Metrology
Metrology
Measurement System Analysis: Digital Amp & Probes
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Measurement System Analysis:
Range, Resolution and Accuracy in Digital Amplifiers and Inductive Probes
George Schuetz, Mahr Federal Inc.

Just as there is no repeatable way to make a perfect part, and there is no repeatable way to make a perfect measurement. Thus, the question is always one of uncertainty: how imperfect is my measurement and how can I make it less imperfect? As imperfection— uncertainty—accumulates throughout the measurement process, it can be lessened by isolating specific causes and minimizing them. Overall, this process is known as Measurement System Analysis. Over the next few columns we'll be looking at various aspects of this process and how they relate ultimately to SPC. We'll start with the question of range, resolution, and accuracy in amplifiers and inductive probes.

Today's digital dimensional bench amplifiers offer many great features. One is the ability to select the digital display resolution, independent of the analog display range. There are advantages and disadvantages to this capability.

The advantage is that a measuring device such as an inductive probe with a large measuring range may be viewed at a fine, or high, resolution. This allows the operator to view an approach to a part more easily. It also allows larger variation in part sizes to be measured.

The disadvantage is that high resolution does not mean high accuracy. Many times the display resolution is assumed to reflect the measuring system's accuracy. This is far from the case. For example, a probe might have a linearity error of 3.0µm/120µin near the ends of its ±2mm measuring range. The bench amplifier can display a digital resolution as fine as 0.01µm/1.0µin (0.000001in). Thus the danger! The probe may be able to achieve errors less than 0.25µm/10µin, but it can only do so over a very short measurement range near its null or electrical zero position where it is most linear. (Of course, inductive probe specifications vary, so make sure the probe selected is sufficient for the application.)

Most bench amplifiers help reduce this potential error. As a higher resolution is selected, the amount of visible measuring range is limited. This is done to keep the accuracy of the system more in line with the readings being displayed at the higher resolution.

There are some amplifiers that have two or more digital ranges. When using resolution settings down to 0.1µm/5µin the digital display range is basically infinite and dependent on the available range of the inductive probe. When selecting a resolution of 0.01µm/1.0µin the digital range is limited to ±0.20mm/±0.008in. This limiting of the range is necessary to ensure that the user is aware of the type of measurement they are making. It simply makes no sense to measure at a 1µin resolution if the gage, fixture, environment, instrument, and part cannot achieve accuracy enough to justify the high resolution selection.

While we don't usually like rules of thumb, it makes no sense to have very high resolutions on relatively loose tolerances. All you tend to get is a lot of numbers, adding confusion to the operator. A general rule for operator ease of use is to limit the digital resolution to no more than 1/40th the total part tolerance. Some might think that it should be as little as 1/10th of the part tolerance, but this can limit the use of measurement data. This is especially true when collecting data for process control, as SPC analysis may require higher resolution to ensure enough variation. 
 
The basic identity of an inductive probe is its electrical signal output curve which is characterized by the "S" shape as shown in Figure 1. The graph plots the deviation error from an ideal straight line over the entire measuring range of the probe. All inductive analog probes have increasing deviation, or linearity error, as the reading moves away from their null or electrical zero position. Thus, these are called zero, or center based systems.

The "S" shape of the curve is a result of the mechanical movement as the spindle positions the core (mounted on spindle) up and down inside a coil assembly. The electrical signal of the coil is balanced at an electrical zero output when the core is precisely in the middle of the coil assembly. Slight inaccuracies tend to be at the extremes of the measuring range and thus the curve becomes "S" shaped. This is why amplifier readouts allow the raw probe signal to be displayed in special absolute modes so that the most accurate part of the probe can be set against a zero or nominal master. This type of measurement system is called a comparative system because it compares the difference between a part and its master. When set up correctly, very accurate readings can be obtained.

Probe ranges are set by their mechanical design. Inductive probe resolution is limited by the amplifier design, display instrumentation, and mechanical design. Electronically, an inductive probe's resolution is infinite, but mechanically it is limited to repeatability and hysteresis characteristics. Thus the first step in selecting the right probe for an application is the application itself, which we'll talk more about next month.


 

Figure 1. The basic identity of an inductive probe is its electrical signal output curve which is characterized by an "S" shape. This graph plots the deviation error from an ideal straight line over the entire measuring range of the probe. All inductive analog probes have increasing deviation, or linearity error, as the reading moves away from their null or electrical zero position.

A gage set up with four probes built into a fixture for small taper parts. Inductive probe resolution is limited by the amplifier design, display instrumentation, and mechanical design. But resolution does not mean high accuracy. Many times the display resolution is assumed to reflect a measuring system's accuracy, but this is far from the case.