Appliance Design - August 2007 - (Page 18)

SWITCHES & RELAYS the resonance from the cavity will decay, or ring down, at a predictable rate, depending upon its material and geometry. Touching the outside surface of a resonant cavity has a damping effect that accelerates the rate at which the resonance decays, just as someone touching a struck bell damps the vibrations and hastens the end of the ringing sound. The damping occurs when the surface is touched by a finger, gloved finger, or any acousticabsorbing object. The time-to-decay rates for both states of the resonant cavity — touched and untouched — are stored within a microcontroller, which can then determine whether the cavity is in a touched or untouched state by comparing the measured decay rate with the decay-rate profiles stored in memory. In a novel application of the piezoelectric effect, the resonance from a cavity is detected by the same piezoelectric transducer that initially generated the ultrasonic energy. The transducer, which is bonded to a switch surface, serves two roles, enabled by the two-way characteristics of piezoelectric materials. Using the indirect piezoelectric effect, the transducer converts electrical pulses into ultrasonic pulses when energized. When de-energized, the transducer acts as a sensor, using the direct piezoelectric effect to convert the reflected ultrasonic vibrations into electrical pulses. In other words, the piezo element “rings the bell,” then pauses to listen to it, determining whether anyone has touched it. This cycle of ringing and listening occurs nearly every microsecond. The microcontroller manages the whole communication and can be easily multiplexed to monitor multiple switch positions. It can ring many such bells and listen to them simultaneously. This constant cycling makes it an active system, hence its name, ActiveTouch. Though operating in an “always on” mode, the system’s power draw is minimal, with 25 microamps being typical of most designs. Contoured regions on the switch plate create resonant cavities that trap the ultrasonic energy. Micro managing Despite the use of acoustic resonance as a principle, the performance of the sensors is not affected by external sources of vibration. This is partly due to the high frequency used by the system, but due in larger part to the microcontroller’s program that recognizes only those specific waveforms pertinent to the system’s design. Management by microcontroller offers a variety of advanced feature options that can be specified by the design engineer. Switch activation can be designated as normally open (NO), normally closed (NC), normally high (NH), normally low (NL), momentary, latched (maintained), or proportional. Outputs can be serial, USB, RS232, CAN bus, in series or in parallel, or any number of customer-specified outputs. The design engineer can specify the sensitivity threshold from near zero to much higher levels, and can select the length of damping time needed to output a signal. The microcontroller can also be programmed to self-calibrate the system to adapt to changing conditions such as temperature swings or deformations in surface material. Because it is an active system, it can learn to identify a new waveform as the nominal, untouched state, and still distinguish it from the waveform of the touched state. It can also apply that learning capability to self-diagnostic functions. For example, upon identifying a changed nominal state, a new baseline, the system can be programmed to communicate an error signal, generate an alarm, call for service, or simply deactivate the abnormal switch area and transfer its function to a backup location. When necessary, these features permit faster troubleshooting and repair to reduce or eliminate system downtime. Defining distinctions Because an ActiveTouch switch employs the piezoelectric effect, both direct and indirect, it is important to distinguish its differences from a standard, commonly used piezoelectric switch, which is also a solid-state component. A piezo switch uses the direct piezoelectric effect, where a mechanical force exerted upon the piezo element causes it to generate an electrical pulse. This signal is then read by a controller that interprets the signal as a touch. The piezo switch can also be placed behind a front panel and still register the touch. The behind-the-panel approach provides design advantages such as sleek styling and protection of the elements. Piezo switches, however, have some limitations compared to ActiveTouch switches. The elements in piezo switches require a small amount of mechanical deflection, which creates two issues involving material thickness and spacing. Typically, the thickness of the material over the piezo element must be 0.030www.applianceDESIGN.com When the ultrasonic transducer operating at 1 MHz vibrates the resonator, a twisting motion is induced. It is known as “trapped torsional mode” because the motion is confined to the shape of the cylinder. 18 applianceDESIGN August 2007 http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - August 2007

Contents Editorial Shipments/Forecasts News Watch A Reed Switch is Simople, yet Versatile, and Designers are Finding New Uses for this Time-Tested Technology A Novel Actuation and Sensing Technology can Register a Light Touch, but Resist Blows from a Hammer Tracing the Source of Noise is Made Easier with Noise Source Identification Methods Appliance Noise can be Annoying, but Designers have Many Material Choices to Help Rein in the Racket Nanocomposites are No Longer a Technology for the Future. These Plastic Materials are Already on the Job A New High-Strength, Low-Density Glass-Microsphere Additive can Improve Plastic Part and Molding Performance A New Electromagnetic Weld/Bonding Process Helps Ensure Long-Term Integrity of Joints in Plastic Parts Snap-Based Technology Eases Installation and Makes Products More Serviceable Two-Part Silicone Adhesives Provide a Rapid-Cure Alternative to Traditional One-Part Silicones Classified DesignMart Advertiser’s Index Association Report: GAMA

Appliance Design - August 2007

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