Linear motors: Day Dreaming about future printers.

JoergS5

What I also like very much is the idea to combine air-beared sliding with a solid construction like taking hardstone: https://www.jfa.de/html/en/produkte/p/53/guidances-with-air-beared-slides-ll-s/

dc42

@bot said in Linear motors: Day Dreaming about future printers.:

Most listings I find for "linear motor" are AC induction motors, so totally not what I'm looking for

That's probably a more practical solution. It should mean that the bar can be a simple steel strip (possibly laminated) with an optical encoder strip mounted on it, instead of needing precisely-milled and epoxy-filled teeth. I think this could be done as a DIY project. Linear encoders can be bought for around $50, see for example https://www.rls.si/en/rlb-linear-component-magnetic-encoder-system.

If it was built as a 2-phase motor, there is a possibility that our upcoming closed loop stepper motor driver could drive it, depending on how much power it needed.

bot

@dc42 said in Linear motors: Day Dreaming about future printers.:

@bot said in Linear motors: Day Dreaming about future printers.:

Most listings I find for "linear motor" are AC induction motors, so totally not what I'm looking for

That's probably a more practical solution. It should mean that the bar can be a simple steel strip (possibly laminated) with an optical encoder strip mounted on it, instead of needing precisely-milled and epoxy-filled teeth. I think this could be done as a DIY project. Linear encoders can be bought for around $50, see for example https://www.rls.si/en/rlb-linear-component-magnetic-encoder-system.

If it was built as a 2-phase motor, there is a possibility that our upcoming closed loop stepper motor driver could drive it, depending on how much power it needed.

Very interesting. I will broaden my casual search. I had no idea this would be a viable option, let alone a superior option to the stepper types.

Typically, though, I'm finding very large AC inductor motors, whereas I want something compact with similar force to that generated by a typical motor/pulley/belt arrangement.

Hmm, here we go. Something like this might be suitable in conjunction with the closed-loop duet expansion: https://www.nipponpulse.com/products/motors/motors-linear-servo

dc42

I remember when I was a teenager, I saw Eric Laithwaite on playing with linear induction motors and magnetic levitation. I got magnetic levitation working - which uses a similar induction principle - but I never got round to building a linear induction motor. I seem to recall that he reckoned transverse flux motors were better than the original longitudinal type, but they were wider.

Thinking about it some more, the stator would be a steel bar faced with fairly thin copper sheet. I think all those years ago, I reckoned that the active bit could be made from laminated steel or ferrite E- or C-cores, which are readily available. You would need to maintain a small air gap between the faces of the cores and the copper face of the stator.

theruttmeister

I looked into linear steppers for 3D printing about 10 years ago.

I was quoted about $1000 per axis, with a 300mm stroke IIRC. That was at volume too.
Most of the companies making them are aiming for 1 micron repeatability, for machines were $1000 is a small cost.

I know of at least 1 person who was working on a low cost linear servo, but he was trying to raise money for it as part of a robotics startup.
You can build something very similar to those NPM linear servos for very little money, the electronics are not super complicated... the hardest part is the control.
With a bit of imagination, you could probably even build a linear stepper in the same style. Thrust output might not be great, but if you can make it a stepper at least control is simple.

Visionary

@dc42

If it was built as a 2-phase motor, there is a possibility that our upcoming closed loop stepper motor driver could drive it, depending on how much power it needed.

Is it possible to drive DIY 2-phase closed loop linear motor with (new) Duet hardware directly (or with additional hardware)? Btw I'm not so sure that you can get enough force from a linear motor without using strong magnets instead of steel bar and a copper sheet. Some example demo videos on Youtube make such a linear motor seem quite weak.

dc42

@Visionary said in Linear motors: Day Dreaming about future printers.:

Is it possible to drive DIY 2-phase closed loop linear motor with (new) Duet hardware directly (or with additional hardware)?

That would depend on the interface t the closed loop motor.

