Goals

The Demonstrator setup inherits various latencies when the process of regulating the paper ring's position takes place:

• The information gathered by the sensor has to be processed by the first system.
• The packet has then to find its way through the network and to the board with the controller software.
• The controller must calculate the electromagnet's next state whilst sharing the board resources with other programs and services.
• The resulting values again are to be sent over the network to finally reach the actor's system to get applied at the right moment.

With the system as a whole now being to slow to maintain a stable floating position the individual components have now to be coordinated in real time.

Subsequently, we realized our PID-loop as follows:

• By using PTP (IEEE 802.1AS), the internal clocks of our boards were synchronized within in the margin of less than a microsecond.
• Critical latencies within the software environment were tracked down and solved if necessary.
• The positioning sensor is being read synchronous to the PTP timer and a respective time stamp gets applied; the daten then needs about 600 µs from sensor to actor.
• The PID controller runs at a frequency of 1.25 kHz / 800 µs.
• Lastly, the actor's software has to take care that the new sVtate of the electromagnet is applied at the right time.

The result can be seen in the accompanying pictures.

Next steps

The Demonstrator setup allows for first insights into the different technologies around TSN under Linux. During the development a range of current SoCs was evaluated regarding their TSN capabilities: Not all features are available at every hardware, so the SoC has to be chosen specific to the task at hand. Additional room for improvements also exists on the software-side - many of the used technologies are still young and quickly evolve from Kernel to Kernel. It is therefore recommended to always use a current version of Linux.