Thursday, August 16, 2012

Three Heads are Better Than One

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Measurement System Analysis, Part II: Electronic Gage Heads
George Schuetz, Mahr Federal Inc.

Last month we began a series looking at various aspects of the electronic gaging process with an eye towards reducing uncertainty and making measurements "less imperfect." This month, we'll look at probes.
Users of electronic amplifiers can choose from a number of gage head types to generate the measurement signal. The three most common—cartridge, pantograph, and lever-type gage heads—differ from each other mainly in the orientation of their sensitive contacts, and the mechanisms by which contact movement actuates the transducer. Other types of dimensional sensing devices, such as capacitance gages and laser devices can also be integrated into electronic amplifier-based systems. These are useful in applications that require non-contact sensing, but most metalworking applications are satisfied with the three common "mechanical," or contact-type gage heads. Each offers particular advantages for different applications.
The cartridge probe, or pencil-type gage head, is a compact cylindrical device, usually 3/8" or 8 mm in diameter and less than 3" (75 mm) long. Not coincidentally, these are about the same dimensions as dial indicator stems, which cartridge probes were originally designed to replace. Like dial indicators, the probe's spindle, or sensitive contact, has an axial motion. Readily incorporated into fixture gages and in-process gages, several cartridge probes can be positioned within close proximity, to measure closely spaced part features. For even tighter spacing requirements, some manufacturers offer special miniature probes, with diameters as small as 6 mm and lengths below 20 mm.
Most cartridge gage heads operate on an LVDT principle. The linear variable differential transducer is 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 by the amplifier's ability to amplify and display the results. Ranges vary from ±0.010" to ±0.100" (±0.250 mm to ±2.500 mm), with linearity from 0.5% to 0.05% over the nominal range. Longer ranges are also available, but they are usually not applied for tight-tolerance measurements. (Linearity is typically a trade-off against longer range.)
Some LVDT probes are signal conditioned to produce a digital format; others have a digital scale built in for extended range. But whatever the actual electrical means of working they pretty much produce the same results. Moreover, even standard-duty LVDTs are very rugged, and heavy-duty versions are capable of extended use in the harshest environments.

Numerous options and variants are available to increase flexibility of application. The standard plain bushings that support the spindle tend to be quite durable, but in applications that subject the spindle to significant side-loading, ball-bearing bushings can provide longer life cycles. The signal output cable is normally supplied straight and plastic-jacketed, but coiled cable is available for use on hand-held gages, and armored cable is available for harsh environments. Cable may exit the probe from the back of the cartridge (axially), or at a right angle—a small detail that occasionally makes mounting the probe much more convenient.
Most cartridge heads are splash-proof, with a protective rubber boot surrounding the probe's stem. Hermetically sealed versions are also available, for use in extremely harsh environments: for example, for in-process gaging during a grinding operation.
Cartridge heads tend to have relatively heavy gaging pressure—about 3.5 oz. (99 g)—but here too, optional specifications are available from some manufacturers. Another handy option is a pneumatic retraction accessory, to minimize side-loading on the spindle when inserting a workpiece into a gage fixture.

Pantograph, or reed-spring gage heads, are most often used in bench top height comparators where both ruggedness and extremely high accuracy are required. The gage's contact is suspended by a pair of reed springs, which provide virtually force-free and friction-free measurement. (External springs or deadweights can be added if a specific gaging pressure is required.) Pantograph gage heads offer a measurement range of ±0.010" (±0.250 mm) with repeatability of <0.5 µin (<0.01 µm). They are more accepting of side-loading than cartridge-type gage heads, and can be repaired more easily and economically if side-loading damage does occur.
Where the cartridge gage head replicates the action of a dial indicator, lever-type gage heads are functional replacements for test indicators. Electronic lever-type gage heads are typically used in connection with a height stand, often for surface plate work. When mounted on a tiltable extendable cross-bar, they can be positioned with a great deal of latitude relative to the workpiece. A clutch on the swivel further assists in positioning convenience, allowing the contact to be repositioned by as much as 20° without moving the body of the gage head. Their ability to measure in both directions further enhances versatility. In contrast, cartridge and pantograph gage heads are unidirectional, and must be positioned perfectly in-line with the dimension being measured.
The extended, pivoting contact of the lever-type gage head provides good access to working surfaces that may be hard to reach with other contact styles. Contacts with special shapes, and diameters as small as 0.010" (0.250 mm), can be specified for use on really inaccessible workpiece surfaces. Repeatability can be as good as <4 µin (<0.1 µm), and gaging pressure, at <0.14 oz. (<4 g), is light, making these heads well suited for high-resolution measurements on delicate surfaces or compressible materials.
All three types of electronic gage heads can be combined in a single fixture gage or application, and many amplifiers will accept all three interchangeably. All three can also be readily integrated with either digital or analog amplifiers. These features help make electronic gaging a very flexible approach to high accuracy inspection.

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