A sense of haptic touch

People appreciate the subtlety of touch when interacting with everyday devices because it so often removes the need to pay close visual attention to how the fingers or hands move. Users don’t have to keep looking at a physical keyboard or keypad to know a key has been pressed. Conversely, it is easy to miss or unintentionally press the wrong key on the unyielding virtual keyboard, with such mistypings often being translated into perfectly spelled gobbledegook, sometimes with amusing or embarrassing results.

Smooth glass is also an unnatural surface for us to use creatively; the slight friction of paper and canvas is often easier to deal with when writing or drawing with a pen or with a finger. Who hasn’t tried to sign for a package on a handheld terminal proffered by a delivery worker only to have the stylus skid haplessly across the smooth surface of the glass? A better solution would be to translate the tangibility of real-world surfaces into the domain of virtual interfaces, and technologies are appearing that will help do that.

1st gen haptics

Physical feedback was a (small) part of the mobile phone experience even in the early days. Realising that users did not want to disturb others during meetings but still be aware of incoming phone calls and text messages, manufacturers provided the ability for the device to vibrate more or less silently - unless it was sitting on a table - instead of playing a tune through the speaker. An off-centre mass on a small electric motor, known as an ERM is often used to provide the vibration for this primitive form of haptics.

Although the vibration alarm is useful for alerting users to a call, the range of sensations it can produce are limited as it has a strong resonant frequency and vibrates the whole device. All that can be done is to modulate the duty cycle to create signal variance, but it is difficult to disguise this as more than just a spinning weight on a motor.

Other applications demonstrate the value even of these simple haptic interfaces. Vibration has been used in fly-by-wire aircraft for some time to simulate the resistance of traditional controls and the effects of buffeting as the aircraft approaches the dangerous condition of a stall. Road vehicles are acquiring similar features. Lane departure warning systems watch the road to see if the vehicle is moving out of its lane and the driver seems to have lost concentration. A device similar to the stick shaker of an aircraft vibrates the wheel or the seat to simulate the effect of running over a road’s rumble strip.

Haptic evolution

For similar reasons, it is possible to incorporate 1st gen haptics into games. For example, in a racing game, if the player collides with another vehicle or runs off the road, vibrating the handheld device adds another dimension of realism to the game.

But vibrating the entire device enclosure has limitations; not least in terms of power consumption, which is high due to the energy required to shake the mass of the entire device. And, while it is ok for a smartphone, this has practical limitations when it comes to a household appliance. Shaking the microwave to tell you that you have pressed the timer buttons does not make sense.

2nd gen haptics aimed to separate the part of the device being touched from the part that is not. On a mobile phone, the display can be ‘floating’ from the body of the device with a flexible mounting with one or more actuators employed to move just the display. This reduced the energy demands and, in turn increased the sensitivity of the signals that could be imparted, enabling crisper, better- defined key press sensations.

These 2nd gen systems were still limited in the sensitivity of tactile information imparted to the user and still transmitted to an entire panel. Further, it could not cope in a multi-touch environment, where the need is to accentuate the sensation in one location on the panel and diminish it elsewhere. A good example comes from the PC keyboard: for the CTRL/ALT/DEL reset, the keys are pressed sequentially so the sensation needs to be felt only by the finger making the relevant key stroke.

This is where 3rd gen haptic interfaces take over. Techniques including the use of multiple actuators enable localised haptic sensation to be delivered to a specific point of contact. When combined with the more detailed sensations permitted by 2nd gen haptics, the prospects of creating a powerful perception of surface features, textures and responses can add greatly to the user experience in mobile phones and tablets. This is especially true for touch-oriented operating systems, such as Windows 8, and the compelling tactile response from touch panels and displays will only increase.

Haptic perception uses the forces that can be generated by actuators to create the feeling of a surface that is different to the actual physical surface. Haptic perception also relies on the fact that many touch sensations are not just due to the object being touched but the way that limbs and digits move as they press down. For example, a soft rubbery surface will feel different to that of a glass surface because the finger meets a lower force when initially pressing down. The force increases until there is no more compliance in the surface.

Competing techniques

Several techniques are used to deliver tactile sensations to electronic products. The first category, Resonant Mass, includes the ERM described above, linear resonant actuators and some piezoelectric devices can also be enabled with a mass to resonate. Generally these can all be considered as masses on springs and have a strong resonant frequency. Techniques of pulsing or modulating this resonance allow a small range of sensations to be created that are discernible from one another.

The alternative 3rd gen haptic technology uses moving coil actuators to deliver haptic feedback responses under far lower voltages. Termed Bending Waves, it is well known as a means of turning flat panels, such as display cover glass, into loudspeakers and are equally effective for haptics. They exploit low-frequency natural resonances in the touch panel or display itself to provide high-fidelity haptic responses including surface textures and key press feelings, localising the tactile feedback at specific points on the touchscreen’s surface while diminishing it at other points to create a true multi-touch/mutli-feel environment. Piezoelectric actuators, which are made from materials that flex in response to the application of an electrical voltage, can also be used to provide force between the display and its mountings in bending wave systems, as described above, or even distributed across a conformal or flexible surface to create a localised response. Electro-active polymers and other materials also allow vibrations to be created in devices for elementary forms of haptics. As well as improving the usability of mobile phones, user interfaces that integrate bending wave haptics can increase the safety of drivers and industrial-system operators who need to change controls while keeping their attention on the road ahead or the systems they manage.

The interplay between body motion and the surface is key to the use of advanced touch interfaces such as those emerging with 3rd gen haptics. With the actuation technology in place it is possible to go further with high-resolution haptic controllers and build pressure-sensitive interfaces - seemingly compliant surfaces that respond to the way in which the user presses down on the surface.

A good example here is the light switch; when we reach out and press it, we are programmed to expect three things: we feel the switch change state; we hear the click as it does so; and we see light come on. If any one of these fail, we interpret it as a broken system. Moreover, this sensory fusion allows us to discern quality - the difference between a cheap switch and an expensive one, or an old car from a new one. Feel is important and must be integrated into electronic devices to bring out the best perception of quality, purpose and productivity.

By generating a force as the user’s finger approaches a spot, it is possible to simulate this surface compliance and so make it appear to the user that the surface is softer than it physically appears. If the force suddenly gives way as the finger pushes down, it is possible to simulate the action of a physical button being pressed and so create the illusion of touching that object. This rapid-response rendering in relation to force or pressure applied is key to the delivery of advanced, compelling haptics.

This kind of response is helpful in creative applications such as art and photographic manipulation where higher pressure is often used to increase the width of a brush, or to alert the user to hyperlinks so that a firmer press invokes the link to a web page. This also means accidental and false touches can be eliminated.

Force detection can be implemented in several ways, from simple force sensing resistors to printable electroactive inks and computational methods derived from current-gen touch detection technologies. This shift to 4th gen haptic interfaces means the range of applications that can benefit from touch will be greatly expanded, making flat panel user interfaces - which are far more robust - even more natural.