Working of servo motor:

An ac servo motor takes the smooth three-phase stator control of an ac asynchronous induction motor and a permanent magnet rotor from a brushless DC synchronous motor.to create a high performance motor that's ideal for smooth accurate and precise positioning with excellent torque and speed characteristic but how does it actually work l

This rotor from an older servo motor the magnets are potted in some kind of plastic if I hold this magnetic paper up to it yep. We see the magnetsnand it looks like there's eight of them here's a disassembled sure servo 2 motor. On the rotor we can actually see the permanent magnets attached to the shaft under this webbing using webbing instead of all that plastic helps keep the inertia low which makes the motor more responsive and of course we can also see the magnets using the magnetic paper and looks like there's 10 of them on the shaft more magnets equal smoother operation another reason the sure servo2 system performs so well it's important to understand. That the polarities flip with eachnmagnet I have a small neodymium magnet on the end of this aluminum rod. It's attracted to this guy, and then it skips over one and goes directly to the next

Why is that well if this is our rectangular magnet on the rotor the magnetic field radiates out like this and wraps around to the back side of the magnet?

 

 

We can see that if I put a rectangular magnet under this dish and drop some ferrofluid on it all the little spikes are showing us the directionality of the magnetic flux coming off the magnet and a number of spikes shows the intensity if I pull the magnet away the spikes become more distributed or less intense bring the magnet close and the spikes get more intense if I flip the magnet 90 degrees we can see the flux wrapping around to the other side of the magnet this magnetic flux wrapping around the magnet is big because the rectangular magnets are thick the magnets on the rotor need to be thin to keep the rotor inertia as low as possible which means this magnetic flux would be real thin too if I hold a thin magnet up to the ferrofluid yep.

We see a much smaller amount of magnetic flux, so the magnets on the rotor are in this orientation which has the flux lines going out in all directions and that explains why the magnets on the rotor alternate I have these two rectangular magnets with a spacer between them to simulate the space between the magnets on the rotor

What happens when I place those under some ferrofluid bingo we get an enormous magnetic field between the two magnets again instead of little spikes?

 

 

We're seeing the spikes connect and go from one magnet to the next, so it's really the magnetic flux between the two magnets that the stator will be moving around not the magnets themselves if we put the rotor under some ferrofluid yep we can actually see the magnetic flux between the magnets rotating around now I can't rotate it smoothly because the magnets are

grabbing and holding onto the ferrofluid. How cool is it that we can actually see the magnetic fields rotating with the rotor by the way this is the fair fluid I'm using that worked really well on the rotor can we do the same thing with the starter sure, but before we do that we need to.

Understand how the stator is configured. This is the stator that was around the rotor we were just playing with.it has the three uvw wires plus chassis. Ground coming out.it's wired like this the drive's whole. Purpose in life is to adjust the currents in these three. Windings to create just the right magnetic field in a stator and of course this has to be a zero-sum game that is if the drive forces three. Quarters of an amp in here and another quarter amp in here.

It better take one amp out here the three of these. Always have to sum to zero now the magic.happens.and how those are wound around the. Stator notice that the Saturn has 12.poles to wrap wire around it's hard to see here because it's all sealed, but that looks like this. Where the wires get wrapped around these guys. But why are there 12 of these seems like there should only be six.a north and a south pole for each wire.

Well.remember on the rotor how it was the side by side magnets that created the big magnetic field for us. The stator uses the same trick if one pole is wound in a clockwise direction using the u wire. The one next to it is wound counterclockwise with the same wire.to create that coupling of the magnetic fields that gives us that side-by-side magnetic effect.

Let's label this pole u and u bar and. This one little u.and little u bar indicating its reverse round likewise v and w are done the same way, so now we'll get a strong magnetic flux between each of these poles on a stator and by injecting more or less current in each winding the drive can very accurately control the amount of magnetic flux between all these poles.

For example what we would expect to see is a really strong flux here smalle here? Smaller here and the inverse of that on the other side to create a super strong magnetic field to position the rotor let's see if the status magnetic field. Can be viewed using our ferrofluid trick I added these lines.to show where the gap is between each of the saddler windings I'll drop a spacer in here to hold this 3d printed plastic cup at the right level, and we'll put some ferrofluid in the cup this rotor with the encoder attached is plugged into the encoder port of the sureservo2 drive and this starter is plugged into the. Motor uvw port on the drive and if you're thinking this is going to confuse the drive.

