In Ground Vehicle Detectors for Automatic Gates
Used primarily for 'Automatic Exit', which is opening a gate automatically when a vehicle drives up to it from inside a property. In ground vehicle detectors can be either a loop of wire cut into a groove in a driveway or a probe buried next to the driveway. Either way they are basically a metal detector so pick up vehicles only not pedestrians so if some one jumps the fence they cannot use it to open the gate.
Ground Loop Vehicle Detector
Are normally made by cutting a groove in a concrete or bitumen driveway 25mm deep x 3mm wide in a rectangle 1m x 1.5m set about 6m from the gate. The wire loop is made from 1mm2 or 1.5mm2 conduit wire the goes around the rectangular groove three times and then is a twisted pair back to an electronic loop detector that is normally located in the gate motors control box. The wire loop and twisted pair held in the groove with silicone and run in conduit alongside the driveway back to the gate motors control box.
When a vehicle drives over the wire loop in the driveway the loop detector senses the change in the magnetic field like a metal detector does and opens the gate.
Loops detectors are the devices used for controlling traffic signals and can also be used to replace photocells on boom gates or barrier arms where the gate isn't near the ground. If the gate is near the ground the metal in the gate will trigger it off so photocells must be used for a safety device. The sensitivity of a loop detector needs to be correct as well because if too sensitive they can false trigger and cause the gate to open by itself.
Vehicle Detector Probe
Are a tubular device buried 200mm to 600mm in the ground along side the driveway detecting vehicles moving within a 4m radius. The advantage of a probe is you don't need to cut a groove in your driveway and they are more cost effective.
The disadvantage of a probe is you cannot set a specific detection area you're stuck with the 4m radius so if your driveway is close to others you may pick up vehicles from next door. They also detect movement only so if a vehicle pauses too long the gate may close on them. Loop detectors can be set to sense the presence of a vehicle rather than the movement so is more suitable for Gated Communities or Commercial Buildings where there is a lot of users.