10. Gating the track

Sweepnik follows a track by finding the angle at which a one millimetre long line of light most closely matches that track. When two paths cross over there will be a measured 'match' to the line at more than one angle. Sweepnik sees the path it is following, but also measures a good fit on the path which is crossing it. To avoid Sweepnik being distracted by this crossing path, the line that Sweepnik follows has to fall within a 'gate'.

The gate is a range of angles automatically set by Sweepnik. When Sweepnik is first set on a track the gate is 180 degrees, meaning that the direction the track is going in has to fall within the gate. Upon finding a track this gate is reduced to 45 degrees, and when Sweepnik has followed it for a few steps the gate is further reduced to 22.5 degrees. Sweepnik will ignore paths that fall outside its gate. It is unlikely that another track will cross the track Sweepnik is following at a sharp enough angle to fall within the gate.

If two paths do cross at such a tight angle, Sweepnik moves by 'dead reckoning', moving at a constant curvature until one of the paths leaves the gate. It will then continue to follow the remaining path. This means Sweepnik rarely 'loses' its correct track, as most tracks in the bubble chamber photographs are either straight lines or spiral tracks of nearly constant curvature.

If Sweepnik does makes a mistake the operator can press a 'reverse' button to go back to the error, and then manually tell Sweepnik which path to follow. If Sweepnik cannot find a path to follow it asks the operator for instructions.

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