Your phone vibrates using a tiny, unbalanced metal weight that spins at high speeds.

The technology behind phone vibrations primarily relies on two types of hardware: Eccentric Rotating Mass (ERM) motors and Linear Resonant Actuators (LRA). The ERM motor, which was the standard for decades, uses a DC motor to rotate a semi-circular weight. Because the weight is asymmetrical, it creates a displacement of the center of mass, resulting in a centrifugal force that we perceive as a vibration.The first mobile phone to feature a vibration alert was the Motorola StarTAC, released in 1996. Before this, pagers were the primary devices using haptic alerts to notify users silently. These early motors were relatively simple, but modern smartphones require much more sophisticated haptic feedback to simulate the tactile click of a button or the scroll of a wheel.Modern devices, like the iPhone since the 6s model, use a Taptic Engine which is a form of Linear Resonant Actuator. Instead of spinning a weight, an LRA moves a magnetic mass back and forth on a spring, driven by voice coils similar to how a speaker works. This allows for much faster start and stop times, typically under 50 milliseconds, compared to the 100-200 milliseconds required for a traditional ERM motor to spin up.Research from the Haptics Group at the University of Pennsylvania highlights that these precise vibrations are essential for user interface design. By varying the frequency and amplitude of the movement, engineers can create 'haptic icons' that convey different meanings. This technology has evolved from simple alerts to complex systems that can mimic the physical sensation of different textures and materials on a flat glass screen.