Wireless systems that coexist

A number of different wireless standards – such as WLAN and Bluetooth – have emerged for industrial automation applications. When several wireless systems are operated simultaneously in close proximity, sporadic interference can result from overlapping signals. The adaptive frequency-hopping technique (AFH) with its frequency limiting functions provides a suitable solution for such problem areas. It permits different wireless applications to be operated in parallel without interfering with each other. The availability, bandwidth, range and latency remain unchanged.

Weidmüller uses the AFH technique for its SAI Active Universal Wireless module. The module features an industry-compliant IP67 protection level and is equipped with an integrated antenna that has isotropic directional characteristics. The Bluetooth stack implemented for its wireless communication already uses the AFH frequency hopping technique for limiting frequency usage. This ensures that the facility can reliably use other wireless systems even when these systems are able to communicate on the same wireless frequencies.

The SAI Active Universal Wireless system consists of a wireless PROFIBUS gateway and one or more wireless slave modules. Thanks to its advanced configuration functions, up to 31 gateway modules, each with six wireless slave modules, can be operated simultaneously and in parallel. This allows up to 3,348 digital signals to be processed in a wireless automation application.

Despite the low transmission power of just a few milliwatts, ranges of up to 100 meters are possible. The standard Bluetooth SPP profile is used and a CANopen-based communication protocol is run over it. The integrated wireless functionality ensures interference-free data communication and allows wireless systems, such as WLAN or Bluetooth, to coexist.

The benefits of AFH and frequency usage limitation

If, for example, a WLAN system is operated in parallel with a Bluetooth system within a single facility, you would need a "warm up" time after every cold start during which the system must detect the busy frequencies. This is where Bluetooth AFH and frequency usage limiting can be used to supplement the system.

Performance can be significantly increased by making a simple firmware upgrade to the Bluetooth device. Because the wireless system transmits on a fixed channel within a constant frequency band, these frequencies are already filtered out in the Bluetooth device. The list of these frequencies is stored permanently in the device so there is no longer the "warm up" time during the facility start.

But because of its AFH functionality, this system is able to adapt quickly to other wireless systems and maintain a continuous communication link. The frequencies which should not to be used are specified using the engineering system. The GSD file (device description file) provided with the SAI Active Universal Wireless gateway specifies all the necessary parameters that can be customised by the user. Even when a device is replaced, these parameters can be uploaded from the engineering system to the new module without the need for any proprietary software.