May, 2019
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Proper Selection of HMI Components... Continued from previous page
ing. Minor periodic arcing with silver contacts is actually beneficial because it keeps contacts free from dirt and corrosion. Another challenge in switch design is contact
“bounce” or “chatter,” a condition in which a con- tact rebounds for several milliseconds before it finally closes. This is not a concern for power cir- cuits but causes problems in logic circuits that may interpret on-off bounces as data streams.
Mechanical Solutions How contacts make and break can minimize
arcing, welding, and bounce. Switch designers minimize bounce by reducing the kinetic energy of the contact, incorporating buffer springs, air, or oil shock absorbers to dampen contact recoil, or by using wiping or sliding type contacts. Bifurcated, fork-shaped contacts minimize con- tact bounce, while providing redun- dancy and higher
reliability.
Commercial switches generally limit contact bounce duration from less than 1 to approximately 100 millisec- onds.
The force holding movable and
stationary contacts together is criti- cal, 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 con- tacts 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 contam- ination to keep contact resistance low. On the other hand, slow make/break switching elements, usu- ally used in emergency stop switches and in high-power applications, employ rigid contact arms that force necessary contact separation to over- come contact welding. Many switches today are modu-
lar, comprised of quickly assembled components including actuators, switching elements,
illumination
blocks, lenses, marking plates, and mounting systems.
Electrical Ratings Electronic switches are rated for
current, voltage, voltage type (AC, DC), and load type (inductive, resis- tive). They are categorized into three power levels. Level 1, “Signal Level,” typically falls in a range up to 42V and 100 mA. Level 2, “General Purpose,” switches range from 42 to 250V and up to 5A. Level 3, “Power,” switches typically exceed 250V and 5A. Level 1 is for low-power, logic-
level applications while levels 2 and 3 are for resistive and inductive power requirements. Continuous current capabilities range from up to 100 mA to 10A or more over the three levels. Switches are supplied with appropri- ate contacts for their current-carry- ing capabilities. Contacts in modular switches are housed in a switching element that includes connection ter- minals and an attachment port for the actuator. The switching element also has
contact terminals for connecting sys- tem electrical wiring. Several differ- ent terminal methods are used and many are offered as options when specifying a given switch. The most popular connectors include choices in screw, soldering, quick-connect, plug- in, push-in, or PCB styles.
Actuation Preferences The actuator assembly, including
the front lens touch surface, is the part of a switch that directly interacts with a user. Its form is closely related to its
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function. Pushbuttons, keylocks, and lever/selector switch actuators provide one or a combination of switching actions. With a momentary-action switch,
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an activation force moves contacts to a new position. When the activation force is removed, the actuator and contacts return to the original position. With a maintained or alternate action switch,
contacts move to a new position and remain there until the switch is activated a second time, which returns actuator and contacts to the original posi- tion. For safety applications, E-Stops are a type of maintained-action switch that requires a pull, twist, or key to reset to the original position. Touch-sensitive switches are totally electron-
ic and use capacitive, high frequency, or piezo tech- nology rather than mechanical actuators to sense when they have been touched and initiate a response. Switches come in ultra-miniature, sub-minia- ture, miniature, and standard sizes for various
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