The term Human-Machine Interface (HMI) varies in meaning depending on the context. It can describe anything from a basic instrument panel with switches to a highly advanced IIoT (Industrial Internet of Things) module that integrates software and hardware to link all modern automation systems. 

These HMI screens integrate seamlessly with SCADA (Supervisory Control and Data Acquisition) systems, MES (Manufacturing Execution Systems), and ERP (Enterprise Resource Planning) systems, along with cloud services. In this guide, we focus on those unique pieces of hardware that connect to or integrate with a type of visual display screen. This connection may or may not include advanced IIoT functionalities.


Furthermore, the broad definition of HMI screens sometimes extends to control panels equipped with pushbuttons and switches, emphasizing their versatility. However, we limit our discussion to HMIs that feature a graphical display. These displays provide information about the machine and offer control options. By doing so, we aim to clarify the scope and utility of HMIs in modern automation and control systems.

Keeping the definitions in mind, let’s explore the more integrated control panels, often referred to as machine front panels. These panels not only incorporate visual-display HMIs but also feature a variety of control elements: electromechanical pushbuttons, switches, analog dial selectors, keylocks, ruggedized keyboards, and toggles. Collectively, we sometimes refer to these as discrete tactile-control components. They enable diverse modes of human-machine interaction.

Nowadays, many HMI screens incorporate such components, including analog indicators, LED signal towers, and sound alarms, connecting into systems via serial bus connections. Despite being fairly mature in design, these components are known for their reliability, handling modest currents and voltages.

It’s important to note that some suppliers refer to discrete tactile-control components as actuators. To avoid confusion with motor-based actuators used in automated machinery’s motion systems, we will not use this term.

Machine builders, particularly those in high-tech sectors like pharmaceutical and medical device manufacturing, have increasingly replaced traditional control-panel components with advanced HMIs. This shift is driven by the recognition that, despite their higher cost, advanced HMI features can reduce overall control-panel expenses. They achieve this by decreasing the number of parts and the need for extensive wiring. 

However, it’s crucial to acknowledge that all HMI screens, including sophisticated touchscreen models, have their limitations. For instance, a traditional mushroom-head emergency-stop button is significantly more effective and legally required for immediate machine shutdowns than a virtual button on an HMI’s home screen.

In industrial settings, the preference for mechanical controls over touchscreen HMIs is pronounced among plant personnel, especially those who must wear gloves. Mechanical buttons and selectors offer a reliable alternative in environments where capacitive touchscreens may trigger falsely due to high sensitivity settings.

Such false triggers can be caused by proximity without direct contact or by environmental factors like water droplets and electromagnetic interference (EMI). Moreover, resistive touchscreen HMIs are prone to wear, hazing, and scratches under harsh conditions, diminishing their effectiveness over time.

Despite the familiarity expert operators may have with the graphical user interfaces (GUIs) of HMI screens, electromechanical controls present a more intuitive option for less experienced personnel. This intuitiveness is particularly beneficial for controls designed with a single, clearly defined function, making them straightforward to use even for those with limited experience.

The incorporation of electromechanical controls such as pushbuttons into control panels offers a robust solution to the limitations faced by touchscreen HMIs.

Unlike HMIs, these electromechanical components do not suffer from false triggering caused by external factors like water, EMI, or debris. Additionally, their design does not rely on light emissions, making them ideal for environments where visibility and distraction are concerns, such as in the operation of automated mobile equipment.

Traditional Pushbuttons and HMI Screens

Traditional electromechanical pushbuttons are crucial in control panels due to their simplicity, efficiency, and durability. They can withstand harsh conditions, such as the IP69K high-pressure washdowns in food processing environments. The operation of these pushbuttons involves a slight displacement of internal contactors that close a circuit, generating an output signal in response to a sufficient input-force magnitude.

Short-Travel Tactile Components for HMI Screens

Some electromechanical pushbuttons fall into the category of short-travel tactile control components. These are designed with an activation surface that deflects a small yet satisfying amount, typically 1 to 4 mm, with the device actuating when the input is displaced about halfway. The low profile of these components, especially when gasketed or membraned, simplifies disinfection—a critical feature in settings like meat processing plants and operating rooms.

The tactile feedback provided by these components, characterized by physical displacement and an audible click at the end of stroke, is invaluable for machine operators. In contrast, piezo-based pushbuttons, which have a low-profile construction suitable for washdowns, displace their surfaces by a much smaller percentage, often incorporating LEDs to signal activation.

