QuoteDotflyer Thanks Apsu! What kind of mechanical arrangement have you chosen for your machine? Well, I was working on an inverted cantilever delta, where the arms are attached to the bottom frame and bend over the bed. I have a thread in this very subforum discussing it in depth. Unfortunately, I ran into some very difficult to solve problems with respect to motor placement, actuation mechaniby Apsu - Look what I made!
I could make a video if necessary, but if you look at the last diagram I posted, with the crossed belts, what happens is very simple. As one belt moves closer (and the other end of it moves away), the two speeds they each move is not the same, because they're crossing the hypotenuse of each triangle half inside the scissor linkage. This means the belt either keeps tightening until it binds, or itby Apsu - Look what I made!
Well, that actuation attempt was a bust I'm not that great at geometry, at least in an intuitive sense, and I totally forgot/missed the differing rates of hypotenuse expansion/collapse for the opposite corner pairs. Belts do not, in fact, work I think the next step would be to go back to gears and switch the motor position to the back of the parallel arm, much like the belted layout, but thereby Apsu - Look what I made!
This is great! I'm also designing and building a new printer, and have to say you did fantastic work, especially considering your (initially) limited background. The quality of work, considering, is extremely polished. Keep us updated!by Apsu - Look what I made!
Another way to think about this without compensating is to compute the height for the angle, and factor in the deg/step of the motor and teeth/radius of the drive pulley, to see what the minimum step/effector angle is. Thinking it through out loud, I think it looks something like this: 1.8deg/step motor with 20T pulley with 2mm pitch GT2 belt, using standard steps/mm formulas with worst-case fuby Apsu - Look what I made!
Quoteo_lampe Thanks for taking the time to explain it to a limited german mind But you mentioned the scissor lift which was actually what I had in mind too, when I wrote my last reply. A car jack is similar to a scissor lift, right? When you try to lift a heavy car ( and they are all overweight nowadays ) you feel it very difficult to turn the crank at the beginning. Later, when the jack is almby Apsu - Look what I made!
Quoteo_lampe The last sketch is pretty simple and straight forward, but it also has a drawback: Lengths of the hypotenuse of P_P triangle specifies the "gear ratio" and thereby accuracy, but it is very low when you print on the bed and the arms are almost folded completely. ( Jeez, I hope you got my point? ) Also the longer it is, the more inertia the motor will produce. I thought through thisby Apsu - Look what I made!
Alright, did some rendered animated joint studies. These clearly show that the basic idea of the shoulders and straight cantilever can reach a significant portion of its volume. It's nearly an armlength-radius x armlength-height cylinder. Study 1 Study 2 Then I thought about the problem a lot more, and I think I've come up with a really good solution to motor placement and actuation system, allby Apsu - Look what I made!
Yep, it's all about the elbow/shoulder angles. I whipped up a quick motion simulation with the relevant joints in place and it moves as expected over a relatively large volume (compared to the base). YouTube Link As you can see, plenty of clearance for a 100mm tall 100mm radius cylinder. The trick is that the elbow and shoulder joints need to hit up in the 60-70º deflection range. Which meansby Apsu - Look what I made!
Alright, after setting up the new brackets and shoulders I've realized that my dimensions aren't quite right for the general size of frame I want to achieve, and for the arm angle limitations due to my current motor placement. Here's a picture of the current setup The arms are just too long given the -40º to +40º arm angle range the motor placement limits me to, so I think I'm going to model uby Apsu - Look what I made!
Alright, I came up with a two-piece shoulder design and a new frame bracket for it to sit in. The frame bracket has open ends now so the extrusions can be slid farther through the bracket to resize the arm locations as necessary given what extrusion you have on hand, or just want to resize it to adjust build envelope.by Apsu - Look what I made!
