polled at a rate of 20 Hz, the total delay time is 0.25 seconds between acquisition of the

stair data and detection of a stair inflection.

 The stair counter assembly also has an "end of world" capability, meaning that it can

be used for detection of drop-offs and ledges. The system uses this function to determine

the edge of a landing when descending a set of stairs. The edge is detected by monitoring

the output of the infrared sensors: the distances measured will remain constant until the

sensor reaches a point when it is directly over the first step. At this point, there will be a

drastic change in the measured distance, which the system interprets as the edge of the

landing and the beginning of the stairs.



 1500



 DWT
 Andros /1 n
 / \ // Sensor
 Chassis \--o
 Sensor
 Cable



 -- - 650



 Auxiliary
 Track .
 \ T- \ _Auxiliary
 S/ Track
 Assembly



 00
Figure 2-10: Auxiliary track position system setup

2.3.2 Auxiliary Track Positioning System

 A Draw-Wire Transducer (DWT) sensor, manufactured by UniMeasure, is used to

measure the angle of the Andros auxiliary tracks, one each for the front and rear tracks.