Why not brushless motors in direct drive extruders?
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Brushless motors require some kind of feedback system to tell the controller their position. It could be an encoder or an accelerometer or something like that.
Then you need a reasonably complex circuit to read the position and the desired position, calculate three different signals and send them to a simple, yet slightly costly driver circuit to drive the coils on the brushless DC motor.
It's certainly possible, but considering that a stepper motor IS a brushless DC motor I'm not sure it would have any advantages.
A 30gm motor might run a camera gimbal, but can it produce enough torque to push filament through a hot-end? Probably not. And by the time you get a motor large enough to do that it will probably be just the same size and weight of a stepper motor used for extruder.
Maybe you can do some experiments and build us a much better lightweight extruder motor?
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@alankilian I´m not an engineer but I have spent time with electric motors, drones and rc-planes. I understand that we have a complex issue here. But why do we need to be so accurate? Extruder is not as critical as xyz -drives. The motor needs to know the direction and rpm. Now I´m talking about brushless motors.
Your suggestion of developing a stepper motor to a lightweight one is interesting. Maybe the spinning iron could be replaced by tens of small super magnets? That´s roughly what is the difference between brushed and brushless motors. I´d like to know if someone has already done this (as usual
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@pertti A stepper motor is the most simple model of brushless motor. The ones that you think of, as used in RC models, have 3 phases (they have 3 coils) and are designed for a very high power/weight ratio and quite high RPMs. Also they produce a lot of heat that must be somehow removed - not a real problem in any fast moving model/drone.
The steppers are simpler as they use just two phases (they have just two coils), are designed for much lower RPM and much higher torque and are cheaper as they need just iron (high quality magnets are crazy expensive since the whole EV craziness - but this off topic!). Not to mention the significant precision. Also, the overall required power is significantly lower and also is the produced heat.
While you don't see why filament extrusion requires precision, just think about a very finely detailed object that needs printing. That implies a lot of quite dramatic direction changes, so the extruder precision is important in order not to over- or under-extrude. If you follow the various topics on this forum, you will see a serious debate on various extruding rate estimation/models. If you consider the 1.75mm filament and a 0.3mm nozzle, for every 1mm of printer head movement you need to extrude just under 0.03mm of filament.
What you are suggesting is to take a motor that literally has only much fewer steps - twice the number of magnets on the rotor, design a microstepping controller for it and use it instead of the well proven steppers. The ESC in any RC model is, in the end, just an oscillator with 3 outputs with 120° between their phases and a frequency matched to the required RPM. It doesn't have all the blows and whistles in any cheap stepper driver!
So, while not impossible, it sounds more like reinventing the wheel. And based on my own experience I bet that the final result, for the same performance, will be more complex, heavier and a lot more expensive!
P.S. Please don't get me wrong! I'm not against new solutions. Even at work I'm known for looking first of all for cons in any technical suggestion and I have my own share of "exploring the uncharted territories" activities. It's just that I'm used to look at all aspects of any alternative technical suggestion. Some of them I gladly embrace myself and help testing/developing!
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Brushless DC motors and steppers have a lot of overlap.
They both use permanent magnet rotors and wound stators.
(Steppers do not have plain iron cores - if you turn one, you can feel the rotor magnet jumping from alignment with one stator pole to the next).The multi-pole type used for such as drones with open-loop control are little different to low step count steppers, though with three phase drive rather than quadrature.
Like steppers, the drive system needs acceleration and torque limits, otherwise they lose steps and "slip".
For position control, eg. servo applications, BLDC motors generally have just three pairs of stator poles and a rotor with a single pair of north-south poles, so one full power input cycle equals one full rotation. To maintain position control and avoid slipping, those must have a feedback system so the drive can see how far out of position the rotor is, and apply the required current to the correct combination of windings to pull the rotor in to position.
And note that extruder drive systems are extremely powerful; the Hemeras I have on my CoreXY machine are rated at 100N load, just over 10Kg force, with very fast reaction times to accurately start and stop extrusion in the correct places.
Robert J.
Printers: Overlord pro, Kossel XL+ with Duet 6HC and "Frankentron", TronXY X5SA Pro converted to E3D toolchange with Duet 6HC and 1LC toolboards.
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@catalin_ro said in Why not brushless motors in direct drive extruders?:
The ESC in any RC model is, in the end, just an oscillator with 3 outputs with 120° between their phases and a frequency matched to the required RPM. It doesn't have all the blows and whistles in any cheap stepper driver!
There are fancy brushless controllers. I saw for example an application with an odrive where it's set for a constant torque and behaves as spring in a rolling measuring tape. Hi believe it was with a high pole count / low KV pancake motor.
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Here's a video I made probably 7 years ago where I was building a system using a BLDC motor in "position control" mode.
(It looks funny because I have a BLDC motor connected to a stepper motor as an inertial load, so don't look too closely)
This was probably US$500 worth of hardware and a TON of firmware development for me to get it this far.
I loved the TI InstaSpin product and their Piccolo motor-control CPUs and if you're interested also, I would recommend looking into it. (I haven't looked in about 6 years, so this might not be supported anymore.)
