Episode #23: Motion Control: The Effect of Accuracy & Repeatability on a Solution

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

The Motion Control Show

I have tried to impart on you the knowledge about repeatability and accuracy and other technical terms to make sure you are specifying stages correctly.  If you don't get them right, it can really be expensive, and I've tried to make that point clear.  But if you don't believe me, maybe you will believe Ray Marquiss, one of our Senior Application Engineers.  I've asked him to give an example of the effects of getting that terminology wrong.  I am Corey Foster from Valin Corporation.  I hope this helps.  We are going to bother Ray while he is on vacation on the beach in Hawaii and have him give us an example:

RAY:  Sometimes customers will come to us looking for actuators for their applications, and they don't always have complete information.  But whenever they do give us information, we want to make sure we have the right information.  Let us say that a customer we have worked with before wants to use manufacturer “A” because they used them before.  They like their products and so they come to us with the specification like this with a 10-kilogram payload, 10-micron accuracy, and 2-meter stroke.  It is not enough to go on to make a full specification to get him his required actuator, but we can zero in on that 10-micron accuracy.  Even without all the other things we need to know, we know we have to find an actuator that can do that before we do anything else.  If we focus on that accuracy, we have to make sure that what he really needs is accuracy and not repeatability.  Here is why: Accuracy can cost you money.  Repeatability can save you money.

COREY: Any guesses as to the difference in cost?

RAY: Probably it is going to be around $4,000 - $5,000 per axis, if he has a multi axis system.  Manufacture “A” is out because they do not specify accuracy in their published specifications.  They specify repeatability.  So, we have to go to manufacturer “B”.  The first solution with manufacturer “B” is a high precision linear motor stage which is going to be about $6k, including the servo drive, but it gets him the accuracy he wants.  We can try another choice with manufacturer “B”, which is a high precision ball screw.  Slightly lower performance than the linear motor, but a little bit less cost at maybe $4 to $6 thousand.

If we convince the customer that he wants repeatability instead of accuracy, we can go back to manufacturer “A” and get a medium precision ball screw which is going to have a little bit less performance than the linear motor.  But the cost is going to be about $1250 or almost $3000 to $4000 less than the other solutions and it gets him 5-micron repeatability. 
So, if we do not pay attention to the difference between accuracy and repeatability, it can cost a lot of money!

COREY: You are saying that if the customer uses the wrong word for the specification, it can cost them thousands of dollars?

RAY: If we do not catch it, yes.  If we do not catch that, or if we do not dig in and decide whether he really means accuracy, because sometimes customers do mean accuracy, but in my experience, most of the time when they say accuracy, if we talked a little bit, what they really need is repeatability. 

COREY: So, we have to sound like we are asking stupid questions to make them save them money? 

RAY: Yes, sometimes we get that look.  I have done that picture of the dartboard so many times, it is just automatic.  I can look at the customer while I am drawing it! 

COREY: They must have a lot of dart boards out there on the beach in Hawaii, huh?  Well, don't get sunburn!  Thanks for the input, Ray.  I'm Corey Foster of Valin Corporation.  That was Ray Marquiss.  I hope this helps.  Don't forget to check out this link and check out the episodes in the past.                                                                                                                                                                                                                                 
If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.