Exit sensor technologies

      Exit sensor technologies

More technologies are being used for detection of vehicles. We examine the options.

Author  :    Huw Jones

Why do we need exits sensors?

So why do we use an exit sensors when all automated gates have remote controls? There is the 'wow' factor that impresses guests, like the brass door knobs and gold taps. 

Automated gates in europe seldom have exit sensors or access keypad. They have security gates on relatively small forecourt areas on suburban properties. It is likely the gates are the first line of security for vehicles and the property.

Exit sensors in UK are likely to be on larger residential properties where there is an access control alternative to remote controls. One can assume keypad users are occasional trusted visitors or regular tradesmen servicing the property. The exit sensor exists to let out those who entered using the access keypad.

Vehicles only?

Strictly speaking, exit sensors are about the detection of vehicles for the purpose of activating a gate or barrier. But many technologies have varying degrees in the ability to discriminate between a vehicle or any other moving targets. They have been included here with caveats because they are described as exit sensors.

An exit loop can be like dangling the key to your front door. It is an invitation to steal a vehicle, though applying a time clock or twilight sensor for daytime only activation limits the risk.

Pet owners in particular need an exit sensor that does not let their dogs out on the road. In rural areas, that can also apply to horses and other livestock getting out, or letting in deer. 

When considering an exit sensor, consider the vehicle trapped inside a property when a sensor doesn't work as expected, or the gate that flies open randomly at the wrong time. Don't let a cheap technology cause a disatisfied customer.

Traditional loops

Traditional road loops are reliable and infinately adjustable. The installation process though is also time consuming and messy. And if the client wants to change the position, it is do it all again.

Road loops should be kept for presence (safety) applications where position change is unlikely.

There are car park applications where an exit loop is justified because of concrete re-inforcement, vandalism or public access. The multicore loop technique shifts the effort and mess onto  the groundworkers where it belongs.

Miniloops

Diablo's Miiniloop is a simplification of an induction loop, but for exit loops only. It's range and applications are narrow.

High gain versions can be set at the side of the road making installation super quick, but then a childs bicycle ridden close by the sensor could open the gate.

Magnetometer

This technology uses the earths magnetic field as the test medium. They are the only single point sensor to offer pulse output and presence. It is a modern technology that will become more effective an cheaper in time.

They do not work well where the magnetic field is disturbed by large ferrous objects, or concrete re-inforcement, or in a steel framed multi-storey car park. 

Two photobeams

An old school approach is to use a pair of photobeams across the road. They are quick to install, easy to move, and reliable.

Set them a distance apart to trigger any car, but too far apart for an animal. Set at 500-700mm high. Battery beam transmitters can keep the cabling to one side of the drive.

A pair of beams can also be operate in one direction only if required. Trigger only when E1 is crossed before E2. So what qualifies them as a vehicle sensor is only triggering within a time period, and having them set at a height and separation that works for large objects.

Any beam type will do. Retro- reflective beams or battery beams will reduce wiring across the road.

Single ToF

These are single point optical sensors that look for a reflection off an object, so clearly not a vehicle only sensor. But as shown with the use of two photobeams, some risks can be excluded by intelligent setting.

ToF beams have distance measuring settings. Some will trigger on distances below a setting. Some will trigger on distance between two settings. ToF beams can be selective because they are narrow and focused.

Microwave

Microwave sensors get a mass of indecipherable reflected noise off everything. Doppler effect is a rising tone from reflections off an object as it approaches, or falling tone as it moves away

On their own, microwave sensors only respond to movement in one axis, and that can be humans as well. They have adjustable sensitivity but no selectivity. 

The reason for consideration in vehicle detection is when they are combined with another technology such as a ToF in the same device.

Microwave sensors are the most common sensor for automatic door activation. They read through plastic so can be completely weatherproof.

PIR sensors

These "Passive Infra Red" sensors are old tech home movement sensors. The operation principle is to compare heat radiation received on two sensors. In practice, an intricate Fresnel lens splits  viewing area into 8 or more vertical zone strips, summing all the odd strips and all the evn strips.

When a body moves across the angle of view, it moves from odds to evens to odds. The output is activated when the balance changes.

As a heat sensing device, a warm engine on a cold day is easy to detect, but so are people. Works well on sunny days. Not very predictable.