See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/285176112
Basics of process: the on-off control system
Article · November 2015
CITATIONS
READS
2
11,753
3 authors:
Antoni Ryniecki
Jolanta Wawrzyniak
Poznań University of Life Sciences
Poznań University of Life Sciences
35 PUBLICATIONS 260 CITATIONS
34 PUBLICATIONS 331 CITATIONS
SEE PROFILE
SEE PROFILE
Agnieszka A. Pilarska
Poznań University of Life Sciences
73 PUBLICATIONS 515 CITATIONS
SEE PROFILE
Some of the authors of this publication are also working on these related projects:
Biogas as Renewable Energy Source View project
The influence of selected microbiological carriers addition on biogas production in the methane fermentation process View project
All content following this page was uploaded by Agnieszka A. Pilarska on 01 December 2015.
The user has requested enhancement of the downloaded file.
TECHNICS – TECHNOLOGY
Antoni
Ryniecki
Jolanta
Wawrzyniak
November 2015 z tome 69
Process Control Systems
Basics of Process Control:
the On-Off Control System
Agnieszka
Anna
Pilarska DOI 10.15199/65.2015.11.6
Food technology engineers have to depend not only on their
knowledge of microbiology, food processing etc. but also on
the basic principles of automatic control engineering [4, 5, 10].
Knowledge in this area can help them to work with control engineers and design control systems for the food industry. Unit
processes in the technological lines of food processing cannot
be conducted without proper supervision and automatic control
because they should run in strictly defined ways. The course of
the processes is frequently disturbed by various factors, therefore
there is a need for constant control. Controlling of the course of
unit processes can be assigned to a man or a machine. Assigning control of processes to a machine is called automation of
these processes, i.e. self-performing (automatos in Greek). Selfperformed are mostly activities which are either dangerous, done
in difficult conditions for a man or simple and monotonous and
thus troublesome.
BASIC TERMS
KEY WORDS:
The first aim of this article is to explain the basics of
automatic control with minimal usage of mathematics based
on literature database. The basis for analysis of automatic
control is the foundation provided by control theory, which
assumes a cause-effect relationship for the components of
a system. Basic terms in the process control discipline are
the object of control, process variable (PV), control variable
(CV), disturbances, feed-back, control system and on-off
control [5, 6, 9, 11, 13].
An object of control most often is a dynamic process
whose significant parameter or state is subject to control.
In practice, an object of control usually refers to a piece of
equipment in which the process of control occurs [1, 8].
Figure 1 depicts schematic diagram of bioreactor – the object
of control as it is understood in practice in which it is possible to find several dynamic processes – objects of control
from the point of view of the theory of automatic control:
heat flow, air flow, alkali/acid flow and nutrient flow.
automatic process control,
feed-back control, on-off
control, properties of the
controllers and objects
of control, quality of the
on-off control
SŁOWA KLUCZOWE:
sterowanie automatyczne,
sterowanie ze sprzężeniem
zwrotnym, regulacja dwupołożeniowa, właściwości
regulatorów i obiektów
regulacji, jakość regulacji
dwupołożeniowej
26
Fig. 1. Schematic diagram of bioreactor
SUMMARY:
This article explains the basic terms
of process control: dynamic process as
an object of control, process variable
called also controlled variable, control
(or manipulated) variable, control
system, control with the feed-back
loop as well as the principle of
operation and quality indicators of
the on-off control. To show indicators
determining the quality of the on-off
control, changes of process variable
STRESZCZENIE:
Artykuł wyjaśnia takie pojęcia,
jak: proces dynamiczny jako obiekt
sterowania, wielkość regulowana
(kontrolowana), wielkość sterująca,
system sterowania, sterowanie ze
sprzężeniem zwrotnym, jak również
zasadę działania i wskaźniki jakości
regulacji dwupołożeniowej. W celu
przedstawienia wskaźników jakości
regulacji dwupołożeniowej wykreślono
zmiany wielkości regulowanej w tego
typu regulacji. Dokonano tego na
podstawie: przedstawionej charakterystyki statycznej typowego regulatora
vs. time during this type of control
were drawn. This was done based on:
depicted static characteristic of a typical on-off controller, and parameters
that characterize the dynamics of the
object - a typical process of heat flow
in a tank. Two parameters of the object
(time-constant and time-delay) were
drawn graphically using the step-response characteristic of the above
mentioned thermal-process.
o działaniu dwupołożeniowym oraz
parametrów, które charakteryzują
dynamikę obiektu - typowego procesu
przepływu ciepła w zbiorniku. Dwa
parametry obiektu (stałą czasową
i czas opóźnienia) wyznaczono
graficznie przy użyciu charakterystyki skokowej wyżej wymienionego
procesu cieplnego.
