Knowing the inertia of your system is important. Why? Because the more inertia your system
has, the more force will be required to get the system moving. And that is what tells you
what size motor you are going to need. So, it all starts with inertia.
The bad news is inertia isn’t always the easiest thing to figure out – especially
in a typical real-world complicated system. The good news is, the Sure Servo 2 Pro software
has a built-in inertia estimator that measures the actual inertia it sees in your system.
Let’s do a quick example so you can see how this works. I’m using the same hardware
as we used in the quick start video except I added a 4-and-a-half-pound steel pulley
to give us plenty of inertia. This pulley has a 5/8 inch bore and the servo motor has a 14mm
shaft, so I just printed a cylindrical spacer to make up the difference for our quick little
demo. I’ve already launched the SureServo 2 Pro
software and connected to the drive. When I click on inertia estimator I get a dialog
telling me it is going to switch the drive to position mode, it’s gonna mess with these
parameters, and that it will put everything back when the drive is done.
And it warns us that if things don’t complete normally the drive will be left in an unknown
state, so you should re-boot it to get things back. I would also suggest that you save a
copy of the parameters to disk before you start, just in case …
The software sees my drive isn’t enabled. I can enable it here or using my systems panel
switch. I prefer to do it here because the software will automatically turn it off when
the estimator is done. This reminds us that if we have a motor with
a brake make sure it isn’t engaged. Here we want to specify the motor speed and
ramps. You want to pick numbers here that are representative of what your system will
be doing. I can tell you 20 rpm is WAY too slow to get a good inertia estimate. That’s
only a third of a revolution per second – that’s really slow. So I’ll crank that up to 1000
rpm for our simple little demo. The ramps need to be fast enough to give us
a good bump when we change direction or better yet, need to emulate what your system will
be doing. This is fine for us, so I’ll leave it alone.
And if you are using an S-curve put that number here. I’m going to leave that at zero so
we get a good hard change in direction to put a little extra stress on the system.
Download those values to the drive. Now we need to specify two positions. In a
linear motion example you would start at position 1, ramp up to the jog speed specified up here,
and then ramp down to position 2. It’s important that you leave enough time for the system
to get up to speed. If you don’t then the ramps won’t have time to complete and the
inertia estimator will throw a bunch of errors at you and shut down.
Looks like my motor is currently at this position, and I could move that around using these jog
buttons. I don’t have anything that needs to be moved so I’ll copy that to position
1 by clicking on this button. Next, you hold down the jog buttons to move
your system to the next position. For our demo I’ll just rotate the pulley a few times
and copy that to position 2. Just make sure you get at least one full rotation – the
estimator won’t work unless you do. Hit the Start Moving button. The drive will
move the motor back and forth between position 1 and 2 over and over until it has a good
handle on the inertia. I did a factory reset on this drive so it is not tuned which means
it will take a little while to come up with an inertia estimate. If you do this on a tuned
drive, it will come back real quick with an answer. When the estimator feels it has a
good handle on the inertia, you get a green check mark.
J stands for Inertia, so this says the load inertia is around 37 times the motor’s inertia!
That’s incredible and way beyond the capabilities of most servo systems. Normally you want to
keep this mismatch to 3:1 or 5:1 or maybe even 10:1. So it’s pretty amazing that our
little demo can handle this large of a mis-match. You can also see this in the Auto Tune video
where we tune the servo for this exact pulley. And it’s the SureServo2’s huge 3.1KHz
bandwidth that makes all of this magic happen. Hit this Download button to update the parameters
and while I didn’t do it here, you will want to read the new drive parameters back
into the SureServo2 Pro workspace so it has the updated parameters when you save it.
This is just a simple example of what can be done – how much inertia mismatch your
system will handle will depend on your acceleration and deceleration ramps, your speed, system
configuration, etc. The good news is you now have a quick and easy way to measure it!
But, how accurate is this? Just for fun, I went to the AutomationDirect
website and looked up the pulley we are using. One of the cool things about the AutomationDirect
website is you can view the part in 3D right on the website AND download it in any of 60
different CAD file formats. I downloaded one and brought it up in a CAD program called
OnShape. The cool thing about OnShape is it is entirely web-based so you don’t need
a fancy computer to use it and you can use it for free. I specified the material to be
steel, and then clicked this guy to get the inertia matrix.
We want the moment about the Y axis. If I then go to the servo’s datasheet,
I see the motor’s inertia is 48. So, the math says in a perfect world we should have
a 35 to 1 ratio between the pulley’s inertia and the motor’s inertia. The estimator saw
around 37 to 1 which includes real word things like friction of the motors bearings, imbalances,
inefficiencies, etc. So yeah, that’s pretty darn close!
The point is, the SureServo 2 system does a really great job of handling extreme inertia
mis-matches like this because of its 3.1KHz bandwidth. Which is just one more reason it
is such an incredible value. Click here to learn more about the SureServo
2 system and to find more videos like this one. Click here to subscribe to our YouTube
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