PowerBook Trackpads: Finger-Tracking Good

You move your finger, it follows your command. If only your kids could respond as well as the trackpad on your PowerBook. In this small black fingerboard, Apple has built a more compact way of moving the cursor than the old trackball--not to mention one that's less susceptible to the whims of dust. As I move my finger around in the physical realm, my cursor follows suit inside the machine. Pretty cool.

So what causes the trackpad to exhibit such doglike fealty? A strong, authoritative finger? A devotion to discipline that stems from a strict upbringing? Does it just want to be loved?

Try a little technology called coupling capacitance.

Yes, I know--coupling capa-what? But if you've ever seen a car pull up to a traffic light, you've watched a similar principle at work.

Your trackpad has two layers of electrodes. Vertical electrode strips compose the top layer, and horizontal strips make up the bottom. These two layers constitute the coupling. Underneath them, an integrated circuit measures the capacitance from each of the horizontal electrodes to each of the vertical ones.

OK, so what's capacitance? It's a measurement of electrical charge--the ratio of stored charge in coulombs to the applied potential difference in volts.

Both your finger and the air around it are dielectric materials--in other words, both conduct electricity poorly and carry a constant charge. Since your finger has a different dielectric constant than air, the electrodes in the trackpad can tell the two apart.

As you move your finger around the trackpad, the differential between your finger and the air modifies the capacitance between the electrodes. In other words, the charge between the two layers changes as your finger moves over them.

Think of traffic lights and the sensors underneath the street that trigger them. When cars drive over the sensor, the charge below changes and sends a signal to the traffic light to switch. The change in charge as your finger moves over the trackpad lets the integrated circuit know exactly where your finger is. The circuit then relays that information to your operating system, which moves the cursor around your screen to follow your finger.

The same principle also explains why your cursor jumps around like Michael Flatley with a squirrel in his skivvies if any moisture gets on the trackpad. Moisture creates yet another differential charge. That confuses the electrodes, and the cursor darts back and forth to follow the water spots--kind of like your kids when you ask them to clean up their rooms.

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