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

20
Network-Based Control
Josep M. Fuertes
Universitat Politècnica
de Catalunya
Mo-Yuen Chow
North Carolina
State University
Ricard Villà
Universitat Politècnica
de Catalunya
Rachana Gupta
North Carolina
State University
Jordi Ayza
Universitat Politècnica
de Catalunya
20.1 Introduction.....................................................................................20-1
20.2 Mutual Concepts in Control and in Communications.............20-2
20.3 Architecture of Networked-Based Control.................................20-2
Connection Types
20.4 Network Effects in Control Performance....................................20-6
20.5 Design in NBC.................................................................................20-6
Design Constraints in the Network Side. •. Design Constraints
in the Control Side. •. Network and Control Co-Design
20.6 Summary..........................................................................................20-7
References....................................................................................................20-8
20.1 Introduction
Industrial control applications use controllers, sensors, and actuators to close control loops. Sensors
and actuators are usually distributed topologically through the controlled plant. As ubiquitous embedded
controllers, sensors, and actuators are more and more common, while inexpensive and reliable
communication systems are ready for use in the industry, many of the control loops are closed using
industrial communication systems. A large part of the industry control loops are discrete in nature,
where a sequence of discrete actions are done after a sequence of sensings, e.g., after a displacement has
been completed, a motor has to stop. From the control side, a discrete system has as major constraint,
the system response time, or the maximum time that can be accepted between occurrence of a stimulus
and the reaction on the plant. Previous chapters of the book address how the industrial communication
systems attain the goal of sending messages keeping this constraint.
Continuous control loops differ from discrete control loops in that the continuous signal measurement
is used by the controller to calculate (using differential equations) the continuous signal to be send to the
actuator. Those continuous signals can be properly approximated by time-discretized sequences of values
that maintain the dynamic characteristics of the sampled system. A goal when connecting through a communication
network sensors, controllers, and actuators in a continuous-time control loop is to maintain
the dynamic characteristics of the extracted data from the sequence of messages.
Communication networks for feedback control are in increasing use. A network-based control
(NBC) system is composed of distributed nodes sharing feedback control-related information through
a communication channel. The basic configuration is given by one sensor node that collects process
sensor data, one controller node that receives the sensor data, calculates the control action and sends
20-1
© 2011 by Taylor and Francis Group, LLC