Module 2- The Feedback Control Loop and its Dynamics Flashcards
What are the two types of controllers
On-Off controller
Proportional Controller
What is the other name for the on-off controller
Two Position Controller
Explain how the on- off controller works
It has two extremes/ position of operation. It is either totally on or totally off ( fully opened or fully closed)
Explain how the Proportional controller works
It opens or closes in proportion to the difference between the Setpoint and the PV
Define Feedback Control
measuring the controlled variable after a change has occurred, then signaling an additional corrective change
What is critical in determining the type of control strategy used
The type of process and its speed
Control Loop Four Basic Functional Blocks
Process
Measurement
Automatic Controller
The Final Control Element
What is the additional control loop functional block
Transmission Media
Define Transmission Media
technology used for transmitting signals from 1 loop device to another
What is the purpose of the process block
to make a product of a desired quality and or quantity
For the purpose of control theory define the process
an action in which material and or energy is modified to a different form
In majority of control loops what dictates the behavior of the loop and as a result it is important to….
the process functional block, this is why it is important to include the characteristics of the processes in process control theory
Measurement block purpose
to measure the value of the
process output variable (in the case of the feedback loop), and to convey the value to the automatic controller.
The measurement block is an interface between
the process and the controller
and
the process and human operator
Measurement block usually comprises of… state its purpose as well
a transmitter
transmits an analog or a digital signal whose value is indicative
of the magnitude of the process’s output variable
Some times measurement may require more than one device to… give an example
measure a single variable
Flow rate is measured with head type flow element such as an orifice plate, differential pressure transmitter and a square root extractor
What does the square root extractor do
linearize the measurement wrt to flow
The square root extractor can be in what two places
the transmitter itself ie the measurement block (smart transmitter) or the mounted in the control room (automatic controller) despite the location it is still considered part of the measurement block
Define local controller
A controller located in the process area and does need the value of the pv to be transmitted to the control room
When may transmitters not be necessary
when low level electronic signal generating devices are used such as Thermocouples
Define Final Actuator
the device that regulates the supply of material and/or energy to affect the desired value of the controlled variable
The primary function of the automatic controller is to
continually compare the measurement signal to the desired value or setpoint, of
the controlled variable
What doe the controller do if there is a difference in the SP and measurement signal
he controller takes corrective action by changing its output,
which in turn adjusts the final actuator
What does the final actuator change and why
The final actuator changes the supply of material and/or energy to
the process, in order to bring the controlled variable closer to the
setpoint
The automatic controller can sometimes be made up of two different blocks they are
a summer
and
a control modes block
Another name for summer
error detector
deviation detector
offset detector
Function of error detector
compares the setpoint to the measured value
The summer must have… for the loop to function properly since…
algebraic signs included since the algebraic sign of the error is important
The error detector in a pneumatic controller is
a mechanical flapper nozzle mechanism
The error detector in a electronic controller is an
electronic solid state amplifier
In a microprocessor-based the error is developed by
the numerical difference between the sp and the measurement signal
what does the controller mode block do
it acts on the error signal to determine the behavior of the controller’s output signal
What is executed in the controller mode block
the on-off or proportional control with its integral (reset) and derivate(rate) modes are executed
What does the derivative / rate mode handles
how frequent the error occurs
rate of change of the error
integral in addition to proportional mode deals with
duration of error
PID Control
Proportional Integral Derivative
Proportional mode deals with
the magnitude of the error
PI Control
Proportional control and integral control
Why is it important to have calibrated measurement devices
to maintain the efficiency of the process
Other not commonly used, 4 types of transmission media
Sonic, hydraulic, capillary and electromagnetic
A pneumatic signal is transmitted over a distance through….,
using a range….
cubing
using a range of 20-100KPa (3-15 psig )
In pneumatics the air/gas is
filtered and compressed to be at the required pressure and and cooled, therefore it is conditioned for this purpose
In pneumatics sometimes the signal range is often changed to… in order
a multiple of the standard 20-100 kPa range
eg 40-200kPa or 60-300kPa
in order to increase the forces applied to the valve actuator
How is pneumatic response time increased
by a valve positioner/air amplifier supplying a larger volume of air
The electronic media often uses a…for which a signal range …. is used
current loop
of 4-20mA
The most common voltage range used is … and how is it obtained
1-5V
he 1 - 5 V range is developed by passing 4 - 20 mA through a 250 ohm resistor.
What magnitude of electronic signals are referred to as high level signals
1-5 V
4-20mA
0-10V
Another form of signal, which can be considered electronic is
the transmission of values in the form of digital codes
define fibre optics
Light beams, modulated with digital codes, are used as the carrier
frequency.
Whatever the transmission medium, it has two design features which enhance the dynamics of the feedback control loop:
It must be instantaneous. or very nearly so.
* It must be compatible with the controller, transmitter and valve type
Ti
Temperature indicator
TiT
Temperature indicating Transmitter
TiC
Temperature indicating Controller
TV
Temperature valve
TY
current to pneumatic transducer
Control loop signals designations
b=
measurement signal
Control loop signals designations
c=
controlled variable
Control loop signals designations
e=
error, deviation or offset
Control loop signals designations
m =
manipulated variable
Control loop signals designations
m꜀=
controller output
Control loop signals designations
r=
setpoint or desired value
Control loop signals designations
q=
load variable
the success of the controller to maintain the efficiency of the process depends on
how accurately it can reduce
error to zero and maintain the controlled variable at the desired value aka sp
What four factors determine the loop dynamics
- Response time of the process
- Dead time of the process
- Single capacity/single resistance process
- Multiple capacity/multiple resistance process
why does the control strategy of a control loop need to consider response time
The control strategy of any feedback loop has to consider the response time of the process to make changes to itself
Define response time of a process
the total process time delay
When it comes to response time what does the controller have to consider
the time delay by the manipulated variable
If the response time is not consider what can happen
the loop will be poorly designed and can oscillate out of control and the controller will over correct making things worse
How can lo0ps be designed to be stable in regards to the response time
by mathematically determining their response time, to aloe
the valve and controller characteristics to minimize effects of response time.
loops can be designed so they are stable and their response time mathematically
determined to allow valve and controller characteristics to minimize the effects of
response time.
Define Dead time
the duration of time where nothing happens
the amount of time after a change is made in the input to the process before there is any change in the process output measurement