The drive is not going to understand what's going on it expects to apply a certain amount of current into the starter and get a specific response from the encoder reacting to that here we have the scope monitoring the motor current and I've adjusted this scale to around plus or minus three amps. We can tell the drive to enable the servo and tell it to move the rotor position. We need to reset any faults when we confuse the drive, and we'll want to see the Saturn in the rotor here we go run the scope enable the servo and nothing the feral fluid didn't do anything

 

 

what happened well when you first power up the drive

It assumes that wherever the rotor is where you want it to be, so it doesn't do anything we can see the total motor current is zero right now if I tell the drive to move the rotor by pressing the jog button then look what happens the total motor current increases and the drive builds up a magnetic field in the stator to try and move the rotor which isn't actually there and where is the magnetic field between the two windings just like we saw with the rotor magnets and look at the shape of the magnetic field it looks kind of sinusoidal with the opposite pole mirroring the other one exactly what we expect to see now the magnetic field is staying there because there's no rotor in the stator to react to the magnetic fields and move to encode the drive is going to keep pushing until it gets what it wants, or it faults out if I press jog to increase the current even more than eventually the drive faults out the current drops to zero and the magnetic fields go away, and the drive gave us an overcurrent fault.

Reset the fault and jog a bit to reestablish the magnetic field if I gently rotate the rotor really the encoder in the direction of drive wants it to go it's really touchy so I'll finish it off by jogging to zero amps. We see the current reduce, and magnetic fields drop perfect if I jog in the other direction get the same magnetic fields but opposite polarity to move the rotor in the other direction. It's a shame we can't see the direction of the fields with the ferrofluid and I'll rotate the rotor in that direction to bring it back towards zero current and I'll finish by jogging to a positive current. Now I'm going to attempt to hold the jog button down and rotate the rotor at the same time to simulate what the drive would normally be doing as I do that you can actually see the stator magnetic fields rotating around.

How cool is that and that's the beauty of the three-phase ac input to the stator the drive can smoothly move the rotor to any position simply by adjusting the currents at each of the three phases to create whatever magnetic flux it needs here on a stator to move the rotor wherever it wants we call that synchronous because the rotor is always going to follow the stator's magnetic flux exactly unlike the asynchronous ac induction motors where the rotor always lags the Stator flux now I say it simply adjusts the currents but realize that requires a phenomenal amount of processing basically all the stuff in the current loop block and all of that is helped by the high bandwidth and extreme resolution of the sureservo2 system.

All of which explains why the sureservo2 system works so well hopefully this gives you a little better understanding of how an ac servo motor works and why ac synchronous motors are what you needn when creating a fast accurate and precise positioning system, and you'll have a tough time finding a servo system.

 

 

Control the servo motor

 

 

By the Blink app through Wi-Fi. The servo motor control signal is sent from the Blink app to Raspberry Pi and then to the servo motor. The first step is wiring the pieces of hardware. You will need to connect the servo power supply from 5V on and GROUND from Raspberry Pi also the PWM pin to the GPIO17 of the Raspberry Pi For the Blink app you will need a simple

project with only one button. In the project setting choose your device choose your Raspberry Pi at the new device. Here I used the Raspberry Pi 2. And then in the project add one

button that's connected to the virtual pin V4.

You can change the mode of the button to from push to switch, but it's really not important. That's it so that's all for the project. The last thing to set is the code to run on your Raspberry Pi So I need to import the python library of Blink into my code plus other library for the GPO control. And don't forget to add your token the Blink token to talk to your Raspberry Pi here. You can assign what you want to do with the servo motor when the event happens here. In this example, I change the duty cycle so the motor can rotate back and forth. It is just for the demo purpose I assume that you already know how to control the servo motor. The Blink is added to the

infinite loop to wait for the event Now you are all set let's run the code in the Raspberry Pi. You can go to Build and Execute if you have terminal you can run it directly from your terminal. I just run it here since I already had the editor opened. It's running, and you will see the welcome board of the Blynk. Now let's go to your app and run the app too. Let's see how it runs on the servo motor.

If I click on the button it rotates back and forth, and you see the print uh the value of V4 is printed here. Let's do it again You see the value here is switching between 1 and 0 because that's the value of the push button. So that's all. That is how you control the servo motor using Blink app.