Enhancing your touch screen design with haptic tactile feedback - Embedded.com

Enhancing your touch screen design with haptic tactile feedback


The popularity of touch screens as a user input device has grownquickly. Some of the demand, as in the Apple iPhone, reflect thecompetitive market advantage of a superior user interface. For otherapplications, safety concerns, distraction issues and other usabilityproblems need to be overcome.

Customers from all market segments – industrial, commercial, as wellas consumer – continue to demand better human-computer interfaces. Oneof the latest advances for the touch-activated interface is tactilefeedback, or haptic technology, which can provide immediate,unmistakable confirmation to the user.

The feature has been found to improve user performance andsatisfaction. Also, product designers may be able to minimize featurecomplexity by providing intuitive tactile cues. This article describeshow to add tactile feedback to a product's touch-activated interface.

How it works
Haptic-enabled touch interface systems rely on actuators to producetactile sensations. Advancements in actuator and control technologieshave produced actuators capable of supporting tactile feedback in touchpanels and touch screens ranging from the very small to the very large,and for products from mobile phones to widescreen touch monitors.

Moreover, the processor load to support a haptic system today istrivial, touch-input technologies are virtual commodities, andelectromechanical solutions are available off the shelf.

Figure1: Displacement vs. time for an A300 actuator.

Figure2: Acceleration vs. time for an A300 actuator.

A common initial assumption about haptic technology fortouch-activated controls is that, to be effective, the full motion of abutton or switch has to be replicated. However, the human finger is notthat discriminating.

Thousands of hours of research have revealed that neurons in thehuman finger detect very small amounts of motion – if combined withmoderate acceleration. Just 0.1mm of motion, combined with anacceleration of at least 1.5g, supplies a sensation that can beperceived as a confirming response.

The low threshold of 1.5g of acceleration, however, does notnecessarily produce the best tactile feedback. A more effective hapticeffect is created by generating an acceleration and displacementprofile that induces a stronger stimulus.

These acceleration and displacement profiles (Figures 1 and 2 above) can bevisualized through “phase portraits” such as shown in Figure 3 below . Phase portraits canbe generated after successful electromechanical integration of haptictechnology into a touch interface device.

Figure3: Acceleration and displacement profiles can be visualized throughphase portraits.

The mechanics
Haptic feedback system architecture typically includes one or moreactuators—either DC electromagnetic or larger customdesigneddevices—that must be correctly mounted to the touchscreen; hapticcontrol software which may either be installed on a control board orembedded in the product's main microprocessor; a haptic effectslibrary; and a programming interface for calling haptic effects fromthe host application. Incorrect implementation of any of these elementsoften leads to a failed design.

Actuation . Using an actuatorspecifically designed to produce haptic effects is the ideal approachbecause re-purposing a generic motor or solenoid can be challenging.

Actuators designed for a haptic application convert the hapticsignal received from the controller into mechanical motion described bya specific phase portrait. Apart from providing a good dynamicresponse, the chosen actuators need to meet exacting specifications forpower, efficiency and reliability.

Two types of actuators commonly used in mobile phones are alsoexcellent for small touch surface implementations (up to about 7 inchesdiagonal). These are eccentric rotating mass motors and linear resonantactuators, in which a mass vibrates between two magnets. Largerproducts with touch interfaces from 7 inches to 36 inches requirelarger actuators (Figure 4, below ).

Figure4: Larger products, those with touch interfaces from 7-inches up to36-inches require larger actuators. The Immersion A100 (top) and A300are two examples.

All actuators should be optimized for generating high forces withsmall displacements, and thoughtfully selected to meet performance andlife requirements. The type and number of actuators required by thedesign depend on the size, weight and implementation of the touchscreenor panel.

Poor implementation usually occurs by misusing simple solenoids ormotors in order to generate a haptic effect. Latency, or slow actuatoracceleration, excessive displacement, or a lack of precise control overdisplacement due to unsuitable actuator performance characteristics,are common problems.

Poor actuator mounting can also be a common problem with hapticsystems. Poor mounting can cause the entire system to resonate, ratherthan the touch interface alone. In handheld devices, this may bedesired.

But in fixed devices, overly strong resonance produces an effectmore like an earthquake than a friendly confirmation. The other extremecan occur as well, where a mounting configuration compresses theinterface to such an extent that the acceleration and displacement aredamped beyond the point of detection.

Correct mounting of the actuators enables displacement to beeffectively transferred to the user's fingertip. Touchscreen displaysare mounted to a bezel, and this connection is then flexibly sealed.

Haptic actuators (Figure 5, below )can provide the main attachment mechanism between the display and thebezel, allowing the touch surface to “float,” which allows the maximumamount of energy to be transferred to the human finger.

Figure5: Positioning and mounting actuators correctly allows motion to beeffectively transferred to the user's fingertip.

Control system.. Proper control of the actuators is achieved through software andelectronics that process the touch input and provide appropriateinstructions to the actuators. Control instructions should be optimizedfor the relevant actuator technology and for a desired phase portrait,one that allows suitable confirmation of a user's input while notinterfering with the primary focus.

Providing this response requires selecting a processor with anappropriate drive output, spare processing capacity of 0.25MIPS andusing a suitable amplifier for the target actuator. The entirecommunication path (from user input to the start of a haptic response)should be less than 30ms.

Haptic effectslibrary. The tactile effects library should include a widevariety of effects so that the feel of various touchscreen controls canbe clearly distinguished and functions differentiated. It is best ifthe haptic development system gives user interface designers aconvenient method for experiencing effects, so they can choose the besteffect from the haptic library.

Programminginterface. Software integration can be helped by a streamlinedAPI to call the tactile effects from the host application. A softwaredevelopment kit may also be useful. One such kit from Immersion givesdesigners several programming options including a Windows ActiveXcontrol, a cross platform API in source code form, and communicationssupport for custom interfaces. Sample code and a full description ofthe process of adding tactile feedback to the host application are alsoincluded.

Today's new haptic interfaces can provide a more familiar, engaging,satisfying user experience for touch-activated controls. Fortunately,enabling a haptics system is now technologically simple and theelectromechanical integration is well understood.

The key parameters needed to respond to this burgeoning designrequirement are actuation integration, mounting, haptic control andprogramming. When implemented following the guidelines presented here,a haptic system can make a clear difference in how a touch-activatedcontrol feels – and how intuitive, satisfying, and natural userinteractions can be.

Steve Kingsley-Jones is Directorof Product Management at Immersion Corp.

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