SCADA and HMI Systems – Differences, Similarities, Purpose

Does SCADA Systems are Really Different from HMI Applications?

One of the thorniest topics in the Automation Industry is that “How is SCADA different from HMI ” some Engineers think that they are used for the same thing and they both are identical, the only difference just in what we call it, but unfortunately that is totally wrong.

The reason for this wrong thought that the both are Graphically Representation of your system or plant also the both are used in monitoring and controlling, but you should be aware about that they are totally different.

So, what is the issue and what is the similarities and dissimilarities between the both, that we are going to talk about in the next few Lines.

What is the Need for SCADA & HMI?

Before we begin discussing the similarities and dissimilarities of SCADA and HMI, let me explain what is the exact function of both systems, so assume that we have a big plant that controlled by many (Machines – sensors – valves …) and all of those as we know controlled by maybe one or several PLCs.

The plant equipments are talking with each other through a Binary Language regardless of all types of communication and signal processing modules and which type of controller is used.

All of this system data is finally converted to 0s and 1s, this is a very low-level language which is really difficult for a human to understand also another issue the machine cannot understand human language.

So, here are the reason for using (SCADA, HMI) these systems just make an Easy interface between the operator and the plant, if you tell the machine to stop it will not stop till you press the OFF button to send Zero for the machine then it will stop. It is just a simple Gateway that converts the machine language (0 and 1 logic) into a unique graphical form so that the operator or maintenance engineer can understand it.

This graphical representation helps us in:

1. Reduce the process cost of the plant.
2. It can also support us with safe interaction between operator and machine.
3. Eliminating the maintenance time and downtime of the machine.

All of this reasons very enough to make SCADA and HMI systems essential parts of the process.

NOTE SCADA and HMI are not adding any type of logic into your process or plant, Just Visual Representation.

The Meaning of HMI systems

HMI stands for Human Machine Interface, it might be an application (software) or also a hardware device

unlike the SCADA it is just software, so we can say that it allows the operator or engineers to communicate and exchange data with a machine.

Note the type of data that is supported by an HMI application is totally different from the data of SCADA system, this data can be called the low level of information that is considered by an operator or technician or either an engineer this data could be ( state of level sensors – overload indication of motor – alarms …).

The Practical Meaning of SCADA systems

First of all, and regardless of the theoretical definition of the SCADA system let us discuss the real meaning of SCADA system, as all of us knows SCADA stands for Supervisory Control and Data Acquisition, or we can say it is a software that is installed into a PC (that is responsible for controlling of the plant), this software supports you with Higher Level of monitoring and control.

So, assume we have an application that supports us with some detailed information about the machine such as ( Motor Amps – Machine state – Rated Temp and Pressure …) this information is not considered as Supervisory information, it may be related to the operator or the maintenance engineer but information like that will not be considered by the manager of the plant, this is could be a low level of monitoring but a supervisor like a manager wants to know somethings like ( amount of production/day – the usage of power – does there is any problem or not …. ) this how the SCADA should be built Just for Higher Level of Monitoring.

Also, the very critical point is the Data Acquisition the manager of the plant or of the production process is not spending all of his time on the SCADA screen, he just opens the history of performance of the plant and checks what is really mean to him.

Differences between SCADA and HMI

After we have illustrated clearly the meaning of the both systems, we can now speak about differences between the both:

The HMI system is a low level of monitoring and control it supports us with a detail information about the process like setpoints, state of machine, load, and so on unlike the SCADA we can say that it’s a higher level for monitoring that scopes to support a very basic and general information about all the process that scaping a lot of details that you really do not need.

The HMI is one – to – one relationship that is between the operator and the machine but the SCADA system can involve and handle a lot of several machines and processes.

One important feature that the SCADA has is that it can store data along with the time SCADA system always need a historical server that can contain its database unlike the HMI it usually does not store any data.

SCADA usually located away from the machine, it should be at the control room in the other hand we found that HMI is usually attached to the machine.

Similarities between SCADA and HMI

Here we are going to say that as the both systems have some differences, they also have a lot of similarities that we are going to talk about:

1. We can say that Basically, they both serve the same purposes (Graphical Representation)
2. Both can be developed using the same designing environment
3. The two systems can be run from a PC
4. Both two systems can communicate with the controlling device (PLC) using the same protocols

Finally, I hope the idea is clear for now and also Note that is a general rule of designing that would really help you a lot if you are an Automation Designer but you should also be aware of that you might find some of this criteria are broken in the field, but I just want the concept to be clear for you.

Author: Hussien Ghareb

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