Pushbuttons, Switches Still Compete in Controls
These days, the term “operator interface” conjures up thoughts of multiple touchscreens, color graphics, an air-conditioned control room, and a sprawling processing plant. Software-enabled control systems represent the latest in HMI technology, and, although they are becoming more common as new plants are built and older systems are updated, they are still far from dominant in factory automation and control.
Conventional control panels—the simple original electrical operator interface design (pushbuttons, switches, and pilot lights)— still link the operator to the process in many, if not most, facilities. Judging by the responses generated in a survey conducted in December 2004 by Control Engineering and Reed Research Group, both part of Reed Business Information (www.reedbusiness.com), progress in pushbutton and switch technology is still on the minds of the control engineers polled.
The e-mail/Web survey queried subscribers about their preferences in pushbuttons and switches, including technological preferences and purchasing patterns. Results were based on 201 qualified responses of readers involved in specifying, recommending, or buying these products.
A device for everyone
Because pushbuttons and switches are used in a wide variety of applications, they have evolved into a device for everyone. They come in many colors, materials, sizes, and power ratings; they can be illuminated or not, withstand harsh environmental and RFI/EMI conditions, and provide “front of panel” safety with Class I, Div. 1 and 2 and intrinsically safe ratings. Will the ability to handle all these conditions stave off the encroachment of touchscreens and other operator interface panels into areas once the domain of pushbuttons? It remains to be seen.
In the survey, 65% of respondents reported replacing pushbuttons with touch panels and other operator interface (OI) devices; 9% planned to do so in the next 12 months. The rest (25%) reported that they have no plans to exchange pushbuttons or switches with other types of OI devices. Does this portend the end of pushbuttons in control circuitry as we know it?
“The answer is no, certainly not completely,” according to Mike Shulim, chief technical officer for DST Controls (www.dstcontrols.com), a system integrator specializing in drug manufacture, safety critical systems, and power substation monitoring and control. “Manual applications with only a few inputs and outputs will still employ pushbuttons, switches, and pilot lights for the foreseeable future. But a real question for pushbutton manufacturers might be how long will single input and output devices have mechanical (versus solid-state) internal workings?”
Shulim notes that pushbutton’s installed base is eroding. “Even now, if the control system requires more than 5 or 6 inputs or indicating devices, system designers could spec touchscreens in most, if not all, cases. Industrial touchscreens have become sufficiently inexpensive, tough, and small,” he says.
Deciding on the type of input devices involves several factors. “Among these are customer preferences, codes and regulations, ergonomics, available panel space, specialized field requirements (nasty environments call for larger, more robust devices, scalability requirements, maintenance considerations), and installed cost,” Shulim adds.
Frank Graninger, principal engineer for Rockwell Automation, also sees replacement of mechanical internal workings of Allen-Bradley products with solid-state technologies having direct isolation. “With pushbuttons, mechanical isolation is required for many, possibly most applications. Some type of voice recognition might be a solution, but would probably not replace the standard pushbutton. It would be in addition to it,” says Graninger. “Any replacement would need to match the cost and performance characteristics of the standard pushbutton, which is very hard to do. A switching contact that inherently provides direct isolation will be around for a long time.”
The end is not likely near. “There have been many alternate solutions to standard pushbuttons over the past 30-some years I have been involved, but these all have complemented the standard pushbutton and not replaced it,” Graninger adds.
For those still specifying pushbuttons and switches for automation projects, the survey provides insight into the reasons for picking one brand over another. The top five factors considered most important by survey respondents were: quality (84%), availability (68%), ease of installation (51%), termination type (40%), and choice of illumination type and/or color (40%). These choices mirror results of the previous survey done in 2001.
Considering their “front line” status, these devices continue to stand up to repeated use and the punishment in industrial environments. And considering the sheer numbers used, it would be difficult for any control engineer to disregard replacability or installation costs. See the accompanying diagram for other decision factors survey respondents deemed important.
When asked about the status of the selection criteria over five years, Graninger says, “Quality of product and ease of installation are the critical components of supplying the lowest installed cost pushbuttons for the user, a factor that is generally the main concern. All things equal, cost is the driving force. Installed cost takes into account simplicity of installation as well as installation reliability and performance.”
Wish list revisited
Respondents to the 2005 survey were asked to predict which pushbutton/switch functional attributes would be “very important” to them in the next three years. Of the attributes cited, network-ready (28%) was most desirable. On-board intelligence (21%) placed second, and wireless capability (15%) third. Also rated were cage-clamp terminal (12%), increased use of plastic construction (9%), fitted with Hall-effect sensors (9%), and fingerless operation (8%).
