Table my solution. Then I will also explain

Table of Contents
Introduction. 2
Background. 2
Aims/Goals. 3
Issues. 3
Piping and Instrumentation Diagram.. 4
PLC systems. 5
Human Machine Interface. 8
Conclusion. 10
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Introduction

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This report contains information about my course project
called “Controlling level of liquid in the horizontal tank”. This control
system is designed in TIA portal which integrates Simatic Step 7 for PLC
programming and WinCC for Human Machine Interface. Firstly, I will explain the
problem statement and the main background for this problem. Then I will
introduce Piping and Instrumentation Diagram of this control system showing the
main devices that help to implement my solution. Then I will also explain the
ladder logic diagram, SCADA system and graphical interface that I built in TIA
portal. At the end I will give my conclusion about my solution and how it can
be developed and improved further.

 

Background

In this task I need to build a control system that
effectively control the level of the liquid in the horizontal tank by opening
and closing the draining valve. 
A control system manages, commands, directs, or regulates the
behavior of other devices or systems using control loops. It can
range from a single home heating controller using a thermostat controlling
a domestic boiler to large Industrial control systems which are used for
controlling processes or machines.

For
continuously modulated control, a feedback controller is used to automatically control a
process or operation. My control system continuously compares the value or
status of the process variable (level of the liquid in the tank) being
controlled with the desired value or set point (this can be varied according to needs),
and applies the difference as a control signal to bring the process variable
output of the plant to the same value as the set point. For sequential and
combinational logic, software logic, such as in a programmable logic
controller, is
used.

There
are two common classes of control action: open loop and closed loop. In an
open-loop control system, the control action from the controller is independent
of the process variable. An example of this is a central heating boiler
controlled only by a timer. The control action is the switching on or off of
the boiler. The process variable is the building temperature. This controller
applies heat for a constant time regardless of the temperature of the building.
However, in a closed-loop control system, the control action from the controller
is dependent on the desired and actual process variable. In my case, for
example the output of the control system (level of the liquid) is continuously
measured and is sent as a feedback to the PLC to compare the value with the set
point and make corrective actions according to the error.

In this
task proper selection of the level sensor, valves, pumps, PLC and communication
system is important and will be discussed further.

 

Aims/Goals

Main objectives of this task are listed here:

·       
Pumping the liquid into the horizontal tank
continuously with pump

·       
Measure the level of the liquid in the tank by using
proper radar level sensor

·       
Creating start and stop buttons

·       
Connecting the drainage (output) of the tank to a
valve with actuator

·       
Opening and closing the valve automatically according
to the set point with PLC

·       
Showing high and low alarms to increase safety

 

Issues

There are some main points that should be considered before
designing the control system. When designing the control system instrumentation
of the system should be carefully chosen in order to make it reliable and easy
to use. Also proper control strategy should be selected to meet the
requirements of the task stated. Control strategy can be different types: logic
control (with relays and timers), linear control (using linear negative
feedback), on-off control (bang-bang control), proportional control, PI
control, PID control and so on. However, we should choose our control strategy
with care in order to make the system work properly while keeping cost and
complexity of the system minimal. It means that if our application is not
hazardous and does not contain any valuable substances we can minimize the cost
by applying simple techniques. In my task we simply need the controlling level
where there is very little potential hazards and we can confidently use on-off
control in order to make thing simple and easy to configure while working
properly. Although the PV will have some slight oscillation around set-point
when it suddenly changes but it will be in tolerable amount and on-off control
matches our requirement. In my task, there is direct relationship between level
of liquid and flow rate in the drainage valve. So, there should be direct
control in this control system. It means that if the level increases above the
set-point the flow rate in the drainage valve also should increase and this
shows direct control.

 

Piping and
Instrumentation Diagram

A piping and instrumentation diagram (P&ID) is
a detailed diagram in the process industry which
shows the piping and vessels in
the process flow, together with the instrumentation and
control devices. So P&ID shows the interconnection of process equipment and
the instrumentation used to control the process. In the process industry, a
standard set of symbols is used to prepare drawings of processes. P&ID of
my task is given below:

Here we can see horizontal vessel having one input pipe and
one output pipe. Input of this vessel is connected to a pump which will send
the liquid into the tank when the start button is pressed. In this diagram LG
stands for level gauge which directly shows the level of the liquid in the task
in the field. LT 135 is a radar type level transmitter which measures the level
of the liquid by sending pulses into the tank and according to their reflection
this transmitter can send 0-10 V or 4-20 mA signal which can be scaled in PLC
and processed to control the level. LI 135 is a level indicator of the LT 135
sensor. LIC135 is the level controller which has auxiliary location accessible
to operator. We can set the set-point (SP) on this controller. LY is the level
relay which converts electrical signal into 20 PSI pneumatic signal which will
control the final control element- actuator to open and close the valve. LV 135
is the controlled valve which will change the flow of draining, thus altering
the level of the liquid in the tank.

