Amplifiers: More than Just Readout Devices

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

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 work. Analog gaging devices also provide more information than digital ones. Just watching the sweep of the needle across the dial of an analog amplifier, from “a little on the plus side” to “a little more on the plus side,” may provide all the information a machinist needs to make the right decision to maintain control over his process—even if he doesn’t actually read any numbers from the dial. So in spite of the benefits of digital instruments (more on this later), analog systems still have an important role to play. 

Analog amplifiers excel in “dynamic” applications, where the gage head moves relative to the part (or vice versa). For example, when “exploring” parts for flatness using surface-plate methods, the user slides the gage stand around on the plate and quickly observes the amount of variation in the part. If the user had a digital amplifier, he would position the stand, wait for a moment to read the value on the display, reposition the stand, read the display a second time and so on, until a sufficient number of data points had been collected.

The problem many analog amplifier die-hards face today is that these devices are headed to obsolescence in the not-too-distant future. It’s not the electronics of the system but rather the hardware of the analog meter itself. This hardware is becoming harder and harder to come by, and, as such, more and more expensive. Thus, many manufacturers of analog bench amplifiers are discontinuing these products.

However, manufacturers do realize the importance of this visual readout and have tried to simulate the look of the analog dial using digital substitutes. The issue here is that the digital version may or may not have the speed and smoothness of a true analog meter. For layout and surface-plate work, this really is a key factor. A fast, clear display that best simulates the original meter is best for these applications. 

The same principle applies to measuring out of roundness, in which the part is turned on a V-block beneath a stationary gage head. Using an analog amplifier, the user can directly observe the amount of variation, compare part size to the mastered dimension, and see whether the variation is all on the plus or minus side, or balanced around zero.

With a fast and easy-to-read display and digital memory capabilities, most digital amplifiers incorporate dynamic measurement functions, including minimum (min.), maximum (max.), and total indicator reading (TIR). 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 inches but you’ve only got gage blocks handy for 2.00000 inches. No problem. Simply set your zero at 2.00000 inches, master the gage, program a deviation of +0.00020 inch 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 0.00010-inch above nominal, the display will read 1.99990 inches.

Digital 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 part-sorting efficiency or bad-part identification in high-volume applications.

Besides these enhancements to the gaging process, the most important and widely used feature on modern amps is the data output port for data collection. Whether it be RS-232, USB or other formats, the data represented on the units is easily available. Using 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 do incorporate some of them. When selecting a new amplifier, it’s easy enough to identify the product features needed to meet the requirements of the application. For those who are currently using amplifiers to simply take comparative measurements, it may be worthwhile to review the owner’s manual and look for built-in functions that can enhance your productivity.