Microstepping and direct drive extruder

With 1/16 step instead of 1/2 step controller on the extruder, do we still need gears on the extruder? I think that microstepping solves the problem of resolution but I'm not sure if it has enough torque to drive a 3 mm feed. The Up! uses direct drive on 1.75 filament so I guess that's one way to do it.

My reason for doing this is to gain the lost build height when using gears. There are many cases where I need to print long parts in the vertical direction.

No, microstepping makes the problem WORSE not BETTER.
You need the gearing to get the torque needed.
The higher the microstepping the less torque that you have.

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

So, do you think we have to use 1.75 mm filament to avoid using geared extruders?

When using 1/16 microstepping I don't know if will be enough of a difference but certainly 1.75 mm filament requires less torque and more speed I just don't know if it would be enough. I know Adrian is experimenting with 1.75 mm filament but I am assuming that he is still using his geared extruder design. Since the 1.75 mm filament currently costs much more per gram than the 3 mm I haven't yet been motivated to move to it especially since I have 12 rolls (different colors) of 3 mm 4043D PLA. I don't know if nophead has experimented with 1.75 mm filament or not but if he has I am sure he can give you good ballpark figures for how much less force is needed to push the filament. But you have to push it much faster to get the same amount of plastic through the nozzle.

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

Why not make a bowden extruder?

That way you remove the entire extruder structure from the printer... gaining much more extra height than you would using a direct drive extruder.

I haven't tried 1.75mm yet but is should need (1.75/3)^2 times less torque, i.e. about 1/3, so yes with a small pinch wheel you could have direct drive. UP uses a motor with a very small gear on the shaft. The downside of that is the flow rate is very dependent on how far the filament is pushed into the teeth.

BTW in my opinion it is a myth that micro stepping reduces the torque. With a chopper driver you just set the current 1.4 times higher so the two coils on state has the same current as full step. Same motor dissipation, same torque.

---

[www.hydraraptor.blogspot.com]

Quote
nophead
Same motor dissipation, same torque.

Won't that make it run hotter?

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

No compared to a full step driver, a microstepping driver sets the "two coils on" current value to 70% of the reference current. If you set the reference current 1.4 times higher it puts exactly the same current in the two coil state as a full step driver would, so on the full step position you have exactly the same current, torque and power dissipation.

In the half step positions it puts one coil on with the specified current, which can be up to 1.4 times the motor rating. In the full step positions each coil only contributes 70% to the torque but at the half step positions the single coil gives a 100% torque contribution. So 2 x 0.7 = 1.4 x 1, i.e. same torque. Power dissipation is 2 x I^2R versus 1 x (1.4 I)^2, so again exactly the same.

So a microstepping driver set to 1.4 times the motor's rated current will give the same torque and power dissipation as a full step driver set to the rated motor current. You only lose torque by microstepping if you don't have enough supply voltage to give the 1.4 times current at the single coil positions, which is the case for the 14V motors Makerbot use, but not the low voltage motors Reprap uses.

---

[www.hydraraptor.blogspot.com]

@nophead: Thanks for the clear explanation.

---

Bob Morrison
Wörth am Rhein, Germany
"Luke, use the source!"
BLOG - PHOTOS - Thingiverse

Not really knowing the working details of a stepper motor, I'm still trying to digest his explanation.

The thing you have to remember is the torque generated is proportional to the vector sum of the two coil currents. [www.zaber.com]

The power dissipation is proportional to the sum of the squares of the two currents.

---

[www.hydraraptor.blogspot.com]

would you not just want to leave the current settings the same, but raise the max voltage to the motors, and change the rate of decay? only do this on a stepper with a chopper circuit, preferably one with pwm.

The torque curve may be extended by using current limiting drivers and increasing the driving voltage. the drive voltage increases the torque. the current limiting enables the higher voltage to not damage the motor.

[en.wikipedia.org]



If changing the current settings has an effect on torque of motor, then your motors are running undercurrent, and getting a lower voltage as a side effect.

IMHO.

It seems more likely to me that the future is to go with an even higher gear ratio (like the 1:246 gearbox here) and a smaller motor, probably a cheap "tin-can" stepper with only 12 or 24 steps per revolution. That would be much lighter and much cheaper.

The thing is that we're vastly under-utilizing the stepper motor in current designs. It's got way more speed than we need, it's a little light in the torque department, and it doesn't have the resolution that we want. It's also expensive. All of that could be changed by going to a stepper like this one (only $4.48 each) and a fancy (but printable) gearbox. Backlash is a concern, but should be quite small with a planetary gearbox such as the one pointed to. The smaller motor takes much less current, and it would then be possible to use cheaper electronics, and would possibly be worthwhile to move to a 5-wire stepper just for the even cheaper electronics.

That particular stepper is 48 steps/rev, but it's the cheapest one a quick ebay search turned up. I can't help but think that there should be even cheaper ones available with a little searching.

---

--
I'm building it with Baling Wire

I made an extruder with a tiny 24 stop motor and a high ratio gearbox [hydraraptor.blogspot.com]. I used it recently to print a large object, actually a 200mm diameter filament spool using 0.8mm filament. The quality was not bad but not as good as my other extruders because it can't reverse as fast.

