HMI (human-machine interface)

An HMI (Human-Machine Interface) is a screen, usually a touchscreen, mounted next to a machine or line, through which the operator can see what is happening in the installation and give it commands.

A PLC controls the installation, but it has no screen or buttons through which a person could communicate with it. That is the role of the HMI panel. It is connected to the controller and works in both directions:

• it displays information: a diagram of the machine or section of the line, current temperatures, pressures and tank levels, the status of pumps and valves, the process stage and a counter of units produced,
• it accepts commands: the operator can start or stop the machine, select a recipe, change settings such as speed or temperature, start cleaning or acknowledge an alarm.

The panel also displays alarms and messages, for example about excessive temperature, a lack of raw material or a pump failure, often with information on what has happened and what needs to be done. Access to some functions is protected by a password or card, so that, for example, only the process technologist can modify recipes, while the operator can only select them.

HMI panels are mounted directly next to machines, in enclosures resistant to dust, water and cleaning. In food plants, they often have stainless steel enclosures. They differ from a SCADA system in that an HMI usually serves a single machine or section of a line and is located next to it, while SCADA runs on computers in the control room and shows the entire installation or the whole plant at once, with data archiving.

Example: An HMI panel is mounted next to the jar filling machine in a jam production plant. At the start of the shift, the operator selects the recipe “strawberry jam, 280 g jar” on the screen, and the machine automatically sets the fill quantity and operating speed. During production, the panel shows the number of jars filled and the jam temperature. When the product in the tank above the machine is running low, a “low product level” alarm appears on the screen, and the operator requests delivery of the next batch.