Closed-loop control for low-cost 3D printers

The topic above is deliberately open-ended and proposes nothing more than closed-loop control, hopefully to encourage people to think laterally and very widely instead of being shackled by a specific construction.  The range of possibilities is enormously varied, probably infinite.

I will however express my own preferences, which are much narrower and more tightly directed.  Please don't be constrained by the following.

Personally, I think closed-loop 3D printers need to head in the direction of direct drive, avoiding intermediate transmission components as much as possible.  Not only would this eliminate loss of rigidity and the severe problems of slip and play and backlash, but it would also open up the possibility of printing our own motors using pancake designs (effectively linear motors arranged in a circle).

This direction is not in the slightest bit easy, but the elimination of transmission components would make this approach more viable at MEMS scales, which are on the path towards which all engineering is leading:  nanotechnology.  The machinery which builds the machinery which builds the machinery which builds the machinery ... of nanoscale systems is in our grasp right now.  It's going to be an interesting voyage.

Morgaine.

so your suggesting that you use servos with encoders? thats dandy and all but its much more complex to set up being that as far as i know (which is verry little i will admit) there arent any programs as side from mach3 that will read encoder data not only that but the reason most people sue steppers today is because they are much cheaper and simpler to use in simple hobbycraft

if you are talking about for the wave of commercial 3d printers? well Great! i definitely see the benefit to useing encoders to determine position it will provide more accurate movement and possibly a better print quality

I'm talking about making the work head's position be determinable to high levels of accuracy --- the distinction between accuracy and precision is important in this context, because we need to know the true position of the hot end when printing.  Inferring head position from the commands we've sent to the motors is absolutely not adequate --- that's open loop operation.  Whatever means are used to move the work head (it's not limited only to hot ends), the motive force should be part of a closed-loop system to reduce the error signal between where you are and where you want to be, as determined by the work head and not by the motors.

There is no difference in setup complexity.  In fact closed-loop systems generally require less calibration since the whole idea is that negative feedback should compensate for the work head being in an unexpected place --- that includes being in the wrong place because of latitude at assembly time.  Good design of positional sensing corrects a huge range of cumulative errors quite automatically.

Commercial servomotors containing encoders employ the same approach but are not needed here.

My interest lies entirely in individual empowerment, and whether the commercial sector picks it up or not isn't particularly interesting except that it might lower the cost of components.  In the end, the future is in our own hands, regardless of where companies want to go for profit.  It's unlikely and would be surprising if the same solutions were best for both.

Morgaine.

Hi Morgaine,

It's what Nanotec is already selling for industrial use: stepper motors driven as 2-phase brushless DC motors. Very nice features as high torque, high accuracy, low noise....

It's a lot to hope for that the open source/hardware community could easily solve problems which have been at the core of machine tool design for the last 300 years (or more). However there are some modern developments that might help a bit.

The traditional machine tool approach relies on stiffness of the mechanical structure and this makes the design of the feed back control system much simpler - so my conventional HAAS CNC milling machine uses a cast iron framework, ball screws and positional feedback. I can (slowly) manage the same precision as the mill when doing a task under the microscope using soft biological structures, visual feed back and a lot more processing. The RepRap and all the similar machines use cheap versions of the traditional machine tool structure but without the feedback (as you mentioned in your first post).

There are two ways you could make cheap and much more precise 3D manipulators - one is to try to reduce the cost of the traditional stiff structure and the other is to attempt a 'soft' machine with much better control systems. People have been trying the first approach for ages and I don't think you will see the cost of a machine with a 30cm cube working area drop much below a £10k - so we had better look elsewhere.

The RepRap approach is dogged by issues with backlash, friction, steppiness of the stepper motors, frame rigidity etc etc - I'm not convinced that it's the best place to start.

Reasearch into soft manipulators doesn't seem to have resulted in any commercial products (can anyone tell me of one - I'd like to be wrong) so I'll offer my own cheapo idea:

How about a suspended hexapod -  I think you can design it so all 6 legs are always in tension so they can be strings wound up and let out by brushless motors - now if someone can suggest how to measure the position accurately enough you're almost there - apart from the control system.

You might prefer hydraulic actuators (much faster, bipolar forces etc but also much more expensive). There are biological prototypes for both schemes.

