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

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


Welcome Back. In part one we saw it was pretty easy to calculate
the number of pulses needed, the step resolution and the motor speed. In this video we’ll learn how to calculate
the torque required and then use that and the speed to select a motor. We’ll also take a look at how to use that
information to so the same thing for other types of mechanisms. Here we go. Step 4: How much Torque do we need to move
the carriage on the linear slide? The total torque we need to worry about is
how much Torque it takes to keep things moving and how much torque it takes to accelerate
the load. And acceleration torque is inertia times the
change in speed over the change in time. What exactly is inertia? It’s just a measure of how much an object
resists being moved. And it’s solely dependent on the mass of
the object. For example, which is harder to get moving
– a clay brick or a Styrofoam brick? The clay brick of course. Why? Because it has more mass. And because it has more mass it takes more
force to get it moving. That force has to be large enough to overcome
what? The inertia of the object. So inertia is just a measure of how much an
object doesn’t want to be moved. And since we are rotating stuff here, we need
torque to get it moving. Get what moving? Well, for a linear slide: The motor has to
rotate, the gearbox has to rotate, the coupler has to rotate, the screw has to rotate, and
the carriage has to move. And all of those things have mass so they
all have inertia – which means they are all going to work against us when we try to
get things moving. So we need to sum all of those inertia’s
to get the total inertia. The gearbox impacts the inertia of everything
behind it by the square of the gear ratio. In our example we don’t have a gear box
so there is no inertia and the gear ratio is 1 to 1 so that gets rid of this term. If you look in the SureStep user manual, Appendix
C there’s a whole bunch of scary looking equations that show you how to calculate all
of this stuff. And while you CAN do all that math that if
you want to, there is a MUCH easier way to do it. For the motor inertia, we just go to the spec
sheet and see the inertia of the rotor is 0.56 oz.-inches squared. If you don’t know what motor you are using
yet, just leave it blank. You can add it in later once you have selected
one to make sure it doesn’t affect anything. And if we go to the lead screw spec sheet
– look at this! It’s already calculated the inertia of everything
associated with the linear slide AND it gives us a factor we multiply the payload mass by
to give us that inertia. Be careful here – is this the weight in
pounds? No, it’s the mass. And since force is mass times acceleration,
we just divide the payload weight by the acceleration – which is gravity in this case, 32.2 feet
per second per second, or since we need inches in this example that would be 386.4 inches
per second per second. This slide can handle 110 pounds, so let’s
divide that by the acceleration to get our mass number, multiply that by the handy inertia
factor and we now have the inertia for this payload and the system inertia of the carriage
and coupler. Let’s add those to our chart. If we had a gear box, same thing – just
get the inertia from that datasheet. We don’t have a gear box but if we did,
we would put it’s here and divide the downstream inertias by the square of the gear ratio. This is really important: We need for all
of our inertia values to be in the same units. For this demo we want ounce inch seconds squared
so when we’re done we can just read the answer right off the motor curves which are
in ounce inches of torque. The only problem is all of these units are
different and how in the world do you convert these units with inches squared to these units
with seconds squared – that doesn’t seem right does it? Here’s the trick. Go out on the web and search for an inertia
units converter. Put the units you have here, the units you
want here and there’s the answer. Easy. Here’s all of our inertia’s converted
to the correct units. The bottom line is we didn’t have to do
any math did we? AutomationDirect provides all the inertias
we need in the data sheets and we used an on line calculator to do the units conversions. So to get the acceleration torque, we just
sum those, multiply by the change in speed in revolutions per second – which we found
back in step one of this video series – and divide that by the change in speed in seconds
– which is our ramp time. To get rid of this revolutions term we just
multiply by 2 pi radians per revolution. We now know the Torque needed to overcome
acceleration inertia. The other Torque term is just the torque needed
to overcome friction, gravity and any other external influences. Gravity is important if you are lifting something
– we aren’t so we’ll set that to zero. Friction is if there is any additional drag
on the system – that was built into the numbers we got from the datasheet, so we can
set that to zero. The External Toruqe is to account for any
other additional drag that might be placed on the system. Maybe you’re pulling something out from
a stack and the weight of the stuff above it is adding drag for example – we don’t
have any of that so we’ll set that one to zero. So we just looked up the inertia numbers in
the datasheets, summed them, and then multiplied by the change in speed and divided by the
change in time. That’s it. We now have the torque required by the motor
and the max speed, so we can flip over to the motor curves and see how we stack up. Our max level is at 720 RPM and .35 oz-in
of torque. As you can see, we are not even coming close
to stressing this motor – even with the full 110 lbs of load on the slide! And that’s not really a surprise because
all of the weight is being supported by the slide – not the motor – and the 0.2 pitch
screw has a lot of mechanical advantage. Which also explains why we didn’t need a
gear box for this application. For applications where more torque IS required,
we recommend that you leave yourself 50% head room on these charts – treat these curves
as the absolute maximum that you do not want to exceed. What if you wanted to use belts and pulleys
instead of a lead screw. Well, it’s the same thing right? You just gather up the inertias calculate
the torque and lookup which motor you need to do the job. The only difference is you’re not going
to find a lot of the inertia values for pulleys and indexing tables, so you will have to fall
back to the equations in the user manual to calculate those. The good news is there are step by step examples
in the user manual showing you exactly how to do it for all three – linear slides,
belts and pulleys and indexing tables. By the way, did you know that AutomationDirect
has a free tool that helps you with all of this? It’s the Visual Sizer and it’s a free
download from here. It’s intended for servo systems, but it
works perfectly for Steppers too. You just build your system, enter each devices
specs, setup your velocity profile, specify your motor and you instantly see how much
torque you will need. And it even handles S-Curves and triangular
and trapezoidal profiles. You can then view your system performance. Here’s what the motor is capable of peak
and rated and what your system requires, peak and RMS. If you do a lot of this kind of stuff it’s
worth spending the time to learn how to use this really cool tool. Regardless of how you calculate the inertia,
please remember that these calculated numbers are just a good starting point. You still need to apply some experience and
common sense. So, how close are these calculated numbers
to the actual measured numbers? Well, check out the next video where we compare
these calculated numbers with real live results …
If you need any help with selecting an AutomationDirect Stepper Motor please contact AutomationDirect’s
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 and questions directed at
AutomationDirect’s support staff there, they don’t monitor the forums on a regular
basis.

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