Two technologies are used to build haptic trackpad modules: linear resonant actuators (LRA) and piezoelectric actuators. Both create a click, but they actuate in completely different ways, and that changes how thin the module can be, which axis the feedback moves along, how sharp the click feels, how much power it draws, and whether you need a separate force sensor. This article compares the two on each of those points and covers the mechanical integration decisions you have to make at the start of a trackpad project rather than partway through.
Key Takeaways
- A mechanical trackpad is typically over 3 mm thick. An LRA module lands around 3 mm because the actuator itself is the thickest part. A piezo module can go as thin as 1.8 mm.
- Thinner LRAs exist, but using one costs you feedback quality: weak, soft clicks that make the whole trackpad feel off.
- To stay thin, an LRA has to lie flat, which makes it vibrate on the X or Y axis. The trackpad slides sideways. A piezo actuator lying flat pushes on the Z axis, against the finger, which is how a real button behaves.
- Piezo rise time, the time to go from 10% to 90% of full amplitude, is far shorter than an LRA's. That is what makes the click sharp, where an LRA's slower ramp adds unwanted noise at the start and end of every effect.
- One piezo actuator can both sense the press and generate the click across the whole trackpad surface, with no separate sensing electronics, when paired with a CapDrive® driver.
- Mechanical integration decides the outcome. Too much friction or rigidity will make a perfectly capable actuator feel weak, and you cannot swap actuator technology partway through a project.
Haptic computer trackpads have real advantages over traditional designs, which is why computer OEMs are replacing mechanical trackpads with haptic hardware. The question is which haptic technology to build on.
Piezo Haptic Trackpad Modules Are Thinner
The biggest motivation for computer OEMs to pursue haptic trackpads is thinness. Mechanical trackpads typically measure more than 3 mm. Haptic trackpads can come in below 3 mm, but how far below depends entirely on the technology you choose.
LRA module designs usually land around 3 mm thick, and the thickest part of the module is the LRA itself. You can find thinner LRAs, but you buy that thinness with a considerable trade-off in feedback quality. Poor feedback means weak, unrefined, soft clicks that annoy the user and make the trackpad feel off. Since the whole point is the user experience, the actuator choice deserves care. In practice, an LRA needs to be at least 3 mm thick to reach an acceptable performance level in a trackpad module.
Piezo actuators come in a range of form factors, and several types can be used to build a trackpad module. They can be much thinner than an LRA, which means a piezo-based trackpad module can go as thin as 1.8 mm.
Piezo Actuators Deliver Better Haptic Feedback Quality
The other significant difference between an LRA and a piezo trackpad is the quality of the click itself, and it comes down to two things: the vibration axis and the actuator's rise time.
The only way to fit an LRA into a trackpad module and keep it thin is to lay the actuator flat. That makes it vibrate laterally, along the X or Y axis, so the trackpad moves left and right, or toward and away from the user. The movement is quick, but it does not produce a satisfying button click, because we are used to buttons pushing back against our fingers along the Z axis.
A piezo actuator lying flat on a surface creates its feedback in the Z axis. The trackpad module pushes against your finger, exactly as a real button does. It simply feels more natural.
The two also convert energy into movement very differently. An LRA uses magnetism to move a mass back and forth at its resonant frequency. A piezo actuator uses the piezoelectric effect and deforms almost instantaneously under high voltage. That means the rise time, the time required for a pulse to go from 10% to 90% of full amplitude, is significantly shorter for piezo than anything an LRA can achieve, and shorter rise time translates directly into a sharper click. Because an LRA takes longer to reach full amplitude, its effect carries unwanted noise at the beginning and the end, and that noise is what the user perceives as softness or lack of sharpness.
CapDrive® Driven Piezo Actuators Consume Much Less Power Than LRA
Piezo actuators are excellent on power efficiency, but there is a catch worth understanding. The actuator itself is efficient; it also requires high voltage to produce usable feedback, and the driver you choose to generate that voltage has an enormous effect on the module's total consumption.
Boréas designs piezo drivers from the ground up for battery-powered mobile applications, with power efficiency as the primary goal. CapDrive® drivers use an architecture that exploits the capacitive nature of piezo actuators to recover and reuse energy rather than dissipating it. A piezo haptic solution powered by a CapDrive® driver can consume up to 10 times less power than the same actuator paired with a competing piezo driver, and the comparison against LRA-based solutions is similarly favourable.
Piezo Actuators Can Both Sense Force and Create Haptic Feedback
The piezoelectric effect lets piezo elements generate an electric charge when they are stressed, which means you can measure the actuator's electric potential to determine how much force has been applied. That is why force sensors so often use piezo materials. The effect is also reversible: the material deforms when it is under an electric charge, and that is how we generate haptic feedback.
A piezo haptic trackpad module can use both directions of the effect at once, employing the actuator to generate high-quality haptics and to sense input force. With a CapDrive® piezo driver you do not need additional electronics for this, because integrated pressure sensing comes built in. Our trackpad demo uses a single piezo actuator to both sense and generate haptics across the entire trackpad surface.
Be Careful: Your Mechanical Integration Has a Significant Impact
The factor that decides whether a haptic trackpad succeeds is the mechanical integration of the actuator, and that holds whichever technology you pick. Integrating an LRA is very different from integrating a piezo actuator, so choose early. Switching actuator technology in the middle of a project is painful, because the module has to be designed from the ground up around the actuator you intend to ship.
