What is an HMI?

What is an HMI?

HMI stands for Human-Machine Interface, a device that allows an operator or user to interact with a machine or an industrial process. The HMI provides a graphical interface, either a touchscreen or buttons and a display, through which the operator can view and control process variables such as temperature, pressure, level, flow, etc.

The HMI is an important component of an industrial automation system, because it makes communication between people and machines easier, increasing the efficiency, quality and safety of production. The HMI also allows the operator to monitor the status of field equipment, send commands to it, and view charts, alarms, reports, etc.

HMIs can be applied in many industrial sectors and processes, such as energy, water, gas, oil, metallurgy, chemicals, pharmaceuticals, food, automotive, etc. A simple analogy is the HMI of an ATM, where the user interacts with the touchscreen graphical interface and, based on their commands, the machine can, for example, dispense cash from their account.

How does an HMI work?

The HMI works by communicating with the programmable logic controller (PLC), the device that controls the industrial process or machine. The PLC receives signals from the sensors, which measure the process variables, and sends signals to the actuators, which drive the field equipment. The PLC also runs a program that defines the control logic of the process, that is, the conditions and actions the PLC must carry out.

The HMI receives data from and sends data to the PLC through a communication protocol, which can be serial, Ethernet, wireless, etc. The HMI displays the PLC data on its graphical interface, which is configured by the HMI programmer using specific programming software. The HMI programmer can define the layout, colors, symbols, buttons, fields, etc. of the graphical interface according to the operator's needs and preferences.

The operator can view and control the process variables through the HMI by touching the screen or pressing the buttons. The HMI sends the operator's commands to the PLC, which executes them, changing the actuator signals. The PLC also sends the updated process variable data back to the HMI, which displays it on its graphical interface, closing the communication cycle.

What are the functions of an HMI?

An HMI has many functions, which can vary according to the model, the manufacturer and the HMI programming software. Some of the main functions of an HMI are:

• Monitoring and controlling the variables of the industrial process or machine through the graphical interface;
• Receiving and displaying the status of field equipment, such as on, off, running, faulted, etc;
• Sending commands to field equipment, such as start, stop, open, close, adjust, etc;
• Displaying process variables in charts, such as bar, line, pie charts, etc;
• Displaying system alarms, which can be audible, visual or text-based, and which indicate abnormal or critical process conditions;
• Monitoring key performance indicators (KPIs), which are measures that evaluate the efficiency, quality and productivity of the process;
• Running recipes, which are sets of predefined process parameters that the operator can select according to the desired product or batch;
• Logging system events, which are the actions performed by the operator or by the PLC, such as changes in setpoint, operating mode, recipe, etc;
• Managing users, who are the people authorized to access and operate the HMI, and who can have different permission levels, such as administrator, supervisor, operator, etc;
• Storing historical process data, which are the values of the process variables over time, and which can be saved to a memory card, a USB flash drive, a database, etc;
• Generating reports, which are documents that present process data, such as charts, tables, statistics, etc, and which can be printed, emailed, exported to a file, etc.

Conclusion

An HMI is a device that allows an operator or user to interact with a machine or an industrial process. The HMI provides a graphical interface, either a touchscreen or buttons and a display, through which the operator can view and control the process variables. The HMI works by communicating with the PLC, the device that controls the industrial process or machine. The HMI has many functions, which are configured by the HMI programmer using specific programming software.