A linear resonant actuator (LRA) creates vibration by moving a spring-suspended magnetic mass with a voice coil. It was the industry's answer to the weaknesses of eccentric rotating mass (ERM) motors, and for years it was the most efficient haptic technology available. This article is part two of our three-part Haptic Technologies Showdown. It compares LRA against a piezo actuator driven by Boréas' CapDrive® architecture on the five factors that decide a design: acceleration, response time, power consumption, frequency bandwidth and integrated force sensing.
Key Takeaways
- LRA improved on ERM in every respect: higher acceleration, faster response, crisper feedback and lower power. It held the title of most efficient haptic technology for years.
- An LRA has to accelerate a suspended mass, so it needs about 25 ms to reach its optimal frequency and a similar period to decelerate. An ERM needs around 50 ms. A piezo actuator starts playing an effect in 0.3 ms.
- An LRA is efficient only in a narrow band around its resonant frequency, which limits the range of effects it can render. A piezo actuator covers the full haptic bandwidth.
- In mobile devices an LRA reaches 1 to 1.7 G. A similar-sized piezo actuator reaches 2.5 to 5 G.
- An LRA consumes 4 to 10 times more power than a piezo actuator driven by a CapDrive® IC.
- CapDrive® senses force and creates feedback from the same piezo actuator, so a button design can drop its separate force-sensing hardware.
Haptic technologies are evolving. We saw how eccentric rotating mass (ERM) motors compare to piezo haptics in the first part of this series. Vibration motors are fine if you need a low-cost solution, but they are limited in feedback quality and produce a low-end rumble instead of clear, crisp tactile effects. High-end device manufacturers turned their attention to another haptic technology as a result.
Linear resonant actuators (LRA) are now used in many devices to create better haptic feedback than an ERM can achieve. They offer higher acceleration, faster response time and crisper feedback. LRAs are based on mass movement, just like ERM motors. The difference is that the mass is suspended on springs and moved by a magnetic field. Form factors vary, with some LRAs rectangular and some round. Some manufacturers have developed their own version for better results, Apple's Taptic Engine being the best-known example. So how do they compare to piezo haptics?
Piezo Trumps Both ERM and LRA Acceleration
LRA haptic solutions offer a clear performance improvement over ERM motors. They reach a higher acceleration, and therefore create stronger haptic feedback, since a higher acceleration value means stronger feedback.
While offering higher acceleration than ERM motors, linear resonant actuators are still held back by the same constraint as their lower-cost alternative: they need to move a mass to create vibration. An LRA reaches its optimal frequency peak faster than an ERM, 25 ms instead of 50 ms, but it still needs that time to get there and to decelerate afterwards.
A piezo actuator reaches its optimal peak in 0.3 ms. That near-instantaneous acceleration makes it a better-performing solution than an LRA for feedback strength. LRAs used in mobile devices reach acceleration values between 1 and 1.7 G. A similar-sized piezo actuator reaches between 2.5 and 5 G.
LRA Consumes More Power Than Piezo Actuators Coupled With Our Driver
Another advantage LRA has over ERM motors is lower power consumption. An LRA consumes far less power than an ERM solution, which makes it better suited to battery-powered mobile devices.
In fact, LRA was the most efficient haptic solution for years, more efficient than piezo haptics before we released CapDrive® technology. Piezo haptics had been held back by high-power drivers until the BOS1901 integrated circuit arrived.
Today an LRA consumes between 4 and 10 times more power than a piezo actuator coupled with a CapDrive® piezo driver. That is worth rethinking if you are specifying haptics for a battery-powered device.
LRA Offers Better Tactile Feedback than ERM Motors, but No HD Haptics
ERM motors only generate unrefined rumble. An LRA generates crisper haptic feedback, mainly thanks to its better acceleration.
That crispness comes from the resonant frequency, the narrow range where vibration is amplified to give optimal acceleration and displacement of the actuator's mass. The catch is in the word narrow. Because an LRA operates efficiently only within a small band around that frequency, its bandwidth is limited and it struggles to create distinct tactile effects. A piezo actuator works across the full haptic bandwidth instead, which opens up a far wider library of effects.
LRA Response Time is Faster Than ERM Motors, but Still Slower Than Piezo Actuators
The second reason LRA outperforms ERM is response time. An LRA, like an ERM, has to move a mass to create tactile feedback. It improved on its lower-cost alternative, but it still needs up to 25 ms to reach its optimal frequency range, and the same again to decelerate once the effect ends.
A piezo actuator starts playing its haptic effect in 0.3 ms. That difference is what makes crisper, more refined feedback possible.
Save Hardware with Piezo Integrated Force Sensing
Another advantage of the CapDrive® architecture is integrated piezo force sensing. The BOS1901, our first piezo driver IC based on CapDrive®, senses force and creates tactile feedback from the same piezo actuator. When your application needs force sensing, as a button replacement does, you can remove the separate sensing hardware from the design.
LRA Takes More Space Than Piezo Actuators
Linear resonant actuator drivers are more compact than piezo drivers. It would be reasonable to conclude that they suit small devices better, but the picture changes once you count the actuator itself.
An LRA, like an ERM, is bigger than a piezo actuator of equivalent acceleration. The space you save on the actuator more than makes up for the space the piezo driver takes.
To keep the overall footprint small, the BOS1901 needs only 7 discrete components to operate, which gives it the industry's smallest footprint.