Diversity in Tactile Control Components for HMI Screens

Beyond pushbuttons, the realm of short-travel tactile components extends to devices such as computer mice and keypads with dome-shaped or conductive rubber keys. Emergency-stop buttons, particularly those with mushroom-head designs, are designed to be conspicuous and mechanically triggered, in compliance with safety regulations. Keylock switches add a layer of security by preventing unauthorized machine operation, accommodating different mechanical key versions for varied access levels.

Direct Command Components for HMI Screens

For direct manual control over motor-driven motion axes, components like joysticks, trackballs, and touchpads are crucial. Joysticks are particularly favored for their versatility in signal types, actuation forces, and ergonomic designs. They meet various design objectives, including ruggedness and tactile feedback for sophisticated applications.

In the evolving landscape of control panel design, the integration of discrete tactile-control components poses a significant challenge due to the extensive wiring required. Recognizing this, suppliers have innovated several solutions to adapt these components for modern control panels by:

  • Reducing their size for more compact arrangements.
  • Introducing easier and more reliable wiring solutions, like tapered screw-down or spring-loaded push-in wire receptacles.
  • Offering gasketed snap-in designs for quick installation into pre-machined panel openings, enhancing assembly efficiency.

Additionally, there are now more ways to customize things like lights, colors, and textures on control panels. This makes it easier for users to interact with and integrate them with other systems. The main innovations driving these changes are softkeys and piezo pushbuttons.

Softkeys represent a significant advancement in control panel design, offering a flexible and intuitive interface that complements traditional HMIs. These tactile-control components are mechanical buttons. Manufacturers position them around the HMI screen’s perimeter, like on the sides or bottom, to boost user interaction and efficiency.

Customization and Flexibility

One of the key advantages of softkeys, especially when integrated with HMI screens, is their adaptability. Programmers can configure these buttons to execute various functions depending on the HMI screens’ current display, offering dynamic control options that adapt to changing operational contexts. This flexibility makes softkeys an invaluable addition to HMI screens within control panels, providing users with immediate access to context-sensitive commands and functions without the need to navigate through multiple menu layers.

Aesthetic and Functional Integration

Manufacturers have leveraged advancements in design and technology to make softkeys more appealing and functional. Modern softkeys come with a range of customization options, including variable lighting, color coding, and textured surfaces, enhancing both the aesthetic appeal and the usability of control panels. These features not only improve the operator’s interaction with the machinery but also contribute to a more coherent and user-friendly interface design.

Complementing Traditional HMIs

Softkeys serve as a perfect complement to HMIs by offering a tactile feedback option that is often missing in touch-screen interfaces. This tactile response is particularly beneficial in environments where operators wear gloves or where touch sensitivity may be compromised. Additionally, the physical presence of softkeys can provide a quicker and more reliable means of executing commands, reducing the likelihood of errors associated with touch-screen misinterpretations.

Piezo pushbuttons are a modern tactile-control component that have gained popularity in various industrial settings, offering a robust and vandal-proof option for machine builders and designers. Applications that demand rugged solutions, like outdoor ticketing kiosks, parking meters, and HMIs control panels within industrial settings, are increasingly adopting these solid-state switches for their durability and reliability.

Design and Functionality

Constructed typically from stainless steel, piezo pushbuttons house a piezo element beneath a thin, depressible surface. This design allows for the surface to deform under the pressure of a normal human finger force, generating a signal whose pulse width varies with the rate of force application. Fast finger taps are especially effective, making these switches highly responsive to user inputs. The internal electronics of the switch convert the piezo element’s charge output into a control signal, facilitating easy integration into various systems.

These switches are designed to accommodate the average human fingertip, with standard diameters of 16, 22.5 (the most common), and 30.5 mm. This ensures a comfortable and intuitive user experience.

Durability and Environmental Resistance

Beyond their traditional roles, piezo pushbuttons are finding their way into more sophisticated, smart designs that benefit from their durability and low maintenance requirements. Their solid-state nature means there are no moving parts to wear out, making them ideal for high-use, public-access applications.

Moreover, piezo pushbuttons excel in harsh environments, boasting IEC 60529 International Protection (IP) ratings suitable for aerospace, food and beverage, and medical laboratory machinery. Their resistance to corrosion and heat allows for placement in demanding locations, such as near ovens or commercial dishwashers, where traditional HMIs cannot operate effectively.

Complementary to HMIs

Piezo pushbuttons, known for their distinct tactile feedback mechanism, frequently complement HMIs to create a complete control interface This combination ensures that operators have access to both visual feedback from the HMI and the reliable, physical interaction provided by piezo switches, enhancing overall system usability and reliability.