A haptic actuator is made of two parts: an engine that generates acceleration, and a mass for that engine to move. Eccentric rotating mass (ERM) and linear resonant actuator (LRA) units ship with both parts inside the same component. A piezoelectric actuator is only the engine, which is why bonding one where an LRA used to sit produces a faint noise instead of a click. This article explains the mechanics behind all three actuator types, shows how to read the acceleration figures in haptic comparison charts, and covers what changes in your mechanical design when you integrate piezo haptics.
Key Takeaways
- Every haptic actuator is an engine plus a mass. The engine generates acceleration, the mass turns it into a force you can feel (F = ma).
- An ERM carries an eccentric mass on a DC motor; an LRA carries a suspended magnetic mass driven by a voice coil. Both arrive with their mass built in.
- A piezo actuator has no built-in mass. It is the engine only, so bonding one into an ERM or LRA footprint produces little more than light noise.
- ERM and LRA are chosen to vibrate a whole device for notifications and alerts. Piezo is chosen for localized feedback at the point of touch and to replace mechanical buttons. Moving from one to the other is a mechanical redesign, not a part substitution.
- Acceleration values in comparison charts are measured against the same reference mass, so they rank actuators fairly, but a piezo figure is measured with the mass sitting on the actuator and does not transfer directly to a different mounting.
- Choosing the mass is a design lever: a small mass gives localized button feedback, a heavier mass or the device structure itself vibrates the whole product.
The launch of our low-power CapDrive® piezo haptic driver architecture revived the industry's interest in piezo-based tactile effects. Because piezo technology works differently from ERMs and LRAs, a few mechanical fundamentals are worth covering before you integrate it. Replacing an ERM or an LRA is not a matter of attaching a piezo actuator in the same spot with adhesive. Doing that produces disappointing results and points your team in the wrong direction.
Piezo haptics operate under a different mechanical concept and need a different approach to reach a successful integration with better tactile effects. Here is how piezo haptics differ from ERMs and LRAs, and what that means for your integration.
Understanding Haptic Actuator Comparison Charts: Making Sense of Acceleration
Even if haptic technologies are trying to create advanced effects, they cannot escape the laws of physics. To understand how haptic actuators work, we need some of the most fundamental laws of classical mechanics, Isaac Newton's laws of motion:
First law
In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
Second law
In an inertial frame of reference, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma. (It is assumed here that the mass m is constant.)
Third law
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.[1]
Comparison charts in haptics are a little bit flawed. The feedback force we feel is the force of motion from the actuator, so why does the industry refer to acceleration to compare feedback strength? The acceleration values you see in comparison charts are based on accelerometer measurements on the same mass for every haptic actuator. Since each actuator is compared on the same mass basis, we can isolate the acceleration value. It is therefore true that the actuator with the highest acceleration value creates the strongest feedback, but their performance is not measured in exactly the same way.
While it might be true that bonding an ERM or an LRA to a given mass is enough to move it, you cannot expect the same result from bonding a piezo actuator. The piezo actuator's acceleration value is measured with the mass on top of the actuator. Here is why that difference matters, and why you need a different approach to integrate piezo haptics successfully in your product design.
Haptic Actuator Components
We have seen that haptics are governed by Newton's laws of motion. To create a force, haptic actuators need two components: a mass, and an engine to accelerate it. When you compare haptic technologies, you are ultimately comparing the type of engine they use to generate acceleration.
Haptic Actuator = Engine + Mass
Let's look at the components behind each haptic actuator, and why you cannot directly replace an ERM or an LRA with a piezo actuator.
Eccentric Rotating Mass (ERM) Actuator Components
ERM vibrations are created by rotating an eccentric mass with a DC motor (the engine). The mass movement creates an unbalanced force, and therefore vibration.
While it is complicated to calculate the movement axis of the eccentric mass, all we need to understand is that the force created by the rotation of the unbalanced mass is transferred to the DC motor. The ERM actuator is bonded to the device at the base of the DC motor, so the vibration induced by the motor is transmitted to the device.
Newton's principles are respected: the eccentric mass and the engine's rotation create a moving force that transfers from the actuator to the device it is bonded to.
Linear Resonant Actuator (LRA) Components
LRA vibrations are created by a suspended magnetic mass driven by a voice coil (the engine). Driving current through the voice coil creates a magnetic field that moves the magnetic mass along the axis where the springs are aligned.
The magnetic mass needs to oscillate near its natural frequency in order to resonate and create the strongest force. That resonant frequency is set by the mass and the rigidity of the suspension springs.
Here again, Newton's principles are respected: the suspended magnetic mass (m) moving under the voice coil's acceleration (a) creates a force that is transferred to the device through the springs.
Piezoelectric Actuator Components
For this example we will use the TDK PowerHap™ piezo actuator line-up, since they all share the same design basics. These piezo actuators do not work like piezo benders. Instead, PowerHap™ actuators mechanically contract when they are under tension (high voltage). The contraction of the piezo material forces the metal cymbals to expand, and that is how the actuator pushes against a mass. If you are pushing a mass on one side of the actuator, the other side needs to lean against a surface that transmits the force to the device, which has a mass of its own. Once your device is subjected to a force and has a mass, Newton's second law dictates that it will vibrate.
The reason piezo actuators cannot replace legacy technologies directly is that they are not haptic actuators, they are engines. Just like the ERM's DC motor and the LRA's voice coil, piezo actuators have no built-in mass.
Looking at Newton's principles, the piezo actuator is the engine and generates acceleration (a), but the mass (m) needed to create an impactful motion force is missing.
