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Learn how to select the right stepper motor for your application. Includes helpful hints, and live demos. Part I is the basics, Part II is how to calculate torque and inertia and use that to select a motor. Part III looks at real world examples and shows why calculations don't always match real world results.
Welcome back. In part II we calculated that it would take about 1.7 ounce inches of torque to move a 110 pound weight on this linear slide. When we did that, did we take into account real world issues like friction and other things that might cause additional drag on our system? No, in that equation we used, we zeroed all of those terms out, right? How much difference do you think that made? Well, Let’s try it and find out! This is the exact slide we used in those calculations. I removed the motor and coupler and added a 2 inch nylon disk so we have a one inch radius. According to our calculations, at a one inch radius we should need 1.7 ounces to move the carriage – right? I have a 2 ounce lead weight here, that should be more than enough … hmmm . not moving. Not even close … Ok, lets add some more lead weights … nope .. how about a little more weight …there we go – now it wants to move. So, it took about 4 ounces to get this to move easily once we got past the initial “stiction” – that’s the extra force required to get it moving. And remember, this is just the slide! What if we add the 110 pounds of normal load we used in the calculations? Let’s add some more lead weights … nope .. a little more weight … nope .. a little more weight …there we go. That was around 12 ounces. So our calculated number was off by 700%. It’s pretty impressive that we only needed a few ounces to move this 110 pound load, but why is it so much different than the number we calculated? Well, the bottom line is the calculated numbers don’t take into account the real world. For example, the drag of the carriage is adjustable using these set screws. You can loosen them up to reduce friction and drag, but then things get looser and you lose accuracy. For some systems that’s fine, for others that need precision motion these screws will have to be tighter which will create more drag. How well rails and screw are lubricated can make a difference. How tight the screw nuts are can make a difference. How long the slide have been in operation makes a difference – it’s going to be a lot easier to move once it’s broken in – right? And of course, this is an extreme example where just a few ounces are moving 110 pounds. So while we were off by 700% remember that it was really just a few ounces. On larger machines where you aren’t dealing with such small numbers you’ll find the calculations won’t be off by such a large percentage. So there are lots of items here that are complicating things and there’s no way we could take them into account because they all depend heavily on how the system is setup and maintained. That’s why it is so important to understand that the calculated numbers are just a starting point to help you get in the ball park. No one knows your system better than you do, so you are the only one that can decide how close those numbers may or may not be. All is not lost though. Remember, when we compared those numbers to the torque chart, we found that this motor should be able to EASILY move the load, right? So even if we were off by an order of magnitude, we should still have a great chance of this working. Let’s prove it. I setup a productivity 2000 controller to tell the motor to implement a profile identical to the one we used in the calculations except I lengthened the travel to 3 inches so it would be easier for us to see in the video. And I did it in both directions so it can run continuously back and forth. I filled up a 5 gallon bucket full of dense marble stone and then put some dumbbells on top of that to get us the 110 lbs we need. It’s just teetering there but hopefully its stable enough for our little proof of concept. Well, let’s try it. Enable the PLC – and look at that, this little motor has no issues moving this 110 pound load. Awesome. In fact, I actually leaned on this weight adding maybe another 30-40 lbs. and the system still had no problem. No missed steps or anything. So it looks like we even have plenty of margin, which again, our calculations and curves implied from the beginning. Well, if nothing else, hopefully you can see why making sure you have PLENTY of margin after doing your calculations is so important. You just never know what the real world is going to throw at you. By the way, there is absolutely nothing wrong with setting up an experiment like this to simply measure the torque you need or on larger systems just use a torque wrench to measure the torque required. But remember, even with that real measured number, we still suggest you leave at least 50% margin … again, you never know what the real world is going to throw at you. One more thing to be aware of: Inertia Mismatch. In a perfect world you want to match your motors rotor inertia and the system inertia. That will give you the absolute best possible motor performance. But in practice, if you can keep within a 3 to 1 or 5 to 1 range you’ll be in pretty good shape. How did we do with our system? Well, our motor inertia is this, and the system inertia the motor sees – we call that the inertia reflected back to the motor – is this. That’s a 37 to one ratio. Which just means that while the motor is perfectly capable of moving this load, it won’t be able to get anywhere near close to performing as well as what the curves show. Which, if that is all you need, then no problem. But if you do need that performance, is there anything you can do about that? Sure! You can get a bigger motor or you can insert a gear box or belt and pulley or anything that implements a gear ratio of some kind. Why? Remember from our equations – the inertia reflected back to the motor is modified inversely by the square of the gear ratio. So by adding some kind of gear reduction, we can drastically change the inertia the motor sees. For example, if we take our system, but add in a 3 to 1 gear box or belt and pulley system the inertia behind the gear box gets divided by 9 which brings it down to an acceptable mismatch level that will allow the motor to perform. Of course you still have to add in the inertia of the gear box. That’s why we call it the reflected inertia – it’s the modified inertia sent to – or reflected to - the motor after gearing. So the bottom line is: You really need two things to determine how well your stepper motor will perform: does it have plenty of margin on the torque curves and does it have a decent inertia mismatch. If you need any help with selecting an AutomationDirect Stepper Motor, please contact our free award winning support team during regular business hours. They will be happy to help. And don’t forget the forums. There are lots of folks there that love to share their years of experience. Just don’t post any questions directed at AutomationDirect’s support team there, they don’t monitor the forums on a regular basis.