Comments on RepRap: Builders: BAMbot (bamboo robot)

This is the best thing I've seen in ages! I'll be ...

2009-10-13T03:05:40.531-07:00

This is the best thing I've seen in ages! I'll be watching...

(you've probly thought about this already, but the bowden extruder would lend itself well to this, decoupling the motor and all)

Hi, I think it's a great idea to allow fabrication...

2009-10-01T01:10:24.075-07:00

Hi,
I think it's a great idea to allow fabrication of larger pieces than the largest of components in the replicator !
Also, the machine is a lot simpler than a cartesian bot.

However, this type of robotic arm might be even easier to power and control: http://en.wikipedia.org/wiki/SCARA
(but probably moving the linear part to the first joint, instead of the last to reduce arm inertia)

The static loads on the motors will be zero, and vertical bending in the arms can be minimized by constructing the arms as torsion boxes.

I was intending to build a rep(st)rap/cnc extruder with a cartesian architecture and steppers, but your post inspired me to go SCARA style with DC motors and encoders!

I'm not sure about the utility of worm drives in a SCARA though, the non-reverse drive feature is not needed and high friction and low speed might not be desirable

I'll try to keep people updated here or start my own site.

--Jonathan

BTW Do you have a blog that I can follow your prog...

2009-09-29T05:10:28.423-07:00

BTW Do you have a blog that I can follow your progress on ??

Wow, power to your elbow. A big challenge sure, b...

2009-09-28T08:42:41.481-07:00

Wow, power to your elbow.

A big challenge sure, but definitely worth the effort.

It is pretty much doable, FPGA should give you quite fine grain control too.

Must admit I would have been tempted to try RC servo motors as an intial experiment.

As a person who favours servo over stepper I wish you every success.

Cheers

aka47

BeagleFury, Yea, I thought about this a while bac...

2009-09-24T10:01:37.444-07:00

BeagleFury,

Yea, I thought about this a while back. I've think I've come up with a way to make scalable encoders that increase with accuracy as they increase in size. Therefore the accuracy at the end point will remain constant as we scale up.

It's in a more recent post.

One thing to keep in mind for "build big stuff the...

2009-09-21T06:34:58.798-07:00

One thing to keep in mind for "build big stuff then worry about detail stuff later".. you'll require some minimum of accuracy based on filament size.

Even if you want to build a big couch, if you are building it with .5 mm thread, you'll need accuracy, probably at least to probably half of that (.25mm) to ensure the thread you lay down actually combines to make the big object you intend, without gaps, holes, etc.

Theoretically, if you can lay down bigger thread (say, 1cm thread), your accuracy requirements will probably loosen. However, I don't think current reprap techniques will scale too large.. heating and cooling, for example, is a harder problem to solve with larger blobs of plastic.

One way you might be able to overcome this, since RepRap doesn't require a lot of force in it's positioning system, is to figure out a way to precisely measure the point of extrusion (without measuring joint angles). I think you'll be in a much stronger position to get the accuracy you need, as long as your feedback system is responsive enough and you can avoid introducing feedback degeneration.

Put a threaded rod coming out the back of both maj...

2009-09-20T21:11:26.140-07:00

Put a threaded rod coming out the back of both major arm segments and use that to attach washers/metal plates/cast cement to act as a counterbalance. Your dynamic max performance will drop but you'll also offload most of the static power requirements.

Yea, if I was starting over from scracth I would u...

2009-09-20T20:41:26.078-07:00

Yea, if I was starting over from scracth I would use a worm drive or a linear actuator to power the arm motion. Hopefully I can get a small arm working without doing this. Then the next generation (larger version) would use these.

I'll be using GM17's from solarbotics and the arms will be ~250 mm long. I've shortened them considerably to make this work.

Ok, some things to keep in mind. I tried something...

2009-09-20T20:03:33.584-07:00

Ok, some things to keep in mind. I tried something similar and discovered that servos, or at least the cheap ones I was using weren't strong enough or accurate enough to move correctly. Also, inverse kinematics for arms, aka mapping 3 space coordinates to joint angles isn't an easy task. I blew a lot of time trying and failing there.

If I was to do it all over again I would look into worm geared motors because they aren't back drivable which means that there isn't a load on a dc motor at rest. The only problem is that if your not careful or don't have the fancy ones with internal dampening springs, you can damage the whole setup by bumping into something too hard. But its a repstrap right?

Also, watch out for power issues. What you have there is in essence an unsupported crane without the counter balanced weight. Its going to take a hefty current to move it about with the tool head attached. In short, decouple the processor and motor driver power supplies.

And finally, good luck and post your progress!

Bottom line is negative comments not backed up by ...

2009-09-20T17:18:02.652-07:00

Bottom line is negative comments not backed up by hands on experience aren't worth spit. That's why I always pay very close attention to guys like nophead who gets his hands dirty all the time and pretty much ignore all the arm chair critics. :-)

Hey Forrest! Thanks for the support. :D However...

2009-09-20T13:28:49.427-07:00

Hey Forrest!

Thanks for the support. :D

However, I respect everyones comments, even if they are negative. If I'm doing something stupid that won't work, I'd rather know right away rather than figure it out on my own.

Also, negative comments are like grit, which polishes my design and highlights future problems.

