In a recent Technology Tuesdays podcast, Brian Scott, application engineer at Nippon Pulse America, talked to me a little bit about linear motors including iron core, U-channel, and tubular linear motors … and the benefits and drawbacks of each. He also shed some light on the major design objectives OEMs cite when migrating axes to linear motors.
Click here or scroll down for an audio recording of the full interview with Scott.
Here are some of the highlights of what Scott had to say about switching from traditional linear motion to linear motors.
1. According to Scott, cost-benefit analysis demonstrates that linear-motor technologies often outperform other linear-motion design arrangements. The way linear motors reduce the number of components in a design tends to reduce maintenance issues — and the direct-drive setups essentially minimize design complexity to make precision control easier as well.
2. OEM engineers switching to linear motors often need a certain amount of force generation for the movement of load in a set time — so linear-motor acceleration or deceleration comes into play here. Other application needs dictate where the design goes from there. “You’ll have some designers just trying to move a load quickly, and some who are trying to move a load very accurately. Some will want to try and do both — to quickly move loads with some accuracy and repeatability — though linear-motion designs that do that drive up cost.”
3. The final design objective that drives OEMs to linear motors — and in a lot of cases, tubular linear motors as the ones Nippon Pulse America manufactures — is form factor. Iron-core motors tend to be fairly wide to deliver on force output. U-channel motors have a long rectangle geometry that’s not very tall but wide. In contrast, tubular linear motors are a compact rectangular form that often fits into even compact applications.
May 23, 2017 Technology Tuesdays podcast: Here’s the recording of my chat with Scott and his answers to more detailed questions about three leading linear-motion technologies. Listen in on the conversation by clicking on the SoundCloud bar below, and then tell us what you think by commenting below.
Linear motors deliver on controllability, and their output-accuracy-to-cost ratio is often justified. So while they’re costlier, their controllability and accuracy capabilities is significantly greater than traditional technology. “The quality of output parts and the way a machine works is directly related to that controllability and accuracy,” said Scott.
There are some major differences between iron core motors, U-channel linear motors, and tubular linear motors. So as Scott explains, the three different linear-motor technologies serve distinct applications.
Iron-core motors have significant iron in the motor, so they’re very stiff in terms of their ability to move loads — and the forces don’t distort the moving part of the motor. In addition, the iron contributes significantly to force generation thanks to its ferromagnetism. This is a characteristic that means the material generates a greater (additive) magnetic field due to the magnetic domains within the iron itself. Iron-core motor drawbacks include the fact that they exhibit cogging — and it’s the very iron that contributes to their extra force that causes this ‘cogginess’ in the motor.
“In short, permanent magnets in the motor attract the iron, which tends to want to lock onto individual magnets. That means every move includes a force vector that acts to pull the iron back over a magnet. What’s more, the iron in iron-core motors contributes to another problem,” added Scott.
Here’s that additional problem: Any time-varying magnetic field creates eddy currents, and these currents generate another magnetic field that opposes the magnetic field being generated by the coils in the motor. Eddy currents — being, after all, current in the iron of such motors — also generate heat, which is wasted energy. Plus the currents induce attraction between the forcer’s iron parts and the permanent magnets, so the latter draw the iron towards the magnetic way or track … necessitating support structures or guides to resolve the forces.
U-channel motors don’t have iron so avoid some of the problems of iron-core motors. No iron means there’s no cogging or attractive forces between the coils and forcer and the magnets. The drawback is that such motors have a lot less stiffness. These use an epoxy backbone to mount the coil and the epoxy’s not a very stiff material … so when the coils are exerting some force in relation to the permanent magnets, the epoxy can deform under that force.
Tubular linear motors contain magnets within a tube, and forcer coils surround that tube.
“Tubular linear motors have the best of all worlds in that efficiently use magnetic flux from the permanent magnets … which you don’t get in the other two types. Plus the coils are structured in a cylinder with inherently stiff cylindrical geometry — so there’s actually a fair amount of stiffness and none of the deformation with movement or application of force.”
Tubular linear motors also efficiently deliver very strong magnetic fields and (again, thanks to their structure) a reasonably decent amount of cooling — something one doesn’t necessarily get with U-channel linear motors.