Literature review for
the design of a SCADA system
Doug Schumann, James Fietsam, Eric Rochel, Kyle Dixon, Marc Imbayan, Mike Ibayan
Abstract:
In this paper we go over the different concepts that are required for understand and designing an
industrial control system. The different concept that we cover are:
1. Piping and instrumentation Diagrams
2. Programmable able Logic Controllers
3. Ladder Logic
4. Programmable Automation Controllers
5. Networking
6. Transducers
7. SCADA
8. Actuary Control of Output
9. One-line Diagrams
10. Wireless communications technologies
I. INTRODUCTION
In the design of a Supervisory Control & Data Acquisition (SCADA) system that would allow Ameren
Control Center (C/C) to securely and safely start, stop, and monitor two gas turbines generators located at
the Meramec location a clear understanding of the workings of each combustion turbine and the currently
implemented control systems is needed so that a design proposal would be able to meet the needs of the
client.
II. PIPING AND INSTRUMENTATION DIAGRAMS
In this design project the basic objective is to clearly understand the workings of the combustion
turbine generators and their control systems and then propose an edited version allowing remote access
and control. To fully comprehend the functioning of the plant it is necessary to first be able to read the
blueprints on which the schematics are laid out. In the engineering industry there are many varying flow
charts and diagrams used to represent differing aspects of a given design. As it relates to this project, the
schematics for the turbine generators are developed using piping and instrumentation diagrams. Piping and
instrumentation diagrams are similar to other schematics such as process flow diagrams or block diagrams. A
process flow diagram gives a general sense of the flow of the system and is more in the realm of conceptual
understanding [2]. Piping and Instrumentation diagrams are much more elaborate in their design. Details left
out of most flow diagrams are interwoven in the schematic using a collection of symbols and acronyms. Tag
numbers are letters and numbers placed within the diagram with specified meanings [2]. For example “TRC”
would translate to Temperature Recording Controller [2]. Shapes, in the form of circles, squares, triangles,
and the like, play an intricate roll in Piping and instrumentation diagrams. For example the presence or
absence of a line has relevance to the location of the otherwise determined device. Symbols are also used
to define properly the method with which the transition is made from one device to the other. Symbols
and abbreviations are also used to provide data related to equipment, pipe lines, valves, fittings, drains,
vents, gauges, transmitters, etc [3]. It is necessary then to have an informative bank of symbols and their
corresponding meanings to reference in order to decipher the logic involved in the diagrams [4]. The next
step in the process is to link together a string of these symbols and acronyms into a process. In this case the
logic is used in the thermodynamic processes of the combustion turbine generators.
III. PROGRAMMABLE LOGIC CONTROLLERS
In the design of thermodynamic, mechanical, and electrical systems it is often necessary to translate
an input of one system to its equivalent in another. Transducers and actuators are common terms for these
devices. Taking a closer look reveals a device that must take an input, run a program, and create a subsequent
output [8]. These computers are programmable logic controllers (PLC’s). Programmable logic controllers
were first developed to replace electromechanical relays [8]. They take a signal from a sensor as an input,
perform a logic decision using some type of programming, and send an output signal to a device like a light
bulb. PLCs are gaining populatirty because of the advantages they offer [7].
● cost effective for controlling complex systems
● flexible and can be reapplied to control of other systems quickly and easily
● computational abilities allow more sophisticated control
● trouble shooting aids make programming easier and reduce downtime.
● reliable components make these likely to operate for years before failure
Modern day programmable logic controllers made by companies such as Allen Bradley and General Electric ,
have specialty modules including thermocouples, servo and stepper motor controls, high speed counters, and
resistive temperature detectors [10].
IV. LADDER LOGIC
To ease the use of PLC’s they are programmed using ladder logic format. Ladder logic is a visual
representation of a set of inputs and outputs. Ladder Logic format resembles familiar hardware systems
and and doesn’t require additional training for technicians or engineers with backgrounds in these hardware
systems. The name is derived from the fact that the diagram upon completion resembles a ladder. A Ladder
Logic diagram consists of a vertical line on the left hand side, known as the hot rail, and a vertical line on
the right hand side, known as the neutral rail [7]. They are connected by lines, known as rungs; with several
different symbols each represents an input or output [7]. The logic is following the path and determining
if the rung is true. For the rung to be true it is necessary to be able to flow across closed contacts to the
opposite rail. If the rung is true the output is then considered true. Figure 3-1 is sample ladder logic diagram
[11].