TYTUŁ:
Podstawowe pojęcia o systemach
sterowania procesami: regulacja
dwupołożeniowa
An object to be controlled, can be represented by a block
(Figure 2). Such a block possesses an input and output. The
input-output relation represents the cause-and-effect relationship of the object.
Process variable (PV), called also a controlled variable
or measured variable, it is an output of dynamic process
(object) that can be controlled. For example, in a bioreactor
we can find the following PVs: temperature in the process
of heat flow, soluble O2 (oxygen) in the process of air flow
or pH (the acidity or alkalinity) in the process of alkali/acid
flow. In a typical fixed-value control such a parameter is
stabilized on the fixed desired level.
Control variable (CV), called also a manipulated variable is the input variable of object that cause a strong effect to
the value of PV in the object. In other words, CV is the variable that can be changed to bring about a desired outcome
[10]. For example, by changing the flow rate of heat put into
the bioreactor, its temperature will change. Changing the
flow rate of air input to the bioreactor, the value of another
PV, soluble O2, will change.
Disturbances are variables that cause undesired effects
to the value of PV in the object that are not caused by an
PRZEMYSŁ SPOZYWCZY
tome 69 z November 2015
operator or a control mechanism. For example, temperature
change in a bioreactor in Figure 1 can be the result not only
of the CV change but also the result of the change in the
flow rate of heat exit through walls from the bioreactor or
the change in the flow rate of heat generated/consumed
inside the bioreactor.
Fig. 2. Block diagram of the object of control
Controlling of a process can happen in open loop or
closed loop systems [5, 8]. In open loop systems, as in Figure 2,
feedback between the output and the input of the object
of control does not exist. An example of such a system can
be heating without thermo-regulation. Such a system cannot respond to disturbances. However, if we would like to
respond to disturbances we have to build a feed-back loop
between the output and input of the object of control [9,
11, 12]. Commanding in a system with a feed-back loop is
called the feed-back control. Such an influence conducted
by a device usually called controller is the most used way
of automatic controlling of technological unit processes.
The object of control together with a measurement element, actuator and controller compose the control system (Figure 3). The overall task of the control system with
feed-back loop is to maintain the PV as close as possible
to the desired value (called set-point) - irrespective of all
disturbances. Using other words we can say that the main
task of the feed-back control system is to maintain the
process error (e = y0 - y) as close to zero as possible irrespective of disturbances.
Fig. 3. Block diagram of the feed-back control system; from the point of view
of controller, an actuator and a measurement element are parts of an object
Automatic control may either be discontinuous, e.g.
on-off control or continuous, e.g. PID control. On-off
controllers are employed most frequently with the aim to
stabilise PVs that change slowly, e.g. temperature, pressure
or the level of liquid in tanks as well as other parameters
which occur in unit operations of production processes.
By far, the on-off control is the most common type of
control used in industry because discontinuous actuators,
like relays or solenoid valves, are simpler and cheaper
than actuators used in the continuous type control systems. On-off controllers are important components of
industrial as well as of laboratory apparatus. The second
aim of this article is to explain the on-off and feed-back
control system that can stabilize temperature in the tank
of a bioreactor.
PRZEMYSŁ SPOZYWCZY
PRINCIPLE OF OPERATION
of the on-off control
Temperature in bioreactors is typically stabilized using
electronic controllers [2]. However, to present how on-off
control works in the simplest way it has been decided that
the measurement element and controller will be a glass
thermometer equipped with electric contacts (in food processing plants mercury glass thermometers should not be
used). A schematic diagram of the on-off control system with
a contact thermometer (CT) is shown in Figure 4. Two thin
wires acting as contacts are put in a mercurial thermometer:
the bottom wire – contact 1 is permanently immersed in
mercury, while the top wire – contact 2 is situated inside
the thermometer capillary over the mercury thread. The
position of contact 2 can be changed and it sets the value
of the desired temperature t0. The CT turns on or off the
control circuit.
