Communication is an integral part of every networked control system. The ability to communicate wirelessly has many benefits over its cable-driven counterpart. Lower installation and maintainance costs come along with a tremendous increase in flexibility as, for example, robots or other parts in an automated factory can move around freely.
Aspects like communication reliability, delay and latency which are unproblematic in a wired communication system become challenging in a wireless approach. The flexibility gained by a wireless solution allows entities to move around freely which may result in very dynamic networks that could also extend over multiple communication hops. Depending on the application task, different communication patterns may be needed spanning the whole range from one-to-one to all-to-all communication. To be able to control also fast mechanical systems, a fast and reliable communication mechanism is needed. The solution should be cheap, energy-efficient and come with a small form factor as, for example, drones are not able to carry around bulky and heavy additional hardware. Overall we aim for the following properties, which our proposed solution achieves:
- high reliability with packet reception rates over 99,9%
- communication latencies in the range of tens of millisecond
- support for dynamic networks with permanent topology changes
- support for arbitrary communication patterns between entities in the system
- multi-hop capabilities to communicate over long distances
- low cost, low power off-the-shelf hardware
Our proposed solution
The hardware basis is a dual processor platform built out of off-the-shelf components. This platform features two ultra-low-power microcontrollers (CC430, MSP432), one dedicated to communication tasks and the other to control-related (application specific) tasks. The two microcontrollers are connected via a dedicated processor interconnect (Bolt) that enables a predictable message exchange between these two.
Synchronous transmissions are the basic building block of our wireless protocol. In particular, we use a series of Glossy floods, a communication primitive that has proven to achieve fast and reliable message exchange in dynamic multi-hop networks, to deliver all messages during periodically scheduled communication rounds. In addition to that, the wireless protocol also time synchronizes all entities in the network with sub-microsecond accuracy.
Communication and application tasks, this is, operations executed on the communication and application processor, are precisely scheduled according to an offline calculated schedule that is optimal with respect to latency and energy-efficiency. In combination with the accurate time synchronization, the latency of the data exchange and other relevant time intervals, such as update intervals of the controller, become very predictable with negligible variation.