Journey to the center of the plant: SCADA systems and the big picture
The instrumentation, control, and automation fields are becoming much more diversified than ever before in their capabilities, underlying technologies, and areas of application. Many professional specialists of varying backgrounds are joining this field. However, many fail to develop a clear understanding of the "big picture" of the automation systems and supervisory control and data acquisition (SCADA) applications due to this diversification.
Some professionals are specialized instrumentation engineers but have no clue how exactly the process variables are scaled by the programmable logic controller (PLC) or know the difference between a communication protocol and a network interface. Some professionals specialize in network communication but have no idea what an object linking and embedding (OLE) for process control (OPC) server is, and some can write down a perfect PLC program and know nothing about how the SCADA software tags work and how they are different from OPC items.
The purpose of this four-part series of articles is providing a simplified step-by-step illustration of the journey of information beginning as physical variables in the field (flow rates, levels, pressures, etc.), the several stages and "transformations" the information goes through, and the final point that information reaches the operator at the SCADA screen.
SCADA system big picture
Figure 1 shows an overview of the information flow within SCADA systems. It will be illustrated in full detail in this series of articles.
For the sake of simplicity, Figure 2 is an example of a tank full of fluid that needs its temperature controlled; fluid is heated by opening a solenoid valve allowing steam to go through the heating coil and become cooled by turning off the valve and allowing the fluid to cool off naturally.
A PLC is carrying out the required control functionality (along with many other jobs), and both the temperature transmitter and the solenoid valve are connected to appropriate input/output (I/O) modules in the PLC. The PLC is connected to a computer via some sort of a network, and that computer has installed on it SCADA software packages. Finally, another computer is connected to the first computer via a local area network (LAN), and it is designed to run a client version of the SCADA software.
The example looks at one variable through its journey, the tank’s temperature, from the field up to the SCADA screen, then back to the process with the control signal commanding the solenoid valve.
Eye on the process
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 this example, the thermocouple type T temperature sensing element (transducer) is used, which is producing an mV signal corresponding to a temperature range from -200 to 350 C (-328 to 662 F). The thermocouple is inserted in a thermowell, which provides protection and a secure means of installation into the tank. The output signal is then fed into a temperature transmitter.
Analog signal transmitters
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.
In most cases the measuring instrument combines both a transmitter and a transducer in one device. In some special cases the term transducer is used for a combined transducer and transmitter such as the case of compact pressure transducers. In this example, a head mounted temperature transmitter is used, it receives the mV signal of the thermocouple, and produces a 4-20 mA signal that is corresponding to a temperature range between 0 to 100 C (0 to 212 F).
Marshaling panel, signals at the gates
By means of a twisted pair cable, the temperature transmitter signal runs for a couple of hundred meters, and now it is at the bottom entry of the PLC panel. Typically, the signal is never connected directly to the PLC analog input module; it has to go through the marshaling part of the panel first.
Marshaling provides an easy way to connect, identify, and segregate the incoming cables to the control panel. While the marshaling 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.
PLC analog input module
The PLC analog input (AI) module uses analog-to-digital converter circuits to convert the standard electrical analog signals into raw binary values. For this example, there are four channels, 12-bits, 4-20 mA AI Module, which means:
This AI module can handle up to four signals.
All are in the range of 4-20 mA.
Each is converted into a binary value with the width of 12-bits.
Each signal’s value is placed in a separate location or "address" and identified as AI0, AI1, AI2, and AI3; this address is accessible by the PLC CPU and the control program stored in it.
PLCs are handling digital information in bits, bytes (8-bits), words (16-bits), or double words (32-bits). As shown in Figure 5, this AI module uses four words for the four channels (AI0 to AI3), and from each word it uses only 12-bits to store the analog signal value. The four remaining bits are not used.
Figure 6 focuses on the temperature transmitter signal only. The value represented in the AI module follows:
- 4 mA will be represented as a binary value of 0000 0000 0000, corresponding to a decimal value of 0.
- 20 mA will be represented as binary value of 1111 1111 1111, corresponding to a decimal value of 4,095.
- Any value between 4 mA and 20 mA will be represented linearly between 0 and 4,095; 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 4,095), regardless of the actual ranges or units of the physical variables. And this is where scaling comes into the picture.
Analog signals scaling
Scaling is the mathematical operation of converting the RAW analog binary value 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 through software configuration of the AI module, most PLCs require the user to write a program to do so. Some models even provide a specific function block in the library to do it. In all cases, scaling is just a matter of a simple cross-multiplication equation.
The scaled variables are located in a memory location, or a register, that has an address selected by the programmer. In this example, the temperature value is now waiting in MW100 for further manipulation, and in this case, communication.
In the next article the journey continues by going through network communication, OPC servers, and more.
– Shady Yehia is the Founder and Author of The Control Blog. He is the Instrumentation, Control, and Automation Proposals & Engineering manager in a process technology integration company based in Qatar and operates in the EMEA region. This article originally appeared on The Control Blog. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, email@example.com.
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