Does that mean it is impossible to replace LRAs and ERMs with piezo haptics to vibrate a complete device? No. It means you need to give the piezo actuator a mass to move, and we consider that an advantage of the technology. It gives you more flexibility to tune piezo haptics for the tactile effect you actually want. Combining a piezo actuator with a small mass is ideal for localized feedback and for replacing mechanical buttons. Combined with a heavier mass, piezo actuators create enough force to vibrate the whole device they are bonded to. The device can also be the mass itself, as with an automotive display. Legacy haptic technologies do not give you that level of flexibility.
Boréas Technologies Mechanical Engineering Team
Need help integrating your piezo haptics? Boréas Technologies has a mechanical engineering department ready to help. Our team can help you get better tactile feedback out of piezo actuators.
Our mission is to drive the haptic revolution, and we mean it. We design low-power electronics and support you through the design process, from prototype to production. Our expertise can help you improve your device's HMI and user experience.
Tell us about your project and we will help you integrate HD haptics. You can reach us at sales@boreas.ca.
Frequently asked questions
What are the components of a haptic actuator?
An ERM combines a DC motor with an off-center mass. An LRA combines a voice coil with a spring-suspended magnetic mass. Both arrive as complete units with their moving mass inside. A piezo actuator works differently: it has no built-in mass. The piezo ceramic changes shape when a driver applies voltage, and the mechanical design around the ceramic determines whether the user feels anything.
What are ERM, LRA and piezo actuators each used for?
ERM and LRA actuators are used to vibrate an entire device. They drive notifications, alerts and incoming calls, the kind of buzz a user feels anywhere on the product. Piezo actuators create localized effects at the point of touch and can replace mechanical buttons with solid-state ones. The two families answer different design questions, so the right comparison is between the interactions you want to deliver, not between the parts on a spec sheet.
Can a piezo actuator replace an ERM or an LRA?
It depends on the application and on what you want the user to feel. In a trackpad, yes. An LRA-based trackpad can be redesigned around a piezo actuator, and the result is a noticeably better experience: a sharper click, localized under the finger instead of felt across the whole chassis. You can't treat it as a part swap. The mechanical integration is completely different and has to be rethought from the start. And if all you need is a notification buzz felt across the whole device, an LRA already does that job well and there is no reason to change.
What does a piezo driver actually do to the actuator?
It bends it. A Boréas driver applies a controlled voltage waveform that deflects the piezo ceramic, and that deflection is what the fingertip perceives as a click. Nothing is spun or shaken. The sensation comes from the ceramic changing shape directly under the finger, which is why the effect stays localized to the surface being touched rather than spreading through the whole device.
Why do haptic comparison charts use acceleration instead of force?
The feedback you feel is a force of motion, and force depends on both mass and acceleration. To make different technologies comparable, chart authors measure every actuator against the same reference mass, which isolates acceleration as the single variable. On that basis, the actuator with the highest acceleration value does produce the strongest feedback. The caveat is the one stated above: the technologies are not all measured in exactly the same way.
What is the hardest part of integrating piezo haptics?
The mechanical design, not the electronics. The ceramic's deflection is small, and it has to reach the user's finger through the surface, the mounting and everything else in the stack-up. Get that path wrong and the effect feels weak no matter how good the driver is. A piezo design starts from the mechanical integration rather than ending with it, which is why Boréas keeps a mechanical engineering team alongside the IC design team.
Glossary
Haptic actuator: An electromechanical component that converts an electrical signal into tactile feedback.
Engine (haptics): The part of an actuator that generates motion, such as an ERM's DC motor, an LRA's voice coil, or a piezoelectric element.
Eccentric rotating mass (ERM): A vibration motor that spins an off-centre weight to create an unbalanced force, and therefore vibration.
Linear resonant actuator (LRA): An actuator that drives a spring-suspended magnetic mass along a single axis, near its resonant frequency, using a voice coil.
Piezoelectric actuator: A component that changes shape when a voltage is applied, converting electrical energy into precise mechanical motion with no moving mass.
Cymbal actuator: A piezo actuator design, used in the TDK PowerHap™ range, where the contraction of the piezo material forces surrounding metal cymbals to expand outward.
Resonant frequency: The single frequency at which a spring-mass system such as an LRA oscillates most efficiently, set by its mass and spring rigidity.
Localized feedback: Tactile feedback delivered at the point the user touches, rather than a vibration felt across the whole device.
Acceleration (g): The metric used to compare haptic actuator strength, measured with an accelerometer against a fixed reference mass.
CapDrive®: Boréas' piezo driver architecture that recovers energy from the actuator on every cycle, enabling high-definition haptics at low power.
Related reading
How to Choose a Haptic Actuator: Piezo vs LRA vs ERM
Boréas Technologies' Piezo Driver IC vs Competitors' Piezo IC: Haptic Technologies Showdown, part 3
Piezo vs. LRA Haptic Computer Trackpads: Which Technology Should You Select?
Boréas Blueprint: piezo integration engineering
Next steps
Review the CapDrive® piezo driver architecture → https://www.boreas.ca/pages/capdrive-technology
De-risk the mechanical integration with our Blueprint co-development program → https://www.boreas.ca/pages/boreas-blueprint-engineering
Prototype piezo feedback with a development kit → https://www.boreas.ca/products/bos1921-kit-c01
Discuss your mechanical integration with our engineering team → sales@boreas.ca
[1] Newton's laws of motion. (2020, February 14). Retrieved from https://en.wikipedia.org/wiki/Newton's_laws_of_motion
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