Figure 3-1
V. PROGRAMMABLE AUTOMATION CONTROLLERS
Although PLC’s have been adequate in the past, modern industrial applications are requiring a more
highly developed machine. Automation manufacturers have responded to the increased scope of
requirements with a device that blends the PLC logic with the flexible configurations and integration strengths
of a PC-based system [12]. This device is known as a programmable automation controller (PAC). PACs are
notable for their modular designs and open architectures that allow it to expand and interconnect with other
devices when necessary [12]. Other characteristic advantages of PACs are that they operate using a single
platform, employ a single development platform, tightly integrate controller hardware and software, are
programmable using software tools, and provide efficient processing and I/O scanning [12]. PACs also have
similarities with PC-based data acquisition system (DAQ). The differences can be seen in the physical
appearance and in their software integrations. Where a PC-based DAQ normally includes a mouse and
monitor as most are accustom to seeing, PACs often have no direct human interface [13]. PACs do however
usually have some limited interactions such as a “No error at this time” type status and some may even give
complete control to an external interface [13]. An example would be, an external computer setting a
temperature set point or target RPM for the PAC to create through its design. PACs create financial benefits
because of their integration capabilities, making most adapters no longer necessary [12]. PACs create a link
between systems including motion control, PID loops, serial I/O, operator interface, and more [12].
VI. NETWORKING
Ethernet/IP has become widely popular because of its ease of configuration and operation, high data
throughput, and straightforward connectivity [14]. Ethernet/IP comes in both hardwire and wireless
applications. Wireless Ethernet I/P eliminates costly wiring and provides a high degree of convenience and
mobility [14]. Limitations do exist for wireless Ethernet I/P including operation latency and necessary message
repeats in noisy environments [14]. In general, wireless Ethernet I/P is applicable for data collection,
equipment maintenance and non-critical SCADA designs[14]. Delays are the most prominent problem with
the wireless version. It is this fact that makes time critical commands a problem because they could lead to
safety risks. Because Ethernet I/P was not developed to be deterministic, or in other words have defined
system response times, Rockwell Animation has made design recommendations to maximize performance
[14]. Wireless Ethernet I/P may very well be an ideal communication system so long as performance
expectations are carefully considered.
DeviceNet is an open network that reduces the cost and time required to install industrial automation
devices and still providing interchangeability of components from multiple vendors [15]. DeviceNet is based
on Controller Area Network (CAN), and is a cost-effective way to solve low-level device networking [15]. It is
also effective in providing access to the intelligence present in those devices. DeviceNet allows for connection
of devices directly to the plant without hard-wiring each device to and input/output module [15]. DeviceNet
networks reduce start-up time and downtime associated with maintenance, while operating on a single
network [15].
ControlNet is a control-layer network that provides high-speed time critical input/output data and
messaging data, upload/download of programming and configuration data, and peer-to-peer messaging on a
single physical media link [16]. ControlNet is deterministic, or in other words it has predetermined response
times, and redundant, or repeatable [16]. Its high speed control and data capabilities allow it to have better
input/output performance and peer-to-peer communications to ensure dependable, synchronized, and
coordinated performance [16]. ControlNet allows multiple controllers to control input/output performance
on the same wire, providing a significant advantage over other networks [16].
VII. TRANSDUCERS
A transducers is a device that sense different type of energy as an input and produces a different type
of energy as an output. Basically it converts one physical quantity to another [17]. The purpose of having
transducers is to get measurements or transfer information. They are many types of transducers such as
tempera. Sensor is a type of transducer that sent signal from a measurable parameter to another form.
Sensors have two kinds that are passive and active. The active sensors are self generating devices and the
passive does not need any additional energy source and directly generates an electric signal in response
to an external stimulus [17]. Also, analogue sensors and digital sensors send signal from physical state to a
measurable data. For example, thermocouple which continuously responds to a temperature change sending
the data through signal conditioner to get analogue output signal [17].
VIII. SCADA
A SCADA system is the overall system used to monitor and control an electricial-mechanical process
[20]. There are three different parts to developing a SCADA system:
1. Developing Master Terminal Unit (MTU) with an Human Machine Interface (HMI)
2. Developing the PLCs based on the Remote Terminal Unit (RTU)
3. Implementing the standard SCADA communication protocols
One of the new concepts proposed is a PC-to-PC (PtP) SCADA system which involves a PC base RTU that is in
charge of the PLCs and directing and storing data. The PC based RTU communicated necessary information to
another PC with the MTU. The paper suggest that this implementation of SCADA would require less resources
than a traditional system [21].
