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Signal Processing Jeremy Wyatt Intelligent Robotics School of Computer Science.

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1 Signal Processing Jeremy Wyatt Intelligent Robotics School of Computer Science

2 Sonar Sound wave transmitter/receiver Sound wave bounces back off objects Counter measures time of flight Calculates the distance assuming speed of sound is ~335ms -1 Errors due to specular reflection

3 Typical data We gathered 100 samples from a sonar in three positions relative to a wall You get a figure for time of flight (T) in units of 0.5 microseconds To convert to metres Metres per millisecond 0.5  secs in each millisecond Sound travels twice the distance to the object

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6 Median and mean Order the values by size take the central value, here 1.425 mean is ~1.458 median is more robust to outliers 1.41, 1.38, 1.71, 1.45, 1.42, 1.39, 1.43, 1.47 1.38, 1.39, 1.41, 1.42, 1.43, 1.45, 1.47, 1.71

7 Mapping We took 25 sonar readings each at 100 equally spaced angles between 0 and 360 degrees

8 Plot range versus angle Range in metres Angle in degrees

9 Convert to Cartesian frame x y r 

10 r r  (x,y)

11 Finding Walls Assume walls are straight lines Each point is on many possible lines Each line can be described by an equation

12  w

13 Hough Transform Create  and w for all possible lines Create an array A indexed by  and w for each point (x,y) for each angle  w = x*cos(  )+ y*sin(  ) A[ ,w] = A[ ,w]+1 end where A > Threshold return a line

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18 Summary Use median Hough finds lines But because of width of sonar beam gives bad results May be better with Infra Red Smooth in Hough Space Use property of sonar

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