QuoteApsu Please note that without endstops, I'm telling the IK where the arms are to start, but I'm manually positioning them to roughly that spot, which appears to account for the minor Z variations during X/Y moves visible in the video. That and the fact that these long, 20% infill Carbon Fiber PETG arms have a bit of flex to them, which might account even more for the variation as the arms arby Apsu - Look what I made!
Ok, new video! After tuning steps/mm and accel settings in conjunction with my IK code that pretends to be Cartesian, I was able to vastly increase the smoothness and behavior of the prototype. YouTube Link Please note that without endstops, I'm telling the IK where the arms are to start, but I'm manually positioning them to roughly that spot, which appears to account for the minor Z variationsby Apsu - Look what I made!
Thinking about it, I *do* see a way to add a planetary gearbox in here, and I think I can do it in the same space the current spur gear is using! Theory is to halve the height of the spur and pinion, make a ring fixed to the lower link underneath it, attach the sun to the current spur, and attach the planet carrier to the parallel arm. If I keep the same 20T count on sun and planets, and 60T onby Apsu - Look what I made!
Quoteo_lampe Maybe the cycloidal gearbox would be helpful here? You might end up with three NEMA14 pancakes like you wanted to in the beginning... Oh man I love cycloidal gearboxes and eccentric drive systems. But as far as I know, there's not a great/cheap way to fix these disadvantages: . Great idea though! Maybe a compact, compound planetary gearbox might be doable. I'm thinking about possibby Apsu - Look what I made!
Added a page on thingiverse to track the project as well, and so others can see the media and model files more easily.by Apsu - Look what I made!
Another quick update, I completed the Inverse Kinematics using some very clean Python code to generate alpha/beta/gamma "distance" gcode which corresponds to arm angles. Once I get the physical prototype a little more refined in design, I'll figure out how to port it to Smoothieware. The code is here at the moment. There's some vestigial scaling trappings in there too, trying to figure out how tby Apsu - Look what I made!
QuoteMechaBits Quoteo_lampe What if you'd skip the parallelogram arms and use a leadscrew to move the upper vertical arm directly? Don't know about the added complexity involved, but the resolution would be better. Yeah I was thinking of this for the scara, which is similar to the arms here if flipped 90 deg Sure, for something with the orientation flipped and SCARA-like where the arm extensionby Apsu - Look what I made!
Well the primary reason for the parallelogram/parallel linkage configuration is so the actuator doesn't have to deal with triangularly-varying angles and distances as the elbow angle changes. I'm not sure I see how to attach a leadscrew in a way that wouldn't physically bind as the angle varies without mounting the motor on a proportionally-varying link. Care to diagram what you're suggesting ifby Apsu - Look what I made!
It lives! Short Video At least an early alpha incarnation of it Lots of work left to do, but my controller works, motors work, old ATX PSU I chopped into a bench supply 10 years ago works... off to a pretty good start for first prototype build! Also, I was off on the IK simplification above. We can only consider one axis for an arm, yes, but that ignores the shoulder-arm and shoulder-baseby Apsu - Look what I made!
Quoteo_lampe Pretty interesting design! What steps/mm could you achieve with the 20/60 gears and a common 1.8°stepper? Thanks, I and a lot of other folks think so too! Quick update and I'll try to answer your question. I have three arm assemblies built and ready to go as of this morning: And my motors arrive today, so I'm pretty excited. Still have to figure out the "wrist" design a little bby Apsu - Look what I made!
Hey folks, I'm not sure what the policy on cross-posting across subforums is here, but I posted about a new design I'm working on in the Look What I Made section, here. I wasn't sure if it should belong there, here or the design section, so I'll just mention it hereby Apsu - Mechanics
Howdy folks! I've been a regular in the #RepRap IRC channel for a few months now and caught the design/building bug which matches my skillset well. I got interested in the GUS Simpson Nicholas Seward spearheaded a few years ago and Solidus Labs' GDR Dogleg variation. Reading through the design evolution and analysis threads, I saw the potential for solving some of the early problems in a differeby Apsu - Look what I made!