1. UNIVERSITÀ DEGLI STUDI DI SALERNO
Bachelor Degree in Chemical Engineering
Course: Process Instrumentation and Control
(Strumentazione e Controllo dei Processi Chimici)
AUTOMATIC FEEDBACK CONTROL
CONCEPT OF FEEDBACK CONTROL
COMPONENTS AND VARIABLES OF THE FEEDBACK CONTROL
Rev. 2.2 – May 22, 2019

2. Process Instrumentation and Control - Prof. M. Miccio
INTRODUCTION
For an adequate management of a chemical process/plant, it is necessary to check its operation and operate it according to the operating conditions of the project (nominal conditions) or vice versa lead it to operate according to alternative regimes dependent on appropriate and specific processing needs.
Control systems therefore play a fundamental role in the management of a process plant, since they influence not only the key variables of the process itself, but also other factors connected to them and employees, such as those
environmental,
commercial quality of the products,
worker safety,
ecc.
A control system is any physical system that establishes a correspondence relation, according to a predetermined law, between an input quantity (called "reference" or set point) and an output quantity, which constitutes the controlled variable, also in presence of other inputs acting as disturbances.
It must primarily meet the following three specifications/needs :
suppression of external disturbances on the process,
assurance of the operational stability of the process and
process performance optimization.
See also:
Ch.1 - Stephanopoulos, “Chemical process control: an Introduction to theory and practice”, Prentice Hall, 1984
Process Instrumentation and Control - Prof. M. Miccio
27/09/2019

3. Automatic Control of house heating
Thermostat
controller TC
Temperature
sensor/transmitter TT
heat loss
(disturbance)
set point
Control
signal
Measurement
Computation/ Decision
Action
furnace
fuel flow
valve
adapted from:
Ch. 1 “Fundamental Principles of Process Control” in Cooper D. (2008), "Practical Process Control using Loop-Pro Software", PDF textbook
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

4. What idea of AUTOMATIC CONTROL?
MEASUREMENT
CALCULATION
ACTION
THouse
House temperature
House is colder than the set point temperature?
TSP - THouse > 0
Positive error
Open the fuel valve
House is hotter than the set point temperature?
TSP - THouse< 0
Negative error
Close the fuel valve
TSP = Thouse
Zero error
No actions
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

5. Process Instrumentation and Control - Prof. M. Miccio
BLOCK DIAGRAM FOR THE PROCESS CONTROL: FEEDBACK LOOP Example of the house heating
adapted from:
Ch. 1 “Fundamental Principles of Process Control” in Cooper D. (2008), "Practical Process Control using Loop-Pro Software", PDF textbook
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

6. BLOCK DIAGRAM FOR THE PROCESS CONTROL: FEEDBACK LOOP General case
o(t)
y(t)
d(t)
ym(t)
controlled
comparator
ysp(t)
m(t)
adapted from:
Ch. 1 “Fundamental Principles of Process Control” in Cooper D. (2008), "Practical Process Control using Loop-Pro Software", PDF textbook
See also:
Ch.14 - Stephanopoulos, “Chemical process control: an Introduction to theory and practice”, Prentice Hall, 1984
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

7. CHARACTERISTIC COMPONENTS OF THE CONTROL
An automatic control and regulation system is generally made up of the following components of the control loop:
MEASURING INSTRUMENTS: an proper device for the measurement of a physical quantity of the process. It is usually a temperature, a pressure, a flowrate or a livel of a liquid in a tank.
TRANSMITTER: connected to the measureing device, it consists of a specfied instrument which transforms the measured physical property in a signal trasmitted to a riceiver at a distance. The more common transmitters are pneumatic, using compressed air, or electric and electronic.
CONTROLLER (or REGULATOR): it amplifies and compares the measured signal to the set point value, evaluating the error of the process. It sends a correction signal which acts on the final control element by means of a servomotor. Also controllers can be pneumatic and electronic, other than hydraulic and oleodynamic.
ATTUATOR (or SERVOMOTOR): device, commonly mechanic, which acts on the final control element as a consequence of the signal received from the controller.
FINAL CONTROL ELEMENT: component which determines the value of the manipulated variable following the correction signal delivered by the controller. in ossequio al segnale di correzione emesso dal controllore. It usually consists o a control valve which operates automatically rather than manually by means of the servomotor at which is directly connected. It cam be also a pump, a fan, a compressor, a heating or cooling element, etc.
TRANSMISSION LINES (or CONNECTING ELEMENTS): they transmit with proper signals controller instructions, measured variables, etc.
PROCESS for which the automatic control is required.
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

