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Log 2: Overall electronics design

A project log for Squidpad

Replace lots of stick notes and scraps of paper around the house with one Slim Quality Uncostly Intuitive Drawing Pad - [WIP]

arthur-admiraalArthur Admiraal 08/11/2015 at 20:030 Comments

Before I actually started to ponder the nitty-gritty details of the hardware and software design for the project, I thought up some overall solutions to the problems.

First and foremost, accurate localisation of the tip of a stylus without detection of hands is arguably the most important part of this project. When looking over Wikipedia’s list of touchscreen technologies, I couldn’t really find anything suitable. All technologies would detect either specifically human hands, or all objects that touch their surface.

So, I tried to think up something better. Eventually, I figured that a system using trilateration could be used to determine the position. If you haven’t heard about trilateration before, it is the calculation that is used to localize GPS modules.---------- more ---------

It works like this: you have a number of base stations with a known location. The number depends on the number of axis on which you want to localize. Now, you measure your distance to these stations. You don’t use a long piece of measuring tape of course, but rather you can calculate your distance to a base station by measuring the propagation delay.

You see, the speed of sound or light through a certain medium is mostly constant. So if we give both the device that needs to be localized and the base station synched clocks, we can send a signal to a base station, and then time how long it takes the signal to reach it. We can then just calculate the distance between the objects using a simple formula, which may look familiar if you have done any physics at a high school level or higher:

In which d is the distance between the objects in m, v is the speed of the signal through the medium in m/s and t is the time it took the signal to reach the base station in s.

So now we’ve gotten a few distances. So what? Well, we can use these distances to construct imaginary circles around our base stations (or spheres if you’re doing this in 3D), like in the picture below.

The device is at the intersection of the circles. That is how trilateration works in a nutshell. The more you know.

So my idea is to put a small ultrasound transducer in the stylus, which will vibrate the tip. It must be ultrasound so that the device won’t annoy people too much. Now if the stylus is placed on some medium – I guess it has to be glass since you need to be able to see a screen under it – the sound wave will be coupled in to it. If I place some ultrasound microphones around the perimeter of the glass plate, I can trilaterate the position of the source of the ultrasound signal from the data I get from them. They’re basically the base stations in the above example.

Of course when I googled my idea, I found that it had existed for a long time and that currently versions are being integrated in tablets. This is also when I found out about drawing tablets, and with them the technology that enables them. But that didn’t stop me from pursuing this design, as it seems like it is the easiest to implement of all the drawing tablet designs.

There are two catches with this design. Firstly, you need to know the time at which the stylus emits the signal. Secondly, the glass plate needs to be really thin, or else you’ll need to do more complicated 3D calculations.

Normally I would’ve implemented an rf communications link from the squidpad to the stylus, on which a signal could be sent to let the stylus emit an ultrasound signal. Because the speed of light is much higher then the speed of sound, any latency on the signal would be negligible in the calculation. But doing this would necessitate some rf design, which I’ve not done yet. It would make everything quite complicated. Instead, I think it is possible to let the stylus periodically emit the signal, and then calculate the exact time at which it was emitted.

You may know that to solve a system of equitations, you need to have as much equitations as variables that you want to solve for. Since the stylus only needs to be located on two axis, two distance measurements are needed to locate it. However, the time of the ultrasound signal emission must also be calculated, and thus adds a third variable. Because of this a third distance measurement is needed to get the third equitation needed. This is probably nothing new either, since something like the shot spotter system would need to perform these types of calculations.

Quick note here: in the figure above I used drew 3 circles. That's because having only two distance measurements results in getting two answers. Mathematicians still consider this finite set of solutions a solution. Normally, this would mean that we needed to add an extra microphone, but we know a range in which the coordinates must fall, so we won't need it.

A good source for strong, high quality thin glass can actually be found in screen protectors. The thinner they are, the less noticeable they are for the users, and the high quality models are usually made from gorilla glass to prevent scratches. I think that I could easily laser cut them to the correct size.

The system needs to be easily available to start drawing on any moment. Because of this, it needs to have a long battery life, and a really good sleep mode from which it can be awaken at any moment. Also, it would be nice to have the screen be a little more pleasant than a regular TFT LCD screen. An e-ink display can solve both of those issues. Not only is it the lowest power display technology currently available (at least that I know off, there might be some specific cases where it isn’t), but also looks a lot like real paper. All the half-decent e-readers have it. It is the perfect fit for this project. The only downside is that I have no idea where to get one, or how to drive one if I even get one. To Google!

Okay, it turns out to be easier then I thought. Petteri Aimonen of Essential scrap has reverse engineered the ED060SC4 display than can be bought of off e-bay for around $20. The coolest thing is that he has documented the entire interface, allowing me to design the display into this project. The ED060SC4 has just the right size and the right prize for this type of device. I will definitely use it as for the display. There is one catch though. (Again!?)

The big downside of e-ink displays is their low update rate. Loading a full frame takes a long time. However, I don’t need to load a full frame. I just have to add a few pixels every time the user draws a line, as e-ink displays keep their contents (they call this a bi-stable display). I should be able to do that quick enough to not form a nuisance to the user.

Oh, and the e-ink display isn’t a fancy coloured one, but I don’t think color is really needed for simple notes. It might be possible to implement some greyscale grading though.

And since this system needs to be able to be used detached from a computer, it needs some sort of battery. The only acceptable choice for a product like this is a rechargeable one, for obvious reasons. I’m going with a Lithium battery, because of the high energy density and somewhat low price point.

If a user can make drawings on the go, they need to be saved somewhere. I think the easiest way to add a lot of memory is to put a micro-SD card socket on the PCB. The SPI interface is easy to use, the memory density is really high and high capacity micro-SD cards are quite cheap these days. This way a user could also swap it out if he/she wanted more memory, or if it ran out of space.

Then that leaves the recharging and the interface to the computer. These two things could either be done wireless, or wired. I don’t see a huge advantage in being able to constantly stream data to the computer without a wired connection, as drawing could be done on the go, so whenever connectivity with the computer is required, the user would probably be right in front of it to see the screen. Then a wireless connection will only suck the energy out of the poor battery, so there should be a wired connection for communication. The most suitable protocol for this is USB, and if that’s plugged in, why not also charge the device with it? A USB port it is then.

Those are the overall design decisions I made regarding the electronics; I think I will post about overall design decisions for the mechanical side of the project tomorrow.

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