Surface Haptic Technology Development
“Surface haptics” is the creation of programmable haptic effects on physical surfaces such as touch screens. Our lab has pioneered an approach to surface haptics based on controlling the shear force at each fingertip. This enables fingertips to interact with physics-based virtual environments, much like force feedback devices enable the whole hand to do. Ultrasonic vibrations and electrostatic fields are used to produce the fingertip forces.
Active Force Feedback on Fingertips
The LateralPaD is a surface haptic device that generates lateral (shear) force on a bare finger, by vibrating the touch surface simultaneously in both out-of-plane (normal) and in-plane (lateral) directions at the same ultrasonic frequency. The force that develops on the finger can be controlled by modulating the relative phase of the two resonances.
The ActivePaD is a surface haptic interface that combines a variable friction device (the Large Area TPaD) with an impedance controlled planar mechanism. This device conﬁguration allows control of the frictional force in the static friction regime, control of the direction of force in the kinetic friction regime, as well as a degree of control over the transition between the two regimes.
The ShiverPaD is a haptic surface capable of controlling shear force on a bare finger. At the heart of the ShiverPaD is the TPaD variable friction device. It modulates the friction of a glass surface by using 39 kHz out-of-plane vibrations to reduce friction. To generate shear forces, the TPaD is oscillated in-plane (i.e., “shivered”) while alternating between low and high friction within each cycle.
Electrostatic Tactile Displays
With the ever-increasing presence of touch interfaces on mobile devices, purely electronic tactile displays show promise of practical application. This research pertains to the development of a new display using the principle of electrostatic attraction of fingertip skin to a charged surface. By taking advantage of electrical properties of human fingertips, a force can be generated on the finger with the application of an electric field.
A thin transparent conductor (ITO) runs just beneath the outer surface of the glass, which is charged to attract the finger to screen. Electrostatic displays use the attraction forces to increase the friction force on a finger sliding across the surface. The focus of this work is to fully understand the electrostatic attraction of a finger to a charged surface and its effect on finger-surface interaction. This work will ultimately lead to optimization in design of future tactile displays using electrostatic attraction.
A clear understanding of both the interaction between the surface and the fingertip and of the propagation of the acoustic energy to the skin and surrounding tissues is crucial for improving the surface-haptic devices and increase forces produced by acoustic emission.
Our lab is equipped with a custom made tribometrer that is able to control the velocity and the normal force of a fingertip exploring a surface. The output of this instrument helps us understand the underpinning of the fingertip friction and its modulation under electrostatic attraction and acoustic emission. On the other hand, we use elastography techniques to measure the transmission and the propagation of acoustic energy into the tissues. Both surface waves and body waves measurement informs on the nature of the tissues and their ability to capture the energy coming provided by the interface.
The output of this research will provide key design parameters for the to the improvement of future surface-haptic displays. Refined models will help us not only to increase the force output but also to be less sensitive to individual specificity and provide the same level of stimulation for everyone.