8. Process Instrumentation and Control - Prof. M. Miccio
EXAMPLE OF COMPONENTS
SENSORS/MEASURING INSTRUMENTS:
Temperature,
Flowrate
Pressure
Level
Concentration pH
ecc.
FINAL CONTROL ELEMENT:
valve, solenoid, pump, compressor, heating and cooling element, ecc.
CONTROLLERS/REGULATORS:
ON/OFF, PID, cascade, feedforward, model-based Smith predictor, multivariable, sampled data, parameter scheduled adaptive control
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

9. Process Instrumentation and Control - Prof. M. Miccio
EXAMPLE OF COMPONENTS
SERVOCONTROLS or ATTUATORS:
They consist of devices (motors, relays, solenoids, etc.) which drive the final control elements using electric or mechanical energy or pressure.
It can be:
1st CLASSIFICATION
PNEUMATIC
OLEODYNAMIC
MECHANIC (motor)
ELECTRIC (relay, solenoid, switches, etc.)
2ND CLASSIFICATION
LINEAR DISPLACEMENT
ROTATIONAL DISPLACEMENT
WITH THE INTERRUPTION OF THE CIRCUIT
3rd CLASSIFICATION
according to the OPERATION (see the following table):
ALTERNATIVE
ON/OFF
proportional
unidirectional
with 2 directions of range
with a limited travel
without limit
(if rotational, they can rotate more than 360°)
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

10. Process Instrumentation and Control - Prof. M. Miccio
STANDARD SIGNALS
Standard electric signals in direct current
In process control systems and small and medium mechanical equipments exchanging measurements and instructions, transmitter signal outputs are commonly current of 4÷20 mA, which allow to identify the interruption of the circuit from the null signal (“LIVE ZERO”), if compared with the signal 0÷20 mA.
The variable current, never lower than 4 mA, power transmitters with a operating voltage between 4.8÷24 V.
Advantages of the electric transmission:
Greater insensitivity to the noises of the measurement;
Transmission to high distances. Distances depends on:
Sources of noise;
Efficacy of screens;
Magnitude and frequency of noise.
Standard electric signals in direct voltage
In laboratory, transmission signals of 0÷5 V; 0÷10 V, ±10 V are used.
Standard pneumatic signals
Compressed air in the range 3÷15 psig.
Digital signals
Cable (Profibus o Fieldbus), wireless.
§ 4.2.6
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

11. CHARACTERISTIC VARIABLES OF THE CONTROL
TIME DOMAIN
LAPLACE DOMAIN
ERROR
ε(t) = ySP (t) –ym(t)
ε(s)
CONTROLLER OUTPUT
o(t)
CO(t) in Control Station®
o(s)
MANIPULATED VARIABLE
(CONTROLL VARIABLE in Magnani®)
m(t)
m(s)
CONTROLLED VARIABLE or PROCESS VARIABLE
y(t)
y(s)
MEASURED VARIABLE
ym(t)
ym(s)
SET POINT
ysp(t)
ysp(s)
DISTURBANCE or LOAD
d(t)
d(s)
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

12. BLOCK DIAGRAM FOR THE PROCESS CONTROLy d x ym
ysp + -
PROCESS e u
CONTROLLER
§ 7.2
Magnani, Ferretti e Rocco (2007)
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

13. PI&D for the PROCESS CONTROL
Test N.2 of
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

14. BLOCK DIAGRAM FOR PROCESS CONTROL: (FEEDBACK LOOP)
Final
control
element
PROCESS
SENSOR
PID Controller
ySP(s)
ε(s)
o(s)
m(s)
d(s)
ym(s)
y(s) + -
see:
Ch.14 - Stephanopoulos, “Chemical process control: an Introduction to theory and practice”, Prentice Hall, 1984
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

15. BLOCK DIAGRAM FOR MIMO PROCESS CONTROL (Multiple Input Multiple Output)
General configuration of the feedback control system
Disturbances
Process
Controller
Set points
Unmeasured outputs
Manipulated
variables
outputs
See Ch.2:
Stephanopoulos, “Chemical process control: an Introduction to theory and practice”, Prentice Hall
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

