With digital production and the increasing networking of machines and systems, the need for flexible, scalable transmission technologies is growing. Up to now, wired fieldbus systems have mainly been used for signal transmission between machines, tools, systems and robots, as cables are still considered the ultimate solution due to their safety and reliability.

However, wired communication quickly reaches its limits in modern and dynamic environments. In the case of connections between sensors and actuators, for example, cables severely restrict the freedom of design and movement. The same applies to fast-moving tools and machines. In some scenarios, such as human-robot interaction, cables cannot be used at all.

Wireless connections offer a decisive advantage in terms of flexibility and freedom. However, there is also a downer: existing wireless solutions are not suitable for safety-critical areas with high demands on robustness and low latency. Most applications of wireless communication in industrial environments are therefore currently limited to non-critical scenarios where the transmission characteristics of standard wireless technologies such as WLAN or Bluetooth are sufficient.

Many applications also have special requirements in terms of information security. However, the use of air interfaces opens up new possibilities for attacks that can be exploited by hackers, for example. When transmitting wirelessly, the data streams must therefore be specially secured to protect them against unauthorized access. This is the only way to guarantee data security and the safe operation of production.

Thanks to star topology, the individual device nodes can always send their messages to several echo nodes simultaneously. The number of echo nodes involved in the respective communication is freely definable.

© Newtec

The requirements are therefore by no means trivial. For this reason, wireless technologies have only been used in isolated and special cases for critical process control tasks or safety-relevant scenarios. As a rule, these are proprietary systems for special applications, which are superior to standard radio systems due to their individual properties. In other respects, however - such as scalability - they do not achieve the performance characteristics that are desirable for flexible, broader use and transmission in line with requirements.

To close this gap, Newtec and Offenburg University of Applied Sciences have developed a Bluetooth Low Energy (BLE)-based wireless technology for safety-critical applications.

Innovative star topology

From the outset, the aim of the development was to create a wireless technology that enables energy-efficient data transfer in latency- and safety-critical applications. It quickly became clear that the attempt to improve the classic point-to-point connection would not have the desired result. A completely innovative approach was needed to meet all the requirements for flexibility, speed and robustness. This was found in the development of a new architecture.

The core of the new solution is a star-shaped wireless network topology with a variable number of cooperating echo nodes and a definable number of individual device nodes. This topology also gave rise to the name NTStarEcho.

The solution uses the TDMA (Time Division Multiple Access) time-division multiplexing method for signal transmission. Recurring TDMA sections (time slots) are available for the exchange between echo nodes (EK) and device nodes (DK). In order to transmit the signals with minimum latency, these time slots are variable (asynchronous). The star topology now makes it possible for the individual device nodes to always send their messages to several echo nodes simultaneously. The number of echo nodes involved in the respective communication is freely definable.

Safety and latency-critical applications

Compared to a conventional point-to-point connection, this architecture offers decisive speed advantages, as the first available echo node - i.e. the node with the next common time slot - always forwards the message to the addressed device node(s). A special scheduling algorithm makes it possible to shift the TDMA time slots between a device node and several echo nodes against each other (asynchronous method) in such a way that a maximum latency gain is achieved. Latencies of 0.5 ms can be achieved with this technology.

Device node A sends a message to device node B via a desired number of echo nodes (here EK1-EK3). The echo node that has the first common time slot with B (marked green) retransmits the message. If only the echo node EK1 were included here, there would be a significantly higher latency (gray arrow).

© Newtec

The redundant signal transmission not only ensures a high transmission speed, but also a high level of transmission reliability: if an echo node or a signal path should fail, the next available connection is used. The communication protocol specially developed at Offenburg University of Applied Sciences enables use even in safety- and latency-critical applications, as it recognizes safety- and latency-critical data packets and sends them preferentially to the receiver(s). A special logical link control layer also guarantees a fixed residual error probability and detects bit errors and faulty physical connections.

Thanks to these properties, NTStarEcho is very well suited for many areas of industrial automation, especially for human-robot collaboration, where the high requirements previously made the use of wireless technologies impossible.

Industrial robots that are approved for collaboration with humans are still extremely inefficient. This is because, in order to minimize the risk to humans in the danger zone, these robots are only allowed to move at a maximum speed of 250 mm/s. For higher speeds, it is essential to transmit the data of detected people and objects to the robot with low latency and high reliability. Thanks to its topology and the use of the asynchronous TDMA method, it is now possible to transmit sensor data virtually in real time using the new technology. The redundant echo nodes, the number of which can be flexibly expanded depending on the required security level, ensure the necessary transmission security.

The application scenarios

Keyword flexibility: thanks to the configurable architecture, energy consumption, latency and transmission reliability can be coordinated and designed to meet requirements. For example, it is possible to reduce latency and increase transmission reliability by increasing the number of integrated echo nodes. As this requires additional transmission power, the energy consumption for the device nodes increases. If, on the other hand, the transmission reliability and latency requirements are lower, the energy requirement can be reduced accordingly by reducing the number of echo nodes. In any case, maximum energy efficiency is ensured thanks to Bluetooth low-energy technology.

Due to these flexibly adaptable properties, the technology is suitable for a wide range of applications in production. Where latency and robustness play a subordinate role - for example in the data transmission of fast-moving tools for optimized maintenance - energy consumption can be significantly reduced as described. However, when it comes to monitoring critical industrial applications or sending alarm messages in the event of emergencies, the required security and robustness is simply achieved by increasing the number of echo nodes.

Even when it comes to networking existing systems, the new wireless technology can significantly reduce the effort and costs compared to wired connections. The high scalability of its basic architecture also enables production to be upgraded at any time as required with new, additional functionalities or the inclusion of newly added machines and systems.

Safety & Security

The wireless solution can be set up with various commercially available components. However, the echo nodes should be sufficiently powerful with a space-saving design to process large amounts of data in real time and with the lowest possible latency.

The author: Matthias Lai is a system architect and wireless specialist at Newtec.

© Newtec

Newtec recommends an architecture based on its NTSecureNodes BLE 100 as echo nodes. This enables a particularly fast, cost-effective setup of a corresponding wireless network and also allows high functional safety requirements (safety up to SIL 3) to be implemented. Last but not least, the use of these nodes in an NTSecureCloud environment enables secure end-to-end encryption. This means that the transmitted data can be securely protected against unauthorized access and manipulation.