In many control applications, a trade-off exists between dynamic response and accuracy. This conflict arises in mouse manipulation with a graphical user interface. With the TrackPoint pointing device, a carefully designed transfer function is used to convert the user's pointing force into cursor motion. This transfer function embodies the semantics of pixel and character selection as well as human limits of eye-tracking and force control. But until recently this transfer function was completely static: it made no use of the dynamic information inherent in the forces applied to the TrackPoint.
The dynamics of cursor movement are important for fast and accurate manipulation. With static transfer functions, the movement is always somewhat sluggish. It takes a finite time for the user to apply and remove force from the TrackPoint. Therefore the cursor motion always starts and stops slowly, as if the cursor had inertia -- it takes time to get it moving and get it to stop again. Negative Inertia solves this problem by counteracting that inertia. This effect is accomplished by exaggerating changes in the input force.
When the negative inertia transfer function detects changes in the input force, it assumes that those changes are the beginning of a greater change. Therefore it makes larger changes than those being commanded. The result is a cursor which takes off and stops quickly, even backing-up slightly to correct for overshoot. These exaggerations are controlled by a nonlinear response which avoids "hunting" problems that can result from too-quick movements near a target.
Negative Inertia is a standard feature of the TrackPoint III pointing device which comes with the ThinkPad 755 series of notebook computers from IBM. If you would like to check your TrackPoint to see if it does have Negative Inertia, just throw the cursor against the side of the screen and you will see it bounce back a little. This bounce is a result of the fast stopping feature of Negative Inertia.
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Last updated: Thursday, 21-Jul-2011 15:25:03 PDT
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