Help with finding solutions for getting an AT-AT walk cycle like in the movies.

[BrickBear]

46 minutes ago, Nelson said:

I made some adjustments to the cam contact areas and got more lift. This helps the inside "toe" clear the ankle stabilizer. I may make the toes hinged so they clear better and look more accurate. (I had that feature on an early model.) I'll just have to see if it compromises stability. The cadence is still a bit off. I'm pretty sure this is a result of the mechanism's geometry being imperfect and the limited range of the ankle joint. The rear foot can't flatten out when the leg comes down because the ankle only has a flex of a few degrees. This is a fine line to get right. I could try to get a bit more flexibility range, but then risk the walker falling back on its rear. (I believe @BrickBear was experiencing this phenomenon.)

I think the next step is to try and add some body panels and a rudimentary head so I can see how the design handles the weight and balance changes. 

There's also the issue of the legs which begin to split apart after a while. I'm bummed about that. I spent a massive amount of time getting these legs designed (with strength in mind) and I feel like the options of build styles I have for getting better longevity will compromise the ascetic. But hey. That's part of the challenge.

 

 

I reckon you could fix the cadence problem (i.e only one foot off the ground) not by tweaking the gearing and mechanism but by adding a spring or two to each foot. 
 

With the spring pushing the previous foot forward a little it should in theory touch the ground as the next foot lifts, giving the illusion all is in sync.

As for my AT-AT’s balance it’s more the issue is that is has the potential to fall forward left or forward right if the weight isn’t on the feet on the ground but i’m hopefully going to remedy this with the seesaw mechanism. 

[Nelson]

@BrickBear I'm not following. Are you saying the whole foot would be attached via springs? Would the overall length of the leg-foot assembly vary as the springs compressed and relaxed? I don't see how that would work. It seems like it would introduce a slew of other issues.

[BrickBear]

1 hour ago, Nelson said:

@BrickBear I'm not following. Are you saying the whole foot would be attached via springs? Would the overall length of the leg-foot assembly vary as the springs compressed and relaxed? I don't see how that would work. It seems like it would introduce a slew of other issues.

Yes, basically the (soft) spring(s) would be half a pin hole longer than the current pivot point for the foot when fully extended. This would give the illusion that the foot has touched the ground when the next foot lifts.

As the leg moves the spring could then contract to the position that you currently have the feet in (adding a stopper to prevent it contracting more) and that may well solve the problem of the foot not quite touching the ground upon its step forward, being the soft kind of spring as well it should not place too much pressure on the ground if any to affect the balance as the weight of the walker would compensate.  
 

 

[Nelson]

17 minutes ago, BrickBear said:

Yes, basically the (soft) spring(s) would be half a pin hole longer than the current pivot point for the foot when fully extended. This would give the illusion that the foot has touched the ground when the next foot lifts.

As the leg moves the spring could then contract to the position that you currently have the feet in (adding a stopper to prevent it contracting more) and that may well solve the problem of the foot not quite touching the ground upon its step forward, being the soft kind of spring as well it should not place too much pressure on the ground if any to affect the balance as the weight of the walker would compensate.  
 

 

Interesting idea, but I think I'll explore other options for now. There is only about 2.5 studs worth of lift happening between the foot and the ground. I think a system like that would negate a good percentage of that clearance. It would also be tough to keep the ankle struts' functionality working. Aside from looking accurate, those bars are critical for stability.

[Nelson]

@BrickBear I had an enlightening result from my attempt to add a bit more ankle flex. The whole design almost completely failed when I added even the slightest bit more range. Previously, the foot could only flex about 2-3 degrees at the ankle. I bumped that up to about 5 degrees and suddenly the model pitches heavily toward the lifting leg, nearly falling over. It sounds a lot like what you're experiencing in your design. Maybe try an experiment where you make the ankles of your model quite rigid, just a couple degrees of range. I'm curious to see if that negates some of that leaning you mentioned. I was not expecting that much difference...or any difference really. It was quite surprising and counterintuitive. I would not have suspected that a joint perpendicular to lateral stability would have such an effect. I guess that stiffness was a key component to my current model's success. Looks like I'll be putting it back the way it was. I'm trying to decide if I'm satisfied with the way this version walks. It's certainly not perfect, but it's closer than Lego's version. I'm going to do a few more tests this weekend, then put it on the shelf for a while. There are some other projects calling and I need a break on this. I'll keep checking in on this thread to see how things are progressing.

 

Edited January 18 by Nelson

[BrickBear]

5 hours ago, Nelson said:

@BrickBear I had an enlightening result from my attempt to add a bit more ankle flex. The whole design almost completely failed when I added even the slightest bit more range. Previously, the foot could only flex about 2-3 degrees at the ankle. I bumped that up to about 5 degrees and suddenly the model pitches heavily toward the lifting leg, nearly falling over. It sounds a lot like what you're experiencing in your design. Maybe try an experiment where you make the ankles of your model quite rigid, just a couple degrees of range. I'm curious to see if that negates some of that leaning you mentioned. I was not expecting that much difference...or any difference really. It was quite surprising and counterintuitive. I would not have suspected that a joint perpendicular to lateral stability would have such an effect. I guess that stiffness was a key component to my current model's success. Looks like I'll be putting it back the way it was. I'm trying to decide if I'm satisfied with the way this version walks. It's certainly not perfect, but it's closer than Lego's version. I'm going to do a few more tests this weekend, then put it on the shelf for a while. There are some other projects calling and I need a break on this. I'll keep checking in on this thread to see how things are progressing.

 

That is useful to know. Alas I cannot test your theory this weekend due to being ill but I shall stare longingly at my model from my bed.