Once you have defined HMI functionality, you are ready to investigate control technologies. Each technology has advantages and disadvantages related to the HMI system, equipment, and application.
This is the second installment of a two-part article. Part 1 can be found here.
by John J. Pannone, VP Sales, HMI Systems
Choosing the Best Control Technologies
Once you have defined HMI functionality, you are ready to investigate control technologies. Each technology has advantages and disadvantages related to the HMI system, equipment, and application.
Cursor Control (Trackball, joystick, keypad, touchpad, etc.)
The selection between different control technologies is primarily determined by the resolution of control that is required by the application. A trackball or joystick enables granular, pixel-by-pixel control, a far higher resolution than possible with a typical PC point-and-click controller.Switches (Pushbutton, rocker, slide, keylock, rotary, etc.)
Pushbutton switches allow the option of illumination to indicate open/close switch status when a quick visual indication is desired. They are also useful in machinery and machine tools, electronic production, rail and bus transportation, medical treatment and diagnostics, or other environments for easier manipulation when gloves are worn.
Rotary-switch and keylock technologies serve best when the application requires position indicators such as those used in heater or fan control. Keylocks provide an additional layer of security to the application. Rotary switches also can be used for an application requiring multiple positions.
Slide switches are the technology of choice when ease-of-use and low-cost switching is desirable – commonly found on notebook cases and handheld on/off functionality.
Short travel technologies (Conductive rubber, membrane, keyboard, keypad, etc.)
Short travel technologies have been developed for industries where ease of cleaning or disinfecting is mandatory, for example pharmaceutical, chemical, and food processing, or in a hazardous environment where a sealed system is required. Short travel technology can include cost effective, conductive rubber keys in a typical keyboard, dome keys under an overlay, or a multi-layer membrane.
Touch and switching technologies, (Capacitive, Piezo, high frequency, etc.)
Applications operating in aggressive environments such as public access or, for example, soda dispensing, where the syrupy liquid tends to get into crevices and gum up the machinery – require a rugged, completely sealed surface. Piezo, capacitive, and high frequency technologies all offer rugged switch technology with long life cycles and low maintenance costs.
Capacitive or high-frequency signals electronically activate an on/off function by changing capacitive load. Capacitive/high-frequency technologies require the use of nonconductive front panel materials which can be up to 15 mm thick, for example those operating under protective glass within hazardous environments.
Display technologies (LCD, Active Matrix, OLED, FED, Plasma, etc.)
The basic function of displays in HMI applications is to provide an information source – operators interact to obtain information or to prompt for the next screen. Display technology choices are dictated by the HMI System environment and its degree of ambient illumination, as well as by color requirements. Active matrix LCD technologies are commonly used for color functionality, while legacy LCD technology is used in applications where monochromatic feedback is sufficient. OLEDS, organic (carbon-based) light-emitting diodes can currently support smaller displays.
Interactive Displays, Touchscreen
Touchscreen technologies offer a range of functionalities and characteristics that govern HMI Systems choice according to application and environment. It is important to determine which touch technology will be used in the early stages of the design cycle as the different options offer quite unique electrical and mechanical requirements.
Capacitive touchscreen transmit 75 percent of the monitor light (compared to 50 percent by Resistive touchscreens), resulting in a clearer picture. They use only conductive input, usually a finger, in order to register a touch.
Infrared touchscreen technology projects horizontal and vertical beams of infrared light over the surface of the screen. When a finger or other object breaks those beams, the X/Y coordinates are calculated and processed. These cost-effective touchscreens can also be used by workers with gloves and are relatively impervious to damage.
Resistive touchscreen technology offers cost-effective, durable performance in environments where equipment must stand up to contaminants and liquids, such as restaurants, factories, and medical environments. When touched, the conductive coating on the screen makes electrical contact with the coating on the outer layer, the touch coordinates are registered by the controller to activate the on/off function.