MK

What a great thread. Unrelated, I was recently trying to figure out how to measure across a small distance (about 10mm) and I was thinking of using a cheap micrometer end and some sort of camera measurement since they usually have their markings etched quite clearly.

For a larger platform for a 3D printer, if the string or wire is always wound on a cylinder (like an elevator), a normal rotary encoder could be used maybe? This assumes the wire rolls flat of course and not in a bunch - no idea if that would happen :-(

Or, replace the wire with flat metal tape of a controlled thickness - then it can roll on top of itself, and the software can compensate for the thickness as more tape is rolled up.

EDIT: I think it would be worth an experiment in one dimension, if some tape could be found!

Michael Kellett wrote:

 

It's a lot to hope for that the open source/hardware community could easily solve problems which have been at the core of machine tool design for the last 300 years (or more).

I'm not sure why you say it's a lot to hope for --- it's the same brains in action in both cases, and companies have a much higher hill to climb.  They have to create a design that not only works properly, but that also employs such cheap components and manufacturing techniques that the profits from sales can pay the huge expenses of company premises and equipment, the salaries of the 90% of their staff that do nothing technical, many times that to pay for the yachts and mansions of directors, not to mention corporation tax, and I've only just begun with the list.  It's a miracle that they manage to get anything to market at all, because the odds are heavily stacked against it.

Individuals and communities in contrast primarily just need to think.  Think of alternative designs that accomplish their desires without needing to be viable commercial products, think how to overcome the hurdles of not having deep resources, think how to share ideas with the whole world so that different groups can reinforce each other (FOSS and OSHW are excellent examples of that) instead of acting as enemies, and think how best to harness those resources which are not in short supply, like their own manpower.

Fortunately thinking is cheap, and everyone can do it.  It's curious though how many fall for the propaganda that they are valueless unless they offer their labour to corporations, at which point they are magically empowered to do great things.  Actually, no, all people can do great things, and they don't need to be serfs in a machine designed to funnel benefits to the top of the pyramid to do it.  The success of FOSS highlights this very well, so much so that the very concept of closed source software is starting to sound retro and misguided even to VCs.

I certainly agree with the rest of your post.  There is nothing that we can't achieve if we don our thinking caps.

Morgaine.

Victor Sluiter wrote:

 

It's what Nanotec is already selling for industrial use: stepper motors driven as 2-phase brushless DC motors. Very nice features as high torque, high accuracy, low noise....

The NEMA 23 steppers in my Shapercube still-in-construction are from Nanotec, model ST5918M1008-A.  Do you have the model number of the ones to which you're referring to hand?

Morgaine.

You'd have to be able to put an encoder on the rear shaft of the servo, and then use their drivers. Here's some more background info:

http://en.nanotec.com/products/1034-smci33-stepper-motor-drive-with-closed-loop-controller/

http://en.nanotec.com/support/application-notes/closed-loop-description/

Victor Sluiter wrote:

 

You'd have to be able to put an encoder on the rear shaft of the servo

In other situations, sure, but not for our purposes here.  It's not the shaft position that we're trying to control closed-loop, but the position of the work head.  A servomotor with attached encoder could certainly be used as a shaft driver instead of a dumb motor, but in that case the work head is being driven open loop by the servosystem, not closed loop.  This is what I meant in my answer to Chris.  Just because some component of a system uses feedback internally doesn't make the overall control regime closed loop.

To provide a work head with closed loop positioning, the work head needs to generate the positional information itself, and that is fed back to the control amplifiers and drivers.  The exact physical arrangement will depend on mechanical construction, but a very common design is to scan a linear graticule lying along each axis of movement of the work head, for example optically.  Success requires the tiniest movement of the work head in any one dimension to be detectable as a proportional change in the corresponding sensor output, regardless of whether the corresponding drive motor has moved at all.  This would produce a feedback error signal even if caused by unintended coupling between axes, non-rigid assemblies, thermal and other environmental creep, and so on.

Encoders on motor shafts solve almost nothing in 3D printing other than detecting lost steps if steppers are being used, which is the least of our problems.  In the context of the work head, that's still open loop operation.

Morgaine.

Hi Morgaine,

I catch your drift, and you're right in most of what you say. Has anyone ever looked at the (incremental) postion feedback which can be found in inkjet printers?