IX. ACTUARY CONTROL OF OUTPUT
An actuator is a mechanical device for moving or controlling a mechanism or system. It is operated by
a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure,
and converts that energy into some kind of motion. Mechanical actuators operate by conversion of rotary
motion into linear motion, or vice versa. Conversion is commonly made via a few simple types of mechanism
including Screw jack, ball screw and roller screw actuators all operate on the principle of the simple machine
known as the screw. By rotating the actuator's nut, the screw shaft moves in a line. By moving the screw
shaft, the nut rotates. In engineering, actuators are frequently used as mechanisms to introduce motion, or to
clamp an object so as to prevent motion. In electronic engineering, actuators are a subdivision of transducers.
They are devices which transform an input signal, mainly an electrical signal into motion. Specific examples
include pneumatic actuators which converts energy, typically in the form of compressed air into motion. The
motion can be rotary or linear, depending on the type of actuator [21]. A Pneumatic actuator mainly consists
of a piston, a cylinder, and valves or ports. The piston is covered by a diaphragm, or seal, which keeps the
air in the upper portion of the cylinder, allowing air pressure to force the diaphragm downward, moving the
piston underneath, which in turn moves the valve stem which is linked to the internal parts of the actuator.
Pneumatic actuators may only have one spot for a signal input, top or bottom, depending on action required.
Another type of actuator is a linear actuator [23]. A linear actuator converts energy from an available source
into force and motion in a linear plane[22]. This is similar to the rotational conversion of an electric motor.
Valves require little pressure to operate and usually double or triple the input force. The larger the size of
the piston, the larger the output pressure can be. Having a larger piston can also be good if air supply is low,
allowing the same forces with less input. These pressures are large enough to crush object in the pipe. On
100 kPa input, you could lift a small car easily, and this is only a basic, small pneumatic valve. However, the
resulting forces required of the stem would be too great and cause the valve stem to fail. This pressure is
transferred to the valve stem, which is hooked up to either the valve plug, butterfly valve etc. Larger forces
are required in high pressure or high flow pipelines to allow the valve to overcome these forces, and allow it
to move the valves moving parts to control the material flowing inside. Valves input pressure is the control
signal. This can come from a variety of measuring devices, and each different pressure is a different set point
for a valve.
X. ELECTRICAL ONE-LINE DIAGRAMS
An Electrical one-line diagrams is a single line drawing services, where in a circuit component
chips and hardware is set to be drawn in a diagram of the exact circuit with using a symbols of the each
component and wire. It is also used when information about the circuit is required but detail of the actual wire
connections and operation of the circuit are not. A line (ladder) diagram is a diagram that shows the logic of an
electrical circuit or system using standard symbols. It is also used to show the relationship between the circuits
and their components but not the actual location of the components. Line diagrams provide fast, easy
understanding of the connections and use of the components. Electrical one-line diagrams is also the blueprint
for the Electrical Systems Design Analysis. These one-line diagrams is the first steps in preparing a critical
response plan, allowing us to became more familiar with electrical distribution system layout and design in our
facility [19]. One-Line diagram is a simplified notation for representing a three phase power system. Electrical
elements such as circuit breakers, Transformers, capacitors, bus bars, and conductors are shown in a
standardized schematic symbols. Figure 3-2 is an example of a One-Line Diagram.
XI. WIRELESS COMMUNICATIONS
Wireless communication is a transfer of information between two more points that are physically
not connected. Wireless communications also permits services, such as long range communications, that
are impossible or impractical to implement with the use of wires [20]. The distances can be short, as a few
meters as in television remote control, or long range from a thousand to millions of kilometers from around
the world. Wireless communication could be found in various types of fixed, mobile, and portable two-ways
radios, cellular telephones, Personal Digital Assistants (PDA), and wireless networking. Other examples of
wireless technology include GPS units, wireless computer accessories such Bluetooth keyboard, mouse,
headsets, radio receivers, satellite television, broadcast television and other wireless device [1].
Wireless communications is rapidly growing segment of the communications industry, with the
potential to provide high-speed and high-quality information exchange between portable devices located
around the world. With a device such as Bluetooth, Wi-FI(Wireless Fidelity), Networking, Satelite, attena,
Broadband, 3G/4G connectivity, Infrared device, Radio-Signals. Wireless communication is making the world’s
technology faster and easy.
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