16. SYNOMPTIC OF THE INDUSTRIAL PROCESS CONTROL
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

17. SINOTTICO PER IL CONTROLLO DI PROCESSO INDUSTRIALE
SCRUBBER
27/09/2019
Strumentazione e Controllo dei Processi Chimici - Prof M. Miccio

18. OPEN LOOP vs. CLOSED LOOP
Let us consider the generalized SISO process showed in figure. It has:
an output: y
an input (represented by a disturbance): d
another input (represented by a manipulated variable): m
Process m y d
OPEN LOOP
Measuring
Device
Controller mechanism
Controller
Final
Control
element
Process
ysp ym ε c m d y + 
CLOSED
LOOP
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

19. Open-loop Control Block diagram of an open-loop control system
Without feedback !!
blocks in “series“
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

20. Process Instrumentation and Control - Prof. M. Miccio
OPEN-LOOP RESPONSE
it can also be realized starting from closed loop
The controller is present, but operates in “manual” mode
Block Structure in ”series” 
Open Loop operation
Final
control
element
PROCESS
SENSOR
PID Controller
ySP(t)
ε(t)
o(t)
m(t)
d(t)
ym(t)
y(t) + - Kc
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

21. Process Instrumentation and Control - Prof. M. Miccio
CLOSED-LOOP RESPONSE
The controller operates in “automatic” mode.
The output is called as closed loop response.
External inputs are the disturbance and the set point.
CONTROLLER
FINAL CONTROL ELEMENT
PROCESS
MEASURING DEVICE
ySP(t)
ε(t)
o(t)
m(t)
d(t)
ym(t)
y(t) + -
CLOSED
LOOP
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

22. Process Instrumentation and Control - Prof. M. Miccio
OPEN vs. CLOSED LOOP
Type of system
Advantages
Disadvantages
Open-loop control system
1. Simple construction and ease of maintenance.
2. Less expensive than a corresponding closed-loop control system
3. There is no stability problem
4. Convenient when output is hard to measure or measuring the output precisely is economically not feasible.
1. The system response very sensitive to external disturbance and internal variations in system parameters.
2. Recalibration is necessary from time to time in order to maintain the required quality in the output
Closed-loop control system
Makes the system response relatively insensitive to external disturbance and internal variations in system parameters.
Possible to use relatively inaccurate and inexpensive components to obtain the accurate control of a given plant.
Better control of transient & steady-state response
Increased accuracy
Increased ability to reproduce output with varied input
1. Risk of BIBO instability
2. Complexity in analysis and implementation and expensive
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

23. Process Instrumentation and Control - Prof. M. Miccio
REGULATOR PROBLEM
DISTURBANCE CHANGING WITH TIME
SET POINT CONSTANT WITH TIME
CONTROLLED (OUTPUT) VARIABLE CHANGING WITH TIME BUT CLOSE ENOUGH TO THE SET POINT (IDEALLY COINCIDENT)
SERVO PROBLEM
DISTURBANCE CONSTANT WITH TIME
CHANGING SET POINT WITH TIME
CONTROLLED (OUTPUT) VARIABLE CHANGING WITH TIME
see:
Ch.??? - Stephanopoulos, “Chemical process control: an Introduction to theory and practice”, Prentice Hall
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

24. Process Instrumentation and Control - Prof. M. Miccio
EXAMPLES of feedback control in process industry
APPENDIX
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

25. Measurement/control of the grinder power Ex. for Food Engineering
Taken from Doebelin E.O., Strumenti e metodi di misura, Copyright © The McGraw-Hill Companies srl
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

26. Process Instrumentation and Control - Prof. M. Miccio
Measurement/control of dissolved O2 in a tank of activated sludge 1st ex. for Enviromental Engineering
Taken from Metcalf & Eddy, Ingegneria delle acque reflue – Trattamento e riuso, 4/ed., Copyright © The McGraw-Hill Companies srl
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio

27. Measurment/control of the level of activated suldge 2nd ex
Measurment/control of the level of activated suldge 2nd ex. for Enviromental Engineering
SS = suspended solids
Taken from
27/09/2019
Process Instrumentation and Control - Prof. M. Miccio