fingers

High Impedance, Warm Heart?

My touch screen phone hates my fingers; I have to jab at the screen multiple times before it acknowledges my presence. Here is my quest to find out why my digits are not touch technology friendly.

How do mobile phone touch screens work?

Most modern touch screen phones work using projective mutual capacitance. This means they use capacitors. In order to get my head round this I have had to go back to basics and re-learn everything I ever forgot about electricity. Hopefully I will be able to explain it to you using my amazing circuit diagrams…

What is a capacitor?

A capacitor is a material that does not conduct electrical charge (an insulator) sandwiched between two materials that do conduct electrical charge. It is probably easiest to understand using the model of a parallel plate capacitor.

Here is a circuit diagram showing a battery connected to two paralell metal plates (the parallel plate capacitor).

Capacitor 1

If we close the switch, electrons will move round to the bottom plate…

Capacitor 2

This will make the bottom plate negatively charged. This negative charge will then repel the electrons in the top plate and cause them to move away.

Capacitor 3

The top plate now has fewer electrons than the bottom plate and therefore has a positive charge. Electrons cannot move between the plates because there is an insulating material between them. This difference in charge between the two plates generates an electric field (seen here in green).

Capacitor 4

Most modern mobile phones use two layers of electrodes separated by a non conductive material underneath a layer of cover glass. These electrode layers are either arranged as two perpendicular rows to form a grid or as an interlocking diamond pattern. At each point where the electrodes cross a capacitor is formed because the two electrodes are separated by the non conductive material. This generates a small electric field. Human fingers conduct electricity so when you touch the screen you form a bridge between the electrodes which means that they no longer function as a capacitor and their electric field is disrupted. This electric field disruption is then detected by the phones microcontroller and used to plot the location of your finger. The really interesting thing is that using current technology you could make a touch screen which detects even more subtle disruptions in the electric field. This would mean that you wouldn’t even need to touch the screen to trigger it, you would just need to bring your finger near to the screen.

Flashgun

Why doesn’t my finger work?

The ability of fingers to conduct electricity is the basis for modern touch screen technology therefore my fingers must be poor conductors. The conductivity of my fingers will depend on the conductivity of the individual tissues within them (bone, tendon, muscle) as well as the electrolytes in my blood stream and the conductivity of my skin. Skin conductivity is something that has been well studied over the years and is the basis for lie detector tests. The galvanic skin response is the change in conductivity of the skin in response to certain stimuli. Activation of the sympathetic nervous system leads to sweat production in the eccrine glands in the skin; this lowers the skin’s resistance meaning it conducts electricity more easily. This is the change that is picked up when you try to lie during a lie detector test. Maybe my poor skin conductivity means I am immune to lie detectors?

Capacitive fingerprinting

At the moment touch screens can detect multiple fingers but cannot tell who these fingers belong to. This means that if you want to play a game against someone on the same touchscreen the programme will not be able to distinguish between you and your opponents’ fingers. Disney’s technology research laboratory have come up with a novel solution to this by developing a touch screen that measures the impedance of individual people. This is known as capacitive fingerprinting.

fingerprint

When a voltage is applied to a material, impedance is the measure of how much the material opposes the current passing through it. Disney’s device works by passing a small electrical current through a person and using this to determine their unique impedance profile. This impedance profile is then used to differentiate them from other users. A persons’ individual impedance is affected by a whole variety of factors including blood volume, amount of fat, muscle mass and bone density. This gives everyone a unique signature. The electrical current passes all the way through a person to the ground so the impedance profile is also affected by how well a person is grounded. Therefore a persons’ signature will change according to what shoes they are wearing and how they are sitting.

Unfortunately capacitive fingerprinting cannot permanently identify you based on your impedance. Not only do your posture and clothing affect your impedance profile but also your impedance actually changes throughout the day. This is probably due to changes in things like blood pressure and hydration status.This means that the touch screen can only differentiate between individuals, it cannot identify them.

Capacitive fingerprinting also currently does not work on projective capacitance screens because the conductive electrodes interfere with the small current used to detect impedance. Disney’s prototype uses an infra-red based touch screen, however they believe with more integrated hardware it should be possible to develop a projected capacitive screen with both touch and impedance sensing.

The simplest solutions are always the best

Anyway, all this talk of conductivity has allowed me to come up with a novel solution to increase my fingers’ conductivity. I call it the “foil finger hat” and although it is not stylish it does make my touch screen phone much more responsive to my poorly conductive digits…

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Comments
One Response to “High Impedance, Warm Heart?”
  1. Twitter23 says:

    Hi, just wanted to say i liked this article. it was practical. keep on posting.

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