Tolerance band

It is often deemed that quality assurance serves prevention and quality control detection, but a control installed to detect failure before it occurs serves prevention, such as reducing the tolerance band to well within the specification limits. So quality control can prevent failure. Assurance is the result of an examination whereas control produces the result. Quality assurance does not change the product, quality control does. 

There are apparently customers who soldered on ceramic capacitors in their power supplies and found the clock was just too low. They figured the capacitance was above the guaranteed upper tolerance band (a rare event with commercial ceramics ), and shipped them right back to their manufacturers. But the problem was only that as soon as the PCBs went through the soldering process, age reset (or de-aging) occurred and so capacitance rose. If only they had waited for some more time, their clocks would have been right on However, I would have preferred SMD him capacitors if stability was so important. 

It should be stressed that LTS regression does not throw away a certain percentage of the data. Instead, it finds a majority lit, which can then be used to detect the actual outliers. The purpose is not to delete and forget the points outside the tolerance band, but to study the residual plot in order to find out more about the data. For instance, we notice the star 7 intermediate between the main sequence and the giants, which might indicate that this star is evolving to its final stage. 

Design and model the sensor function on the basis of a comprehensive set of model parameters (i.e., geometrical dimensions and material properties) and calculate tolerance bands for all layout-specific model parameters (including those for wafer-level packages) so that functional specs are safely met (see Section 4.1). 

Identify functionally critical model parameters for which the tolerance bands of available batch fabrication processes do not comply with those calculated above. Consider alternative processing technologies and redesign the sensor so that the number of critical parameters is minimized (see Section 4.1). 

Obviously, this gyro design requires tightly controlled surface micromachining processes with respect to the thicknesses of the polysilicon and sacrificial layers and to stress gradients within the polysilicon rotor. However, the tolerance band widths and process capability indices available for these model parameters were not indicative of significant yield losses in a factorial analysis. 

Defects include (dust) particles on or within suspended interdigitated seismic masses, leading to electrical shorts and to deviations in moments of inertia. Another example relates to the appearance of crystallographic defects such as dislocations or stacking faults in epitaxial superstructures, causing leakage currents that might impair sensor reliability in the field. Defect densities may of course vary within a wafer, from wafer to wafer, and from lot to lot. Unfortunately, at present not many models are available that link tolerance bands of functional parameters to specific defects and their tolerable density distributions. 

Any specification, including the current limit, either set by the user or fixed internally in the IC, will have a certain inherent tolerance band — that includes spreads over process variations and over temperature. All these variations are combined together inside the electrical tables of the datasheet of the device, under its MIN and MAX limits. In a practical converter design, a good designer learns to pay heed to such spreads. 

For maintaining the accuracy of ultraprecision machines under working conditimis, environmental control is imperative, that is, vibration isolation of the machine and cmitrol of humidity and of ambient temperature within a tolerance band of at least 0.5 °C. 

Variation within the process should always be smaller than the tolerance and rather be in the center of the tolerance band, so that the products do not immediately exceed the tolerance, during process deviations. 

Fig. 6.148. Tolerance band within the switching stage of a hybrid amplifier during dual-state control...

As can be seen from equation 10.3, a square wave exceeds the limits on the third harmonic by a factor of over 6, as well as others. So, how can a purer sinusoidal voltage and current waveform be generated Two approaches are used, pulse-width modulation and the more modem tolerance-band pulse-inverter technique. 

Figure 10.13 Typical voltage/time waveforms when using the tolerance band pulse inverter technique.

In Figure 10.13, the on/off cycle is shown in (a) for a wide tolerance band and in (b) for a narrow tolerance band. It should also be appreciated that the resistance and inductance of the load will also affect the waveform, and hence the frequency at which switching occurs. This is thus an adaptive system that always keeps the deviation from a sine wave, and hence the unwanted harmonics, below fixed levels. 

The only significant disadvantage of the tolerance band regulation method is that it is possible for the pulsing frequency to become very high. Because most of the losses in the system occur at the time of switching, while the transistors move from off to on and on to off, this can lead to lower efficiency. The well-tried pulse-width modulation method is still... 

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