Controlling in
a system with
a feed-back loop
can respond to
disturbances.
Fig. 4. Schematic diagram of a control system with a contact thermometer (CT) as
the example of an on-off controller (in food processing plants electronic temperature sensors and controllers are used); t0, ta – set-point and ambient air temperature,
respectively
How does the above presented system of temperature
control work? The task of the thermoregulation system is to
maintain temperature in the object (e.g. liquid temperature
in the bioreactor) as close to the desired value (set-point)
of temperature as possible, irrespective of all disturbances.
The task is achieved by the system in a way described below.
Let us assume that the temperature in the bioreactor at the
beginning is identical with the temperature of ambient air
ta and lower than the set-point desired temperature t0 (as in
Figure 4). In this situation, the control circuit is open, electrical current does not flow through coil R of the relay and,
therefore, the working contact of the relay marked in Figure 4
by small r is closed. The closing of the switch S in Figure 4
results in the closing of the power circuit (which closes
between points U2 and N), passage of current IH through
the heater and increase of the temperature in the bioreactor. This, in turn, leads to the increase of the level of the
mercury thread which represents the level of the measured
temperature t. When temperature t reaches the value of t0,
the thread of mercury will touch the contact wire 2. The
control circuit will close and electrical current (much lower
than the heater current IH) will flow through electromagnetic
coil R. As a result this will cause the opening of contact r and
break of the power circuit of supply of heater. For a short
period of time, temperature t will still rise above the value of
t0, until the heater is cooled down. Since the heat is continually being lost from the bioreactor, the temperature inside
will begin to drop after this short period of time. The drop
in temperature leads to the opening of contacts 1-2 of the
Automatic
control may
either be discontinuous, e.g.
on-off control or
continuous, e.g.
PID control.
27
TECHNICS – TECHNOLOGY
November 2015 z tome 69
Process Control Systems
mand signal and the process error e, we obtain the static
characteristic of the controller [3, 11]. Such characteristic
is shown in Fiure 6. For even better clarity, Figure 6 presents
temperature t in the neighbourhood of its set-point value
instead of the temperature difference e.
It is evident that this characteristic has the zone of
ambiguity, known as the zone of hysteresis – the opening
and closing of the power circuit takes place at two different
temperatures. The width of the zone of hysteresis is marked
with letter h. The heater is switched off when the increasing
temperature reaches the value of toff, and it is switched on
when the decreasing temperature reaches the value of ton.
DYNAMIC PROPERTIES OF THERMAL PROCESS
Fig. 5. Block diagram of the feed-back control system with a contact thermometer
CT shown in Fig. 4; t – current value of temperature
The static characteristic of controller it
is the relationship
between the output
of controller and its
input.
contact thermometer CT, closure of contact r, and closure
of the power circuit. For a short period of time, temperature
t will still drop below the value of t0, until heat from the
heater is transferred to the contact thermometer CT. After
this short period of time, measured temperature will start
to increase and the cycle of changes will start to repeat itself.
The temperature of the liquid in the bioreactor will fluctuate
around the desired value of t0. The schematic diagram of
a control system with the contact thermometer CT, shown
in Figure 4, can be transferred into a block diagram (Figure 5).
DYNAMICS OF THE ON-OFF CONTROL
STATIC CHARACTERISTIC OF THE CONTROLLER
The relationship between the output of controller (e.g.
command signal) and its input (process error) in steadystates is called the static characteristic of controller. In the
control system depicted in Figure 4 and 5, in which the object
Dynamic properties of the thermal-processes characterise their behaviour in unsteady states, i.e. in situations when
the flow rate of heat flowing to or out of such an object changes in time. The dynamics of a thermal-process described in
this article can be determined by conducting the analysis of
temperature changes in the bioreactor during the time from
the moment the bioreactor is subjected to a step change of
the command signal Sr [7]. The graphic picture of the PV
to the step change of the input of object is called the stepresponse characteristic of object. The following two parameters of the object that characterize its dynamic properties
were determined graphically using the object step-response
characteristic (the curve th in Figure 7): time constant (τch) and
dead-time delay (τdh). Both parameters were determined with
the assumption that the changes in time of the temperature
th can be simplified to the first order system with a dead-time
delay. The rate of the temperature increase is determined by
the time-constant τch. The time-constant τch is the time counted from the τdh moment after which temperature increase
would reach the maximum value, if it were a linear function.
