As in occupational safety the RFID system is usually a safety system, the demands made of the system features are different from those made of an access system, for example.

The first important point is that the transponder is worn on the body of the person to be protected, i.e. the human body (90% water) is not permitted to have any influence on the functionality of the transponder and the body must to a certain extent be transparent for the transponder. The transponder must also function if the person should fall and end up lying on the transponder. If this happens, it could be pointing in any direction, i.e. the transponder must be able to recognise the protective magnetic field in three dimensions.

Both of these requirements show that traditional transponders, such as those used to date in inventory tracking, access controllers or automotive toll systems, are not suitable for safety applications.

Abb. 2

Figure 2 illustrates the basic principle of operation of an RFID system used for occupational safety. The controller module is the logistical brains of the system. It creates the protective field that scans the entire danger area with the antenna. So that this can be classed as a safety system, it must be ensured that the magnetic field is always present and cannot be disrupted by a fault or error, such as a fractured cable, for example. This is achieved by constant measurement of the protective field. One way of doing this is to attach a reference transponder. If this loses the protective magnetic field, it reports this to the controller module which then switches off the machine via its controller output, for example, so that there is no longer any danger. The same happens when the transponder enters the protective field.