Episode #42: Types of Linear Motors

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The Motion Control Show

Linear motors are our next stop on our tour of electric motors.  They can come in small ones.  They can come in really big ones.  After watching this, tell me that California Screaming, now known as Incredicoaster, at Disneyland’s California Adventure in Anaheim, isn't a linear motor…a really big powerful linear motor!  I geek out on things like this.  I hope you do, too, after you watch this.  If you need any help, reach out to us at the website and email address here.  I'm Corey Foster at Valin Corporation.  Let’s see what we can learn.

First off, a linear motor is pretty much the same technology as a rotary motor, just opened up and unfurled like a sail.  It generates a force, a linear force, as opposed to a rotary torque.  And then we no longer call it a stator and a rotor.  Some people call it a primary and secondary.  Others call it the magnets and forcer.  Personally, I just prefer the magnets and forcer.  “Magnets” is a little clearer as to what it actually is. 

If we start by looking at conventional linear mechanics, we have the machine bed here, and a ball screw or lead screw perhaps, and a motor and encoder, a slide or a carriage, and then maybe a linear encoder here along the side of it.  Each one of these points creates a point of elasticity, or some compliance, where there can be a little springiness to it.  If we compare that to a linear direct drive where we might have magnets here, some copper coil here, a slide, and still a linear encoder, the only compliance here is between the linear encoder and the carriage or the slide.  So, you really get some better performance when you have that direct drive of the linear motor.  That'll speak to why we want to have a linear motor in our applications at times.

If we take a closer look at a typical screw-based table, here are the linear guides holding and carrying the load, and then a drive screw.  Again, it could be a ball or a lead screw and then the motor couples to that through the motor mount and the motor may or may not have an encoder on the back of it.  Then the carriage sits on top of the linear bearings and that supports the load.  If we look at the linear motor-based table here, we still have a carriage or table.  We still have the linear bearings here, but now instead of the ball screw or the lead screw we have a linear motor.  Then we have to have pretty much a linear encoder along the side unless it is a linear stepper.  Those do exist.  We're not going to be talking about linear steppers much today.  We're really just talking about linear servos.  Linear steppers do exist and have for a long, long time.  They just aren't very common.  They're not really popular.  But, there are even ones that go both in the X and Y directions.  So, that's pretty cool.  But the three types that we're going to talk about today of linear servo motors are iron core, ironless, and slotless.  These aren't all the types, and there's variations on them, but they give you a good idea of the different pros and cons and different possibilities of the designs.  So, let's go into these in more detail.

Here's the iron core design, where the forcer is up here, or the carriage here with the forcer and the coils.  These are the copper coils as part of the carriage and then it has a single row of the magnets along the bottom.  And then there's an iron back-plate down here that really helps increase the magnetism. 

Then there's the ironless design where you have two rows of magnets along the side forming a “U”.  Then the forcer goes up and down the middle like an “I” or a “T”.  It's non-ferrous.  There's no magnetism in the forcer going up and down.  It's all just the copper coils that are allowing the control along the magnet track.  And you can see the pros and cons here.  There's really no attractive forces when the coils are off and there's no cogging.  Their heat management is a little bit of an issue because you actually are putting the heat into the copper windings in the middle of that “U”.  So where does the heat have to go?   The different designs from different manufacturers will have different ways of dealing with that heat management.

Then there's a slotless design which is shaped a lot like the iron core design.  You still have this single row of magnets down here, and you still have a forcer going on top of it.  There are a couple differences though.  One is that there's no iron back-plate to the magnets so you don't get that huge magnetic attraction.  Also, in this case, rather than the windings being just these individual windings next to each other, this happens to be the very same basket winding that I referred to in the rotary servo motors episode where, rather than being turned into a basket around the stator in the rotary motor, it is just laid out flat, again, just use a rolling pin to lay it out flat.  These windings actually overlap a lot more than a typical one and so you get very low cogging, pretty much no cogging, action there as the motor moves over it.  This does show a laminated back-iron, but it's not nearly the same as a heavy back-iron of an iron core assembly.  Then you have the epoxy that holds the windings in place just like the other linear motors.

Here is a summary of these three different types with the good, better or best.  You see here that the slotless is really “better” across all of them.  There are places where the cost is the best on the iron core, but the velocity ripple is the best on ironless and the slotless is somewhere in between.  So, the slotless is a good average design in between all the best and goods you can see.  Some general ranges are here.  The iron core is really good for heavy duty applications where a lot of ton of force is needed.  The ironless is going to be better where you need that high precision and higher acceleration rates and then the slotless is in between. 

You might be wondering when you're going to use linear motors: 

• High speeds 3 to 5 meters per second.  The speed is really limited by the servo loop of the electronics and the linear bearings that are holding the load. 
• High precision.  It is really important for linear motors.  It's going to be controlled by your feedback device, so your resolution really is dependent upon that feedback device.  You're not going to get the repeatability or the accuracy necessarily of that feedback device because you're limited by mechanics, servo tuning, mechanical stiction and magnetic stiction.  But usually you can get really close to whatever that resolution of that linear encoder is. 
• Then just overall great performance.  You get fast, responsive, and zero backlash.  You get the stiffness of the system.  Low maintenance because you don't have the ball screw in there.  You have smooth motion, low velocity ripple and just a much quieter environment because the ball screw can be pretty darn noisy.  Leadscrews are even noisier usually. 

A few gotchas to be aware of:

• Usually the cost is an issue.  Linear motors are usually more expensive.  However, I have been in applications where I'm working on sizing a ball screw and we want a little more precision out of it, so we put a linear encoder along the ball screw.  By the time we do that, it's actually more expensive than a linear motor often times.  So, if you really trying to squeeze more precision out of the ball screw, you might be better off just jumping to that linear motor. 
• The force per package size: the linear motors just don't get that force per package size that a ball screw does, just because it doesn't have the mechanical advantage of that ball screw. 
• Heating can be an issue. 
• Magnetism can be an issue. 
• No friction can be an interesting issue because if we're trying to do a vertical axis, a Z-axis, and if you turn off the power, that linear motor coil is just going to drop like a rock unless you put a shaft brake on there.  Or you can put a counterbalance, even pneumatic or magnetic or mechanical counterbalance to hold it up.  Or maybe you just don't care, and you just put something on the bottom to just kind of catch it like a spring. 

Interesting story: I was once trying to tune an XY linear motor stage where it had an X-stage and “T” where the Y that was hanging off of it like a “T”.  On the end of the T were some air bearing chucks.  This was on a big granite stage.  When that load was out at the end of the T over those air bearing chucks, I could not tune it.  It could not settle just because there was no friction to the system either on those bearings or in the linear motors themselves until I finally figured out that if they turned down the pressure on those air bearings from 80 PSI down to 40 PSI to induce just a little bit of friction in it, then we could tune it the same way every time.  But sometimes you have to induce friction into a system with linear motors in order to get the tuning performance that you want, because without that, the only friction in there is really the bearings themselves. 

I hope this helps.  I'm Corey Foster at Valin Corporation.  If you have any questions just reach out to us here and we're happy to help.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.