Resolution—The Oft Misunderstood Third Specification

Editor's Note: This is the first in a monthly series of Machine Automation columns covering subjects such as motion control, machine control, intelligent sensors and precision mechanical devices. Please e-mail comments, questions and ideas for future column subject matter

by: Matt Johnson, Industrial Devices Corp.

At first glance, "speculation" isn't such an ugly word.

Speculation regarding positioning technology may even lead one to adopt the optimum positioning system for their motion application, but I wouldn't bet on it. A less-than-complete understanding of application requirements will more than likely lead to an overly expensive motion system, or disappointing results, or both.

In my estimation, accuracy, repeatability and resolution (see Understanding Positioning Performance for detailed information on accuracy and repeatability) are three of today's most commonly misused and misunderstood positioning terms. Additionally, it is overly simplistic to consider these specifications for each individual system component on their own merits. Specifications provide meaningful application information only when they are used to describe the performance of a complete positioning system. In this case, the sum is greater than the parts. But back to the issue at hand.

Accuracy is the difference between the target point and the point actually attained by the motion system, which is described by the mean of the dispersion plus the width of the dispersion that yields the largest absolute sum. In the case of either linear or angular motion, repeatability defines the variation in a series of moves or, more analytically, the width of the dispersion about the mean for a significant number of positioning trials.

"Well, if that's what accuracy and repeatability are, what the heck is resolution?" Resolution is probably the most frequently misunderstood specification, often confused with both accuracy and repeatability.

There are two types of resolution for motion systems. The first is electrical resolution, which is the resolution of the system encoder(s). This is the smallest incremental movement that a positioning system can detect. The second is system resolution, which is the smallest incremental motion that a motion system can actually achieve. Systems with high friction (stiction) or inertia can have system resolution that is 5 to 10 times larger than the electrical resolution.

Finer resolutions are most commonly achieved with finer lead ball screws or lead screws, finer drive resolutions (step motors) and finer resolution encoders (rotary or linear for step motors and servomotors). Resolutions on the order of less than 1 micron usually require piezo-ceramic drives. But remember that specifying fine resolution components does not guarantee that the system will actually be able to move in such small increments, as the actual system resolution must still be measured. Motion systems that must be positioned relative to an external gauge or sensor may only require high resolution, without requiring high accuracy or repeatability!

It is important to note that a finer lead, higher resolution lead screw or ball screw will not become more accurate because of its finer lead. As an example, a JIS grade C3 ground ball screw could have as much as six microns of lead error in one revolution, regardless of ball screw lead. Screw lead error should be mapped for applications requiring high accuracy and resolution, because some controllers can use the error mapping information to compensate for the actual lead error.

Additionally, applications requiring high accuracy and resolution should also measure the periodic error of the positioning system, as this helps gauge errors caused by the mechanical periodic error (e.g. from a ball screw or lead screw), as well as the interpolation errors from an encoder and controller.

Knowing and understanding the terminology necessary to specify the optimum positioning system for a particular application can be intimidating. But when compared to the cost of over or under specifying a system, it cannot be ignored.

Matt Johnson is product manager for the Precision Systems Division of Industrial Devices Corp. of Petaluma, Calif. He has experience as a Product Manager and Sales Manager working for Hiwin and Yaskawa. His previous experience also includes aerospace engineering, finance and consulting. Matt has an MBA, as well as a Master's Degree in Mechanical Engineering. E-mail: site: