ERM Motor
An eccentric rotating mass motor creates tactile vibration by spinning an unbalanced weight. Vibration strength and frequency depend on the motor's speed, the mass and offset, mounting and the assembly being shaken.
The rotating imbalance produces a periodically changing force. One rotation gives one vibration cycle, so frequency in hertz equals speed in revolutions per minute divided by 60. ERM designs include brushed and brushless motors. Changing drive conditions changes speed and therefore both frequency and vibration strength; the PWM switching frequency is not the same as the mechanical rotation frequency.
In plain terms
Like an unbalanced washing-machine load, the rotating weight keeps pulling in a changing direction and shakes the surrounding structure.
Why designers use it
- Providing a tactile alert with a compact rotating actuator.
- Creating vibration patterns through a suitable driver.
- Choosing from coin, cylindrical and other mounting formats.
Best for
- Toys
- Cheap fitness bands
- Notification alerts
Key specifications
- Drive: Motor type, supply range, starting voltage and current
- Vibration frequency: Mechanical RPM divided by 60
- Vibration strength: Check the stated mounting and normalization conditions
- Dynamics and mechanics: Startup/braking behaviour, dimensions and retention
When not to use it
- When its acceleration and braking response cannot produce the required haptic effect.
- When the required frequency and amplitude must be controlled independently beyond what the selected ERM permits.
Common mistakes
- Confusing PWM switching frequency with the motor's mechanical vibration frequency.
- Driving motor current directly from an unsuitable GPIO.
- Ignoring minimum starting voltage and changes as the battery discharges.
- Assuming the same motor feels identical in differently sized or mounted enclosures.
- Choosing a drive circuit without identifying whether the motor is brushed or brushless.
Where you will find it
- A notification device can use an ERM and suitable switching driver to produce a repeated tactile alert.
- Precision Microdrives gives a 5,400 RPM motor example: dividing by 60 gives a 90 Hz mechanical vibration frequency. The felt effect still depends on the attached assembly.
A short history
ERM actuators use rotating imbalance to create vibration. Brushed, brushless and different mechanical formats allow different drive and durability trade-offs. A linear resonant actuator instead moves a mass back and forth near resonance; the two types should not be treated as interchangeable drive loads.
Good to know
- Increasing an ERM's speed changes the pitch of its mechanical vibration as well as its strength.
- An actuator's own rating is not automatically the acceleration felt by the whole product.