08/21/2014 at 06:42 •
As it turns out, making a stable USB link is not as easy as it might look. I would like to emulate a mouse with the pic18f4550, I am still seachring how ! In the mean time, I am using the HID library in Mikroc for Pic which simplifies everything !
Actually, I am able to send data through the usb port ... getting in the right track !
08/19/2014 at 07:43 •
What happens when you touch the surface ?
- The mechanical energy propagates inside the material
How can the piezo find out where the finger landed ?
- Using the time of flight :
The energy wave arrives at the different sensors at time t1, t2 and t3.
The only problem is that you cannot determine t1 as there is no t=0s origin.
If there was a way to find t1, you would only need two piezo.
The only information we can find are the time differences. But that's more than enough because every time difference describes a hyperbola which goes through the point where the energy wave came from. The intersection between the 3 hyperbolas that can be drawn gives you an unique point : the origin of the energy wave.
08/19/2014 at 07:19 •
The signal comming from a piezo is really uneven. First of all in order to be able to compare the signals, they need to have some similarities. I
found that the signal is almost a sinewave (with decreasing amplitude). Here is a screenshot of my (little) oscilloscope :
My idea was to rectify the signal so that I only have to care about amplitude.
I used a rectifying bridge (Gretz Bridge) in order to do that.
What a noisy signal !
After passing it through an amplifier and an RS latch :
Now we are talking !
08/19/2014 at 07:07 •
In order to calculate where the finger touched the surface, I needed a powerful enough microcontroller. It also had to support USB in order to be able to communicate to a computer. I am planning to emulate an HID mouse with the microcontroller. I had a PIC18F4550 lying around with seems to fit to the requirements : it supports USB and has plenty of RAM !
But I cannot connect the piezos directly to the microcontroller, I first need to process the signals and make them a little more readable !
07/21/2014 at 07:50 •
Finaly the maths behind everything are done. Electronics are also done but I am planning to change the whole thing, the second version is in progress.