THEORY:
Sequence and Logic Control
Many control applications do
not involve analog process variables, that is, the ones which can assume a
continuous range of values, but instead variables that are set valued, that is
they only assume values belonging to a finite set. The simplest examples of
such variables are binary variables that can have either of two possible
values, (such as 1 or 0, on or off, open or closed etc.). These control systems
operate by turning on and off switches, motors, valves, and other devices in
response to operating conditions and as a function of time. Such systems are
referred to as sequence/logic control systems. For example, in the operation of
transfer lines and automated assembly a machine, sequence control is used to
coordinate the various actions of the production system (e.g., transfer of
parts, changing of the tool, feeding of the metal cutting tool, etc.).
Typically the control
problem is to cause/ prevent occurrence of
- Particular values of outputs process variables
- Particular values of outputs obeying timing
restrictions
- Given sequences of discrete outputs
- Given orders between various discrete outputs
Note that some of these can
also be operated using analog control methods. However, in specific
applications they may be viewed as discrete control or sensing devices for two
reasons, namely,
- The inputs to these devices only belong to two specific sets. For
example in the control of a reciprocating conveyor system, analog motor
control is not applied. Simple on-off control is adequate. Therefore for
this application, the motor-starter actuation system may be considered as
discrete.
- Often the control problem considered is supervisory in nature, where
the problem is provide different types of supervisory commands to automatic
control systems, which in turn carry out analog control tasks, such that
overall system operating modes can be maintained and coordinated to
achieve system objectives.
Example Explaining Sequence / Logic Control
The die stamping process is shown in
figure below. This process consists of a metal stamping die fixed to the end of
a piston. The piston is extended to stamp a work piece and retracted to allow
the work piece to be removed. The process has 2 actuators: an up solenoid and a
down solenoid, which respectively control the hydraulics for the extension and
retraction of the stamping piston and die. The process also has 2 sensors: an
upper limit switch that indicates when the piston is fully retracted and a
lower limit switch that indicates when the piston is fully extended. Lastly,
the process has a master switch which is used to start the process and to shut
it down. The control computer for the process has 3 inputs (2 from the limit
sensors and 1 from the master switch) and controls 2 outputs (1 to each
actuator solenoid). The desired control algorithm for the process is simply as
follows. When the master switch is turned on the die-stamping piston is to
reciprocate between the extended and retracted positions, stamping parts that
have been placed in the machine. When the master switch is switched off, the
piston is to return to a shutdown configuration with the actuators off and the
piston fully retracted.
Evolution of Control System
The
control system development after the advent of digital computers now called
PC’s took place in the following order as the technology as well as the
difficulties faced by each of them was realized.
1.
Open Loop
: offline
2.
Closed
Loop : offline
Here set point values were calculated but still
manually set by plant operator, thus offline closed loop control was formed.
This was only acceptable when timing condition of process control is not severe
as manual intervention leads to introduction of time delay in the control of
the process.
3.
Open Loop
: online
In this era of 50’s computers were provided for
process interface for data acquisition and process control, by connecting
inputs directly to the computer. But still set point values of the controller
were not being done, thus online open loop control.
4.
Closed
Loop : online
Here in the end of 50’s era output elements were
also connected to the computers for online process monitoring as well as
controlling. Thus there was data transfer in both the direct making it the
first stepping stone towards online closed loop control and advance control
strategies thereby developed.
5.
Distributed
Dedicated Computers
In the first half of 60’s computers were used for
dedicated functions i.e. their functions were clearly defined like data
processing, data acquisition etc. with no interconnection between them. Data
interexchange was only possible via a transportable medium.
6.
Centralized
Dedicated Computers
Here the information interexchange which was not
possible in the distributed dedicated computer control was possible by
introducing another central computer in which data from all the dedicated
computers come which can be shared later on.
This led to the information exchange but with
computational speed and reliability of computer at stake. This was the advent
of PLC (Programmable Logic Controller)
7.
Decentralized
Computer System
In the beginning of the 70’s it was accepted that
the central computer will be solving central automation problem only leaving
peripheral computers to solve local problems in their close surrounding,
because of which a two stage hierarchical automation system structure called
Decentralized Computer System was introduced . This was the advent of
Distributed Control System (DCS).
Programmable Logic Controllers
(PLC)
A modern controller device
used extensively for sequence control today in transfer lines, robotics,
process control, and many other automated systems is the Programmable Logic
Controller (PLC). In essence, a PLC is a special purpose industrial
microprocessor based real-time computing system, which performs the following
functions in the context of industrial operations
•
Monitor Input/Sensors
•
Execute logic, sequencing, timing, counting
functions for Control/Diagnostics
•
Drives Actuators/Indicators
•
Communicates with other computers
Evolution of PLC
Before the advent of
microprocessors, industrial logic and sequence control used to be performed
using elaborate control panels containing electromechanical or solid-state
relays, contactors and switches, indicator lamps, mechanical or electronic
timers and counters etc., all hardwired by complex and elaborate wiring. In
fact, for many applications such control panels are used even today. However,
the development of microprocessors in the early 1980’s quickly led to the
development of the PLCs, which had significant advantages over conventional
control panels.
Advantages of PLC
•
Programming the PLC is easier than wiring physical
components; the only wiring required is that of connecting the I/O terminals.
•
The PLC can be reprogrammed using user-friendly
programming devices. Controls must be physically rewired.
•
PLCs take up much less space.
•
Installation and maintenance of PLCs is easier, and
with present day solid-state technology, reliability is greater.
•
The PLC can be connected to a distributed plant
automation system, supervised and monitored.
•
Beyond a certain size and complexity of the process,
a PLC-based system compare favorably with control panels.
•
Ability of PLCs to accept digital data in serial, parallel
and network modes imply a drastic reduction in plant sensor and actuator
wirings, since single cable runs to remote terminal I/O units can be made.
Wiring only need to be made locally from that point.
•
Special diagnostic and maintenance modes for quick
troubleshooting and servicing, without disrupting plant operations.
SCADA
The
full form of SCADA is Supervisory Control and Data Acquisition. As the name
suggests this is used for higher level controls. SCADA is software which is
used for monitoring the status of the field which is being controlled. As our
main topic is about PLC which is hardware, existence of PLC without SCADA is
now days not possible and feasible.
PLC
being hardware we are not able to visualize what exactly is the condition of
the process which we are controlling. SCADA along with PLC solves this problem.
SCADA is software which has several graphics available within it which can be
arranged in a form that can simulate the process which we need to control. This
provides us the feature of monitoring. Based on the visualization provided by
the SCADA (which is internally combined with the program in PLC) we can decide
how to control and what to control i.e. we can provide the control from SCADA
itself for the process which in turn will go to the PLC program and after
execution of program will be implemented and updated status of implementation
can be again viewed in SCADA, thus verifying our command.
As we
go further we get to know more about SCADA by performing various experiments on
the same.
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