Metrology
Metrology
It's an Analog World
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IT'S AN ANALOG WORLD
George Schuetz, Mahr Federal Inc.

 In spite of what some Internet addicts may think, the world is analog, not digital.  A simple example that proves this statement is what happens when we try to cross a busy intersection on foot.  If the world were digital, we'd be limited to working with simple "on/off" information, indicating "car present/not-present."  If we took several readings over time, we would be able to extrapolate, but not directly detect, a car's direction, speed, and acceleration.  By the time we had done all that, the car would be long gone, and we would have to start collecting the next data set.  Because the world is analog, a brief glance is all we need to detect presence, distance, direction, speed and acceleration.  This enables us to react safely, either by staying put or crossing, choosing our rate of acceleration somewhere along a continuous but finite scale of values.

 Analog gaging devices also contain more information than digital ones.  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 a machinist with all the information he needs to make the right decision to maintain control over his process—even if he doesn't actually read any numbers from the dial. 
In spite of the benefits of digital instruments (more on this below), 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 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.

 Machine tool setup is another valuable application for analog amps.  For example, to ensure good centering when preparing to final bore a hole, a lever-type gage head is mounted on the machine spindle with the contact against the inside of the bore.  By turning the spindle back and forth by hand and making cross-slide adjustments while watching the movement of the amplifier needle, the user can readily center the bore directly under the spindle.  The same principle applies to positioning a fixture on a machine table by means of a reference "button" on the fixture.

 Some analog amplifiers accept inputs from level-sensing devices in addition to dimensional transducers.  Electronic levels are of value when installing a new machine or truing up an existing one: the analog display enables the user to watch the effects of leveling adjustments in real time.  If the amplifier has dual inputs, two levels can be used in tandem for "differential" measurements to check parallelism or squareness between surfaces.

 When selecting an amplifier for dynamic applications, look for adequate response speed to display change as soon as it occurs.  Also, consider the needle's tendency to overshoot the measurement during rapid changes; some amps control this better than others.

 Other important features include: switchable "normal/reverse" settings to make setup and interpretation easier; a dial with selectable range/resolution settings to accommodate a variety of tolerance specifications; and an analog output port for collecting data on a strip chart recorder.

 None of this is to imply that analog amplifiers are always the best choice.  Digital systems are superior for the purposes of data processing and output and the generation of feedback for the control of CNC machines.  Some digital amps incorporate "dynamic" features that automatically capture minimum or maximum readings or calculate the difference between the two (i.e., TIR).  Although the digital amp user can't see the sweep of a needle, he can still obtain "variable" information fairly readily. 

      In a nutshell, digital devices are generally preferable where:

  • The measurement is static—neither gage nor the part is moving.
  • Output for data collection or analysis is desired.
  • High resolution over long range is required.

     
     Analog devices are preferred when:
  • You want to observe trends or rates of change, as in approach-to-size, leveling, positioning, flatness, and out-of-roundness measurements.
  • High resolution over short range is required.
  • The measurement is "dynamic," involving a moving part or gage.