Thomas Stevic is a controls engineer at Star Manufacturing, an engineering and production company in Cincinnati. Contact him at [email protected].
The original human-machine interface (HMI) devices—buttons, selector switches, indicator lamps, buzzers and horns—are still widely used on equipment even if the main control interface is a computer-controlled graphical device. In some applications, a graphical interface will not withstand the physical or environment abuse the equipment may be subject to. In other applications, it may be desirable to have a simple red light to indicate a fault condition or start-stop buttons that are always available to the operator.
If the operator needs to wear heavy protective gloves, a large push button may be easier to press than a touch-sensitive area of an LCD panel.
After determining that some type of button or indicator is required for an application, environmental protection for the device and the type of enclosure in which it is installed must be considered for a successful application. Industrial buttons and indicators should have either a NEMA or an IEC (IP) rating that describes the degree of protection the device provides for the enclosure in which the device is mounted. NEMA and IEC are similar but have somewhat different definitions of the degree of protection.
Also read: What the ability to design or program a simple start/stop circuit says about you
The standards are widely available on the Internet and in the manufacturer’s literature. In addition to environmental protection, NEMA and IEC require the devices pass certain minimum electrical requirements. IEC also addresses exposure to the possibility of electrically live parts with the back-of-hand or finger-safe specifications. Care must be given to the sourcing of this equipment. There are cases of imitation devices sold that are nearly identical to the genuine products but lack the production quality of the genuine products.
Three main hole sizes are widely used worldwide: 16 mm, 22 mm and 30.5 mm. North America has conventionally used 30.5 mm, Europe 22 mm, and Asia 16 mm, but with equipment being designed and built in many countries, there is no de facto standard. Larger devices may offer a greater variety of contact arrangements than smaller switches.
Larger switches also require a larger enclosure. This could provide more space to mount additional equipment such as a terminal strip or remote I/O module. Smaller devices allow a greater density in a given space. They will also require greater precision from the operator using them. Independent of training, some operators tend to repeatedly and aggressively press buttons in the belief that it will make the control system react faster. A case in point is the repeated pressing of a door close button on an elevator when someone is in a hurry.
When selecting between products with higher metal content or higher plastic content, often the metal device will offer a degree of toughness that plastic does not. However, metal is subject to corrosion and can transfer an electrical shock in the event of a short circuit or loose wire. Heavy industry such as steel foundries or mining, often subject equipment to more abuse than lighter industry such as electronic assembly or processing. Personal protective equipment (PPE) may make the actuation of a switch more difficult and require a larger and stronger switch. Emergency stop buttons should not be less robust than other controls on the machine because they need to operate in extreme situations.
Stack lights and indicators are used to give an operator a quick snapshot of the state of a machine. Green lights most often signal everything is in order; red typically indicates some type of problem. NFPA 79 lists some recommended colors for different situations, but it has been my experience that different industries—manufacturing, processing, utilities—all have their own way of indicating different states of a process. Within an industry, larger manufacturers will often have company-wide specifications.
More than one machine condition can be indicated by the same set of indicators. Making a light flash at different speeds can indicate the severity of an error. Using four different colors can indicate 16 different states if combined in binary fashion. If an exotic light combination is created, a legend can be created to explain each sequence. Indicators cannot replace the fault screen of a graphical display, but they can provide a very quick visual reference of what may be happening with a process.
The physical placement of lights and indicators warrants some consideration. Line-of-sight for every person concerned with the operation of a machine is always the ideal. Locating the indicators near the location where most operator activity occurs could be a good compromise if line-of-sight is not possible.
Buzzers and horns are used to alert an operator of supervision to some condition of a machine, typically when it is changing start from stop to run or when a fault occurs. While much of the research on human hearing is subjective, it is generally agreed that a human ear can detect a change of about 1 dB. The sound made by an audio signaling device must be louder than the ambient background noise or it will fail to do what it is intended. Choosing the correct loudness of a buzzer or horn needs to take into consideration not only the furthest distance a human will be, but also the closest.
If the audio signal is too loud and annoying, or if it occurs too often, some operators can be very creative is finding ways to muffle the sound. Measuring the ambient noise in the machine’s environment should be the first step when choosing a device. A human can hear a sound that is of a different frequency than ambient at a lower intensity than one of the same frequency. Consider an oscillating sound rather than just a louder sound.
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