An eccentric rotating mass (ERM) actuator creates vibration by spinning an off-center weight with a small electric motor. The design dates to the 1960s, it is the cheapest way to make a device buzz, and it is still in millions of products today. This article is part one of our three-part Haptic Technologies Showdown. It compares ERM against a piezo actuator driven by Boréas' CapDrive® architecture on the five factors that decide a design: acceleration, response time, power consumption, footprint and integrated force sensing.
Key Takeaways
- An ERM spins an off-center mass with an electric motor, a design essentially unchanged since the 1960s and still used wherever cost matters most.
- The motor has to overcome the mass's inertia and ramp up to its optimal frequency, which takes up to 50 ms, then decelerate again afterwards. That is why an ERM produces an unrefined rumble rather than a defined click.
- A piezo actuator does not rotate anything. The material deforms as soon as high voltage reaches it, so the effect starts in 0.3 ms.
- An ERM can consume up to 20 times more power than a piezo actuator driven by a CapDrive® IC.
- A piezo actuator is smaller than an ERM producing comparable feedback, and the space saved on the actuator outweighs the larger driver.
- Because the piezoelectric effect is reversible, a CapDrive® driver senses force and creates feedback through the same actuator. An ERM can only play feedback, so force detection needs separate hardware.
Haptic feedback isn't new. You have to go back to the 1960s to find the first devices using vibration to send information to a user, and the system they used then is still what creates basic tactile feedback in low-cost devices now. An eccentric rotating mass system, often just called a vibration motor, is an electric motor spinning an off-center mass at high frequency. The concept has not changed; the technology was simply miniaturized to fit smaller devices.
ERM actuators are still used in many devices because of their low cost, but better-performing technologies are steadily replacing them. We live surrounded by touchscreens, and companies everywhere are developing ways to improve how their devices feel, which is what has driven the development of better haptic technology.
One of those higher-performing options is the piezoelectric haptic actuator. Rather than rotating an off-center mass, a piezo solution sends high-voltage power through the actuator material with a piezo driver, deforming it to create tactile feedback. Here is what separates the two, and how to decide which belongs in your application.
Piezo Haptics Create Stronger Tactile Feedback
Acceleration, in haptics, relates to the strength of the feedback you can feel. The higher the acceleration an actuator reaches, the stronger the haptic effect it generates.
ERM systems are limited in the acceleration they can reach, and it is easy to see why. The motor has to accelerate a mass to create vibration, which means breaking the mass's inertia and ramping the speed up to the optimal frequency. That acceleration is not instantaneous, and the result is an unrefined rumble.
A piezo actuator has no such mechanical limitation. The vibration does not depend on rotating a mass: the actuator moves as soon as the high voltage reaches the piezo material. That is what lets piezo produce stronger and crisper feedback than an ERM.
ERM Solutions Consume Up to 20 Times More Energy Than Piezo with CapDrive®
Power consumption is another area where piezo has a decisive advantage. Piezo actuators are very power efficient, but for years that efficiency was cancelled out by underperforming drivers. That changed with CapDrive® technology. The BOS1901, our first piezo haptic driver IC based on CapDrive®, made piezo the lowest power-consuming haptic technology available.
An eccentric rotating mass uses a lot of power. It can draw up to 20 times more than a piezo actuator paired with a CapDrive® driver.
Source: data extracted from Haptic Energy Consumption, Application Report SLOA194, Texas Instruments, May 2014. The data point for Boréas was extrapolated from comparative measurements between the TI DRV8662 and the Boréas driver.
If you are integrating haptics into a device with a limited power source, such as a mobile product, piezo is worth looking at before an ERM.
Vibration Motors Have Slow Reaction Times While Piezo Is Near Instantaneous
Haptic feedback needs precise timing to feel right, and faster reaction times translate directly into crisper, more refined effects. A vibration motor needs time to accelerate its rotating mass up to peak frequency, the zone where the feedback is actually usable, and time to decelerate it again once the effect ends. An ERM typically takes up to 50 milliseconds to reach peak frequency. Those acceleration and deceleration delays create small, soft, unwanted vibrations at both the start and the end of every effect.
A piezo actuator reacts in 0.3 milliseconds, which means none of that leading and trailing mush. The limiting factor in a piezo design is the driver rather than the actuator, and the BOS1901 has the fastest response time in the industry at under 6 milliseconds.
Piezo Haptics Save Space Over ERM Systems
An ERM haptic system takes up more room than a piezo solution. ERM drivers are smaller than piezo drivers, but the meaningful difference is in the actuator: a piezo actuator is smaller than an ERM and delivers stronger feedback, and that more than offsets the larger driver.
If you need a compact solution, piezo beats a vibration motor.
Taking Full Advantage of the Piezo Effect to Offer Integrated Force Sensing
The piezo effect is reversible. You can apply high voltage to a piezo material to make it move, and the same material produces an electrical charge when it is deformed. Pressing on a piezo actuator deforms the material slightly, and the actuator generates a current. CapDrive® drivers detect that current, which means they can sense force and trigger feedback through the same actuator.
