Everyday more and more industries move towards centralized plant monitoring and control through SCADA systems. Clear understanding of how process information is flowing through an automation system managed by SCADA is a KEY skill for every instrumentation and control specialist, whether he is responsible for the design, integration, installation or maintaining such systems. This understanding can make the difference between a seamlessly integrated system and a complete chaos.
SCADA Systems Information Flow from Field Instruments to Control Room – Part I
During the past year, I was privileged to have the opportunity to work in several EPC (Engineering – Procurement – Construction) projects for sewage treatment plants and water desalination plants. As I was managing the entire instrumentation and automation scope, I had the chance to meet with many instrumentation and control specialists working for my organization, the clients’ organizations, the ICA specialized sub-contractors and within the consulting firms.
What I noticed during this time, that even that all of them were very professional in their fields with years of experience, very few of them, and I mean “Really Very Few” had a clear understanding of the “Big Picture” of the automation systems and SCADA applications. Understanding the “Big Picture” was crucial in those projects, as each of them involved integration of several control systems from different vendors.
Some were very specialized instrumentation engineers, but they had no clue how exactly the process variables are scaled by the PLC logic, or what are the differences between a communication protocol and a network interface, some SCADA programmers who were specialized in only one vendor’s SCADA and PLC had no idea what an OPC server is “which really surprised me, as they actually worked for an automation market leader manufacturer”, and some could write down a perfect PLC program while they didn’t know nothing about how the SCADA software TAGs work and how they are different from OPC ITEMs.
So I decided to do something about it by writing this post, for the purpose of providing a simplified step-by-step illustration for the journey of information starting from being merely physical variables (flow rates, levels, pressures … etc.) in the field, explaining the several stages and “transformations” it goes through until it reaches the operator at the SCADA screen.
SCADA System Big Picture, the Journey Made Short
The below figure shows an overview of the information flow within SCADA systems, it will be illustrated in full details in this post and the following posts. “Journey to the Center of the Plant, from FIELD to SCADA and back! – Part 2 – Network & OPC”.
For the sake of simplicity, we will use an example of a simple process shown in the figure below, a tank full of fluid that we wish to control its temperature, we heat the fluid by opening a solenoid valve allowing steam to go through the heating coil, we cool the fluid by turning off the valve and allowing the fluid to cool off naturally.
We got a PLC to carry out this job (along with many other jobs of course), both the temperature transmitter and the solenoid valve are connected to appropriate I/O modules in a PLC, that PLC is connected to a computer via some sort of a network and that computer has installed on it SCADA software packages. Last but not least, another computer is there that is connected to the first computer via a LAN, and this computer is supposed to run a client version of the SCADA software.
We will concentrate on only one variable as we try to follow its journey, from the field up to the SCADA screen, in our example we will use the tank temperature. Then, we shall follow our steps back to the process with the control signal commanding the solenoid valve.
Now let’s start from the beginning, and try to measure that tank temperature.
Sensing Elements, the Eye on the Process
You will hear a lot of terms in this area, transducer, sensor, sensing element, and maybe more even. Don’t let those terms confuse you, as you only need to focus on definition.
The Transducer, sensor or the sensing element, is a basic device that converts physical quantities such as temperature or pressure into an electronically measurable quantity (such as mV, µA or Ohms).
In our example here we use a thermocouple type T temperature sensing element (transducer) which is producing mV signal corresponding to a temperature range between -200 °C and +350 °C, the thermocouple is inserted in a thermowell that is providing protection and secure means of installation into the tank. The output signal is then fed to our temperature transmitter.
Analog Signal Transmitters, Oh, it Talks too!
The Transmitter is an electronic device that converts the transducer output into a “standard electrical signal” measured in Volts or mA, and is capable of TRANSMITTING that signal for a relatively long distance (few hundred meters).
Mostly the term measuring instrument combines both a transmitter and a transducer in one device. Also some special cases use the term transducer for a combined transducer and transmitter such as the case of compact pressure transducers, but with little experience you will be able to tell the difference by just looking at the output signal in the datasheet. Note that we are not discussing SMART instruments in this article.
