This is the second post in a series on the basics of haptic technology and perception. This installment focuses on vibrotactile (vibration) feedback. Subscribe to our newsletter to get these delivered straight to your inbox.
In a previous post, I described how haptics can be divided into a series of “sensory channels” – tactile, vibrotactile, force, and thermal. I focused on tactile feedback in that post. This time, let's move on to vibrotactile (vibration) feedback.
Vibrotactile feedback devices are by far the most widespread class of commercial haptic devices. Almost everyone reading this article has experienced vibrotactile feedback in one form or another – the buzz of a cellphone, the rumble of a game controller. Some VR enthusiasts may even have tried one of the latest generation of vibrotactile feedback wearables like Gloveone or Manus. In fact, in many cases vibrotactile feedback has become synonymous with haptic feedback, leading to some misconceptions about the nature and capabilities of vibrotactile feedback devices.
Vibrotactile feedback: feeling sounds
Put simply: vibrotactile feedback is feeling sounds. Sound is a form of mechanical energy that consists of back-and-forth motion of a material. Usually, we think of sound waves travelling through air, but they can also travel through almost any other material, including human skin.
We have a specialized type of receptor in certain parts of our skin called a Pacinian Corpuscle that is tuned to detect these vibrations. These little onion-like receptors are exquisitely sensitive to vibrations of up to 1000 hertz (cycles per second, a measure of frequency). For comparison, the fundamental frequency of human speech is in the low hundreds of hertz. Under the right conditions, our skin can actually feel many of the sounds we hear, including speech!
But, what’s the use of feeling sound? Sound vibrations provide subtle cues about many objects we interact with. For example, when you run your finger over a surface, the interaction of your skin and the surface produces a sound which gives you information about whether that surface is rough or smooth.
Similar vibrations are also produced when using tools. Think, for example, of the vibrations produced when striking an object with a hammer. Without vibrotactile feedback, it would be difficult to distinguish between hitting a rubber surface and hitting a metal surface.
Like tactile feedback devices, vibrotactile feedback devices also consist of one or more actuators in contact with the skin. You may recall that tactile feedback devices are challenging to construct because these actuators must be closely spaced and must exhibit high displacement relative to their size. The good news with vibrotactile feedback is that both requirements are significantly lessened.
The receptors in the skin that detect vibrations have much larger receptive fields than those that detect tactile stimuli. This means that the number of vibrotactile actuators required for a given area of skin is only a small fraction of the number of tactile actuators required in the same area. The displacement requirements are also much more forgiving.
You may recall that tactile actuators require displacements of up to about 2 centimeters to accurately reproduce pressures on the skin surface. On the other hand, due to the sensitivity of the skin to vibration, vibrotactile actuators can get away with just a fraction of a millimeter of displacement.
The limits of vibrotactile feedback
If vibrotactile feedback devices are relatively easy to construct, and vibration feedback is important for many parts of tactile perception, why do we need any other sensory channels in haptics? Unfortunately, as anyone who has tried a vibrotactile feedback device can attest, vibrotactile feedback on its own tends to be quite limited in its realism and richness.
In the real world, vibrations are usually created as a secondary part of a holistic haptic interaction. Imagine swinging a hammer in virtual reality. When it strikes the surface of a glowing hot sword, a high-quality vibrotactile actuator would accurately simulate the vibration of the strike. Except, without the other sensory channels you would be missing the weight and shape of the hammer in your hand, the pressure of the hammer against your skin as you grasp it, and the force of the blow stopping your arm and hand. Without these cues, the virtual hammer wouldn’t feel much like a real one despite the accurate vibrations.
Having vibrotactile feedback without tactile or force feedback is like having icing without a cake. It’s still a valuable (and delicious!) component of a larger meal, but it’s ultimately not very satisfying on its own.
While we needed to develop novel technology to simulate tactile, force, and thermal feedback in HaptX, vibrotactile feedback is the one sensory channel for which high-quality hardware already exists. Currently, we integrate our HaptX with off-the-shelf vibration actuators, like the one found in your cellphone. These actuators produce high-fidelity vibration feedback in a compact and inexpensive form. The real challenge with vibrotactile feedback is on the software side.
We're developing software that simulates the complex physical interactions that produce haptic vibrations. By faithfully simulating these interactions, we can produce more accurate and realistic vibrotactile sensations.
Next post: things are going to heat up (and cool down) as we talk about thermal feedback. Check back soon for more!