An ERM solution can only trigger feedback. If you need to detect force, you need a complete force-sensing subsystem alongside the haptic one.
So if your application triggers feedback when force is applied, as a mechanical button replacement does, you can eliminate the sensing hardware and rely on a single piezo actuator paired with one of our drivers. That is exactly what we did with our SmartClik buttonless phone prototype.
High-Definition Haptics vs. Basic Rumble
Piezo actuators generate far more advanced effects. Like an audio speaker, they take an electrical waveform from an amplifier, in this case the piezo driver, and can be operated across a broad range of frequencies and amplitudes to produce a wide library of distinct tactile effects. You can tune the feedback to the application and create rich, detailed sensations. That is what we mean by high-definition haptics.
An ERM has a very narrow frequency range where its feedback is usable, which means a very limited set of possible effects. Vibration motors are confined to low-quality rumble.
Piezo opens up effects an ERM cannot reproduce, which matters for applications that need several distinct sensations: AR and VR, automotive safety alerts, button replacement and more.
Boréas Technologies' Piezo IC CapDrive® Technology vs. Eccentric Rotating Mass (ERM)
| Features | ERM | Piezo - CapDrive® |
| Acceleration (g)* | 0.6 | 2.5 |
| Power Consumption | High | Low |
| Start-Up Time [ms] | 50 | 0.3 |
| Footprint Size | Big | Miniature |
| Integrated Force Sensing | No | Yes |
| HD Haptics | 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 an ERM actuator and how does it work?
An eccentric rotating mass actuator is a small electric motor spinning a deliberately off-center weight. Because the weight is unbalanced, spinning it shakes the motor, and that vibration transfers into whatever the motor is bonded to. It is the oldest haptic technology still in production, essentially unchanged since the 1960s, and it remains the cheapest way to make a device buzz.
Why does an ERM feel like a rumble instead of a click?
Because it takes time to spin up and time to stop. The motor has to overcome the mass's inertia and reach its optimal frequency, which takes up to 50 milliseconds, then decelerate once the effect ends. Those ramps produce soft unwanted vibration at the start and finish of every effect, so the sensation smears into a rumble rather than landing as a defined click.
How much power does an ERM use compared with piezo?
An ERM can consume up to 20 times more power than a piezo actuator paired with a CapDrive® driver. Piezo actuators were always efficient, but for years that was masked by inefficient drivers. CapDrive® recovers the energy stored in the actuator's capacitance rather than dissipating it, which is what makes piezo the lowest power-consuming haptic option available today.
How fast is an ERM compared with a piezo actuator?
An ERM needs roughly 50 milliseconds to reach peak frequency. A piezo actuator starts producing its effect in 0.3 milliseconds, because nothing has to be spun up: the material deforms as soon as voltage reaches it. That difference of more than two orders of magnitude is the main reason piezo feedback reads as crisp and ERM feedback does not.
Is an ERM still worth using?
Yes, in the right product. If all you need is a notification buzz felt across the whole device and cost is the dominant constraint, an ERM does that job and does it cheaply. It stops being the right answer when you need feedback localized at the point of touch, several distinct effects rather than one, force detection, or a design that cannot spare the power or the volume.
Can an ERM do force sensing?
No. An ERM can only play an effect. If your product needs to know how hard the user pressed, an ERM design requires a separate force-sensing subsystem alongside the haptic one. A piezo actuator can do both, because the piezoelectric effect is reversible and the same actuator generates a measurable voltage when it is deformed.
What is HD haptics and why can't an ERM produce it?
High-definition haptics means effects with enough frequency range and speed to be felt as distinct sensations rather than one generic vibration. A piezo actuator works like a speaker, taking a waveform from its driver across a broad range of frequencies and amplitudes. An ERM has a narrow usable frequency band and a slow response, so it can only deliver low-quality rumble regardless of how it is driven.
Glossary
Eccentric rotating mass (ERM): A vibration motor that spins an off-center weight to create an unbalanced force, and therefore vibration.
Linear resonant actuator (LRA): The second-generation alternative to an ERM, which drives a spring-suspended magnetic mass along a single axis near its resonant frequency.
Piezoelectric actuator: A component that changes shape when a voltage is applied, converting electrical energy into precise mechanical motion with no moving mass.
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 usable 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 one generic vibration.
Reversible piezoelectric effect: The property by which a piezo material both deforms under applied voltage and generates a voltage when deformed, which is what makes force sensing possible through the same actuator.
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 on every cycle, enabling high-definition haptics at low power.
Related reading
Piezo vs LRA: How Linear Resonant Actuators Compare to Piezo Haptics (Showdown, part 2)
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
How to Select the Right Haptic Piezo Driver and Actuator Combination
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
Get integration help through the Boréas Blueprint → https://www.boreas.ca/pages/boreas-blueprint-engineering
Discuss replacing an ERM in your design with our applications engineering team → sales@boreas.ca
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