We should preface this blog post with the disclaimer that we are not a collection of biomedical engineers and that the details below are based on our own experience and our customer feedback!
Having said that, we are often asked what vibration frequency is best for haptic feedback devices. The enquiry is looking at which of our actuators operate at the frequency our skin is most receptive to. So, of course, we should begin with a discussion of where our skin is most sensitive.
The current general consensus is that there are 4 different sensory receptors in our skin that respond to mechanical pressure (mechanoreceptors). Each has different characteristics that make them better at detecting different modalities, we’ve listed them below:
|Mechanoreceptor||Best at Sensing (Modality)||Frequency Range|
|Merkel’s Cells||Pressure (slower movements)||0.4 – 100 Hz (5 – 15 Hz peak)|
|Ruffini Ending||Pressure (slower movements)||7 Hz|
|Meissner’s Corpuscle||Touch (faster movements), Vibration||10 – 200 Hz (10 – 50 peak)|
|Pacinian Corpuscle||Vibration||40 – 800 Hz (200 – 300 Hz peak)|
Vibration amplitude of an ERM is dependant upon the speed of the motor, the faster the motor the higher the amplitude. As our small vibration motors have a limited eccentric mass they use a higher speed to compensate, leading to an increased vibration frequency. Our slowest is the 324-102 which is rated at 2,800 RPM (46.7 Hz), you can see that it is the larger motors that tend to operate at lower speeds:
However, there are two large caveats to the above. Firstly, as the frequency increases the overall displacement caused by the motor is reduced. It is important to note that the figures above are a result of measuring action potentials from the skin (quantitative), not through asking subjects about what they felt (qualitative). This means the increased displacement (lower frequency) may feel stronger to the user despite their mechanoreceptors being more ‘active’ at a higher frequency.
Secondly, in our experience people are more sensitive to the vibration strength and are particularly poor at differentiating between frequencies.
Our design advice is to concentrate on achieving the right amplitude for your device and how to mount/drive the motor effectively. Once you have covered these aspects of your haptic feedback, if you still have options vibration frequency then use the information above to start some qualitative tests.