SeemeCNC Rostock Max V3 converted to V3.2 with a Duet2 Ethernet Firmware 3.2 and SE300
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@zapta said in Why not brushless motors in direct drive extruders?:
Oh MAN, those look nice! Thanks for the reference.
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@alankilian said in Why not brushless motors in direct drive extruders?:
Youtbue says it's private and can't be viewed.
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@zapta Extruding is all about torque at relative low RPM. This is what I see for those really interesting solutions:
- Maximum torque 1.99Nm or 3.86Nm, but that comes with a peak current of no less than 65A!!!
- A big note - "*Note that torque and current ratings are with Extremely good forced air cooling"
- RPM at no load - 5760RPM or 8640RPM.
The torque I get, at much lower speeds of course, from a rather standard NEMA 23 at 2.4A and no need for extra cooling.
And the pricing... the driver is a merely 179EUR. You get 3 or even 4 decent Leadshine drivers for that amount!
Again, that is exactly what I have stated... it is a solution, bot not for extruding the filament! I would see it more likely for the gantry movement where, in a 3D printer, the required torque is very low but the speed is high. But, still, it might not justify the extra costs...
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@rjenkinsgb said in Why not brushless motors in direct drive extruders?:
They both use permanent magnet rotors and wound stators.
(Steppers do not have plain iron cores - if you turn one, you can feel the rotor magnet jumping from alignment with one stator pole to the next).Unless they are variable reluctance ones!
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@catalin_ro said in Why not brushless motors in direct drive extruders?:
Unless they are variable reluctance ones!
Yes, but only permanent magnet types are used in such as 3D printers, as far as I'm aware?
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@catalin_ro said in Why not brushless motors in direct drive extruders?:
Extruding is all about torque at relative low RPM
I am not an expert but isn't this what gears do, trading between torque and speed?
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@catalin_ro said in Why not brushless motors in direct drive extruders?:
And the pricing... the driver is a merely 179EUR
This is probably a matter of demand. With sufficient demand, this can be a much lower cost IC. The question is, is it useful for large scale applications?
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@zapta said in Why not brushless motors in direct drive extruders?:
I am not an expert but isn't this what gears do, trading between torque and speed?
Of course, but those gears and their supporting plates start adding to the weight. And the whole discussion started from the 30g BLDC motor.
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@zapta said in Why not brushless motors in direct drive extruders?:
This is probably a matter of demand. With sufficient demand, this can be a much lower cost IC. The question is, is it useful for large scale applications?
Demand builds up when there is a real need for such a solution. I see a need for it in completely different applications, but not in a 3D printer, not even in a prosumer one. In a professional one, maybe!
Has anyone complained about the steppers limiting the performance or the quality of the 3D printers on this forum? I have not personally checked, but from already 4 years of browsing I don't recall anything significant. Of course, I'm not discussing about faulty steppers or super cheap steppers assembled in a barn in China (by the pigs grown up for feeding the family over the next year) or poorly chosen (wrong size for the job) ones. There are very good quality steppers at very decent prices, most of them manufactured in China. And they are not even difficult to find.
So with a relatively low demand, combined with the high current involved in those drivers (120A peak current! those transistors are anything but cheap!) I don't see the solution getting significantly cheaper. Overall you must also factor in the high current power supply. 24V at 60A is almost 1.5kW peak power for one of these motors. While the average required power is significantly lower, the PSU should be ready to handle those peak currents. Look at this just for reference - https://www.onlinecomponents.com/en/mean-well-usa/rsp200024-43879729.html.
As for recognized professional solutions, check this servo with integrated driver - https://www.sorotec.de/shop/JMC-Servo-Motor-with-integrated-driver-100-Watt---36-Volt---3000-1-min.html. Significant torque with very high speed as 36V and 6A peak current.
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@catalin_ro, I don't know much about the technical aspects or the market potential, to have an opinion.
BTW, Skyentific on youtube often deal with low speed, high torque motors for his robotic actuators. It's an interesting channel. These are high pole count motor, sometimes with additional gearing.
https://www.youtube.com/results?search_query=Skyentific+actuator
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@zapta Quite interesting. Those might be suitable for axis 4 and 5 in a prosumer CNC...
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This video has a teardown of such an actuator. Plenty of poles and a planetary gear for torque, and a smart builtin controller.
https://www.youtube.com/watch?v=Mhxz2Bj2RXA&t=615s
It's not light and not cheap but interesting.
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@zapta said in Why not brushless motors in direct drive extruders?:
Skyentific on youtube often deal with low speed, high torque motors for his robotic actuators.
Those are clones or descendants of the MIT "Cheetah" robot actuators.
They were originally a modified open-loop motor for a large drone, with a hall position sensor and epicyclic gearbox included, to allow them to function as fully commutated BLDCs.
And, they specifically have the ability to be back-driven to accept the weight and shock loading in a robot limb application to emulate biological muscle & limb elasticity, without a high-ratio gearbox that would be damaged by such loading.
The originals are clearly visible in this clip:
https://www.youtube.com/watch?v=xkfXW6XMXaoJames Bruton came up with a lower cost DIY version, using generic drone motors with belt reduction rather than the custom epicyclic gearing:
https://youtu.be/Id11PWxnd4M?list=PLuzW8n0FM__UQutfHUF7jf4jbZwmWM1L6&t=196
