CAPÍTULO 2. MATERIALES Y MÉTODOS
2.3 Propuestas de procedimientos para localizar aproximadamente puntos de fuga
3.4.1 Briefing on stylus
The contact-probe/stylus instruments have been widely used in the surface metrology field. The first diamond stylus of 90opyramid shape was developed in the UK which obtained two dimensions 2μm and 7μm at the tips (Whitehouse,
2002).
This type of stylus worked well on surfaces with an appreciable lay. The US subsequently invented a diamond cone which had an angle of 60° (Whitehouse, 1997).
Figure 3.6: Stylus shapes: cone (left), pyramid (right)
As Figure 3.6 presents: the two shapes of stylus are those of cone and pyramid. Both of these stylus tip shapes have been suitable for a great many jobs. They are able to report accurately on the structure of fairly coarse structures with ‘open’
texture, however, they are not suited to the smaller scale texture more much more commonly found in industry. So the stylus slope has to be considered, as regards which the tip can be infinitely sharp so that the stylus can go into the valleys to get more precise measurement results.
3.4.2 Noise and Vibration
Noise and vibration can be one of the major sources of interference for doing the contact-probe/stylus measurement on rough surfaces. This can be caused by inherent instrument noise, environment or by sound. For instance, when the contact-probe through the roughness surface caused some vibration and electrical noise in the pickup line. The most typical noise source is sinusoidal which has a range from 50Hz to 60Hz inclusive, short noise peaks (Mainsha, 2001). Noise contribution can be formally written as:
(3.8)
nx, nynzare the noise components,x,yandzpresented the directions and t indicates
time. The first two noise components would influence length and height measurement on x and y directions. And nzaffects the height measurement on z
direction and also provides an indirect influence on determining the length. Value
nz will be considered and added into this project topic as affecting the height
measurements. In sum, noise is clearly a critical major effect especially when measuring small dimensions, because the dimension of the object being measured is closer to the noise level.
3.4.3 Stylus errors and distortion
Despite the points discussed previously, stylus methods still retain a most significant role in surface metrology because of the more precise measuring and the fact that they are relatively robust against dirt compared with optical methods (non-contact ways).The probe stylus is one of the most critical and pivotal parts in surface metrology (Mainsha, 2001). The probe has to be sharper than the perturbation being measured. If a suitable probe is not selected for the rough surface measuring, damage on the surface and on the probe itself might occur. According to the work done by Church and Takacs (1990) because of effects on tip size, smooth surfaces are likely to be reported as rough, while rough surfaces are smoothed.
The stylus method is sometimes criticized because of claims that it can damage the surface (Whitehouse, 2002). Whitehouse mentioned the drawbacks to the theories’ algorithms method on stylus techniques, for example possible distortion and stylus damage. He mentioned that there is a simple way to obtain a criterion for a stylus to achieve no surface damage. Whitehouse’s method was focused essentially on kinematic theory not on the material of the stylus. Damage index was the crucial factor in this theory in which it was defined that its value must be in the acceptable range of no damage occurring. Whitehouse pointed out, drawing on (Schmalz, 1936), which itself used the well-known Hertz contact theory, that a damage index
can be used:
Where H is the material’s hardness being measured, E is the elastic modulus, and R is the radius of the stylus’ tip, W is the normal force applied to the surface by the stylus, which is often taken as the basic (static) load from the instrument but should really include dynamic (inertial) effects as well. No damage should occur if < 1. During the period when Whitehouse’s research was mainly focused on the kinematics theory the researches on different materials of stylus were also in progress. To follow on from the previous work, because of internal springs and probe acceleration driven by the surface, forces tend to be larger near to the peaks. So far there is no useful guideline beyond the simple damage index.
Drawing on analogy between a ball contact (as in the E-system) and a stylus tip, it is obvious that any surface features characterised by a dimension smaller or similar to that of the tip will be distorted. Small valleys will be bridged, sharp peaks made to appear non-rounded and so on. The stylus acts as a highly non-linear filter of short-wavelength features.
(McCool, 1984) was one of the first to study theoretically the influence of stylus tip radius on surface roughness measurements. One simulation model had been developed by him that magnitude of distortion can be assessed as well as the effect of record sampling frequency and length with tracing surface profiles. However, these are realizations of random processes having an indicated spectrum and do not apply generally.
These have been plenty of other attempts at modelling stylus contact since McCool, but we still have little practical understanding of how to predict the level of distortion in general.
Several publications were proposed that the measured surface could be “corrected” for distortion based on knowledge of the tip size. Just as in work done by Schwarz (Schwarz, 1994), the suggestion on different techniques for in-situ characterization of probe was presented which can minimise the probe distortions (Whitehouse, 2000). While valid for a few special measurement conditions, it is at best dubious for general surface arrangement because (1) in the highly non-linear distortion measuring there will not be a single inverse operation and (2) in practice the tip size and shape are not known to good precision.