Civil Engineering Reference
In-Depth Information
Somewhat similar to this process in a typical building control system, each sen-
sor which has its own specifi c network address generates a signal sensed from the
fi eld and sends it to data link layer at MAC sub layer of the building control system.
MAC sub-layer receives the data and sends it up through the different layers to the
application layer where the direct digital control software is resides. In application
layer, the received signal from MAC layer will be compared against a prepro-
grammed desired quantity (set point) and based on the degree of deviation of the
received data from the preprogrammed set point data a command will be generated
that will travel down the network layers and to the MAC layer. MAC layer then
directs the response to the proper actuator (proper address) and cause an adjustment
at a control device such as a damper or a valve in which as a result attempts to pro-
vide the proper condition in the space in which the original data was sensed via
local sensor.
Another communication protocol which is more useful in connecting hosts to
internet is called Transmission Control Protocol/Internet Protocol (TCP/IP). This
protocol generally is used by Internet applications to connect different devices to a
common network (Internet). The difference in structure of TCP/IP model and OSI
model is that in TCP/IP a four layer model instead of the seven layer model devel-
oped in OSI model is used. TCP/IP layers are (1) link layer (similar to physical and
data link layers in OSI model), Internet layer (similar to network layer in OSI
model), transport layer (similar to transport layer in OSI model), and application
layer (similar to session, presentation, and application layers in OSI model). It
should be noted that OSI model is basically a theoretical reference model, while
TCP/IP is a practical model that is developed around the Internet functions.
9.3
Control Algorithms
Control algorithms generally are defi ned to be one of proportional (P), proportional
plus integral (PI), and proportional, plus integral plus derivative (PID). To better
understand how each of these logics act, imagine you are driving your car on a road
with a speed of 45 miles per hour, which is your desired speed (set point) as well.
Originally you are satisfi ed with the car speed, until you reach an uphill section of
the road, and your car speed drops to 40 miles per hour. At the moment you recog-
nize that your speed has dropped you push more on the gas pedal which increases
your speed which after some time reaches to 47 miles per hour. Since you want to
keep the set point (45 miles per hour) speed, you will decrease the pressure on the
gas pedal and the car speed drops to 44 miles per hour. You will continue this
increasing and decreasing pressure on the gas pedal until you reach the desired 45
miles per hour again.
In each instant you are fi ghting to overcome the discrepancies from the desired
speed set point with your controlling actions. Discrepancies at each instance are the
sum of (1) how much the real speed deviates from the set point (proportional), (2) a
weighted average of the undesired speeds and the time spent in these speeds from