I'm a pretty stubborn guy. I'll make this work one way or another. It might take longer than I thought and it might be harder than I thought (usually is) but that's what makes it a worthwhile achievement.

So, if you have a negative comment, bring it on. The more eyes looking at the design and poking holes in it, the better the finished product.

Hi, I don't know how to reply directly to a post ...

2009-09-20T13:22:48.682-07:00

Hi,

I don't know how to reply directly to a post on blogger. This is a reply to Erik de Bruijin.

I just feel that G code is a bad intermediary step. It looses all of the information in the model. I feel that this is quite useful information and shouldn't be discarded when figuring the toolpaths.

From what I understand it's a two step process. Generate G code from model in PC. Generate motion profile from G code in ATMega. I think these two steps should be combined because it will create better designs and allow for more capabilities.

I also think some of that intelligence should be moved into the design tools so that the user can specify tolerances of paths or infill or other TBD functions in the modeling tools.

-Brian

Yea, I think it will make a lot more sense to prin...

2009-09-20T12:44:28.010-07:00

Yea, I think it will make a lot more sense to print out big simple things first and then small complex things later.

Things like furniture seems like it would be a really good application. It's a high dollar item that is relatively simple. We could even incorporate lighting into furniture with our current technology.

Then houses, car bodies... things like that. Large complex shapes which are composed of one or two materials.

I'd eventually like to shift print materials though. Plastic is kinda expensive. I think a mix of Grancrete and recycled paper would do very well and be low cost.

Neat little machine! I especially like the scaleab...

2009-09-20T11:16:32.436-07:00

Neat little machine! I especially like the scaleability of the design. I already envision a much larger version of this arm bolted to the ceiling of my garage and printing my own car.... :-)

You are on the right track.

@letsburn00, When operating as a servo the curre...

2009-09-20T09:12:32.446-07:00

@letsburn00,
When operating as a servo the current is controlled to match the static load when stationary. As long as the load is not massive, after the gearing is taken into account, then the holding current will not be much, so the motor will be fine.

It is not the same as stalling a motor connected to an uncontrolled supply, when indeed the current does increase massively.

Hey, talk is the cheapest thing around the Reprap ...

2009-09-20T09:01:23.225-07:00

Hey, talk is the cheapest thing around the Reprap project. Brian is to be warmly commended for getting out there and actually building something. That is infinitely more valuable than all of the pompous verbosity that passes for thought experiments around here. :-D

I have a similar software design where the g-code ...

2009-09-20T08:05:50.235-07:00

I have a similar software design where the g-code is parsed on the computer and turned into something very similar to your t-code, in my case it's short strings of step data.

I've noticed a few things that might lead to a slightly different design:

1. The smarter the process parsing the geometric data into motion control, the bigger bandwidth you need to the motion controller. You can help this somewhat by adding a buffer, but in the end bandwidth is better.

2. My laptop is overkill for the geometry to motion control parsing. An ARM or potentially a ARM-softcore on an FPGA, could easily handle the geometry to motion control parsing.

3. Having the laptop tethered to the machine isn't very much fun. Especially once you get to the point where you have several devices running, now your laptop is a single point of failure.

4. While GCode isn't pretty, it's the standard geometry description language. It also has the benefit of being semi easy for humans to generate. While your t-code sounds like binary, gcode is more like assembly. Supporting it as an input format or as an interactive (shell) format seems like a good thing to do.

So I'm leaning towards taking an arm core, slapping on some memory and running the device on that.

> I don't natively support g-code. I hate g-code. ...

2009-09-20T05:07:37.238-07:00

> I don't natively support g-code. I hate g-code. It does not contain any acceleration/velocity data.

Actually the 5D G-Code interpreter firmware contains a linear acceleration capability and of course still the feed rate.
Have a look here:
http://blog.reprap.org/2009/06/acceleration-code-released.html

I've been using it to reduce stringing because you can achieve much higher speeds.

If the problem is stalling, maybe a solution would...

2009-09-20T05:03:36.471-07:00

If the problem is stalling, maybe a solution would be to have a "brake", like a friction pad on a bicycle, perhaps operated by a linear electric motor, which can clamp down on the joint to stop it from moving when it doesn't have to.

I'm sceptical about accuracy, but people have been...

2009-09-19T22:03:51.901-07:00

I'm sceptical about accuracy, but people have been talking about this forever, and it just might work :)

Aren't servo's just DC gear motors? I must be mis...

2009-09-19T20:48:21.628-07:00

Aren't servo's just DC gear motors?

I must be missing something.

From what I understand, one of the big issues that...

2009-09-19T20:13:40.598-07:00

From what I understand, one of the big issues that you'll have is that DC motors when they aren't moving but still holding a weight(basically a stall) will start taking huge current loads and burn out much faster, steppers and servos's avoid this problem.

On the other hand, if you can get this to work (it's very difficult and many have tried) then kudos. One way for you to maintain good control would be to have a motor pulling each axis both ways. They pull against each other and that is a way to get better control. It's the way muscles work and aparently is how Disney do it in all their anamatronics.

a design basis might be gleaned from this http://www.thingiverse.com/thing:781 printable arm that uses servo's that Simon Kirkby designed.