Confusion surrounding Supervisory Control and Data Acquisition (SCADA) systems and Human Machine Interfaces (HMIs) is not a rarity. HMIs are simply part of a SCADA system, but the confusion is understandable considering the HMI is a part of the system people most frequently use directly.
A SCADA system is a type of Industrial Control System (ICS). They are characteristically very large systems that cover very large areas, usually an entire site. Subsystems make up the SCADA system as a whole, though. Different Programmable Logic Controllers (PLCs), Remote Terminal Units (RTUs), software systems, analytical instruments, and HMIs monitor and control sites. Data is then gathered and reported back to a central location. PLCs and RTUs perform most of the routine actions, but operators can control the SCADA system as a whole by controlling supervisory actions. For example, a PLC might control the valves and pumps in a hydraulic system, but higher up the line an operator can adjust the conditions that the PLC responds to.
Once the data from the subsystems is collected, you’re close to involving the HMI. The HMI doesn’t collect the data, but it reports it to operators visually. Usually a mimic diagram shows the conditions on the floor. Diagrams can range from simple stop lights that are actually being monitored, to multi-screen systems showing a full transportation system. Operators can then use the data and its analysis to make changes and overrides in the individual PLCs, etc. Logistic information for management and maintenance, diagnostic data, and troubleshooting guides can also be added to the HMI.
If a SCADA system is thought of as a car, the HMI is only the dashboard. The car is made up of many smaller systems and machines, but all of the information from those parts is reported to the driver on the dashboard. From there, the driver can make the needed adjustments.