VibroMat Application and Research

We’ve recently been in contact with one of our customers at the National ICT Institute of Australia (NICTA) who have been using our coin vibration motors in their research.

The team is working on a project called VibroMat, which looks to present a “2D view of the world” using an array of vibration motors to users with visual impairment. In total 96 vibration motors are used to translate complex visual information into tactile information as a form of ‘sensory substitution’ – i.e. vibrations are used to replace visual information by varying the vibration amplitude. This is closely related to the Bionic Eye project, also from NICTA.

The “complex visual information” is actual levels of brightness, translated into grayscale. The more levels of grayscale available, the higher quality the image. Here’s an example below of an original colour image, the same image with 8-bit grayscale (256 levels), and again with 1-bit grayscale (2 levels):

Full Colour; 8 bit Grayscale; 1 bit Grayscale

Full Colour; 8-bit Grayscale; 1-bit Grayscale

More levels are better. It’s worth noting that the images above are at a much higher resolution than the matrix of motors in VibroMat, with a reduced resolution the drop in quality is even more pronounced. So a vibration motor that has more discernable levels of vibration is important for producing a high-quality equivalent.

As part of the project, the team have been conducting research tests into the suitability of coin vibration motors. In particular, the study is of great interest to us at Precision Microdrives for two main reasons:

The VibroMat Project

The VibroMat Project

  • We can quantify our motors very accurately with our advanced testing, but there is little information available about the perception of vibrations. Vibration amplitude may not be the only factor for how strong a vibration feels to the user. As we’ve discussed before, an increased frequency decreases the displacement – which may have a negative impact. This study addresses that issue by focusing on the perceived levels of vibration.
  • Again, we have good knowledge of the dynamic range of vibrations but how does that translate to the real world? For example, if a motor can range between 1.5 G and 3.1 G, how many different vibration intensities can the user determine? The study addresses this by measuring the number of discrete levels identified by users.

Coin motors were the focus of testing because they have several positives that make them suitable for the application. They are light and have no external moving parts, so they can be easily worn by a user. Also, they are inexpensive, easy to implement, and energy-efficient so the system is less complicated. The research team highlighted that this made them much more attractive to users because user-friendliness is critical to the adoption of a device.

A matrix of coin motors was worn by subjects on their lower back for two different experiments. The first experiment compared three Precision Microdrives coin motors to test for the motor’s number of ‘Just Noticeable Differences’ (JND) – which equals the number of grey levels that can be produced.

This was tested by pulsing the motors for 200ms at 256 different voltages, from 0 – 5 V in equal steps. 8mm, 10mm, and 12mm coin motors were used to offer a range in vibration frequency, their results were as follows (remember, more JND = more grey levels = better!):

Model 308-100 310-113 312-101
Diameter 8mm 10mm 12mm
Typical Normalised Amplitude 0.7 G 1.34 G 2.6 G
Rated Speed (Frequency) 12,000 RPM (200 Hz) 12,200 RPM (203 Hz) 12,500 RPM (208 Hz)
Median Number of JNDs 8 10 15

The second experiment used the 312-101 and tested different pulse lengths to determine if they affected the number of JND. The first experiment used only 200 ms pulses, now the 312-101 was also being tested with 100 ms and 50 ms pulses.

As you might expect, the longer the pulse the more JND could be picked out by users. Interestingly, the relationship was almost 1:1 – halving the pulse duration also halved the number of JND. Although, as noted in the research, extending the length of overdrive (only 1 ms was used) could improve the results for shorter pulses, which we would agree with.

The study (tentatively) concludes that coin motors are “potentially able to convey a larger number of discernable grey levels compared to state-of-the-art retinal implants”, and thanks to their cost and size benefits they are suitable candidates for tactile vision substitution.

We’re glad to hear our vibration motors have performed so well, but of course, we always knew they would!

For more information on the project, you can use the following links:

National ICT Institute of Australia

The VibroMat Project

The Bionic Eye Project

H. C. Stronks, D. J. Parker, J. Walker, P. Lieby, N. Barnes (2015) ‘The Feasibility of Coin Motors for Use in a Vibrotactile Display for the Blind’, Artificial Organs

Or email H. Christiaan Stronks directly at [email protected]