Metrology
Metrology
Amplifiers: More Than Just Readout Devices
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Amplifiers: More Than Just Readout Devices
George Schuetz, Mahr Federal Inc.

  Electronic gaging amplifiers are one of those devices whose full potential is rarely appreciated by their owners, sort of like Range Rovers that never leave the pavement. Gaging amplifiers are often used simply as replacements for dial indicators where a higher degree of resolution is required. This is to ignore numerous opportunities to make gaging more efficient and productive.

  Some amplifiers, for example, incorporate dynamic measurement capabilities, including Minimum (Min.), Maximum (Max.), and Total Indicated Reading (TIR) functions. The amplifier "remembers" the highest and lowest points measured on a part, and displays either or both of them, or subtracts the Min. from the Max. to calculate TIR. 

  This is useful when gaging round parts in a V-block fixture, or measuring the height of a flat surface. The operator can quickly turn a shaft through a complete revolution, or move a flat part around under the gage head, without pausing to read the display. When manipulation of the workpiece is complete, the operator may select to display the maximum or minimum ID, OD, height, depth, or runout.

  Other advanced functions can speed gaging setups. The "auto zero" function is the electronic equivalent of the rotating bezel on mechanical dial indicators: the operator brings the gage head into rough contact with the master, and simply zeroes the amplifier, eliminating the need for ultra careful positioning of the gage head. A "master deviation" function allows the addition of a fudge factor to the zero setting. Say your spec calls for a nominal dimension of 1.99980", but you've only got gage blocks handy for 2.00000". No problem. Simply set your zero at 2.00000", master the gage, program in a deviation of +.00020" to all measurements, and voila! Quick and easy mastering, without the hassle of post measurement arithmetic.

  A "preset value" allows switching between comparative and absolute measurements. In other words, instead of gaging deviation from nominal, the amplifier displays actual part dimensions. (In the above example, if a part is .00010 above nominal, the display will read 1.99990".)

  Many amplifiers accept signals from two or more gages. This means that more than one part feature can be measured on a multi gage fixture, by simply "toggling" between the inputs. Somewhat more sophisticated is the capability for differential measurements, in which the amplifier subtracts the reading of one gage head from the other: for example, you can derive straightness by calculating the difference in height of two co-linear points on a shaft.

  Amplifiers also allow the user to establish tolerance limits, and some incorporate green, amber, and red lights to indicate "in tolerance," "approaching limits," and "out of tolerance" conditions. Alternately, the lights can indicate different part size categories for match gaging applications. Through digital output ports, the same electronics can be used to drive large accessory lights, enhancing parts sorting efficiency or bad part identification in high volume applications.

  These digital output ports represent a great benefit of modern bench top gaging amplifiers. Through them, gaging data can be used to control production machinery on an in-process basis, replacing expensive, dedicated closed loop controllers at a fraction of the cost.
                 
  In one real, representative application, a gage head is positioned to measure a workpiece while it's still on a grinder. The user of this system assembled it using an off the shelf, hermetically sealed gage head, and a standard bench top amplifier connected via the digital I/O ports to the grinder's computer numeric controller. The grinder shifts to a shallower depth of cut when the gaged data approaches the specified dimension, and stops automatically when the spec is reached. As long as the system is calibrated adequately, no post process gaging is required.

  Besides these enhancements to the gaging process, the most important and widely used feature on modern amps is the RS-232 port for data collection. Now, through SPC, intelligent decisions can be made about the sample lot or the process. Amplifiers also provide analog output to drive strip chart recorders for continuous part measurement.

  Not all gaging amplifiers incorporate all of the features listed here, although most modern amps incorporate some of them. When selecting a new amplifier, one can readily enough identify the product features needed to meet the requirements of the application. For those who are currently using amplifiers simply to take comparative measurements, it may be worthwhile to review the owner's manual, to look for built in functions that can enhance your productivity.