Episode #4: What Are Resolution, Repeatability and Accuracy?

Contact Valin today for more information at (855) 737-4716, or fill out our online form.

The Motion Control Show

As we dive further into sizing and selecting mechanics, there are a couple of terms that are extremely important in understanding the requirements of an application.  They center around the precision requirements for the application.  I'm Corey foster of Valin Corporation. 

A customer will typically say that they need precision of some sort.  But, there are a couple different types of precision that kind of fall under that umbrella.  

Let's start with resolution.  Resolution is the smallest amount of distance that you can move or command.  If you look at this travel, and the micrometer driving this stage, the smallest little bit is the resolution that you can actually move or command this actuator to go.  But, that's a far cry from what you can actually effectively use in an application.

Repeatability: the definition is “the precision with which a positioner can reproduce a given movement over a number of attempts.”  I'll explain this a little bit more, but it's how repeatedly you can go to the same location.  

Accuracy is the difference between where you command the actuator to go versus the actual location it goes to.  There's going to be some error between them.  Again, we’ll dive into this a little bit further out.  

This classic dartboard is commonly used in discussing the repeatability versus accuracy.  The very first dartboard there shows results that are not accurate and they're not repeatable.  The second one shows accurate because they're centered around the bull's eye, but they're not repeatable because we're not repeatedly hitting the bull's eye.  The third one, it's repeatable because they're all clumped together up there in the corner, but it's not accurate because we're not hitting the bull's eye.  The fourth one is repeatable and accurate because we're hitting the bull's eye pretty repeatedly. 

Here's another way of looking at it.  The resolution of this actuator is the smallest amount that it can be actually moved.  Repeatability is how you can repeatedly go to the same location, but it has to be the same location.  So, if you're talking about discrete locations along an actuator you can talk about repeatedly hitting those discrete locations.  But, if you want to look at an infinite number of locations along a travel, you have to talk about accuracy.  It’s how accurately can you go to any one of an infinite number of locations.  And there's the comparison against the dartboards.  

Keep in mind that each of these measurements are taken at a point of measurement which is typically some distance above the carriage.  Now you can't put a load really right on the carriage unless it's a piece of paper, but usually there's some load.  So, there's an assumption that maybe it's an inch, twenty-five millimeters, maybe it's two inches, above the carriage.  But, there's some point of measurement that's taken and that's where that piece is moved along the travel.  So, as I moved, there may be some abbe offset errors which you can have some angular offset.  There's a pitch error and there's an angular error there.  If that actually has some pitch and yaw to it and roll as it goes along the movement, because it's not perfect, that point an inch or two above the carriage is going to have a little bit of abbe offset error.  How does that affect it as you go along the travel of an actuator?  Along the straightness, you're going to get some motion back and forth just a small amount.  And the flatness, you're going to get some up-down and it's going to be aggravated by being that inch or two off the carriage.

Orthogonality is another term that we have to use if we're talking about XY or YZ axes, two axes that are placed on top of each other.  We can't put them perfectly on top of each other and have them perfectly square.  There's going to be some orthogonality and if you want it to be as super tight and square as possible, that's going to be harder to do then if you just slapped two actuators together.  

When we are looking at rotational stages, rotary axes, there’s a concentricity.  That red circle there shows the perfect circle, but the green one is the actual travel path.  The difference between the two is the concentricity.  And run out is, if you're looking at that same rotational stage from the side, it may not be perfectly flat.  Nothing is perfect therefore it has some run-out as it rotates.  It's going to have that that change in vertical position as it goes around the circle.  It's also going to have some wobble as it goes around.  It's going to have that wobble back and forth. 

There's some basic terminology about precision, specifically the repeatability and accuracy.   Those are very important.  I discuss the differences between those with customers all the time because one is much harder to attain than others.

Contact Valin today for more information at (855) 737-4716, or fill out our online form.