Gear/Drive Train Design In the Audrey Braille Display

In a nutshell, the gear train in the Audrey Braille display is THE machine. Although the mechanics are simple – move a rack gear forward/backward to position, then move over to the next one, repeating until all are set – the details of this gear train are rather involved.

For example, why this particular gear design – and why so big?

The design of the display is an inverted rack and pinion gearing, with the racks moving over the gear, positioning the Braille half-characters under a detecting window. Since there are eight possible half character patterns (three dots either on or off, meaning two to the third power), we need to move as many as eight positions to go from one character to another. While that’s the maximum, fortunately, the average is four (and by positioning the primary characters near the center of the rod, we can improve further on that).

So since moving is important, I decided to use one gear tooth per character position, or eight teeth to move the entire rod. I could have used fractions of that (for example, two gears per character), but there were problems:

  • When more than one gear is used per tooth, positioning becomes an issue – is the ‘top’ position we detect for the correct position, or is it one off? When using a single tooth per character position, when that tooth is at top (12 o’clock or 0 degrees) then we have a character displayed – simple.
  • More teeth means smaller gears, which streamlines the device – however, it means more travel to reach each position. This actually slows down the device, rather than speeds it up.

This latter point is important; although this device was never meant to compete with piezoelectric devices with near-instant updates, it still needs respectable update times. And so what would they be?

The estimates I am working off of (provide by a local Blind techhead) are that most blind people read Braille anywhere from 30 to 120 words per minute, and at an upper limit, a few can read 150 words per minute. Despite Grade 2 Braille not being a straight character for character translation of text, I’m guesstimating that 150 words per minute translates into 750 characters per minute (assuming 6 chars/word on average, including spaces), or about 15 characters a second. For the Audrey display, with its half characters, that means a top speed to aim for is 30 rods updated a second!

So, one way to get these times is by using a big gear, with fewer teeth, and less turning needed to move the racks. I’m currently using a 20 tooth gear not just for speed, but also to match my stepper motors. Using 1.8 degree steppers, which have 200 positions/steps per revolution, I need to evenly divide the teeth into this number, or I risk being out of alignment, since a tooth always must end up at the topmost/0 degree position so that a character is lined up properly (try the problem with a 48 position stepper of 7.5 degree steps, and you’ll see the problem, and the need for a different tooth count like 16). And books I’ve read recommend at least 15 teeth per gear for performance – ergo, the 20 tooth gear was born.

Now, with the 20 tooth gear, on average, a rack/rod/pin change is 4 movements of each tooth – which is 4/20=1/5 of a revolution, or 72 degrees, or 40 steps. So to get that 30 updates a second, I’d need to move 30 times that, or six complete revolutions on average. Six per second ends up being 360rpm, pretty perky for a stepper motor! And that doesn’t include movement from side to side as it shifts to the next rack.

There are solutions to this problem:

  • Use larger gears. While my 20 tooth gear is a compromise, you can use more. Imagine a 200 tooth gear: now each step moves a character position, and you are moving 10 times faster in updates. However, a 200 tooth gear would be about 10 times larger, so now you’d be looking at a floor model device!
  • You could replace the solid gear with a flexible belt. But remember that the speed of updates is how fast the MOTOR turns. So the belt would only update as fast as the wheel driving it can turn. One advantage however is you can now place the wheel on its side with some careful twisting of the belt, allowing you to have a shorter display device, and larger (=faster) gear wheel.
  • Use multiple motors. One goal in my design is to make it possible to use multiple motors and gears in the display. Since parallel gears can update simultaneously this way, I effectively divide the work evenly between motors. In effect, four motors turns a 40 character display into a 10 character one, in terms of speed of updates.
  • Settle for slower. 150 words per minute is the upper limit, but don’t forget, the 30 words per minute lower limit is 20% of the maximum speed, equivalent to about 60rpm, or one revolution a second, and much more manageable. So while speed ‘readers’ will chomp at the bit at that speed, it will be fast enough to use.

The good news is that, with one or more of these options, I believe the Audrey Device can meet its goal. While it won’t have the speed of a $7,000 device, it won’t have the price tag either. Plus, being Open Source, others can take the device and improve it (including speeding it up). And I am confident that once the device gets “out there”, people WILL speed it up, proving the benefit of the Open Source model yet again.

3 thoughts on “Gear/Drive Train Design In the Audrey Braille Display

  1. I hate to tell you, but that max estimate is low. I’ve been a student at, and later done some work for, a blindness training center. We had students reading much higher. 150 is probably closer to the average. One of the highest reading rates I know of was around 129. If you need, contact me, and I can either find out, or put you in touch with people that can give you better estimates.

  2. Yesterday I was on a site that talked about teaching kids – and getting them up to about 200wpm before they left school! So yes, I guess my estimate was way too low. However, I still need a target to aim for, and right now I’m well under 150wpm, so I’m good for awhile – but if I ever reach 150, I’ll be sure not to rest on my laurels!