Surface Acoustic Wave (SAW) touch technology sends acoustic waves across a glass surface from one transducer to another positioned on an X/Y grid. The receiving transducer detects if a wave has been disrupted by touch and identifies its coordinates for conversion to an electrical signal. SAW serves well in outdoor and harsh environments because it can be activated by a heavy stylus or gloved fingers.
Motion Control
Motion control most often employs joystick technology for applications requiring macro control, such as controlling the bucket on a payloader, a robotic arm, or directional control for a piece of materials handling equipment, or pull mechanisms.
Connecting/Communicating with an HMI System
Once you have established how your HMI will look, feel, and operate, you need to consider how the HMI will connect to and communicate with the core equipment or system under control. Typically, communication can be achieved through several approaches: hard wired connection, serial bus connection, or wireless connection.
Hard-Wired Connections
Conventional, hard wired systems are still used in many transportation and industrial legacy systems. Hard wired systems require no special tools and are simple, visible, and easy to understand, especially where the HMI interface controls a single machine.
There are many drawbacks, including difficulty integrating changes or new features – new features require new wiring. Conventional wiring also requires more space due to the number of wires and the actual size of the wires and larger connectors due to higher pin counts.Serial Bus Systems
As equipment and control systems became more complex and data hungry, transmission of data became a critical issue. To facilitate faster data transmission rates, devices incorporated serial bus connections – especially in electronics, semiconductor, machining, industrial, process and transportation. A serial bus approach eliminated data transmission slowdowns due to cable length and delivered reliable, real-time operations and work-in-process feedback.
Bus systems provide many advantages over hard wired connections, including easy addition of new functionality – typically through software – without adding or replacing hardware. Wiring is much simpler and more flexible with smaller cables and connectors allowing for more compact design, and easier hardware updating and relocation.
Field bus protocols evolved for interconnecting industrial drives, motors, actuators and controllers. Field buses include: PROFIBUS, DeviceNet, ControlNet, CAN/CANOpen, InterBus, and Foundation Field Bus.
Higher level networks connect with field bus protocols primarily across variations of Ethernet. These include: PROFINET, Ethernet/IP, Ethernet Powerlink, EtherCAT, Modbus-TCP and SERCOS III.
Wireless Connections/Communications
Industrial applications have employed wireless technologies over the last 20 or so years, primarily to take advantage of real-time data transmission, application mobility, and remote management capabilities. Interference, reliability, and security continue to present difficulties for wireless in the HMI universe.
Safety Considerations
For HMI Systems design, safety considerations are a critical part of the system. Human error is a contributing factor in most accidents in high-risk environments. Clear presentation of alarms as well as the ability to report errors, are crucial elements in any HMI.
In addition, emergency stop switches, generally referred to as E-Stops, ensure the safety of persons and machinery and provide consistent, predictable, failsafe control response. A wide range of electrical machinery must have these specialized switch controls for emergency shutdown to meet workplace safety and established international and domestic regulatory requirements.
International and U.S. Standards for HMI Systems
Key to the entire HMI System design cycle is a thorough knowledge of federal, industry, ergonomic, safety, and design standards. These include Human Engineering standards, such as MILSTD-1472F, which establishes human engineering design criteria for military systems, subsystems, equipment, and facilities; federal standards like those set by the Americans with Disabilities Act; and industry guidelines such as those from SEMI, the global semiconductor industry association, covering HMI for semiconductor manufacturing equipment.
Additional HMI specifications are furnished by ANSI, IEEE, ISO, and others. The EU provides specifications in its EU Machinery Directive for any equipment for domestic, commercial, or industrial applications that have parts actuated by a power source other than manual effort. Meeting this directive earns the equipment a CE mark.
There are also standards for public access HMI Systems, including security and cryptography standards for systems that handle payment cards; specific flammability standards and test procedures for transportation systems, and medical device and equipment standards.
Depending on the ultimate product application, observing appropriate standards assures that a product will meet industry criteria. This includes placement of components, legend size and color, emergency stop switch configuration and guards, and other ergonomic factors that improve usability, efficiency, and safety.