I've looked at the possibility of using a DIY linear induction motor to drive axes of 3D printers - I have some knowledge of them from experiments many years ago. For the Z axis, I doubt they are practical or that they would offer any significant advantages. For X and Y, I don't see any arrangement other than a bed-slinger that wouldn't require one of the motors to be mounted on a moving axes, which is undesirable because then the moving mass would be considerable.

Visionary

@dc42

Peopoly uses one linear motor on each X and Y axis and two linear rail blocks on each Y-axis rail. On their printer it seems to work even though there is some cantilever pulling/pushing going on. Thomas Sanladerer shows some details on the printer on his review video.

STōN-WoLF printer also has kinematic configuration that could be a bit more suitable for linear motors.

I've had (Chinese) ball screws on my Z-axes and they are ok, but they still have slight Z-wobble and they do have noticeable amount of backlash. Using linear motors also on Z-axis would definitely be expensive, but I'm curious if there would be better in every other way (speed, noise, precision, no moving parts, no grease). Preventing bed from dropping hard when unpowered is an issue too.

Many of the examples of DIY linear motors on YT and on Internet use 3-phase setup and drive the motor with VFD or what I assume is a BLDC drive. If it would be possible to use 2-phase setup instead and use directly say 6HC stepper drivers then that would very simple if possible. Or could ODrive Pro-board be used through CAN-connection to Duet to drive a 3-phase linear motor? I do realise that this would be expensive, but e.g. one Clearpath servo imported to Europe costs >400€ and that doesn't even include belt drive components.

o_lampe

@Visionary said in Linear motors: Day Dreaming about future printers.:

Or could ODrive Pro-board be used through CAN-connection to Duet to drive a 3-phase linear motor?

I don't know about O-drive (they are closed source now), but I've used simpleFOC with open source driver boards to drive sensored BLDC motors via step/dir interface.
If it would be easier to DIY a 3-phase linear motor, that would be a cheap solution.

mrehorstdmd

@Visionary said in Linear motors: Day Dreaming about future printers.:

I've had (Chinese) ball screws on my Z-axes and they are ok, but they still have slight Z-wobble and they do have noticeable amount of backlash. Using linear motors also on Z-axis would definitely be expensive, but I'm curious if there would be better in every other way (speed, noise, precision, no moving parts, no grease). Preventing bed from dropping hard when unpowered is an issue too.

A linear motor powering the Z axis doesn't fix anything. It would have to remain energized to stop the bed from falling. When power is cut, the bed will drop.

I don't know how ball screws in the Z axis could have backlash unless there's an alignment problem (screws not parallel to guide rails, guide rails not parallel, maybe both). Belts don't wobble, and a simple worm gear drive will prevent bed motion when power is cut. I use a 30:1 worm reducer in my printer with a 695 mm belt lifted Z axis. The motor is driven directly by the Duet2 Wifi board, and I don't need any special config for brakes, etc. It uses two linear guides and two belts tied to a common 8mm drive shaft. The two belts are just simple loops. There's no backlash. Full step distance is 20 um and maximum speed is about 15 mm/sec. It just works, every time.

Visionary

@mrehorstdmd

The backlash in my case may have come from linear rails combined with a ball screw and a stepper motor. Stepper motors can't quite perfectly instantly reach and stay at commanded position. The torque is very low when stepper motor rotor is very close to position it is suppose to be and this causes some springiness, which shows as backlash. The particular linear rails that I used when I tested this were quite stiff and not very smooth, which caused more load on the system making the backlash more pronounced. This kind of backlash isn't big problem and doesn't show on prints because normally you are only going one way on Z-axis during printing. However, this could cause problems for Z-hopping and nonplanar printing.

I can't remember the exact backlash but I think it was around 0.01-0.03MM (using SFU1605 + Stepper + disc shaft coupling).

Also I had my endstops at max end and therefore I had to home to max on Z-axis, which took maybe like 20 seconds to do due to low speeds. My Z-motors would stall if I went any faster.