[Magic Mouse Teardown (4)] 'Never-seen-before' Electrode Structure
The lattice-like touch-sensor electrode of the Magic Mouse seems to be based on a capacitive detection method, which utilizes matrix to detect the location where capacity distribution is changed by an approaching finger.
The Magic Mouse currently detects up to two points at the same time, but it is technically possible to detects more than two points, said an engineer who is well-versed in touch panels.
Apple Inc has employed a capacitive detection method for touch panels that can detect finger movements, such as the one used for the iPhone. But the shape of the electrode formed on the flexible substrate for the Magic Mouse's touch sensor is exceptional. The electrode consists of 15 4.0 x 4.0mm electrode elements in the longitudinal direction and 10 elements of the same size in the horizontal direction. And they are arranged at intervals of about 4.5mm.
"I have never seen such a touch panel structure," an engineer said. "It looks like that the electrode elements for touch switches are arranged in a matrix. And it is totally different from the electrode structures of the iPhone and such."
When the touch sensor (flexible printed circuit board) is seen from the sensing side (the top side of the Magic Mouse), the electrode element looks like a combination of one H-shaped electrode and two E-shaped electrodes. When we measured the voltage applied to the electrode with a tester, it was as low as about 1.65V.
"Probably because the width of a finger is about 15mm, Apple arranged elements at intervals of 4.5mm so that the position of a finger can always be detected by three or four elements," an engineer said. "I think it aimed to ensure a high detection accuracy and enhance sensitivity at the same time by arranging blocks with a large electrode area at intervals."
The top cover of the Magic Mouse is about 2.45mm in thickness, which makes it difficult to use a touch sensor. That is why Apple employed such a method, he said.
For example, the company embedded a touch sensor in the Wireless Mighty Mouse, the predecessor of the Magic Mouse, to enable it to distinguish right and left clicks. The Wireless Mighty Mouse is equipped with a combination of jagged electrodes on each of the right and left frontal areas of the mouse.
In the case of the Wireless Mighty Mouse, it is possible to use electrodes with a wide area because they are used only to determine whether a finger is in the right or left area. In the case of the Magic Mouse, however, it is necessary to detect the accurate position and movement of a finger, and the electrode area of each element is smaller.