How Traditional Rumble Works
The rumble effect we all know and love uses a simple yet ingenious method to create vibrations in the controller. There are usually two electric motors, one in each grip, that spin an unbalanced weight. They are known as eccentric rotors and the principle is the same as having an unbalanced washing machine. As the unbalanced weight spins around it wobbles, making the controller rumble. By varying the speed of the motor, the intensity and vibration frequency can be varied.
In modern controllers that use this design, such as the Xbox One controller shown below, the two rumble motors have different sized weights.
This allows a greater range of rumble types and blending of rumble types. The motors themselves have also become much stronger and more precise over the years, so the haptic experience from a modern controller such as the Xbox Series controllers feels much more refined and powerful than, for example, a PlayStation 2 DualShock controller.
However, VR controllers, the PlayStation 5 DualSense, and the Nintendo Switch Joy-Cons don’t have these rumble motors. Instead, they use a device known as a Linear Actuator also known as a Linear Resonant Actuator, or sometimes a “voice coil motor”.
How Linear Resonant Actuators Work
A linear actuator gets its name from how it moves its mass. Where the traditional rumble motor spins its mass around, a linear actuator moves its mass back and for along an axis. Linear actuators are usually labeled depending on which axis they are aligned to. They are commonly used in smartphones for haptic feedback.
Linear actuators use a voice coil that’s the same as the one found in a loudspeaker. A voice coil is essentially an electromagnet. In a loudspeaker, the voice coil drives the speaker cone using magnetism. By altering the frequency at which the voice coil drives the cone, sound can be reproduced in the air.
In a linear actuator, the voice coil drives a spring that has a resonant frequency. When driven at its resonant frequency, the spring moves a mass within the actuator with maximum strength. The further from that resonant frequency the coil is driven, the less efficient it becomes. Linear actuators can only be driven within a specific frequency range centered around that optimal resonant frequency of the spring. Going too far outside of that range means the vibration needs too much power to be usable.
Within that frequency range, linear actuators can recreate vibrations with astounding levels of detail. In fact, developers of games for the Nintendo Switch literally convert audio files into the rumble files for the console’s HD Rumble feature.
In the PlayStation 5’s DualSense controller, the built-in speaker can be used to augment the linear actuators at the edges of their frequency ranges. So the speaker is used to both augment haptic vibration and produces audible sound.
Why Linear Actuators Are the New Gold Standard in Haptics
Linear actuators look like they’re becoming the most popular choice for haptic feedback. Among current-generation consoles, Microsoft’s Xbox Series consoles are the only systems to stick with eccentric rumble motors. In the world of VR, current generation controllers all use linear actuators, and in mobile devices, they’re the only practical option thanks to their compact size and low power requirements.
More than anything, we think the push for linear actuators is driven by more than just their cost, power, and refined detail advantages. Since the software that we use has become so detailed and sophisticated, a new generation of haptic technology is needed.
As electronics become ever more solid-state, with fewer moving parts or mechanical components, linear actuators are the logical choice for thin and light systems. While they can’t match the outright power of traditional rumble motors, there’s nothing quite like feeling your hand “bump” against something in VR, feeling raindrops fall through your controller, or being able to tell the texture of the road through your hands when racing in a simulation.
