electroguard]

Be lovely if that had been sending Pelco commands over RS485 Tony, but I think it was probably more likely to have been a motorised usb webcam (I used to have something similar but called something like Hawkeye or EagleEye).

The problem with motion tracking is that the target still needs to appear into the cameras field of view first to gets its attention, whereas triggered sensors can command the camera to look behind to its blind spots.

The auto-tracking camera is mounted high up and normally looking forward towards the driveway and gates, and it does a fairly good job of following visitors in and under and behind on down through the main yard, but when its looking forward at the gates its oblivious to anyone already in the yard or on the property behind.

I can get over that by having sensors in the yard causing the camera to look behind when triggered, but normal PIRs don't even cover half the yards width, and long range external PIRs aint cheap, so if underground cable sensing had been feasible it might have offered a good solution.

I was also considering the cheap doppler radar sensors which are now available, but I don't think they would offer any benefit over normal PIRs, and would still pose their own problems.

BTW, the reason I'm using the Arduino to control the PTZ CCTVs using an Infra-red controller and by sending it IR command sequences, is because I haven't been able to find a Pelco RS485 library for the Arduino - nor even any practical PC freeware for that matter, which is particularly frustrating when you can pick up USB and serial RS485 adapters off ebay for 99p - but I thought I'd mention it because I still live in hope!

 

[Tony Stewart]

Doppler might suffer from near field blind zone due to response time after burst, whereas measuring return loss is full duplex sensing of CW reflected energy. All antenna have dead zones though so mixing antennae zone could be problematic from Fading.

Generally pulse IR carrier is the most sensitive omnidirectional motion sensing used for garage door openers with BPF and AGC and also cheap for wide coverage. Not sure how rapid PIR detectors can respond at distances.

 

[dr pepper]

I have some doppler modules, with a directional horn they work over 20m or so.

 

[Mike Maynard]

We manufacture a Perimeter Intrusion Detection System which uses RG-11 coaxial cables buried in parallel (or cable to antenna). This system uses RF technology and transmits a set of frequencies from a Tx cable to an Rx cable (or Rx antenna) and the disturbance results in alarm annunciation. This system is installed in very high profile applications and there are a few other companies in the world who also manufacture similar PIDS. This system would probably not be cost effective for a garden.

 

[Tony Stewart]

When I used a direction coupler to measure return loss with a hot carrier diode and DMM on VHF dipole antenna 1 to 2m above the ground, I found I could detect motion of people within 10m easily within the radiation pattern of the antenna, when the person moves over 1/4 wavelength which modulates the reflected signal.

However this would work great for hallways not a perimeter motion detection would require many antenna and results in deadspots of signal cancellation. The setup would require distance tuning of each antenna to null reflections ( multiples of 1.2 wavelength) so that any moving targets trigger reflections above the null.

PIR thermal detectors, which is fairly sensitive but much slower in response can be strung be radially wired or networked as a perimeter detection method effectively.

But these are common methods for intrusion security detectors.

The next consideration is laser diode or narrow beam LED restricted path loss from emitter to detector across a path, where bandwidth of signal is inversely related to distance like a remote control with beam path limited to prevent false reflections from path interruption. THese can be effective for medium range paths.

THese are the cheapest methods to detect the dielectric reflection , optical and thermal of humans and thermal emission respectively.

NExt choice is a low cost video system with motion detection. When I got my first WebCamGo from Logitech in 1999, it had on-screen zone motion detection free software included with FTP snapshots uploaded.