Pseudo-Frequency Modulation

When we slide our skin across a real textured surface, contact with surface asperities evoke vibrations in our skin.

These vibrations have a temporal frequency, which depends not only on the texture’s spatial frequency, but also on our sliding speed. These temporal changes in skin vibrations form the basis of our texture perception.

Extending this understanding of texture perception to virtual environments, vibration feedback provided by virtual textures should also respond dynamically to the user’s speed by proportionally changing the temporal frequency. 

However, implementing a continuous change in vibration frequency (Continuous Frequency Modulation or CFM) is not trivial. Wide-band vibration motors are large, expensive, and have a variable frequency response. Linear Resonant Actuators or LRAs are small and cheap, but narrowband.

Is there an approximation to CFM that is easier to implement? We found that a signal that morphs from one frequency to another, merely by amplitude modulation of the two components, creates a perception of continuous frequency change. We call this technique Pseudo-Frequency Modulation or PFM. The following figure shows an example of CFM and PFM temporal waveforms, along with their respective spectrograms.

This finding, in addition to our ongoing psychophysical research, points to a multi-LRA configuration as a viable path towards broadband textures.

Check out our paper, published in Eurohaptics 2024, to learn more about the technique, and its applicability in rendering of spatially constant haptic textures.