The upper part of Figure 7 shows the step increase of the
command signal Sr. The response to the step change of Sr is
the change of PV – temperature th(τ). Additionally, the bottom part of the graph shows the rate of temperature change
in time (dth/dτ). It can be noticed that the curve th(τ) inflexion
point (IP) occurs when dth /dτ reaches its maximum value.
Fig. 6. Static characteristic of an on-off controller; Sr – state (position) of the
relay contact r as the command signal (0 = open, i.e. the electrical current to the
heater doesn’t flow, 1 = closed – see also Fig. 4 and 5), h – hysteresis zone of on-off
control, ton, toff – switch on and off temperature, respectively
The graphic picture
of the PV to the step
change of the CV is
called the step-response characteristic
of object.
28
is defined as the process of heat flow in a bioreactor, the CV
is the electric current IH flowing through the heater which
has the direct influence on the heater temperature. Heater
is an actuator. Electric current IH depends directly on the
state (position) of the relay contact r, signed as Sr. The Sr is
a command signal, output from the controller, i.e. input to
the actuator. We know that Sr can be in one of the following
two states: closed which is conventionally marked as “1” or
open – marked as “0”. The input quantity to the controller
is the process error – the difference between the set-point
value and the measured value of the process variable PV,
e = t0 – t. By determining the relationship between the com-
Fig. 7. Typical time response of a thermal-process to a step-increase of input
and method of determination of its parameters: time constant (τch ) and deadtime delay (τdh), IP – inflection point, line AB – tangent at the IP, tmax – maximum
temperature, τ – time
QUALITY OF THE ON-OFF CONTROL
Knowing the static characteristic of the controller and
the step-response characteristic of the object, the drawing
PRZEMYSŁ SPOZYWCZY
tome 69 z November 2015
‘1’ but the temperature continues to decline for τdh time.
The next exponential increase begins at time τon+τdh and the
change cycle begins to repeat. A characteristic feature of the
on-off control is the occurrence of a set cycle of changes of
the PV close to the set-point value.
Usually, it is the oscillation amplitude, Δt = tu – tb, that is
taken as an important indicator determining the quality of
control in a system with an on-off controller (see in Figure 8).
The second indicator of quality is the so called mean error:
G = t – t0, where t = (tu + tb)/2 - is the mean value of temperature oscillation [8]. The quality of the on-off control can
be improved, especially the value of the oscillation amplitude
(Δt) can be reduced, when: (a) the time constant of object
(τc) can be increased, (b) the dead-time delay of object (τd)
can be decreased and (c) the hysteresis of controller (h) can
be decreased.
CONCLUSIONS:
Summing up, we can say that the on-off temperature control in the tank of
a bioreactor is a good case study in explaining the basic terms of process control with
minimal usage of mathematics. It is a simple example of a relay-type control in which
the CV assumes only two discrete values. A system of the on-off control can be applied
in a situation when: a periodical fluctuation in the PV is acceptable and oscillations vary
within reasonable limits, acceptable in a particular case. This fluctuation results from
periodical step changes in values of the CV. This is acceptable when the object of control
is to exhibit a ‘smoothing’ action, i.e. when it is to be characterised by a slow response
to the step changes of the CV. Such requirements are easily fulfilled in the case of temperature control of thermal objects or in the control of the level of liquid or pressure in
containers where the PV values can only change slowly. The advantage of the relay in
the control system is the ease of switching over of actuators of high electric power at low
power used for the control itself.
