Haptic Actuators: How LRA and ERM Stack Up With Piezo Actuators

Three haptic actuator types have been used in haptic applications to provide tactile feedback: eccentric rotating mass actuators (ERM), linear resonant actuators (LRA), and piezoelectric actuators. ERMs were famous once due to their low cost. 

But LRA replaced them by offering improved performance in terms of better acceleration. 

Thanks to ever-evolving haptic technology, a piezo actuator can replace both the LRA and the ERM. It can surpass haptic actuators in acceleration, quick frequency response, and crisper haptic feedback. Let’s explore how this technology works and what else it can do to improve your users’ experience.


Haptic Actuators: LRA vs. ERM vs. Piezo

erm vs lra vs piezo direction

ERM uses a vibration motor that rotates an off-centered mass at a high frequency to produce motion across two axes. But they come with mechanical limitations. 

The mass rotation relies on vibration, which takes more time to accelerate the mass and reach peak frequency. It has an optimal peak time of 50 ms and 0.6 G of average acceleration. ERM also produces unrefined rumble during acceleration.

LRA also uses a vibration motor with a magnetic field to spin or move a mass up and down. The mass is attached to a spring and pressed against a voice coil. Under the influence of AC voltage, the coil moves the mass to create a motion across a single axis. 

LRAs offer higher acceleration than ERM and crisper haptic feedback due to their resonant frequency. The resonant frequency is the frequency range at which the LRA is at its most efficient, producing maximum vibration to amplify the acceleration of the actuator mass. 

LRAs can reach their optimal frequency peak in 25 ms and consumes less power than ERM during acceleration. 

In contrast, a piezoelectric haptic actuator works on the principle of the piezoelectric effect by converting an electrical signal into mechanical displacement. Unlike LRA and ERM, it has no mass to move. The actuator will squeeze or expand when you apply voltage, generating motion and force to produce high frequency and quick response time.

How to select the right piezo driver and actuator combination


The Limitations of Linear Resonant Actuators and Eccentric Rotating Mass Actuators

There are certain limitations to LRA and ERM in terms of haptic waveforms range, power consumption, and response time, including the following:


ERM Devices Have Slower Haptic Response Times

ERM devices use rotating motors to move a mass. Due to the inertia of the mass, an ERM takes time to accelerate and decelerate and reach peak frequency. 

The peak frequency is the optimal zone for feedback, and the motor speed (frequency) determines the output strength (amplitude).

Therefore, if the motor’s speed is less, the response time is also slow, making it difficult for the ERM device to produce more complex haptic waveforms.


LRA Actuators Have Only One Resonant Frequency and Lack Bandwidth

The mass spring in an LRA is a constant resonant system. Hence, LRA requires a precise resonant frequency closer to the spring constant to achieve good acceleration. Different tactile feedbacks require a large bandwidth. 

Due to a narrow resonant frequency, LRA has a limited bandwidth, making it challenging to produce various haptic events. It can only operate over 170 Hz to 180 Hz range.


Both LRA and ERM Actuators Require a Higher Power Consumption

ERM and LRA haptic actuators require high operating voltage to rotate the motor and move the mass. In addition, the more time it will take in acceleration to reach peak frequency and then decelerate, the more power the motor will consume. 

Also, the large size of ERM and LRA causes heat dissipation, another reason behind higher power consumption.


Why Piezoelectric Actuators Have the Advantage

3 persons running after someone on a bike

A piezoelectric haptics actuator provides an improved haptic experience. Unlike LRA and ERM, which have limited frequency ranges, it has a broad operating frequency range from 0 Hz to 500 Hz. 

Coupled with a haptic driver and depending on the application, the piezo actuator can perform with custom waveforms to produce different tactile effects. Thus, you can customize the amplitude and frequency of displacement to create complex and detailed signals. 

A haptic piezo actuator has no motor to move mass and, as a result, does not take time in acceleration and deceleration. This feature allows it to reach its optimal peak in 1.5 ms and produce high-definition haptic feedback. 

Furthermore, the undesirable vibration of an ERM during acceleration and deceleration delays does not plague a piezo actuator. Because of the instantaneous reaction time of a piezo actuator, it does not create undesirable vibration.

ERM and LRA consume 20 times more power than a piezo actuator. But the small size of a piezo actuator results in a smaller footprint. 

A piezo actuator will reduce heat dissipation and consume even less power when coupled with a piezo driver. A smaller size also means it will take less space in an application, allowing a slimmer design. 


Are You Ready To Transform Your Piezo Haptic Devices?

ERM and LRA have various restrictions that do not allow achieving the performance levels necessary for immediate haptic feedback. Piezo haptic actuators open up multiple possibilities for advanced haptic effects in many haptic devices. 

If you are ready to transform your piezo haptic devices, the most convenient method will be to adopt a complete piezoelectric solution. 

A complete piezo haptic solution needs a piezo actuator and piezo driver to boost voltage. The high power consumption once associated with a piezo haptic solution is not a problem now. 

Boréas Technologies offers improved haptic sensation with its haptic driver integrated circuit that combines force sensing with improved haptic feedback. A start-up time of fewer than 300 µs makes these piezo drivers ideal for producing high-definition haptic effects. 

Built-in force sensing in the driver helps create feedback and force sensing from the actuator. Due to the patented CapDrive technology, the piezo drivers consume four to ten times less power than the competitor piezo drivers. 

The power efficiency also means less heat dissipation, making the piezo drivers from Boréas perfect for small applications such as mobile devices. Moreover, they offer high energy recovery and substantial throughput to drive piezo actuators. 

Integrate piezo drivers into your haptic devices today to unlock their full haptic potential. 


How to select the right piezo driver and actuator combination

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