SYSTEM
BOUNDARY
INPUTS OUTPUTS
SYSTEM
ENVIRONMENT
●
A SYSTEM is a combination of elements (parts, components) acting together to perform a specific
objective.
●
A system can be very large and complex. It can be comprised of sub-systems.
●
The BOUNDARY of a system can be real or imaginary.
●
The outside of the boundary is called the ENVIRONMENT.
●
The elements of a system interacts with each other as well as with the environment.
●
Some of the interactions are caused directly by the environment and are not affected by the
changes of the system. These interactions are called the INPUTS of the system.
● CONTROL INPUTS are put into the system intentionally.
● DISTURBANCE INPUTS are usually present. We try to minimize their effects.
●
The interactions of interest, which are monitored, are called the OUTPUTS of the system.
SYSTEM DYNAMICS
BOUNDARY
INPUTS OUTPUTS
SYSTEM
ENVIRONMENT
●
A system is STATIC if its output at any given time depends only on the input at that time. The
output remains constant if the input does not change.
●
A system is DYNAMIC if its present output depends on past inputs. In a dynamic system, the output
changes with time if it is not in a state of equilibrium.
SYSTEM DYNAMİCS aims:
1. Understanding the nature of dynamic systems.
“How does the system behave/respond if the inputs change”
2. Improving system performance.
“How can the system be modified so that the output properties improve”
ALSO; system dynamics employs a generic approach for the analysis of similar systems from
different domains (e.g. mechanical, electrical, fluidic, thermal)
MODELING
●
Any attempt to design a system must begin with a prediction of its performance. This prediction is
based on a description of the system’s dynamic characteristics. Such a description is called a
MODEL.
●
The model used most frequently is the MATHEMATICAL MODEL, which is an idealization of the
actual physical system, described in terms of mathematical relations.
● For many physical systems, these relations consist of differential equations.
●
Predicting the performance from a model is called ANALYSIS. A compromise must be made
between the simplicity of the model and the accuracy of the analysis results.
● In determining a reasonably simplified model, we must decide which physical variables and
relationships are negligable and which are crucial to the accuracy of the model.
● Experience is an important factor here.
●
IMPORTANT !!! A model is an approximate mathematical description of the physical system.
● It is not the physical system itself !
● In reality, no mathematical model can represent any physical compoment or system
completely.
MODELING
●
The physical properties or characteristics of a dynamic system are known as PARAMETERS.
● In general, real systems are continuous and their parameters are distributed.
● DISTRIBUTED PARAMETER MODELS require partial differential equations, in which the
independent parameters are the spatial coordinates and time.
●
However, in most cases, it is possible to replace the distributed characteristics of a system by
discrete ones by ignoring the spatial dependence and chosing a single representative value.
● This process is called lumping and the model of a lumped element or system is called LUMPED
PARAMETER MODEL.
● Lumped parameter models consist of easier-to-solve ordinary differential equations, which
include time derivatives only.
CONTROL
CONTROL
INPUT SIGNAL OUTPUT
CONTROLLER SYSTEM
desired manipulated controlled
value variable variable
●
The CONTROLLED VARIABLE is the quantity or condition that is measured and controlled.
●
The MANIPULATED VARIABLE is the quantity or condition that is varied by the controller so as to
affect the value of the controlled variable.
●
CONTROL means applying the manipulated variable to the system to correct or limit deviation of
the controlled value from a DESIRED VALUE.
CLOSED-LOOP (FEEDBACK) CONTROL
CONTROL
INPUT ERROR SIGNAL OUTPUT
+- CONTROLLER SYSTEM
FEEDBACK
●
A control system that maintains a prescribed relationship between the output and the reference
input by comparing them and using the difference as a means of control is called a CLOSED-LOOP
(FEEDBACK) CONTROL SYSTEM.
TREF TREF-TOUT FUEL TOUT
X FUEL VALVE FURNACE
MANUAL CONTROL: Controlled by human beings. (X = OPERATOR)
AUTOMATIC CONTROL: Controlled by devices. (X = THERMOSTAT)
MAK 305 System Dynamics and Control Dr. Yiğit Taşcıoğlu
http://groups.google.com/group/mak305 Spring 2014-2015
OPEN-LOOP CONTROL
●
The control systems in which the output has no effect on the control action are called OPEN-LOOP
CONTROL SYSTEMS.
●
The output is neither measured nor fed back for comparison with the input.
●
To each reference input there corresponds a fixed operating condition. Therefore; the accuracy of
the control system depends on calibration.
●
In the presence of (unpredictable) disturbances, an open-loop control system will not perform the
desired task.
●
Any control that operates on a time basis is open loop (e.g. traffic lights, washing machine)
TREF FUEL TOUT
X FUEL VALVE FURNACE
NOT
MEASURED !!!
INFORMATION BASED ON
PAST EXPERIENCE
MANUAL CONTROL: Controlled by human beings. (X = OPERATOR)
AUTOMATIC CONTROL: Controlled by devices. (X = THERMOSTAT)
