Data from the process – values, alarms, and events
How can humans understand what is happening at any given point in time in a machine? The human machine interface (HMI) provides all such information in the form of alarms, warnings, and status information.
Elementary operator interface – switches, dials, and meters
An operator needs to know what the machine is doing at any given point. A machine needs commands from the operator. So, every machine needs an HMI.
On a washing machine, for example, there is a small lamp next to the step of the cycle that is running. There may be a rotary switch to command which cycle step to execute. There may also be a power light to indicate that the machine is switched on. There will also be a power switch. In a more sophisticated machine, there may be a small two-digit display to indicate how many minutes are remaining in the current step. There can also be a small LED display to indicate which operation is in progress, and how...
