Optimizing Industrial HMI Design with Modular 22 mm Pilot Devices

The design and implementation of the Human-Machine Interface (HMI) for industrial machinery and control systems present a unique set of engineering challenges. Beyond basic functionality, today's HMI components must meet stringent requirements for operational reliability, environmental resilience, and design flexibility. For electronics engineers and system integrators, component selection has a direct impact not only on the end-user experience but also on the total cost of ownership (TCO), affecting factors such as inventory management, installation time, and long-term maintenance.

This technical article will explore how a modular approach to 22 mm pilot devices can address these industrial challenges. Using the Eaton M22 series as a technical case study, this article will examine how modularity, field-configurable functionality, and robust construction provide engineers with a versatile and reliable toolkit for HMI design.

The foundation of this approach is a system where a wide range of operators are built from a common set of back-panel components. Figure 1 provides a representative overview of such a component family, illustrating the variety of operators and the building-block nature of their assembly.

Figure 1: An exploded-view diagram illustrating the components of a modular 22 mm pilot device product line. (Image source: Eaton)

Challenge 1: durability and reliability in harsh industrial environments

A primary challenge in industrial HMI design is ensuring the survivability and long-term reliability of components in harsh environments. Exposure to liquids, dust, oil, corrosive agents, and significant vibration can cause premature failure in standard electromechanical components.

Traditionally, specifying components for harsh environments required sourcing specialized, often expensive, parts rated for specific conditions. Standard components with lower Ingress Protection (IP) ratings are unsuitable for applications involving regular cleaning with high-pressure water jets or those with a high concentration of airborne particulates. This forces designers to either over-specify for all applications, increasing costs, or manage multiple component tiers for different environmental conditions.

A more efficient solution is a component line where high environmental resilience is a baseline feature. The Eaton M22 series, for example, is designed with high IP ratings across its standard offerings. Many operators, including the M22S-PV push-pull emergency stop (Figure 2) and the M22S-DR-S non-illuminated pushbutton (Figure 3), are rated to IP67 and IP69K.

Figure 2: The M22S-PV modular push-pull emergency stop operator. (Image source: Eaton)

Figure 3: The M22-DR-S RMQ-Titan pushbutton. (Image source: Eaton)

IP67 protects against temporary immersion in water. This rating is crucial for components on machinery exposed to standing water, heavy splashing, or various outdoor weather conditions.

IP69K protects against close-range, high-pressure, high-temperature water jets. This is a critical requirement in industries such as food/beverage, pharmaceuticals, and heavy equipment, where stringent and frequent sanitation protocols are commonplace.

By standardizing on components with these ratings, engineers can design systems capable of withstanding harsh conditions without needing to source niche parts, thereby simplifying the design and procurement process.

Another aspect of reliability is the light source for illuminated operators. Conventional incandescent bulbs are susceptible to failure due to mechanical shock and vibration, and have a limited operational lifespan, necessitating regular replacement. The M22 line, with operators like the M22-DL-X illuminated pushbutton body shown in Figure 4, addresses such issues by exclusively using LED illumination.

Figure 4: The M22-DL-X modular pushbutton with LED illumination. (Image source: Eaton)

Solid-state LEDs are inherently resistant to shock and vibration, offering a significantly longer operational life, rated for up to 100,000 hours. This longevity eliminates the need for bulb replacement over the typical lifespan of a machine, reducing maintenance schedules and improving the TCO. Furthermore, the optical design of the M22 lenses is matched to the output of the LEDs, ensuring a clear and bright indication that is not easily washed out by ambient light.

Challenge 2: optimizing inventory and reducing total cost of ownership

For OEMs and system integrators, the cost of a component extends beyond its initial purchase price. The complexity and cost of managing inventory for a wide range of functionally different but physically similar parts can be substantial. A typical control panel may require momentary pushbuttons, maintained pushbuttons, and various selector switch configurations, each traditionally corresponding to a unique part number.

