Wandering thinkpoint is disappeared?!

[fefrie]

In doing a couple of part exchanges, I switched the keyboards of 2 t42 thinkpads.

I've just noticed, that I don't have to fight the thinkpoint anymore. So it is a hardware based problem! Or crumb or something like that, but the important thing I'm glad for is that it's mostly gone!

[flipteo]

Reading your post, I'm realizing that my wandering trackpoint/cursor issues have gone away recently as well. The only upgrades I've done recently are RAM, and (what it must be), a new trackpoint cap. With my old, extremely worn and dried out cap, I had to practically put my body weight into it while moving it, even with the sensitivity turned way up. They must not like that.

Did the act of you swapping keyboards result in you having a new(er) trackpoint cap?

[fefrie]

I kept the old trackpoint because it's a soft rim, but what I notice is that there can be tiny amounts of crud in the part that's attached to the keyboard. I've scraped it out and vacumed it, and it wanders even less now.

When it wants to go on it's own trip it's a very short one now whereas before it was a real fight before I let the trackpoint go, and then it would go for a long long ride to the side of the screen where it will still migrate up the side of the screen.

Now, not so much. Alot better in fact.

[yak]

Perhaps it would be good to explain the cause for wandering.

The TrackPoint sensor always reads some force, even if not touched. To detect when the TrackPoint is actually pushed by a finger, the driver looks at fluctuations of the applied force. If the TrackPoint is not touched, the force will stay constant and the driver assumes the TrackPoint is in idle position. It aligns itself to this constant force and if it detects any deviations, it moves the pointer. If the new force value remains constant for a second or so, the driver aligns to this new position (and stops the pointer).

You can see this algorithm by applying constant force to the TrackPoint. The pointer will start moving but will soon stop. The driver assumes the constant force means the TP is not touched so the pointer shouldn't be moving. Now if you release it, the force suddenly changes so the pointer starts moving again (with your finger lifted) but after a second or so, it will notice the constant force again and realign.

This explains why having crumbs under the cap causes the pointer to wander. The crumbs prevent the TrackPoint from returning to the same position after you lift your finger. It takes about a second for the driver to realign to this new position and stop the pointer. But after you touch it, the force changes again and the pointer wanders some more.

The effect is most commonly seen when resting a finger on the TrackPoint. Sometimes the finger will apply some minimal but constant force causing the driver to align to it.