Servo systems need to be tuned and tuning can be a real hassle. Fortunately, the SureServo2 provides
one of the best tuning algorithms in the industry, and most of the time it will be all you ever need.
But, if you need more control over the auto tune process,
the SureServo 2 system gives you several options.
Tuning mode 1 allows you to manually specify the Frequency Response level.
Mode 2 also gives you control over the load inertia instead of letting auto tune estimate it.
And mode 3 adds control over the command responsiveness.
Of course, if you are some kind of servo savant, you can always manually tune the drive but that
means taking control of 3 or 4 dozen inter-related parameters yourself. Good luck with that.
The point is, the SureServo 2 system lets you work at whatever level you are comfortable with.
For this video, we’ll stick with the auto tune which does an excellent job, and again,
most of the time it’s all you will ever need. I’m using the same hardware that we used in
the quick start video, but I also added this four-and-a-half-pound steel pully
to give us some inertia and shimmed its 5/8 bore down to the 14mm motor shaft size
using a 3D printed cylindrical spacer. I reset the drive to factory default,
so you know exactly where I am starting from. Don’t forget to do a power cycle after a
factory reset – that ensures any residual non-volatile parameters get cleared out.
Auto tune is completely self-contained so you don’t need to do anything to prepare for it.
You CAN start auto-tune from the drive’s keypad – the user manual shows step by step how to do
that - but it is much easier to use the free Sure Servo2 Pro software and a USB cable.
I’ll connect to the drive and read the servo info, so you can see what hardware I am using.
Click on Auto tune. You have two choices. You can have your controller control the motor’s motion
or have the drive do it. You would use the controller option if you have a servo
in a system and a controller already driving it along the intended path.
That way you can tune it to your exact application.
I don’t have a controller connected, so we’ll let the drive control the motor during auto tune.
The software warns us to make sure the emergency brake works, no one is near the machine
and it warns us that the system may resonate and do some funny stuff so don’t be alarmed.
If you have a braking motor, make sure the brake is not engaged.
The software sees I haven’t turned the motor on yet. I can do that via my panel switch or just
do it here. I prefer to do it here because then I don’t have to remember to turn the panel switch
off when I am done. Again, the software warns us to make sure everything and everyone is safe. Yep.
Now we need to tell it how fast to spin and how fast the ramps should be. You want to emulate
what your system will be doing. It’s important to make sure the speed is fast enough to get a
good bump when the motor changes directions so auto tune can get a good feel for the inertia
it has to deal with. A jog speed of 20 rpm is WAY too low to do that – that’s only a third
of a rotation per second. So I’ll put that at 1000 rpm for our little spinning pulley demo.
Likewise, to get a good bump when the motor changes directions, we need for the
ramp times to be reasonably fast, so I’ll leave those at 200ms. If you can do faster,
then make sure this is as small as is realistic for your system.
Again, you want to simulate the kind of jerking around your system will be giving the motor as
best you can. If you will be using S-Curve in your application and know what that value is,
then add that in here. We’ll skip that one for our demo to give us a good hard change in direction.
We’ll skip these for now, just note those are the same parameters the different modes
we mentioned a moment ago give you control over when doing the semi-automatic tunings.
Hit the download button to send those to the drive.
Now we need to tell the drive how far the motor needs to go between direction changes.
When we specify these two positions for the motor to bump back and forth between,
they need to make sense. That is if we start here at position 1, ramp up at this rate,
run for a while, ramp down to position 2, these positions or this total duration needs to be far
enough apart to give the motor time to ramp up to speed. If you don’t give it enough time, then it
won’t have time to ramp up to speed, and auto tune will throw some errors at you and will shut down.
So, I’m going to take my current position and copy that to position 1 by clicking this button.
Now by holding down these jog buttons, I can move my system to a new position that is far
enough away that the motor has time to ramp up and back down. That makes more sense in a linear
motion example, but for us, I’ll just let it rotate a few revolutions. Ok,
that ought to be enough time to get up to speed. Again, nothing precise here, just need to go far
enough for it to ramp up to speed, or better yet, to match the system you will be using it in.
Just make sure you get at least one full revolution – auto tune won’t work if you don’t.
Now I just copy that position to position 2 by hitting this button.
And I can still modify these two parameters if I want to and download them to the drive.
And again, we’ll skip that. Everything is set up,
so we hit the start moving button. This gives us a chance to see if we
believe the system is getting up to speed and everything is working as expected. If not,
then we would hit stop and go back and modify things and try again.
Looks like the motor is getting up to speed so I hit NEXT. The software reminds us one more
time to make sure everyone is safe. And auto tune begins. If you watch these parameters, you can see
it is taking less and less time for things to stabilize and the overshoot is getting smaller
and smaller as the tuning gets better and better. Looks like it found a harmonic resonant frequency
and applied a notch filter to suppress the mechanical resonance the auto tune found.
Cool. And we’re not getting any overload warnings. So far so good.
You can also see how long it is taking up here. Looks like the motor current
is around 260% of rated – that tells us we’re trying to handle a large load!
The Max Overshoot is in PUU – that Pulse User Units. That’s determined by the electronic gear
ratio of parameters 1.044 and P1.045 which by default is 100,000 PUU per revolution.
I’ll fast forward the video a bit and when these numbers get nice and small, auto-tune is done!
When I hit OK, we see a list of all of the parameters auto tune optimized.
This is the parameter number, what it was, what it is now, and a brief description.
These are all of the parameters YOU would have to manage if you did this manually yourself. Yeah,
now you see why I’m such a big fan of auto tune! Parameter 1.037 tells us that auto tune calculated
roughly a 36 or 37 to 1 inertia ratio mismatch. It’s crazy that the SureServo2 system can handle
that. Most servo systems need the mismatch to be lower in the 3 to1 or 5 to 1 or maybe even 10 to
1 range. The reason the SureServo2 can handle such a large mismatch is because it has a 3.1
KHz bandwidth which makes it responsive enough that it can handle extreme inertial loads.
Now DON’T Miss this STEP! Auto tune has calculated all of these for us but they have NOT been written
to the drive. I really wish this button was a LOT bigger or highlighted or something because
it is easy to miss. I’ll hit update and now all the new tuning parameters are in the drive.
Don’t miss this step either: make sure you read the drive parameters back into the SureServo2
Pro workspace so when you save it to disc so you have a copy of the new auto tuned parameters.
Auto tune returns the drive to whatever mode you were using
except you will now find your SureServo2 system is more responsive and accurate.
One of the really cool things you can do with the SureServo 2 Pro software is you can run
the Scope while auto tune is doing its thing. In this example, I tracked the commanded position
vs the actual encoder position. If we zoom in on the early data, we see the white is the commanded
position, and the purple trace of the actual shaft position lags by about this much and misses the
mark by about this much. If I zoom back out and then in on one of the latter commands, we can see
the lag is only this now and the tracking is much better! All courtesy of auto tune and the
built-in o-scope. Very cool. Just make sure you set the o-scope window width to something wide,
use 32-bit numbers and start the scope before you run auto tune. You won’t be
able to run it once auto tune has started. There’s a whole separate video teaching
you all about the o-scope so check that out when you get a chance.
If auto tune has trouble completing due to the high inertia load, then try slowing
down the jog speed and increasing the ramp times. That will usually get things working.
And of course, you can also adjust these guys. Also, if it is really struggling, it could be
because you simply have too much inertia for the motor to drive. Check out this video on how to use
the built-in inertia estimator. It’s an awesome feature of the free SureServo2 Pro software that
figures out the inertia of the system for you. Well, that should be enough to get
you started with auto tune. Meanwhile, click here to learn
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