Eccentric rotating mass motors start and stop slowly, so short pulses are hard

What it is

An eccentric rotating mass (ERM) motor produces vibration by spinning an off-center weight. Because inertia drives it, acceleration and deceleration take time: tens of milliseconds to ramp into a perceptible buzz, and a decaying tail after the stop command. Those two intervals are what make crisp short pulses hard.

Why it happens

Amplitude tracks rotation speed, which cannot change instantly given inertia and motor torque. A "click" needs a very short rise, hold, and fall, but ERM output is a smoothed envelope with both ends stretched. The consequence is that two nearby pulses blur into one and long versus short gaps become hard to tell apart. The motor also offers essentially one controllable dimension, amplitude, because frequency and speed are coupled and cannot be adjusted independently.

Studying it

Measure the physical response rather than trusting impressions: log acceleration from command to threshold amplitude, and the residual ring-down after stopping, to derive the minimum usable pulse width and minimum gap. At the behavioral level, use two-point and rhythm discrimination tasks to see whether users can separate counts and order at given gaps. Report the motor model and drive scheme, since drive voltage changes response time markedly.

Where it stops holding

The disadvantage concentrates where fine temporal structure is required—keypress feedback, rhythmic game haptics, or separating short taps from long buzzes. For "something happened" cues that only need presence, slow start and stop are not a problem and the sustained buzz is easier to notice. Low-end devices still ship ERM widely, so the judgment must follow the specific hardware rather than a blanket dismissal.

Applying it

• Confirm whether the target hardware is ERM before designing patterns that rely on tens-of-milliseconds distinctions.
• Set a minimum inter-pulse gap for ERM so adjacent pulses cannot merge physically.
• Encode meaning through duration and repetition count rather than crispness of onset and offset.
• Verification: record an acceleration curve on the target device, measure the smallest discriminable gap and shortest perceptible duration, and revise the haptic pattern table accordingly.

Related

• Within the group: D3.01.2 Linear resonant actuators start and stop fast for a crisp tap · D3.01.3 Motor type bounds the achievable haptic vocabulary
• Adjacent: D3.02.1 Too short a duration falls below the perceptibility threshold · D3.14.2 A haptic design may not reproduce at all on low-end devices
• Search terms: ERM actuator · eccentric rotating mass · haptic latency