  Rotational speed is one of the important metrics to be measured for
calibrating the electric motors in manufacturing, monitoring engine during car
repairing, faults detection on electrical appliance and etc. However, existing
measurement techniques either require prohibitive hardware (e.g., high-speed
camera) or are inconvenient to use in real-world application scenarios. In this
paper, we propose, EV-Tach, an event-based tachometer via efficient dynamic
vision sensing on mobile devices. EV-Tach is designed as a high-fidelity and
convenient tachometer by introducing dynamic vision sensor as a new sensing
modality to capture the high-speed rotation precisely under various real-world
scenarios. By designing a series of signal processing algorithms bespoke for
dynamic vision sensing on mobile devices, EV-Tach is able to extract the
rotational speed accurately from the event stream produced by dynamic vision
sensing on rotary targets. According to our extensive evaluations, the Relative
Mean Absolute Error (RMAE) of EV-Tach is as low as 0.03% which is comparable to
the state-of-the-art laser tachometer under fixed measurement mode. Moreover,
EV-Tach is robust to subtle movement of user's hand, therefore, can be used as
a handheld device, where the laser tachometer fails to produce reasonable
results.