---

[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> I made an extruder with a tiny 24 stop motor and a
> high ratio gearbox
> [hydraraptor.blogspot.com]
> der.html. I used it recently to print a large
> object, actually a 200mm diameter filament spool
> using 0.8mm filament. The quality was not bad but
> not as good as my other extruders because it can't
> reverse as fast.


Interesting. That would imply that we are actually using the speed. I wonder if it'd be possible to get an even faster "reverse" via a completely different mechanism? Solenoid-driven spacing between the hot and cold ends possibly?

---

--
I'm building it with Baling Wire

Interesting idea. Perhaps it would work if combined with a Bowden cable design so we don't have to lift the motor with the solenoid. That would also minimize the problem of lag with the bowden cable as the cable and filament compresses and stretches during restart/retract.

Whatever you use to rewind and fast forward needs to move quickly and apply a lot of force which implies a lot of power, be it a motor or a solenoid.

---

[www.hydraraptor.blogspot.com]

Right. A quickly and with a lot of force does imply a lot of power.

So any useful (i.e. cheaper but with same quality of printing) design would have to concentrate force in time - i.e. wind up and "click" into place. Which would imply a second motor and a solenoid. Which is getting far too complicated, time to give it up as a bad idea.

I suppose it'll have to be left at "yes, it can be made cheaper with this technique, but with slightly lower quality due to slower retraction"

---

--
I'm building it with Baling Wire

actually the speed is determined by pps (pulses per second) for the motors.


the tiny motors can have 2500 full step pulsed per second over-driven to 24v. A typical 48oz/inch motor hybrid design can do 10000. this is of course without a load attached. you can probably reason that forward or backwards the tiny steppers are not going to move fast with the gear reduction. Still i purchased 5 of them to play around with.... But not to be used for extruder. maybe as an actuator.

48oz motor 200 step per revolution motor
[www.farnell.com]

3.5 oz 48step per revolution motor
[www.jameco.com]

One thing I haven't tried yet is using acceleration on my extruder motor. A few steps of acceleration will probably allow much faster rewind compared to instantaneously changing direction. On my list of things to try.

---

[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> Whatever you use to rewind and fast forward needs
> to move quickly and apply a lot of force which
> implies a lot of power, be it a motor or a
> solenoid.


The amount of force required depends on the mass of what needs to be moved. With a bowden cable, we just need to move the point of contact between the cable and the hot end upwards and then downwards by around a mm or so. There's not much mass there so it should be possible to do this with a solenoid.

Mass isn't the issue its the force to push the filament, something in the order of 50N for 3mm filament.

---

[www.hydraraptor.blogspot.com]

nophead Wrote:
-------------------------------------------------------
> Mass isn't the issue its the force to push the
> filament, something in the order of 50N for 3mm
> filament.

You mean the solenoid would have to be able to take this force while extruding so that it doesn't give way. Agreed. I was referring to the force during retract/restart that the solenoid need to apply. As I think you pointed out, this requires a lot less force than when extruding.

Yes the solenoid has to hold 50N, then let it go and then exert it again for the fast forward. Obviously the force will ramp to 0 and back again, but at the end of the fast forward stroke it will be at least full pressure.

---

[www.hydraraptor.blogspot.com]

Speed Vs. Torque Vs. Resolution.

A small stepper that can do 1000 steps/sec is the same relative speed/resolution as a larger stepper that can do 1000 steps/sec. How many steps per revolution a stepper is only determines what it's torque requirements will be and what sort of gearing is needed for that specific stepper.

So the significant figures are "power per step" which is some mishmash of torque and steps/rev, and steps per second.

Anything else can be made up with gearing. (Ok, it may add a little backlash, but I think that'd be overwhelmed by the backlash already inherent in the stored pressure of the extruder)

Based on that, I was looking at a spec sheet for some tin can steppers, and it listed 24 and 48 steps/rev motors. I'm assuming that these specs are fairly typical, or at somewhat indicitive of comparable-priced motors.

The 24 step/rev motor has slightly lower torque and starting pulse rate that's more than half. So the 24step/rev one is faster and higher power per step.

comparing a hybrid stepper from the same site, the tin can steppers poop out at around 1000 steps/sec, while the data sheet for a random hybrid motor I pulled up still has torque up over 3000 steps/sec. If that's also typical, tin can steppers will never be as good in the extruder because of their speed limits.

That's certainly not the conclusion that I expected to reach!

---

--
I'm building it with Baling Wire

Yes tin can (permanent magnet) steppers are cheaper and lower performance than hybrid. You get what you pay for. Their positional accuracy is poor because the pole pieces are just stamped and folded metal. Also they are optimised for cheap systems, so tend to be unipolar and high voltage to allow cheap constant voltage electronics to be used.

---

[www.hydraraptor.blogspot.com]

jgilmore Wrote:
-------------------------------------------------------
> The thing is that we're vastly under-utilizing the
> stepper motor in current designs. It's got way
> more speed than we need, it's a little light in
> the torque department, and it doesn't have the
> resolution that we want. It's also expensive. All
> of that could be changed by going to a stepper
> like this one (only $4.48 each) and a fancy (but
> printable) gearbox. Backlash is a concern, .

Hi Folks

Newby here - I'm thinking about building a wolfstrap with threaded rod drive for the 3 axis's - what I need is cheap, large step steppers - these may be just the thing!!

Cheers

CS

Edited 2 time(s). Last edit at 06/08/2011 04:01PM by currystomper.