In this example, we are using head mounted temperature transmitter (the red connector in the figure below) that receives the mV signal of the thermocouple and configured to produce a 4-20mA signal that is corresponding to a temperature range between 0 °C to 100 °C.
Marshalling Panel, Signals at the Gates
By means of a twisted pair cable, the temperature transmitter signal ran for a couple of hundred meters and now it is at the bottom entry of the PLC panel, we typically never connect the signal directly to the PLC Analog Input module, it has to go through the marshalling part of the panel first.
The marshalling provides an easy way to connect, identify and segregate the incoming cables to the control panel, and while the marshalling function has nothing to do with the value of the incoming signal, it provides several benefits such as:
- Protection for the PLC I/O modules, using fused terminal blocks.
- Disconnecting individual signals, by means of knife-disconnect terminal blocks.
- Isolation, by means of interface relays, signal isolators and barriers.
Now we are sure our signal is connected to the control panel in a safe way, time to see our PLC.
PLC Analog Input Module
The discussion on PLC input modules goes a long way, so let’s focus on our example here, the PLC Analog Input (AI) Module uses Analog to Digital converter circuits to convert the standard electrical analog signals into RAW binary values.
For the sake of this example let’s consider we have 12-bits, 4 channels, 4-20mA AI Module.
What’s that supposed to mean? It means that:
- This AI module can handle up to 4 signals,
- All of them are in the range of 4-20mA,
- It converts each of them into a binary value with the width of 12 bits,
- Placing each signal’s value in a separate location or “address” and identify them as AI0, AI1, AI2 and AI3, this address is accessible by the PLC CPU and the control program stored in it.
Note that PLCs are handling digital information in bits, bytes (8-bits), words (16-bits) or double words (32-bits). So as explained in the figure above, this AI module uses 4 words for the four channels (AI0 to AI3), and from each word it uses only 12-bits and the remaining 4-bits are not used.
- 4mA will be represented as 0000 0000 0000, corresponding to a decimal value of 0.
- 20mA will be represented as 1111 1111 1111, corresponding to a decimal value of 4095.
- Any value between 4mA and 20mA will be represented linearly between 0 and 4095; simple cross multiplication can determine the decimal value corresponding to any mA value.
Note that up to this stage, all the process variables are represented in the same way (a number between 0 and 4095), regardless of the actual ranges or units of the physical variables. And this is where scaling comes into the picture.
Analog Signals Scaling
So what exactly is scaling?
Scaling is the mathematical operation of converting the RAW analog binary value in an AI register to its corresponding meaningful engineering value, and placing this value in a known memory location “register” in the PLC memory, for further use or manipulation.
Though some PLC models offer scaling simply through software configuration of the AI module, most PLCs require the user to write a program to do so, some of them even provide a specific function block in the library to do it.
To cut it short, it’s a simple cross multiplication equation. DO NOT let anybody convince you with something else. They will talk about different data types, floating point vs. integers, that’s alright, we will cover that in another post soon, but for now, this is all you need to know.
The scaled variables are located now in a memory location, or a register that has an address selected by the programmer, so for our example, the temperature value is now waiting for us in (MW100) for further manipulation, and in our case, communication.
In the next post “Journey to the Center of the Plant, from FIELD to SCADA and back! – Part 2 – Network & OPC” we will continue our journey by going through the Network Communication, OPC Servers and more.
This post is also published in Control Engineering Magazine.
July 6, 2015 at 5:17 pm
Well done shady, thanks for great effort it was very refreshing to read such complicated technical details in a very interesting way.
July 7, 2015 at 9:34 am
Simple and rich information which really complete the picture for any one who wants to know about the entire system.
Well done and really great effort.
July 7, 2015 at 9:41 am
Thanks Amr, don’t forget to subscribe to receive our updates by email.. Part 2 is coming soon..
July 7, 2015 at 8:12 am
Excellent explanation, I appereciate
July 7, 2015 at 9:33 am
Soon i will release the second part.. make sure you subscribed to get email notifications..
July 7, 2015 at 3:28 pm
Well done , great job , simple and to the point , waiting for the next blog , very interesting
July 7, 2015 at 10:16 pm
many thanks for very well written post. It would be great to expand little on Signal Isolators as well to help people understand about dirty and safe environment. Nevertheless, this is a very useful information indeed!!
