Tracker ball rolls on tip of fingers

Tracker ball rolls on tip of fingersSteve Bush
A solid-state ‘tracker ball’ has been demonstrated by a team from the University of Bologna.
It is a chip, 3.8 x 3.8mm, whose surface is stroked by the user’s finger tip. The chip resolves the direction of stroking in two axes and develops a signal like that from a mouse or tracker ball.  
 
Actual detection is made by 49 on-chip cross-shaped sensors, each with five 65 x 65?m pixels.
The size of the sensors is deliberately set to be comparable with the ridges and grooves of fingerprints. Each pixel capacitively detects the transition from groove to ridge as the fingerprint moves over the surface of the chip. A transition over the centre pixel of any sensor triggers the sensors own state machine. This then records over which of the four surrounding pixels the transition leaves, determining stroke direction.
Sensor electronics adjusts for common-mode variations due to skin condition by setting a pixel trigger threshold which is the average of all pixel capacitance, plus a noise-rejecting offset.
Multiple sensors allow the effects of skin damage, scaring and skin elasticity to be rejected. An on-chip ‘motion processor’ does the decision making.
Passivation on the surface of the chip is said to reliably protect it from non-abusive use and the delicate bond wires of the prototype are protected with epoxy.
Initial results show the output is somewhat noisy, as you would expect with a sensor detecting 65?m movements. This noise however is amenable to filtering and the device is easily usable to control a pointing cursor.
Power in the 0.7?m CMOS prototype was 900?W at 5V.
Applications are foreseen in palmtops and credit-card sized computers.


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