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Chapter 7 of 12Part 2: Inside a control system · 3 min
HMI and SCADA: the screens people watch
The PLC does the deciding, but people still need to see what is going on and give orders. For that, they use screens.
HMI: a screen on the machine
HMI stands for human–machine interface. It is usually a touch screen fixed to the machine. It shows pictures of the tanks and pumps, with live readings. The person running the machine, called an operator, presses buttons on it to start and stop things.
SCADA: the control room
SCADA stands for supervisory control and data acquisition. It is the big picture: screens in a control room that show a whole factory at once. For a water company, that can be hundreds of sites across a whole region.
SCADA keeps a record of everything. It raises an alarm when something goes wrong, so someone can deal with it.
Who does what
- Sensors notice. Motors and valves act.
- The PLC decides, many times a second.
- HMI and SCADA screens let people watch and give orders.
- If a screen breaks, the PLC carries on. The deciding never depends on the screen.
Try it
You are the operator. Start the pump and watch the tank fill. What happens when it gets too full?
Event record
SCADA keeps a record like this of everything that happens.
Nothing has happened yet.
New words
- Operator
- The person who runs a machine day to day.
- HMI
- Human–machine interface. A screen on the machine for watching it and giving orders.
- SCADA
- A control-room system that watches a whole site, or many sites, and records everything.
- Alarm
- A warning that something needs a person's attention.
Quick check
A water company watches 200 pumping stations from one control room. What is that system called?