We discovered in the first year of Grapheel’s existence that it’s actually quite tricky to move things forward and backward, especially if you have size constraints on the mechanism (called an actuator).

There are several types of actuator, but in the past refreshable braille displays most often¬†used a “piezoelectric actuator”. Piezoelectric crystals have an extremely interesting property in that they will build up an electric charge when subjected to mechanical stress. The more you squeeze, the bigger the charge!¬† What’s more, the reverse effect also applies, so if you apply a current to these crystals, they will expand by a very small amount.

You can see why they’re used as actuators; just attach this crystal to something, pass some current through it and watch it move backwards and forwards, right?! Unfortunately it’s not quite that simple; these crystals expand by such a tiny amount that a lever is required to magnify the movement, and these levers take up a lot of space.

A diagram showing how using a lever allows the very small movement of the piezoelectric crystal to be magnified into a much larger movement
Magnifying piezoelectric expansion [Made by Piezotechnics GmbH]
This is the reason that braille displays have always been so bulky, they need to have a long lever to get any noticeable movement at all. Not only that, but piezoelectric crystals are very expensive, causing braille displays to cost many thousands of pounds.

But we’re in 2017 now! So called “smart materials” are becoming more common and much easier to produce. We’ve recently come across a particularly interesting one developed by Professor Hod Lipson and his team at Columbia University. In a recently published paper and news article, a cheap and easy to produce “soft actuator” is described as a new way to build resilient robots with similar muscle systems to the human body.

The way it works is actually very simple; the actuators are made of ethanol suspended in a silicon mix and so when current is passed through a resistive wire inside the actuator, the wire heats up, the ethanol evaporates and causes the silicon mix to expand. When you stop putting current through the wire, the ethanol condenses again and the silicon returns to its original shape.

Professor Lipson has kindly made a tutorial video on how to produce the actuators, so we will be attempting to develop these into something useful for future tactile graphics displays.

Soft and squishy: The development of silicon based actuators
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