Boréas Technologies' Piezo IC CapDrive® Technology vs. Linear Resonant Actuator (LRA)
| Features | ERM | LRA | Piezo - CapDrive® |
| Acceleration (g)* | 0.6 | 1.7 | 2.5 |
| Power Consumption | High | Medium | Low |
| Start-Up Time [ms] | 50 | 25 | 0.3 |
| Footprint Size | Big | Big | Miniature |
| Integrated Force Sensing | No | No | Yes |
| HD Haptics | No | No | Yes |
*with 100 g mass. Acceleration relates to feedback strength: a higher number means stronger feedback.
Experiment with Piezo Haptics Today
A BOS1921 development kit is available to test what piezo haptics can do. It comes with a selection of piezo actuators and GUI software so you can launch tactile effects within minutes.
Frequently asked questions
What is the difference between an LRA and a piezo haptic actuator?
An LRA moves a spring-suspended magnetic mass with a voice coil, so it has to accelerate and then decelerate that mass on every effect. A piezo actuator has no moving mass: the ceramic changes shape when the driver applies voltage. That difference is why an LRA needs about 25 ms to reach its optimal frequency while a piezo actuator starts in 0.3 ms, and why an LRA works best in a narrow band around one resonant frequency while piezo covers the full haptic bandwidth.
Is piezo better than LRA?
It depends on what you want the user to feel. On raw specifications piezo wins on acceleration, response time, power draw, bandwidth and footprint, and it adds force sensing an LRA cannot offer. But an LRA is a proven, low-cost way to shake a whole device for a notification or an incoming call, and if that is all your product needs, there is no reason to change. Piezo earns its place when you want localized feedback at the point of touch, a range of distinct effects, or a solid-state button in place of a mechanical one.
How much power does an LRA use compared to a piezo actuator?
An LRA consumes between 4 and 10 times more power than a piezo actuator driven by a CapDrive® IC. That ranking is recent. For years an LRA was the most efficient haptic technology available, more efficient than piezo, because piezo drivers of the day were power-hungry. CapDrive® reversed the comparison by recovering energy from the actuator rather than dissipating it.
Why is an LRA limited to a narrow frequency range?
An LRA relies on resonance. Its suspended mass and spring stiffness set a single natural frequency, and the actuator is only efficient within a narrow band around it. Outside that band the amplitude falls off sharply. That is what caps the variety of tactile effects an LRA can render, and why it cannot deliver HD haptics.
How fast is an LRA compared with an ERM and a piezo actuator?
An ERM needs roughly 50 ms to reach its optimal frequency. An LRA halves that to about 25 ms. A piezo actuator starts playing an effect in 0.3 ms. The slower technologies also need a comparable period to decelerate once the effect ends, which is why they feel like a lingering buzz rather than a defined click.
Can one piezo actuator both sense force and produce feedback?
Yes. A CapDrive®-based driver such as the BOS1901 senses how hard the user presses and generates the tactile response through the same piezo actuator. For an application that needs force detection, such as replacing a mechanical button, that removes the separate sensing hardware an LRA design would require.
Is Apple's Taptic Engine an LRA?
Yes. The Taptic Engine is Apple's own linear resonant actuator, developed in-house for better results than an off-the-shelf LRA. It shares the underlying limits of the technology: a suspended mass that has to be accelerated and stopped, and a narrow resonant band.
Glossary
Linear resonant actuator (LRA): An actuator that vibrates a spring-suspended magnetic mass along one axis using a voice coil, operating near its resonant frequency.
Eccentric rotating mass (ERM): A vibration motor that spins an off-center weight to create an unbalanced force, and therefore vibration.
Piezoelectric actuator: A component that changes shape when a voltage is applied, converting electrical energy into precise mechanical motion with no moving mass.
Resonant frequency: The single frequency at which a spring-mass system such as an LRA vibrates most efficiently, set by its mass and spring stiffness.
Haptic bandwidth: The range of frequencies a fingertip perceives as distinct tactile feedback, and the range an actuator must cover to render varied effects.
Acceleration (g): The measure used to compare haptic feedback strength, taken against a fixed reference mass. A higher value means stronger feedback.
Start-up time: How long an actuator takes to reach its optimal output after the signal arrives. It determines whether an effect feels like a click or a buzz.
HD haptics: High-definition tactile feedback, meaning effects with enough bandwidth and speed to be felt as distinct sensations rather than a single generic vibration.
Integrated force sensing: The ability of a driver to detect how hard a surface is pressed through the same actuator that produces the feedback, removing the need for a separate sensor.
CapDrive®: Boréas' piezo driver architecture that recovers energy from the actuator on every cycle, enabling high-definition haptics at low power.
Related reading
Haptic Technologies Showdown, part 1: Boréas' Piezo Driver IC vs. Eccentric Rotating Mass (ERM)
Haptic Technologies Showdown, part 3: Boréas' Piezo Driver IC vs. Competitors' Piezo IC
How to Choose a Haptic Actuator: Piezo vs LRA vs ERM
Mechanical Fundamentals of Piezo Haptic Actuators
Piezo vs. LRA Haptic Computer Trackpads: Which Technology Should You Select?
Haptics in Wearables: Why Small Actuators Make Your Device Feel Cheap
Next steps
Review the CapDrive® piezo driver architecture → https://www.boreas.ca/pages/capdrive-technology
See the BOS1921 piezo driver specifications → https://www.boreas.ca/collections/piezo-haptic-drivers/products/bos1921-piezo-driver
Prototype piezo feedback with a development kit → https://www.boreas.ca/products/bos1921-kit-c01
Discuss replacing an LRA in your design with our applications engineering team → sales@boreas.ca
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