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First steering test deemed tentative sucess

A project log for Landbeest walking robot

Terrain-going quadruped. 3D printable. Two servo drive.

Dejan RisticDejan Ristic 02/05/2019 at 22:330 Comments

I've now built and tested the LB-101 prototype. I spent very little time on this one, and took a lot of design shortcuts.  

Mainly I wanted a practical test of turning by means of articulated steering. I also wanted to find out if this steering method is feasible using only 3D printed parts.

I figured that if this very rough draft could manage to turn left and right without immediately breaking down, then there might be some merit to this this type of solution. I'd hate to have spent weeks on a solution that ends up being a dead-end when finally put to the test. 

My hopes started waning during assembly and I sort of suspected whole thing would tear itself apart immediately. So I was somewhat surprised it actually worked as well as it did.

Results

Here's LB-101 executing left turns at 15 and then 30 degrees:

The 15° turn performed fairly well, but the robot gets quite unbalanced at 30°. You can also tell the servos are really struggling at 30°. Much steeper than that and they'd stall completely. I took power from an laptop USB output, which I believe maxes out at 500 mA. So given a bit more juice they'd probably have managed to soldier on at a steeper angle. But still there's clearly a lot of friction in the moving parts.  

Here's a closer look at the engineering horror story that makes this thing tick:

Compared to the Zero prototype, the two actuators are now about twice as long and have three jointed segments along their travel across the chassis hinge.

On the first run the actuators got misaligned and the whole thing stalled. To keep them moving along the right track I hot glued some bits of plastic under the front carriage. I also put lithium grease on joints and moving parts, but I doubt that makes much of a difference. 

Conclusions

Clearly the chassis is too long—left and right legs needs to be spaced further apart. This design error results in the robot leaning over to one side when turning. Since I based LB-101 on the previous prototypes 3D model I just kept the same chassis width of 80 mm while almost doubling the length to 190 mm. Even though I realized this might be a problem, I ignored it in a fit of overzealous shortcut-taking. A reasonable chassis width to length ratio would probably be about 3:4.

Another issue is that the jointed actuators were pretty difficult to print and needed a lot of cleanup before assembly. Some of that is just sloppy design work on my part, but a bit of complexity is just inherent in the concept. I'd prefer that one wouldn't need a perfectly calibrated 3D printer in order to make this thing. 

On the whole I'd say articulated steering is a viable method provided I put a lot more work into the design. However I'm still not convinced it's the ideal solution. The turning radius can be improved a bit but it's never going to be tight. And the internal friction will make the robot prone to breaking down after weeks of wear and tear.

The next prototype is going to be all about the new drive train discussed in the previous log. If that works out as well as I hope I will have some more options for controlling the legs movement patterns. Steering by leg movement alone has its own set of challenges but at least it will be more mechanically robust than this method. 

In any case I'll put the steering design work on hold for now, and plan to take a fresh look at it in a few weeks. . 

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