Where I have to correct you a bit, is that "it solves almost nothing", because it does reduce the 'jerkiness' of movement, which reduces vibrations. Also the resolution can be improved (<<0.9°). Especially the first one can be a major benefit.

The problem with "motor/some kind of leadscrew/sensor for feedback" linear positioning systems is much more difficult to solve than just having a good position sensor at the end. Indeed with simple leadscrews overall feedback may give no benefit compared with motor position feedback. This is because overall feedback can't deal with backlash, dead zones or mechanical hsytereisis very well. To overcome these problems you need to use ball screws and/or preload but these are expensive. As I referred to earlier we have literally hundreds of years of refinement built into modern machine tools.

At least some inkjet printers work in a slightly different way which would be applicable to some additive 3D techniques but not all. No attempt is made to precisely control the position of the print head but it is traversed at about the right speed and the sensor is used to measure exactly where it is at any given instant and the ink squirted accordingly. This is a nice technique where it can be applied because it sidesteps the issue of backlash and precise position control.

It wouldn't work with toothpaste style 3D printers because the position of the head must be precisely and accurately controlled at all times and it won't work where the load is variable.

MK

The problem with "motor/some kind of leadscrew/sensor for feedback" linear positioning systems is much more difficult to solve than just having a good position sensor at the end. Indeed with simple leadscrews overall feedback may give no benefit compared with motor position feedback. This is because overall feedback can't deal with backlash, dead zones or mechanical hsytereisis very well. To overcome these problems you need to use ball screws and/or preload but these are expensive. As I referred to earlier we have literally hundreds of years of refinement built into modern machine tools.

At least some inkjet printers work in a slightly different way which would be applicable to some additive 3D techniques but not all. No attempt is made to precisely control the position of the print head but it is traversed at about the right speed and the sensor is used to measure exactly where it is at any given instant and the ink squirted accordingly. This is a nice technique where it can be applied because it sidesteps the issue of backlash and precise position control.

It wouldn't work with toothpaste style 3D printers because the position of the head must be precisely and accurately controlled at all times and it won't work where the load is variable.

MK

Hi Michael,

Good point, thanks!

The feedback at the motor axle could help here because the speed (or better: accleration) of the motor could be controlled much more, thus making the movement smoother, and if you know what the next G-code is, you could take care to slow down the head in time to prevent overshoots. Don't know in how far that's implemented in current 3D-printers. Dead zones might also be somewhat compensated if known... I agree that that's a very hideous EE-domain fix for a Mechanical domain problem....

Michael Kellett wrote:

 

overall feedback can't deal with backlash, dead zones or mechanical hsytereisis very well.

It certainly can deal with backlash, so I'd be interested to read anything suggesting that it can't do it very well.

The reality is actually the opposite:  overall feedback returns an error function that contains (as a component) the actual backlash experienced by the work head, instead of a simplified error function that encompasses only a limited set of transmission components and/or which deals with each axis separately.  The needed information is certainly there.  It's a lot harder to interpret this actual feedback of course compared to simplistic single-axis feedback obtained nearer the motors, but it's the real thing, not an approximation.

In contrast, motor or feedscrew-level feedback isn't really closing the loop at all from the PoV of the work head, and so it can't compensate for unplanned interactions between axes and other structural problems, as it simply gets no data on them.  The consequence of this is that very high quality construction is still mandatory to avoid the unmonitored anomalies from appearing in the first place in such a half-open-loop system, which of course precludes its use for the subject of the topic, namely "low-cost 3D printers".

Morgaine.

First let's get our definitions in order, from Wiki:

in mechanical engineering, backlash, sometimes called lash or play, is clearance or lost motion in a mechanism caused by gaps between the parts. One source defines it as the maximum distance through which one part of something can be moved without moving a connected part. An example, in the context of gears and gear trains, is the amount of clearance between mated gear teeth. It can be seen when the direction of movement is reversed and the slack or lost motion is taken up before the reversal of motion is complete.

Note that bit about "can be moved without moving a connected part" - if the actuator is pushing the load along and needs to reverse the force (for whatever reason) the actuator must move back by the backlash distance. In any real system this takes time and no amount of feedback and no amount of cleverness of control algorithm can do anything about it (the time is set me purely physical constraints). You can try to minimse the effect of backlash by having a very fast slewing actuator but this often results in instability and increases costs. The problem is that in the backlash or dead zone the actuator is not connected to the load so in the time it takes to reconnect the load is uncontrolled.