Stocking dozens of fixed-function stock keeping units (SKUs) results in high inventory carrying costs, increased warehouse complexity, and the risk of production delays if a specific, less-common variant is out of stock.

A modular system with field-convertible operators provides a direct solution to this logistical challenge. A maintained pushbutton from the M22 series, such as the M22S-DR-S, can be converted to a momentary function by the installer. Figure 5 shows that this is achieved by adjusting a small locking ring accessible on the side of the operator body, a feature designed for both flexibility and convenient installation. This single feature means one part number can fulfill the role of two, halving the inventory requirement for basic pushbuttons.

Figure 5: The field-convertible function selector (left) and anti-rotation tab for secure mounting (right). (Image source: Eaton)

For key-operated switches, such as the M22-WRS (Figure 6), this system determines the positions at which the key can be removed. Similarly, a standard two-position knob selector, such as the M22-WKV (Figure 7), can have its maintained function modified in the field, further reducing the need for multiple SKUs with different operational logic. This adaptability allows a few base operator SKUs to serve a vast number of application-specific logic requirements.

Figure 6: The M22-WRS modular two-position key-operated selector switch. (Image source: Eaton)

Figure 7: The M22-WKV modular selector switch operator. (Image source: DigiKey)

By adopting a modular and convertible component family, organizations can significantly reduce the number of unique part numbers in their inventory, leading to lower TCO, simplified procurement, and greater design flexibility.

Challenge 3: integrating advanced control and safety in limited space

As industrial machinery becomes more sophisticated, the density of control functions on HMI panels tends to increase. However, the physical space on a control panel is often a fixed and valuable commodity. Engineers are frequently tasked with providing more control options in a smaller footprint without compromising usability or safety.

The conventional approach of using one standard 22 mm operator for each function quickly consumes available panel real estate, especially in applications that require multi-axis directional control.

Specialized operators that consolidate multiple functions into a single 22 mm cutout are an effective solution.

• Joystick Operators: For applications requiring directional control, such as moving a gantry or conveyor, a joystick operator like the M22-WJ2H in Figure 8 can replace two individual pushbuttons. This saves space and provides a more intuitive control interface for the operator.

Figure 8: The M22-WJ2H pushbutton joystick operator. (Image source: Eaton)

• Double Pushbuttons: For high-density start/stop controls, a double pushbutton, such as M22-DDL-GR-GB1-GB0 (Figure 9), combines two buttons (typically for START/STOP) and a central indicator light into one unit. These components are purpose-built to maximize control density.

Figure 9: The M22-DDL-GR-GB1-GB0 pushbutton for start/stop control. (Image source: Eaton)

Safety is a non-negotiable aspect of HMI design. Emergency stop circuits must be reliable and fail-safe. A key technical requirement for components in these circuits is the concept of "positive opening" (or direct opening action) for normally closed (NC) contacts, as defined in International Electrotechnical Commission (IEC)/EN 60 947-5-1. This ensures that the contacts are forced open mechanically by the actuator's movement, even in the rare event of a contact weld.

The M22 system includes a range of emergency stop operators designed to work with these safety-rated contact blocks. Actuator types include standard push-pull (M22S-PV) and key-release models, such as the M22-PVS45P, as shown in Figure 10. The key-release version provides an additional layer of security, ensuring that a machine cannot be restarted after an emergency stop without authorization from personnel holding the key.

Figure 10: The M22-PVS45P modular key release emergency stop operator. (Image source: Eaton)

Conclusion

The challenges facing electronics engineers in industrial HMI design require a solution that is more holistic than simply selecting individual components based on a single specification. A modular and field-configurable system of pilot devices, such as the Eaton M22 series, provides a comprehensive toolkit that addresses these challenges systemically.

By leveraging features such as high ingress protection, long-life LED illumination, field-convertible functionality, and a wide range of specialized operators, engineers can design HMI systems that are not only reliable and robust but also cost-effective to build and maintain. This system-level approach to component selection ultimately leads to more efficient design cycles, streamlined logistics, and safer, more durable end products.