Compared to the top three in 2001, onboard intelligence saw “wish list” fortunes. Wireless capability at the pushbutton/switch level, on the other hand, seems to have caught the interest of the control engineering community. Considering the inroads made by industrial wireless communications in general, it seems logical that adapting this technology to local control circuit design would have advantages.
Tim Reilly, senior product manager at StacoSwitch, concurred, but with some environmental reservations. “Our primary application in the control engineering arena has been in high-reliability applications, such as nuclear power plants, subway/railway controllers, water treatment plants, etc. Due to the cost of our lighted pushbutton switches and the nature of the items controlled by the switches, wireless applications have not been considered by our traditional customers. They could not allow any potential disruption of the switch interface due to wireless signal transmission problems. Also, many of our switches are sold with EMI/RFI shielding, so making a switch of this type emit a signal is counterintuitive.”
Reilly continues, “Low-cost, molded elastomer switch assemblies that complete a circuit on a printed circuit board [and] are used to interface with less critical equipment, such as CNC machines or metering pumps, have limited potential. One area [in which] StacoSwitch sees huge growth is wireless keyboards. It is very possible that control rooms would want a similar capability.”
Richard Chung, global product manager, industrial pilot devices, at Eaton’s Cutler Hammer division, also sees wireless expansion. “Popularity of wireless pushbuttons seems to be growing based on the practicality, feasibility, and increased safety redundancy of their installations and applications. In environments where the cost of installing wire and conduit are high, such as in hazardous locations, the use of wireless pushbutton control stations is a practical and effective way of providing remote control for motor starters. As wireless technology becomes more ubiquitous, the cost of transmitters and receivers is expected to decline so that wireless installations in normal applications become more,” Chung says.
Interest in wireless switch interfaces seems to revolve around convenience and flexibility. Bluetooth, IR, and RF wireless options allow designers to pick the best technology for an application. Less costly, lower-power-consumption components have made wireless products more feasible. Some markets cannot apply them universally, but for others they have great intrinsic value. “As with any technology, correct application is key,” Reilly adds.
Pushbutton and switch products
For more information on pushbuttons and switches, visit the Web sites of the manufacturers of the products below.
Allen-Bradley 800F brand pushbuttons are designed and manufactured to meet demanding performance specifications. They feature easy one-person mounting. Lower front-of-panel profile enhances equipment appearance. Available in a wide variety of colors, styles, and sizes, 800F pushbuttons meet most commercial-grade and industrial application requirements. Advanced and dependable sealing techniques increase protection and corrosion resistance for IP65/66 and Type 4/4X/13 environments. These devices are UL listed, CSA rated, CE marked, and third-party approved and comply to KEMA and RoHS Directives. www.ab.com
Failsafe E-stop operation
E22CB1M self-monitoring-circuit contact block is said to improve reliability of the conventional e-stop circuit. Under normal circumstances, actuating the E-stop opens the normally closed contact, which interrupts the control circuit, and shuts off the machine. If the normally closed contact were separated from the E-stop actuator, the circuit would remain uninterrupted, the E-stop actuator would not open the normally closed contact, and the machine-safeguarding circuit would be lost, resulting in a hazardous situation. E22CB1M device has an additional circuit wired in series with the normally closed contact. This normally open circuit monitors the contact block’s attachment to the E-stop operator. When attached, the normally open self-monitoring-circuit contact is closed and is in series with the normally closed emergency stop circuit. If the contact block were separated from the E-stop operator, this circuit would open and shut off the machine. www.eaton.com
Voltage-dimmable LED pushbuttons
Series 90 products include voltage-dimmed white light emitting diodes (LEDs) for the light source, replacing incandescent bulbs. A chip handles dimming, which allows LED units to replace incandescent units in a voltage-dimmed system with no additional hardware, the company says. Such LEDs eliminate complex circuitry usually associated with voltage-dimmed devices and provide reduced energy use, lower heating, and brighter legends. www.stacoswitch.com
Durable, cost-effective single-switch application
