INTRODUCTION TO SUPERVISORY CONTROL
The idea of integrating logic comes with continuous dynamics in the control of complex systems is certainly not new. Let us consider for example an industrial setting where a human operator periodically adjust the set points of an array of PID controllers to account for changes in our environment. One easily recognizes the human operator as a component of the feedback loop that adjusts the continuous dynamics using the logic-based decision rules. The basic problem lies here is the control of complex systems for which traditional control methodologies based on a single controller do not provide satisfactory performance. In the switching control, one builds a bank of alternative candidate controllers and switches among them based on measurements collected online. The switching is orchestrated by a very specially designed logic that uses the measurements to assess the performance of the candidate controller currently in use and also the potential performance of some alternative controllers.
CONTROL LOOPS FOR SIMPLE SYSTEMS AND DYNAMICS AND THEIR STABILITY
Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems with inputs. The external input of a system is known as the reference. When there are one or more output variables of a system they follow a certain reference over time, a controller then manipulates the inputs to a system to obtain the desired effect on the output of the system.
The general objective of a control theory is to calculate solutions for the proper corrective action from the controller that result in system stability, that is, the system will hold the set point and not oscillate around it.
The inputs and outputs of a continuous control system are generally related by differential equations. If now these are linear with constant coefficients, a transfer function which relates the input and output can be obtained by taking their Laplace transform. If the differential equations are nonlinear and have a known solution, it might be possible to linearize the nonlinear differential equations at that solution. If the linear differential equations have constant coefficients one can take their Laplace transform to obtain a transfer function.
THE PLC’S AND DDC’S
PLC’s or programmable logic controller devices use current in terms of control, while DDC’s or digital direct controllers use voltage variations to achieve control.
Generally if the HVAC system is a small one, a low cost PLC for example take-a Koyo could be cost effective as far as hardware is concerned. However the programming would not be successful. The HVAC Controls companies have canned algorithms for their DDC units that basically only require i/o addressing to set them up. Therefore this programming is much less expensive that the PLC programming. For large Systems, PLCs are not usually viable. The HVAC control companies have large numbers of dedicated modules to control the various types of HVAC equipment. These modules communication with their own DDC units and require very little setup to do so. The DDC units use either a propitiatory network or in some cases Bacnet to communicate with each other or with unit controllers. These companies sometimes also have programs available for scheduling equipment and trending alarming. Some very large industrial companies have already used PLC’s in HVAC control and the installed cost ran 2 to 4 times the cost of dedicated HVAC controls systems.
CONVERSION OF EXISTING CONTROL SCHEMES IN OPERATING PLANTS
Today more than ever, the supply and demand requirements within the energy industry arecontinually evolving requiring new and much more creative methods in advanced technology to
produce power more efficiently with less environmental impacts. Our nation expects from the public
and private sectors to be at the forefront in providing a safe, reliable, and the economical source of
energy supply that makes existing market and forecasted environmental demands to lead us
successfully into the 21st century. We should continue to focus our efforts on understanding the
current state-of-the-art technologies, operational issues, and few emerging trends by monitoring the
pulse of the industry through some proactive research and development.
THE THREE COMPONENT SYSTEM
States of matter in physics are the distinct forms that different phases of matter take place. There are four states of matter observable in everyday life: solid, liquid, gas, and plasma. Many other states are known such as Bose–Einstein condensates and neutron-degenerate matter but these only occur in extreme situations such as ultra cold or ultra dense matter.
Historically, the distinction is made based on qualitative differences in properties. Matter in its solid state maintains a fixed volume and shape, with the component particles (atoms, molecules or ions) close together and fixed into place. Matter in its liquid state maintains a fixed volume, but has a variable shape that adapts itself to fit its container. The particles are still close with each but move freely. Matter in its gaseous state has both variable volume and shape, adapting both to fit in its container. Its particles are neither close together nor fixed in place. Matter in its the plasma state has variable volume and shape, but as well as some neutral atoms, it also contains a significant number of ions and electrons, both of which moves around freely. Plasma form is the most common form of visible matter in the universe.