July 7, 2015 at 10:21 pm
Very good point, i wished i could elaborate more in several areas, but then I remrmbered, its a blog post not a book, so I had to stop myself, and even split the post into two parts.. I am planning to write a couple of more separate posts just about the I/O wiring strategies.. Maybe that will be a better place to include your comment..
Thank you very much..
July 7, 2015 at 10:35 pm
Thanks for your prompt, gentle and candid response. Certainly i shall look forward to receiving your further posts. Please let me know if i can be of any assistance to you in this noble work.
July 8, 2015 at 8:41 am
Thanks for your information. currently am working in battery plant..i want to make it as central monitoring system through out the plant..let me know the steps and what the information required for naking the same.
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July 8, 2015 at 8:48 am
You are welcome Jani Basha..
Soon I will publish the 2nd part of the subject, it shall cover the remaining area.. Regarding your project, maybe i will be able to support you if you share Soy ingeniero Electronico en control de procesos, alta experiencia en Instrumentacion y automatizacion PLC, HMI, VFD, DCS, Project Manager, Puesta en Marcha, Busco Empleosome more information about your application and what’s your strategy to make it work.. Glad you enjoyed the post.
July 8, 2015 at 9:29 pm
Well expalined and nice work…but the most difficult task of an instrumentation engineer from indian origin that to find a job as fresher.i hope next time your article will facilate information regarding that too…..
July 8, 2015 at 9:47 pm
Thank you Rajendra for your interest, very good point. Even that I am starting now with purely technical subjects, my plan is to expand to business and career posts as well. Will keep your comment in mind..
July 10, 2015 at 10:52 am
But you didn’t explain about scada
July 10, 2015 at 12:50 pm
In the next post I will explain about SCADA and will start the journey back to the field, If you don’t want to miss it you can follow the blog so you receive email notifications with every new post.
July 10, 2015 at 6:06 pm
Well done shady, very interesting
July 12, 2015 at 6:16 am
Mr shady thank you verymuch for such powerful info, some engineers make this scada system to seem complicated i realise they just dnt wanna share info. My the lord bless u going forward achieving all you disire. I’ll be looking forward to ur next blog.
Thank you Sir
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July 12, 2015 at 9:42 am
Amen.. Many thanks Vuyo 🙂
July 15, 2015 at 6:35 pm
Good explanation Mr. Shady. I am working as a freshman C&I engineer in Thermal Power Plant project. And your blog really helps me to understand from the basic (to make sure that I don’t miss some important points). It is very easy to understand even non-experience on this field. Looking forward to every new posts.
Thank you for valuable post !
July 15, 2015 at 10:27 pm
You are most welcome my friend.. I hope you like and enjoy the coming posts.. After finishing the Signal journey in SCADA I am planning to discuss a couple of control applications in water & waste water treatment.. waiting your further comments..
July 20, 2015 at 5:27 am
Mr Yehia, thanks for that wonderful info, here in Venezuela we work with almost all the stages of automation due the lack of workers but we do not become in experts in any area because we jump from programing plc’s to work with scada and then to solve issues in a 4-20 mA wiring, so, we win in seeing the whole picture as you said but if someone ask me, in which area I consider myself an expert the answer is: in none!!
Great info, it is nice to learn and refresh some terms and perfect to teach to some new intern the automation world.
July 20, 2015 at 6:27 am
Antony you are most welcome. I have to disagree with you in one point, I believe that if you work long enough with “The Big Picture” you will become an expert in everything.. I worked with those called experts in one part, they fail to deliver solutions to complex problems because they know nothing about how the other parts work.. Keep going as a generalist and soon you will be incomparable.. good luck.. and stay with us for part 3 & 4..
July 24, 2015 at 8:56 pm
Thank you Mr.shaddy for presenting initially Automation concept with great simplicity,thus making it more interesting..Keep posting n motivating
July 24, 2015 at 8:59 pm
I have around 2 years of experience in PLC programming n SCADA development.Your posts boost my interest into the subject.Thanks.
July 25, 2015 at 12:24 am
Thanks Rupal for your comment, I hope you like the next posts as well..
