Tuesday, January 26, 2016

Restrictive Capacitive Touch Screen Guide

Recently at work we received MC40 mobile computers to perform inventory tasks.  Just one problem.  The interface was designed by Lucifer himself.  The image below shows the problem.  The on screen keyboard is located in the red area at the bottom of the screen and the scan button to read bar codes is shown as well.  Repeatedly moving your thumb back and forth between the two areas causes pain in the first joint of the thumb.  Ideally the keyboard should be at the top of the screen in the green area.  That way the thumb can move across without having to bend.

All of that wouldn't be too bad if it wasn't for the other issue, the custom on screen keyboard can be swiped to the left or right to get a full keyboard.  So when typing with your thumb bent back in an awkward position, if you move even slightly to the left or right while contacting the screen the keyboard starts to move left or right and the key press isn't registered.  The units the MC40 replaced were old, but they had physical buttons with audio feedback.  This made it easy type without looking at the keyboard.  The MC40 forces you to touch type like someone who's never seen a keyboard before.  If there was only one lesson to learn from technology over the last 10 years it's what Apple taught us, study how users interact with the product.  Put more than 10 seconds of thought into things.  I've partly already solved the problem by creating bar codes that automate common function that I used to perform on the keyboard.  At an estimate I've cut my key presses by about 95%.  Can we do more though?

handheld computer
MC40 layout
What's needed is a physical object to give tactile feedback of the button locations and limit the movement of the thumb when it touches the screen.  It's a capacitive touch screen so as long as I use a non conductive material I should be fine.  So what I plan to do is 3D print a guide that goes over the screen and is held in place by rubber bands.  It's a prototype just to prove a concept,  I could have had a PCB with holes made to do the same thing, but I want to try 3D printing.  It also allows me to add guides to hold the rubber bands. and round some of the edges.  As this part is designed to be touched, it can't have edges that will wear away at the skin.

On screen keyboard
On screen short cut keyboard
I quickly realised that Sketchup wasn't up to the job.  I settled on FreeCad and I'm happy with it.  There was a week of swearing and watching video tutorials but I got up to speed after I figured out how parametric modelling works.  (Another thing to add to resume)  The result is below.

3D model
Touch-screen guide
When placed over the screen you can see how it's meant to work.  Two rubber bands that run between the guides at the top and bottom of the part hold it in place.  It allows you to feel where the buttons are without looking and it restricts the movement of the thumb so the keyboard cant start swiping to the left or right.  Imagine the hole in the end of a ruler, when pressing it with a thumb the skin protrudes the most in the middle.  This will be the contact point.  I'm not sure that I have the geometry right though.  If the holes are too big the structure will become weak, if they're too small the thumb won't fit through.  If the part is too thin it will break, if it's too thick the thumb won't touch the screen.  You can see that it will be sensitive to geometry.  Consider this a minimum viable product.  Once I have it I'll be able to make some informed iterations.

3D model
Guide in place over screen
To see how the 3D printing would be completed I installed Repetier Host and sliced the model as a test.  I don't have a printer, a friend does, so I want to make sure I sort out any problems to make things run smoothly.

3D model
Sliced model
I then came up with another crazy idea.  What if I were to make a negative of the design and use it as a mould?  I could make rubber version of the design to prevent it slipping around the screen.  I'm no polymer expert, but some silicone RTV for gaskets from a local automotive store should do the trick for a prototype.  Just a thought.

3D model
Mould negative

I may appear to be down on the MC40, but the scan engine in that thing is a beast.  It's a newer (2012, that's new compared to most of our equipment) camera based design and it can read bar codes so fast you're not even sure you fully pressed the button.  The images are processed so quickly I'm 99% sure the algorithms it uses are in custom silicon.  I'd love to see how it works.

This isn't a project that needs to be done and 3D printing may not be the way to go, but at least I'll have some experience with 3D modelling and printing.  Excited.

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