  Vibro-tactile feedback is, by far the most common haptic interface in
wearable or touchable devices. This feedback can be amplified by controlling
the wave propagation characteristics in devices, by utilizing phenomena such as
structural resonance. However, much of the work in vibro-tactile haptics has
focused on amplifying local displacements in a structure by increasing local
compliance. In this paper, we show that engineering the resonance mode shape of
a structure with embedded localized mass amplifies the displacements without
compromising on the stiffness or resonance frequency. The resulting structure,
i.e., a tuned mass amplifier, produces higher tactile forces (7.7 times)
compared to its counterpart without a mass, while maintaining a low frequency.
We optimize the proposed design using a combination of a neural network and
sensitivity analysis, and validate the results with experiments on 3-D printed
structures. We also study the performance of the device on contact with a soft
material, to evaluate the interaction with skin. Potential avenues for future
work are also presented, including small form factor wearable haptic devices
and remote haptics.
