Surfaces and their Measurements
This book is primarily intended to help designers and inspectors to understand the fundamentals of surface metrology and, where appropriate, point to relevant procedures for specification.
TABLE OF CONTENTS Surfaces and their Measurements
3.6: Comments on digital areal analysis
3.6 Comments on digital areal analysis
There is another consideration to take into account when discussing the use of areal parameters in place of profile parameters. This is digital analysis. Some of the practical differences are shown below. Most important is the digital representation of summits (the areal equivalent of a peak).
Figure 3.47 shows various sampling patterns to describe a summit. Obviously, they are not the same as each other and they are not the same as defining a peak on the profile: there is much more flexibility, which can cause problems. The areal flexibility can at the same time be an aid to understanding or means of generating confusion.
Figure 3.47: Digital forms of summit appearance
Wavelet filters (space frequency)
The fundamental idea is to analyse the waveform according to scale (or resolution).
It is equivalent to an octave band filter bank, each one using the same shape 'wavelet' - but each of different scale.
Originally, the wavelet was a Gaussian pulse but now all shapes can be used. The difficulty is to make the system orthogonal i.e. each band-independent.
There is potential for using the wavelet method with fractals, as both use different scales.
The actual identification of a summit depends on how it is defined. There are a number of ways the summit can appear in digital form (Figure 3.47).
| (a) | Triagonal | 3 points at 120 must be lower than the centre one | |
| (b) | Rectangular | b(l) | 4 points surrounding centre one |
| b(2) | 8 points surrounding... |
Copyright Taylor Hobson Ltd. 2002 under license agreement with Books24x7
