  Surface vibration tactile feedback is capable of conveying various semantic
information to humans via the handheld electronic devices, like smartphone,
touch panel,and game controller. However, covering the whole device contacting
surface with dense actuator arrangement can affect its normal use, how to
produce desired vibration patterns at any contact point with only several
sparse actuators deployed on the handled device surface remains a significant
challenge. In this work, we develop a tactile feedback board with only five
actuators in the size of a smartphone, and achieve the precise vibration
pattern production that can focus at any desired position all over the board.
Specifically, we investigate the vibration characteristics of single passive
coil actuator, and construct its vibration pattern model at any position on the
feedback board surface. Optimal phase and amplitude modulation, found with the
simulated annealing algorithm, is employed with five actuators in a sparse
array. And all actuators' vibration patterns are superimposed linearly to
synthetically generate different onboard vibration energy distribution for
tactile sensing. Experiments demonstrated that for point-wise vibration pattern
production on our tactile board achieved an average level of about 0.9 in the
Structural Similarity Index Measure (SSIM) evaluation, when compared to the
ideal single-point-focused target vibration pattern. The sparse actuator array
can be easily embedded into usual handheld electronic devices, which shows a
good significant implication for enriching their haptic interaction
functionalities.
