How does Multitouch work? By. Branch Education
Have you ever thought of your smartphone’s multitouch screen
as functionally similar to your body’s skin?
Stay with me here: both are able to sense multiple touches,
and then both are able to make decisions and act on those inputs.
We take for granted being able to feel and sense the environment around us
but translating these senses to electronic devices and
building something like multi touch is no trivial task.
In this branch episode we are going to discuss how engineers
created screens that could discern multiple touches in smartphones.
This episode is a branch of the episode how does a touchscreen display work?
which goes into the physics and basic structure behind projected capacitive touchscreens
as well as OLED displays and toughened glass,
so I recommend you take a look if you haven’t seen it.
*Intro Music*
Ok, so let's dive in!
This touch screen is made of 40 rows colored in blue
and 80 columns colored in yellow
The result is a grid with over 3 thousand intersections,
and your phone can sense a touch at each of those intersections.
We’ll get into how it does that shortly, but for now-
when you type out a message, your phone detects a pattern that looks like this.
When you swipe right, your phone detects this, and multi touch zooms look like this.
Even if you have large fingers, your smartphone can find the center of your touch.
It can see the size, shape and location of the tap or gesture
and then calculates the X, and Y coordinates. It also determines whether you left your finger
or fingers on the screen and if you swiped in any particular direction.
Let’s quickly review how the screen detects a touch at one intersection.
Electrons with a negative charge are applied to the blue plate
which builds an electric field and causes positive charges to gather on the yellow plate.
When a conductive material like your finger tip comes close,
it interrupts the electric field and the change in positive charge on the yellow plate is measured.
You might be wondering how does my touchscreen work through the glass and screen protector.
Well, this type of touchscreen is called a projected capacitive touchscreen
because the electric field rises above the surface of the glass,
so the touchscreen will work through protective covers
as well as thin plastic or rubber gloves. Test it out!
grab a few plastic bags or saran wrap
and see how many layers of plastic your touchscreen can sense through.
Before we continue, let's simplify the grid from thirty two hundred intersections
down to two hundred intersections.
Everything conceptually works the same, it's just easier to visualize.
Ok- so with all the rows charged negatively and the electric fields built up,
and the columns actively measuring to see changes in the electric field,
we have a touchscreen, right? Well not exactly.
See, if we continuously measure each of these columns
while all the rows of electric fields are active, we run into a few issues.
Say I tap here. Well, because the entire yellow column is connected,
it can’t distinguish whether there was a tap here, or here.
This is not a good touch screen. We have to measure the entire column simultaneous,
because if we break up the column and measure individual diamonds,
we will end up measuring thirty-two hundred points, and that is too many points.
so then how do we get it to work properly?
Well, the solution that engineers implemented was to scan or
sweep the electrid field along the rows veerrryy quickly
- at a rate of about 10 microseconds per row. At any given time only one row is on,
thus only one row of intersections can detect a touch at a time.
Circuitry in the smartphone controls this sweep of the electric field across each of the rows,
and at every row a measurement along each column is recorded.
Before the electric field moves to the next row, each column resets its measurement.
The smartphone then correlates when the row was active
with the output of each column in order to reconstruct a complete grid.
This scanning happens within a couple millisconds
and it allows for the smartphone to distinguis between 3 or more touches
in all different locations. The drawback is that a new time delay is introduced
and if the touchscreen gets too big, say the size of a small table,
there would be a rather noticeable delay from the scanning across so many rows.
However, with a touchscreen the size of your smartphone,
it can quickly determine all types of touches and gestures.
That pretty much sums it up for multi touch in your smartphone
There are many other types of touchscreens like resistive, or self-capacitive touchscreens
to be detailed in other episodes. This episode is related to the structure of a touchscreen display
as well as electric fields, and capacitors. And how to measure capacitance
Thanks for watching! Don’t forget to subscribe, comment with your questions and thoughts,
and tell your friends and family about something you learned today.
And remember to think about conceptual simplicity and structural complexity.
Thanks!