Absolute Zero

Author Steve Pike
December 01, 1997 - 11:00am

When digital calipers replaced traditional mechanical measuring tools, users learned that certain time-consuming prodedures had to be followed to ensure accurate measurements. But today, users can take advantage of digital calipers equipped with an absolute-measurement feature that overcomes most of the shortcomings of conventional digital calipers and gives users new confidence in the accuracy of their measurements. As the name implies, absolute-reading digital calipers always indicate the absolute distance from zero.

Excitement about a measuring tool that provides an absolute or total reading may seem strange to readers who grew up using traditional mechanical measuring tools featuring etched or engraved scales that provided such readings as a matter of course. However, most digital calipers currently in use are capable of providing measurements relative to the last at-rest position. The advantage of always having the reading from zero was lost in the transition from mechanical calipers to digital calipers.

For example, if the jaws of a conventional digital caliper were already open 1.500" when the caliper was initially turned on, and the user opens them an additional 0.500" to measure a 2.000" diameter bar, the digital display will read 0.500" instead of 2.000" To get around this problem, users of relative-reading digital calipers must close the jaws to zero to gage before taking measurements. If the jaws remain slightly open when the user zeros the gage, or if he forgets to zero the gage altogether, the tool will display an inaccurate measurement.

Absolute-reading digital caliper technology is the latest in a long chain of improvements in capacitance-type measuring devices.

Conventional capacitance-type devices are widely used for linear and angular measurements. They usually include two support members or scales, on which are mounted arrays of discreet, capacitively coupled electrodes. The electrodes are displaced relative to one another when the tool is opened or closed to measure a part. The change in the position of one scale relative to the other is determined by sensing the resulting change in the capacitance pattern created by the arrays of electrodes.

Typically, the capacitance pattern is sensed by applying periodic signals to one of the electrode arrays and measuring the shift in the signal resulting from the transfer from one array of electrodes to another.

While conventional capacitance-type measuring devices have become increasingly popular, users have had to put up with some fundamental limitations. As mentioned earlier, conventional capacitance-type measuring devices have been capable of taking only relative measurements. That is, measurements are taken by sensing the relative change in position of the scales with respect to a reference position, which requires continuous sensing of the change in the capacitance pattern created by the electrode arrays so that pattern repetitions can be counted.

Absolutely Accurate 
Absolute-reading digital calipers operate on the same general principles as relative-reading units. However, absolute-reading units employ more complex and more sophisticated electrode arrays and transmitter/receiver circuitry to enhance performance.

For example, with some absolute-reading digital calipers, second and third receiver electrode arrays produce complementary first and second outputs, respectively, in response to excitation signals applied to the first transmitter electrode array. The caliper's signal-processing unit selectively combines the respective first and second outputs to produce measurement signals equivalent to single signals of the type produced by electrode configurations common to relative-reading calipers.

While the relative-reading digital caliper arrives at a reading with a single measurement, the absolute-reading unit derives its reading from a combination of a coarse, a medium, and a fine measurement.

No Speed Limit 
In addition, the rate at which the scales of relative-reading measurement devices can be displaced with respect to each other has been limited by the speed of the signal processing. Simply put, if the user moves the scales too quickly in the process of taking a measurement, miscounting can occur. The caliper no longer knows where it is and its display registers an overspeed error message. When this happens, the user must zero the gage and remeasure the dimension. Manufacturers can build relative-reading gages that can handle rapid movements, but this ability requires the use of high-frequency signals and more sophisticated signal-processing circuitry, which increases the cost of the measuring device.

When the slide is moved too rapidly, there is also a danger of skipping a count. Some tools are designed to alert the user to such an occurence, but without such a warning, the reading will be off by an amount that depends on the resolution of the measuring tool.

Absolute-reading digital calipers do not have to count pulses as the slide is moved. Therefore, the user cannot go too fast for an absolute-reading caliper. There is no danger of skipping a count or losing measuring accuracy by over-running the ability of the tool to keep up. An absolute-reading device not only provides more accurate measurements, but it also allows the user to have more confidence in his readings. That makes for faster measurements each time, because the user doesn't feel the need to double- or triple-check a measurement or to zero the gage again because he can't remember having done it.

There is also a modest savings in operating costs for absolute-reading digital calipers. Unlike conventional digital calipers, which use battery power to count the number of pulses from the previous position, an absolute-reading unit uses battery power only when it is in its final measuring position. This greatly reduces power requirements, so that batteries last longer. In fact, power requirements are so low that solar-powered models are now available. These models can operate in low-light conditions down to 60 Lux.

This greater efficiency might not seem significant at first glance, but for a large firm with numerous employees measuring parts with digital calipers, the cumulative benefits of faster, more consistently accurate measurements certainly justify the expense of moving to the newer technology.

Just as digital-display technology eliminated gage-reading errors commonly encountered with manual calipers, absolute-reading technology eliminates measurement errors associated with the failure to use digital calipers properly, moving the industry one step closer to foolproof hand measuring tools.

About the Author 
Steve Pike is precision-measuring-tools product manager with Mitutoyo Corp., Aurora, IL.


Steve Pike is precision-measuring-tools product manager with Mitutoyo Corp., Aurora, Illinois.