A cyber-physical system is formed by a network of interacting sensors and actuators. With the availability of small inexpensive sensor elements, such a network may contain thousands of interacting sensors. This makes such systems a little bit similar to biological systems. For example, the human body has 3 billion touch sensor cells. Still a notable difference though.
However, even for a few thousand sensors, conventional communication methods come into trouble. With a typical network, it takes too long to transmit such a large number of sensor measurements.
In the paper “A Scalable and Efficient Approach for Obtaining Measurements in CAN-based Control Systems” scientists from Portugal and Austria present a solution to this problem by polling many sensors at once while actually transmitting only one representative measurement.
The idea is based on so-called priority-based medium access control protocols:
- the message consists of a number of bits, each bit being 0 or 1 (any message can be encoded that way)
- when the bits are sent via transmission medium, one of the states can overwrite the other (for example if 1 overrides 0, then a synchronously transmitted 1 and 0 result in a 1)
- all sensors start their transmission at the same time, for example triggered by a query message
- a transmitting sensor monitors its own transmission; if a bit gets overwritten it immediately stops transmission. This way it is possible to get the largest value (or by inverting the messages, the lowest value) of a number of sensors with a single query. If there are thousands of sensors, the protocol definitely pays off.
Andersson, Pereira, Elmenreich, Tovar, Pacheco and Cruz employ this method to get a map of sensor readings with just a few messages. The priority-based protocol is here used to select the sensor messages with high information content. This approach greatly reduces the time for obtaining sensory information, which makes cyber-physical systems ready for use in fast control systems.
The full paper is available as a technical report. It was published in the highly respected IEEE Transactions on Industrial Informatics:
- B. Andersson, N. Pereira, W. Elmenreich, E. Tovar, F. Pacheco, and N. Cruz. A scalable and efficient approach for obtaining measurements in CAN-based control
systems. IEEE Transactions on Industrial Informatics, 4(2):80–91, May 2008.