Knee motor issue

Hello Robotical team and everyone,

I have managed to print my Marty. Calibration after many attempts has been completed. But right now, I have an issue with the movement of the right or left knee. I tested all motors using all Scratch commands and it worked fine. However, using the calibration tool when I click walk or through Scratch when I click a command like Walk 2 steps etc., I have this issue that you can see in the video attached. When I hold Marty in the air, motors work fine, but on the table, it seems that the knee needs a support in order to return back to the straight position.

I assume that this kind of support only the foot-springs can offer. But I think the 3d printed parts (foot in this case) are different from the mass production parts and they do not have the clippers where you can attach the foot springs.

Do you think that I can find solution without foot springs? Have you faced the same problem? Is that the right solution to my issue or do you think I can support the knee with other solution?

Thank you very much in advance

Regards, John

ps1. here ( you can see the video with the knee issue ps2. here ( and here ( I have attached the 3d printed foot that is quite different from the one of the mass production


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2 Responses

trojan OP

Hi Sandy,

Thank you so much for the immediate answer. My Marty thanks you for the compliment :). Yes, I had to clean up all the 3d printed parts, especially the links and the bores on the feet and servo holders in order to have a smooth and free moving. Actually, I used a soldering iron to widen the bores instead of a drill that sometimes produces some notches. So, I will follow your suggestions about the springs. I have already ordered some springs and I hope that they will work. I will let you know when I have some good news.

Thank you,


16:06:58, 12th February 2018   |   Permalink   |   View Source
sandy STAFF

Hi John,

Your Marty is looking great! Love the colour scheme!

Thanks for linking to the video and photos. You're right that the 3D printed version uses a different foot and different springs to the production version - that's because the springs we get for the production version are custom made for us. With the 3D printed version you can use off the shelf extension springs to give the extra lift, as shown in the instructions here: 3D printing instructions

I bought a mixed spring kit from Maplin to do the first Martys, but I think some of these would probably work. You can connect them to Marty as shown in the 3d print instructions

One more thing to check though - how free moving are the lower leg links? If there are prominent irregularities on the 3D printed parts that can cause too much friction, and also lead to that behaviour. Ideally the links should be movable without much force at all (except to backdrive the motor when it's connected).

I always have to spend a little while with a craft knife cleaning up the link parts on a 3D printed Marty!

Let me know how you get on,


12:53:06, 12th February 2018  *   |   Permalink   |   View Source