I want to begin with a question about the Xerox mouse. What sorts of problems did it have that needed to be solved to make a mouse that could be mass-producible mouse?
Yurchenco: Well, the Xerox mouse that we examined was, I suspect, one of a relatively small number of actual physical mice that had been made. It had a couple problems.
The first was the way in which the ball was actually mounted and tracked on the table. They had a relatively small ball, and it was installed in a gimballing mechanism that required some bearings and some high-precision parts. These added cost and complexity, and caused reliability problems. They were operating under the assumption that the ball had to be pushed down onto the table, but one of the things we discovered in our research-- and this became fundamental to the design of all mice-- was that the ball could be held in contact with the table by gravity. It doesn't need to push, it can float.
The second was that the Xerox mouse was doing its encoding by means of mechanical commutators: they had little wires that brushed onto a commutator similar to what you'd find on a motor. It was expensive to do this: you needed two sets of these commutators, there were potential wear problems, and certainly reliability problems, in an environment in which dirt could be easily picked up.
Now, you think, motors have commutators and they work fine. Well, yes, true, but the amount of friction in the system in a motor is much less important than the amount of friction of a mouse, where you just have a ball that's picking up rotational energy from contact with the table. So the commutators had to be very delicate, and very lightly forced. By going away from a mechanical commutator to an optoelectronic system, we eliminated almost all the friction in the mouse. That also made the manufacturing simpler and reduced the cost.
One of my experiences later on with mice, when I designed the Microsoft mouse, was that their supplier, Alps Electric, actually had developed a very low-friction, relatively low-cost commutator which they were using in their mice. Alps was a company that specialized in building potentiometers, and the technology they had and the knowledge they had in-house that allowed them to do this; but they were the only people I've ever seen since the Apple mouse came onto the marketplace that has gone away from the optoelectronic encoder and used a commutator for this type of mouse design.
But basically, everybody builds them with optoelectronic encoders: they're reliable, they're inexpensive, they're easy to clean, they have relatively loose tolerances, and they're very, very easy to manufacture.