Appliance Design - February 2009 - (Page 14)

CONTROLS & SENSORS total applied force rather than calculating where the force was being applied. Force-based touch screens are conceptually attractive. Because they do not rely on surface mounted films, they can be robust, comparatively easy to manufacture, and attractively priced. Typical configurations use four force sensors placed in each corner of a rectangular touch surface. The sensors are combined with a processing unit, software, and other offthe-shelf components. There are no complex manufacturing issues to overcome and product yields are consistent. So why is this seemingly simple touch screen technology not widely implemented? As it turns out, accurately recognizing and measuring a touch or stylus tap with a force-based system is much more difficult than it would appear. In order to ensure that the correct coordinate is calculated, each sensor needs to report a change in force of tenths of grams or less. This level of accuracy places very tough requirements on the sensitivity and performance of the force sensor, as well as the other parts of the touch screen system. The difficulty of detecting such a small Fig. 2. Illustration shows how force sensing can be combined with an LCD screen. force on an object that is many orders of magnitude greater in weight is compounded by the need for isolation from external forces. The entire system must not be impacted by friction or energy loss from bending. Furthermore, the software must be able to filter out unwanted sensor noise from changes in forces and weight distribution caused by environmental impact, such as temperature, movement, or vibrations. As F-Origin continued its research, it became clear that the mechanical construct itself prevented the system from taking advantage of the rather precise output from the piezoresistive force sensors used. The touch lens must be rigid, the sensors placed between the rigid lens and a rigid structure or back plane, and the touch lens movement must be fixed in the xy-plane while allowing a close to frictionless movement (few tenths of 1 mm) in the z-plane. Depending on the exact mechanical suspension and sensor placement, the mechanical system can help to reduce the need for vibration dampen14 applianceDESIGN February 2009 AD11084Ark.indd 1 10/10/08 11:41:00 AM ing in the software. With a new mechanical approach to manage the suspension of the touch surface over the force sensors, the system was able to take advantage of the precise sensor input. When developing a forcebased touch screen, there are several sensor types to choose from: strain-gage, based on conductive material, or semiconductor-based sensors such as a force-sensing resistor, transducer, or load cell. F-Origin has found that a piezoresistive silicon strain gage can deliver the optimal combination of performance, cost, and physical size for its customers’ applications. As noted previously, forcesensing technology provides a unique combination of benefits over other touch technologies. The core system components are simply the four force sensors, signal amplification, and a processor or controller with analogto-digital signal conversion. Not only can the overall system cost be kept very low, manufacturing and assembly is a straightforward and scalable process. www.applianceDESIGN.com http://www.ark-plas.com http://www.ark-plas.com http://www.appliancedesign.com

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Appliance Design - February 2009 Contents Editorial Shipments/Forecasts News Watch Force-Sensing Technology Offers Designers an Alternative Approach to Creating Touch-Control Interfaces A Flammable Vapor Sensing Technology that was Originally Developed for use in Water Heaters has Evolved for use in a Broader Array of Applications Latches, Hinges, and Slides Must Reliably Secure Parts Together, Sometimes for Hundreds of Thousands of Cycles. Beyond their Utilitarian Functions, They Can Also Change Consumer Perceptions Elastomeric Electrical Connectors Can Solve Space Constraint Problems in Electronic Products and Provide Some Cost Advantages as Well A Multi-Layer Ceramic Capacitor with a Novel Electrode Design Can Suppress Electromagnetic Interference, Improve Circuit Performance, and Reduce the Number of Needed Components Design Marts Association Report: AHRI Advertisers’ Index

Appliance Design - February 2009

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