Electronic Gaging Basics

Mechanical gages are familiar and economical. Air gages offer non-contact measurement and ease of use. But for the highest levels of accuracy and performance, it's hard to beat electronic gaging. No other method combines all this: extremely high resolution; relatively long range; adjustable magnification; programmability; digital output; and flexibility to move from job to job.

The "basic" electronic gage consists of three elements: a gage head; an amplifier; and a fixture or a stand to position the gage head relative to the workpiece. We'll touch on each separately.

The most common gage head type is the LVDT (linear variable differential transducer), an electromechanical device consisting of a primary coil, flanked by two secondary coils connected in series, all surrounding a movable magnetic core (the spindle) which provides a path for magnetic flux linking the coils. When the primary coil is energized by a sinusoidal signal from the amplifier, voltage of opposite polarity is induced in the secondary coils. The device's net output is the difference between the voltages of the two secondary coils, so when the core is centered, net output is zero.

The null or zero position is very stable, making LVDTs ideal for high repeatability comparative measurements. And because the LVDT works on an inductive principle, its resolution is, in theory, virtually infinite. In practice, it is limited the amplifier's ability to amplify and display the results. Ranges vary from +0.010 inch to +0.100 inch (+0.250 mm to +2.500 mm), with linearity from O.5 percent to 0.05 percent over the nominal range.

Heavy-duty LVDTs are capable of extended use in the harshest environments. Less than three inches (75 mm) long and about the diameter of a pencil, they can be laid out with great flexibility in fixture gages.

Gaging amplifiers are made in analog and digital versions. Analog amps are preferred where highest resolution is required (at short range); where multiple-range capability is desirable in a single task; where measurement involves watching trends (such as approach-to-size); or where part motion or exploration is required (for example, measuring flatness over a large area). Digital amps are preferable where high resolution and relatively long range are required; where the measurement is static (no motion between part and gage); and where digital output is required for data collection or machine control. In general, analog amps tend to be used in machine setup and surface plate measurements, while digital amps are used in high-volume inspection applications.

Many amps have two input channels, and the ability to combine signals from two transducers into one measurement is another benefit of electronic gaging. In "differential" mode, the amplifier is programmed to add or (more commonly) subtract one signal from the other. This gives the gage the flexibility to measure parts when mechanical references are difficult or impossible to establish, or when two variables may exist simultaneously: for example, on nominally round parts that are subject to dimensional variation and out-of-roundness. Depending on gage setup, the amplifier can be programmed to display either or both variables.

A typical digital amplifier might offer a choice of three measurement range/resolution combinations: +0.100 inch/0.0001 inch; +0.010 inch/10 microinches; and +0.001 inch/1 microinch (+2 mm/0.001mm; +0.200 mm/0.0001 mm; and +0.020mm/0.00002 mm). It is important to remember that microinch/sub-micron resolution on the display does not necessarily mean that level of accuracy in practice. The accuracy of the fixture and the master, the geometric consistency of the workpiece, the stability of the gaging environment and other conditions will influence gaging results. In practice, accuracy below 10 microinches (0.000025 mm) generally requires a laboratory environment.

In order to achieve the best accuracy under shop-floor conditions, it is essential that the gage head stand be as sturdy and stable as possible. Platen, post, arm, and mounting bracket must all be totally rigid when locked. For high-precision height gaging, the arm assembly is often equipped with a lead screw mechanism, because the extra-sturdy arm is too heavy to be conveniently raised or lowered manually.

Another great benefit of electronic gaging is the ease with which components can be swapped around for different jobs. An amplifier that is used with two LVDTs for differential measurements in a fixture gage today may be hooked up to a single lever-type gage head in a comparator stand for height gaging tomorrow. Likewise, a gage head may be used with analog amplifier one day, and with a digital amp the next. No matter what your dimensional gaging problem, an electronic gage can probably be configured to handle it without too much fuss.