Eccentric Rotating Mass (ERM) Motor

The Eccentric Rotating Mass (ERM) motor is the most common type of vibration motor commonly found in a wide variety of personal, insudrial and medical devices. It is comprised of a rotating DC motor with an offset mass attached to the shaft. As the motor rotates, the centripetal force of the offset mass is asymetric, creating a net centrifugal force, causing a displacement of the motor. When rotating at high speeds, the rapid displacements are perceived as vibration.

Generally small and inexpensive, ERM motors are commonly used in cell phones, mobile devices, and an ever increasing range of electronics devices as a source of haptic feedback.

Types of Vibration Motors

Image1) Info
ERM/“Pager” Motor Most common type of ERM motor; typically used in mobile phones and personal notification devices
Coin (Pancake) Motor Most popular type of ERM used in musical applications; small size and flat form factor make it easy to mount in small devices
Encapsulated Vibration Motor img10-erm-encapsulated.jpg Sealed, dust and water resistant; for use in harsh and/or wet environments
Enclosed Vibration Motor img11-erm-enclosed.jpg No external moving parts; larger than pager-style ERMs; commonly used for industrial alerting and vibration-aided sorting
Brushless Vibration Motor img12-erm-brushless.jpg Brushless motors cause less mechanical wear and afford longer lige space than other ERMs; available in cylinder and coin form factors
Linear Resonant Actuator* not actually an ERM, but commonly classified alongside coin-style ERMs as they are similar in physical form and vibrational output;

How ERMs Work

  • An ERM motor is a standard DC motor with an off-center load attached to the shaft.
  • During rotation, the unbalanced mass creates centripetal force. When the motor is attached to an object, the force acts on the object and causes it to displace.
  • The motors rotate at over 100x per second, which is felt as vibration.

Driven Harmonic Vibration

ERM vibration motors generate forced vibration, otherwise known as “Driven Harmonic Vibration”. This means that an external force (excitation) causes the system to vibrate. While DC voltage is applied to spin the motor, this technically isn't the source of the excitation. Rather, the unbalanced mass rotating around the central shaft is the excitation introduces energy into the system and creates the vibration.

The amount of vibration created is a result of the speed of motor rotation, weight of the rotating mass, and size, weight and stiffness of the system to which it is attached. These combined factors can be modeled and analyzed as system having one degree of freedom (DOF). This one DOF model contains a mass connected to a spring with damping factor, and driven by a sinusoidal input. The ability to model such a system allows for the designer to calculate the optimal size, voltage and weight ERM motor(s) to produce the desired vibrational effect for a new system.

In practice, this combination of factors in a system leads to resonant points (nodes) in the system as the motor increases rotational speed.

ERM Rotation Position as a sine wave Figure 1: ERM Rotation Position as a sine wave

ERM model with one DOF Figure 2: ERM model with one DOF

Vibration Frequency and Vibration Amplitude

  • The force is dependent on the size of the mass, the distance between the mass center of gravity and the motor shaft and the speed of the motor.
  • BUT: The total vibration amplitude also depends on the size of the object the motor is attached to.
  • The vibration amplitude is a measurement of acceleration. Instead of using the SI unit (m/s2), it is expressed in G - the acceleration caused by the earth's gravitational pull.

ERM Motor Power Ratings

ERM motors are usually quoted using the “Typical Normalised Amplitude” rating.

This is the level of vibration that the motor will produced when the motor is powered at its rated voltage (typically around 3.0V for a haptic feedback application) and attached to a 100g mass.

This allows for easy comparison of motor strengths and quick estimation of vibration strength needed in a design.

Varying vibration amplitude

  • The easiest way to vary the amplitude is to change the voltage applied to the motor.
  • This changes the speed of the motor and therefore the centripetal force.
  • However this also causes the vibration frequency to change, as it too directly relates to the motor speed

For ERM vibration motors you cannot vary the amplitude and frequency independently.

Active Braking

* A basic circuit to power an ERM motor will drive it in a single direction based on the polarity of the circuit. * When power is cut the motor rotation will slow to a stop as the centripetal force dissipates. Depending on the size and amplitude range, the typical stop time can be from 65 to 150 ms. * An active braking circuit can be implemented so that the polarity can be momentarily reversed to create a brake for the momentum of the eccentric mass. This can effectively cut the typical stop time in half.

**H-bridge drive circuit designed for active braking. (www.precisionmicrodrives.com)

Common Vibration Motors

Pager Motors

  • Most common ERM motors
  • Used in cell phones and other devices to provide vibration alerts and haptic feedback.
  • Range from 4 - 7mm diameter
  • Designed for equipment with weight range of 25 ~ 200g.
  • Common form factors include PCB Mount Motors and Encapsulated ERM Motors.

Coin (Pancake) Motors

  • Popular and very versatile because they have no moving parts and are very small.
  • Most common type used in musical applications.
  • Typically 8, 10, or 12mm diameters
  • Amplitude ranges from 0.5 G to 2.6 G
  • Most popular form factor includes long life adhesive backing for easy installation.

Linear Resonant Actuators (LRAs)

  • Longer life span as they don’t have internal brushes susceptible to wear.
  • Consume lower power than ERM equivalents.
  • Compact size and adhesive backing for easy installation.
  • The input signal’s amplitude and frequency are independent of each other, allowing the input to have a more complex waveform than with an ERM.

Pros and Cons of different types

Pager Motors Coin Motors LRAs
Pros Inexpensive and easily sourced Inexpensive and easily sourced Ability to separate frequency from amplitude
Available in encapsulated and enclosed housings Compact & flat form factor Strongest vibration rating for power consumption
Best response time (ramp up/ramp down) Adhesive backing makes for easy installation Compact & flat form factor
Stronger perceived vibration than coin type Can employ active braking Adhesive backing makes for easy installation
Can employ active braking
Cons Need to mount to project securely Longer response time than pager type More expensive than ERMs
not as compact as coin type Not as strong vibration as pager type No active braking
Fixed resonant frequency

Sources

References

1) Images courtesy Precision Microdrives
actuators/eccentric_rotating_mass_erm_motor.txt · Last modified: 2016/04/28 15:34 by sullivanj
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