Clever strategies to try to and reduce the impact are possible (and routinely and instinctively used by manual machine tool operators.). Under some conditions it is possible to ensure that the direction of applied force never changes in a critical place but these restrictions are often inconvenient.

A low cost mechanism may easily be so poor in it's mechnical performance that there is no gain in improving the control. Stepper motors are very poor at fast dynamic control so I can easily believe that there are many cases where control of the stepper motor shaft is no worse than attempting to control the load position, but at least has the advantage of cheapness and robustness (robust in control stability sense).

MK

Michael Kellett wrote:

 

Note that bit about "can be moved without moving a connected part" - if the actuator is pushing the load along and needs to reverse the force (for whatever reason) the actuator must move back by the backlash distance. In any real system this takes time and no amount of feedback and no amount of cleverness of control algorithm can do anything about it (the time is set me purely physical constraints). You can try to minimse the effect of backlash by having a very fast slewing actuator but this often results in instability and increases costs. The problem is that in the backlash or dead zone the actuator is not connected to the load so in the time it takes to reconnect the load is uncontrolled.

That's a very actuator-focused view (instead of being concerned mainly with the work head), and it's also a time-focused view whereas our main concern is that the work head be in the desired position (time is not the key requirement).  It's entirely normal to trade off time (speed of operation) to lower the cost of end products --- this is why low-end 2D printers are slower than higher end ones.

In order to get the work head to a desired position in axis X, the only requirement on the drive train is that this position is reachable in a monotonic traversal using actuator X.  It is not a requirement that the position be reachable by tiny bidirectional increments of the actuator which would litter the workspace with dead zones --- backlash in a given axis is taken up at the start of a movement in one direction in that axis.  The control loop doesn't even need to know that backlash exists in the drive train, because all it's concerned with is reaching its seek position --- backlash just looks like some extra sluggishness in reaching the desired point.  If the backlash is severe enough that it's noticeable at the work head as bumpiness in the extruded plastic at points where there is change of direction, then the control software just needs to reduce the feed rate at the start of changes of direction.  This is no big deal, and it shouldn't be presented as a terminal stumbling block.

I think we may be looking at two very different goals, one (the topic of this thread) which is creating 3D printers at very low cost by closing the loop at the work head to compensate for less than ideal components, and two, making industrial machinery with good open-loop performance, which is extremely interesting in its own right and contributes hugely to the thought process, but is not the end purpose in this thread.

Morgaine.

It's the difference between kinetics and kinematics.

If the dead zones are known, then you can easily add that in your control scheme. Not perfectly, but still. Otherwise, adding an accelerometer to the end of your tool head might tell you a lot about the accelerations in the head, and can at least tell you when it starts moving. That, combined in a filter with a stepper motor and a control algorithm might already give a great performance boost.

Victor Sluiter wrote:

 

Otherwise, adding an accelerometer to the end of your tool head might tell you a lot about the accelerations in the head, and can at least tell you when it starts moving.

Yes!  That's an excellent additional piece of information that the printer firmware can use to do a better job, and at very little cost.  It's probably true to say that just about every kind of data acquired at the work head can be of benefit as feedback.

Morgaine.

PS.  As an example, low cost physical construction can result in very nasty undamped vibration modes, which can be annoying, harmful to effective resolution, and reduce lifetime.  Accelerometers with adequate bandwidth can help you detect and avoid equipment resonances.

By ignoring the accumulated experience of machine tool design you are doomed to repeat all the mistakes of the past. 3D toothpaste style printers have to control speed and precise position of the print head in order to manage the 'thread' of semi molten plastic that they are depositing. They absolutely must cope with direction changes while maintaining positional accuracy and they must control speed because it isn't possible to change the extrusion temperature rapidly.. They don't have to move fast but extrusion type 3D printers are already very slow.

Adding an accelerometer won't help unless you have an actuator which can operate in the backlash zone. If you consider biological systems they use multiple sensors and multiple actuators, very complex control systems (brains) and require a vast (multi million cycles) amounts of training. So far we have had very little success in copying such systems but done quite well in finding alternative methods. For example replacing panel beating with press tools.

