Episode #24: The Effect of Moment Loading on a Solution

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The Motion Control Show

Transcription from Corey's conversation with Kent in Episode 24:

COREY: We've talked about loading and moment loading and what we need to know in order to size and select an actuator properly. Interestingly enough, what we do as Application Engineers is to help you fill in all the gaps, all the what-you-don’t-knows. So, I've asked one of our Senior Application Engineers, Kent Martins, to go through a sizing and show you what he does and he’s done and what happens when he’s not given all the information he should be given right up front. But, listen carefully as Kent describes what happens when he isn’t given all the important information. I’m Corey Foster at Valin Corporation. Let’s see what we can learn. Kent, would you start with an application that is 25 pounds, 24-inch stroke, but then show what happens when you are not told that it is about 12 inches off to the side for the center of gravity?  Walk us through your process, please, Kent…

KENT: When I size an actuator for a customer, I usually reach for a tool, similar to this tool here from this vendor, that I can select the appropriate type of actuator, in this case a rodless actuator.  And it shows an illustration of it and I place my, say, 25-pound load and it's at, say, the origin so it's right here on the center of the carriage.  And then I tell it some arbitrary 30-inch stroke to make sure that, in my move, I make sure I don't hit the ends.  So, I then put in my motion profile where I tell it I'm going to move 24 inches in 1 second.  Then I put in a negative 24-inch move in 1 second and it's a trapezoidal move.  Both of them are trapezoidal moves.  So down here below, it has an animation showing me what's going to happen.  It is going to start at zero.  It's going to move to 24, is going to end.  2 seconds and it'll be at zero again when it's done.  So there's more information that can be entered in here relative to the LOSTPED, but at this point there's sufficient information entered into this that I can use the selection tool on the right-hand side to select an actuator that would be appropriate for at least what I've put into this design so far.  And in this case, the vendor has placed the least expensive alternative, which is very common among the vendors, at the top of the list and I can see it's relative cost is, in this case, single dollar sign out of five.  I would say this might be an appropriate choice and I could use this for the design of their system.  

But, if the customer comes back, or for some reason I find out later down the line, that, uh, this load isn't really here, it's off to the side here and we've gotten, you know, half a month down the road with this design.  And now I'm finding out it’s 12 inches over to the right-hand side of this module, as illustrated in this picture here.  And when you do that, it automatically updates all the available modules that are recommended for this application.  The motion profile is still all the same, but the list now no longer has our single dollar-sign selection available at the top of the list.  In fact, it's actually gone to a 2.3 dollar-signs selection, which is probably something close to two times what the other module would have cost.  

And so, one important thing is that we now figured out that we selected the module originally without the 12 inches that probably wouldn't work.  If it did work, it would have a limited capacity, there’d be something that would be wrong.  It either would have some sort of harmonics, there'd be maybe binding, it may not last longer than a year, there would be all sorts of mechanical grief, and then you'd end up having to make compromises like going down and changing the velocity here to half of the velocity.  That may not even work.  Or you may have to even design in counterbalances or things like that just to compensate for the fact that you have this 12-inch offset on this load.  So getting the correct information up front is really important, but if it does change, it's just as important to come back and give it to the people who originally sized the module so that we can update all our calculations because this may affect not just this module, but if it's in the cartesian system where you have an XY and Z and they're all connected together, if the mass of this module increases because the mass has been moved to the right of another module, and all of a sudden all of your modules may be under sized, and you've got a whole system that needs to be resized.  And that, that is…it's a lot of work.  But it's a lot better to catch it and to realize it in the field.  

COREY: Something like that could be a pretty expensive omission of details if, uh, if you don't get told all that information.

KENT:  It can be. It can be miserable as a matter of fact, and it happens.  It happens more often than you'd like. 

COREY: Do you ever feel like you're asking stupid questions of customers in order to get all the information you need?  

KENT: You know, that's part, it's part of the process, I guess, of doing this is, if, if you're an Application Engineer, you send out countless emails, it seems, is asking the same questions.  They say it's going to move 24 inches in 1 second, is just a hypothetical one.  Well, you don't know what the accelerations are.  You don't know this or that, so you just throw in a trapezoid 'cause you know it's got to accelerate but, you know it could be a triangular motion profile, or it could be some other motion profile with a lot of jerk because they're moving something that's delicate and it can't chatter or something like that. 

COREY: Thanks, Kent, for that insightful information.  Appreciate it.  I hope that makes you think about your application more, what you need to know, what you don't need to know, and make sure you, if you're working with people, give them all the information up front and entertain what may seem like silly questions to you up front.  There's reasons for them.  In the meantime, check out my show by clicking on the link below.  We're happy to help.

The Motion Control Show: https://www.valin.com/motion-control-show
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 at https://www.valin.com/contact-us.