The reason we push this point is simple: inadequate mechanical integration will significantly degrade the performance of a perfectly good actuator. A design with too much friction or rigidity will convince you the actuator is not strong enough when the actuator was never the problem. Avoiding that mistake matters most during evaluation, because otherwise you can rule out the right technology or lose months of development time. Our team can help you start by sharing best practices for evaluating the technology and building a convincing demonstrator.
Starting Your Piezo Haptic Trackpad Project
We believe piezo haptic hardware makes the best trackpad modules. We are not neutral, since we design piezo haptic drivers, which is exactly why we built a development kit for your team to test the claims rather than take our word for them.
The BOS1921 development kit is a low-cost evaluation kit that lets you test haptic feedback quality and integrated force sensing, measure system power consumption, and accelerate development. It is available worldwide on our website and through our distributors.
Haptic technologies are almost impossible to compare without feeling them, so the most useful thing you can do to start a trackpad project is get a kit in your hands. We will set you up with an onboarding program built specifically around piezo haptic trackpads.
Frequently asked questions
How thin can a haptic trackpad module be?
A mechanical trackpad typically measures more than 3 mm. An LRA-based haptic module usually lands around 3 mm, because the actuator is the thickest element and an LRA needs roughly that depth to perform acceptably. A piezo-based module can reach 1.8 mm. Thinner LRAs are available, but they trade away feedback quality, producing weak and soft clicks.
Why does an LRA trackpad vibrate sideways instead of up and down?
Because of how it has to be mounted. Keeping the module thin means laying the LRA flat, and a flat LRA vibrates along its lateral axis, so the trackpad moves left and right or toward and away from the user. A piezo actuator lying flat produces its force along the Z axis, pushing up against the finger the way a mechanical button does, which is why a piezo click reads as more natural.
What is rise time and why does it make a click feel sharp?
Rise time is how long a pulse takes to go from 10% to 90% of full amplitude. A piezo actuator deforms almost instantaneously under voltage, so its rise time is very short and the click lands as a single defined event. An LRA has to accelerate a suspended mass to full amplitude, which takes longer and adds unwanted vibration at the start and end of the effect. That leading and trailing noise is exactly what users perceive as a soft or mushy click.
Can you replace an LRA trackpad with a piezo one?
Yes, and it is one of the clearest cases where piezo wins: you gain a thinner module, a Z-axis click, a sharper effect and integrated force sensing. What you cannot do is treat it as a component swap. The mechanical integration is completely different and the module has to be redesigned around the piezo actuator from the start, which is why the decision belongs at the beginning of a project rather than partway through.
Why does my piezo actuator feel weak in my trackpad prototype?
Usually the mechanical design rather than the actuator. Too much friction or too much rigidity in the module will absorb the actuator's output before it reaches the user's finger, and the symptom looks identical to an underpowered actuator. This is the single most common reason a piezo evaluation gives a misleading result, so it is worth reviewing the stack-up before concluding the technology is not strong enough.
How much power does a piezo trackpad use compared with an LRA?
It depends far more on the driver than on the actuator. Piezo actuators are efficient but need high voltage, and generating that voltage is where most piezo drivers waste power. A CapDrive® driver recovers and reuses the energy stored in the actuator's capacitance, so the same actuator can consume up to 10 times less power than it would on a competing driver, and the result compares favourably against LRA-based modules too.
Glossary
Haptic trackpad: A trackpad with no mechanical click mechanism, where the click sensation is generated by an actuator and the press is detected electronically.
Linear resonant actuator (LRA): An actuator that moves a spring-suspended magnetic mass along one axis using a voice coil, operating near its resonant frequency.
Piezoelectric actuator: A component that changes shape when voltage is applied, converting electrical energy into precise mechanical motion with no moving mass.
Rise time: The time a pulse takes to go from 10% to 90% of full amplitude. Shorter rise time produces a sharper, better-defined click.
Z-axis feedback: Feedback delivered perpendicular to the surface, pushing back against the finger the way a mechanical button does, as opposed to lateral X or Y axis movement.
Module stack-up: The full mechanical assembly of the trackpad, including the surface, the actuator mounting and everything between them. It determines how much of the actuator's output reaches the user.
Integrated force sensing: A driver's ability 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's capacitance on every cycle, enabling high-definition haptics at low power with integrated force sensing.
Related reading
Piezo vs LRA: How Linear Resonant Actuators Compare to Piezo Haptics
How to Choose a Haptic Actuator: Piezo vs LRA vs ERM
Mechanical Fundamentals of Piezo Haptic Actuators
How to Select the Right Haptic Piezo Driver and Actuator Combination
How to Build the Perfect Trackpad Click
Boréas Blueprint: piezo integration engineering
Next steps
Evaluate haptic feedback and force sensing with a development kit → https://www.boreas.ca/products/bos1921-kit-c01
Review the CapDrive® piezo driver architecture → https://www.boreas.ca/pages/capdrive-technology
De-risk the mechanical integration with the Boréas Blueprint → https://www.boreas.ca/pages/boreas-blueprint-engineering
Talk through your trackpad module design with our engineering team → sales@boreas.ca
Leave a comment