SureStep Stepper Motors – How to Choose and Use (Part III)

SureStep Stepper Motors – How to Choose and Use (Part III)


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.

9 Replies to “SureStep Stepper Motors – How to Choose and Use (Part III)”

  1. Cool, thank you. Now I know that the last thing to be selected when purchasing the main part of my system is the stepper motor, after I calculated the inertia and torque… Thank you again.

  2. I am trying to build linear slider with the help of NEMA 17 for this i have bought the linear motion bearing and all that stuff which has small friction, my motor has v and I rating 12 V and 1.3 amp and its holding torque is 0.3 Nm will it be possible to pull 5 Kg mass with this motor and set up ????
    and one more question
    they have mentioned location torque
    what does this exactly means????

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