Episode #19: LOSTPED: What To Know About Selecting Mechanics

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The Motion Control Show

We are diving deeper into sizing and selecting your own mechanics.  We talked about a lot of terminology you need to know about back in Episodes 3 through 8.  Now we are going to dive deeper into each of those and kind of bring it together.  I'm Corey Foster at Valin Corporation.  Let's see what we can learn.  If you like what you are learning, follow #MotionControlShow and check out this link here.

Let's jump into summarizing LOSTPED.  LOSTPED is an acronym that is used in the industry where LOSTPED stands for Load, Orientation, Speed, Travel, Precision, Environment and Duty Cycle. 

Load: weight or mass applied to the system
Orientation: position or direction of force applied
Speed: speed and acceleration
Travel: distance or range of motion
Precision: travel or positioning accuracy or repeatability
Environment: surrounding conditions
Duty Cycle: cycle time divided by total time with rest

But, in each one of those, there is a lot to it.  For instance, if I ask you for the load, you might just give me 1 pound.  But, I need to know more about that 1 pound:  Where is it located?  What is the orientation?  How fast are you moving it?  How far do you need to move it?  But, even at 1 pound, I need to know more a lot about it.  So, what does LOSTPED really mean?

Let's talk about the first two: Load and Orientation.  Remember, if you go back to an earlier episode, Force = Mass x Acceleration and Moment = Force x Distance.  If we look at that, and what Load and Orientation actually mean, here is an example of a simple diagram of an actuator.  Here is the load in the middle and, if it's off-center, that ends up creating some moments.  It's not just about the load down, but it is about where that is located and what moments that can cause.  I'm not going to go into all these equations, but, different scenarios will cause different loading.  We will talk about static loading sometimes, which is the load while there's no motion.  Just sitting here, this 1 pound is 1 pound down on the stage.  But, while it's moving, that can cause a dynamic load because the mass times acceleration can cause a force.  For example, here are some catalog specifications from one manufacturer: it talks about Moment A, Moment B, and Moment C.  Those could be XYZ for that matter.  It talks about the allowable overhang.  This is just an example of what stage manufactures will give you for some sort of dynamic loading.  That is what it might look like.  Again, this is all based upon the load and the bearing life curves that I talked about back in Episode 8.  Here is one orientation: it's just horizontal.  We have a load, but when this load is sitting on top of this, it creates a static load.  When you start moving, it can create an acceleration forwards or backwards which causes a moment loading.  Here are some equations for this orientation.  If that load is way off here to the side, it can cause some other loads.  I talked about these in the earlier episode, but that's what the Load and Orientation aspects of LOSTPED are.  If we look at this one we'll get some more loading here and if you get vertical with it hanging off, you get a moment loading this way.  These are all the equations involved.

Speed is an interesting thing because people will say “I need to go 1 meter per second.”  Does that mean you care about making sure you are at 1 meter per second?  Or you want to move 1 meter in a second?  Because if we look at a curve here, this graph is velocity versus time.  Distance is the area under the curve.  Acceleration is this ramp here.  If we do a constant velocity the whole time, this acceleration is instantaneous.  But, if we want to make this acceleration ramp more of a ramp as opposed to instantaneous which is not possible, then we're going to end up with a higher speed.  If we want to keep the same distance and area under the curve.  If we want to go a lower acceleration rate, then we might end up with a higher speed to keep that same distance under the curve.  This is called a triangular profile.  This is called a trapezoidal profile.  Again, if you say you want to go 1 meter per second, do you care about moving one meter in a second?  Or do you care about the actual 1 meter per second and you don't care how long it takes to get up there or how long you actually move?  What is important?  If you really care about smoothness, then we might be taking a look at an S-curve profile in order to reduce the amount of jerk caused by the points where the acceleration changes.  In which case, we end up with a really steep acceleration ramp here which is more force or torque at the motor and so we have to take that into account.  The speed really has to be about the motion profile, has to be defined with possible accelerations, distance versus time, what's important.  Often times, I'm just asked “I want to go this speed.”

velocity distance time

We have gotten through the LOS of LOSTPED.  Let's take a look at T, the travel or the distance.  What's the real distance requirement?  Some people will talk about the length of the actuator: I need an actuator that's 1 meter long.  Or I need to move a travel that's 1 meter.  Depending upon your stages, or your actuators, these numbers can be a little bit different.  For example, for some actuators we have to define a safety margin, or an over-travel margin, so that 1 meter of an application’s requirement might be here in the middle and there might be, let's say, a hundred millimeters on each end.  That means now we're looking at 1.2 meters of travel.  Now the overall actuator is closer to 1.4 meters.  So, when you say the distance, what is the actual distance you're talking about?  If you define a 1 meter of travel hard-stop to hard-stop, you may have a hard time using that full 1 meter in your application.  You have to take that and travel into account.

Precision: I had a whole episode on this back in Episode 4.  What's the real precision requirement?  I talked about resolution, talked about repeatability, and I talked about accuracy.  You can go back and look at that for more details, but what that actually means in your application is hugely important.  There are a lot of components that affect the precision.  Don't forget those.  You have to go through those.  There’s the encoder.  There is the possible gearbox, belt and pulley, and coupler.  All the products there affect the precision in the end.  Don't forget all the types of precision.  I talked about linear, rotational, abbe error, straight line, orthogonality, concentricity, run-out, rotational wobble.  And, of course, where is the point of measurement?  Is it at the actuator itself?  Is it an inch above?  Is it 2 inches above?  Is it off to the side?  Where is that point of measurement?  That's actually important.  

Then the E in LOSTPED is the Environment.  For a lot of people this may not seem important, but there may be a washdown environment where there is fluid that cause rust and corrosion.  It may be dusty and dirty which can cause the mechanics to gum up and jam.  It may be a vacuum which causes plastics and other materials to outgas.  It may be extreme temperatures which causes materials to freeze or melt.  Or radiation which causes decay of materials.  I've worked on mechanics and other products through all these environments and these are a lot of things that we come up against.  If you have no extreme environment, then it doesn't matter, but it begs the question: what's your environment?

The last one is Duty Cycle. Duty cycle is something people forget about because they will just think about the motion profile.  But, there could be a dwell time after it.  If we look at a distance here, maybe you're moving out and then moving back.  The time in between moves gives you a dwell time.  If you're moving out and back, out and back, with no dwell time in between, then you have a hundred percent duty cycle.  But, if you are moving out and waiting and then moving back and waiting, your duty cycle goes down and that changes.  It changes the lifespan of your mechanics, changes the actual torque requirements on the motors.  It makes a big deal.

I hope that summarizes LOSTPED and kind of packages everything nicely into one episode for you.  I think it would be a good point of reference in the future, so remember to follow my #MotionControlShow and check out this link for other episodes.  Thanks for watching.  I'm Corey foster of Valin Corporation.

If you have any questions about this give Valin a call today at (855) 737-4716, or fill out our online form.