Control
Function Block Diagrams
EGR 386
8/26/2011
Block diagram: terminology
Feedback Basic Terminology
• Process: system or central component whose output is to be
controlled
• Actuator: Device that can influence the process and change the
process output
• Plant: Combination of Plant and Actuator
• Controller (or compensator): Device that computes the control
signal/variable : Combination of Comparator and Compensator
• Senor: Provides an electrical output proportional to variable to be
measured
• Reference: the desired value for the output of a system,
sometimes called Reference Input or Input or Set point
• Output: the process variable of a system to be controlled
• Comparator: Computes the difference between Reference and
Output
• Transfer Function: function when multiplied by an Input provides
an Output for the component or system being modeled (usually
thought of in terms of Laplace transforms for continuous systems)
Temperature Control Function Block
Diagram: Open Loop System
Disturbance
Heat flow
in/out QDist
Compensator
Desired
Input
TSET
Temperature
Actuator
QIn
dV/dt
Fuel
Flow
Control
Furnace
Fuel
Rate
Plant
Room
Output
TActual
Temperature Control Function Block
Diagram: Closed-Loop Continuous
System
Reference
Input
Temp
TSET
+
_
Compensator
Error
Actuator
dV/dt
Fuel
Flow
Control
TMeasured
Furnace
Disturbance
Heat flow
in/out QDist
QIn
Room
Fuel
Rate
Temperature
Sensor
Output
TActual
Function block diagram of a room
temperature control system, digital output
Room temperature and furnace output vs. time
Plant and Controller
• Plant ~= Actuator + Process
• Controller ~= Comparator + Compensator
Plant
Controller
Reference
Input
Temp
TSET
+
_
Compensator
Error
Actuator
dV/dt
Fuel
Flow
Control
TMeasured
Furnace
Disturbance
Heat flow
in/out QDist
QIn
Room
Fuel
Rate
Temperature
Sensor
Output
TActual
Why is feedback needed?
• Disturbances ( weather changes for example)
• Unmodeled plant or actuator characteristics
• Changes plant or actuator
What is required to control a system
Open Loop?
• Compensator + Actuator
• Model of system response to control input
What additional is required to control
a system via feedback control?
• Output sensor
• Comparator
Modeling of Systems
• Needed to design an appropriate compensator
(whether feedback is used or not)
• Characterize the system performance or best
possible performance
Modeling: Dynamic Models
Mechanical Systems: Car Suspension
Electro/Mech. Systems : Disk Drive
Segway
Electronic circuits
DC Motor
DC motor
• (a) electric circuit of the armature;
• (b) free-body diagram of the rotor
Fluid systems
Thermal Systems (heat transfer)
Steam Engine + Control
Operating parts of a fly-ball
governor