A haptic effect begins as a request from software, something like "play a click," and ends as a voltage across a coil or a piece of ceramic. The waveform in between decides most of what the user feels. Two products built around the same actuator can feel completely different, one crisp and one mushy, because of how each one drives it.
What a haptic driver does
A haptic driver is the power stage between the processor and the actuator. At its simplest it's an H-bridge that can push current in either direction, which an ERM needs for braking and an LRA needs for its alternating drive signal. Dedicated haptic driver chips add more on top, including a library or buffer of stored effects the processor can trigger with one command, calibration routines that characterize the actuator at startup, and closed-loop control that watches the actuator while it plays. Piezo drivers also include a boost converter to generate the high voltage the ceramic needs.
A dedicated chip isn't mandatory. A microcontroller with a PWM output and a small motor driver can run an ERM or LRA open loop, and many low-cost products work that way. The difference shows up as inconsistency, because open-loop drive can't correct for an LRA whose resonance has drifted or an ERM that spins up slowly when the battery voltage sags.
Overdrive and active braking
Much of what makes a click feel crisp comes from what the driver does at the start and end of each effect. A spring-mass actuator takes several cycles to build up to full amplitude at its rated voltage, so drivers apply a brief overdrive pulse at a higher voltage to get the mass moving and then drop to the steady level. At the end of the effect, the driver reverses the phase of the drive so it pushes against the moving mass and stops it within a cycle or two. ERMs get the same treatment with a short reverse-voltage pulse that halts the spinning weight.
Without braking, an LRA keeps oscillating after the drive ends, and the decaying tail turns a short click into a soft buzz. With well-tuned overdrive and braking, the same part can produce a click only a few cycles long. The timing depends on knowing the actuator's resonance and damping, which is why calibration matters, and overly aggressive braking can overshoot and create a rebound that feels like a faint second tap.
Resonance tracking and closed-loop control
An LRA's resonant frequency varies between units and shifts with temperature, wear, and how tightly the part is mounted. Drive it even slightly off resonance and output drops, sometimes sharply, because the response peak is narrow. Closed-loop drivers handle this by sensing back-EMF, the voltage the moving magnet induces in the coil, and using its zero crossings to lock the drive frequency to the actuator's real resonance from cycle to cycle.
The same back-EMF signal tells the driver how fast the mass is moving. That lets the driver brake at the right moment and hold amplitude steady as the battery drains or the device warms up. Some drivers apply a similar idea to ERMs, regulating motor speed so the buzz stays consistent across supply voltages.
Building effects from primitives
Above the driver sits the effect design layer. Most systems describe an effect as an amplitude envelope applied to a carrier signal, with a short, steep envelope for a click, a longer one for a buzz, and a rising ramp for something that feels like it's building. Haptic driver chips often ship with a library of preset effects, and phone platforms expose their own vocabularies to app developers.
Apple's Core Haptics framework describes events as either transient or continuous, each with intensity and sharpness parameters. Android's VibrationEffect API offers predefined effects such as click and tick, and on supported hardware it lets apps compose primitives like a click, a thud, or a quick rise into longer patterns. How much of that vocabulary a device can render depends on its actuator. A narrowband LRA can vary amplitude and duration well but has little room to change frequency, so effects that differ mainly in sharpness may feel alike on it, while a wideband actuator gives the effect designer more range.
Common drive problems and their causes
A buzzy tail after a click usually points to missing or mistuned braking. Output that's strong on some units and weak on others often means open-loop drive at a fixed frequency that doesn't match each part's resonance. Audible noise can come from square-wave drive, whose harmonics land in the audible range, from loose parts rattling inside the enclosure, or from an enclosure resonance the actuator happens to excite.
Effects that feel weaker during exercise or in a moving car aren't always a drive problem, since ambient motion masks subtle output. Several of these issues only show up on a test rig, and the guide to measuring haptic quality covers how to catch them. The actuator comparison explains how each actuator family responds to drive, and the article on nonlinear actuators describes a mechanism that avoids ringing by design.