Fig. 8. Changes of the PV vs. time during on-off control; tb , tu – the bottom and
the upper limit of temperature oscillation, respectively, ∆t – the amplitude of
temperature oscillation; additionally changes of command signal Sr vs. time are
depicted.
of changes of PV vs. time were drawn in Figure 8 [8]. Such
a drawing is important if we want to learn more about the
on-off control and determine the indicators of quality. In
the analyzed control system the PV is the liquid temperature in the bioreactor. We assume that the heat flow in the
bioreactor is a typical thermal-process and the temperature,
after switching the heater on without the controller, would
increase exponentially to the maximum temperature tmax in
accordance with curve th(τ) presented in Figure 8 as a dashed
line, and after switching off, it would decline exponentially
to the temperature of ambient air ta in accordance with
curve tc(τ). A mathematical model of the step-response of
thermal-process during heating can be simplified to the
step-response model of the first order system with a deadtime delay, presented below:
3URIGUKDELQŮ$5\QLHFNLGULQŮ-:DZU]\QLDN
GU LQŮ $ $ 3LODUVND ² 3URFHVV (QJLQHHULQJ
DQG $SSDUDWXV LQ WKH )RRG ,QGXVWU\ *URXS 3R]QDĻ
8QLYHUVLW\RI/LIH6FLHQFHV
REFERENCES:
(1)
For cooling from tmax to ta this model has the following
form:
(2)
At the time of switching on of the supply voltage U1 and
U2 in Figure 4, no increase in temperature can be observed
during the dead-time delay τdh, despite the fact that electric
current flows through the heater. The τdh time is the time of
heat transport from the heater to the thermometer. Once
the lag time is over, the temperature begins to increase exponentially. At time τof , when the temperature exceeds the
toff value, the controller changes the position of the working
contact, Sr, from ‘1’ to ‘0’, as shown on the static characteristic
of the controller presented on the left part of the graph as
well as on the time function of the command signal Sr(τ) in
the bottom part of the Figure 8. However, the temperature
keeps rising during the τdc lag time until the moment τoff+τdc.
Next, it begins to decline exponentially. At time τon, when
the temperature decreases to the ton value, the controller
changes the position of the working contact, Sr, from ‘0’ to
PRZEMYSŁ SPOZYWCZY
View publication stats
The oscillation
amplitude is an
important indicator
of the quality of the
on-off control.
[1] Dobrzycki, J. (1991). Automation in the Sugar Industry (in Polish: Automatyzacja
w przemyśle cukrowniczym). WNT, Warszawa.
[2] Guwy, A.J., Hawkes, F.R., Wilcox, S.J., Hawkes, D.L. (1997). Neural network and on-off
control of bicarbonate alkalinity in a fluidised-bed anaerobic digester. Water Research,
31 (8), 2019-2025.
[3] Horla, D. (2015). Basics of Control Engineering (in Polish: Podstawy automatyki). Wyd.
Politechniki Poznańskiej.
[4] Ludwicki, M. & Ludwicki, M. (2015). Control of Technological Processes in the Food
Production (in Polish: Sterowanie procesami technologicznymi w produkcji żywności).
PWN, Warszawa.
[5] Mittal, G.S. (ed.). (1997). Computerized Control Systems in the Food Industry. Marcel
Dekker, New York, USA.
[6] Raven, F.H. (1978). Automatic Control Engineering. McGraw-Hill, New York, USA.
[7] Roots, W.K. (1969). Fundamentals of temperature control. Academic Press, New York,
USA, London, UK.
[8] Ryniecki, A. (1990). Basics of Processes Automation in the Food Industry (in Polish:
Podstawy automatyzacji procesów w przemyśle spożywczym). Wyd. Akad. Roln.,
Poznań.
[9] Scientific Encyclopedia: 1983. Van Nostrand Reinhold, London, UK, New York, USA.
[10] Singh, R.P., Heldman, D.R. (2009). Energy and Controls in Food Processes, Ch. 3 in: Introduction to Food Engineering. Academic Press, London, UK, San Diego, USA.
[11] Stephanopoulos G. (ed.). (1984). Chemical Process Control. Prentice-Hall, USA.
[12] Teixeira, A.A., Shoemaker, C.F. (1989). Computerized Food Processing Operations. An
AVI Book, Van Nostrand Reinhold, London, UK, New York, USA.
[13] Tuszyński, W., Sitkiewicz, W., Skierkowski, K. (1989). Basics of Processes Automation in the Food Industry (in Polish: Podstawy automatyzacji procesów w przemyśle
spożywczym). WNT, Warszawa.
29