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Network Control Systems (NCSs) are spatially distributed
systems in which the communication between sensors,
actuators and controllers occurs through a shared
band-limited digital communication network. The use of a
multi-purpose shared network to connect spatially
distributed elements results in flexible architectures and
generally reduces installation and maintenance
costs. Consequently, NCSs have been finding application in
a broad range of areas such as the automotive and
aerospace industries, mobile sensor networks, remote
surgery, automated highway systems, and unmanned aerial
vehicles.
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This diagram shows the general architecture of an
NCS. Encoder blocks map
measurements into streams of “symbols”
that can be transmitted across the network. Encoders
serve two purposes: they decide when to sample a
continuous-time signal for transmission, and what to
send through the network. Conversely, decoder
blocks perform the task of mapping the streams
of symbols received from the network into continuous
actuation signals. One could also include in the
diagram encoding/decoding blocks to mediate the
controllers’ access to the network. We do not
explicitly represent these blocks because the
boundaries between a digital controller and
encoder/decoder blocks are often blurry. |
The interest in NCSs has been steadily rising due to several factors:
- Low-cost, low-power, small embedded
processors are widely available, which permits
endowing sensors and actuators with local processing and
sophisticated network protocols.
- Low-power, high-bandwidth digital
communication is widely available to interconnect
a large number of sensors, actuators, and controller
nodes. Wireless connections are especially attractive
because they have minimal installation costs (although
they can be severely constrained in terms of
bandwidth).
Inexpensive computation and ubiquitous embedded sensing,
actuation, and communication provide tremendous
opportunities for societal impact, but also great
challenges in the design of networked control systems,
because the traditional unity feedback loop that operates
in continuous time or at a fixed sampling rate is not
adequate when sensor data arrives from multiple sources,
asynchronously, delayed, and possibly corrupted. Moreover,
the design of NCSs poses novel questions that lie at the
intersection of control, communication, and signal
processing:
- How often/When should a sensor transmit a measurement
to a control unit?
- How often/When should a control unit send a control
update to an actuator?
- What forms of error correction/routing/data
compression are most adequate for control
applications?
- Which wireless medium-access methods are most
effective for control applications?
- How can one implement a control/estimation algorithm within
an embedded processor so as to minimize energy
expenditure?
Our research on NCSs is motivated by the following observations:
- At essentially every layer of the protocol stack, the
protocols needed for networks of embedded systems are
fundamentally different than those needed for bulk data
transfer or even for other “real-time”
applications such as voice-over-ip or live video
streaming.
- Fundamental research is needed to solve multiple open
problems in the area of networked embedded
systems. Adhoc approaches without a strong theoretical
underpinning will fail to find appropriate solutions to
these problems.
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The
widely used Controller Area Network (CAN) bus
standard manages the medium access for wireline
connections of electronic control units
(ECUs). Originally developed for automobiles, the
CAN bus was specifically designed to be robust in
electromagnetically noisy environments such as those
arising in the Supervisory Control And Data
Acquisition (SCADA) systems used to monitor or to
control chemical or transport processes, in
municipal water supply systems, to control electric
power generation, transmission and distribution, gas
and oil pipelines, and other distributed
processes. The CAN standard uses a priority-based
collision-free medium access protocol. Image from
Robert Bosch GmbH, the developer of CAN.
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A good starting point to learn about the design of
controllers for NCSs is the following survey:
J. Hespanha, P. Naghshtabrizi, Y. Xu. A Survey of Recent Results in Networked Control Systems. Proc. of IEEE Special Issue on Technology of Networked Control Systems, 95(1):138—162, Jan. 2007. [bibtex] [pdf]
Publications on this work can be found at the following URL:
http://www.ece.ucsb.edu/~hespanha/published.html#4NetworkedControlSystems
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