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HMI system design – more than just a touch

02 February 2017

Designing intuitive and reliable HMI Systems requires in-depth specialist knowledge about display technology, electromechanical design, ergonomics and relevant industry standards.

Proper HMI design requires the ideal combination of the application, environment, technology and perfect ergonomics. This article provides a gateway for better understanding the fundamentals of designing an innovative, intuitive and reliable HMI system.

HMI systems take many forms. From a simple two-button instrument panel to an extensive software/hardware-based SCADA system at the heart of factory automation. It is critical to understand and appreciate the importance of an HMI as the link between user and machine regardless of their size or scale. 

Operational requirements

These are at the core of every new design project and can include the user environment, the operating temperature and factors of influence such as humidity, dust or vibration. 

There are many operational needs, all of which should be considered by the designer when making material and functional choices.

Selection of control elements

Displays can be a core element of modern HMIs. Fast, easy-to-read graphical information provides the ability to control multiple system functions from a single point or location. 

Common displays include:

LCD and LED non-touch displays: LCD displays remain competitive for simple numeric displays. LED screens work well in bright conditions and are preferred for alphanumeric and graphic displays

Touch displays: Touch screen type plays a critical role in interface functionality. Operator and environmental characteristics affect the advantages and disadvantages of a touch screen.

Resistive touch screens: Generally used in industrial applications, they can be activated with a variety of objects including a finger or stylus. They are relatively inexpensive and resistant to moisture, dust, oil and cleaning agents, but vulnerable to damage from sharp objects.

Capacitive technology: The second-most frequently used touch screen in industrial applications. This technology has enormous popularity in the consumer segment due to its robust glass surface making it resilient to scratches and chemicals. 

Surface Acoustic Wave (SAW)/Projected Capacitive Technology (PCT): SAW ultrasonic waves are transmitted across the screen and picked up by sensors which change when the screen is touched. PCT screens operate within an electrostatic field and are commonly used in consumer mobile devices. They support multi-touch input allowing for the use of two fingers to pinch, expand, tap or rotate a screen. Both SAW and PCT technologies offer improved image clarity, as well as high resistance to surface contaminants, liquids, scratches and impacts. 

Challenges of touch displays

False triggering is one of the biggest safety and productivity concerns of HMI systems. 

Unwanted reactions can be triggered by moisture, electrical interference or oversensitivity. For these and other reasons, touch displays are not suitable for all applications. Tactile control elements are essential when an operator does not have an HMI in his direct field of vision, such as critical control actions often found within lifting and moving applications. 

Mixed technology solutions

Combining touch screens with tactile elements makes it possible to overcome these disadvantages. Mixed technology solutions provide benefits such as intuitive operation, discrete pushbuttons, illumination and softkeys.

With help from a mixed technology solutions expert such as EAO, you can generate real added value during each step of your HMI building process, from the initial idea through the manufacturing stage.

Electromechanical devices

Electromechanical devices such as pushbuttons, emergency stops, selector switches, potentiometers, keylock switches and optical and acoustic signaling devices remain an important part of most control systems. They will continue to be used in the future due to their intuitive use, high degree of safety, robustness and reliability. 

When selecting electromechanical devices it’s important to consider the following attributes:

Modern construction: Modern actuators are easy to install, provide flexible communications options, consume less power and deliver a long service life of up to 10 million switching cycles.

Materials: Stainless steel and special-purpose plastics are crucial for providing HMIs with chemical and UV radiation resistance.

Modularity and configurability: These devices should be modular and configurable to suit a variety of specific requirements.

Mounting: Modern mounting systems feature a tight seal into which the actuator holder can be pushed or “snapped” in with only one hand and are critically important to providing secure attachment. 

Environmental conditions: Reliability of HMI functions must not be affected by dust, water, oils, caustic solutions and extreme environmental conditions. IP69k front protection is usually selected for harsh environments such as food and beverage applications that require high-pressure/temperature wash-downs. 

Short travel technologies: These are cost-effective, conductive rubber keys in a typical key-board, dome keys under an overlay, or a multi-layer membrane. They offer HMI design flexibility and efficient haptic feedback. 

Motion control devices: Trackballs, touchpads and joysticks should be considered when precise, intuitive movement and control is required. 

Safety and security devices

It is also important to consider all available safety mechanisms when selecting an HMI system:

Emergency stop switches: Emergency stop switches are regulated by strict safety regulations and must be actuated via a mechanical action. The actuated state must be clearly visible, and the release must be performed manually. At this time, guidelines do not allow for a touch-based “certified” emergency stop system  

Keylock switches: Key-operated switches prevent accidental or unauthorised access. Each key can provide authentication of employee groups or individuals and can even record operator access and time. 

Communication and interfaces

HMI systems were traditionally connected using hard-wired point-to-point connections. However, modern pushbuttons and signaling devices are becoming increasingly connected via serial bus communication cables to control systems. Interfaces such as Profibus and IO-link are common interfaces providing pluggable communication links to reduce initial wiring time and make expansion/fault detection easier. 

Standards and regulations 

Standards are immensely important within HMI system design as both general and industry-specific requirements dictate features, functional attributes and design elements. Even the placement of components, markings and component colors can be influenced by regulations. 

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