Induction loop
An induction loop (also inductive loop , contact loop or ring loop) is in principle a simple wire loop that is used for various purposes, in particular for signaling and information transmission using the principle of electromagnetic induction .
Use in road traffic
In traffic engineering , an induction loop is used to identify vehicles on the route. Large cable loops embedded in the road surface or laid under it are used, preferably in front of the stop lines of demand-controlled traffic lights , to switch on red light surveillance cameras or to open and lower barriers in parking garages or at toll stations, etc.
functionality
The induction loop acts as a coil . It is connected in parallel to a capacitor and thus forms an oscillating circuit whose resonance frequency depends on the capacitance of the capacitor (fixed) and the inductivity of the coil (the induction loop). If there is a metallic object (e.g. a vehicle) in the area of the induction loop, the inductance and the quality of the coil are changed: field displacement and eddy currents reduce the quality and inductance, ferromagnetic (sheet metal) parts increase the inductivity. This changes the resonance frequency and the damping of the resonant circuit. This is evaluated by electronics. This principle is suitable for the static detection of vehicles.
The following principles are also described, but they can only register the arrival / departure of the vehicle:
- A constant direct current flows through the loop / coil. If, in particular, a ferromagnetic body enters the resulting magnetic field, this changes, which changes the coil voltage.
- Vehicles deflect the earth's magnetic field by bundling it in their iron mass. Due to the change in the magnetic flux ( induction principle ), a voltage pulse is generated on the coil when the vehicle moves.
Utility functions
Traffic- dependent traffic light controls activate the relevant direction of travel when a vehicle is reported via the induction loop. In addition, depending on the type of control, the release can be extended until a gap is detected in the vehicle current.
An induction loop laid at the stop line can be used to determine whether a vehicle has entered the intersection. Red light violations can be recognized automatically (see speed monitoring ). In addition, induction loops are used to count traffic, including mobile versions that are simply placed over the route.
By evaluating the measured envelope curve , it is possible to differentiate the recorded vehicles by type (e.g. car, motorcycle, truck). With a double loop (two induction loops that were laid one behind the other at a short distance), in addition to a finer type differentiation (e.g. small versus large vehicles or trailers), the speed can also be determined.
By designing the reception sensitivity to specific frequencies (around 10 kHz ), for example, ambulance vehicles with appropriate signaling devices can preferably set the traffic light signals for their direction of travel to green in order to move faster.
Induction loops for controlling traffic lights are generally optimized for motor vehicles. Whether cyclists are also detected by the induction loop depends on the sensitivity set. The metal content of the bike also plays a role. To achieve the highest possible disturbance effect and thus the highest possible probability of detection, it is recommended to drive over the edge of the loop.
Application in rail transport
In rail transport, induction loops are used for data transmission in train protection and train control systems such as SELCAB or in line train control . The strung together loops are referred to here as line cables because of the straight, continuous routing within a cable.
Application for the hard of hearing
An induction loop system enables hearing aid wearers to wirelessly receive audio signals such as music in cinemas and theaters , speeches at events and lectures, e.g. in churches, etc. via the hearing aid . For this purpose, a hearing aid must have a so-called telephone coil (T-coil for short) that absorbs the electromagnetic alternating field of the induction loop.
See also
Web links
- Induction loops for controlling robots
- Illustration of an information sign for the identification of induction loop systems
- Induction loop in churches (PDF; 17 kB)
Individual evidence
- ↑ How the loop works
- ↑ Martin Spletter: City of Essen now counts cyclists. In: WAZ. Retrieved January 22, 2016 .