Tracking Down Tracking Errors

Tracking Down Tracking Errors
George Schuetz, Mahr Federal Inc.

In the world of dimensional measurement, electronic gages make up a class of instruments that are capable of detecting extremely small dimensional variations on a surface element. The electronic transducer can operate in a number of different ways, typically LVDT (Linear Variable Differential Transducer) or through a digital scale-based technology. The resultant signal can be amplified or counted, and is then available for display on an electronic readout, or fed directly into a computer-based software system.

Most of these electronic instruments are used in short-range comparative gages and can have sub-micron or millionth of an inch resolutions. Besides having extremely high resolutions, fast response and the ability to collect data, another benefit of electronic gaging is that transducer signals can be combined in mathematical formulas to display dimensional or geometric conditions. The most common example is a differential measurement where two transducers combine to produce a reading of part size without regard to part position.

When parts are measured under ideal conditions—for example, when a part with perfectly flat and parallel dimensions is placed on the reference table of a gage that is extremely flat and perfectly perpendicular to the part—then results should be pretty good without the need for differential measurement. But, since neither we, our parts, nor our gages live in a perfect world, differential gaging can be a decided benefit to the application.

This type of signal combination is usually used on simple bench stands or on complex fixture gages having multiple checks, where the part position cannot be well controlled. In this case, any out-of-position is seen by both heads, and since the heads are combined differentially, this shift is canceled out and only the change of size is displayed. However, there is a catch.

There is a potential source of error that must be accounted for when trying to make very precise measurements using these high resolution transducers. Each transducer has its own performance characteristics. These are repeatability, linearity and calibration accuracy—all performance terms we have discussed many times in previous articles. Knowing the performance characteristics of the transducers is important, but what you need to remember is that every additional transducer adds another source of error to the measurement.

These errors may or may not be additive, but when they are, they can produce significant errors in extremely demanding measurements. This 'new' error is called tracking error. Normally two heads are added differentially to produce a diameter measurement. But sometimes, because of the manufacturing process, a two point measurement will not suffice, and 3, 4 or even 6 transducers are combined to produce an average diameter, and this can result in tracking error.

So how do you track tracking error? It's really not that hard. It requires a way to hold the transducers, a display, and a means of displacing all the gage heads equally. Let's start with a basic two-headed differential check. After each transducer is calibrated with the amplifier, place them in a gage calibrator or bench stand where gage blocks can be used to move each transducer precisely the same amount. If using gage blocks, select a set that will allow you to record at least 3 equally spaced points on each side of the transducer's zero.  

Mechanically position both transducers at electrical zero on the zero gage block. Then switch the heads to the amplifier's differential mode and note the result in the display. Now replace the zero block with the series of gage blocks and record the values (or if using the calibrator, move it to displace the transducers at a number of points in their travel). The results should provide a linear trace, but there will likely be more error than when each head is tracked individually. This check should be performed with all the heads that would be used in the mathematical calculation to determine the worst case error of the system. In extreme cases it may be necessary to calibrate on the total system response, but mostly you are apt to see non-linear results over different areas of the signal's range.

While it may not always be possible to correct for these errors over the short range, knowing where they are may help you optimize the position of the transducers, or look for a different combination of transducers that provide the best results.

Tracking error is a potential source of error that must be accounted for when trying to make very precise differential measurements using these high resolution transducers.