Human-Machine Interface (HMI) vs. LCD

The terms LCD and HMI are often confused as being synonymous, but their relationship is closer to the comparison of squares and rectangles.

When we talk about LCD modules, the first thing that pops up in our minds is a television. The term LCD TV is so widely recognized that people pretty much think this is what LCD technology is exclusively for. But in reality, LCD technology builds a bridge between humans and machines, not just a TV.

LCD modules can be integrated into different machines across different industries to display crucial information about the machines and give you control over the parameters. Such a module is known as a Human Machine Interface (HMI). So, when you come across touch-screen modules, driver cab panels, or ticketing machines, you use an LCD module to interact with a particular machine.

But does this mean LCD and HMI are synonymous? The simple answer is No. HMI is a broader term comprising many components, including LCD modules.

So, let’s try to understand both these terms, their differences, how they function, and how they get together to provide us with a seamless way to interact with modern machinery.

Understanding LCDs

LCD stands for liquid crystal display, a technology that’s very common around us. From televisions to computer monitors to smartphone screens, LCD has transformed electronics in the last two decades, making it convenient for us to control gadgets and access system information. Digital clocks, CD players, microwave ovens, burners, refrigerator panels, lift information panels, car screens, and tons of other devices and machines use LCD screens these days. Contrary to the previous major display technology, which was the CRT (cathode ray tubes), LCDs are much lighter, power efficient, and usable in many applications.  

How Does an LCD Work?

The liquid crystals inside an LCD module possess light-modulating properties, such as reflectivity and birefringence. They’re long, rod-like molecules that orient in one direction and can also flow freely. Since they have a unique molecular appearance, they can control light and produce visuals. They don’t emit light but use a backlight and polarizing layers to produce monochrome or colorful images.

In digital clocks, timers, and other low-information devices, the display usually has large elements, where certain characters and shapes are the main priority. However, matrix or small pixels are used where dynamic or arbitrary visuals are necessary, such as on a computer display or TV.

LCD electronics is a delicate and ingenious topic, not to mention a large part of technology. We will stick to the general mechanism since it’s not essential for the article topic.

Layers of an LCD (Twisted Nematic)

  1. A polarizing filter that redirects light when it enters the film (vertical).
  2. Shaped ITO electrodes that determine the character or shape of the LCD module, vertical ridges.
  3. Twisted nematic liquid crystal
  4. Shaped ITO electrodes with horizontal ridges.
  5. Horizontal polarizing film layer.
  6. Reflective layer that creates the main visuals. A light source is used instead of a reflector in backlit LCD modules.

In most cases, the axes of transmission of the two transparent electrodes that make up each pixel of an LCD are perpendicular to one another. They are often made of indium tin oxide (ITO) and a pair of polarizing filters or parallel and perpendicular polarizers. If there were no liquid crystals between them, the second dipole would block light through the first filter. The alignment at the electrode surfaces determines the arrangement of the liquid-crystal particles before applying an electric field. By controlling the applied voltage, the display changes grey levels.

Exploring HMIs

A human-machine interface (HMI) screen exclusively connects a machine to a human operator. It allows for direct human intervention and supervision of an automated process. HMI screens can control an operation, gain real-time insights into a process, determine progress, and command future courses of action. They can be LCDs, LEDs, OLEDs, TFTs, etc. Their primary purpose is the graphical representation of a machine’s current conditions.

Take the user interface of a modern-day lift, for example. It may or may not have a touchscreen control panel, but it has display screens inside the lift and on all the levels. These displays are usually non-arbitrary because they only show the lift’s position and direction at a specific time. The display inside a lift may show additional information such as current load, maximum load capacity, emergency instructions, etc. All of these are human-machine interfaces that allow control or display of information.

HMI Components

We can break down the components of a human-machine interface into three parts.

  1. Control Panel: A control panel that lets the user directly interact with the system of the machine. they are usually equipped with buttons, knobs, levers, switches, and other input options. with these components, a user can input commands, change settings, manipulate different parameters, start or stop a process, etc. 
  2. Touch Screen: The touch screen is one of the key HMI features. It removes the traditional control panel and provides control within the display itself. You can simply use your fingers or a stylus to manipulate the operations. This is especially helpful if you use a machine with many control parameters and functions. Rather than using dozens of buttons and levers, using a single dynamic screen to control all the operations is much more convenient.
  3. Display Panel: This is the critical component of an HMI system. The display panel acts as a gateway to the system. It shows real-time data, metrics, fluctuations, status indicators, etc. Some advanced panels can also show dynamic graphic analysis and generate predictions based on variable changes.

Differences Between HMIs and LCDs

PurposeFacilitates human-machine interaction with real-time monitoring and control.Used for static or dynamic visual output regardless of user interaction.
Offers live interaction with on-screen elements through touch-screen features.Separate input devices are required.
FunctionalityVisualizes data for industrial purposes. Uses dedicated software and hardware.Plain visual illustration, doesn’t usually have specialized features.
ApplicationUsed in automation, manufacturing, robotics, vehicle production, and other complex industries requiring real-time operation and monitoring.Used in general electronics, advertising, entertainment, information display, and other visual tasks.


As time passes, machines are rapidly becoming more advanced and complicated. Whereas machines from a few decades ago were lever-controlled, analog, and fully labor-driven, modern machines are intelligent and digitally controlled. LCD technology gives us that control by providing a digital interface that controls analog actions via electronics. However, even though LCDs are a big part of this, they do not represent the human-machine interfaces alone.