A simple industrial automation system usually comprises a PLC and HMI / SCADA. The PLC helps in operating the logic by analysing and controlling the field IOs. The HMI / SCADA is a graphical system that shows the processes and it’s parameters in a live state. Both these devices help the operator to control the plant efficiently and reliably. But, have you imagined if a PLC can work without a graphic system? Many engineers raise this question, but get inappropriate answers to this. In this post, we will see a PLC function without an HMI or SCADA.
What is the function of a PLC?
A PLC is a controller that takes hardware inputs from the field, processes the logic written inside through a programming language, and then controls the hardware outputs in the field. It also has communication protocols to connect with real-time data of industrial communication, like Modbus, Ethernet, CAN Open, etc. So, in an industrial setup, a PLC is a device that actually talks with the field devices and connects them further to upper-level automation like SCADA, DCS, or IoT. It can store the field data, set parameters, and other real-time data in its memory for retention and further logic.
What is the function of HMI or SCADA?
An HMI or SCADA is a monitor system that is a graphical representation of the process happening in the plant. Basically, it takes communication data from PLC or other lower-level controllers, and shows them on the screen. The programmer needs to map the data on the screen with graphical symbols and objects, to show it as an animation. This helps the operator to view the live process with real-time data. Apart from graphics, it also has the objective to generate reports, talk with database systems for storing historical data, generate trends, generate alarms, and interact with higher-level automation of IoT and cloud services. An HMI is an individual device like a PLC, whereas a SCADA system runs on a computer or IPC (industrial computer) mounted on a panel.
Can PLC work without HMI or SCADA?
Now that we have understood the roles of both systems, we will dive into our main discussion. Technically speaking, yes, you can run the PLC system without HMI or SCADA. For that, refer to the points below on how it is possible:
- A PLC executes logic based on inputs and controls the outputs. If HMI were there, the graphical representation of inputs and outputs would have been shown on the screen. But if not there, and the system is small with a very small number of IOs, lamps and indications can be used on the electrical panel for the same.
- If there is an application where the set parameter will be entered only once during commissioning, and will not change in the course of life, then the engineer can download the values in the PLC during final commissioning. After which, the system can be handed over to the client without any graphics, because neither they are not going to change any set parameter now, nor do they want to view any current parameter value running inside. As long as the system is running fine, such types of applications can run without an HMI or SCADA.
- Some PLCs have built-in small HMI displays, where you can configure the parameters and status of data. It is enough for small-scale systems where your motive of graphics will be achieved even without an HMI or SCADA. So, for such systems, PLCs can operate without an external display system, which will save you cost to a very large extent.
- In the last, if at all a need arises to view the current parameters inside the PLC or set any parameter without graphics, then PLC’s mostly nowadays have embedded webpage, which can be viewed through PLC’s local Ethernet port. There, you can fulfill your purpose. Also, you can access these data via communication protocols of PLC like Modbus, Ethernet IP etc. You will just require an external computer, to access them whenever required.
So, practically speaking, PLC can run without an HMI / SCADA. But that has its own cons too, which can be described as below:
- HMI or SCADA has great tools or objects designed to view complex displays very easily. All this viewing will not be possible now without them. Also, as these systems perform reporting, alarming display, or trending display, getting real-time data from the PLC is a very hectic task. Yes, some PLCs have features to connect to server databases like SQL Server, but the programming is complex, and if the network lags in between, then real-time data will not be logged in the database.
- As graphics help in quick troubleshooting and maintenance activities, a PLC system without graphics just increases this load to the operator. An increase in such tasks increases manpower, additional costs, and downtime for a system.
In short, it can be recommended to run PLC without HMI or SCADA, but only for small or less complex applications.
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