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.
- 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.