Signal Processing Jeremy Wyatt Intelligent Robotics School of
Signal Processing Jeremy Wyatt Intelligent Robotics School of Computer Science
Sonar • • Sound wave transmitter/receiver Sound wave bounces back off objects Counter measures time of flight Calculates the distance assuming speed of sound is ~335 ms-1 • Errors due to specular reflection
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 msecs in each millisecond Sound travels twice the distance to the object
Median and mean • Order the values by size 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 • take the central value, here 1. 425 • mean is ~1. 458 • median is more robust to outliers
Mapping • We took 25 sonar readings each at 100 equally spaced angles between 0 and 360 degrees
Plot range versus angle Range in metres Angle in degrees
Convert to Cartesian frame x y r q
(x, y) r q r
Finding Walls • Assume walls are straight lines • Each point is on many possible lines • Each line can be described by an equation
w f
Hough Transform Create f and w for all possible lines Create an array A indexed by f and w for each point (x, y) for each angle f w = x*cos(f)+ y*sin(f) A[f, w] = A[f, w]+1 end where A > Threshold return a line
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
- Slides: 19