How Proper Component Selection is Crucial to Enhancing the Human Machine Interface
Designers today face an astonishing range of choices in electromechanical components.
Electromechanical devices, including switches, keypads, keyboards, and other elements such as indicators and alarms, are critical aspects of the human machine interface (HMI) for controlling equipment and systems. HMI component technology has adapted over the years to serve the increasingly specialized needs of industrial, transportation, telecommunications, audio/visual, public access/security, and lifting/moving applications.
Designers today face a truly astonishing range of choices in electromechanical components that encompass not only the type of device, electrical specifications, environmental sealing, and mounting and termination styles, but also ergonomic considerations such as configuration, size, illumination, and tactile feel.
Technology of Switch Design
Through an actuating mechanism, a switch makes or breaks an electrical connection or diverts current from one conductor to another. A basic switch that makes and breaks a single circuit has one pole. The number of poles a switch has represents the number of separate circuits that can be active through the switch.
Switches are available with normally open (NO) contacts, normally closed (NC) contacts, or a combination of both. When a NO contact is activated the contact closes; when a NC contact is activated the contact opens.
Making Better Contacts
Much early work leading to modern switch designs was concerned with perfecting durable contacts to reliably make and break the electrical connection. Chief requirements included corrosion and abrasive wear resistance, electrical conductivity, mechanical strength, affordability, and low toxicity.
Gold and silver contacts are used in most switches due their varying options suitable for numerous power levels and application requirements.
However, three major challenges presented themselves to designers as they searched for a more perfect switch: arcing, welding and bounce:
Arcing and Welding
Arcing is discharge of electricity—a spark—that can occur when contacts make or break. Welding is when contact material melts and fuses, causing contacts to stick. Useful life is reduced during both as contacts are degraded or burned.
As switched current increases, hotter arcs form and the potential for erosion and contact welding is greater, ultimately affecting design. For example, solid-gold contacts are limited to low-current switching as they are more easily melted and eroded by arcing.
Arcing is also more severe when handling high inrush currents. For example, lamps may draw 10 to 12 times their normal operating current when first activated. Switches for higher currents use gold- plated silver or all silver contacts that resist the effects of arcing.
“Bounce” or “chatter,” a condition in which a contact rebounds for several milliseconds before it finally closes. This is not a concern for power circuits but causes problems in logic circuits that may interpret on-off bounces as data streams.
Solving Design Challenges
How contacts make and break can minimize arcing, welding, and bounce.
Bounce is minimized by reducing the kinetic energy of the contact, incorporating buffer springs, air, or oil shock absorbers to dampen contact recoil, or employing wiping or sliding type contacts. Bifurcated, fork-shaped contacts minimize contact bounce while providing redundancy and higher reliability.
The force holding movable and stationary contacts together is critical, ranging from 5 gm in miniature switches to more than 150 gm in heavy-duty, motor-load switches. Snap-action switching elements reduce arcing by rapidly moving contacts from one spring- loaded position to another independent of actuator speed. Self-cleaning contacts slide against each other when making or breaking a circuit, removing contamination to keep contact resistance low. On the other hand, slow make/break switching elements, usually used in emergency-stop switches and in high-power applications, employ rigid contact arms that force necessary contact separation to overcome contact welding.
Making the Right Choice
Switches come in a wide variety of shapes, sizes, ratings, and functions offering significant customization flexibility. High-quality switches are expected to have a mechanical life of 1 million to 10 million operations, minimizing the need for replacements.
Designers can simplify their search for the perfect switch by carefully analyzing their application requirements to determine the following:
Electronic switches are rated for current, voltage, voltage type (AC, DC), and load type (inductive, resistive) and categorized into three power levels. Level 1 is for low-power, logic-level applications while Levels 2-3 are for resistive and inductive power requirements. Continuous current capabilities range up to 100 mA to 10A or more over the three levels. Switches are supplied with appropriate contacts for their current-carrying capabilities. Contacts in modular switches are housed in a switching element that includes connection terminals and an attachment port for the actuator are used, and many are offered as options when specifying a given switch.
The actuator assembly, including the front lens touch surface, is the part of a switch that directly interacts with a user.
Momentary-action switches - An activation force (pushing a pushbutton or twisting a keylock) moves contacts to a new position, until the force is removed and the actuator and contacts return to the original position.
Maintained or alternate-action switches - Contacts move to a new position and remain there until the switch is activated a second time, which returns the actuator and contacts to the original position.
Safety applications (E-Stops) - A type of maintained-action switch that requires a pull, twist, or key to reset.
Tactile feel is a subtle, yet crucial, feature which provides an indication—sensed by human touch—of the point just prior to activation. For example, a light touch is often desirable in large audio/video control consoles, while a heavy-duty industrial E-Stop should require a determined effort to avoid inadvertent shutdown. Touch-sensitive switches are totally electronic and use capacitive, high frequency, or Piezo technology rather than mechanical actuators to initiate a response when they have been touched. This same approach is used in touch-sensitive keyboards and keypads designed for use in harsh environments or where vandal protection is desired.
Physical Configuration and Mounting Needs
Switches come in ultra-miniature, sub-miniature, miniature, and standard sizes for various printed circuit board (PCB) and panel-mount applications. Generally, as switch size decreases, so does the electrical rating. Some panel-mount switches are designed to project above the panel surface, others for flush mounting or even sub-panel mounting. Where space behind a panel is limited, several switch styles are available with small behind-panel depth. PCB switches typically come in two mounting types: straight pin through-board mounting and surface mount devices (SMDs).
Clean, indoor environments not subject to rigorous cleaning, such as recording studios and telecommunications control rooms, generally don’t need protected switchgear. If water, fuel, cleaning solutions, fine dust, and other materials may come in contact with control panels, such as in industrial or transportation environments, it is important to select switch controls that have appropriate environmental sealing dictated by International Ingress Protection (IP) codes.
Illumination, color and graphics
Illuminated switches use incandescent, neon, or light emitting diode (LED) light sources that visually show the state of a switch. Adopting LEDs has been a significant change in switch design as they offer high reliability and energy efficiency, low-temperature operation, and better control over color and brightness compared to incandescent and neon bulbs. Although slightly more expensive, LEDs can last ten times longer and consume half the power of their previous counterparts. They also excel in applications requiring frequent on-off cycling that may cause incandescent bulbs to fail faster.
Additionally, LEDs are perfectly suited to new switch styling trends such as halo lighting and multicolor effects. Switches combining both and using multiple lenses or RGB LEDs can provide even more information by using different colors to indicate specific switch states (i.e., green = on, red = stopped, etc.). Truly intuitive designs can be achieved by synthesizing or blending colors utilizing PWM (pulse-width modulation) techniques which give the ability to create any color in the spectrum.
Legends or graphics that identify a switch or its function are placed on the lens or marking plate behind the lens using engraving, hot stamping, pad printing, and other techniques. Contrasting colors of inks can be used to highlight these legends. Other marking techniques can be used to comply with ADA, such as Braille dots.
The Importance of Component Selection
Today’s advanced HMI components are precisely crafted devices, made to exact design specifications with very close tolerances using high-grade plastics, metals and meticulously calibrated springs. To achieve reliable, long service lives, they are engineered like fine watches with the performance, feel, and look required in modern HMI systems.
Ergonomics play a key role in modern switch design, assuring the right switch for each application—whether it is a flush-mount design to avoid inadvertent actuation, or an emergency-stop switch with a mushroom actuator for fast palm-slap shutdown and safe twist or key release. When designing an HMI system for demanding applications, design engineers must carefully select the appropriate HMI components to ensure the safety, longevity, and ergonomic appeal of their equipment.