 

PLC systems

To
implement a control algorithm for this control system Programmable Logic
Controller will be used. A PLC is an industrial digital computer which has been ruggedized and adapted for the control of manufacturing
processes, such as assembly lines, or robotic devices, or any activity that requires
high reliability control and ease of programming and process fault diagnosis.
PLCs are used in the shop floor of the manufacturing process as a part of
process control. For larger and high reliability systems DCSs are used and they
are efficient for many loops. For our application PLC system is suitable since
our process is not complex and contains one loop.

In my
control system I used Siemens S-300 CPU 315-2 PN/DP which has its own input and
output modules so I do not need to add I/O modules. This PLC is connected to a
PC system SIMATIC PC Station WinCC RT Advanced through HMI connection. So, the
device configuration is finished and the PLC should be programmed to implement
the required logical operations. In this picture below, device configuration of
the system is shown:

After
proper device configuration I started programming the PLC in LAD language and
the code is shown below:

 

In this
ladder logic diagram, network 1 is start stop scheme. Start is normally open
contact when it is closed the Run coil is energized and it closes the auxiliary
contact Run below the Start contact. Stop button is normally closed contact.
So, when we need to stop the process we should open this normally closed
contact and the system will stop operation. In network 2, after the Start is
pressed and the Run coil is energized the Q0.1 pump starts working whose PLC
tag is named as “Filling”. Here closed contact of draining valve (Q0.2) is used
to turn off the filling pump when the set-point is exceeded and the draining
valve is working. So, this pump serves for pumping the liquid into the vessel, but
it will stop working when draining happens.

In
network 3, the control action is shown. Here Run contact is firstly added to
operate only when the Start button is pressed. Then comes a comparator which
compares the Level of the liquid (MW2) with the set-point (MW8). If the level
is higher than the desired set-point the Draining valve Q0.2 will be energized
the valve will be opened to reduce the level of the liquid in the tank. The
value of the level MW2 is simulated by a slider in HMI which will be discussed
later.

So,
continuously, if we increase the level of the liquid above the set-point,
drainage valve will try to decrease the level until it reaches the set-point.
After reaching the set-point the valve will be closed automatically and the
filling will start.

In
network 4 and 5 working of low and high alarms is shown. When the level of the
tank is too low below 10 cm the low alarm will turn to red showing the operator
that the level is too low. When the level is too high above 90 cm the high
alarm will turn to red to make the operator aware about hazardous case.

 

 

 

 

 

 

 

 

Human Machine Interface

The main screen before run:

 

The main
screen after run:

 

 

 

 

 

 

Changes after the Start is pressed: (PV is below the
set-point and low alarm level)

 

 

Changes after the PV exceeds set-point:

 

 

 

 

 

 

 

Main
screen when PV exceeds set-point and high alarm level:

 

The graphical
interface is made simplistic in order to make it easy to monitor the process.
The input from the level sensor is simulated with a vertical slider which
changes from 0 to 100 and this corresponds to the level of the tank from 0 cm
to 100 cm. Start and Stop button is used to run and stop the process. These
buttons have events connected to them with PLC tags. When the start button is
pressed the filling pump will turn to green showing that this pump is running.
So, firstly, low alarm will turn to red because the level will be too low.
After some time level should increase and low alarm will turn to yellow. After
the PV goes above set point (50 cm is set with right side slider) the draining
valve will turn to green from red showing that this valve is open. While the
drainage valve is open the filling pump should be stopped in order to reach the
set-point easily. When the level goes to high alarm level the High alarm will
turn to red showing that the PV is at dangerous level.

User can
easily change the required set-point by using the slider on the right side. In
the screenshots it is set to 50 cm. When we change the set-point the process
will behave differently.

Conclusion

In this project, I implemented on-off control to control the
level of the liquid in a closed horizontal tank. Comparator block is used to
implement on-logic. First of all, SetBits events are attached to Start and Stop
button to start and stop the process. There is also low and high alarms to
monitor the critical levels. Drainage valve is open when the PV exceeds SP.
Level of the liquid inside the tank is shown on the indicator of the tank.
Set-point can be changed by using the slider on the right side. To conclude up,
I would say that the system works properly, but on-off control is not optimal
solution for level control when high quality and precision is crucial. PID
control is better selection, I tried to implement continuous PID control, but I
faced with problems of changing variable types.