July 30, 2015 at 5:19 am
Very good information, but in the first picture I think that the SCADA must be bi-directional, because from the SCADA you can see and change any parameter in the PLC to the process…
July 30, 2015 at 10:27 am
This is what will be explained in Part 4.. it will be released in few days.. So stay with us.. I recommend you subscribe to the blog so you receive email notifications with every new post..
August 7, 2015 at 1:40 pm
Very well done
December 8, 2015 at 2:26 pm
Shady, thank you for the very interesting article. I have a question about scaling. Do you know why only 12 bits are used and 4 are wasted? Looks like a strange decision, resulting in precision loss. Also, is scaling always done by PLC itself? I.e. when I read a register from PLC, is it always a scaled value, say, in the floating point format? Is it possible to read the raw value and scale it in the OPC or HMI?
December 9, 2015 at 12:30 pm
Dmitry, thank you for your comments, I will reply to you point by point..
– About the 12 bits, it’s not always 12 bits, some use 10 bits only, some use 14, and some use the maximum of 16 bits.. It’s all related to what Analog-to-Digital converter (A/D) chip that is the heart of the module, obviously more bits means more expensive chip, on the other side, to have a fast 16-bit A/D Conversion for example for 8 Channels will require a very fast processing, which also leads to more expensive components, manufacturers always balance performance and accuracy in order to remain competitive in their prices.
– Scaling is not always done in PLC itself, the value you read from a PLC is not always scaled.. You can read a RAW binary value in “Integer” format and converted either in the OPC Server or the HMI/SCADA software..
Let me know if you have any more questions..
December 9, 2015 at 1:06 pm
Thank you for the answer, Shady!
I am trying to interpret register values that I see in a communication dump (pcap). As I understand, if the values are scaled, I must interpret them as float16 and be done. If not, I must interpret them as uint16 and apply the scaling function.
Now, if I don’t know whether a register is scaled, my best bet is to try to guess by looking at its values. And if it is not scaled and I don’t know the scaling function, then I could still compare two values, but not make sense of them.
Does the above sound correct? Also, is it possible for some registers on a PLC to be scaled and others not, or is it all or nothing?
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December 9, 2015 at 2:18 pm
Why only fload16? Why not float32?
Can you tell me what type of PLC is this? Do you have access to the PLC program?
Consider this, a float 32 uses two memory locations, so your values will be arranged in the memory in for example Word No. 10, 12, 14 … etc. If you make a mistake and read them as 10, 11, 12 … etc, you will see meaningless data..
Scaling is not mandatory, the programmer of the PLC can scale some variables and leave other values raw, but this is highly unlikely..
Can you tell me more about your application?
December 9, 2015 at 3:03 pm
Thank you for the response! I was just wondering how one could read data from the network packets themselves, without any knowledge of PLCs. You have a very good point in that one cannot always interpret each register as a separate value and absolutely has to know the memory layout to make full sense of the data. Thanks a lot!
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December 23, 2015 at 10:57 am
Very Good Blog Post,SCADA Systems comprise of field data interface equipment normally remote control units or programmable logic controllers.SCADA system also consist of communications system, which is used to move data between various pieces of control units and field data interface device and the computer systems used in SCADA central host.
December 28, 2015 at 1:41 pm
Thanks alot for KISS (keep it simple stupid) work,it is really helpful…
September 25, 2016 at 7:42 pm
more grease to your elbows, you are doing good job. My name is wisdom Kwame Samlafo, I am a beginner when it comes to automation. Until this point, I thought the temperature transmitter would send measurements of the process variables indirectly through industrial protocols like fieldbus, Hart etc without cabling to the marshalling point or the PLC.
But in the example above, you used communication cable to join the transmitter to the marshalling station.
September 30, 2017 at 2:26 am
Very helpful information you share for automation engineers .
October 9, 2017 at 4:27 pm
thank you. share the information with us. we are provided training in PLC, SCADA, and Automation in INDIA
November 28, 2017 at 10:48 pm
Very well explained..thanks. Have subscribed.
October 14, 2018 at 1:03 am
This a comprehensive article, good one for automation engineers. Well done!
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