Supervisory control systems – HMI and SCADA
When we talk about control systems, usually we focus on the field devices (inputs), PLCs or DCSs (controllers), and final control elements (outputs). That makes sense, since you need all three elements for a closed-loop control system. However, without supervisory control systems, you’ll have trouble monitoring and controlling your process.
The supervisory control system translates all the numbers and data generated in the field into a visual interface. It helps the operator see what goes on in the field without having to understand all kinds of coding and data.
Some supervisory control systems only show you what’s happening at your process, but some allow you to control variables straight from the display of your human-machine interface (HMI). Before we start talking about different systems, let’s understand what they are and how they work.
What is a supervisory control system?
Supervisory control systems can be simple, with only a few functions, or complex, with all the tools you need to manage everything in your plant. Regardless of the system’s complexity, it will have one main objective: to provide a graphic user interface (GUI) for at least part of your process.
Your control system handles loads of data each second. However, the jumble of numbers, letters, and colors can make it hard to tell what’s going on. A GUI gives faces, or graphics, to each of your variables. However, this interface only helps if it shows you your plant data. Therefore, you must have your control system connected to your controller. Let’s see how it works with an example.
The automated teller machine
Yes, an ATM makes a great example of a simple supervisory system with an HMI. When you use an ATM, you interact with your accounts using the screen and operation buttons, which act as the supervisory control system. Inside the ATM, you have another control system.
First, the controller waits for the input, the operation you choose. After you tell it what you want, it will then run its control logic. Let’s say you choose to withdraw money. The machine “asks” you for the amount of money you want. Without an HMI, you’d need to send a binary code to tell the controller how much you want. Thanks to the GUI, you just type in the amount.
After that, it may ask you for another input, like type of receipt. After collecting the three inputs, the machine does its magic. It will send output signals telling the actuator to count the money, open the safe door, and push out the notes you need, as well as sending the kind of receipt you chose.
So you’re operating a control system through a supervisory control system every time you get cash from an ATM.
Types of supervisory systems
I mentioned a simple supervisory system, the HMI, when I gave the ATM example. We also have a more complex system we call the supervisory control and data acquisition (SCADA) system. Let’s have a better look at each of these systems.
Human-machine interface is a set of hardware and software that provides a user with a visual depiction of a process. Usually connected to your controller, the HMI gets all the data from your field devices and displays it in a GUI.
Because you have the HMI connected to your controller, you can make changes in your process straight from the HMI, like shutting off a pump or raising a setpoint. This lets the operator interact with the process without needing to understand all the coding behind it.
The HMI will also show alarms set on your PLC logic. And because your HMI is connected to your controller, you can pull the control logic and troubleshoot it through the HMI’s display without needing a PC.
SCADA stands for “supervisory control and data acquisition.” It differs from the HMI mainly with the last bit, data acquisition. Every SCADA will have an integrated HMI. However, a SCADA system can do far more than a simple HMI.
In a SCADA system, you can create a GUI for the interaction between operator and machine. But more than that, a SCADA system can connect to several controllers. You can have a central control room and supervise areas remotely.
The SCADA system can also connect to a Structured Query Language (SQL) or other similar database. Because of that, the SCADA system can not only store data but also feed historians, to allow better trend monitoring and other analytical uses.
Another advantage of SCADA systems is that you can connect more than controllers to the system, like surveillance cameras, for example. This feature gives you better control of your entire plant.
Supervisory control systems connect operator and machine by providing a graphic user interface (GUI) in a human-machine interface (HMI).
For more complex systems you can have a supervisory control and data acquisition (SCADA) system to visually aid the operator and collect data over time. With this data, you can create histories for trend monitoring, alarm history, or other analyses.