wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Network-Based Control 20-7
20.5.1 Design Constraints in the Network Side
The main design constraints in the network side are the network bandwidth assigned to the controlled
system, the resulting sampling to actuation delay, and the probability of message drops. The assigned
bandwidth is used to interchange messages between the sensor node, the control node, and the actuator
node. On a periodic control model, this bandwidth has to provide enough room for at least one sampling
message and one control message in each control period. The network message scheduler has to respect
all of the control loops timing constraints, each one with its own sampling period. Depending on the
type of access scheme and communication relationship, the assignment of loop bandwidth can become
crucial in the resulting network induced delay.
20.5.2 Design Constraints in the Control Side
In the control side, the constraints are mainly given by the dynamic response of the controlled system.
The dynamic response, also known as the control system bandwidth, can be expressed as the range of
frequencies the control system is able to compensate and is directly related to the characteristic polynomial
of the closed-loop system. This control bandwidth, together with the sampling theorem, expresses
the range of sampling periods that can be chosen for the discretized control law. The application of the
sampling theorem to a discrete control system allows to follow one of the accepted rule of thumb that
states that the sampling frequency should be 4–20 times the system’s control bandwidth. The result is
that there is some flexibility in selecting the control period T. Sampling fast (small T) usually allows
better performance, but requires the use of more communication bandwidth. Slow sampling (large T)
degrades the control performance, but reduces the network usage. A trade-off must be considered
between the common resource usage (network bandwidth) and the control performance.
20.5.3 Network and Control Co-Design
From the above constraints, it naturally appears the consideration of co-design of the network and the
control. The assignment of the message scheduling and the control law design is done in parallel as
each one of the indicators depends on the others. The control law can include some estimations of the
sampling to actuation delays, the control period can be chosen depending on the network conditions.
In addition, other considerations and paradigms can be applied. The preceding triggering scheme
for the control loop execution is based on a constant time base, it is time triggered. This scheme has
been proved to be quite efficient for continuous signals, so the result is predictable and deterministic,
but delays are large. An alternative is to use an event-trigger paradigm. In this case, the control messages
are triggered by the occurrence of events, e.g., when a perturbation produces a control error [11]. This
scheme is quite efficient for discrete signals as network bandwidth is used when it is necessary, but
reduced when the controlled system is stationary. The drawback of this scheme is that, although delays
are low in average, it is neither predictable nor deterministic. Guaranteeing the stability and performance
of those event-driven systems requires research and applied effort [12,13].
20.6 Summary
NBC systems are control and communication systems where messages are used to close the loops
between sensors, controller, and actuator nodes through a shared, band-limited, digital communication
network. Designing this kind of system involves the integration of the disciplines of digital communications,
control design, and real-time computing. In this chapter, we have shown the architectures for
NBC systems, the concepts associated with the network side seen from the control side, the concepts
related to the control side seen from the network side, and the principal paradigms and constraints for
designing an NBC system. This NBC chapter aims to provide a description not only of the fundamental
aspects in this practical area but also where some of the principal research directions lie.
© 2011 by Taylor and Francis Group, LLC