Peel-and-Place PushGate Island offers cost savings for applications that use individual pushbutton switches under a hard keycap or plunger. In PushGate Island, switch element modules can be placed individually on a flex circuit or printed circuit board without added wiring and soldering associated with alternative single-key switches, which offers a cost-effective alternative to snap-action or mechanical switches. Construction consists of adhesive on the bottom side of a standard Duraswitch Island, which bonds it to the circuitry. A Tedlar seal on the top protects the switch from contaminants, such as dust and moisture. Peel-and-Place Islands can be used with various actuators including plastic, metal, and elastomer. www.duraswitch.com
Quickly installed, flexible command/signaling devices
Sirius distributed operator stations feature integrated AS-interface connections, reduced installation time, and enhanced modification flexibility. They are shipped with pre-mounted command and signaling devices and AS-interface technology to speed commissioning and reduce change-out times. An AS-interface slave easily snaps into a station’s base and connects to various switching elements and lamp sockets via conductors. Up to 62 slaves can be connected on one master. Plastic and heavy-duty cast metal housings are available. E-stop slaves comply with new NFPA 79-2002 directives for networked safety. www.sea.siemens.com
Siemens Energy & Automation
Explosion-proof operator line features LED pilot
Line of explosion-proof operators now feature an ATEX-certified pilot light with LED lamps. The new LED pilot light completes line of NEMA 7 and 9 pilot devices, which include pushbuttons, selector switches, and potentiometers. All products sold or placed into service in any European Union Member State must be ATEX-certified. ATEX-certified flameproof pilot devices are UL and CSA approved and meet the ATEX Directive for European applications when installed in an ATEX-certified explosion-proof enclosure. These devices are intended for use in control circuits functioning in environments containing hazardous gases, vapors, fibers, dusts, and in weather-tight NEMA 4 applications. www.adalet.com
Adalet, a Scott Fetzer Co.
‘No-tools required’ pushbuttons
A22 family of pushbuttons offers 22-mm illuminated and non-illuminated pushbuttons, knob type selector switches, key selector switches, E-stops, and pilot lights. A22 design reduces panel depth by providing three-row switch units accepting up to six contact circuits (normally closed contacts are approved for safety circuits). Contact blocks mount side by side and are not stackable. They feature two-piece, snap-together construction with a locking lever for “no-tools-required” installation and simplified terminal layout for more efficient wiring. They are IP65 rated and are UL approved and CE marked. A complete offering of nameplates, custom engraving, and pushbutton enclosures is also available.
Omron Electronics LLC
Pushbutton line extended
Pushbuttons include several new non-illuminated and illuminated 22-mm and 30-mm models. The 22-mm plastic models include several choices of push-on and push-off configurations; metal pushbutton models with LED indicating lights are available in extended operator or mushroom operator designs, 24 V dc/V ac or 120 V dc/V ac. The 22-mm metal line offers potentiometers and audible buzzers with LED indication. Larger 30-mm models feature NEMA-rated metal housings and include flush momentary operators and mushroom push-pull operator configurations. Several in each line include a guarded housing for protection from accidental activation or obstruction. www.automationdirect.com
High-quality, low-cost pushbuttons, switches
Cam-operated rotary switches are designed for ON-OFF, changeover, multi-step, ammeter, voltmeter, and motor reversing control and load break applications. They have up to 10 poles and 6 positions, are rated up to 32 amp, and are easy to install in standard 22.5-mm mounting holes. All are certified for UL and CSA standards and are CE marked. They are rated Type 1, 3, 3R, 12, and IP65, and complement industrial controls in 30-mm, 22-mm, 16-mm and 13-mm sizes: pushbuttons, pilot lights, selector switches, potentiometers, cable operators, special operators, and accessories. www.c3controls.com
Key-operated safety interlock switches
GKN Series interlock switches provide features industrial equipment manufacturers require for safety position detection of protective guards or doors in a reliable, cost-effective, industry- standard package. They have positive opening safety contacts and double insulation. Five contact configurations, including two with three contacts, are available. Configurations with three contacts provide cost-effective safety monitoring when used with a safety module. GKN Series also features a rotating head that allows actuator engagement from the front, back, or either of two top positions, and choice of three standard key actuators, including a flexible key actuator for minor misalignment. www.honeywell.com
Honeywell Sensing and Control
Frost-resistant pushbutton switches
IF Series momentary pushbutton switches are designed to withstand frost and icing conditions and meet ergonomic and contemporary cosmetic requirements. IF models feature a 1.180 in. diameter black or red panel mounting bezel, with flexible rubber-like actuator that provides 500,000 cycles of life at full load at -20 to 85micals, pneumatic delivery, outdoor equipment operation, and control panels. www.apem.com
APEM Components Inc.