Back to the 3D printer - you can't always add a better control system at an existing mechanism and get the performance you want - it is almost always necessary to design the mechanics, sensors and controls as a complete system with due reference to the spec.

With that in mind, what is your performance target for your improved 3D printer ?

Michael Kellett wrote:

 

With that in mind, what is your performance target for your improved 3D printer ?

Nanometer resolution at terahertz deposition rates, what else?   The only limits are those we impose on ourselves, or try to impose on others.

Admittedly we're not in that operating area yet, but of one thing I am certain:  saying that we can't do it because 300 years of experience tells us that we can't is totally doomed to failure.  Future 3D printers won't look anything like current ones nor like CNC tools of the past, so quoting past experience is not particularly useful.  In contrast, control theory is extremely robust and applies at all scales, so closed-loop systems is a very good place at which to start.

Michael Kellett writes:

 

Back to the 3D printer - you can't always add a better control system at an existing mechanism and get the performance you want - it is almost always necessary to design the mechanics, sensors and controls as a complete system with due reference to the spec.

That goes without saying, which is why the starting point here is closed loop control, and everything else is up for grabs.  I expect that there will be some desire to retain some of the components of past designs, but my inclination towards direct drive certainly doesn't fall into that category.  I'm fascinated to see what emerges, but it's going to need some lateral thinking.

Morgaine.

Your wilfull misunderstanding of my earlier comments does you no credit.

As you are well aware, I have not, at any time, suggested that improvements are not possible.

"The only limits are those we impose on ourselves, or try to impose on others" - this isn't engineering but nonsense - and as an excuse for ignoring the work of countless other engineers it's arrogant as well.

MK

Michael Kellett wrote:

 

"The only limits are those we impose on ourselves, or try to impose on others" - this isn't engineering but nonsense - and as an excuse for ignoring the work of countless other engineers it's arrogant as well.

That's false as a fact and ridiculous as an accusation.  And your personalizing of the discussion is uncalled for as well.

It's by ignoring past assumed limits that we've put people on the moon instead of still lying huddled and shivering in caves.  The "valued experience" of experts assured us that the earth was flat, and later that it was the centre of the universe --- see how far that got us.  Minds as bright as Einstein's weren't sufficient to bring us quantum mechanics, so other people had to add that to our state of knowledge.  And in the fields of engineering, materials and methods and understanding improve continually.  Everything changes, and limits derived from partial understanding fall away as our understanding improves.  They are entirely self-imposed limits, and to consider them absolute is to not understand how science works.

Arrogance was a pretty negative and irrelevant thing for you to bring up here, but if you're looking for arrogance, consider your own belief that nothing will supersede the experience of the last 300 years or your own knowledge of lessons from CNC --- that's arrogance to the point of comedy.  It's also arrogance to view engineers as high priests preaching unquestionable gospel and wisdom.  There is no such thing --- everything that we know evolves in relevance, and we have so many ad hoc rules of thumb in engineering that treating them as conditional is always advised.

A better approach if one is interested in the future is to embrace the key principle held by scientists and a core M.O. of engineers who don't have have a closed mind --- the scientific method, which in an engineering context equates to "nothing is sacrosanct".  Best engineering practices and the most cherished theories are only as good as the next development that improves upon them.  This is as true in machine tool operation as it is in everything else.  It is expected and unavoidable in domestic 3D printing because we're barely on the first rung of that ladder.

To answer your last point specifically, it's important to take into account existing experience and best practices where relevant, but only where relevant.  Indeed, the whole point of past experience is to identify the conditions that make known difficulties relevant, so that we can bypass them.  If a known limitation is likely to bite us if we head down a certain road, then the answer is not to head down that road.  And that's exactly what this thread is about, since as Ben Heck said (and I agree), there is not much mileage available in the current open-loop designs for making 3D printers significantly cheaper.

I'm not sure why you're trying to naysay future development starting from closed loop design.  You've certainly not presented any argument for why it's not a good way forward, and you haven't bothered to answer previous posts that addressed yours either, so it looks like you're simply begging for a fight as always.

Future development is done in the context of past experience.  Past experience should never be employed as a shackle.

Morgaine.